diff --git a/src/arch/aarch64/CodeGen.zig b/src/arch/aarch64/CodeGen.zig
index 825bf51b1f..e43cbca1c7 100644
--- a/src/arch/aarch64/CodeGen.zig
+++ b/src/arch/aarch64/CodeGen.zig
@@ -23,6 +23,7 @@ const log = std.log.scoped(.codegen);
 const build_options = @import("build_options");
 const RegisterManagerFn = @import("../../register_manager.zig").RegisterManager;
 const RegisterManager = RegisterManagerFn(Self, Register, &callee_preserved_regs);
+const RegisterLock = RegisterManager.RegisterLock;
 
 const GenerateSymbolError = @import("../../codegen.zig").GenerateSymbolError;
 const FnResult = @import("../../codegen.zig").FnResult;
@@ -727,7 +728,7 @@ fn genBody(self: *Self, body: []const Air.Inst.Index) InnerError!void {
             // zig fmt: on
         }
 
-        assert(!self.register_manager.frozenRegsExist());
+        assert(!self.register_manager.lockedRegsExist());
 
         if (std.debug.runtime_safety) {
             if (self.air_bookkeeping < old_air_bookkeeping + 1) {
@@ -910,16 +911,16 @@ pub fn spillInstruction(self: *Self, reg: Register, inst: Air.Inst.Index) !void
 fn spillCompareFlagsIfOccupied(self: *Self) !void {
     if (self.compare_flags_inst) |inst_to_save| {
         const mcv = self.getResolvedInstValue(inst_to_save);
-        switch (mcv) {
+        const new_mcv = switch (mcv) {
             .compare_flags_signed,
             .compare_flags_unsigned,
+            => try self.allocRegOrMem(inst_to_save, true),
             .register_c_flag,
             .register_v_flag,
-            => {},
+            => try self.allocRegOrMem(inst_to_save, false),
             else => unreachable, // mcv doesn't occupy the compare flags
-        }
+        };
 
-        const new_mcv = try self.allocRegOrMem(inst_to_save, true);
         try self.setRegOrMem(self.air.typeOfIndex(inst_to_save), new_mcv, mcv);
         log.debug("spilling {d} to mcv {any}", .{ inst_to_save, new_mcv });
 
@@ -927,6 +928,15 @@ fn spillCompareFlagsIfOccupied(self: *Self) !void {
         try branch.inst_table.put(self.gpa, inst_to_save, new_mcv);
 
         self.compare_flags_inst = null;
+
+        // TODO consolidate with register manager and spillInstruction
+        // this call should really belong in the register manager!
+        switch (mcv) {
+            .register_c_flag,
+            .register_v_flag,
+            => |reg| self.register_manager.freeReg(reg),
+            else => {},
+        }
     }
 }
 
@@ -1048,8 +1058,8 @@ fn trunc(
                 }
             },
         };
-        self.register_manager.freezeRegs(&.{operand_reg});
-        defer self.register_manager.unfreezeRegs(&.{operand_reg});
+        const lock = self.register_manager.lockReg(operand_reg);
+        defer if (lock) |reg| self.register_manager.unlockReg(reg);
 
         const dest_reg = if (maybe_inst) |inst| blk: {
             const ty_op = self.air.instructions.items(.data)[inst].ty_op;
@@ -1135,8 +1145,8 @@ fn airNot(self: *Self, inst: Air.Inst.Index) !void {
                             .register => |r| r,
                             else => try self.copyToTmpRegister(operand_ty, operand),
                         };
-                        self.register_manager.freezeRegs(&.{op_reg});
-                        defer self.register_manager.unfreezeRegs(&.{op_reg});
+                        const reg_lock = self.register_manager.lockRegAssumeUnused(op_reg);
+                        defer self.register_manager.unlockReg(reg_lock);
 
                         const dest_reg = blk: {
                             if (operand == .register and self.reuseOperand(inst, ty_op.operand, 0, operand)) {
@@ -1168,8 +1178,8 @@ fn airNot(self: *Self, inst: Air.Inst.Index) !void {
                                 .register => |r| r,
                                 else => try self.copyToTmpRegister(operand_ty, operand),
                             };
-                            self.register_manager.freezeRegs(&.{op_reg});
-                            defer self.register_manager.unfreezeRegs(&.{op_reg});
+                            const reg_lock = self.register_manager.lockRegAssumeUnused(op_reg);
+                            defer self.register_manager.unlockReg(reg_lock);
 
                             const dest_reg = blk: {
                                 if (operand == .register and self.reuseOperand(inst, ty_op.operand, 0, operand)) {
@@ -1257,8 +1267,17 @@ fn binOpRegister(
     const lhs_is_register = lhs == .register;
     const rhs_is_register = rhs == .register;
 
-    if (lhs_is_register) self.register_manager.freezeRegs(&.{lhs.register});
-    if (rhs_is_register) self.register_manager.freezeRegs(&.{rhs.register});
+    const lhs_lock: ?RegisterLock = if (lhs_is_register)
+        self.register_manager.lockReg(lhs.register)
+    else
+        null;
+    defer if (lhs_lock) |reg| self.register_manager.unlockReg(reg);
+
+    const rhs_lock: ?RegisterLock = if (rhs_is_register)
+        self.register_manager.lockReg(rhs.register)
+    else
+        null;
+    defer if (rhs_lock) |reg| self.register_manager.unlockReg(reg);
 
     const branch = &self.branch_stack.items[self.branch_stack.items.len - 1];
 
@@ -1270,13 +1289,13 @@ fn binOpRegister(
 
         const raw_reg = try self.register_manager.allocReg(track_inst);
         const reg = registerAlias(raw_reg, lhs_ty.abiSize(self.target.*));
-        self.register_manager.freezeRegs(&.{reg});
 
         if (track_inst) |inst| branch.inst_table.putAssumeCapacity(inst, .{ .register = reg });
 
         break :blk reg;
     };
-    defer self.register_manager.unfreezeRegs(&.{lhs_reg});
+    const new_lhs_lock = self.register_manager.lockReg(lhs_reg);
+    defer if (new_lhs_lock) |reg| self.register_manager.unlockReg(reg);
 
     const rhs_reg = if (rhs_is_register) rhs.register else blk: {
         const track_inst: ?Air.Inst.Index = if (maybe_inst) |inst| inst: {
@@ -1286,13 +1305,13 @@ fn binOpRegister(
 
         const raw_reg = try self.register_manager.allocReg(track_inst);
         const reg = registerAlias(raw_reg, rhs_ty.abiAlignment(self.target.*));
-        self.register_manager.freezeRegs(&.{reg});
 
         if (track_inst) |inst| branch.inst_table.putAssumeCapacity(inst, .{ .register = reg });
 
         break :blk reg;
     };
-    defer self.register_manager.unfreezeRegs(&.{rhs_reg});
+    const new_rhs_lock = self.register_manager.lockReg(rhs_reg);
+    defer if (new_rhs_lock) |reg| self.register_manager.unlockReg(reg);
 
     const dest_reg = switch (mir_tag) {
         .cmp_shifted_register => undefined, // cmp has no destination register
@@ -1394,7 +1413,11 @@ fn binOpImmediate(
 ) !MCValue {
     const lhs_is_register = lhs == .register;
 
-    if (lhs_is_register) self.register_manager.freezeRegs(&.{lhs.register});
+    const lhs_lock: ?RegisterLock = if (lhs_is_register)
+        self.register_manager.lockReg(lhs.register)
+    else
+        null;
+    defer if (lhs_lock) |reg| self.register_manager.unlockReg(reg);
 
     const branch = &self.branch_stack.items[self.branch_stack.items.len - 1];
 
@@ -1408,13 +1431,13 @@ fn binOpImmediate(
 
         const raw_reg = try self.register_manager.allocReg(track_inst);
         const reg = registerAlias(raw_reg, lhs_ty.abiSize(self.target.*));
-        self.register_manager.freezeRegs(&.{reg});
 
         if (track_inst) |inst| branch.inst_table.putAssumeCapacity(inst, .{ .register = reg });
 
         break :blk reg;
     };
-    defer self.register_manager.unfreezeRegs(&.{lhs_reg});
+    const new_lhs_lock = self.register_manager.lockReg(lhs_reg);
+    defer if (new_lhs_lock) |reg| self.register_manager.unlockReg(reg);
 
     const dest_reg = switch (mir_tag) {
         .cmp_immediate => undefined, // cmp has no destination register
@@ -1758,7 +1781,10 @@ fn airBinOp(self: *Self, inst: Air.Inst.Index) !void {
     const lhs_ty = self.air.typeOf(bin_op.lhs);
     const rhs_ty = self.air.typeOf(bin_op.rhs);
 
-    const result: MCValue = if (self.liveness.isUnused(inst)) .dead else try self.binOp(tag, inst, lhs, rhs, lhs_ty, rhs_ty);
+    const result: MCValue = if (self.liveness.isUnused(inst))
+        .dead
+    else
+        try self.binOp(tag, inst, lhs, rhs, lhs_ty, rhs_ty);
     return self.finishAir(inst, result, .{ bin_op.lhs, bin_op.rhs, .none });
 }
 
@@ -1815,13 +1841,13 @@ fn airOverflow(self: *Self, inst: Air.Inst.Index) !void {
                         };
                         const dest = try self.binOp(base_tag, null, lhs, rhs, lhs_ty, rhs_ty);
                         const dest_reg = dest.register;
-                        self.register_manager.freezeRegs(&.{dest_reg});
-                        defer self.register_manager.unfreezeRegs(&.{dest_reg});
+                        const dest_reg_lock = self.register_manager.lockRegAssumeUnused(dest_reg);
+                        defer self.register_manager.unlockReg(dest_reg_lock);
 
                         const raw_truncated_reg = try self.register_manager.allocReg(null);
                         const truncated_reg = registerAlias(raw_truncated_reg, lhs_ty.abiSize(self.target.*));
-                        self.register_manager.freezeRegs(&.{truncated_reg});
-                        defer self.register_manager.unfreezeRegs(&.{truncated_reg});
+                        const truncated_reg_lock = self.register_manager.lockRegAssumeUnused(truncated_reg);
+                        defer self.register_manager.unlockReg(truncated_reg_lock);
 
                         // sbfx/ubfx truncated, dest, #0, #bits
                         try self.truncRegister(dest_reg, truncated_reg, int_info.signedness, int_info.bits);
@@ -1922,12 +1948,12 @@ fn airMulWithOverflow(self: *Self, inst: Air.Inst.Index) !void {
 
                     const dest = try self.binOpRegister(base_tag, null, lhs, rhs, lhs_ty, rhs_ty);
                     const dest_reg = dest.register;
-                    self.register_manager.freezeRegs(&.{dest_reg});
-                    defer self.register_manager.unfreezeRegs(&.{dest_reg});
+                    const dest_reg_lock = self.register_manager.lockRegAssumeUnused(dest_reg);
+                    defer self.register_manager.unlockReg(dest_reg_lock);
 
                     const truncated_reg = try self.register_manager.allocReg(null);
-                    self.register_manager.freezeRegs(&.{truncated_reg});
-                    defer self.register_manager.unfreezeRegs(&.{truncated_reg});
+                    const truncated_reg_lock = self.register_manager.lockRegAssumeUnused(truncated_reg);
+                    defer self.register_manager.unlockReg(truncated_reg_lock);
 
                     try self.truncRegister(
                         dest_reg.to32(),
@@ -1977,36 +2003,44 @@ fn airMulWithOverflow(self: *Self, inst: Air.Inst.Index) !void {
                     const lhs_is_register = lhs == .register;
                     const rhs_is_register = rhs == .register;
 
-                    if (lhs_is_register) self.register_manager.freezeRegs(&.{lhs.register});
-                    if (rhs_is_register) self.register_manager.freezeRegs(&.{rhs.register});
+                    const lhs_lock: ?RegisterLock = if (lhs_is_register)
+                        self.register_manager.lockRegAssumeUnused(lhs.register)
+                    else
+                        null;
+                    defer if (lhs_lock) |reg| self.register_manager.unlockReg(reg);
+
+                    const rhs_lock: ?RegisterLock = if (rhs_is_register)
+                        self.register_manager.lockRegAssumeUnused(rhs.register)
+                    else
+                        null;
+                    defer if (rhs_lock) |reg| self.register_manager.unlockReg(reg);
 
                     const lhs_reg = if (lhs_is_register) lhs.register else blk: {
                         const raw_reg = try self.register_manager.allocReg(null);
                         const reg = registerAlias(raw_reg, lhs_ty.abiSize(self.target.*));
-                        self.register_manager.freezeRegs(&.{reg});
                         break :blk reg;
                     };
-                    defer self.register_manager.unfreezeRegs(&.{lhs_reg});
+                    const new_lhs_lock = self.register_manager.lockReg(lhs_reg);
+                    defer if (new_lhs_lock) |reg| self.register_manager.unlockReg(reg);
 
                     const rhs_reg = if (rhs_is_register) rhs.register else blk: {
                         const raw_reg = try self.register_manager.allocReg(null);
                         const reg = registerAlias(raw_reg, rhs_ty.abiAlignment(self.target.*));
-                        self.register_manager.freezeRegs(&.{reg});
                         break :blk reg;
                     };
-                    defer self.register_manager.unfreezeRegs(&.{rhs_reg});
+                    const new_rhs_lock = self.register_manager.lockReg(rhs_reg);
+                    defer if (new_rhs_lock) |reg| self.register_manager.unlockReg(reg);
 
                     if (!lhs_is_register) try self.genSetReg(lhs_ty, lhs_reg, lhs);
                     if (!rhs_is_register) try self.genSetReg(rhs_ty, rhs_reg, rhs);
 
-                    // TODO reuse operands
                     const dest_reg = blk: {
                         const raw_reg = try self.register_manager.allocReg(null);
                         const reg = registerAlias(raw_reg, lhs_ty.abiSize(self.target.*));
-                        self.register_manager.freezeRegs(&.{reg});
                         break :blk reg;
                     };
-                    defer self.register_manager.unfreezeRegs(&.{dest_reg});
+                    const dest_reg_lock = self.register_manager.lockRegAssumeUnused(dest_reg);
+                    defer self.register_manager.unlockReg(dest_reg_lock);
 
                     switch (int_info.signedness) {
                         .signed => {
@@ -2021,8 +2055,8 @@ fn airMulWithOverflow(self: *Self, inst: Air.Inst.Index) !void {
                             });
 
                             const dest_high_reg = try self.register_manager.allocReg(null);
-                            self.register_manager.freezeRegs(&.{dest_high_reg});
-                            defer self.register_manager.unfreezeRegs(&.{dest_high_reg});
+                            const dest_high_reg_lock = self.register_manager.lockRegAssumeUnused(dest_high_reg);
+                            defer self.register_manager.unlockReg(dest_high_reg_lock);
 
                             // smulh dest_high, lhs, rhs
                             _ = try self.addInst(.{
@@ -2071,8 +2105,8 @@ fn airMulWithOverflow(self: *Self, inst: Air.Inst.Index) !void {
                         },
                         .unsigned => {
                             const dest_high_reg = try self.register_manager.allocReg(null);
-                            self.register_manager.freezeRegs(&.{dest_high_reg});
-                            defer self.register_manager.unfreezeRegs(&.{dest_high_reg});
+                            const dest_high_reg_lock = self.register_manager.lockRegAssumeUnused(dest_high_reg);
+                            defer self.register_manager.unlockReg(dest_high_reg_lock);
 
                             // umulh dest_high, lhs, rhs
                             _ = try self.addInst(.{
@@ -2127,8 +2161,8 @@ fn airMulWithOverflow(self: *Self, inst: Air.Inst.Index) !void {
                     }
 
                     const truncated_reg = try self.register_manager.allocReg(null);
-                    self.register_manager.freezeRegs(&.{truncated_reg});
-                    defer self.register_manager.unfreezeRegs(&.{truncated_reg});
+                    const truncated_reg_lock = self.register_manager.lockRegAssumeUnused(truncated_reg);
+                    defer self.register_manager.unlockReg(truncated_reg_lock);
 
                     try self.truncRegister(dest_reg, truncated_reg, int_info.signedness, int_info.bits);
 
@@ -2168,14 +2202,20 @@ fn airShlWithOverflow(self: *Self, inst: Air.Inst.Index) !void {
                 if (int_info.bits <= 64) {
                     const stack_offset = try self.allocMem(inst, tuple_size, tuple_align);
 
-                    if (lhs == .register) self.register_manager.freezeRegs(&.{lhs.register});
-                    defer if (lhs == .register) self.register_manager.unfreezeRegs(&.{lhs.register});
+                    const lhs_lock: ?RegisterLock = if (lhs == .register)
+                        self.register_manager.lockRegAssumeUnused(lhs.register)
+                    else
+                        null;
+                    defer if (lhs_lock) |reg| self.register_manager.unlockReg(reg);
 
                     try self.spillCompareFlagsIfOccupied();
                     self.compare_flags_inst = null;
 
                     // lsl dest, lhs, rhs
                     const dest = try self.binOp(.shl, null, lhs, rhs, lhs_ty, rhs_ty);
+                    const dest_reg = dest.register;
+                    const dest_reg_lock = self.register_manager.lockRegAssumeUnused(dest_reg);
+                    defer self.register_manager.unlockReg(dest_reg_lock);
 
                     // asr/lsr reconstructed, dest, rhs
                     const reconstructed = try self.binOp(.shr, null, dest, rhs, lhs_ty, rhs_ty);
@@ -2184,7 +2224,9 @@ fn airShlWithOverflow(self: *Self, inst: Air.Inst.Index) !void {
                     _ = try self.binOp(.cmp_eq, null, lhs, reconstructed, lhs_ty, lhs_ty);
 
                     try self.genSetStack(lhs_ty, stack_offset, dest);
-                    try self.genSetStack(Type.initTag(.u1), stack_offset - overflow_bit_offset, .{ .compare_flags_unsigned = .neq });
+                    try self.genSetStack(Type.initTag(.u1), stack_offset - overflow_bit_offset, .{
+                        .compare_flags_unsigned = .neq,
+                    });
 
                     break :result MCValue{ .stack_offset = stack_offset };
                 } else {
@@ -2411,14 +2453,18 @@ fn airSliceElemVal(self: *Self, inst: Air.Inst.Index) !void {
         var buf: Type.SlicePtrFieldTypeBuffer = undefined;
         const slice_ptr_field_type = slice_ty.slicePtrFieldType(&buf);
 
-        if (index_is_register) self.register_manager.freezeRegs(&.{index_mcv.register});
-        defer if (index_is_register) self.register_manager.unfreezeRegs(&.{index_mcv.register});
+        const index_lock: ?RegisterLock = if (index_is_register)
+            self.register_manager.lockRegAssumeUnused(index_mcv.register)
+        else
+            null;
+        defer if (index_lock) |reg| self.register_manager.unlockReg(reg);
 
         const base_mcv: MCValue = switch (slice_mcv) {
             .stack_offset => |off| .{ .register = try self.copyToTmpRegister(slice_ptr_field_type, .{ .stack_offset = off }) },
             else => return self.fail("TODO slice_elem_val when slice is {}", .{slice_mcv}),
         };
-        self.register_manager.freezeRegs(&.{base_mcv.register});
+        const base_lock = self.register_manager.lockRegAssumeUnused(base_mcv.register);
+        defer self.register_manager.unlockReg(base_lock);
 
         switch (elem_size) {
             else => {
@@ -2559,8 +2605,8 @@ fn load(self: *Self, dst_mcv: MCValue, ptr: MCValue, ptr_ty: Type) InnerError!vo
         .immediate => |imm| try self.setRegOrMem(elem_ty, dst_mcv, .{ .memory = imm }),
         .ptr_stack_offset => |off| try self.setRegOrMem(elem_ty, dst_mcv, .{ .stack_offset = off }),
         .register => |addr_reg| {
-            self.register_manager.freezeRegs(&.{addr_reg});
-            defer self.register_manager.unfreezeRegs(&.{addr_reg});
+            const addr_reg_lock = self.register_manager.lockReg(addr_reg);
+            defer if (addr_reg_lock) |reg| self.register_manager.unlockReg(reg);
 
             switch (dst_mcv) {
                 .dead => unreachable,
@@ -2573,16 +2619,18 @@ fn load(self: *Self, dst_mcv: MCValue, ptr: MCValue, ptr_ty: Type) InnerError!vo
                     if (elem_size <= 8) {
                         const raw_tmp_reg = try self.register_manager.allocReg(null);
                         const tmp_reg = registerAlias(raw_tmp_reg, elem_size);
-                        self.register_manager.freezeRegs(&.{tmp_reg});
-                        defer self.register_manager.unfreezeRegs(&.{tmp_reg});
+                        const tmp_reg_lock = self.register_manager.lockRegAssumeUnused(tmp_reg);
+                        defer self.register_manager.unlockReg(tmp_reg_lock);
 
                         try self.load(.{ .register = tmp_reg }, ptr, ptr_ty);
                         try self.genSetStack(elem_ty, off, MCValue{ .register = tmp_reg });
                     } else {
                         // TODO optimize the register allocation
                         const regs = try self.register_manager.allocRegs(4, .{ null, null, null, null });
-                        self.register_manager.freezeRegs(&regs);
-                        defer self.register_manager.unfreezeRegs(&regs);
+                        const regs_locks = self.register_manager.lockRegsAssumeUnused(4, regs);
+                        defer for (regs_locks) |reg| {
+                            self.register_manager.unlockReg(reg);
+                        };
 
                         const src_reg = addr_reg;
                         const dst_reg = regs[0];
@@ -2784,8 +2832,8 @@ fn store(self: *Self, ptr: MCValue, value: MCValue, ptr_ty: Type, value_ty: Type
             try self.genSetStack(value_ty, off, value);
         },
         .register => |addr_reg| {
-            self.register_manager.freezeRegs(&.{addr_reg});
-            defer self.register_manager.unfreezeRegs(&.{addr_reg});
+            const addr_reg_lock = self.register_manager.lockReg(addr_reg);
+            defer if (addr_reg_lock) |reg| self.register_manager.unlockReg(reg);
 
             switch (value) {
                 .register => |value_reg| {
@@ -2795,8 +2843,8 @@ fn store(self: *Self, ptr: MCValue, value: MCValue, ptr_ty: Type, value_ty: Type
                     if (abi_size <= 8) {
                         const raw_tmp_reg = try self.register_manager.allocReg(null);
                         const tmp_reg = registerAlias(raw_tmp_reg, abi_size);
-                        self.register_manager.freezeRegs(&.{tmp_reg});
-                        defer self.register_manager.unfreezeRegs(&.{tmp_reg});
+                        const tmp_reg_lock = self.register_manager.lockRegAssumeUnused(tmp_reg);
+                        defer self.register_manager.unlockReg(tmp_reg_lock);
 
                         try self.genSetReg(value_ty, tmp_reg, value);
                         try self.store(ptr, .{ .register = tmp_reg }, ptr_ty, value_ty);
@@ -2856,12 +2904,12 @@ fn structFieldPtr(self: *Self, inst: Air.Inst.Index, operand: Air.Inst.Ref, inde
                 const offset_reg = try self.copyToTmpRegister(ptr_ty, .{
                     .immediate = struct_field_offset,
                 });
-                self.register_manager.freezeRegs(&.{offset_reg});
-                defer self.register_manager.unfreezeRegs(&.{offset_reg});
+                const offset_reg_lock = self.register_manager.lockRegAssumeUnused(offset_reg);
+                defer self.register_manager.unlockReg(offset_reg_lock);
 
                 const addr_reg = try self.copyToTmpRegister(ptr_ty, mcv);
-                self.register_manager.freezeRegs(&.{addr_reg});
-                defer self.register_manager.unfreezeRegs(&.{addr_reg});
+                const addr_reg_lock = self.register_manager.lockRegAssumeUnused(addr_reg);
+                defer self.register_manager.unlockReg(addr_reg_lock);
 
                 const dest = try self.binOp(
                     .add,
@@ -3369,6 +3417,9 @@ fn airCondBr(self: *Self, inst: Air.Inst.Index) !void {
     const parent_compare_flags_inst = self.compare_flags_inst;
 
     try self.branch_stack.append(.{});
+    errdefer {
+        _ = self.branch_stack.pop();
+    }
 
     try self.ensureProcessDeathCapacity(liveness_condbr.then_deaths.len);
     for (liveness_condbr.then_deaths) |operand| {
@@ -3955,8 +4006,38 @@ fn genSetStack(self: *Self, ty: Type, stack_offset: u32, mcv: MCValue) InnerErro
         },
         .register_c_flag,
         .register_v_flag,
-        => {
-            return self.fail("TODO implement genSetStack {}", .{mcv});
+        => |reg| {
+            const reg_lock = self.register_manager.lockReg(reg);
+            defer if (reg_lock) |locked_reg| self.register_manager.unlockReg(locked_reg);
+
+            const wrapped_ty = ty.structFieldType(0);
+            try self.genSetStack(wrapped_ty, stack_offset, .{ .register = reg });
+
+            const overflow_bit_ty = ty.structFieldType(1);
+            const overflow_bit_offset = @intCast(u32, ty.structFieldOffset(1, self.target.*));
+            const raw_cond_reg = try self.register_manager.allocReg(null);
+            const cond_reg = registerAlias(
+                raw_cond_reg,
+                @intCast(u32, overflow_bit_ty.abiSize(self.target.*)),
+            );
+
+            // C flag: cset reg, cs
+            // V flag: cset reg, vs
+            _ = try self.addInst(.{
+                .tag = .cset,
+                .data = .{ .r_cond = .{
+                    .rd = cond_reg,
+                    .cond = switch (mcv) {
+                        .register_c_flag => .cs,
+                        .register_v_flag => .vs,
+                        else => unreachable,
+                    },
+                } },
+            });
+
+            try self.genSetStack(overflow_bit_ty, stack_offset - overflow_bit_offset, .{
+                .register = cond_reg,
+            });
         },
         .got_load,
         .direct_load,
@@ -3983,8 +4064,10 @@ fn genSetStack(self: *Self, ty: Type, stack_offset: u32, mcv: MCValue) InnerErro
 
                 // TODO call extern memcpy
                 const regs = try self.register_manager.allocRegs(5, .{ null, null, null, null, null });
-                self.register_manager.freezeRegs(&regs);
-                defer self.register_manager.unfreezeRegs(&regs);
+                const regs_locks = self.register_manager.lockRegsAssumeUnused(5, regs);
+                defer for (regs_locks) |reg| {
+                    self.register_manager.unlockReg(reg);
+                };
 
                 const src_reg = regs[0];
                 const dst_reg = regs[1];
diff --git a/src/arch/arm/CodeGen.zig b/src/arch/arm/CodeGen.zig
index d463ba9928..02ca66f297 100644
--- a/src/arch/arm/CodeGen.zig
+++ b/src/arch/arm/CodeGen.zig
@@ -23,6 +23,7 @@ const log = std.log.scoped(.codegen);
 const build_options = @import("build_options");
 const RegisterManagerFn = @import("../../register_manager.zig").RegisterManager;
 const RegisterManager = RegisterManagerFn(Self, Register, &allocatable_registers);
+const RegisterLock = RegisterManager.RegisterLock;
 
 const FnResult = @import("../../codegen.zig").FnResult;
 const GenerateSymbolError = @import("../../codegen.zig").GenerateSymbolError;
@@ -734,7 +735,7 @@ fn genBody(self: *Self, body: []const Air.Inst.Index) InnerError!void {
             // zig fmt: on
         }
 
-        assert(!self.register_manager.frozenRegsExist());
+        assert(!self.register_manager.lockedRegsExist());
 
         if (std.debug.runtime_safety) {
             if (self.air_bookkeeping < old_air_bookkeeping + 1) {
@@ -897,16 +898,16 @@ pub fn spillInstruction(self: *Self, reg: Register, inst: Air.Inst.Index) !void
 fn spillCompareFlagsIfOccupied(self: *Self) !void {
     if (self.compare_flags_inst) |inst_to_save| {
         const mcv = self.getResolvedInstValue(inst_to_save);
-        switch (mcv) {
+        const new_mcv = switch (mcv) {
             .compare_flags_signed,
             .compare_flags_unsigned,
+            => try self.allocRegOrMem(inst_to_save, true),
             .register_c_flag,
             .register_v_flag,
-            => {},
+            => try self.allocRegOrMem(inst_to_save, false),
             else => unreachable, // mcv doesn't occupy the compare flags
-        }
+        };
 
-        const new_mcv = try self.allocRegOrMem(inst_to_save, true);
         try self.setRegOrMem(self.air.typeOfIndex(inst_to_save), new_mcv, mcv);
         log.debug("spilling {d} to mcv {any}", .{ inst_to_save, new_mcv });
 
@@ -914,6 +915,15 @@ fn spillCompareFlagsIfOccupied(self: *Self) !void {
         try branch.inst_table.put(self.gpa, inst_to_save, new_mcv);
 
         self.compare_flags_inst = null;
+
+        // TODO consolidate with register manager and spillInstruction
+        // this call should really belong in the register manager!
+        switch (mcv) {
+            .register_c_flag,
+            .register_v_flag,
+            => |reg| self.register_manager.freeReg(reg),
+            else => {},
+        }
     }
 }
 
@@ -1038,8 +1048,8 @@ fn trunc(
                 }
             },
         };
-        self.register_manager.freezeRegs(&.{operand_reg});
-        defer self.register_manager.unfreezeRegs(&.{operand_reg});
+        const operand_reg_lock = self.register_manager.lockReg(operand_reg);
+        defer if (operand_reg_lock) |reg| self.register_manager.unlockReg(reg);
 
         const dest_reg = if (maybe_inst) |inst| blk: {
             const ty_op = self.air.instructions.items(.data)[inst].ty_op;
@@ -1127,8 +1137,8 @@ fn airNot(self: *Self, inst: Air.Inst.Index) !void {
                             .register => |r| r,
                             else => try self.copyToTmpRegister(operand_ty, operand),
                         };
-                        self.register_manager.freezeRegs(&.{op_reg});
-                        defer self.register_manager.unfreezeRegs(&.{op_reg});
+                        const op_reg_lock = self.register_manager.lockRegAssumeUnused(op_reg);
+                        defer self.register_manager.unlockReg(op_reg_lock);
 
                         const dest_reg = blk: {
                             if (operand == .register and self.reuseOperand(inst, ty_op.operand, 0, operand)) {
@@ -1157,8 +1167,8 @@ fn airNot(self: *Self, inst: Air.Inst.Index) !void {
                                 .register => |r| r,
                                 else => try self.copyToTmpRegister(operand_ty, operand),
                             };
-                            self.register_manager.freezeRegs(&.{op_reg});
-                            defer self.register_manager.unfreezeRegs(&.{op_reg});
+                            const op_reg_lock = self.register_manager.lockRegAssumeUnused(op_reg);
+                            defer self.register_manager.unlockReg(op_reg_lock);
 
                             const dest_reg = blk: {
                                 if (operand == .register and self.reuseOperand(inst, ty_op.operand, 0, operand)) {
@@ -1218,15 +1228,15 @@ fn minMax(
                     .register => |r| r,
                     else => try self.copyToTmpRegister(lhs_ty, lhs),
                 };
-                self.register_manager.freezeRegs(&.{lhs_reg});
-                defer self.register_manager.unfreezeRegs(&.{lhs_reg});
+                const lhs_reg_lock = self.register_manager.lockReg(lhs_reg);
+                defer if (lhs_reg_lock) |reg| self.register_manager.unlockReg(reg);
 
                 const rhs_reg = switch (rhs) {
                     .register => |r| r,
                     else => try self.copyToTmpRegister(rhs_ty, rhs),
                 };
-                self.register_manager.freezeRegs(&.{rhs_reg});
-                defer self.register_manager.unfreezeRegs(&.{rhs_reg});
+                const rhs_reg_lock = self.register_manager.lockReg(rhs_reg);
+                defer if (rhs_reg_lock) |reg| self.register_manager.unlockReg(reg);
 
                 const dest_reg = if (maybe_inst) |inst| blk: {
                     const bin_op = self.air.instructions.items(.data)[inst].bin_op;
@@ -1392,12 +1402,12 @@ fn airOverflow(self: *Self, inst: Air.Inst.Index) !void {
                     };
                     const dest = try self.binOp(base_tag, null, lhs, rhs, lhs_ty, rhs_ty);
                     const dest_reg = dest.register;
-                    self.register_manager.freezeRegs(&.{dest_reg});
-                    defer self.register_manager.unfreezeRegs(&.{dest_reg});
+                    const dest_reg_lock = self.register_manager.lockRegAssumeUnused(dest_reg);
+                    defer self.register_manager.unlockReg(dest_reg_lock);
 
                     const truncated_reg = try self.register_manager.allocReg(null);
-                    self.register_manager.freezeRegs(&.{truncated_reg});
-                    defer self.register_manager.unfreezeRegs(&.{truncated_reg});
+                    const truncated_reg_lock = self.register_manager.lockRegAssumeUnused(truncated_reg);
+                    defer self.register_manager.unlockReg(truncated_reg_lock);
 
                     // sbfx/ubfx truncated, dest, #0, #bits
                     try self.truncRegister(dest_reg, truncated_reg, int_info.signedness, int_info.bits);
@@ -1493,12 +1503,12 @@ fn airMulWithOverflow(self: *Self, inst: Air.Inst.Index) !void {
 
                     const dest = try self.binOpRegister(base_tag, null, lhs, rhs, lhs_ty, rhs_ty);
                     const dest_reg = dest.register;
-                    self.register_manager.freezeRegs(&.{dest_reg});
-                    defer self.register_manager.unfreezeRegs(&.{dest_reg});
+                    const dest_reg_lock = self.register_manager.lockRegAssumeUnused(dest_reg);
+                    defer self.register_manager.unlockReg(dest_reg_lock);
 
                     const truncated_reg = try self.register_manager.allocReg(null);
-                    self.register_manager.freezeRegs(&.{truncated_reg});
-                    defer self.register_manager.unfreezeRegs(&.{truncated_reg});
+                    const truncated_reg_lock = self.register_manager.lockRegAssumeUnused(truncated_reg);
+                    defer self.register_manager.unlockReg(truncated_reg_lock);
 
                     // sbfx/ubfx truncated, dest, #0, #bits
                     try self.truncRegister(dest_reg, truncated_reg, int_info.signedness, int_info.bits);
@@ -1526,28 +1536,31 @@ fn airMulWithOverflow(self: *Self, inst: Air.Inst.Index) !void {
                     const lhs_is_register = lhs == .register;
                     const rhs_is_register = rhs == .register;
 
-                    if (lhs_is_register) self.register_manager.freezeRegs(&.{lhs.register});
-                    if (rhs_is_register) self.register_manager.freezeRegs(&.{rhs.register});
+                    const lhs_lock: ?RegisterLock = if (lhs_is_register)
+                        self.register_manager.lockReg(lhs.register)
+                    else
+                        null;
+                    defer if (lhs_lock) |reg| self.register_manager.unlockReg(reg);
 
-                    const lhs_reg = if (lhs_is_register) lhs.register else blk: {
-                        const reg = try self.register_manager.allocReg(null);
-                        self.register_manager.freezeRegs(&.{reg});
+                    const lhs_reg = if (lhs_is_register)
+                        lhs.register
+                    else
+                        try self.register_manager.allocReg(null);
+                    const new_lhs_lock = self.register_manager.lockReg(lhs_reg);
+                    defer if (new_lhs_lock) |reg| self.register_manager.unlockReg(reg);
 
-                        break :blk reg;
-                    };
-                    defer self.register_manager.unfreezeRegs(&.{lhs_reg});
-
-                    const rhs_reg = if (rhs_is_register) rhs.register else blk: {
-                        const reg = try self.register_manager.allocReg(null);
-                        self.register_manager.freezeRegs(&.{reg});
-
-                        break :blk reg;
-                    };
-                    defer self.register_manager.unfreezeRegs(&.{rhs_reg});
+                    const rhs_reg = if (rhs_is_register)
+                        rhs.register
+                    else
+                        try self.register_manager.allocReg(null);
+                    const new_rhs_lock = self.register_manager.lockReg(rhs_reg);
+                    defer if (new_rhs_lock) |reg| self.register_manager.unlockReg(reg);
 
                     const dest_regs = try self.register_manager.allocRegs(2, .{ null, null });
-                    self.register_manager.freezeRegs(&dest_regs);
-                    defer self.register_manager.unfreezeRegs(&dest_regs);
+                    const dest_regs_locks = self.register_manager.lockRegsAssumeUnused(2, dest_regs);
+                    defer for (dest_regs_locks) |reg| {
+                        self.register_manager.unlockReg(reg);
+                    };
                     const rdlo = dest_regs[0];
                     const rdhi = dest_regs[1];
 
@@ -1555,8 +1568,8 @@ fn airMulWithOverflow(self: *Self, inst: Air.Inst.Index) !void {
                     if (!rhs_is_register) try self.genSetReg(rhs_ty, rhs_reg, rhs);
 
                     const truncated_reg = try self.register_manager.allocReg(null);
-                    self.register_manager.freezeRegs(&.{truncated_reg});
-                    defer self.register_manager.unfreezeRegs(&.{truncated_reg});
+                    const truncated_reg_lock = self.register_manager.lockRegAssumeUnused(truncated_reg);
+                    defer self.register_manager.unlockReg(truncated_reg_lock);
 
                     _ = try self.addInst(.{
                         .tag = base_tag,
@@ -1648,14 +1661,20 @@ fn airShlWithOverflow(self: *Self, inst: Air.Inst.Index) !void {
                 if (int_info.bits <= 32) {
                     const stack_offset = try self.allocMem(inst, tuple_size, tuple_align);
 
-                    if (lhs == .register) self.register_manager.freezeRegs(&.{lhs.register});
-                    defer if (lhs == .register) self.register_manager.unfreezeRegs(&.{lhs.register});
+                    const lhs_lock: ?RegisterLock = if (lhs == .register)
+                        self.register_manager.lockRegAssumeUnused(lhs.register)
+                    else
+                        null;
+                    defer if (lhs_lock) |reg| self.register_manager.unlockReg(reg);
 
                     try self.spillCompareFlagsIfOccupied();
                     self.compare_flags_inst = null;
 
                     // lsl dest, lhs, rhs
                     const dest = try self.binOp(.shl, null, lhs, rhs, lhs_ty, rhs_ty);
+                    const dest_reg = dest.register;
+                    const dest_lock = self.register_manager.lockRegAssumeUnused(dest_reg);
+                    defer self.register_manager.unlockReg(dest_lock);
 
                     // asr/lsr reconstructed, dest, rhs
                     const reconstructed = try self.binOp(.shr, null, dest, rhs, lhs_ty, rhs_ty);
@@ -1939,8 +1958,11 @@ fn airSliceElemVal(self: *Self, inst: Air.Inst.Index) !void {
         var buf: Type.SlicePtrFieldTypeBuffer = undefined;
         const slice_ptr_field_type = slice_ty.slicePtrFieldType(&buf);
 
-        if (index_is_register) self.register_manager.freezeRegs(&.{index_mcv.register});
-        defer if (index_is_register) self.register_manager.unfreezeRegs(&.{index_mcv.register});
+        const index_lock: ?RegisterLock = if (index_is_register)
+            self.register_manager.lockRegAssumeUnused(index_mcv.register)
+        else
+            null;
+        defer if (index_lock) |reg| self.register_manager.unlockReg(reg);
 
         const base_mcv = slicePtr(slice_mcv);
 
@@ -1950,20 +1972,20 @@ fn airSliceElemVal(self: *Self, inst: Air.Inst.Index) !void {
                     .register => |r| r,
                     else => try self.copyToTmpRegister(slice_ptr_field_type, base_mcv),
                 };
-                self.register_manager.freezeRegs(&.{base_reg});
-                defer self.register_manager.unfreezeRegs(&.{base_reg});
+                const base_reg_lock = self.register_manager.lockRegAssumeUnused(base_reg);
+                defer self.register_manager.unlockReg(base_reg_lock);
 
                 const dst_reg = try self.register_manager.allocReg(inst);
                 const dst_mcv = MCValue{ .register = dst_reg };
-                self.register_manager.freezeRegs(&.{dst_reg});
-                defer self.register_manager.unfreezeRegs(&.{dst_reg});
+                const dst_reg_lock = self.register_manager.lockRegAssumeUnused(dst_reg);
+                defer self.register_manager.unlockReg(dst_reg_lock);
 
                 const index_reg: Register = switch (index_mcv) {
                     .register => |reg| reg,
                     else => try self.copyToTmpRegister(Type.usize, index_mcv),
                 };
-                self.register_manager.freezeRegs(&.{index_reg});
-                defer self.register_manager.unfreezeRegs(&.{index_reg});
+                const index_reg_lock = self.register_manager.lockReg(index_reg);
+                defer if (index_reg_lock) |lock| self.register_manager.unlockReg(lock);
 
                 const tag: Mir.Inst.Tag = switch (elem_size) {
                     1 => .ldrb,
@@ -2149,8 +2171,8 @@ fn load(self: *Self, dst_mcv: MCValue, ptr: MCValue, ptr_ty: Type) InnerError!vo
         .immediate => |imm| try self.setRegOrMem(elem_ty, dst_mcv, .{ .memory = imm }),
         .ptr_stack_offset => |off| try self.setRegOrMem(elem_ty, dst_mcv, .{ .stack_offset = off }),
         .register => |reg| {
-            self.register_manager.freezeRegs(&.{reg});
-            defer self.register_manager.unfreezeRegs(&.{reg});
+            const reg_lock = self.register_manager.lockReg(reg);
+            defer if (reg_lock) |reg_locked| self.register_manager.unlockReg(reg_locked);
 
             switch (dst_mcv) {
                 .dead => unreachable,
@@ -2162,16 +2184,18 @@ fn load(self: *Self, dst_mcv: MCValue, ptr: MCValue, ptr_ty: Type) InnerError!vo
                 .stack_offset => |off| {
                     if (elem_size <= 4) {
                         const tmp_reg = try self.register_manager.allocReg(null);
-                        self.register_manager.freezeRegs(&.{tmp_reg});
-                        defer self.register_manager.unfreezeRegs(&.{tmp_reg});
+                        const tmp_reg_lock = self.register_manager.lockRegAssumeUnused(tmp_reg);
+                        defer self.register_manager.unlockReg(tmp_reg_lock);
 
                         try self.load(.{ .register = tmp_reg }, ptr, ptr_ty);
                         try self.genSetStack(elem_ty, off, MCValue{ .register = tmp_reg });
                     } else {
                         // TODO optimize the register allocation
                         const regs = try self.register_manager.allocRegs(4, .{ null, null, null, null });
-                        self.register_manager.freezeRegs(&regs);
-                        defer self.register_manager.unfreezeRegs(&regs);
+                        const regs_locks = self.register_manager.lockRegsAssumeUnused(4, regs);
+                        defer for (regs_locks) |reg_locked| {
+                            self.register_manager.unlockReg(reg_locked);
+                        };
 
                         const src_reg = reg;
                         const dst_reg = regs[0];
@@ -2197,8 +2221,8 @@ fn load(self: *Self, dst_mcv: MCValue, ptr: MCValue, ptr_ty: Type) InnerError!vo
         .stack_argument_offset,
         => {
             const reg = try self.register_manager.allocReg(null);
-            self.register_manager.freezeRegs(&.{reg});
-            defer self.register_manager.unfreezeRegs(&.{reg});
+            const reg_lock = self.register_manager.lockRegAssumeUnused(reg);
+            defer self.register_manager.unlockReg(reg_lock);
 
             try self.genSetReg(ptr_ty, reg, ptr);
             try self.load(dst_mcv, .{ .register = reg }, ptr_ty);
@@ -2252,8 +2276,8 @@ fn store(self: *Self, ptr: MCValue, value: MCValue, ptr_ty: Type, value_ty: Type
             try self.genSetStack(value_ty, off, value);
         },
         .register => |addr_reg| {
-            self.register_manager.freezeRegs(&.{addr_reg});
-            defer self.register_manager.unfreezeRegs(&.{addr_reg});
+            const addr_reg_lock = self.register_manager.lockReg(addr_reg);
+            defer if (addr_reg_lock) |reg| self.register_manager.unlockReg(reg);
 
             switch (value) {
                 .dead => unreachable,
@@ -2264,15 +2288,17 @@ fn store(self: *Self, ptr: MCValue, value: MCValue, ptr_ty: Type, value_ty: Type
                 else => {
                     if (elem_size <= 4) {
                         const tmp_reg = try self.register_manager.allocReg(null);
-                        self.register_manager.freezeRegs(&.{tmp_reg});
-                        defer self.register_manager.unfreezeRegs(&.{tmp_reg});
+                        const tmp_reg_lock = self.register_manager.lockRegAssumeUnused(tmp_reg);
+                        defer self.register_manager.unlockReg(tmp_reg_lock);
 
                         try self.genSetReg(value_ty, tmp_reg, value);
                         try self.store(ptr, .{ .register = tmp_reg }, ptr_ty, value_ty);
                     } else {
                         const regs = try self.register_manager.allocRegs(4, .{ null, null, null, null });
-                        self.register_manager.freezeRegs(&regs);
-                        defer self.register_manager.unfreezeRegs(&regs);
+                        const regs_locks = self.register_manager.lockRegsAssumeUnused(4, regs);
+                        defer for (regs_locks) |reg| {
+                            self.register_manager.unlockReg(reg);
+                        };
 
                         const src_reg = regs[0];
                         const dst_reg = addr_reg;
@@ -2356,12 +2382,12 @@ fn structFieldPtr(self: *Self, inst: Air.Inst.Index, operand: Air.Inst.Ref, inde
                 const offset_reg = try self.copyToTmpRegister(ptr_ty, .{
                     .immediate = struct_field_offset,
                 });
-                self.register_manager.freezeRegs(&.{offset_reg});
-                defer self.register_manager.unfreezeRegs(&.{offset_reg});
+                const offset_reg_lock = self.register_manager.lockRegAssumeUnused(offset_reg);
+                defer self.register_manager.unlockReg(offset_reg_lock);
 
                 const addr_reg = try self.copyToTmpRegister(ptr_ty, mcv);
-                self.register_manager.freezeRegs(&.{addr_reg});
-                defer self.register_manager.unfreezeRegs(&.{addr_reg});
+                const addr_reg_lock = self.register_manager.lockRegAssumeUnused(addr_reg);
+                defer self.register_manager.unlockReg(addr_reg_lock);
 
                 const dest = try self.binOp(
                     .add,
@@ -2477,8 +2503,11 @@ fn binOpRegister(
     const lhs_is_register = lhs == .register;
     const rhs_is_register = rhs == .register;
 
-    if (lhs_is_register) self.register_manager.freezeRegs(&.{lhs.register});
-    if (rhs_is_register) self.register_manager.freezeRegs(&.{rhs.register});
+    const lhs_lock: ?RegisterLock = if (lhs_is_register)
+        self.register_manager.lockReg(lhs.register)
+    else
+        null;
+    defer if (lhs_lock) |reg| self.register_manager.unlockReg(reg);
 
     const branch = &self.branch_stack.items[self.branch_stack.items.len - 1];
 
@@ -2489,13 +2518,13 @@ fn binOpRegister(
         } else null;
 
         const reg = try self.register_manager.allocReg(track_inst);
-        self.register_manager.freezeRegs(&.{reg});
 
         if (track_inst) |inst| branch.inst_table.putAssumeCapacity(inst, .{ .register = reg });
 
         break :blk reg;
     };
-    defer self.register_manager.unfreezeRegs(&.{lhs_reg});
+    const new_lhs_lock = self.register_manager.lockReg(lhs_reg);
+    defer if (new_lhs_lock) |reg| self.register_manager.unlockReg(reg);
 
     const rhs_reg = if (rhs_is_register) rhs.register else blk: {
         const track_inst: ?Air.Inst.Index = if (maybe_inst) |inst| inst: {
@@ -2504,13 +2533,13 @@ fn binOpRegister(
         } else null;
 
         const reg = try self.register_manager.allocReg(track_inst);
-        self.register_manager.freezeRegs(&.{reg});
 
         if (track_inst) |inst| branch.inst_table.putAssumeCapacity(inst, .{ .register = reg });
 
         break :blk reg;
     };
-    defer self.register_manager.unfreezeRegs(&.{rhs_reg});
+    const new_rhs_lock = self.register_manager.lockReg(rhs_reg);
+    defer if (new_rhs_lock) |reg| self.register_manager.unlockReg(reg);
 
     const dest_reg = switch (mir_tag) {
         .cmp => .r0, // cmp has no destination regardless
@@ -2593,7 +2622,11 @@ fn binOpImmediate(
 ) !MCValue {
     const lhs_is_register = lhs == .register;
 
-    if (lhs_is_register) self.register_manager.freezeRegs(&.{lhs.register});
+    const lhs_lock: ?RegisterLock = if (lhs_is_register)
+        self.register_manager.lockReg(lhs.register)
+    else
+        null;
+    defer if (lhs_lock) |reg| self.register_manager.unlockReg(reg);
 
     const branch = &self.branch_stack.items[self.branch_stack.items.len - 1];
 
@@ -2606,13 +2639,13 @@ fn binOpImmediate(
         } else null;
 
         const reg = try self.register_manager.allocReg(track_inst);
-        self.register_manager.freezeRegs(&.{reg});
 
         if (track_inst) |inst| branch.inst_table.putAssumeCapacity(inst, .{ .register = reg });
 
         break :blk reg;
     };
-    defer self.register_manager.unfreezeRegs(&.{lhs_reg});
+    const new_lhs_lock = self.register_manager.lockReg(lhs_reg);
+    defer if (new_lhs_lock) |reg| self.register_manager.unlockReg(reg);
 
     const dest_reg = switch (mir_tag) {
         .cmp => .r0, // cmp has no destination reg
@@ -3656,6 +3689,9 @@ fn airCondBr(self: *Self, inst: Air.Inst.Index) !void {
     const parent_compare_flags_inst = self.compare_flags_inst;
 
     try self.branch_stack.append(.{});
+    errdefer {
+        _ = self.branch_stack.pop();
+    }
 
     try self.ensureProcessDeathCapacity(liveness_condbr.then_deaths.len);
     for (liveness_condbr.then_deaths) |operand| {
@@ -4264,8 +4300,36 @@ fn genSetStack(self: *Self, ty: Type, stack_offset: u32, mcv: MCValue) InnerErro
         },
         .register_c_flag,
         .register_v_flag,
-        => {
-            return self.fail("TODO implement genSetStack {}", .{mcv});
+        => |reg| {
+            const reg_lock = self.register_manager.lockReg(reg);
+            defer if (reg_lock) |locked_reg| self.register_manager.unlockReg(locked_reg);
+
+            const wrapped_ty = ty.structFieldType(0);
+            try self.genSetStack(wrapped_ty, stack_offset, .{ .register = reg });
+
+            const overflow_bit_ty = ty.structFieldType(1);
+            const overflow_bit_offset = @intCast(u32, ty.structFieldOffset(1, self.target.*));
+            const cond_reg = try self.register_manager.allocReg(null);
+
+            // C flag: movcs reg, #1
+            // V flag: movvs reg, #1
+            _ = try self.addInst(.{
+                .tag = .mov,
+                .cond = switch (mcv) {
+                    .register_c_flag => .cs,
+                    .register_v_flag => .vs,
+                    else => unreachable,
+                },
+                .data = .{ .rr_op = .{
+                    .rd = cond_reg,
+                    .rn = .r0,
+                    .op = Instruction.Operand.fromU32(1).?,
+                } },
+            });
+
+            try self.genSetStack(overflow_bit_ty, stack_offset - overflow_bit_offset, .{
+                .register = cond_reg,
+            });
         },
         .memory,
         .stack_argument_offset,
diff --git a/src/arch/riscv64/CodeGen.zig b/src/arch/riscv64/CodeGen.zig
index 96d30c31ce..07ce2a9f89 100644
--- a/src/arch/riscv64/CodeGen.zig
+++ b/src/arch/riscv64/CodeGen.zig
@@ -23,6 +23,7 @@ const log = std.log.scoped(.codegen);
 const build_options = @import("build_options");
 const RegisterManagerFn = @import("../../register_manager.zig").RegisterManager;
 const RegisterManager = RegisterManagerFn(Self, Register, &callee_preserved_regs);
+const RegisterLock = RegisterManager.RegisterLock;
 
 const FnResult = @import("../../codegen.zig").FnResult;
 const GenerateSymbolError = @import("../../codegen.zig").GenerateSymbolError;
@@ -937,8 +938,11 @@ fn binOpRegister(
     const lhs_is_register = lhs == .register;
     const rhs_is_register = rhs == .register;
 
-    if (lhs_is_register) self.register_manager.freezeRegs(&.{lhs.register});
-    if (rhs_is_register) self.register_manager.freezeRegs(&.{rhs.register});
+    const lhs_lock: ?RegisterLock = if (lhs_is_register)
+        self.register_manager.lockReg(lhs.register)
+    else
+        null;
+    defer if (lhs_lock) |reg| self.register_manager.unlockReg(reg);
 
     const branch = &self.branch_stack.items[self.branch_stack.items.len - 1];
 
@@ -949,13 +953,13 @@ fn binOpRegister(
         } else null;
 
         const reg = try self.register_manager.allocReg(track_inst);
-        self.register_manager.freezeRegs(&.{reg});
 
         if (track_inst) |inst| branch.inst_table.putAssumeCapacity(inst, .{ .register = reg });
 
         break :blk reg;
     };
-    defer self.register_manager.unfreezeRegs(&.{lhs_reg});
+    const new_lhs_lock = self.register_manager.lockReg(lhs_reg);
+    defer if (new_lhs_lock) |reg| self.register_manager.unlockReg(reg);
 
     const rhs_reg = if (rhs_is_register) rhs.register else blk: {
         const track_inst: ?Air.Inst.Index = if (maybe_inst) |inst| inst: {
@@ -964,13 +968,13 @@ fn binOpRegister(
         } else null;
 
         const reg = try self.register_manager.allocReg(track_inst);
-        self.register_manager.freezeRegs(&.{reg});
 
         if (track_inst) |inst| branch.inst_table.putAssumeCapacity(inst, .{ .register = reg });
 
         break :blk reg;
     };
-    defer self.register_manager.unfreezeRegs(&.{rhs_reg});
+    const new_rhs_lock = self.register_manager.lockReg(rhs_reg);
+    defer if (new_rhs_lock) |reg| self.register_manager.unlockReg(reg);
 
     const dest_reg = if (maybe_inst) |inst| blk: {
         const bin_op = self.air.instructions.items(.data)[inst].bin_op;
@@ -1448,8 +1452,8 @@ fn load(self: *Self, dst_mcv: MCValue, ptr: MCValue, ptr_ty: Type) InnerError!vo
         .stack_offset,
         => {
             const reg = try self.register_manager.allocReg(null);
-            self.register_manager.freezeRegs(&.{reg});
-            defer self.register_manager.unfreezeRegs(&.{reg});
+            const reg_lock = self.register_manager.lockRegAssumeUnused(reg);
+            defer self.register_manager.unlockReg(reg_lock);
 
             try self.genSetReg(ptr_ty, reg, ptr);
             try self.load(dst_mcv, .{ .register = reg }, ptr_ty);
diff --git a/src/arch/x86_64/CodeGen.zig b/src/arch/x86_64/CodeGen.zig
index 2fdce1149c..ee472eeac8 100644
--- a/src/arch/x86_64/CodeGen.zig
+++ b/src/arch/x86_64/CodeGen.zig
@@ -22,6 +22,8 @@ const Liveness = @import("../../Liveness.zig");
 const Mir = @import("Mir.zig");
 const Module = @import("../../Module.zig");
 const RegisterManagerFn = @import("../../register_manager.zig").RegisterManager;
+const RegisterManager = RegisterManagerFn(Self, Register, &allocatable_registers);
+const RegisterLock = RegisterManager.RegisterLock;
 const Target = std.Target;
 const Type = @import("../../type.zig").Type;
 const TypedValue = @import("../../TypedValue.zig");
@@ -42,8 +44,6 @@ const InnerError = error{
     OutOfRegisters,
 };
 
-const RegisterManager = RegisterManagerFn(Self, Register, &allocatable_registers);
-
 gpa: Allocator,
 air: Air,
 liveness: Liveness,
@@ -191,60 +191,12 @@ pub const MCValue = union(enum) {
         };
     }
 
-    fn usesCompareFlags(mcv: MCValue) bool {
-        return switch (mcv) {
-            .compare_flags_unsigned,
-            .compare_flags_signed,
-            .register_overflow_unsigned,
-            .register_overflow_signed,
-            => true,
-            else => false,
-        };
-    }
-
     fn isRegister(mcv: MCValue) bool {
         return switch (mcv) {
-            .register,
-            .register_overflow_unsigned,
-            .register_overflow_signed,
-            => true,
+            .register => true,
             else => false,
         };
     }
-
-    fn freezeIfRegister(mcv: MCValue, mgr: *RegisterManager) void {
-        switch (mcv) {
-            .register,
-            .register_overflow_signed,
-            .register_overflow_unsigned,
-            => |reg| {
-                mgr.freezeRegs(&.{reg});
-            },
-            else => {},
-        }
-    }
-
-    fn unfreezeIfRegister(mcv: MCValue, mgr: *RegisterManager) void {
-        switch (mcv) {
-            .register,
-            .register_overflow_signed,
-            .register_overflow_unsigned,
-            => |reg| {
-                mgr.unfreezeRegs(&.{reg});
-            },
-            else => {},
-        }
-    }
-
-    fn asRegister(mcv: MCValue) ?Register {
-        return switch (mcv) {
-            .register,
-            .register_overflow_signed,
-            .register_overflow_unsigned,
-            => |reg| reg,
-            else => null,
-        };
-    }
 };
 
 const Branch = struct {
@@ -819,7 +771,7 @@ fn genBody(self: *Self, body: []const Air.Inst.Index) InnerError!void {
             // zig fmt: on
         }
 
-        assert(!self.register_manager.frozenRegsExist());
+        assert(!self.register_manager.lockedRegsExist());
 
         if (std.debug.runtime_safety) {
             if (self.air_bookkeeping < old_air_bookkeeping + 1) {
@@ -876,15 +828,21 @@ fn finishAir(self: *Self, inst: Air.Inst.Index, result: MCValue, operands: [Live
         const branch = &self.branch_stack.items[self.branch_stack.items.len - 1];
         branch.inst_table.putAssumeCapacityNoClobber(inst, result);
 
-        if (result.asRegister()) |reg| {
-            // In some cases (such as bitcast), an operand
-            // may be the same MCValue as the result. If
-            // that operand died and was a register, it
-            // was freed by processDeath. We have to
-            // "re-allocate" the register.
-            if (self.register_manager.isRegFree(reg)) {
-                self.register_manager.getRegAssumeFree(reg, inst);
-            }
+        switch (result) {
+            .register,
+            .register_overflow_signed,
+            .register_overflow_unsigned,
+            => |reg| {
+                // In some cases (such as bitcast), an operand
+                // may be the same MCValue as the result. If
+                // that operand died and was a register, it
+                // was freed by processDeath. We have to
+                // "re-allocate" the register.
+                if (self.register_manager.isRegFree(reg)) {
+                    self.register_manager.getRegAssumeFree(reg, inst);
+                }
+            },
+            else => {},
         }
     }
     self.finishAirBookkeeping();
@@ -955,7 +913,15 @@ pub fn spillInstruction(self: *Self, reg: Register, inst: Air.Inst.Index) !void
     const stack_mcv = try self.allocRegOrMem(inst, false);
     log.debug("spilling {d} to stack mcv {any}", .{ inst, stack_mcv });
     const reg_mcv = self.getResolvedInstValue(inst);
-    assert(reg.to64() == reg_mcv.asRegister().?.to64());
+    switch (reg_mcv) {
+        .register,
+        .register_overflow_unsigned,
+        .register_overflow_signed,
+        => |other| {
+            assert(reg.to64() == other.to64());
+        },
+        else => {},
+    }
     const branch = &self.branch_stack.items[self.branch_stack.items.len - 1];
     try branch.inst_table.put(self.gpa, inst, stack_mcv);
     try self.genSetStack(self.air.typeOfIndex(inst), stack_mcv.stack_offset, reg_mcv, .{});
@@ -964,18 +930,32 @@ pub fn spillInstruction(self: *Self, reg: Register, inst: Air.Inst.Index) !void
 pub fn spillCompareFlagsIfOccupied(self: *Self) !void {
     if (self.compare_flags_inst) |inst_to_save| {
         const mcv = self.getResolvedInstValue(inst_to_save);
-        assert(mcv.usesCompareFlags());
+        const new_mcv = switch (mcv) {
+            .register_overflow_signed,
+            .register_overflow_unsigned,
+            => try self.allocRegOrMem(inst_to_save, false),
+            .compare_flags_signed,
+            .compare_flags_unsigned,
+            => try self.allocRegOrMem(inst_to_save, true),
+            else => unreachable,
+        };
 
-        const new_mcv = try self.allocRegOrMem(inst_to_save, !mcv.isRegister());
         try self.setRegOrMem(self.air.typeOfIndex(inst_to_save), new_mcv, mcv);
         log.debug("spilling {d} to mcv {any}", .{ inst_to_save, new_mcv });
+
         const branch = &self.branch_stack.items[self.branch_stack.items.len - 1];
         try branch.inst_table.put(self.gpa, inst_to_save, new_mcv);
 
         self.compare_flags_inst = null;
+
         // TODO consolidate with register manager and spillInstruction
         // this call should really belong in the register manager!
-        if (mcv.isRegister()) self.register_manager.freeReg(mcv.asRegister().?);
+        switch (mcv) {
+            .register_overflow_signed,
+            .register_overflow_unsigned,
+            => |reg| self.register_manager.freeReg(reg),
+            else => {},
+        }
     }
 }
 
@@ -1043,8 +1023,11 @@ fn airIntCast(self: *Self, inst: Air.Inst.Index) !void {
             return self.fail("TODO implement intCast for abi sizes larger than 8", .{});
         }
 
-        operand.freezeIfRegister(&self.register_manager);
-        defer operand.unfreezeIfRegister(&self.register_manager);
+        const operand_lock: ?RegisterLock = switch (operand) {
+            .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+            else => null,
+        };
+        defer if (operand_lock) |lock| self.register_manager.unlockReg(lock);
 
         const reg = try self.register_manager.allocReg(inst);
         try self.genSetReg(dest_ty, reg, .{ .immediate = 0 });
@@ -1071,8 +1054,11 @@ fn airTrunc(self: *Self, inst: Air.Inst.Index) !void {
         return self.fail("TODO implement trunc for abi sizes larger than 8", .{});
     }
 
-    operand.freezeIfRegister(&self.register_manager);
-    defer operand.unfreezeIfRegister(&self.register_manager);
+    const operand_lock: ?RegisterLock = switch (operand) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (operand_lock) |lock| self.register_manager.unlockReg(lock);
 
     const reg: Register = blk: {
         if (operand.isRegister()) {
@@ -1156,16 +1142,22 @@ fn airMin(self: *Self, inst: Air.Inst.Index) !void {
         // TODO improve by checking if any operand can be reused.
         // TODO audit register allocation
         const lhs = try self.resolveInst(bin_op.lhs);
-        lhs.freezeIfRegister(&self.register_manager);
-        defer lhs.unfreezeIfRegister(&self.register_manager);
+        const lhs_lock: ?RegisterLock = switch (lhs) {
+            .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+            else => null,
+        };
+        defer if (lhs_lock) |lock| self.register_manager.unlockReg(lock);
 
         const lhs_reg = try self.copyToTmpRegister(ty, lhs);
-        self.register_manager.freezeRegs(&.{lhs_reg});
-        defer self.register_manager.unfreezeRegs(&.{lhs_reg});
+        const lhs_reg_lock = self.register_manager.lockRegAssumeUnused(lhs_reg);
+        defer self.register_manager.unlockReg(lhs_reg_lock);
 
         const rhs_mcv = try self.limitImmediateType(bin_op.rhs, i32);
-        rhs_mcv.freezeIfRegister(&self.register_manager);
-        defer rhs_mcv.unfreezeIfRegister(&self.register_manager);
+        const rhs_lock: ?RegisterLock = switch (rhs_mcv) {
+            .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+            else => null,
+        };
+        defer if (rhs_lock) |lock| self.register_manager.unlockReg(lock);
 
         try self.genBinMathOpMir(.cmp, ty, .{ .register = lhs_reg }, rhs_mcv);
 
@@ -1200,28 +1192,37 @@ fn genPtrBinMathOp(self: *Self, inst: Air.Inst.Index, op_lhs: Air.Inst.Ref, op_r
     const offset = try self.resolveInst(op_rhs);
     const offset_ty = self.air.typeOf(op_rhs);
 
-    offset.freezeIfRegister(&self.register_manager);
-    defer offset.unfreezeIfRegister(&self.register_manager);
+    const offset_lock: ?RegisterLock = switch (offset) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (offset_lock) |lock| self.register_manager.unlockReg(lock);
 
-    const dst_mcv = blk: {
+    const dst_mcv: MCValue = blk: {
         if (self.reuseOperand(inst, op_lhs, 0, ptr)) {
             if (ptr.isMemory() or ptr.isRegister()) break :blk ptr;
         }
         break :blk MCValue{ .register = try self.copyToTmpRegister(dst_ty, ptr) };
     };
 
-    dst_mcv.freezeIfRegister(&self.register_manager);
-    defer dst_mcv.unfreezeIfRegister(&self.register_manager);
+    const dst_mcv_lock: ?RegisterLock = switch (dst_mcv) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (dst_mcv_lock) |lock| self.register_manager.unlockReg(lock);
 
-    const offset_mcv = blk: {
+    const offset_mcv: MCValue = blk: {
         if (self.reuseOperand(inst, op_rhs, 1, offset)) {
             if (offset.isRegister()) break :blk offset;
         }
         break :blk MCValue{ .register = try self.copyToTmpRegister(offset_ty, offset) };
     };
 
-    offset_mcv.freezeIfRegister(&self.register_manager);
-    defer offset_mcv.unfreezeIfRegister(&self.register_manager);
+    const offset_mcv_lock: ?RegisterLock = switch (offset_mcv) {
+        .register => |reg| self.register_manager.lockReg(reg),
+        else => null,
+    };
+    defer if (offset_mcv_lock) |lock| self.register_manager.unlockReg(lock);
 
     try self.genIntMulComplexOpMir(offset_ty, offset_mcv, .{ .immediate = elem_size });
 
@@ -1306,30 +1307,40 @@ fn genSubOp(self: *Self, inst: Air.Inst.Index, op_lhs: Air.Inst.Ref, op_rhs: Air
     const dst_ty = self.air.typeOf(op_lhs);
 
     const lhs = try self.resolveInst(op_lhs);
-    lhs.freezeIfRegister(&self.register_manager);
-    defer lhs.unfreezeIfRegister(&self.register_manager);
+    const lhs_lock: ?RegisterLock = switch (lhs) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (lhs_lock) |lock| self.register_manager.unlockReg(lock);
 
     const rhs = try self.resolveInst(op_rhs);
-    rhs.freezeIfRegister(&self.register_manager);
-    defer rhs.unfreezeIfRegister(&self.register_manager);
+    const rhs_lock: ?RegisterLock = switch (rhs) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (rhs_lock) |lock| self.register_manager.unlockReg(lock);
 
-    const dst_mcv = blk: {
+    const dst_mcv: MCValue = blk: {
         if (self.reuseOperand(inst, op_lhs, 0, lhs) and lhs.isRegister()) {
             break :blk lhs;
         }
         break :blk try self.copyToRegisterWithInstTracking(inst, dst_ty, lhs);
     };
+    const dst_mcv_lock: ?RegisterLock = switch (dst_mcv) {
+        .register => |reg| self.register_manager.lockReg(reg),
+        else => null,
+    };
+    defer if (dst_mcv_lock) |lock| self.register_manager.unlockReg(lock);
 
-    dst_mcv.freezeIfRegister(&self.register_manager);
-    defer dst_mcv.unfreezeIfRegister(&self.register_manager);
-
-    const rhs_mcv = blk: {
+    const rhs_mcv: MCValue = blk: {
         if (rhs.isMemory() or rhs.isRegister()) break :blk rhs;
         break :blk MCValue{ .register = try self.copyToTmpRegister(dst_ty, rhs) };
     };
-
-    rhs_mcv.freezeIfRegister(&self.register_manager);
-    defer rhs_mcv.unfreezeIfRegister(&self.register_manager);
+    const rhs_mcv_lock: ?RegisterLock = switch (rhs_mcv) {
+        .register => |reg| self.register_manager.lockReg(reg),
+        else => null,
+    };
+    defer if (rhs_mcv_lock) |lock| self.register_manager.unlockReg(lock);
 
     try self.genBinMathOpMir(.sub, dst_ty, dst_mcv, rhs_mcv);
 
@@ -1366,8 +1377,10 @@ fn airMul(self: *Self, inst: Air.Inst.Index) !void {
         // Spill .rax and .rdx upfront to ensure we don't spill the operands too late.
         try self.register_manager.getReg(.rax, inst);
         try self.register_manager.getReg(.rdx, null);
-        self.register_manager.freezeRegs(&.{ .rax, .rdx });
-        defer self.register_manager.unfreezeRegs(&.{ .rax, .rdx });
+        const reg_locks = self.register_manager.lockRegsAssumeUnused(2, .{ .rax, .rdx });
+        defer for (reg_locks) |reg| {
+            self.register_manager.unlockReg(reg);
+        };
 
         const lhs = try self.resolveInst(bin_op.lhs);
         const rhs = try self.resolveInst(bin_op.rhs);
@@ -1458,9 +1471,13 @@ fn airSubWithOverflow(self: *Self, inst: Air.Inst.Index) !void {
 fn airMulWithOverflow(self: *Self, inst: Air.Inst.Index) !void {
     const ty_pl = self.air.instructions.items(.data)[inst].ty_pl;
     const bin_op = self.air.extraData(Air.Bin, ty_pl.payload).data;
-    const result = if (self.liveness.isUnused(inst)) .dead else result: {
-        const ty = self.air.typeOf(bin_op.lhs);
 
+    if (self.liveness.isUnused(inst)) {
+        return self.finishAir(inst, .dead, .{ bin_op.lhs, bin_op.rhs, .none });
+    }
+
+    const ty = self.air.typeOf(bin_op.lhs);
+    const result: MCValue = result: {
         switch (ty.zigTypeTag()) {
             .Vector => return self.fail("TODO implement mul_with_overflow for Vector type", .{}),
             .Int => {
@@ -1477,8 +1494,10 @@ fn airMulWithOverflow(self: *Self, inst: Air.Inst.Index) !void {
                     // Spill .rax and .rdx upfront to ensure we don't spill the operands too late.
                     try self.register_manager.getReg(.rax, inst);
                     try self.register_manager.getReg(.rdx, null);
-                    self.register_manager.freezeRegs(&.{ .rax, .rdx });
-                    defer self.register_manager.unfreezeRegs(&.{ .rax, .rdx });
+                    const reg_locks = self.register_manager.lockRegsAssumeUnused(2, .{ .rax, .rdx });
+                    defer for (reg_locks) |reg| {
+                        self.register_manager.unlockReg(reg);
+                    };
 
                     const lhs = try self.resolveInst(bin_op.lhs);
                     const rhs = try self.resolveInst(bin_op.rhs);
@@ -1504,21 +1523,28 @@ fn airMulWithOverflow(self: *Self, inst: Air.Inst.Index) !void {
                             const lhs = try self.resolveInst(bin_op.lhs);
                             const rhs = try self.resolveInst(bin_op.rhs);
 
-                            rhs.freezeIfRegister(&self.register_manager);
-                            defer rhs.unfreezeIfRegister(&self.register_manager);
+                            const rhs_lock: ?RegisterLock = switch (rhs) {
+                                .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+                                else => null,
+                            };
+                            defer if (rhs_lock) |lock| self.register_manager.unlockReg(lock);
 
                             const dst_reg: Register = blk: {
                                 if (lhs.isRegister()) break :blk lhs.register;
                                 break :blk try self.copyToTmpRegister(ty, lhs);
                             };
-                            self.register_manager.freezeRegs(&.{dst_reg});
+                            const dst_reg_lock = self.register_manager.lockRegAssumeUnused(dst_reg);
+                            defer self.register_manager.unlockReg(dst_reg_lock);
 
-                            const rhs_mcv = blk: {
+                            const rhs_mcv: MCValue = blk: {
                                 if (rhs.isRegister() or rhs.isMemory()) break :blk rhs;
                                 break :blk MCValue{ .register = try self.copyToTmpRegister(ty, rhs) };
                             };
-                            rhs_mcv.freezeIfRegister(&self.register_manager);
-                            defer rhs_mcv.unfreezeIfRegister(&self.register_manager);
+                            const rhs_mcv_lock: ?RegisterLock = switch (rhs_mcv) {
+                                .register => |reg| self.register_manager.lockReg(reg),
+                                else => null,
+                            };
+                            defer if (rhs_mcv_lock) |lock| self.register_manager.unlockReg(lock);
 
                             try self.genIntMulComplexOpMir(Type.isize, .{ .register = dst_reg }, rhs_mcv);
 
@@ -1528,8 +1554,10 @@ fn airMulWithOverflow(self: *Self, inst: Air.Inst.Index) !void {
                             // Spill .rax and .rdx upfront to ensure we don't spill the operands too late.
                             try self.register_manager.getReg(.rax, null);
                             try self.register_manager.getReg(.rdx, null);
-                            self.register_manager.freezeRegs(&.{ .rax, .rdx });
-                            defer self.register_manager.unfreezeRegs(&.{.rdx});
+                            const reg_locks = self.register_manager.lockRegsAssumeUnused(2, .{ .rax, .rdx });
+                            defer for (reg_locks) |reg| {
+                                self.register_manager.unlockReg(reg);
+                            };
 
                             const lhs = try self.resolveInst(bin_op.lhs);
                             const rhs = try self.resolveInst(bin_op.rhs);
@@ -1540,7 +1568,8 @@ fn airMulWithOverflow(self: *Self, inst: Air.Inst.Index) !void {
                         },
                     }
                 };
-                defer self.register_manager.unfreezeRegs(&.{dst_reg});
+                const dst_reg_lock = self.register_manager.lockRegAssumeUnused(dst_reg);
+                defer self.register_manager.unlockReg(dst_reg_lock);
 
                 const tuple_ty = self.air.typeOfIndex(inst);
                 const tuple_size = @intCast(u32, tuple_ty.abiSize(self.target.*));
@@ -1554,8 +1583,10 @@ fn airMulWithOverflow(self: *Self, inst: Air.Inst.Index) !void {
                 };
 
                 const temp_regs = try self.register_manager.allocRegs(3, .{ null, null, null });
-                self.register_manager.freezeRegs(&temp_regs);
-                defer self.register_manager.unfreezeRegs(&temp_regs);
+                const temp_regs_locks = self.register_manager.lockRegsAssumeUnused(3, temp_regs);
+                defer for (temp_regs_locks) |reg| {
+                    self.register_manager.unlockReg(reg);
+                };
 
                 const overflow_reg = temp_regs[0];
                 const flags: u2 = switch (int_info.signedness) {
@@ -1699,18 +1730,19 @@ fn genIntMulDivOpMir(
 /// Clobbers .rax and .rdx registers.
 fn genInlineIntDivFloor(self: *Self, ty: Type, lhs: MCValue, rhs: MCValue) !MCValue {
     const signedness = ty.intInfo(self.target.*).signedness;
-    const dividend = switch (lhs) {
+    const dividend: Register = switch (lhs) {
         .register => |reg| reg,
         else => try self.copyToTmpRegister(ty, lhs),
     };
-    self.register_manager.freezeRegs(&.{dividend});
+    const dividend_lock = self.register_manager.lockReg(dividend);
+    defer if (dividend_lock) |lock| self.register_manager.unlockReg(lock);
 
-    const divisor = switch (rhs) {
+    const divisor: Register = switch (rhs) {
         .register => |reg| reg,
         else => try self.copyToTmpRegister(ty, rhs),
     };
-    self.register_manager.freezeRegs(&.{divisor});
-    defer self.register_manager.unfreezeRegs(&.{ dividend, divisor });
+    const divisor_lock = self.register_manager.lockReg(divisor);
+    defer if (divisor_lock) |lock| self.register_manager.unlockReg(lock);
 
     try self.genIntMulDivOpMir(switch (signedness) {
         .signed => .idiv,
@@ -1755,54 +1787,71 @@ fn genInlineIntDivFloor(self: *Self, ty: Type, lhs: MCValue, rhs: MCValue) !MCVa
 
 fn airDiv(self: *Self, inst: Air.Inst.Index) !void {
     const bin_op = self.air.instructions.items(.data)[inst].bin_op;
-    const result: MCValue = if (self.liveness.isUnused(inst)) .dead else result: {
-        const tag = self.air.instructions.items(.tag)[inst];
-        const ty = self.air.typeOfIndex(inst);
 
-        if (ty.zigTypeTag() != .Int) {
-            return self.fail("TODO implement {} for operands of dst type {}", .{ tag, ty.zigTypeTag() });
+    if (self.liveness.isUnused(inst)) {
+        return self.finishAir(inst, .dead, .{ bin_op.lhs, bin_op.rhs, .none });
+    }
+
+    const tag = self.air.instructions.items(.tag)[inst];
+    const ty = self.air.typeOfIndex(inst);
+
+    if (ty.zigTypeTag() != .Int) {
+        return self.fail("TODO implement {} for operands of dst type {}", .{ tag, ty.zigTypeTag() });
+    }
+
+    if (tag == .div_float) {
+        return self.fail("TODO implement {}", .{tag});
+    }
+
+    const signedness = ty.intInfo(self.target.*).signedness;
+
+    // Spill .rax and .rdx upfront to ensure we don't spill the operands too late.
+    const track_rax: ?Air.Inst.Index = blk: {
+        if (signedness == .unsigned) break :blk inst;
+        switch (tag) {
+            .div_exact, .div_trunc => break :blk inst,
+            else => break :blk null,
         }
+    };
+    try self.register_manager.getReg(.rax, track_rax);
+    try self.register_manager.getReg(.rdx, null);
+    const reg_locks = self.register_manager.lockRegsAssumeUnused(2, .{ .rax, .rdx });
+    defer for (reg_locks) |reg| {
+        self.register_manager.unlockReg(reg);
+    };
 
-        if (tag == .div_float) {
-            return self.fail("TODO implement {}", .{tag});
-        }
+    const lhs = try self.resolveInst(bin_op.lhs);
+    const lhs_lock: ?RegisterLock = switch (lhs) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (lhs_lock) |lock| self.register_manager.unlockReg(lock);
 
-        const signedness = ty.intInfo(self.target.*).signedness;
-
-        // Spill .rax and .rdx upfront to ensure we don't spill the operands too late.
-        const track_rax: ?Air.Inst.Index = blk: {
-            if (signedness == .unsigned) break :blk inst;
+    const rhs: MCValue = blk: {
+        const rhs = try self.resolveInst(bin_op.rhs);
+        if (signedness == .signed) {
             switch (tag) {
-                .div_exact, .div_trunc => break :blk inst,
-                else => break :blk null,
+                .div_floor => {
+                    const rhs_lock: ?RegisterLock = switch (rhs) {
+                        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+                        else => null,
+                    };
+                    defer if (rhs_lock) |lock| self.register_manager.unlockReg(lock);
+
+                    break :blk try self.copyToRegisterWithInstTracking(inst, ty, rhs);
+                },
+                else => {},
             }
-        };
-        try self.register_manager.getReg(.rax, track_rax);
-        try self.register_manager.getReg(.rdx, null);
-        self.register_manager.freezeRegs(&.{ .rax, .rdx });
-        defer self.register_manager.unfreezeRegs(&.{ .rax, .rdx });
-
-        const lhs = try self.resolveInst(bin_op.lhs);
-        lhs.freezeIfRegister(&self.register_manager);
-        defer lhs.unfreezeIfRegister(&self.register_manager);
-
-        const rhs = blk: {
-            const rhs = try self.resolveInst(bin_op.rhs);
-            if (signedness == .signed) {
-                switch (tag) {
-                    .div_floor => {
-                        rhs.freezeIfRegister(&self.register_manager);
-                        defer rhs.unfreezeIfRegister(&self.register_manager);
-                        break :blk try self.copyToRegisterWithInstTracking(inst, ty, rhs);
-                    },
-                    else => {},
-                }
-            }
-            break :blk rhs;
-        };
-        rhs.freezeIfRegister(&self.register_manager);
-        defer rhs.unfreezeIfRegister(&self.register_manager);
+        }
+        break :blk rhs;
+    };
+    const rhs_lock: ?RegisterLock = switch (rhs) {
+        .register => |reg| self.register_manager.lockReg(reg),
+        else => null,
+    };
+    defer if (rhs_lock) |lock| self.register_manager.unlockReg(lock);
 
+    const result: MCValue = result: {
         if (signedness == .unsigned) {
             try self.genIntMulDivOpMir(.div, ty, signedness, lhs, rhs);
             break :result MCValue{ .register = .rax };
@@ -1822,53 +1871,67 @@ fn airDiv(self: *Self, inst: Air.Inst.Index) !void {
             else => unreachable,
         }
     };
+
     return self.finishAir(inst, result, .{ bin_op.lhs, bin_op.rhs, .none });
 }
 
 fn airRem(self: *Self, inst: Air.Inst.Index) !void {
     const bin_op = self.air.instructions.items(.data)[inst].bin_op;
-    const result: MCValue = if (self.liveness.isUnused(inst)) .dead else result: {
-        const ty = self.air.typeOfIndex(inst);
-        if (ty.zigTypeTag() != .Int) {
-            return self.fail("TODO implement .rem for operands of dst type {}", .{ty.zigTypeTag()});
-        }
-        // Spill .rax and .rdx upfront to ensure we don't spill the operands too late.
-        try self.register_manager.getReg(.rax, null);
-        try self.register_manager.getReg(.rdx, inst);
-        self.register_manager.freezeRegs(&.{ .rax, .rdx });
-        defer self.register_manager.unfreezeRegs(&.{ .rax, .rdx });
 
-        const lhs = try self.resolveInst(bin_op.lhs);
-        const rhs = try self.resolveInst(bin_op.rhs);
-
-        const signedness = ty.intInfo(self.target.*).signedness;
-        try self.genIntMulDivOpMir(switch (signedness) {
-            .signed => .idiv,
-            .unsigned => .div,
-        }, ty, signedness, lhs, rhs);
-        break :result MCValue{ .register = .rdx };
+    if (self.liveness.isUnused(inst)) {
+        return self.finishAir(inst, .dead, .{ bin_op.lhs, bin_op.rhs, .none });
+    }
+    const ty = self.air.typeOfIndex(inst);
+    if (ty.zigTypeTag() != .Int) {
+        return self.fail("TODO implement .rem for operands of dst type {}", .{ty.zigTypeTag()});
+    }
+    // Spill .rax and .rdx upfront to ensure we don't spill the operands too late.
+    try self.register_manager.getReg(.rax, null);
+    try self.register_manager.getReg(.rdx, inst);
+    const reg_locks = self.register_manager.lockRegsAssumeUnused(2, .{ .rax, .rdx });
+    defer for (reg_locks) |reg| {
+        self.register_manager.unlockReg(reg);
     };
+
+    const lhs = try self.resolveInst(bin_op.lhs);
+    const rhs = try self.resolveInst(bin_op.rhs);
+
+    const signedness = ty.intInfo(self.target.*).signedness;
+    try self.genIntMulDivOpMir(switch (signedness) {
+        .signed => .idiv,
+        .unsigned => .div,
+    }, ty, signedness, lhs, rhs);
+
+    const result: MCValue = .{ .register = .rdx };
+
     return self.finishAir(inst, result, .{ bin_op.lhs, bin_op.rhs, .none });
 }
 
 fn airMod(self: *Self, inst: Air.Inst.Index) !void {
     const bin_op = self.air.instructions.items(.data)[inst].bin_op;
-    const result: MCValue = if (self.liveness.isUnused(inst)) .dead else result: {
-        const ty = self.air.typeOfIndex(inst);
-        if (ty.zigTypeTag() != .Int) {
-            return self.fail("TODO implement .mod for operands of dst type {}", .{ty.zigTypeTag()});
-        }
-        const signedness = ty.intInfo(self.target.*).signedness;
 
-        // Spill .rax and .rdx upfront to ensure we don't spill the operands too late.
-        try self.register_manager.getReg(.rax, null);
-        try self.register_manager.getReg(.rdx, if (signedness == .unsigned) inst else null);
-        self.register_manager.freezeRegs(&.{ .rax, .rdx });
-        defer self.register_manager.unfreezeRegs(&.{ .rax, .rdx });
+    if (self.liveness.isUnused(inst)) {
+        return self.finishAir(inst, .dead, .{ bin_op.lhs, bin_op.rhs, .none });
+    }
 
-        const lhs = try self.resolveInst(bin_op.lhs);
-        const rhs = try self.resolveInst(bin_op.rhs);
+    const ty = self.air.typeOfIndex(inst);
+    if (ty.zigTypeTag() != .Int) {
+        return self.fail("TODO implement .mod for operands of dst type {}", .{ty.zigTypeTag()});
+    }
+    const signedness = ty.intInfo(self.target.*).signedness;
 
+    // Spill .rax and .rdx upfront to ensure we don't spill the operands too late.
+    try self.register_manager.getReg(.rax, null);
+    try self.register_manager.getReg(.rdx, if (signedness == .unsigned) inst else null);
+    const reg_locks = self.register_manager.lockRegsAssumeUnused(2, .{ .rax, .rdx });
+    defer for (reg_locks) |reg| {
+        self.register_manager.unlockReg(reg);
+    };
+
+    const lhs = try self.resolveInst(bin_op.lhs);
+    const rhs = try self.resolveInst(bin_op.rhs);
+
+    const result: MCValue = result: {
         switch (signedness) {
             .unsigned => {
                 try self.genIntMulDivOpMir(switch (signedness) {
@@ -1888,6 +1951,7 @@ fn airMod(self: *Self, inst: Air.Inst.Index) !void {
             },
         }
     };
+
     return self.finishAir(inst, result, .{ bin_op.lhs, bin_op.rhs, .none });
 }
 
@@ -1954,12 +2018,15 @@ fn airShl(self: *Self, inst: Air.Inst.Index) !void {
         try self.register_manager.getReg(.rcx, null);
         try self.genSetReg(shift_ty, .rcx, shift);
     }
-    self.register_manager.freezeRegs(&.{.rcx});
-    defer self.register_manager.unfreezeRegs(&.{.rcx});
+    const rcx_lock = self.register_manager.lockRegAssumeUnused(.rcx);
+    defer self.register_manager.unlockReg(rcx_lock);
 
     const value = try self.resolveInst(bin_op.lhs);
-    value.freezeIfRegister(&self.register_manager);
-    defer value.unfreezeIfRegister(&self.register_manager);
+    const value_lock: ?RegisterLock = switch (value) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (value_lock) |lock| self.register_manager.unlockReg(lock);
 
     const dst_mcv = try self.copyToRegisterWithInstTracking(inst, ty, value);
     _ = try self.addInst(.{
@@ -2055,8 +2122,11 @@ fn airUnwrapErrErr(self: *Self, inst: Air.Inst.Index) !void {
     const err_ty = err_union_ty.errorUnionSet();
     const payload_ty = err_union_ty.errorUnionPayload();
     const operand = try self.resolveInst(ty_op.operand);
-    operand.freezeIfRegister(&self.register_manager);
-    defer operand.unfreezeIfRegister(&self.register_manager);
+    const operand_lock: ?RegisterLock = switch (operand) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (operand_lock) |lock| self.register_manager.unlockReg(lock);
 
     const result: MCValue = result: {
         if (!payload_ty.hasRuntimeBits()) break :result operand;
@@ -2085,8 +2155,11 @@ fn airUnwrapErrPayload(self: *Self, inst: Air.Inst.Index) !void {
         if (!payload_ty.hasRuntimeBits()) break :result MCValue.none;
 
         const operand = try self.resolveInst(ty_op.operand);
-        operand.freezeIfRegister(&self.register_manager);
-        defer operand.unfreezeIfRegister(&self.register_manager);
+        const operand_lock: ?RegisterLock = switch (operand) {
+            .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+            else => null,
+        };
+        defer if (operand_lock) |lock| self.register_manager.unlockReg(lock);
 
         const abi_align = err_union_ty.abiAlignment(self.target.*);
         const err_ty = err_union_ty.errorUnionSet();
@@ -2154,8 +2227,11 @@ fn airWrapOptional(self: *Self, inst: Air.Inst.Index) !void {
 
         const optional_ty = self.air.typeOfIndex(inst);
         const operand = try self.resolveInst(ty_op.operand);
-        operand.freezeIfRegister(&self.register_manager);
-        defer operand.unfreezeIfRegister(&self.register_manager);
+        const operand_lock: ?RegisterLock = switch (operand) {
+            .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+            else => null,
+        };
+        defer if (operand_lock) |lock| self.register_manager.unlockReg(lock);
 
         if (optional_ty.isPtrLikeOptional()) {
             // TODO should we check if we can reuse the operand?
@@ -2288,8 +2364,11 @@ fn elemOffset(self: *Self, index_ty: Type, index: MCValue, elem_size: u64) !Regi
 fn genSliceElemPtr(self: *Self, lhs: Air.Inst.Ref, rhs: Air.Inst.Ref) !MCValue {
     const slice_ty = self.air.typeOf(lhs);
     const slice_mcv = try self.resolveInst(lhs);
-    slice_mcv.freezeIfRegister(&self.register_manager);
-    defer slice_mcv.unfreezeIfRegister(&self.register_manager);
+    const slice_mcv_lock: ?RegisterLock = switch (slice_mcv) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (slice_mcv_lock) |lock| self.register_manager.unlockReg(lock);
 
     const elem_ty = slice_ty.childType();
     const elem_size = elem_ty.abiSize(self.target.*);
@@ -2298,12 +2377,15 @@ fn genSliceElemPtr(self: *Self, lhs: Air.Inst.Ref, rhs: Air.Inst.Ref) !MCValue {
 
     const index_ty = self.air.typeOf(rhs);
     const index_mcv = try self.resolveInst(rhs);
-    index_mcv.freezeIfRegister(&self.register_manager);
-    defer index_mcv.unfreezeIfRegister(&self.register_manager);
+    const index_mcv_lock: ?RegisterLock = switch (index_mcv) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (index_mcv_lock) |lock| self.register_manager.unlockReg(lock);
 
     const offset_reg = try self.elemOffset(index_ty, index_mcv, elem_size);
-    self.register_manager.freezeRegs(&.{offset_reg});
-    defer self.register_manager.unfreezeRegs(&.{offset_reg});
+    const offset_reg_lock = self.register_manager.lockRegAssumeUnused(offset_reg);
+    defer self.register_manager.unlockReg(offset_reg_lock);
 
     const addr_reg = try self.register_manager.allocReg(null);
     switch (slice_mcv) {
@@ -2356,98 +2438,119 @@ fn airSliceElemPtr(self: *Self, inst: Air.Inst.Index) !void {
 
 fn airArrayElemVal(self: *Self, inst: Air.Inst.Index) !void {
     const bin_op = self.air.instructions.items(.data)[inst].bin_op;
-    const result: MCValue = if (self.liveness.isUnused(inst)) .dead else result: {
-        const array_ty = self.air.typeOf(bin_op.lhs);
-        const array = try self.resolveInst(bin_op.lhs);
-        array.freezeIfRegister(&self.register_manager);
-        defer array.unfreezeIfRegister(&self.register_manager);
 
-        const elem_ty = array_ty.childType();
-        const elem_abi_size = elem_ty.abiSize(self.target.*);
+    if (self.liveness.isUnused(inst)) {
+        return self.finishAir(inst, .dead, .{ bin_op.lhs, bin_op.rhs, .none });
+    }
 
-        const index_ty = self.air.typeOf(bin_op.rhs);
-        const index = try self.resolveInst(bin_op.rhs);
-        index.freezeIfRegister(&self.register_manager);
-        defer index.unfreezeIfRegister(&self.register_manager);
-
-        const offset_reg = try self.elemOffset(index_ty, index, elem_abi_size);
-        self.register_manager.freezeRegs(&.{offset_reg});
-        defer self.register_manager.unfreezeRegs(&.{offset_reg});
-
-        const addr_reg = try self.register_manager.allocReg(null);
-        switch (array) {
-            .register => {
-                const off = @intCast(i32, try self.allocMem(
-                    inst,
-                    @intCast(u32, array_ty.abiSize(self.target.*)),
-                    array_ty.abiAlignment(self.target.*),
-                ));
-                try self.genSetStack(array_ty, off, array, .{});
-                // lea reg, [rbp]
-                _ = try self.addInst(.{
-                    .tag = .lea,
-                    .ops = (Mir.Ops{
-                        .reg1 = addr_reg.to64(),
-                        .reg2 = .rbp,
-                    }).encode(),
-                    .data = .{ .imm = @bitCast(u32, -off) },
-                });
-            },
-            .stack_offset => |off| {
-                // lea reg, [rbp]
-                _ = try self.addInst(.{
-                    .tag = .lea,
-                    .ops = (Mir.Ops{
-                        .reg1 = addr_reg.to64(),
-                        .reg2 = .rbp,
-                    }).encode(),
-                    .data = .{ .imm = @bitCast(u32, -off) },
-                });
-            },
-            .memory,
-            .got_load,
-            .direct_load,
-            => {
-                try self.loadMemPtrIntoRegister(addr_reg, Type.usize, array);
-            },
-            else => return self.fail("TODO implement array_elem_val when array is {}", .{array}),
-        }
-
-        // TODO we could allocate register here, but need to expect addr register and potentially
-        // offset register.
-        const dst_mcv = try self.allocRegOrMem(inst, false);
-        try self.genBinMathOpMir(.add, Type.usize, .{ .register = addr_reg }, .{ .register = offset_reg });
-        try self.load(dst_mcv, .{ .register = addr_reg.to64() }, array_ty);
-        break :result dst_mcv;
+    const array_ty = self.air.typeOf(bin_op.lhs);
+    const array = try self.resolveInst(bin_op.lhs);
+    const array_lock: ?RegisterLock = switch (array) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
     };
-    return self.finishAir(inst, result, .{ bin_op.lhs, bin_op.rhs, .none });
+    defer if (array_lock) |lock| self.register_manager.unlockReg(lock);
+
+    const elem_ty = array_ty.childType();
+    const elem_abi_size = elem_ty.abiSize(self.target.*);
+
+    const index_ty = self.air.typeOf(bin_op.rhs);
+    const index = try self.resolveInst(bin_op.rhs);
+    const index_lock: ?RegisterLock = switch (index) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (index_lock) |lock| self.register_manager.unlockReg(lock);
+
+    const offset_reg = try self.elemOffset(index_ty, index, elem_abi_size);
+    const offset_reg_lock = self.register_manager.lockRegAssumeUnused(offset_reg);
+    defer self.register_manager.unlockReg(offset_reg_lock);
+
+    const addr_reg = try self.register_manager.allocReg(null);
+    switch (array) {
+        .register => {
+            const off = @intCast(i32, try self.allocMem(
+                inst,
+                @intCast(u32, array_ty.abiSize(self.target.*)),
+                array_ty.abiAlignment(self.target.*),
+            ));
+            try self.genSetStack(array_ty, off, array, .{});
+            // lea reg, [rbp]
+            _ = try self.addInst(.{
+                .tag = .lea,
+                .ops = (Mir.Ops{
+                    .reg1 = addr_reg.to64(),
+                    .reg2 = .rbp,
+                }).encode(),
+                .data = .{ .imm = @bitCast(u32, -off) },
+            });
+        },
+        .stack_offset => |off| {
+            // lea reg, [rbp]
+            _ = try self.addInst(.{
+                .tag = .lea,
+                .ops = (Mir.Ops{
+                    .reg1 = addr_reg.to64(),
+                    .reg2 = .rbp,
+                }).encode(),
+                .data = .{ .imm = @bitCast(u32, -off) },
+            });
+        },
+        .memory,
+        .got_load,
+        .direct_load,
+        => {
+            try self.loadMemPtrIntoRegister(addr_reg, Type.usize, array);
+        },
+        else => return self.fail("TODO implement array_elem_val when array is {}", .{array}),
+    }
+
+    // TODO we could allocate register here, but need to expect addr register and potentially
+    // offset register.
+    const dst_mcv = try self.allocRegOrMem(inst, false);
+    try self.genBinMathOpMir(.add, Type.usize, .{ .register = addr_reg }, .{ .register = offset_reg });
+    try self.load(dst_mcv, .{ .register = addr_reg.to64() }, array_ty);
+
+    return self.finishAir(inst, dst_mcv, .{ bin_op.lhs, bin_op.rhs, .none });
 }
 
 fn airPtrElemVal(self: *Self, inst: Air.Inst.Index) !void {
     const is_volatile = false; // TODO
     const bin_op = self.air.instructions.items(.data)[inst].bin_op;
-    const result: MCValue = if (!is_volatile and self.liveness.isUnused(inst)) .dead else result: {
-        // this is identical to the `airPtrElemPtr` codegen expect here an
-        // additional `mov` is needed at the end to get the actual value
 
-        const ptr_ty = self.air.typeOf(bin_op.lhs);
-        const ptr = try self.resolveInst(bin_op.lhs);
-        ptr.freezeIfRegister(&self.register_manager);
-        defer ptr.unfreezeIfRegister(&self.register_manager);
+    if (!is_volatile and self.liveness.isUnused(inst)) {
+        return self.finishAir(inst, .dead, .{ bin_op.lhs, bin_op.rhs, .none });
+    }
 
-        const elem_ty = ptr_ty.elemType2();
-        const elem_abi_size = elem_ty.abiSize(self.target.*);
-        const index_ty = self.air.typeOf(bin_op.rhs);
-        const index = try self.resolveInst(bin_op.rhs);
-        index.freezeIfRegister(&self.register_manager);
-        defer index.unfreezeIfRegister(&self.register_manager);
+    // this is identical to the `airPtrElemPtr` codegen expect here an
+    // additional `mov` is needed at the end to get the actual value
 
-        const offset_reg = try self.elemOffset(index_ty, index, elem_abi_size);
-        self.register_manager.freezeRegs(&.{offset_reg});
-        defer self.register_manager.unfreezeRegs(&.{offset_reg});
+    const ptr_ty = self.air.typeOf(bin_op.lhs);
+    const ptr = try self.resolveInst(bin_op.lhs);
+    const ptr_lock: ?RegisterLock = switch (ptr) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (ptr_lock) |lock| self.register_manager.unlockReg(lock);
 
-        const dst_mcv = try self.copyToRegisterWithInstTracking(inst, ptr_ty, ptr);
-        try self.genBinMathOpMir(.add, ptr_ty, dst_mcv, .{ .register = offset_reg });
+    const elem_ty = ptr_ty.elemType2();
+    const elem_abi_size = elem_ty.abiSize(self.target.*);
+    const index_ty = self.air.typeOf(bin_op.rhs);
+    const index = try self.resolveInst(bin_op.rhs);
+    const index_lock: ?RegisterLock = switch (index) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (index_lock) |lock| self.register_manager.unlockReg(lock);
+
+    const offset_reg = try self.elemOffset(index_ty, index, elem_abi_size);
+    const offset_reg_lock = self.register_manager.lockRegAssumeUnused(offset_reg);
+    defer self.register_manager.unlockReg(offset_reg_lock);
+
+    const dst_mcv = try self.copyToRegisterWithInstTracking(inst, ptr_ty, ptr);
+    try self.genBinMathOpMir(.add, ptr_ty, dst_mcv, .{ .register = offset_reg });
+
+    const result: MCValue = result: {
         if (elem_abi_size > 8) {
             return self.fail("TODO copy value with size {} from pointer", .{elem_abi_size});
         } else {
@@ -2464,34 +2567,44 @@ fn airPtrElemVal(self: *Self, inst: Air.Inst.Index) !void {
             break :result .{ .register = registerAlias(dst_mcv.register, @intCast(u32, elem_abi_size)) };
         }
     };
+
     return self.finishAir(inst, result, .{ bin_op.lhs, bin_op.rhs, .none });
 }
 
 fn airPtrElemPtr(self: *Self, inst: Air.Inst.Index) !void {
     const ty_pl = self.air.instructions.items(.data)[inst].ty_pl;
     const extra = self.air.extraData(Air.Bin, ty_pl.payload).data;
-    const result: MCValue = if (self.liveness.isUnused(inst)) .dead else result: {
-        const ptr_ty = self.air.typeOf(extra.lhs);
-        const ptr = try self.resolveInst(extra.lhs);
-        ptr.freezeIfRegister(&self.register_manager);
-        defer ptr.unfreezeIfRegister(&self.register_manager);
 
-        const elem_ty = ptr_ty.elemType2();
-        const elem_abi_size = elem_ty.abiSize(self.target.*);
-        const index_ty = self.air.typeOf(extra.rhs);
-        const index = try self.resolveInst(extra.rhs);
-        index.freezeIfRegister(&self.register_manager);
-        defer index.unfreezeIfRegister(&self.register_manager);
+    if (self.liveness.isUnused(inst)) {
+        return self.finishAir(inst, .dead, .{ extra.lhs, extra.rhs, .none });
+    }
 
-        const offset_reg = try self.elemOffset(index_ty, index, elem_abi_size);
-        self.register_manager.freezeRegs(&.{offset_reg});
-        defer self.register_manager.unfreezeRegs(&.{offset_reg});
-
-        const dst_mcv = try self.copyToRegisterWithInstTracking(inst, ptr_ty, ptr);
-        try self.genBinMathOpMir(.add, ptr_ty, dst_mcv, .{ .register = offset_reg });
-        break :result dst_mcv;
+    const ptr_ty = self.air.typeOf(extra.lhs);
+    const ptr = try self.resolveInst(extra.lhs);
+    const ptr_lock: ?RegisterLock = switch (ptr) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
     };
-    return self.finishAir(inst, result, .{ extra.lhs, extra.rhs, .none });
+    defer if (ptr_lock) |lock| self.register_manager.unlockReg(lock);
+
+    const elem_ty = ptr_ty.elemType2();
+    const elem_abi_size = elem_ty.abiSize(self.target.*);
+    const index_ty = self.air.typeOf(extra.rhs);
+    const index = try self.resolveInst(extra.rhs);
+    const index_lock: ?RegisterLock = switch (index) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (index_lock) |lock| self.register_manager.unlockReg(lock);
+
+    const offset_reg = try self.elemOffset(index_ty, index, elem_abi_size);
+    const offset_reg_lock = self.register_manager.lockRegAssumeUnused(offset_reg);
+    defer self.register_manager.unlockReg(offset_reg_lock);
+
+    const dst_mcv = try self.copyToRegisterWithInstTracking(inst, ptr_ty, ptr);
+    try self.genBinMathOpMir(.add, ptr_ty, dst_mcv, .{ .register = offset_reg });
+
+    return self.finishAir(inst, dst_mcv, .{ extra.lhs, extra.rhs, .none });
 }
 
 fn airSetUnionTag(self: *Self, inst: Air.Inst.Index) !void {
@@ -2506,12 +2619,18 @@ fn airSetUnionTag(self: *Self, inst: Air.Inst.Index) !void {
     }
 
     const ptr = try self.resolveInst(bin_op.lhs);
-    ptr.freezeIfRegister(&self.register_manager);
-    defer ptr.unfreezeIfRegister(&self.register_manager);
+    const ptr_lock: ?RegisterLock = switch (ptr) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (ptr_lock) |lock| self.register_manager.unlockReg(lock);
 
     const tag = try self.resolveInst(bin_op.rhs);
-    tag.freezeIfRegister(&self.register_manager);
-    defer tag.unfreezeIfRegister(&self.register_manager);
+    const tag_lock: ?RegisterLock = switch (tag) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (tag_lock) |lock| self.register_manager.unlockReg(lock);
 
     const adjusted_ptr: MCValue = if (layout.payload_size > 0 and layout.tag_align < layout.payload_align) blk: {
         // TODO reusing the operand
@@ -2541,8 +2660,11 @@ fn airGetUnionTag(self: *Self, inst: Air.Inst.Index) !void {
 
     // TODO reusing the operand
     const operand = try self.resolveInst(ty_op.operand);
-    operand.freezeIfRegister(&self.register_manager);
-    defer operand.unfreezeIfRegister(&self.register_manager);
+    const operand_lock: ?RegisterLock = switch (operand) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (operand_lock) |lock| self.register_manager.unlockReg(lock);
 
     const tag_abi_size = tag_ty.abiSize(self.target.*);
     const dst_mcv: MCValue = blk: {
@@ -2689,8 +2811,8 @@ fn load(self: *Self, dst_mcv: MCValue, ptr: MCValue, ptr_ty: Type) InnerError!vo
             try self.setRegOrMem(elem_ty, dst_mcv, .{ .stack_offset = off });
         },
         .register => |reg| {
-            self.register_manager.freezeRegs(&.{reg});
-            defer self.register_manager.unfreezeRegs(&.{reg});
+            const reg_lock = self.register_manager.lockReg(reg);
+            defer if (reg_lock) |lock| self.register_manager.unlockReg(lock);
 
             switch (dst_mcv) {
                 .dead => unreachable,
@@ -2815,8 +2937,8 @@ fn store(self: *Self, ptr: MCValue, value: MCValue, ptr_ty: Type, value_ty: Type
             try self.genSetStack(value_ty, off, value, .{});
         },
         .register => |reg| {
-            self.register_manager.freezeRegs(&.{reg});
-            defer self.register_manager.unfreezeRegs(&.{reg});
+            const reg_lock = self.register_manager.lockReg(reg);
+            defer if (reg_lock) |lock| self.register_manager.unlockReg(lock);
 
             switch (value) {
                 .none => unreachable,
@@ -2906,12 +3028,15 @@ fn store(self: *Self, ptr: MCValue, value: MCValue, ptr_ty: Type, value_ty: Type
         .direct_load,
         .memory,
         => {
-            value.freezeIfRegister(&self.register_manager);
-            defer value.unfreezeIfRegister(&self.register_manager);
+            const value_lock: ?RegisterLock = switch (value) {
+                .register => |reg| self.register_manager.lockReg(reg),
+                else => null,
+            };
+            defer if (value_lock) |lock| self.register_manager.unlockReg(lock);
 
             const addr_reg = try self.register_manager.allocReg(null);
-            self.register_manager.freezeRegs(&.{addr_reg});
-            defer self.register_manager.unfreezeRegs(&.{addr_reg});
+            const addr_reg_lock = self.register_manager.lockRegAssumeUnused(addr_reg);
+            defer self.register_manager.unlockReg(addr_reg_lock);
 
             try self.loadMemPtrIntoRegister(addr_reg, ptr_ty, ptr);
 
@@ -2982,8 +3107,8 @@ fn store(self: *Self, ptr: MCValue, value: MCValue, ptr_ty: Type, value_ty: Type
                 => {
                     if (abi_size <= 8) {
                         const tmp_reg = try self.register_manager.allocReg(null);
-                        self.register_manager.freezeRegs(&.{tmp_reg});
-                        defer self.register_manager.unfreezeRegs(&.{tmp_reg});
+                        const tmp_reg_lock = self.register_manager.lockRegAssumeUnused(tmp_reg);
+                        defer self.register_manager.unlockReg(tmp_reg_lock);
 
                         try self.loadMemPtrIntoRegister(tmp_reg, value_ty, value);
 
@@ -3073,8 +3198,8 @@ fn structFieldPtr(self: *Self, inst: Air.Inst.Index, operand: Air.Inst.Ref, inde
                 const offset_reg = try self.copyToTmpRegister(ptr_ty, .{
                     .immediate = struct_field_offset,
                 });
-                self.register_manager.freezeRegs(&.{offset_reg});
-                defer self.register_manager.unfreezeRegs(&.{offset_reg});
+                const offset_reg_lock = self.register_manager.lockRegAssumeUnused(offset_reg);
+                defer self.register_manager.unlockReg(offset_reg_lock);
 
                 const dst_mcv = try self.copyToRegisterWithInstTracking(inst, ptr_ty, mcv);
                 try self.genBinMathOpMir(.add, ptr_ty, dst_mcv, .{ .register = offset_reg });
@@ -3085,24 +3210,27 @@ fn structFieldPtr(self: *Self, inst: Air.Inst.Index, operand: Air.Inst.Ref, inde
                 break :result MCValue{ .ptr_stack_offset = ptr_stack_offset };
             },
             .register => |reg| {
+                const reg_lock = self.register_manager.lockRegAssumeUnused(reg);
+                defer self.register_manager.unlockReg(reg_lock);
+
                 const offset_reg = try self.copyToTmpRegister(ptr_ty, .{
                     .immediate = struct_field_offset,
                 });
-                self.register_manager.freezeRegs(&.{offset_reg});
-                defer self.register_manager.unfreezeRegs(&.{offset_reg});
+                const offset_reg_lock = self.register_manager.lockRegAssumeUnused(offset_reg);
+                defer self.register_manager.unlockReg(offset_reg_lock);
 
                 const can_reuse_operand = self.reuseOperand(inst, operand, 0, mcv);
-                const result_reg = blk: {
+                const result_reg: Register = blk: {
                     if (can_reuse_operand) {
                         break :blk reg;
                     } else {
-                        self.register_manager.freezeRegs(&.{reg});
                         const result_reg = try self.register_manager.allocReg(inst);
                         try self.genSetReg(ptr_ty, result_reg, mcv);
                         break :blk result_reg;
                     }
                 };
-                defer if (!can_reuse_operand) self.register_manager.unfreezeRegs(&.{reg});
+                const result_reg_lock = self.register_manager.lockReg(result_reg);
+                defer if (result_reg_lock) |lock| self.register_manager.unlockReg(lock);
 
                 try self.genBinMathOpMir(.add, ptr_ty, .{ .register = result_reg }, .{ .register = offset_reg });
                 break :result MCValue{ .register = result_reg };
@@ -3118,22 +3246,27 @@ fn airStructFieldVal(self: *Self, inst: Air.Inst.Index) !void {
     const extra = self.air.extraData(Air.StructField, ty_pl.payload).data;
     const operand = extra.struct_operand;
     const index = extra.field_index;
-    const result: MCValue = if (self.liveness.isUnused(inst)) .dead else result: {
-        const mcv = try self.resolveInst(operand);
-        const struct_ty = self.air.typeOf(operand);
-        const struct_field_offset = struct_ty.structFieldOffset(index, self.target.*);
-        const struct_field_ty = struct_ty.structFieldType(index);
 
+    if (self.liveness.isUnused(inst)) {
+        return self.finishAir(inst, .dead, .{ extra.struct_operand, .none, .none });
+    }
+
+    const mcv = try self.resolveInst(operand);
+    const struct_ty = self.air.typeOf(operand);
+    const struct_field_offset = struct_ty.structFieldOffset(index, self.target.*);
+    const struct_field_ty = struct_ty.structFieldType(index);
+
+    const result: MCValue = result: {
         switch (mcv) {
             .stack_offset => |off| {
                 const stack_offset = off - @intCast(i32, struct_field_offset);
                 break :result MCValue{ .stack_offset = stack_offset };
             },
             .register => |reg| {
-                self.register_manager.freezeRegs(&.{reg});
-                defer self.register_manager.unfreezeRegs(&.{reg});
+                const reg_lock = self.register_manager.lockRegAssumeUnused(reg);
+                defer self.register_manager.unlockReg(reg_lock);
 
-                const dst_mcv = blk: {
+                const dst_mcv: MCValue = blk: {
                     if (self.reuseOperand(inst, operand, 0, mcv)) {
                         break :blk mcv;
                     } else {
@@ -3143,8 +3276,11 @@ fn airStructFieldVal(self: *Self, inst: Air.Inst.Index) !void {
                         break :blk dst_mcv;
                     }
                 };
-                dst_mcv.freezeIfRegister(&self.register_manager);
-                defer dst_mcv.unfreezeIfRegister(&self.register_manager);
+                const dst_mcv_lock: ?RegisterLock = switch (dst_mcv) {
+                    .register => |a_reg| self.register_manager.lockReg(a_reg),
+                    else => null,
+                };
+                defer if (dst_mcv_lock) |lock| self.register_manager.unlockReg(lock);
 
                 // Shift by struct_field_offset.
                 const shift = @intCast(u8, struct_field_offset * @sizeOf(usize));
@@ -3186,8 +3322,8 @@ fn airStructFieldVal(self: *Self, inst: Air.Inst.Index) !void {
                     },
                     1 => {
                         // Get overflow bit.
-                        mcv.freezeIfRegister(&self.register_manager);
-                        defer mcv.unfreezeIfRegister(&self.register_manager);
+                        const reg_lock = self.register_manager.lockRegAssumeUnused(reg);
+                        defer self.register_manager.unlockReg(reg_lock);
 
                         const dst_reg = try self.register_manager.allocReg(inst);
                         const flags: u2 = switch (mcv) {
@@ -3229,15 +3365,21 @@ fn genBinMathOp(self: *Self, inst: Air.Inst.Index, op_lhs: Air.Inst.Ref, op_rhs:
     const dst_ty = self.air.typeOf(op_lhs);
 
     const lhs = try self.resolveInst(op_lhs);
-    lhs.freezeIfRegister(&self.register_manager);
-    defer lhs.unfreezeIfRegister(&self.register_manager);
+    const lhs_lock: ?RegisterLock = switch (lhs) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (lhs_lock) |lock| self.register_manager.unlockReg(lock);
 
     const rhs = try self.resolveInst(op_rhs);
-    rhs.freezeIfRegister(&self.register_manager);
-    defer rhs.unfreezeIfRegister(&self.register_manager);
+    const rhs_lock: ?RegisterLock = switch (rhs) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (rhs_lock) |lock| self.register_manager.unlockReg(lock);
 
     var flipped: bool = false;
-    const dst_mcv = blk: {
+    const dst_mcv: MCValue = blk: {
         if (self.reuseOperand(inst, op_lhs, 0, lhs) and lhs.isRegister()) {
             break :blk lhs;
         }
@@ -3247,16 +3389,22 @@ fn genBinMathOp(self: *Self, inst: Air.Inst.Index, op_lhs: Air.Inst.Ref, op_rhs:
         }
         break :blk try self.copyToRegisterWithInstTracking(inst, dst_ty, lhs);
     };
-    dst_mcv.freezeIfRegister(&self.register_manager);
-    defer dst_mcv.unfreezeIfRegister(&self.register_manager);
+    const dst_mcv_lock: ?RegisterLock = switch (dst_mcv) {
+        .register => |reg| self.register_manager.lockReg(reg),
+        else => null,
+    };
+    defer if (dst_mcv_lock) |lock| self.register_manager.unlockReg(lock);
 
-    const src_mcv = blk: {
+    const src_mcv: MCValue = blk: {
         const mcv = if (flipped) lhs else rhs;
         if (mcv.isRegister() or mcv.isMemory()) break :blk mcv;
         break :blk MCValue{ .register = try self.copyToTmpRegister(dst_ty, mcv) };
     };
-    src_mcv.freezeIfRegister(&self.register_manager);
-    defer src_mcv.unfreezeIfRegister(&self.register_manager);
+    const src_mcv_lock: ?RegisterLock = switch (src_mcv) {
+        .register => |reg| self.register_manager.lockReg(reg),
+        else => null,
+    };
+    defer if (src_mcv_lock) |lock| self.register_manager.unlockReg(lock);
 
     const tag = self.air.instructions.items(.tag)[inst];
     switch (tag) {
@@ -3287,8 +3435,9 @@ fn genBinMathOpMir(self: *Self, mir_tag: Mir.Inst.Tag, dst_ty: Type, dst_mcv: MC
                 .register_overflow_unsigned => unreachable,
                 .register_overflow_signed => unreachable,
                 .ptr_stack_offset => {
-                    self.register_manager.freezeRegs(&.{dst_reg});
-                    defer self.register_manager.unfreezeRegs(&.{dst_reg});
+                    const dst_reg_lock = self.register_manager.lockReg(dst_reg);
+                    defer if (dst_reg_lock) |lock| self.register_manager.unlockReg(lock);
+
                     const reg = try self.copyToTmpRegister(dst_ty, src_mcv);
                     return self.genBinMathOpMir(mir_tag, dst_ty, dst_mcv, .{ .register = reg });
                 },
@@ -3318,8 +3467,9 @@ fn genBinMathOpMir(self: *Self, mir_tag: Mir.Inst.Tag, dst_ty: Type, dst_mcv: MC
                 .compare_flags_unsigned,
                 => {
                     assert(abi_size <= 8);
-                    self.register_manager.freezeRegs(&.{dst_reg});
-                    defer self.register_manager.unfreezeRegs(&.{dst_reg});
+                    const dst_reg_lock = self.register_manager.lockReg(dst_reg);
+                    defer if (dst_reg_lock) |lock| self.register_manager.unlockReg(lock);
+
                     const reg = try self.copyToTmpRegister(dst_ty, src_mcv);
                     return self.genBinMathOpMir(mir_tag, dst_ty, dst_mcv, .{ .register = reg });
                 },
@@ -3659,20 +3809,25 @@ fn airCall(self: *Self, inst: Air.Inst.Index, modifier: std.builtin.CallOptions.
         try self.register_manager.getReg(reg, null);
     }
 
-    if (info.return_value == .stack_offset) {
-        const ret_ty = fn_ty.fnReturnType();
-        const ret_abi_size = @intCast(u32, ret_ty.abiSize(self.target.*));
-        const ret_abi_align = @intCast(u32, ret_ty.abiAlignment(self.target.*));
-        const stack_offset = @intCast(i32, try self.allocMem(inst, ret_abi_size, ret_abi_align));
-        log.debug("airCall: return value on stack at offset {}", .{stack_offset});
+    const rdi_lock: ?RegisterLock = blk: {
+        if (info.return_value == .stack_offset) {
+            const ret_ty = fn_ty.fnReturnType();
+            const ret_abi_size = @intCast(u32, ret_ty.abiSize(self.target.*));
+            const ret_abi_align = @intCast(u32, ret_ty.abiAlignment(self.target.*));
+            const stack_offset = @intCast(i32, try self.allocMem(inst, ret_abi_size, ret_abi_align));
+            log.debug("airCall: return value on stack at offset {}", .{stack_offset});
 
-        try self.register_manager.getReg(.rdi, null);
-        try self.genSetReg(Type.usize, .rdi, .{ .ptr_stack_offset = stack_offset });
-        self.register_manager.freezeRegs(&.{.rdi});
+            try self.register_manager.getReg(.rdi, null);
+            try self.genSetReg(Type.usize, .rdi, .{ .ptr_stack_offset = stack_offset });
+            const rdi_lock = self.register_manager.lockRegAssumeUnused(.rdi);
 
-        info.return_value.stack_offset = stack_offset;
-    }
-    defer if (info.return_value == .stack_offset) self.register_manager.unfreezeRegs(&.{.rdi});
+            info.return_value.stack_offset = stack_offset;
+
+            break :blk rdi_lock;
+        }
+        break :blk null;
+    };
+    defer if (rdi_lock) |lock| self.register_manager.unlockReg(lock);
 
     for (args) |arg, arg_i| {
         const mc_arg = info.args[arg_i];
@@ -3891,11 +4046,10 @@ fn airRet(self: *Self, inst: Air.Inst.Index) !void {
     const ret_ty = self.fn_type.fnReturnType();
     switch (self.ret_mcv) {
         .stack_offset => {
-            self.register_manager.freezeRegs(&.{ .rax, .rcx });
-            defer self.register_manager.unfreezeRegs(&.{ .rax, .rcx });
             const reg = try self.copyToTmpRegister(Type.usize, self.ret_mcv);
-            self.register_manager.freezeRegs(&.{reg});
-            defer self.register_manager.unfreezeRegs(&.{reg});
+            const reg_lock = self.register_manager.lockRegAssumeUnused(reg);
+            defer self.register_manager.unlockReg(reg_lock);
+
             try self.genSetStack(ret_ty, 0, operand, .{
                 .source_stack_base = .rbp,
                 .dest_stack_base = reg,
@@ -3926,11 +4080,10 @@ fn airRetLoad(self: *Self, inst: Air.Inst.Index) !void {
     const elem_ty = ptr_ty.elemType();
     switch (self.ret_mcv) {
         .stack_offset => {
-            self.register_manager.freezeRegs(&.{ .rax, .rcx });
-            defer self.register_manager.unfreezeRegs(&.{ .rax, .rcx });
             const reg = try self.copyToTmpRegister(Type.usize, self.ret_mcv);
-            self.register_manager.freezeRegs(&.{reg});
-            defer self.register_manager.unfreezeRegs(&.{reg});
+            const reg_lock = self.register_manager.lockRegAssumeUnused(reg);
+            defer self.register_manager.unlockReg(reg_lock);
+
             try self.genInlineMemcpy(.{ .stack_offset = 0 }, ptr, .{ .immediate = elem_ty.abiSize(self.target.*) }, .{
                 .source_stack_base = .rbp,
                 .dest_stack_base = reg,
@@ -3980,12 +4133,15 @@ fn airCmp(self: *Self, inst: Air.Inst.Index, op: math.CompareOperator) !void {
         // Source operand can be an immediate, 8 bits or 32 bits.
         // TODO look into reusing the operand
         const lhs = try self.resolveInst(bin_op.lhs);
-        lhs.freezeIfRegister(&self.register_manager);
-        defer lhs.unfreezeIfRegister(&self.register_manager);
+        const lhs_lock: ?RegisterLock = switch (lhs) {
+            .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+            else => null,
+        };
+        defer if (lhs_lock) |lock| self.register_manager.unlockReg(lock);
 
         const dst_reg = try self.copyToTmpRegister(ty, lhs);
-        self.register_manager.freezeRegs(&.{dst_reg});
-        defer self.register_manager.unfreezeRegs(&.{dst_reg});
+        const dst_reg_lock = self.register_manager.lockRegAssumeUnused(dst_reg);
+        defer self.register_manager.unlockReg(dst_reg_lock);
 
         const dst_mcv = MCValue{ .register = dst_reg };
 
@@ -4446,22 +4602,31 @@ fn airIsNull(self: *Self, inst: Air.Inst.Index) !void {
 
 fn airIsNullPtr(self: *Self, inst: Air.Inst.Index) !void {
     const un_op = self.air.instructions.items(.data)[inst].un_op;
-    const result: MCValue = if (self.liveness.isUnused(inst)) .dead else result: {
-        const operand_ptr = try self.resolveInst(un_op);
-        operand_ptr.freezeIfRegister(&self.register_manager);
-        defer operand_ptr.unfreezeIfRegister(&self.register_manager);
-        const operand: MCValue = blk: {
-            if (self.reuseOperand(inst, un_op, 0, operand_ptr)) {
-                // The MCValue that holds the pointer can be re-used as the value.
-                break :blk operand_ptr;
-            } else {
-                break :blk try self.allocRegOrMem(inst, true);
-            }
-        };
-        const ptr_ty = self.air.typeOf(un_op);
-        try self.load(operand, operand_ptr, ptr_ty);
-        break :result try self.isNull(inst, ptr_ty.elemType(), operand);
+
+    if (self.liveness.isUnused(inst)) {
+        return self.finishAir(inst, .dead, .{ un_op, .none, .none });
+    }
+
+    const operand_ptr = try self.resolveInst(un_op);
+    const operand_ptr_lock: ?RegisterLock = switch (operand_ptr) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
     };
+    defer if (operand_ptr_lock) |lock| self.register_manager.unlockReg(lock);
+
+    const operand: MCValue = blk: {
+        if (self.reuseOperand(inst, un_op, 0, operand_ptr)) {
+            // The MCValue that holds the pointer can be re-used as the value.
+            break :blk operand_ptr;
+        } else {
+            break :blk try self.allocRegOrMem(inst, true);
+        }
+    };
+    const ptr_ty = self.air.typeOf(un_op);
+    try self.load(operand, operand_ptr, ptr_ty);
+
+    const result = try self.isNull(inst, ptr_ty.elemType(), operand);
+
     return self.finishAir(inst, result, .{ un_op, .none, .none });
 }
 
@@ -4477,22 +4642,31 @@ fn airIsNonNull(self: *Self, inst: Air.Inst.Index) !void {
 
 fn airIsNonNullPtr(self: *Self, inst: Air.Inst.Index) !void {
     const un_op = self.air.instructions.items(.data)[inst].un_op;
-    const result: MCValue = if (self.liveness.isUnused(inst)) .dead else result: {
-        const operand_ptr = try self.resolveInst(un_op);
-        operand_ptr.freezeIfRegister(&self.register_manager);
-        defer operand_ptr.unfreezeIfRegister(&self.register_manager);
-        const operand: MCValue = blk: {
-            if (self.reuseOperand(inst, un_op, 0, operand_ptr)) {
-                // The MCValue that holds the pointer can be re-used as the value.
-                break :blk operand_ptr;
-            } else {
-                break :blk try self.allocRegOrMem(inst, true);
-            }
-        };
-        const ptr_ty = self.air.typeOf(un_op);
-        try self.load(operand, operand_ptr, ptr_ty);
-        break :result try self.isNonNull(inst, ptr_ty.elemType(), operand);
+
+    if (self.liveness.isUnused(inst)) {
+        return self.finishAir(inst, .dead, .{ un_op, .none, .none });
+    }
+
+    const operand_ptr = try self.resolveInst(un_op);
+    const operand_ptr_lock: ?RegisterLock = switch (operand_ptr) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
     };
+    defer if (operand_ptr_lock) |lock| self.register_manager.unlockReg(lock);
+
+    const operand: MCValue = blk: {
+        if (self.reuseOperand(inst, un_op, 0, operand_ptr)) {
+            // The MCValue that holds the pointer can be re-used as the value.
+            break :blk operand_ptr;
+        } else {
+            break :blk try self.allocRegOrMem(inst, true);
+        }
+    };
+    const ptr_ty = self.air.typeOf(un_op);
+    try self.load(operand, operand_ptr, ptr_ty);
+
+    const result = try self.isNonNull(inst, ptr_ty.elemType(), operand);
+
     return self.finishAir(inst, result, .{ un_op, .none, .none });
 }
 
@@ -4508,22 +4682,31 @@ fn airIsErr(self: *Self, inst: Air.Inst.Index) !void {
 
 fn airIsErrPtr(self: *Self, inst: Air.Inst.Index) !void {
     const un_op = self.air.instructions.items(.data)[inst].un_op;
-    const result: MCValue = if (self.liveness.isUnused(inst)) .dead else result: {
-        const operand_ptr = try self.resolveInst(un_op);
-        operand_ptr.freezeIfRegister(&self.register_manager);
-        defer operand_ptr.unfreezeIfRegister(&self.register_manager);
-        const operand: MCValue = blk: {
-            if (self.reuseOperand(inst, un_op, 0, operand_ptr)) {
-                // The MCValue that holds the pointer can be re-used as the value.
-                break :blk operand_ptr;
-            } else {
-                break :blk try self.allocRegOrMem(inst, true);
-            }
-        };
-        const ptr_ty = self.air.typeOf(un_op);
-        try self.load(operand, operand_ptr, ptr_ty);
-        break :result try self.isErr(inst, ptr_ty.elemType(), operand);
+
+    if (self.liveness.isUnused(inst)) {
+        return self.finishAir(inst, .dead, .{ un_op, .none, .none });
+    }
+
+    const operand_ptr = try self.resolveInst(un_op);
+    const operand_ptr_lock: ?RegisterLock = switch (operand_ptr) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
     };
+    defer if (operand_ptr_lock) |lock| self.register_manager.unlockReg(lock);
+
+    const operand: MCValue = blk: {
+        if (self.reuseOperand(inst, un_op, 0, operand_ptr)) {
+            // The MCValue that holds the pointer can be re-used as the value.
+            break :blk operand_ptr;
+        } else {
+            break :blk try self.allocRegOrMem(inst, true);
+        }
+    };
+    const ptr_ty = self.air.typeOf(un_op);
+    try self.load(operand, operand_ptr, ptr_ty);
+
+    const result = try self.isErr(inst, ptr_ty.elemType(), operand);
+
     return self.finishAir(inst, result, .{ un_op, .none, .none });
 }
 
@@ -4539,22 +4722,31 @@ fn airIsNonErr(self: *Self, inst: Air.Inst.Index) !void {
 
 fn airIsNonErrPtr(self: *Self, inst: Air.Inst.Index) !void {
     const un_op = self.air.instructions.items(.data)[inst].un_op;
-    const result: MCValue = if (self.liveness.isUnused(inst)) .dead else result: {
-        const operand_ptr = try self.resolveInst(un_op);
-        operand_ptr.freezeIfRegister(&self.register_manager);
-        defer operand_ptr.unfreezeIfRegister(&self.register_manager);
-        const operand: MCValue = blk: {
-            if (self.reuseOperand(inst, un_op, 0, operand_ptr)) {
-                // The MCValue that holds the pointer can be re-used as the value.
-                break :blk operand_ptr;
-            } else {
-                break :blk try self.allocRegOrMem(inst, true);
-            }
-        };
-        const ptr_ty = self.air.typeOf(un_op);
-        try self.load(operand, operand_ptr, ptr_ty);
-        break :result try self.isNonErr(inst, ptr_ty.elemType(), operand);
+
+    if (self.liveness.isUnused(inst)) {
+        return self.finishAir(inst, .dead, .{ un_op, .none, .none });
+    }
+
+    const operand_ptr = try self.resolveInst(un_op);
+    const operand_ptr_lock: ?RegisterLock = switch (operand_ptr) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
     };
+    defer if (operand_ptr_lock) |lock| self.register_manager.unlockReg(lock);
+
+    const operand: MCValue = blk: {
+        if (self.reuseOperand(inst, un_op, 0, operand_ptr)) {
+            // The MCValue that holds the pointer can be re-used as the value.
+            break :blk operand_ptr;
+        } else {
+            break :blk try self.allocRegOrMem(inst, true);
+        }
+    };
+    const ptr_ty = self.air.typeOf(un_op);
+    try self.load(operand, operand_ptr, ptr_ty);
+
+    const result = try self.isNonErr(inst, ptr_ty.elemType(), operand);
+
     return self.finishAir(inst, result, .{ un_op, .none, .none });
 }
 
@@ -4610,8 +4802,8 @@ fn genCondSwitchMir(self: *Self, ty: Type, condition: MCValue, case: MCValue) !u
         .register => |cond_reg| {
             try self.spillCompareFlagsIfOccupied();
 
-            self.register_manager.freezeRegs(&.{cond_reg});
-            defer self.register_manager.unfreezeRegs(&.{cond_reg});
+            const cond_reg_lock = self.register_manager.lockReg(cond_reg);
+            defer if (cond_reg_lock) |lock| self.register_manager.unlockReg(lock);
 
             switch (case) {
                 .none => unreachable,
@@ -4670,8 +4862,8 @@ fn genCondSwitchMir(self: *Self, ty: Type, condition: MCValue, case: MCValue) !u
 
             if (abi_size <= 8) {
                 const reg = try self.copyToTmpRegister(ty, condition);
-                self.register_manager.freezeRegs(&.{reg});
-                defer self.register_manager.unfreezeRegs(&.{reg});
+                const reg_lock = self.register_manager.lockRegAssumeUnused(reg);
+                defer self.register_manager.unlockReg(reg_lock);
                 return self.genCondSwitchMir(ty, .{ .register = reg }, case);
             }
 
@@ -5158,8 +5350,8 @@ fn genSetStack(self: *Self, ty: Type, stack_offset: i32, mcv: MCValue, opts: Inl
         .register_overflow_unsigned,
         .register_overflow_signed,
         => |reg| {
-            self.register_manager.freezeRegs(&.{reg});
-            defer self.register_manager.unfreezeRegs(&.{reg});
+            const reg_lock = self.register_manager.lockReg(reg);
+            defer if (reg_lock) |lock| self.register_manager.unlockReg(lock);
 
             const wrapped_ty = ty.structFieldType(0);
             try self.genSetStack(wrapped_ty, stack_offset, .{ .register = reg }, .{});
@@ -5260,8 +5452,8 @@ fn genSetStack(self: *Self, ty: Type, stack_offset: i32, mcv: MCValue, opts: Inl
 
             const base_reg = opts.dest_stack_base orelse .rbp;
             if (!math.isPowerOfTwo(abi_size)) {
-                self.register_manager.freezeRegs(&.{reg});
-                defer self.register_manager.unfreezeRegs(&.{reg});
+                const reg_lock = self.register_manager.lockReg(reg);
+                defer if (reg_lock) |lock| self.register_manager.unlockReg(lock);
 
                 const tmp_reg = try self.copyToTmpRegister(ty, mcv);
 
@@ -5350,13 +5542,25 @@ fn genInlineMemcpy(
     len: MCValue,
     opts: InlineMemcpyOpts,
 ) InnerError!void {
-    self.register_manager.freezeRegs(&.{ .rax, .rcx });
+    try self.register_manager.getReg(.rax, null);
+    try self.register_manager.getReg(.rcx, null);
 
-    if (opts.source_stack_base) |reg| self.register_manager.freezeRegs(&.{reg});
-    defer if (opts.source_stack_base) |reg| self.register_manager.unfreezeRegs(&.{reg});
+    const reg_locks = self.register_manager.lockRegsAssumeUnused(2, .{ .rax, .rcx });
+    defer for (reg_locks) |lock| {
+        self.register_manager.unlockReg(lock);
+    };
 
-    if (opts.dest_stack_base) |reg| self.register_manager.freezeRegs(&.{reg});
-    defer if (opts.dest_stack_base) |reg| self.register_manager.unfreezeRegs(&.{reg});
+    const ssbase_lock: ?RegisterLock = if (opts.source_stack_base) |reg|
+        self.register_manager.lockReg(reg)
+    else
+        null;
+    defer if (ssbase_lock) |reg| self.register_manager.unlockReg(reg);
+
+    const dsbase_lock: ?RegisterLock = if (opts.dest_stack_base) |reg|
+        self.register_manager.lockReg(reg)
+    else
+        null;
+    defer if (dsbase_lock) |lock| self.register_manager.unlockReg(lock);
 
     const dst_addr_reg = try self.register_manager.allocReg(null);
     switch (dst_ptr) {
@@ -5390,8 +5594,8 @@ fn genInlineMemcpy(
             return self.fail("TODO implement memcpy for setting stack when dest is {}", .{dst_ptr});
         },
     }
-    self.register_manager.freezeRegs(&.{dst_addr_reg});
-    defer self.register_manager.unfreezeRegs(&.{dst_addr_reg});
+    const dst_addr_reg_lock = self.register_manager.lockRegAssumeUnused(dst_addr_reg);
+    defer self.register_manager.unlockReg(dst_addr_reg_lock);
 
     const src_addr_reg = try self.register_manager.allocReg(null);
     switch (src_ptr) {
@@ -5425,18 +5629,13 @@ fn genInlineMemcpy(
             return self.fail("TODO implement memcpy for setting stack when src is {}", .{src_ptr});
         },
     }
-    self.register_manager.freezeRegs(&.{src_addr_reg});
-    defer self.register_manager.unfreezeRegs(&.{src_addr_reg});
+    const src_addr_reg_lock = self.register_manager.lockRegAssumeUnused(src_addr_reg);
+    defer self.register_manager.unlockReg(src_addr_reg_lock);
 
     const regs = try self.register_manager.allocRegs(2, .{ null, null });
     const count_reg = regs[0].to64();
     const tmp_reg = regs[1].to8();
 
-    self.register_manager.unfreezeRegs(&.{ .rax, .rcx });
-
-    try self.register_manager.getReg(.rax, null);
-    try self.register_manager.getReg(.rcx, null);
-
     try self.genSetReg(Type.usize, count_reg, len);
 
     // mov rcx, 0
@@ -5540,7 +5739,9 @@ fn genInlineMemset(
     len: MCValue,
     opts: InlineMemcpyOpts,
 ) InnerError!void {
-    self.register_manager.freezeRegs(&.{.rax});
+    try self.register_manager.getReg(.rax, null);
+    const rax_lock = self.register_manager.lockRegAssumeUnused(.rax);
+    defer self.register_manager.unlockReg(rax_lock);
 
     const addr_reg = try self.register_manager.allocReg(null);
     switch (dst_ptr) {
@@ -5574,11 +5775,8 @@ fn genInlineMemset(
             return self.fail("TODO implement memcpy for setting stack when dest is {}", .{dst_ptr});
         },
     }
-    self.register_manager.freezeRegs(&.{addr_reg});
-    defer self.register_manager.unfreezeRegs(&.{addr_reg});
-
-    self.register_manager.unfreezeRegs(&.{.rax});
-    try self.register_manager.getReg(.rax, null);
+    const addr_reg_lock = self.register_manager.lockRegAssumeUnused(addr_reg);
+    defer self.register_manager.unlockReg(addr_reg_lock);
 
     try self.genSetReg(Type.usize, .rax, len);
     try self.genBinMathOpMir(.sub, Type.usize, .{ .register = .rax }, .{ .immediate = 1 });
@@ -6017,16 +6215,25 @@ fn airMemset(self: *Self, inst: Air.Inst.Index) !void {
     const extra = self.air.extraData(Air.Bin, pl_op.payload).data;
 
     const dst_ptr = try self.resolveInst(pl_op.operand);
-    dst_ptr.freezeIfRegister(&self.register_manager);
-    defer dst_ptr.unfreezeIfRegister(&self.register_manager);
+    const dst_ptr_lock: ?RegisterLock = switch (dst_ptr) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (dst_ptr_lock) |lock| self.register_manager.unlockReg(lock);
 
     const src_val = try self.resolveInst(extra.lhs);
-    src_val.freezeIfRegister(&self.register_manager);
-    defer src_val.unfreezeIfRegister(&self.register_manager);
+    const src_val_lock: ?RegisterLock = switch (src_val) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (src_val_lock) |lock| self.register_manager.unlockReg(lock);
 
     const len = try self.resolveInst(extra.rhs);
-    len.freezeIfRegister(&self.register_manager);
-    defer len.unfreezeIfRegister(&self.register_manager);
+    const len_lock: ?RegisterLock = switch (len) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (len_lock) |lock| self.register_manager.unlockReg(lock);
 
     try self.genInlineMemset(dst_ptr, src_val, len, .{});
 
@@ -6038,17 +6245,26 @@ fn airMemcpy(self: *Self, inst: Air.Inst.Index) !void {
     const extra = self.air.extraData(Air.Bin, pl_op.payload).data;
 
     const dst_ptr = try self.resolveInst(pl_op.operand);
-    dst_ptr.freezeIfRegister(&self.register_manager);
-    defer dst_ptr.unfreezeIfRegister(&self.register_manager);
+    const dst_ptr_lock: ?RegisterLock = switch (dst_ptr) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (dst_ptr_lock) |lock| self.register_manager.unlockReg(lock);
 
     const src_ty = self.air.typeOf(extra.lhs);
     const src_ptr = try self.resolveInst(extra.lhs);
-    src_ptr.freezeIfRegister(&self.register_manager);
-    defer src_ptr.unfreezeIfRegister(&self.register_manager);
+    const src_ptr_lock: ?RegisterLock = switch (src_ptr) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (src_ptr_lock) |lock| self.register_manager.unlockReg(lock);
 
     const len = try self.resolveInst(extra.rhs);
-    len.freezeIfRegister(&self.register_manager);
-    defer len.unfreezeIfRegister(&self.register_manager);
+    const len_lock: ?RegisterLock = switch (len) {
+        .register => |reg| self.register_manager.lockRegAssumeUnused(reg),
+        else => null,
+    };
+    defer if (len_lock) |lock| self.register_manager.unlockReg(lock);
 
     // TODO Is this the only condition for pointer dereference for memcpy?
     const src: MCValue = blk: {
@@ -6070,8 +6286,11 @@ fn airMemcpy(self: *Self, inst: Air.Inst.Index) !void {
             else => break :blk src_ptr,
         }
     };
-    src.freezeIfRegister(&self.register_manager);
-    defer src.unfreezeIfRegister(&self.register_manager);
+    const src_lock: ?RegisterLock = switch (src) {
+        .register => |reg| self.register_manager.lockReg(reg),
+        else => null,
+    };
+    defer if (src_lock) |lock| self.register_manager.unlockReg(lock);
 
     try self.genInlineMemcpy(dst_ptr, src, len, .{});
 
diff --git a/src/register_manager.zig b/src/register_manager.zig
index 08ac50377b..2c0502e867 100644
--- a/src/register_manager.zig
+++ b/src/register_manager.zig
@@ -45,9 +45,8 @@ pub fn RegisterManager(
         /// Tracks all registers allocated in the course of this
         /// function
         allocated_registers: FreeRegInt = 0,
-        /// Tracks registers which are temporarily blocked from being
-        /// allocated
-        frozen_registers: FreeRegInt = 0,
+        /// Tracks registers which are locked from being allocated
+        locked_registers: FreeRegInt = 0,
 
         const Self = @This();
 
@@ -108,34 +107,70 @@ pub fn RegisterManager(
             return self.allocated_registers & mask != 0;
         }
 
-        /// Returns whether this register is frozen
+        /// Returns whether this register is locked
         ///
         /// Returns false when this register is not tracked
-        pub fn isRegFrozen(self: Self, reg: Register) bool {
+        pub fn isRegLocked(self: Self, reg: Register) bool {
             const mask = getRegisterMask(reg) orelse return false;
-            return self.frozen_registers & mask != 0;
+            return self.locked_registers & mask != 0;
         }
 
-        /// Prevents the registers from being allocated until they are
-        /// unfrozen again
-        pub fn freezeRegs(self: *Self, regs: []const Register) void {
-            for (regs) |reg| {
-                const mask = getRegisterMask(reg) orelse continue;
-                self.frozen_registers |= mask;
+        pub const RegisterLock = struct {
+            register: Register,
+        };
+
+        /// Prevents the register from being allocated until they are
+        /// unlocked again.
+        /// Returns `RegisterLock` if the register was not already
+        /// locked, or `null` otherwise.
+        /// Only the owner of the `RegisterLock` can unlock the
+        /// register later.
+        pub fn lockReg(self: *Self, reg: Register) ?RegisterLock {
+            log.debug("locking {}", .{reg});
+            if (self.isRegLocked(reg)) {
+                log.debug("  register already locked", .{});
+                return null;
             }
+            const mask = getRegisterMask(reg) orelse return null;
+            self.locked_registers |= mask;
+            return RegisterLock{ .register = reg };
         }
 
-        /// Enables the allocation of the registers
-        pub fn unfreezeRegs(self: *Self, regs: []const Register) void {
-            for (regs) |reg| {
-                const mask = getRegisterMask(reg) orelse continue;
-                self.frozen_registers &= ~mask;
+        /// Like `lockReg` but asserts the register was unused always
+        /// returning a valid lock.
+        pub fn lockRegAssumeUnused(self: *Self, reg: Register) RegisterLock {
+            log.debug("locking asserting free {}", .{reg});
+            assert(!self.isRegLocked(reg));
+            const mask = getRegisterMask(reg) orelse unreachable;
+            self.locked_registers |= mask;
+            return RegisterLock{ .register = reg };
+        }
+
+        /// Like `lockRegAssumeUnused` but locks multiple registers.
+        pub fn lockRegsAssumeUnused(
+            self: *Self,
+            comptime count: comptime_int,
+            regs: [count]Register,
+        ) [count]RegisterLock {
+            var buf: [count]RegisterLock = undefined;
+            for (regs) |reg, i| {
+                buf[i] = self.lockRegAssumeUnused(reg);
             }
+            return buf;
         }
 
-        /// Returns true when at least one register is frozen
-        pub fn frozenRegsExist(self: Self) bool {
-            return self.frozen_registers != 0;
+        /// Unlocks the register allowing its re-allocation and re-use.
+        /// Requires `RegisterLock` to unlock a register.
+        /// Call `lockReg` to obtain the lock first.
+        pub fn unlockReg(self: *Self, lock: RegisterLock) void {
+            log.debug("unlocking {}", .{lock.register});
+            const mask = getRegisterMask(lock.register) orelse return;
+            self.locked_registers &= ~mask;
+        }
+
+        /// Returns true when at least one register is locked
+        pub fn lockedRegsExist(self: Self) bool {
+            return self.locked_registers != 0;
         }
 
         /// Allocates a specified number of registers, optionally
@@ -148,15 +183,15 @@ pub fn RegisterManager(
         ) ?[count]Register {
             comptime assert(count > 0 and count <= tracked_registers.len);
 
-            const free_and_not_frozen_registers = self.free_registers & ~self.frozen_registers;
-            const free_and_not_frozen_registers_count = @popCount(FreeRegInt, free_and_not_frozen_registers);
-            if (free_and_not_frozen_registers_count < count) return null;
+            const free_and_not_locked_registers = self.free_registers & ~self.locked_registers;
+            const free_and_not_locked_registers_count = @popCount(FreeRegInt, free_and_not_locked_registers);
+            if (free_and_not_locked_registers_count < count) return null;
 
             var regs: [count]Register = undefined;
             var i: usize = 0;
             for (tracked_registers) |reg| {
                 if (i >= count) break;
-                if (self.isRegFrozen(reg)) continue;
+                if (self.isRegLocked(reg)) continue;
                 if (!self.isRegFree(reg)) continue;
 
                 regs[i] = reg;
@@ -194,8 +229,8 @@ pub fn RegisterManager(
             insts: [count]?Air.Inst.Index,
         ) AllocateRegistersError![count]Register {
             comptime assert(count > 0 and count <= tracked_registers.len);
-            const frozen_registers_count = @popCount(FreeRegInt, self.frozen_registers);
-            if (count > tracked_registers.len - frozen_registers_count) return error.OutOfRegisters;
+            const locked_registers_count = @popCount(FreeRegInt, self.locked_registers);
+            if (count > tracked_registers.len - locked_registers_count) return error.OutOfRegisters;
 
             const result = self.tryAllocRegs(count, insts) orelse blk: {
                 // We'll take over the first count registers. Spill
@@ -205,7 +240,7 @@ pub fn RegisterManager(
                 var i: usize = 0;
                 for (tracked_registers) |reg| {
                     if (i >= count) break;
-                    if (self.isRegFrozen(reg)) continue;
+                    if (self.isRegLocked(reg)) continue;
 
                     regs[i] = reg;
                     self.markRegAllocated(reg);
@@ -416,15 +451,15 @@ test "allocReg: spilling" {
     try expectEqual(@as(?MockRegister1, .r3), try function.register_manager.allocReg(mock_instruction));
     try expectEqualSlices(MockRegister1, &[_]MockRegister1{.r2}, function.spilled.items);
 
-    // Frozen registers
+    // Locked registers
     function.register_manager.freeReg(.r3);
     {
-        function.register_manager.freezeRegs(&.{.r2});
-        defer function.register_manager.unfreezeRegs(&.{.r2});
+        const lock = function.register_manager.lockReg(.r2);
+        defer if (lock) |reg| function.register_manager.unlockReg(reg);
 
         try expectEqual(@as(?MockRegister1, .r3), try function.register_manager.allocReg(mock_instruction));
     }
-    try expect(!function.register_manager.frozenRegsExist());
+    try expect(!function.register_manager.lockedRegsExist());
 }
 
 test "tryAllocRegs" {
@@ -442,17 +477,17 @@ test "tryAllocRegs" {
     try expect(function.register_manager.isRegAllocated(.r2));
     try expect(!function.register_manager.isRegAllocated(.r3));
 
-    // Frozen registers
+    // Locked registers
     function.register_manager.freeReg(.r0);
     function.register_manager.freeReg(.r2);
     function.register_manager.freeReg(.r3);
     {
-        function.register_manager.freezeRegs(&.{.r1});
-        defer function.register_manager.unfreezeRegs(&.{.r1});
+        const lock = function.register_manager.lockReg(.r1);
+        defer if (lock) |reg| function.register_manager.unlockReg(reg);
 
         try expectEqual([_]MockRegister2{ .r0, .r2, .r3 }, function.register_manager.tryAllocRegs(3, .{ null, null, null }).?);
     }
-    try expect(!function.register_manager.frozenRegsExist());
+    try expect(!function.register_manager.lockedRegsExist());
 
     try expect(function.register_manager.isRegAllocated(.r0));
     try expect(function.register_manager.isRegAllocated(.r1));
@@ -475,19 +510,19 @@ test "allocRegs: normal usage" {
 
         // The result register is known and fixed at this point, we
         // don't want to accidentally allocate lhs or rhs to the
-        // result register, this is why we freeze it.
+        // result register, this is why we lock it.
         //
-        // Using defer unfreeze right after freeze is a good idea in
-        // most cases as you probably are using the frozen registers
+        // Using defer unlock right after lock is a good idea in
+        // most cases as you probably are using the locked registers
         // in the remainder of this scope and don't need to use it
         // after the end of this scope. However, in some situations,
-        // it may make sense to manually unfreeze registers before the
+        // it may make sense to manually unlock registers before the
         // end of the scope when you are certain that they don't
         // contain any valuable data anymore and can be reused. For an
         // example of that, see `selectively reducing register
         // pressure`.
-        function.register_manager.freezeRegs(&.{result_reg});
-        defer function.register_manager.unfreezeRegs(&.{result_reg});
+        const lock = function.register_manager.lockReg(result_reg);
+        defer if (lock) |reg| function.register_manager.unlockReg(reg);
 
         const regs = try function.register_manager.allocRegs(2, .{ null, null });
         try function.genAdd(result_reg, regs[0], regs[1]);
@@ -507,16 +542,14 @@ test "allocRegs: selectively reducing register pressure" {
     {
         const result_reg: MockRegister2 = .r1;
 
-        function.register_manager.freezeRegs(&.{result_reg});
-        defer function.register_manager.unfreezeRegs(&.{result_reg});
+        const lock = function.register_manager.lockReg(result_reg);
 
-        // Here, we don't defer unfreeze because we manually unfreeze
+        // Here, we don't defer unlock because we manually unlock
         // after genAdd
         const regs = try function.register_manager.allocRegs(2, .{ null, null });
-        function.register_manager.freezeRegs(&.{result_reg});
 
         try function.genAdd(result_reg, regs[0], regs[1]);
-        function.register_manager.unfreezeRegs(&regs);
+        function.register_manager.unlockReg(lock.?);
 
         const extra_summand_reg = try function.register_manager.allocReg(null);
         try function.genAdd(result_reg, result_reg, extra_summand_reg);
diff --git a/test/behavior/align.zig b/test/behavior/align.zig
index 18a55f6653..6c1122323a 100644
--- a/test/behavior/align.zig
+++ b/test/behavior/align.zig
@@ -7,6 +7,8 @@ var foo: u8 align(4) = 100;
 
 test "global variable alignment" {
     if (builtin.zig_backend == .stage2_c) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
 
     comptime try expect(@typeInfo(@TypeOf(&foo)).Pointer.alignment == 4);
     comptime try expect(@TypeOf(&foo) == *align(4) u8);
diff --git a/test/behavior/byval_arg_var.zig b/test/behavior/byval_arg_var.zig
index b6b972d2d3..d2e8ecb638 100644
--- a/test/behavior/byval_arg_var.zig
+++ b/test/behavior/byval_arg_var.zig
@@ -6,6 +6,7 @@ var result: []const u8 = "wrong";
 test "pass string literal byvalue to a generic var param" {
     if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest;
 
     start();
     blowUpStack(10);