stage2 codegen: proper abstraction for re-using dying operands

closes #6064

src-self-hosted/codegen.zig

@@ -858,6 +858,16 @@ fn Function(comptime arch: std.Target.Cpu.Arch) type {
             }
         }
 
+        fn reuseOperand(inst: *ir.Inst, op_index: ir.Inst.DeathsBitIndex, mcv: MCValue) bool {
+            if (!inst.operandDies(op_index) or !mcv.isMutable())
+                return false;
+
+            // OK we're going to do it, but we need to clear the operand death bit so that
+            // it stays allocated.
+            inst.clearOperandDeath(op_index);
+            return true;
+        }
+
         fn genLoad(self: *Self, inst: *ir.Inst.UnOp) !MCValue {
             const elem_ty = inst.base.ty;
             if (!elem_ty.hasCodeGenBits())
@@ -867,9 +877,8 @@ fn Function(comptime arch: std.Target.Cpu.Arch) type {
             if (inst.base.isUnused() and !is_volatile)
                 return MCValue.dead;
             const dst_mcv: MCValue = blk: {
-                if (inst.base.operandDies(0) and ptr.isMutable()) {
+                if (reuseOperand(&inst.base, 0, ptr)) {
                     // The MCValue that holds the pointer can be re-used as the value.
-                    // TODO track this in the register/stack allocation metadata.
                     break :blk ptr;
                 } else {
                     break :blk try self.allocRegOrMem(&inst.base);
@@ -966,7 +975,7 @@ fn Function(comptime arch: std.Target.Cpu.Arch) type {
             var dst_mcv: MCValue = undefined;
             var src_mcv: MCValue = undefined;
             var src_inst: *ir.Inst = undefined;
-            if (inst.operandDies(0) and lhs.isMutable()) {
+            if (reuseOperand(inst, 0, lhs)) {
                 // LHS dies; use it as the destination.
                 // Both operands cannot be memory.
                 src_inst = op_rhs;
@@ -977,7 +986,7 @@ fn Function(comptime arch: std.Target.Cpu.Arch) type {
                     dst_mcv = lhs;
                     src_mcv = rhs;
                 }
-            } else if (inst.operandDies(1) and rhs.isMutable()) {
+            } else if (reuseOperand(inst, 1, rhs)) {
                 // RHS dies; use it as the destination.
                 // Both operands cannot be memory.
                 src_inst = op_lhs;

src-self-hosted/ir.zig

@@ -42,6 +42,11 @@ pub const Inst = struct {
         return @truncate(u1, self.deaths >> index) != 0;
     }
 
+    pub fn clearOperandDeath(self: *Inst, index: DeathsBitIndex) void {
+        assert(index < deaths_bits);
+        self.deaths &= ~(@as(DeathsInt, 1) << index);
+    }
+
     pub fn specialOperandDeaths(self: Inst) bool {
         return (self.deaths & (1 << deaths_bits)) != 0;
     }

test/stage2/compare_output.zig

@@ -544,6 +544,48 @@ pub fn addCases(ctx: *TestContext) !void {
             "",
         );
 
+        // This catches a possible bug in the logic for re-using dying operands.
+        case.addCompareOutput(
+            \\export fn _start() noreturn {
+            \\    assert(add(3, 4) == 116);
+            \\
+            \\    exit();
+            \\}
+            \\
+            \\fn add(a: u32, b: u32) u32 {
+            \\    const x: u32 = blk: {
+            \\        const c = a + b; // 7
+            \\        const d = a + c; // 10
+            \\        const e = d + b; // 14
+            \\        const f = d + e; // 24
+            \\        const g = e + f; // 38
+            \\        const h = f + g; // 62
+            \\        const i = g + h; // 100
+            \\        const j = i + d; // 110
+            \\        break :blk j;
+            \\    };
+            \\    const y = x + a; // 113
+            \\    const z = y + a; // 116
+            \\    return z;
+            \\}
+            \\
+            \\pub fn assert(ok: bool) void {
+            \\    if (!ok) unreachable; // assertion failure
+            \\}
+            \\
+            \\fn exit() noreturn {
+            \\    asm volatile ("syscall"
+            \\        :
+            \\        : [number] "{rax}" (231),
+            \\          [arg1] "{rdi}" (0)
+            \\        : "rcx", "r11", "memory"
+            \\    );
+            \\    unreachable;
+            \\}
+        ,
+            "",
+        );
+
         case.addCompareOutput(
             \\export fn _start() noreturn {
             \\    const ignore =