x64: track callee and caller saved registers

This is now required to correctly track and spill registers
required for some ops such `mul` or `div` (both required use of
`.rax` and `.rdx` registers).

src/arch/x86_64/CodeGen.zig

@@ -31,6 +31,8 @@ const bits = @import("bits.zig");
 const abi = @import("abi.zig");
 const Register = bits.Register;
 const callee_preserved_regs = abi.callee_preserved_regs;
+const caller_preserved_regs = abi.caller_preserved_regs;
+const allocatable_registers = abi.allocatable_registers;
 const c_abi_int_param_regs = abi.c_abi_int_param_regs;
 const c_abi_int_return_regs = abi.c_abi_int_return_regs;
 
@@ -40,7 +42,7 @@ const InnerError = error{
     OutOfRegisters,
 };
 
-const RegisterManager = RegisterManagerFn(Self, Register, &callee_preserved_regs);
+const RegisterManager = RegisterManagerFn(Self, Register, &allocatable_registers);
 
 gpa: Allocator,
 air: Air,
@@ -3519,6 +3521,10 @@ fn airCall(self: *Self, inst: Air.Inst.Index, modifier: std.builtin.CallOptions.
 
     try self.spillCompareFlagsIfOccupied();
 
+    for (caller_preserved_regs) |reg| {
+        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.*));
@@ -3715,7 +3721,7 @@ fn airCall(self: *Self, inst: Air.Inst.Index, modifier: std.builtin.CallOptions.
     const result: MCValue = result: {
         switch (info.return_value) {
             .register => |reg| {
-                if (RegisterManager.indexOfReg(&callee_preserved_regs, reg) == null) {
+                if (RegisterManager.indexOfRegIntoTracked(reg) == null) {
                     // Save function return value in a callee saved register
                     break :result try self.copyToRegisterWithInstTracking(
                         inst,

src/arch/x86_64/abi.zig

@@ -370,8 +370,13 @@ pub fn classifySystemV(ty: Type, target: Target) [8]Class {
     }
 }
 
-/// These registers need to be preserved (saved on the stack) and restored by the callee before getting clobbered
-/// and when the callee returns.
-pub const callee_preserved_regs = [_]Register{ .rcx, .rsi, .rdi, .r8, .r9, .r10, .r11 };
+/// Note that .rsp and .rbp also belong to this set, however, we never expect to use them
+/// for anything else but stack offset tracking therefore we exclude them from this set.
+pub const callee_preserved_regs = [_]Register{ .rbx, .r12, .r13, .r14, .r15 };
+/// These registers need to be preserved (saved on the stack) and restored by the caller before
+/// the caller relinquishes control to a subroutine via call instruction (or similar).
+/// In other words, these registers are free to use by the callee.
+pub const caller_preserved_regs = [_]Register{ .rax, .rcx, .rdx, .rsi, .rdi, .r8, .r9, .r10, .r11 };
+pub const allocatable_registers = callee_preserved_regs ++ caller_preserved_regs;
 pub const c_abi_int_param_regs = [_]Register{ .rdi, .rsi, .rdx, .rcx, .r8, .r9 };
 pub const c_abi_int_return_regs = [_]Register{ .rax, .rdx };