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x64: optimise element offset calculation if dealing with immediates

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If `index` MCValue is actually an immediate, we can calculate offset
directly at "comptime" rather than at runtime.
This commit is contained in:
Jakub Konka 2022-06-07 18:10:28 +02:00
parent fc015231ad
commit 0c72760454
1 changed files with 16 additions and 2 deletions
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18
src/arch/x86_64/CodeGen.zig
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@ -2100,8 +2100,22 @@ fn airPtrSlicePtrPtr(self: *Self, inst: Air.Inst.Index) !void {
}
fn elemOffset(self: *Self, index_ty: Type, index: MCValue, elem_size: u64) !Register {
const reg = try self.copyToTmpRegister(index_ty, index);
try self.genIntMulComplexOpMir(index_ty, .{ .register = reg }, .{ .immediate = elem_size });
const reg: Register = blk: {
switch (index) {
.immediate => |imm| {
// Optimisation: if index MCValue is an immediate, we can multiply in `comptime`
// and set the register directly to the scaled offset as an immediate.
const reg = try self.register_manager.allocReg(null, gp);
try self.genSetReg(index_ty, reg, .{ .immediate = imm * elem_size });
break :blk reg;
},
else => {
const reg = try self.copyToTmpRegister(index_ty, index);
try self.genIntMulComplexOpMir(index_ty, .{ .register = reg }, .{ .immediate = elem_size });
break :blk reg;
},
}
};
return reg;
}