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backend_x64/EmitX64: Improve emitted code for non-carry ArithmeticShiftRight

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This commit is contained in:
MerryMage 2016-07-14 09:02:27 +01:00
parent 08e848044d
commit 4ab4ca58f9
1 changed files with 7 additions and 13 deletions
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20
src/backend_x64/emit_x64.cpp
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@ -408,23 +408,17 @@ void EmitX64::EmitArithmeticShiftRight(IR::Value* value_) {
if (!carry_inst) { if (!carry_inst) {
X64Reg shift = reg_alloc.UseRegister(value->GetArg(1).get(), {HostLoc::RCX}); X64Reg shift = reg_alloc.UseRegister(value->GetArg(1).get(), {HostLoc::RCX});
X64Reg result = reg_alloc.UseDefRegister(value->GetArg(0).get(), value); X64Reg result = reg_alloc.UseDefRegister(value->GetArg(0).get(), value);
//X64Reg zero = reg_alloc.ScratchRegister(); X64Reg const31 = reg_alloc.ScratchRegister();
// The 32-bit x64 SAR instruction masks the shift count by 0x1F before performing the shift. // The 32-bit x64 SAR instruction masks the shift count by 0x1F before performing the shift.
// ARM differs from the behaviour: It does not mask the count, so shifts above 31 result in zeros. // ARM differs from the behaviour: It does not mask the count.
// TODO: Optimize this. // We note that all shift values above 31 have the same behaviour as 31 does, so we saturate `shift` to 31.
code->MOV(32, R(const31), Imm32(31));
code->CMP(8, R(shift), Imm8(31)); code->MOVZX(32, 8, shift, R(shift));
auto Rs_gt31 = code->J_CC(CC_A); code->CMP(32, R(shift), Imm32(31));
// if (Rs & 0xFF <= 31) { code->CMOVcc(32, shift, R(const31), CC_L);
code->SAR(32, R(result), R(shift)); code->SAR(32, R(result), R(shift));
auto jmp_to_end = code->J();
// } else {
code->SetJumpTarget(Rs_gt31);
code->SAR(32, R(result), Imm8(31)); // Verified.
// }
code->SetJumpTarget(jmp_to_end);
} else { } else {
inhibit_emission.insert(carry_inst); inhibit_emission.insert(carry_inst);