| Issue |
123139
|
| Summary |
Missed optimization(?): LLVM ignores carry flag after a 'shr'
|
| Labels |
|
| Assignees |
|
| Reporter |
shelerr
|
https://godbolt.org/z/zdhPve74e
```
unsigned int bar1();
unsigned int bar2();
int foo(unsigned int num) {
unsigned int temp = num >> 12;
if ((num >> 11) % 2 == 0) {
return bar1() + temp;
} else {
return bar2() * temp;
}
}
```
```
foo(unsigned int):
push rbx
mov ebx, edi
shr ebx, 12
test edi, 2048
jne .LBB0_2
call bar1()@PLT
add eax, ebx
pop rbx
ret
.LBB0_2:
call bar2()@PLT
imul eax, ebx
pop rbx
ret
```
On x86 64, 'shr' sets carry flag to the value of the last bit shifted out of the destination operand. So, I would expect the conditional jump to just be 'jc', insted of 'test'+'jne'. Am I missing something?
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