https://bugs.llvm.org/show_bug.cgi?id=45759
Bug ID: 45759
Summary: Failure to elide useless mov in sign function
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: gabrav...@gmail.com
CC: craig.top...@gmail.com, llvm-bugs@lists.llvm.org,
llvm-...@redking.me.uk, spatel+l...@rotateright.com
int sign(int x)
{
return -((unsigned)x >> 31) | ((unsigned)-x >> 31);
}
With -O3, GCC outputs this :
sign(int):
mov eax, edi
sar edi, 31
neg eax
shr eax, 31
or eax, edi
ret
LLVM outputs this :
sign(int):
mov eax, edi
mov ecx, edi
sar ecx, 31
neg eax
shr eax, 31
or eax, ecx
ret
Most likely this is specific to the X86 backend and its register allocator, so
I marked it as such
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