https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92592
Bug ID: 92592
Summary: Redundant comparison after subtraction on x86
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: antoshkka at gmail dot com
Target Milestone: ---
Consider the example:
int sample(int a, int b) {
unsigned diff = (unsigned)b - (unsigned)a;
unsigned sign_bit = b < a;
return diff + sign_bit;
}
With -O2 and -O3 GCC produces the assembly:
sample(int, int):
mov eax, esi ; <=== not required
xor edx, edx
sub eax, edi
cmp esi, edi ; <=== not required
setl dl
add eax, edx
ret
However, `sub` changes the status flags and there's no need to call `cmp`:
sample(int, int):
xor eax, eax
sub esi, edi
setl al
add eax, esi
ret
The above sample is a minimized version of std::midpoint.
Godbolt playground: https://godbolt.org/z/j6FGq4
