https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108465
Bug ID: 108465
Summary: Optimize (a < b) == (b < a) to a == b
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: antoshkka at gmail dot com
Target Milestone: ---
For GCC 12 the following code with -O2:
int compare_eq(int a, int b) {
return ((a < b) == (b < a));
}
compiles into the following assembly:
compare_eq(int, int):
cmp edi, esi
setl dl
setg al
cmp dl, al
sete al
movzx eax, al
ret
Which is suboptimal. More optimal assembly would be:
compare_eq(int, int):
xor eax, eax
cmp edi, esi
sete al
ret
Godbolt Playground: https://godbolt.org/z/4sfcTjjjb
Motivation: in generic C++ code the comparison is often done via a functor. The
algorithm is only allowed to use that functor:
if (__comp(a, b) == __comp(b, a)) {
return x;
} else if (__comp(b, a)) {
return y;
}
Because of that, with the inlined functor the comparison becomes ((a < b) == (b
< a))
