https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96289
Bug ID: 96289
Summary: Unnecessary saving and re-testing of the carry flag
with __builtin_usub_overflow
Product: gcc
Version: 10.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: josephcsible at gmail dot com
Target Milestone: ---
Target: x86_64-linux-gnu
Consider this C code:
unsigned f(unsigned x, unsigned y) {
if (__builtin_usub_overflow(x, y, &x)) {
x += 100;
}
return x;
}
When compiled with -O3, it produces the following assembly:
f:
xorl %edx, %edx
subl %esi, %edi
setb %dl
leal 100(%rdi), %eax
testl %edx, %edx
cmove %edi, %eax
ret
https://godbolt.org/z/WMo377
But "lea" doesn't affect the carry flag (or any flags for that matter), so
there's no need to save it to a register and then re-test it. It should have
produced this assembly instead:
f:
subl %esi, %edi
leal 100(%rdi), %eax
cmovae %edi, %eax
ret
