https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110438
Bug ID: 110438
Summary: generating all-ones zmm needs dep-breaking pxor before
ternlog
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: amonakov at gcc dot gnu.org
Target Milestone: ---
Target: x86_64-*-*
VPTERNLOG is never a dependency-breaking instruction on existing x86
implementations, so generating a vector of all-ones via bare ternlog can stall
waiting on destination register. GCC should emit a dependency-breaking PXOR,
otherwise it will be a false-dependency-on-popcnt-lzcnt debacle all over again.
#include <immintrin.h>
__m512i g(void)
{
return (__m512i){ 0 } - 1;
}
g:
# waits until previous computation
# of zmm0 has completed
vpternlogd zmm0, zmm0, zmm0, 0xFF
ret
