https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92437
Bug ID: 92437
Summary: Unnecessary register duplication of vector constant in
x86
Product: gcc
Version: 9.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: zingaburga+gcc at hotmail dot com
Target Milestone: ---
Consider the following code example:
#include <x86intrin.h>
void fn(__m128i* in, __m128i* out) {
int i=0;
const __m128i num = _mm_set1_epi8(99);
while(i<100) {
__m128i a = in[i];
__m128i b = _mm_add_epi8(a, num);
if(_mm_movemask_epi8(b))
a = _mm_or_si128(a, num);
if(_mm_movemask_epi8(a))
a = _mm_or_si128(a, num);
out[i] = a;
i++;
}
}
The vector `num` is referenced 3 times in the loop, and GCC seems to load it
into 3 separate registers, when 1 would suffice: https://godbolt.org/z/mP22ez
(in this link, the `99` vector is held in xmm2, xmm3 and xmm4).
This seems to be the case regardless of AVX being enabled or not.
I don't really get what a possible cause for this is, but it seems that the
`if` conditions are necessary to trigger this effect.
