https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72805
Bug ID: 72805
Summary: AVX512: invalid code generation involving masks
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: wen...@mitsuba-renderer.org
Target Milestone: ---
Consider the following minimal program, which initializes an 16 int AVX512
vector with -1 entries, does a componen-twise "< 0" comparison, and prints the
resulting mask.
Since there are 16 entries, the expected output is "65535". GCC trunk prints
"255" (compilation flags: g++-7 -S -mavx512f test.c -o test.s
-fomit-frame-pointer -fno-asynchronous-unwind-tables -fno-exceptions). The
issue goes away when compiling at higher optimization levels, though that is
clearly not a good solution.
#include <immintrin.h>
#include <stdio.h>
__attribute__((noinline))
int test() {
__m512i value = _mm512_set1_epi32(-1);
return (int) _mm512_cmp_epi32_mask(value, _mm512_setzero_si512(), 1 /*
_MM_CMPINT_LT */);
}
int main(int argc, char *argv[]) {
printf("%i\n", test());
return 0;
}
Looking at the assembly reveals the problem:
__Z4testv:
leaq 8(%rsp), %r10
andq $-64, %rsp
pushq -8(%r10)
pushq %rbp
movq %rsp, %rbp
pushq %r10
subq $112, %rsp
movl $-1, -52(%rbp)
vmovdqa64 -176(%rbp), %zmm0
movl $-1, %eax
kmovw %eax, %k2
vpbroadcastd -52(%rbp), %zmm0{%k2}
vmovdqa64 %zmm0, -240(%rbp)
vpxord %zmm0, %zmm0, %zmm0
vmovdqa64 %zmm0, %zmm1
vmovdqa64 -240(%rbp), %zmm0
movl $-1, %eax
kmovw %eax, %k3
vpcmpd $1, %zmm1, %zmm0, %k1{%k3}
kmovw %k1, %eax
movzbl %al, %eax <----- UH OH
addq $112, %rsp
popq %r10
popq %rbp
leaq -8(%r10), %rsp
ret
For some reason, GCC things that the mask is only eight byte wide and uses a
"movzbl" instruction.
At higher optimization levels, many of the moves are elided, and the mask is
directly copied to %eax. Very mysterious.
__Z4testv:
vpternlogd $0xFF, %zmm0, %zmm0, %zmm0
vpxord %zmm1, %zmm1, %zmm1
vpcmpd $1, %zmm1, %zmm0, %k1
kmovw %k1, %eax
movzwl %ax, %eax
ret
