https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88868
Bug ID: 88868
Summary: [SSE] pshufb can be omitted for a specific pattern
Product: gcc
Version: 8.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: wojciech_mula at poczta dot onet.pl
Target Milestone: ---
SSSE3 instruction PSHUFB (and the AVX2 counterpart VPSHUFB) acts as a
no-operation
when its argument is a sequence 0..15. Such invocation does not alter shuffled
register, thus PSHUFB can be safely omitted
BTW, clang does this optimization, but ICC doesn't.
---pshufb.c---
#include <immintrin.h>
__m128i shuffle(__m128i x) {
const __m128i noop = _mm_setr_epi8(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,
12, 13, 14, 15);
return _mm_shuffle_epi8(x, noop);
}
---eof---
$ gcc --version
gcc (Debian 8.2.0-13) 8.2.0
$ gcc -O3 -march=skylake -S pshufb.c
$ cat pshufb.s
shuffle:
vpshufb .LC0(%rip), %xmm0, %xmm0
ret
.LC0:
.byte 0
.byte 1
.byte 2
.byte 3
.byte 4
.byte 5
.byte 6
.byte 7
.byte 8
.byte 9
.byte 10
.byte 11
.byte 12
.byte 13
.byte 14
.byte 15
An expected output:
shuffle:
ret
