https://llvm.org/bugs/show_bug.cgi?id=28892
Bug ID: 28892
Summary: AVX512: fmsub intrinsic generates fmadd instruction
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: wenzel.ja...@epfl.ch
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Consider the following simple snippet which uses the fused multiply subtract
instruction on AVX512 and AVX:
=============
#include <immintrin.h>
__m512 test(__m512 a, __m512 b, __m512 c) {
return _mm512_fmsub_ps(a, b, c);
}
__m256 test(__m256 a, __m256 b, __m256 c) {
return _mm256_fmsub_ps(a, b, c);
}
=============
In the AVX512 case, LLVM trunk generates the following weird code which does a
sign flip first followed by a FMADD instruction. The AVX backend, in
comparison, generates the expected (single FMADD instruction).
__Z4testDv16_fS_S_:
vpxord LCPI0_0(%rip), %zmm2, %zmm2
vfmadd213ps %zmm2, %zmm1, %zmm0
retq
__Z4testDv8_fS_S_:
vfmsub213ps %ymm2, %ymm1, %ymm0
retq
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