CLANG optimizing using SVML and it's autovectorization

问题

Consider simple function:

#include <math.h>
void ahoj(float *a)
{
    for (int i=0; i<256; i++) a[i] = sin(a[i]);
}

Try that at https://godbolt.org/z/ynQKRb, and use following settings

-fveclib=SVML -mfpmath=sse -ffast-math -fno-math-errno -O3 -mavx2 -fvectorize

Select x86_64 CLANG 7.0, currently the newest. This is the most interesting part of the result:

vmovups ymm0, ymmword ptr [rdi]
vmovups ymm1, ymmword ptr [rdi + 32]
vmovups ymmword ptr [rsp], ymm1 # 32-byte Spill
vmovups ymm1, ymmword ptr [rdi + 64]
vmovups ymmword ptr [rsp + 32], ymm1 # 32-byte Spill
vmovups ymm1, ymmword ptr [rdi + 96]
vmovups ymmword ptr [rsp + 96], ymm1 # 32-byte Spill
call    __svml_sinf8
vmovups ymmword ptr [rsp + 64], ymm0 # 32-byte Spill
vmovups ymm0, ymmword ptr [rsp] # 32-byte Reload
call    __svml_sinf8
vmovups ymmword ptr [rsp], ymm0 # 32-byte Spill
vmovups ymm0, ymmword ptr [rsp + 32] # 32-byte Reload
call    __svml_sinf8
vmovups ymmword ptr [rsp + 32], ymm0 # 32-byte Spill
vmovups ymm0, ymmword ptr [rsp + 96] # 32-byte Reload
call    __svml_sinf8
vmovups ymm1, ymmword ptr [rsp + 64] # 32-byte Reload
vmovups ymmword ptr [rbx], ymm1
vmovups ymm1, ymmword ptr [rsp] # 32-byte Reload
vmovups ymmword ptr [rbx + 32], ymm1
vmovups ymm1, ymmword ptr [rsp + 32] # 32-byte Reload
vmovups ymmword ptr [rbx + 64], ymm1
vmovups ymmword ptr [rbx + 96], ymm0
vmovups ymm0, ymmword ptr [rbx + 128]
vmovups ymm1, ymmword ptr [rbx + 160]
vmovups ymmword ptr [rsp], ymm1 # 32-byte Spill
vmovups ymm1, ymmword ptr [rbx + 192]
vmovups ymmword ptr [rsp + 32], ymm1 # 32-byte Spill
vmovups ymm1, ymmword ptr [rbx + 224]
vmovups ymmword ptr [rsp + 96], ymm1 # 32-byte Spill
call    __svml_sinf8
vmovups ymmword ptr [rsp + 64], ymm0 # 32-byte Spill
vmovups ymm0, ymmword ptr [rsp] # 32-byte Reload
call    __svml_sinf8
vmovups ymmword ptr [rsp], ymm0 # 32-byte Spill
vmovups ymm0, ymmword ptr [rsp + 32] # 32-byte Reload
call    __svml_sinf8
vmovups ymmword ptr [rsp + 32], ymm0 # 32-byte Spill
vmovups ymm0, ymmword ptr [rsp + 96] # 32-byte Reload
call    __svml_sinf8
...

It literally avoids any loops and instead creates code for processing 256 items. Could this really be optimal solution considering code cache? When using -mavx512f, it expands even 1024 items :).

Another problem is that with this option current CLANG sometimes generates AVX512 code even if the target is AVX2 making it basically unusable.

来源：https://stackoverflow.com/questions/52562272/clang-optimizing-using-svml-and-its-autovectorization

标签

optimization

clang

llvm

avx2