llvm dev - Nov 2021 - k registers in extended asm

Hi,

I am getting an abort in llc with 'main' and LLVM 13.0 when I run clang
at
-O0 for the following test case:

#include <immintrin.h>
void small(float *b, __m512 c, __mmask16 mask)
{
  __m512 a = _mm512_setzero_ps();
  asm("vfmadd231ps (%1), %2, %0 %{%3%}": "+v"(a):
"r"(b), "v"(c), "k"
(mask));
  _mm512_mask_storeu_ps(b, mask, a);
}

clang -S bug.c -march=skylake-avx512 -O0

bug.c:6:7: error: Register k0 can't be used as write mask
  asm("vfmadd231ps (%1), %2, %0 %{%3%}": "+v"(a):
"r"(b), "v"(c), "k"
(mask));
      ^
<inline asm>:1:36: note: instantiated into assembly here
        vfmadd231ps (%rax), %zmm1, %zmm0 {%k0}


Works: clang -S bug.c -march=skylake-avx512 -O2 // chooses k1

The first observation I have is that there is no write of k0 on the fma
instruction. So even if I wrote my test case to explicitly use k0, there is
still the abort at -O0. This feels like a bug.

The second thing I have -- is it required that I specify the mask register
I want since k0 is "special"? Or is llc incorrectly choosing the k0
register in the first place?

Thanks!

Scott
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Try using "Yk" instead of "k". That should pick from k1-k7.

~Craig


On Thu, Nov 4, 2021 at 9:14 AM Scott Manley via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi,
>
> I am getting an abort in llc with 'main' and LLVM 13.0 when I run
clang at
> -O0 for the following test case:
>
> #include <immintrin.h>
> void small(float *b, __m512 c, __mmask16 mask)
> {
>   __m512 a = _mm512_setzero_ps();
>   asm("vfmadd231ps (%1), %2, %0 %{%3%}": "+v"(a):
"r"(b), "v"(c), "k"
> (mask));
>   _mm512_mask_storeu_ps(b, mask, a);
> }
>
> clang -S bug.c -march=skylake-avx512 -O0
>
> bug.c:6:7: error: Register k0 can't be used as write mask
>   asm("vfmadd231ps (%1), %2, %0 %{%3%}": "+v"(a):
"r"(b), "v"(c), "k"
> (mask));
>       ^
> <inline asm>:1:36: note: instantiated into assembly here
>         vfmadd231ps (%rax), %zmm1, %zmm0 {%k0}
>
>
> Works: clang -S bug.c -march=skylake-avx512 -O2 // chooses k1
>
> The first observation I have is that there is no write of k0 on the fma
> instruction. So even if I wrote my test case to explicitly use k0, there is
> still the abort at -O0. This feels like a bug.
>
> The second thing I have -- is it required that I specify the mask register
> I want since k0 is "special"? Or is llc incorrectly choosing the
k0
> register in the first place?
>
> Thanks!
>
> Scott
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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