Re: [PATCH]AArch64 sve2: Fix expansion of division [PR107830]

[Richard Sandiford <richard.sandiford@arm.com>]

Tamar Christina <tamar.christina@arm.com> writes:
> Hi All,
>
> SVE has an actual division optab, and when using -Os we don't
> optimize the division away.  This means that we need to distinguish
> between a div which we can optimize and one we cannot even during
> expansion.
>
> Bootstrapped Regtested on aarch64-none-linux-gnu and no issues.
>
> Ok for master?
>
> Thanks,
> Tamar
>
> gcc/ChangeLog:
>
> 	PR target/107830
> 	* config/aarch64/aarch64.cc
> 	(aarch64_vectorize_can_special_div_by_constant): Check validity during
> 	codegen phase as well.
>
> gcc/testsuite/ChangeLog:
>
> 	PR target/107830
> 	* gcc.target/aarch64/sve2/pr107830.c: New test.
>
> --- inline copy of patch -- 
> diff --git a/gcc/config/aarch64/aarch64.cc b/gcc/config/aarch64/aarch64.cc
> index 4176d7b046a126664360596b6db79a43e77ff76a..bee23625807af95d5ec15ad45702961b2d7ab55d 100644
> --- a/gcc/config/aarch64/aarch64.cc
> +++ b/gcc/config/aarch64/aarch64.cc
> @@ -24322,12 +24322,15 @@ aarch64_vectorize_can_special_div_by_constant (enum tree_code code,
>    if ((flags & VEC_ANY_SVE) && !TARGET_SVE2)
>      return false;
>  
> +  wide_int val = wi::add (cst, 1);
> +  int pow = wi::exact_log2 (val);
> +  bool valid_p = pow == (int)(element_precision (vectype) / 2);
> +  /* SVE actually has a div operator, we we may have gotten here through
> +     that route.  */
>    if (in0 == NULL_RTX && in1 == NULL_RTX)
> -    {
> -      wide_int val = wi::add (cst, 1);
> -      int pow = wi::exact_log2 (val);
> -      return pow == (int)(element_precision (vectype) / 2);
> -    }
> +    return valid_p;
> +  else if (!valid_p)
> +    return false;

Is this equivalent to:

  int pow = wi::exact_log2 (cst + 1);
  if (pow != (int) (element_precision (vectype) / 2))
    return false;

  /* We can use the optimized pattern.  */
  if (in0 == NULL_RTX && in1 == NULL_RTX)
    return true;

?  If so, I'd find that slightly easier to follow, but I realise it's
personal taste.  OK with that change if it works and you agree.

While looking at this, I noticed that we ICE for:

  void f(unsigned short *restrict p1, unsigned int *restrict p2)
  {
    for (int i = 0; i < 16; ++i)
      {
        p1[i] /= 0xff;
        p2[i] += 1;
      }
  }

for -march=armv8-a+sve2 -msve-vector-bits=512.  I guess we need to filter
out partial modes or (better) add support for them.  Adding support for
them probably requires changes to the underlying ADDHNB pattern.

Thanks,
Richard

>    if (!VECTOR_TYPE_P (vectype))
>     return false;
> diff --git a/gcc/testsuite/gcc.target/aarch64/sve2/pr107830.c b/gcc/testsuite/gcc.target/aarch64/sve2/pr107830.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..6d8ee3615fdb0083dbde1e45a2826fb681726139
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/aarch64/sve2/pr107830.c
> @@ -0,0 +1,13 @@
> +/* { dg-do compile } */
> +/* { dg-require-effective-target fopenmp } */
> +/* { dg-additional-options "-Os -fopenmp" } */
> +
> +void
> +f2 (int *a)
> +{
> +  unsigned int i;
> +
> +#pragma omp simd
> +  for (i = 0; i < 4; ++i)
> +    a[i / 3] -= 4;
> +}