Re: [PATCH] tree-optimization/110979 - fold-left reduction and partial vectors

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

Richard Biener <rguenther@suse.de> writes:
> When we vectorize fold-left reductions with partial vectors but
> no target operation available we use a vector conditional to force
> excess elements to zero.  But that doesn't correctly preserve
> the sign of zero.  The following patch disables partial vector
> support in that case.
>
> Bootstrap and regtest running on x86_64-unknown-linux-gnu.
>
> Does this look OK?

LGTM.

> With -frounding-math -fno-signed-zeros we are
> happily using the masking again, but that's OK, right?  An additional
> + 0.0 shouldn't do anything here.

Yeah, I would hope so.

Thanks,
Richard

>
> Thanks,
> Richard.
>
> 	PR tree-optimization/110979
> 	* tree-vect-loop.cc (vectorizable_reduction): For
> 	FOLD_LEFT_REDUCTION without target support make sure
> 	we don't need to honor signed zeros.
>
> 	* gcc.dg/torture/pr110979.c: New testcase.
> ---
>  gcc/testsuite/gcc.dg/torture/pr110979.c | 25 +++++++++++++++++++++++++
>  gcc/tree-vect-loop.cc                   | 11 +++++++++++
>  2 files changed, 36 insertions(+)
>  create mode 100644 gcc/testsuite/gcc.dg/torture/pr110979.c
>
> diff --git a/gcc/testsuite/gcc.dg/torture/pr110979.c b/gcc/testsuite/gcc.dg/torture/pr110979.c
> new file mode 100644
> index 00000000000..c25ad7a8a31
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/torture/pr110979.c
> @@ -0,0 +1,25 @@
> +/* { dg-do run } */
> +/* { dg-additional-options "--param vect-partial-vector-usage=2" } */
> +
> +#define FLT double
> +#define N 20
> +
> +__attribute__((noipa))
> +FLT
> +foo3 (FLT *a)
> +{
> +  FLT sum = -0.0;
> +  for (int i = 0; i != N; i++)
> +    sum += a[i];
> +  return sum;
> +}
> +
> +int main()
> +{
> +  FLT a[N];
> +  for (int i = 0; i != N; i++)
> +    a[i] = -0.0;
> +  if (!__builtin_signbit(foo3(a)))
> +    __builtin_abort();
> +  return 0;
> +}
> diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
> index bf8d677b584..741b5c20389 100644
> --- a/gcc/tree-vect-loop.cc
> +++ b/gcc/tree-vect-loop.cc
> @@ -8037,6 +8037,17 @@ vectorizable_reduction (loop_vec_info loop_vinfo,
>  			     " no conditional operation is available.\n");
>  	  LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P (loop_vinfo) = false;
>  	}
> +      else if (reduction_type == FOLD_LEFT_REDUCTION
> +	       && reduc_fn == IFN_LAST
> +	       && FLOAT_TYPE_P (vectype_in)
> +	       && HONOR_SIGNED_ZEROS (vectype_in))
> +	{
> +	  if (dump_enabled_p ())
> +	    dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
> +			     "can't operate on partial vectors because"
> +			     " signed zeros need to be handled.\n");
> +	  LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P (loop_vinfo) = false;
> +	}
>        else
>  	{
>  	  internal_fn mask_reduc_fn