Add missing alignment checks in epilogue loop vectorisation (PR 86877)

[Richard Biener]

On Thu, Sep 20, 2018 at 1:42 PM Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> Epilogue loop vectorisation skips vect_enhance_data_refs_alignment
> since it doesn't make sense to version or peel the epilogue loop
> (that will already have happened for the main loop).  But this means
> that it also fails to check whether the accesses are suitably aligned
> for the new vector subarch.
>
> We don't seem to carry alignment information from the (potentially
> peeled or versioned) main loop to the epilogue loop, which would be
> good to fix at some point.  I think we want this patch regardless,
> since there's no guarantee that the alignment requirements are the
> same for every subarch.
>
> Tested on aarch64-linux-gnu (with and without SVE), aarch64_be-elf
> and x86_64-linux-gnu.  OK to install?

OK.

Richard.

> Richard
>
>
> 2018-09-20  Richard Sandiford  <richard.sandiford@arm.com>
>
> gcc/
>         PR tree-optimization/86877
>         * tree-vect-loop.c (vect_analyze_loop_2): Call
>         vect_verify_datarefs_alignment.
>
> gcc/testsuite/
>         PR tree-optimization/86877
>         * gfortran.dg/vect/vect-8-epilogue.F90: New test.
>
> Index: gcc/tree-vect-loop.c
> ===================================================================
> --- gcc/tree-vect-loop.c        2018-09-20 12:39:06.341625036 +0100
> +++ gcc/tree-vect-loop.c        2018-09-20 12:39:14.541555902 +0100
> @@ -1979,20 +1979,21 @@ vect_analyze_loop_2 (loop_vec_info loop_
>    if (!ok)
>      return false;
>
> -  /* Do not invoke vect_enhance_data_refs_alignment for eplilogue
> -     vectorization.  */
> +  /* Do not invoke vect_enhance_data_refs_alignment for epilogue
> +     vectorization, since we do not want to add extra peeling or
> +     add versioning for alignment.  */
>    if (!LOOP_VINFO_EPILOGUE_P (loop_vinfo))
> -    {
>      /* This pass will decide on using loop versioning and/or loop peeling in
>         order to enhance the alignment of data references in the loop.  */
>      ok = vect_enhance_data_refs_alignment (loop_vinfo);
> -    if (!ok)
> -      {
> -       if (dump_enabled_p ())
> -         dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
> -                          "bad data alignment.\n");
> -        return false;
> -      }
> +  else
> +    ok = vect_verify_datarefs_alignment (loop_vinfo);
> +  if (!ok)
> +    {
> +      if (dump_enabled_p ())
> +       dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
> +                        "bad data alignment.\n");
> +      return false;
>      }
>
>    if (slp)
> Index: gcc/testsuite/gfortran.dg/vect/vect-8-epilogue.F90
> ===================================================================
> --- /dev/null   2018-09-14 11:16:31.122530289 +0100
> +++ gcc/testsuite/gfortran.dg/vect/vect-8-epilogue.F90  2018-09-20 12:39:14.537555936 +0100
> @@ -0,0 +1,6 @@
> +! { dg-do compile }
> +! { dg-require-effective-target vect_double }
> +! { dg-additional-options "-finline-matmul-limit=0 --param vect-epilogues-nomask=1" }
> +! { dg-additional-options "-mstrict-align" { target { aarch64*-*-* } } }
> +
> +#include "vect-8.f90"