[PATCH] vect: Tweak comparisons with existing epilogue loops

Richard Biener richard.guenther@gmail.com
Wed Aug 4 11:48:29 GMT 2021


On Tue, Aug 3, 2021 at 3:52 PM Richard Sandiford via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> This patch uses a more accurate scalar iteration estimate when
> comparing the epilogue of a constant-iteration loop with a candidate
> replacement epilogue.
>
> In the testcase, the patch prevents a 1-to-3-element SVE epilogue
> from seeming better than a 64-bit Advanced SIMD epilogue.
>
> Tested on aarch64-linux-gnu and x86_64-linux-gnu.  OK to install?

OK.

Richard.

> Richard
>
>
> gcc/
>         * tree-vect-loop.c (vect_better_loop_vinfo_p): Detect cases in
>         which old_loop_vinfo is an epilogue loop that handles a constant
>         number of iterations.
>
> gcc/testsuite/
>         * gcc.target/aarch64/sve/cost_model_12.c: New test.
> ---
>  .../gcc.target/aarch64/sve/cost_model_12.c    | 19 +++++++++++++++++++
>  gcc/tree-vect-loop.c                          | 10 +++++++++-
>  2 files changed, 28 insertions(+), 1 deletion(-)
>  create mode 100644 gcc/testsuite/gcc.target/aarch64/sve/cost_model_12.c
>
> diff --git a/gcc/tree-vect-loop.c b/gcc/tree-vect-loop.c
> index 0009d0964af..0a5b65adb04 100644
> --- a/gcc/tree-vect-loop.c
> +++ b/gcc/tree-vect-loop.c
> @@ -2778,7 +2778,15 @@ vect_better_loop_vinfo_p (loop_vec_info new_loop_vinfo,
>
>    /* Limit the VFs to what is likely to be the maximum number of iterations,
>       to handle cases in which at least one loop_vinfo is fully-masked.  */
> -  HOST_WIDE_INT estimated_max_niter = likely_max_stmt_executions_int (loop);
> +  HOST_WIDE_INT estimated_max_niter;
> +  loop_vec_info main_loop = LOOP_VINFO_ORIG_LOOP_INFO (old_loop_vinfo);
> +  unsigned HOST_WIDE_INT main_vf;
> +  if (main_loop
> +      && LOOP_VINFO_NITERS_KNOWN_P (main_loop)
> +      && LOOP_VINFO_VECT_FACTOR (main_loop).is_constant (&main_vf))
> +    estimated_max_niter = LOOP_VINFO_INT_NITERS (main_loop) % main_vf;
> +  else
> +    estimated_max_niter = likely_max_stmt_executions_int (loop);
>    if (estimated_max_niter != -1)
>      {
>        if (known_le (estimated_max_niter, new_vf))
> diff --git a/gcc/testsuite/gcc.target/aarch64/sve/cost_model_12.c b/gcc/testsuite/gcc.target/aarch64/sve/cost_model_12.c
> new file mode 100644
> index 00000000000..4c5226e05de
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/aarch64/sve/cost_model_12.c
> @@ -0,0 +1,19 @@
> +/* { dg-options "-O3 -mtune=neoverse-512tvb" } */
> +
> +void
> +f (float x[restrict 10][1024],
> +   float y[restrict 10][1024], float z)
> +{
> +  for (int i = 0; i < 10; ++i)
> +    {
> +#pragma GCC unroll 10
> +      for (int j = 0; j < 10; ++j)
> +       x[j][i] = y[j][i] * z;
> +    }
> +}
> +
> +/* We should unroll the outer loop, with 2x 16-byte vectors and 1x
> +   8-byte vectors.  */
> +/* { dg-final { scan-assembler-not {\tptrue\t} } } */
> +/* { dg-final { scan-assembler {\tv[0-9]+\.4s,} } } */
> +/* { dg-final { scan-assembler {\tv[0-9]+\.2s,} } } */


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