Re: [PATCH] [i386] Prevent vectorization for load from parm_decl at O2 to avoid STF issue.

[Hongtao Liu <crazylht@gmail.com>]

On Fri, Mar 4, 2022 at 3:28 PM liuhongt via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> For parameter passing through stack, vectorized load from parm_decl
> in callee may trigger serious STF issue. This is why GCC12 regresses
> 50% for cray at -O2 compared to GCC11.
>
> The patch add an extremely large number to stmt_cost to prevent
> vectorization for loads from parm_decl under very-cheap cost model,
> this can at least prevent O2 regression due to STF issue, but may lose
> some perf where there's no such issue(1 vector_load vs n scalar_load +
> CTOR).
>
> No impact for SPEC2017 for both plain O2 and native O2 on ICX.
> Bootstrapped and regtested on x86_64-linux-gnu{-m32,}.
> Ok for trunk?
>
> gcc/ChangeLog:
>
>         PR target/101908
>         * config/i386/i386.cc (ix86_load_maybe_stfs_p): New.
>         (ix86_vector_costs::add_stmt_cost): Add extra cost for
>         vector_load/unsigned_load which may have stall forward issue.
>
> gcc/testsuite/ChangeLog:
>
>         * gcc.target/i386/pr101908-1.c: New test.
>         * gcc.target/i386/pr101908-2.c: New test.
> ---
>  gcc/config/i386/i386.cc                    | 31 ++++++++++++++++++++++
>  gcc/testsuite/gcc.target/i386/pr101908-1.c | 12 +++++++++
>  gcc/testsuite/gcc.target/i386/pr101908-2.c | 12 +++++++++
>  3 files changed, 55 insertions(+)
>  create mode 100644 gcc/testsuite/gcc.target/i386/pr101908-1.c
>  create mode 100644 gcc/testsuite/gcc.target/i386/pr101908-2.c
>
> diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
> index b2bf90576d5..3bbaaf65ea8 100644
> --- a/gcc/config/i386/i386.cc
> +++ b/gcc/config/i386/i386.cc
> @@ -22976,6 +22976,19 @@ ix86_noce_conversion_profitable_p (rtx_insn *seq, struct noce_if_info *if_info)
>    return default_noce_conversion_profitable_p (seq, if_info);
>  }
>
> +/* Return true if REF may have STF issue, otherwise false.  */
> +static bool
> +ix86_load_maybe_stfs_p (tree ref)
> +{
> +  tree addr = get_base_address (ref);
> +
> +  if (TREE_CODE (addr) != PARM_DECL
> +      || !tree_fits_uhwi_p (TYPE_SIZE (TREE_TYPE (addr)))
> +      || tree_to_uhwi (TYPE_SIZE (TREE_TYPE (addr))) <= MAX_BITS_PER_WORD)
> +    return false;
> +  return true;
> +}
> +
>  /* x86-specific vector costs.  */
>  class ix86_vector_costs : public vector_costs
>  {
> @@ -23203,6 +23216,24 @@ ix86_vector_costs::add_stmt_cost (int count, vect_cost_for_stmt kind,
>         if (TREE_CODE (op) == SSA_NAME)
>           TREE_VISITED (op) = 0;
>      }
> +
> +  /* Prevent vectorization for load from parm_decl at O2 to avoid STF issue.
> +     Performance may lose when there's no STF issue(1 vector_load vs n
> +     scalar_load + CTOR).
> +     TODO: both extra cost(2000) and ix86_load_maybe_stfs_p need to be fine
> +     tuned.  */
> +  if ((kind == vector_load || kind == unaligned_load)
> +      && flag_vect_cost_model == VECT_COST_MODEL_VERY_CHEAP
> +      && stmt_info
> +      && stmt_info->slp_type == pure_slp
> +      && stmt_info->stmt
> +      && gimple_assign_load_p (stmt_info->stmt)
> +      && ix86_load_maybe_stfs_p (gimple_assign_rhs1 (stmt_info->stmt)))
> +    {
> +      stmt_cost = ix86_builtin_vectorization_cost (kind, vectype, misalign);
> +      stmt_cost += 2000;
> +    }
> +
>    if (stmt_cost == -1)
>      stmt_cost = ix86_builtin_vectorization_cost (kind, vectype, misalign);
>
> diff --git a/gcc/testsuite/gcc.target/i386/pr101908-1.c b/gcc/testsuite/gcc.target/i386/pr101908-1.c
> new file mode 100644
> index 00000000000..f8e0f2e26bb
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/i386/pr101908-1.c
> @@ -0,0 +1,12 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-slp-details" } */
> +/* { dg-final { scan-tree-dump {(?n)add new stmt:.*MEM \<vector\(2\) double\>} "slp2" } } */
> +
> +struct X { double x[2]; };
> +typedef double v2df __attribute__((vector_size(16)));
> +
> +v2df __attribute__((noipa))
> +foo (struct X* x, struct X* y)
> +{
> +  return (v2df) {x->x[1], x->x[0] } + (v2df) { y->x[1], y->x[0] };
> +}
> diff --git a/gcc/testsuite/gcc.target/i386/pr101908-2.c b/gcc/testsuite/gcc.target/i386/pr101908-2.c
> new file mode 100644
> index 00000000000..7f2f00cebab
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/i386/pr101908-2.c
> @@ -0,0 +1,12 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-slp-details" } */
> +/* { dg-final { scan-tree-dump-not {(?n)add new stmt:.*MEM \<vector\(2\) double\>} "slp2" } } */
> +
> +struct X { double x[2]; };
> +typedef double v2df __attribute__((vector_size(16)));
> +
> +v2df __attribute__((noipa))
> +foo (struct X x, struct X y)
> +{
> +  return (v2df) {x.x[1], x.x[0] } + (v2df) { y.x[1], y.x[0] };
> +}
> --
> 2.18.1
>


-- 
BR,
Hongtao