Re: [PATCH] match.pd: extend the MIN/MAX narrowing through a cast to vectors

On Tue, Aug 4, 2026 at 3:00 AM <[email protected]> wrote:
>
> From: Kyrylo Tkachov <[email protected]>
>
> The rule folding (type) minmax ((wide_type) a, (wide_type) b) to
> minmax (a, b) is restricted to scalars.  Extension is monotone, so it
> commutes with the comparison and the outer truncation is exact, and the
> argument is lanewise, so it holds for vectors unchanged.
>
> The vector case additionally needs the narrow operation to be available,
> otherwise vector lowering would scalarise what used to be a single wide
> instruction.
>
>   typedef int  v2si __attribute__((vector_size (8)));
>   typedef long v2di __attribute__((vector_size (16)));
>   v2si h (v2si a, v2si b)
>   {
>     v2di x = __builtin_convertvector (a, v2di);
>     v2di y = __builtin_convertvector (b, v2di);
>     return __builtin_convertvector (x < y ? x : y, v2si);
>   }
>
> aarch64 -O3 before:
>
>         sshll   v0.2d, v0.2s, 0
>         sshll   v1.2d, v1.2s, 0
>         cmgt    v2.2d, v1.2d, v0.2d
>         bsl     v2.16b, v0.16b, v1.16b
>         xtn     v0.2s, v2.2d
>
> after:
>
>         smin    v0.2s, v0.2s, v1.2s
>
> Require the wide vector MIN or MAX to have one use.  A shared wide
> result would otherwise gain a second narrow operation.
>
> Bootstrapped and tested on aarch64-none-linux-gnu.
> Ok for trunk?
> Thanks,
> Kyrill
>
> gcc/ChangeLog:
>
>         * match.pd ((type) minmax ((wide_type) a, (wide_type) b)): Accept
>         vector types, using element_precision and requiring the narrow
>         operation to be supported.
>
> gcc/testsuite/ChangeLog:
>
>         * g++.dg/tree-ssa/vec-narrow-1.C: New test.
>         * g++.dg/tree-ssa/vec-narrow-minmax-2.C: New test.
>
> Signed-off-by: Kyrylo Tkachov <[email protected]>
> ---
>  gcc/match.pd                                  | 20 +++++++++++++------
>  gcc/testsuite/g++.dg/tree-ssa/vec-narrow-1.C  | 11 ++++++++++
>  .../g++.dg/tree-ssa/vec-narrow-minmax-2.C     | 18 +++++++++++++++++
>  3 files changed, 43 insertions(+), 6 deletions(-)
>  create mode 100644 gcc/testsuite/g++.dg/tree-ssa/vec-narrow-1.C
>  create mode 100644 gcc/testsuite/g++.dg/tree-ssa/vec-narrow-minmax-2.C
>
> diff --git a/gcc/match.pd b/gcc/match.pd
> index e0ec2626842..adbdda91b51 100644
> --- a/gcc/match.pd
> +++ b/gcc/match.pd
> @@ -4649,17 +4649,25 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
>
>  /* (type) minmax ((wide_type) a, (wide_type) b) -> minmax (a, b)
>     when type matches the type of a and b, and wide_type is a wider
> -   type with the same signedness as type. */
> +   type with the same signedness as type.  Extension is monotone, so it
> +   commutes with the comparison, and the truncation is then exact.  The
> +   same holds lanewise for a vector, where the narrow operation has to be
> +   available so that lowering does not scalarise it.  The wide operation
> +   must become dead before a narrow vector operation is introduced.  */
>  (for minmax (min max)
> +     MINMAX (MIN_EXPR MAX_EXPR)
>   (simplify
> -  (convert (minmax:c (convert@2 @0) (convert@3 @1)))
> -  (if (INTEGRAL_TYPE_P (type)
> -       && INTEGRAL_TYPE_P (TREE_TYPE (@2))
> +  (convert (minmax:c@4 (convert@2 @0) (convert@3 @1)))
> +  (if (ANY_INTEGRAL_TYPE_P (type)
> +       && ANY_INTEGRAL_TYPE_P (TREE_TYPE (@2))
>         && types_match (type, TREE_TYPE (@0))
>         && types_match (type, TREE_TYPE (@1))
>         && types_match (TREE_TYPE (@2), TREE_TYPE (@3))
> -       && TYPE_PRECISION (TREE_TYPE (@2)) > TYPE_PRECISION (type)
> -       && TYPE_UNSIGNED (TREE_TYPE (@2)) == TYPE_UNSIGNED (type))
> +       && element_precision (TREE_TYPE (@2)) > element_precision (type)
> +       && TYPE_UNSIGNED (TREE_TYPE (@2)) == TYPE_UNSIGNED (type)
> +       && (!VECTOR_TYPE_P (type)
> +          || (single_use (@4)
> +              && target_supports_op_p (type, MINMAX, optab_vector))))
>     (minmax @0 @1))))
>
>  /* max (a, a + CST) -> a + CST where CST is positive.  */
> diff --git a/gcc/testsuite/g++.dg/tree-ssa/vec-narrow-1.C b/gcc/testsuite/g++.dg/tree-ssa/vec-narrow-1.C
> new file mode 100644
> index 00000000000..de738e33c30
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/tree-ssa/vec-narrow-1.C
> @@ -0,0 +1,11 @@
> +// { dg-do compile }
> +// { dg-options "-O2 -fdump-tree-optimized" }
> +// Extension is monotone, so it commutes with the comparison and the outer
> +// truncation is exact.  The argument is lanewise, so a widened vector
> +// MIN/MAX feeding a truncating conversion narrows.
> +typedef int  v2si __attribute__((vector_size (8)));
> +typedef long long v2di __attribute__((vector_size (16)));
> +v2si f (v2si a, v2si b)
> +{ v2di x = __builtin_convertvector (a, v2di), y = __builtin_convertvector (b, v2di);
> +  return __builtin_convertvector (x < y ? x : y, v2si); }
> +// { dg-final { scan-tree-dump-not "vector\\(2\\) long" "optimized" } }


This testcase fails on x86_64.

> diff --git a/gcc/testsuite/g++.dg/tree-ssa/vec-narrow-minmax-2.C b/gcc/testsuite/g++.dg/tree-ssa/vec-narrow-minmax-2.C
> new file mode 100644
> index 00000000000..d7fb7f06f16
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/tree-ssa/vec-narrow-minmax-2.C
> @@ -0,0 +1,18 @@
> +// { dg-do compile }
> +// { dg-options "-O2 -fdump-tree-optimized" }
> +
> +typedef int v2si __attribute__((vector_size (8)));
> +typedef long long v2di __attribute__((vector_size (16)));
> +
> +v2si
> +f (v2si a, v2si b, v2di *p)
> +{
> +  v2di x = __builtin_convertvector (a, v2di);
> +  v2di y = __builtin_convertvector (b, v2di);
> +  v2di z = x < y ? x : y;
> +  *p = z;
> +  return __builtin_convertvector (z, v2si);
> +}
> +
> +// A shared wide MIN must not gain a second narrow MIN.
> +// { dg-final { scan-tree-dump-times "MIN_EXPR" 1 "optimized" } }
> --
> 2.50.1 (Apple Git-155)
>