On Tue, Jul 7, 2026 at 10:00 AM Pengfei Li <[email protected]> wrote:
>
> Normally, SAD_EXPRs with identical first and second operands are not
> constructed, since the absolute difference value is zero. However, this
> can still appear when vectorizing small iteration-count reduction loops
> with an absolute difference operation.
>
> This patch adds a match.pd rule to fold the redundant SAD_EXPR to its
> accumulator operand.
>
> Bootstrapped and tested on aarch64-linux-gnu and x86_64-linux-gnu.
The pattern looks good to me, but I don't see how the testcase exercises it?
> gcc/ChangeLog:
>
> * match.pd: Simplify SAD(x, x, acc) into acc.
>
> gcc/testsuite/ChangeLog:
>
> * gcc.target/aarch64/sve/sad_2.c: New test.
> ---
> gcc/match.pd | 7 +++++++
> gcc/testsuite/gcc.target/aarch64/sve/sad_2.c | 16 ++++++++++++++++
> 2 files changed, 23 insertions(+)
> create mode 100644 gcc/testsuite/gcc.target/aarch64/sve/sad_2.c
>
> diff --git a/gcc/match.pd b/gcc/match.pd
> index a7cec25dbad..4520a23a026 100644
> --- a/gcc/match.pd
> +++ b/gcc/match.pd
> @@ -1229,6 +1229,13 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
> (mult (absu@1 @0) @1)
> (mult (convert@2 @0) @2))
>
> +#if GIMPLE
> +/* Simplify SAD(x, x, acc) -> acc since the absolute difference is zero. */
> +(simplify
> + (sad @0 @0 @1)
> + @1)
> +#endif
> +
> /* cos(copysign(x, y)) -> cos(x). Similarly for cosh. */
> (for coss (COS COSH)
> (for copysigns (COPYSIGN)
> diff --git a/gcc/testsuite/gcc.target/aarch64/sve/sad_2.c
> b/gcc/testsuite/gcc.target/aarch64/sve/sad_2.c
> new file mode 100644
> index 00000000000..d46517f10d8
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/aarch64/sve/sad_2.c
> @@ -0,0 +1,16 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O3 -msve-vector-bits=128 -mtune=neoverse-v2" } */
> +
> +#include <stdint.h>
> +
> +int foo (uint8_t *pix1, uint8_t *pix2)
> +{
> + int sum = 0;
> + for (int x = 0; x < 48; x++)
> + {
> + sum += __builtin_abs (pix1[x] - pix2[x]);
> + }
> + return sum;
> +}
> +
> +/* { dg-final { scan-assembler-times {\tudot\t} 3 } } */
> --
> 2.43.0
>
