On Tue, Jul 7, 2026 at 11:49 AM Pengfei Li <[email protected]> wrote:
>
> Hi Richard,
>
> On 07/07/2026 09:38, Richard Biener wrote:
> > 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?
>
> In this testcase, SAD_EXPR is expanded to AArch64 uabd + udot
> instructions. Without the pattern, the uabd is eliminated but SAD_EXPR
> still generates an extra udot with a zero vector, e.g.
>
> movi d31, #0
> ...
> udot z29.s, z31.b, z28.b
>
> The 48-iteration loop needs 3 udot, instead of 4. That's what the
> testcase checks. Ok for trunk?
OK.
>
> Thanks,
> Pengfei
>
> >
> >> 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
> >>
