Re: [PATCH] match.pd: fold the inequality spelling of divisibility

On Fri, Aug 14, 2026 at 6:23 AM <[email protected]> wrote:
>
> From: Kyrylo Tkachov <[email protected]>
>
> GCC folds:
>
>   x / y * y == x
>
> to:
>
>   x % y == 0
>
> Apply the same simplification to inequality.
>
>   int divisible_by_3 (unsigned int x)
>   {
>     return x / 3 * 3 != x;
>   }
>
> aarch64 -O2:
>
> before:
>
>         divisible_by_3:
>                 mov     w1, 43691
>                 movk    w1, 0xaaaa, lsl 16
>                 umull   x1, w0, w1
>                 lsr     x1, x1, 33
>                 add     w1, w1, w1, lsl 1
>                 cmp     w1, w0
>                 cset    w0, ne
>                 ret
>
> after:
>
>         divisible_by_3:
>                 mov     w1, 43691
>                 movk    w1, 0xaaaa, lsl 16
>                 mul     w0, w0, w1
>                 mov     w1, 1431655765
>                 cmp     w0, w1
>                 cset    w0, hi
>                 ret
>
> Bootstrapped and tested on aarch64-none-linux-gnu.
> Ok for trunk?
> Thanks,
> Kyrill
>
> gcc/ChangeLog:
>
>         * match.pd (x / y * y == x): Extend to inequality.

Ok.

>
> gcc/testsuite/ChangeLog:
>
>         * gcc.dg/tree-ssa/divmul-mod-1.c: New test.
>
> Signed-off-by: Kyrylo Tkachov <[email protected]>
> ---
>  gcc/match.pd                                 |  9 ++---
>  gcc/testsuite/gcc.dg/tree-ssa/divmul-mod-1.c | 38 ++++++++++++++++++++
>  2 files changed, 43 insertions(+), 4 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/divmul-mod-1.c
>
> diff --git a/gcc/match.pd b/gcc/match.pd
> index 877e449d7d8..33eec29f53a 100644
> --- a/gcc/match.pd
> +++ b/gcc/match.pd
> @@ -6036,16 +6036,17 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
>     (plus:c (mult:c (div @0 @1) @1) (mod @0 @1))
>     @0))
>
> -/* x / y * y == x -> x % y == 0.  */
> -(simplify
> -  (eq:c (mult:c (trunc_div:s @0 @1) @1) @0)
> +/* x / y * y == x -> x % y == 0, and likewise for !=.  */
> +(for cmp (eq ne)
> + (simplify
> +  (cmp:c (mult:c (trunc_div:s @0 @1) @1) @0)
>    (if (TREE_CODE (TREE_TYPE (@0)) != COMPLEX_TYPE
>         && (!VECTOR_MODE_P (TYPE_MODE (TREE_TYPE (@0)))
>            || !target_supports_op_p (TREE_TYPE (@0), TRUNC_DIV_EXPR,
>                                      optab_vector)
>            || target_supports_op_p (TREE_TYPE (@0), TRUNC_MOD_EXPR,
>                                     optab_vector)))
> -   (eq (trunc_mod @0 @1) { build_zero_cst (TREE_TYPE (@0)); })))
> +   (cmp (trunc_mod @0 @1) { build_zero_cst (TREE_TYPE (@0)); }))))
>
>  /* ((X /[ex] C1) +- C2) * (C1 * C3)  -->  (X * C3) +- (C1 * C2 * C3).  */
>  (for op (plus minus)
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/divmul-mod-1.c b/gcc/testsuite/gcc.dg/tree-ssa/divmul-mod-1.c
> new file mode 100644
> index 00000000000..f65a28d2335
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/divmul-mod-1.c
> @@ -0,0 +1,38 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-optimized" } */
> +
> +typedef int v4si __attribute__ ((vector_size (16)));
> +typedef unsigned int v4ui __attribute__ ((vector_size (16)));
> +
> +int
> +eq (unsigned int a, unsigned int b)
> +{
> +  return a / b * b == a;
> +}
> +
> +int
> +ne (unsigned int a, unsigned int b)
> +{
> +  return a / b * b != a;
> +}
> +
> +int
> +nes (int a, int b)
> +{
> +  return a / b * b != a;
> +}
> +
> +v4si
> +veq (v4ui a, v4ui b)
> +{
> +  return a / b * b == a;
> +}
> +
> +v4si
> +vne (v4ui a, v4ui b)
> +{
> +  return a / b * b != a;
> +}
> +
> +/* { dg-final { scan-tree-dump-not " \\* " "optimized" } } */
> +/* { dg-final { scan-tree-dump " % " "optimized" } } */
> --
> 2.50.1 (Apple Git-155)
>