On Fri, Sep 6, 2024 at 2:44 PM <[email protected]> wrote:
>
> From: kelefth <[email protected]>
>
> The following function:
>
> int foo(int *a, int j)
> {
> int k = j - 1;
> return a[j - 1] == a[k];
> }
>
> does not fold to `return 1;` using -O2 or higher. The cause of this is that
> the expression `4 * j + (-4)` for the index computation is not folded to
> `4 * (j - 1)`. Existing simplifications that handle similar cases are applied
> when A == C, which is not the case in this instance.
>
> A previous attempt to address this issue is
> https://gcc.gnu.org/pipermail/gcc-patches/2024-April/649896.html
>
> This patch adds the following simplification in match.pd:
> (A * B) + (-C) -> (B - C/A) * A, if C a multiple of A
>
> which also handles cases where the index is j - 2, j - 3, etc.
>
> Bootstrapped for all languages and regression tested on x86-64 and aarch64.
>
> PR tree-optimization/109393
>
> gcc/ChangeLog:
>
> * match.pd: (A * B) + (-C) -> (B - C/A) * A, if C a multiple of A.
>
> gcc/testsuite/ChangeLog:
>
> * gcc.dg/pr109393.c: New test.
>
> Tested-by: Christoph Müllner <[email protected]>
> Signed-off-by: Philipp Tomsich <[email protected]>
> Signed-off-by: Konstantinos Eleftheriou <[email protected]>
> ---
> gcc/match.pd | 15 ++++++++++++++-
> gcc/testsuite/gcc.dg/pr109393.c | 23 +++++++++++++++++++++++
> 2 files changed, 37 insertions(+), 1 deletion(-)
> create mode 100644 gcc/testsuite/gcc.dg/pr109393.c
>
> diff --git a/gcc/match.pd b/gcc/match.pd
> index 621306213e4..9d971b663c6 100644
> --- a/gcc/match.pd
> +++ b/gcc/match.pd
> @@ -4216,7 +4216,20 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
> ? wi::max_value (TYPE_PRECISION (type), SIGNED)
> : wi::min_value (TYPE_PRECISION (type), SIGNED))))))
> && single_use (@3))
> - (mult (plusminus @2 { build_one_cst (type); }) @0))))))
> + (mult (plusminus @2 { build_one_cst (type); }) @0)))
Please move the pattern outside of the enclosing (for plusminus (..) loop
> + /* (A * B) + (-C) -> (B - C/A) * A, if C is a multiple of A. */
> + (simplify
> + (plus (mult:cs@3 integer_nonzerop@0 @2) INTEGER_CST@4)
> + (if (TREE_CODE (type) == INTEGER_TYPE
> + && wi::neg_p (wi::to_wide (@4)))
> + (with {
> + wide_int c1 = wi::to_wide (@0);
> + wide_int c2_abs = wi::abs (wi::to_wide (@4));
I think you need to exclude the most negative number for @4
> + /* Calculate @4 / @0 in order to factorize the expression. */
> + wide_int div_res = wi::div_trunc (c2_abs, c1, TYPE_SIGN (type));
> + tree div_cst = wide_int_to_tree (type, div_res); }
Please build the tree node (and perform the division) only when
wi::multiple_of_p.
> + (if (wi::multiple_of_p (c2_abs, c1, TYPE_SIGN (type)))
> + (mult (minus @2 { div_cst; }) @0))))))))
(minus @2 INTEGER_CST) isn't canonical, it's going to be
folded back to a plus with the negative constant. Can't you
work with a signed division on the original possibly "signed" value?
You have to be careful to not introduce new undefined overflow
with the B - C/A expression which can happen for A == 1 and B == 0
for C == INT_MIN. I think fold_plusminus_mult_expr documents all
problematical cases.
Thanks,
Richard.
>
> #if GIMPLE
> /* Canonicalize X + (X << C) into X * (1 + (1 << C)) and
> diff --git a/gcc/testsuite/gcc.dg/pr109393.c b/gcc/testsuite/gcc.dg/pr109393.c
> new file mode 100644
> index 00000000000..17bf9330796
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/pr109393.c
> @@ -0,0 +1,23 @@
> +/* PR tree-optimization/109393 */
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fdump-tree-optimized" } */
> +
> +int foo(int *a, int j)
> +{
> + int k = j - 1;
> + return a[j - 1] == a[k];
> +}
> +
> +int foo2(int *a, int j)
> +{
> + int k = j - 5;
> + return a[j - 5] == a[k];
> +}
> +
> +int bar(int *a, int j)
> +{
> + int k = j - 1;
> + return (&a[j + 1] - 2) == &a[k];
> +}
> +
> +/* { dg-final { scan-tree-dump-times "return 1;" 3 "optimized" } } */
> \ No newline at end of file
> --
> 2.46.0
>
