Thanks for the feedback.
I have sent a new version (
https://gcc.gnu.org/pipermail/gcc-patches/2024-September/663104.html).
On Tue, Sep 10, 2024 at 2:53 PM Richard Biener <richard.guent...@gmail.com>
wrote:
> On Fri, Sep 6, 2024 at 2:44 PM <konstantinos.elefther...@vrull.eu> wrote:
> >
> > From: kelefth <konstantinos.elefther...@vrull.eu>
> >
> > 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 <christoph.muell...@vrull.eu>
> > Signed-off-by: Philipp Tomsich <philipp.toms...@vrull.eu>
> > Signed-off-by: Konstantinos Eleftheriou <
> konstantinos.elefther...@vrull.eu>
> > ---
> > 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
> >
>
