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 | 21 ++++++++++++++++++++-
gcc/testsuite/gcc.dg/pr109393.c | 23 +++++++++++++++++++++++
2 files changed, 43 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/gcc.dg/pr109393.c
diff --git a/gcc/match.pd b/gcc/match.pd
index 5566c0e4c41..6aeb92cdad0 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -4245,7 +4245,26 @@ 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)))))
+ /* (A * B) + (-C) -> (B - C/A) * A, if C is a multiple of A. */
+ (if (!ALL_FRACT_MODE_P (TYPE_MODE (type)))
+ (simplify
+ (plus (mult:cs integer_nonzerop@0 @1) INTEGER_CST@2)
+ /* Exclude the case that @2 == min to prevent UB when calculating abs
+ and (B - C/A). */
+ (if (TREE_CODE (type) == INTEGER_TYPE
+ && wi::neg_p (wi::to_wide (@2))
+ && wi::to_wide (@2) != wi::min_value (TYPE_PRECISION (type), SIGNED))
+ (with {
+ wide_int c0 = wi::to_wide (@0);
+ wide_int c2 = wi::to_wide (@2);
+ wide_int c2_abs = wi::abs (c2); }
+ (if (wi::multiple_of_p (c2_abs, c0, TYPE_SIGN (type)))
+ (with {
+ /* Calculate @2 / @0 in order to factorize the expression. */
+ wide_int div_res = wi::sdiv_trunc (c2, c0);
+ tree div_cst = wide_int_to_tree (type, div_res); }
+ (mult (plus @1 { div_cst; }) @0))))))))
#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
