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[v2] match: Change (A * B) + (-C) to (B - C/A) * A, if C multiple of A [PR109393]

Message ID 20240917074215.2978559-1-konstantinos.eleftheriou@vrull.eu
State New
Headers show
Series [v2] match: Change (A * B) + (-C) to (B - C/A) * A, if C multiple of A [PR109393] | expand

Commit Message

Konstantinos Eleftheriou Sept. 17, 2024, 7:42 a.m. UTC 
From: kelefth <konstantinos.eleftheriou@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.muellner@vrull.eu>
Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
Signed-off-by: Konstantinos Eleftheriou <konstantinos.eleftheriou@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
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Patch

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