[PATCH] match: X % Y < Y -> Y >= 0 and X % Y >= Y -> Y < 0 [PR125737]

[Kael Andrew Franco]

>From 5ff3dfadc6294f452aa8332dfdd9663dfbe4db0f Mon Sep 17 00:00:00 2001
From: Kael Andrew Alonzo Franco <kaelfand...@gmail.com>
Date: Tue, 23 Jun 2026 14:34:29 -0400
Subject: [PATCH] match: X % Y < Y -> Y >= 0 and X % Y >= Y -> Y < 0 [PR125737]

LLVM trunk optimizes those patterns so add those patterns to GCC.

Bootstrapped and tested on x86_64-pc-linux-gnu

        PR tree-optimization/125737

gcc/ChangeLog:

        PR tree-optimization/125737
        * match.pd: Optimize X % Y < Y -> Y >= 0 and X % Y >= Y -> Y < 0

gcc/testsuite/ChangeLog:

        PR tree-optimization/125737
        * gcc.dg/pr125737.c: Move test to pr125737-1.c.
        * gcc.dg/pr125737-1.c: Add test for (a % b) >= b when a and b are 
nonnegative.
        * gcc.dg/pr125737-2.c: New test for (a % b) {<,>=} b.

Signed-off-by: Kael Franco <kaelfand...@gmail.com>
---
 gcc/match.pd                                      | 10 ++++++----
 gcc/testsuite/gcc.dg/{pr125737.c => pr125737-1.c} | 10 +++++++++-
 gcc/testsuite/gcc.dg/pr125737-2.c                 | 14 ++++++++++++++
 3 files changed, 29 insertions(+), 5 deletions(-)
 rename gcc/testsuite/gcc.dg/{pr125737.c => pr125737-1.c} (51%)
 create mode 100644 gcc/testsuite/gcc.dg/pr125737-2.c

diff --git a/gcc/match.pd b/gcc/match.pd
index 8c410c2f3b3..7319be803f5 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -1730,11 +1730,13 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
      (convert (bit_and (negate (convert:utype { pmop[0]; }))
                       (convert:utype @1)))))))

-/* X % Y is smaller than Y.  */
-(for cmp (lt ge)
+/* X % Y < Y -> Y >= 0
+   X % Y >= Y -> Y < 0  */
+(for ltge (lt ge)
+     gelt (ge lt)
  (simplify
-  (cmp:c (trunc_mod tree_expr_nonnegative_p@0 tree_expr_nonnegative_p@1) @1)
-  { constant_boolean_node (cmp == LT_EXPR, type); }))
+  (ltge:c (trunc_mod @0 @1) @1)
+  (gelt @1 { build_zero_cst (TREE_TYPE (@1)); })))

 /* x | ~0 -> ~0  */
 (simplify
diff --git a/gcc/testsuite/gcc.dg/pr125737.c b/gcc/testsuite/gcc.dg/pr125737-1.c
similarity index 51%
rename from gcc/testsuite/gcc.dg/pr125737.c
rename to gcc/testsuite/gcc.dg/pr125737-1.c
index 93d0fcd215a..85347a63c7f 100644
--- a/gcc/testsuite/gcc.dg/pr125737.c
+++ b/gcc/testsuite/gcc.dg/pr125737-1.c
@@ -9,4 +9,12 @@ f (unsigned short a, unsigned short b) {
   return c < bb;
 }

-/* { dg-final { scan-tree-dump-times "return 1;" 1 "optimized" } } */
+int
+g (unsigned short a, unsigned short b) {
+  int aa = a;
+  int bb = b;
+  int c = aa % bb;
+  return (c >= bb) == 0;
+}
+
+/* { dg-final { scan-tree-dump-times "return 1;" 2 "optimized" } } */
diff --git a/gcc/testsuite/gcc.dg/pr125737-2.c
b/gcc/testsuite/gcc.dg/pr125737-2.c
new file mode 100644
index 00000000000..0090fa04ca8
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr125737-2.c
@@ -0,0 +1,14 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+_Bool
+a_mod_b_lt_b (int a, int b) {
+  return ((a % b) < b) == (b >= 0);
+}
+
+_Bool
+a_mod_b_ge_b (int a, int b) {
+  return ((a % b) >= b) == (b < 0);
+}
+
+/* { dg-final { scan-tree-dump-times "return 1;" 2 "optimized" } } */
-- 
2.54.0