match.pd: Remove "+ 0x80000000" in int comparisons [PR94899]

Expressions of the form "X + CST < Y + CST" where:

* CST is an unsigned integer constant with only the MSB set, and
* X and Y's types have integer conversion ranks <= CST's

can be simplified to "(signed) X < (signed) Y".

This is because, assuming a 32-bit signed numbers,
(unsigned) INT_MIN + 0x80000000 is 0, and
(unsigned) INT_MAX + 0x80000000 is UINT_MAX.

i.e. the result increases monotonically with signed input.

This means:
((signed) X < (signed) Y) iff (X + 0x80000000 < Y + 0x80000000)

gcc/
	PR tree-optimization/94899
	* match.pd (X + C < Y + C -> (signed) X < (signed) Y, if C is
	0x80000000): New simplification.
gcc/testsuite/
	* gcc.dg/pr94899.c: New test.

2 files changed, 62 insertions, 0 deletions

diff --git a/gcc/match.pd b/gcc/match.pd
index a63b649..4a57089 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -2089,6 +2089,19 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
   (if (ANY_INTEGRAL_TYPE_P (TREE_TYPE (@0))
        && TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (@0)))
    (op @0 @1))))
+
+/* As a special case, X + C < Y + C is the same as (signed) X < (signed) Y
+   when C is an unsigned integer constant with only the MSB set, and X and
+   Y have types of equal or lower integer conversion rank than C's.  */
+(for op (lt le ge gt)
+ (simplify
+  (op (plus @1 INTEGER_CST@0) (plus @2 @0))
+  (if (INTEGRAL_TYPE_P (TREE_TYPE (@0))
+       && TYPE_UNSIGNED (TREE_TYPE (@0))
+       && wi::only_sign_bit_p (wi::to_wide (@0)))
+   (with { tree stype = signed_type_for (TREE_TYPE (@0)); }
+    (op (convert:stype @1) (convert:stype @2))))))
+
 /* For equality and subtraction, this is also true with wrapping overflow.  */
 (for op (eq ne minus)
  (simplify
diff --git a/gcc/testsuite/gcc.dg/pr94899.c b/gcc/testsuite/gcc.dg/pr94899.c
new file mode 100644
index 0000000..9fad057
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr94899.c
@@ -0,0 +1,49 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+typedef __INT16_TYPE__ int16_t;
+typedef __INT32_TYPE__ int32_t;
+typedef __UINT16_TYPE__ uint16_t;
+typedef __UINT32_TYPE__ uint32_t;
+
+#define MAGIC (~ (uint32_t) 0 / 2 + 1)
+
+int
+f_i16_i16 (int16_t x, int16_t y)
+{
+  return x + MAGIC < y + MAGIC;
+}
+
+int
+f_i16_i32 (int16_t x, int32_t y)
+{
+  return x + MAGIC < y + MAGIC;
+}
+
+int
+f_i32_i32 (int32_t x, int32_t y)
+{
+  return x + MAGIC < y + MAGIC;
+}
+
+int
+f_u32_i32 (uint32_t x, int32_t y)
+{
+  return x + MAGIC < y + MAGIC;
+}
+
+int
+f_u32_u32 (uint32_t x, uint32_t y)
+{
+  return x + MAGIC < y + MAGIC;
+}
+
+int
+f_i32_i32_sub (int32_t x, int32_t y)
+{
+  return x - MAGIC < y - MAGIC;
+}
+
+/* The addition/subtraction of constants should be optimized away.  */
+/* { dg-final { scan-tree-dump-not " \\+ " "optimized"} } */
+/* { dg-final { scan-tree-dump-not " \\- " "optimized"} } */