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/
* match.pd (X + C < Y + C -> (signed) X < (signed) Y, if C is
0x80000000): New simplification.
gcc/testsuite/
* gcc.dg/pr94899.c: New test.
---
gcc/match.pd | 13 +++++++++
gcc/testsuite/gcc.dg/pr94899.c | 48 ++++++++++++++++++++++++++++++++++
2 files changed, 61 insertions(+)
create mode 100644 gcc/testsuite/gcc.dg/pr94899.c
---
v2: https://gcc.gnu.org/pipermail/gcc-patches/2022-February/589709.html
Notes on v3, based on Richard and Jakub's review comments:
1. Canonicalized the match expression to avoid having to use ":c".
2. Redefined MAGIC in the test to avoid running afoul of 16-bit int
machines.
Richard has approved this patch for inclusion in GCC 13:
https://gcc.gnu.org/pipermail/gcc-patches/2022-February/589852.html
diff --git a/gcc/match.pd b/gcc/match.pd
index 3e9572e4c9c..ef42611854a 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -2080,6 +2080,19 @@ 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 INTEGER_CST@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 00000000000..685201307ec
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr94899.c
@@ -0,0 +1,48 @@
+/* { 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 constants above should have been optimized away. */
+/* { dg-final { scan-tree-dump-times "2147483648" 0 "optimized"} } */
--
2.35.3
