SUB (a, b) underflows precisely when a < b. Convert (compare (minus (a b)) a)
to (compare (a b)) to match *sub<mode>_3 pattern.
PR target/122518
gcc/ChangeLog:
* config/i386/i386.cc (ix86_canonicalize_comparison): Convert
(compare (minus (a b)) a) to (compare (a b)) to
match *sub<mode>_3 pattern.
gcc/testsuite/ChangeLog:
* gcc.target/i386/pr122518.c: New test.
Bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.
Uros.
diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
index 587b2bd0c1d..6b6febc8870 100644
--- a/gcc/config/i386/i386.cc
+++ b/gcc/config/i386/i386.cc
@@ -598,6 +598,20 @@ ix86_canonicalize_comparison (int *code, rtx *op0, rtx
*op1,
}
}
+ /* SUB (a, b) underflows precisely when a < b. Convert
+ (compare (minus (a b)) a) to (compare (a b))
+ to match *sub<mode>_3 pattern. */
+ if (!op0_preserve_value
+ && (*code == GTU || *code == LEU)
+ && GET_CODE (*op0) == MINUS
+ && rtx_equal_p (XEXP (*op0, 0), *op1))
+ {
+ *op1 = XEXP (*op0, 1);
+ *op0 = XEXP (*op0, 0);
+ *code = (int) swap_condition ((enum rtx_code) *code);
+ return;
+ }
+
/* Swap operands of GTU comparison to canonicalize
addcarry/subborrow comparison. */
if (!op0_preserve_value
diff --git a/gcc/testsuite/gcc.target/i386/pr122518.c
b/gcc/testsuite/gcc.target/i386/pr122518.c
new file mode 100644
index 00000000000..2791889c45d
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr122518.c
@@ -0,0 +1,15 @@
+/* PR target/122518 */
+/* { dg-do compile } */
+/* { dg-options "-O2" } */
+
+inline unsigned min (unsigned a, unsigned b)
+{
+ return (a < b) ? a : b;
+}
+
+unsigned uminsub (unsigned a, unsigned b)
+{
+ return min (a - b, a);
+}
+
+/* { dg-final { scan-assembler-not "cmp" } } */
