i386: Canonicalize (compare (minus (a b)) a) to (compare (a b)) [PR122518]

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.

diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
index 587b2bd0c1d2d057d707f84cb4c7f6a1f5fcc91b..6b6febc88709d9396d3c091a1ba1d825f518e520 100644 (file)
--- 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 (file)
index 0000000..2791889
--- /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" } } */