(subst, case PLUS): Simplify (plus (comp A B) -1), etc.

From-SVN: r4853

diff --git a/gcc/combine.c b/gcc/combine.c
index fb63387b4f11d66457e2332db46cf3b15bf4ee45..b91558a9832136708a0dbfebdc517b5b819d0ce9 100644 (file)
--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -3394,7 +3394,22 @@ subst (x, from, to, in_dest, unique_copy)
          goto restart;
        }
 
-      /* If only the low-order bit of X is possible nonzero, (plus x -1)
+      /* (plus (comparison A B) C) can become (neg (rev-comp A B)) if
+        C is 1 and STORE_FLAG_VALUE is -1 or if C is -1 and STORE_FLAG_VALUE
+        is 1.  This produces better code than the alternative immediately
+        below.  */
+      if (GET_RTX_CLASS (GET_CODE (XEXP (x, 0))) == '<'
+         && reversible_comparison_p (XEXP (x, 0))
+         && ((STORE_FLAG_VALUE == -1 && XEXP (x, 1) == const1_rtx)
+             || (STORE_FLAG_VALUE == 1 && XEXP (x, 1) == constm1_rtx)))
+       {
+         x = gen_binary (reverse_condition (GET_CODE (XEXP (x, 0))),
+                         mode, XEXP (XEXP (x, 0), 0), XEXP (XEXP (x, 0), 1));
+         x = gen_unary (NEG, mode, x);
+         goto restart;
+       }
+
+      /* If only the low-order bit of X is possibly nonzero, (plus x -1)
         can become (ashiftrt (ashift (xor x 1) C) C) where C is
         the bitsize of the mode - 1.  This allows simplification of
         "a = (b & 8) == 0;"  */