[PATCH] Fix up simplify_truncation (PR rtl-optimization/56494)

[Jakub Jelinek]

Hi!

Optimizing (truncate:A (subreg:B (truncate:C X) 0)) into
(truncate:A X) is wrong if C mode is narrower than A mode.
The following patch fixes that and simplifies it into
(subreg:A (truncate:C X 0)) instead.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

2013-03-04  Jakub Jelinek  <ja...@redhat.com>

        PR rtl-optimization/56494
        * simplify-rtx.c (simplify_truncation): Don't optimize
        (truncate:A (subreg:B (truncate:C X) 0)) into (truncate:A X)
        if C is narrower than A.  If subreg is paradoxical, optimize
        it into (subreg:A (truncate:C X) 0) instead.

        * gcc.dg/pr56494.c: New test.

--- gcc/simplify-rtx.c.jj       2013-02-12 11:23:37.000000000 +0100
+++ gcc/simplify-rtx.c  2013-03-04 11:32:44.111721542 +0100
@@ -757,8 +757,18 @@ simplify_truncation (enum machine_mode m
       && SCALAR_INT_MODE_P (GET_MODE (SUBREG_REG (op)))
       && GET_CODE (SUBREG_REG (op)) == TRUNCATE
       && subreg_lowpart_p (op))
-    return simplify_gen_unary (TRUNCATE, mode, XEXP (SUBREG_REG (op), 0),
-                              GET_MODE (XEXP (SUBREG_REG (op), 0)));
+    {
+      rtx inner = XEXP (SUBREG_REG (op), 0);
+      if (GET_MODE_PRECISION (mode)
+         <= GET_MODE_PRECISION (GET_MODE (SUBREG_REG (op))))
+       return simplify_gen_unary (TRUNCATE, mode, inner, GET_MODE (inner));
+      /* If subreg above is paradoxical and C is narrower
+        than A, return (subreg:A (truncate:C X) 0).  */
+      if (GET_MODE_PRECISION (op_mode)
+         > GET_MODE_PRECISION (GET_MODE (SUBREG_REG (op))))
+       return simplify_gen_subreg (mode, SUBREG_REG (op),
+                                   GET_MODE (SUBREG_REG (op)), 0);
+    }
 
   /* (truncate:A (truncate:B X)) is (truncate:A X).  */
   if (GET_CODE (op) == TRUNCATE)
--- gcc/testsuite/gcc.dg/pr56494.c.jj   2013-03-04 11:37:16.285155928 +0100
+++ gcc/testsuite/gcc.dg/pr56494.c      2013-03-04 11:36:59.000000000 +0100
@@ -0,0 +1,13 @@
+/* PR rtl-optimization/56494 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -ftracer -w" } */
+
+char a;
+short b;
+void bar (int);
+
+void
+foo (void)
+{
+  bar ((!!b ? : (a *= a / 0)) >= (a = b));
+}

        Jakub