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


On Mon, Mar 04, 2013 at 09:08:36PM +0000, Richard Sandiford wrote:
> ...truncations can't be to a narrower mode (already asserted at the top
> of the function) so in some ways I think it would be clearer to have an
> "if ... else" rather than two "if"s.

So like this instead?

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

	PR rtl-optimization/56494
	* simplify-rtx.c (simplify_truncation): If C is narrower than A,
	optimize (truncate:A (subreg:B (truncate:C X) 0)) into
	(subreg:A (truncate:C X) 0) instead of (truncate:A X).

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

--- gcc/simplify-rtx.c.jj	2013-03-04 12:10:39.007677516 +0100
+++ gcc/simplify-rtx.c	2013-03-04 22:23:49.450894108 +0100
@@ -757,8 +757,17 @@ 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));
+      else
+	/* If subreg above is paradoxical and C is narrower
+	   than A, return (subreg:A (truncate:C X) 0).  */
+	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 22:23:10.016124685 +0100
+++ gcc/testsuite/gcc.dg/pr56494.c	2013-03-04 22:23:10.016124685 +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


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