[backport from gcc-4.8/trunk r187648 <mikpe@it.uu.se>]

gcc/

2012-05-18  Meador Inge  <meadori@codesourcery.com>

	PR rtl-optimization/53352
	* cse.c (equiv_constant): Ignore paradoxical subregs.

gcc/testsuite/

2012-05-18  Meador Inge  <meadori@codesourcery.com>

	PR rtl-optimization/53352
	* gcc.dg/pr53352.c: New test.

--- gcc-4.6.3/gcc/cse.c.~1~	2011-06-14 17:01:10.000000000 +0200
+++ gcc-4.6.3/gcc/cse.c	2012-06-09 23:04:53.000000000 +0200
@@ -3849,8 +3849,12 @@ equiv_constant (rtx x)
 	    }
 	}
 
-      /* Otherwise see if we already have a constant for the inner REG.  */
+      /* Otherwise see if we already have a constant for the inner REG,
+	 and if that is enough to calculate an equivalent constant for
+	 the subreg.  Note that the upper bits of paradoxical subregs
+	 are undefined, so they cannot be said to equal anything.  */
       if (REG_P (SUBREG_REG (x))
+	  && GET_MODE_SIZE (mode) <= GET_MODE_SIZE (imode)
 	  && (new_rtx = equiv_constant (SUBREG_REG (x))) != 0)
         return simplify_subreg (mode, new_rtx, imode, SUBREG_BYTE (x));
 
--- gcc-4.6.3/gcc/testsuite/gcc.dg/pr53352.c.~1~	1970-01-01 01:00:00.000000000 +0100
+++ gcc-4.6.3/gcc/testsuite/gcc.dg/pr53352.c	2012-06-09 23:04:53.000000000 +0200
@@ -0,0 +1,41 @@
+/* { dg-do run } */
+/* { dg-options "-O1" } */
+
+#include <stdlib.h>
+
+typedef union
+{
+  struct
+  {
+    unsigned char a;
+    unsigned char b;
+    unsigned char c;
+    unsigned char d;
+  } parts;
+  unsigned long whole;
+} T;
+
+T *g_t;
+
+void bar (unsigned long x)
+{
+  if (x != 0)
+    abort ();
+}
+
+int main ()
+{
+  T one;
+  T two;
+  T tmp1, tmp2;
+
+  one.whole = 0xFFE0E0E0UL;
+  two.whole = 0xFF000000UL;
+  tmp1.parts = two.parts;
+  tmp2.parts = one.parts;
+  tmp2.parts.c = tmp1.parts.c;
+  one.parts = tmp2.parts;
+
+  g_t = &one;
+  bar (0);
+}