[backport from gcc-4.7/trunk r183752 <mikpe@it.uu.se>]

gcc/

2012-01-31  Richard Guenther  <rguenther@suse.de>

	PR tree-optimization/51528
	* tree-sra.c (sra_modify_assign): Avoid copy-in/out for aggregate
	assigns.

gcc/testsuite/

2012-01-31  Richard Guenther  <rguenther@suse.de>

	PR tree-optimization/51528
	* gcc.dg/torture/pr51528.c: New testcase.

--- gcc-4.6.2/gcc/testsuite/gcc.dg/torture/pr51528.c.~1~	1970-01-01 01:00:00.000000000 +0100
+++ gcc-4.6.2/gcc/testsuite/gcc.dg/torture/pr51528.c	2012-02-05 21:47:04.000000000 +0100
@@ -0,0 +1,46 @@
+/* { dg-do run } */
+/* { dg-options "-fno-early-inlining" } */
+
+extern void abort (void);
+
+union U
+{
+  int i;
+  _Bool b;
+};
+
+_Bool gb;
+
+void  __attribute__ ((noinline))
+use_bool (union U u)
+{
+  gb = u.b;
+}
+
+union U
+bar (void)
+{
+  union U u;
+  u.i = 0xFFFE;
+  return u;
+}
+
+union U  __attribute__ ((noinline))
+foo (void)
+{
+  union U u,v;
+
+  u.b = 1;
+  use_bool (u);
+  u = bar ();
+
+  return u;
+}
+
+int main (int argc, char **argv)
+{
+  union U u = foo ();
+  if (u.i != 0xFFFE)
+    abort ();
+  return 0;
+}
--- gcc-4.6.2/gcc/tree-sra.c.~1~	2012-02-05 21:45:41.000000000 +0100
+++ gcc-4.6.2/gcc/tree-sra.c	2012-02-05 21:47:04.000000000 +0100
@@ -2934,9 +2934,14 @@ sra_modify_assign (gimple *stmt, gimple_
 	      sra_stats.deleted++;
 	      return SRA_AM_REMOVED;
 	    }
+	  /* Restore the aggregate RHS from its components so the
+	     prevailing aggregate copy does the right thing.  */
 	  if (access_has_children_p (racc))
-	    generate_subtree_copies (racc->first_child, lhs, racc->offset,
-				     0, 0, gsi, false, true, loc);
+	    generate_subtree_copies (racc->first_child, racc->base, 0, 0, 0,
+				     gsi, false, false, loc);
+	  /* Re-load the components of the aggregate copy destination.
+	     But use the RHS aggregate to load from to expose more
+	     optimization opportunities.  */
 	  if (access_has_children_p (lacc))
 	    generate_subtree_copies (lacc->first_child, rhs, lacc->offset,
 				     0, 0, gsi, true, true, loc);