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

gcc/

2011-12-22  Bin Cheng  <bin.cheng@arm.com>
	Richard Guenther  <rguenther@suse.de>

	PR tree-optimization/43491
	* tree-ssa-pre.c (eliminate): Don't replace global register
	variable when it is the RHS of a single assign.

gcc/testsuite/

2011-12-22  Bin Cheng  <bin.cheng@arm.com>

	PR tree-optimization/43491
	* gcc.dg/tree-ssa/pr43491.c: New test.

--- gcc-4.6.2/gcc/testsuite/gcc.dg/tree-ssa/pr43491.c.~1~	1970-01-01 01:00:00.000000000 +0100
+++ gcc-4.6.2/gcc/testsuite/gcc.dg/tree-ssa/pr43491.c	2011-12-23 16:03:42.000000000 +0100
@@ -0,0 +1,42 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-pre-stats" } */
+
+#define REGISTER register
+
+#if defined __arm__
+# define REG1 asm("r4")
+#elif defined __i386__
+# define REG1 asm("ebx")
+#elif defined __mips__
+# define REG1 asm("s0")
+#elif defined __x86_64__
+# define REG1 asm("rbp")
+#else
+# undef REGISTER
+# define REGISTER
+# define REG1
+#endif
+
+REGISTER long data_0 REG1;
+long data_3; 
+
+long foo(long data, long v)
+{
+	long i;
+	long t, u;
+
+	if (data)
+		i = data_0 + data_3;
+	else {
+		v = 2;
+		i = 5;
+	}
+	t = data_0 + data_3;
+	u = i;
+	return v * t * u;
+}
+/* We should not eliminate global register variable when it is the RHS of
+   a single assignment.  */
+/* { dg-final { scan-tree-dump-times "Eliminated: 2" 1 "pre" { target { arm-*-* i?86-*-* mips*-*-* x86_64-*-* } } } } */
+/* { dg-final { scan-tree-dump-times "Eliminated: 3" 1 "pre" { target { ! { arm-*-* i?86-*-* mips*-*-* x86_64-*-* } } } } } */
+/* { dg-final { cleanup-tree-dump "pre" } } */
--- gcc-4.6.2/gcc/tree-ssa-pre.c.~1~	2011-07-16 13:33:28.000000000 +0200
+++ gcc-4.6.2/gcc/tree-ssa-pre.c	2011-12-23 16:03:42.000000000 +0100
@@ -4195,28 +4195,40 @@ eliminate (void)
     {
       for (gsi = gsi_start_bb (b); !gsi_end_p (gsi); gsi_next (&gsi))
 	{
+	  tree lhs = NULL_TREE;
+	  tree rhs = NULL_TREE;
+
 	  stmt = gsi_stmt (gsi);
 
+	  if (gimple_has_lhs (stmt))
+	    lhs = gimple_get_lhs (stmt);
+
+	  if (gimple_assign_single_p (stmt))
+	    rhs = gimple_assign_rhs1 (stmt);
+
 	  /* Lookup the RHS of the expression, see if we have an
 	     available computation for it.  If so, replace the RHS with
-	     the available computation.  */
+	     the available computation.
+
+	     See PR43491.
+	     We don't replace global register variable when it is a the RHS of
+	     a single assign. We do replace local register variable since gcc
+	     does not guarantee local variable will be allocated in register.  */
 	  if (gimple_has_lhs (stmt)
-	      && TREE_CODE (gimple_get_lhs (stmt)) == SSA_NAME
+	      && TREE_CODE (lhs) == SSA_NAME
 	      && !gimple_assign_ssa_name_copy_p (stmt)
 	      && (!gimple_assign_single_p (stmt)
-		  || !is_gimple_min_invariant (gimple_assign_rhs1 (stmt)))
+		  || (!is_gimple_min_invariant (rhs)
+                      && (gimple_assign_rhs_code (stmt) != VAR_DECL
+                          || !is_global_var (rhs)
+                          || !DECL_HARD_REGISTER (rhs))))
 	      && !gimple_has_volatile_ops  (stmt)
-	      && !has_zero_uses (gimple_get_lhs (stmt)))
+	      && !has_zero_uses (lhs))
 	    {
-	      tree lhs = gimple_get_lhs (stmt);
-	      tree rhs = NULL_TREE;
 	      tree sprime = NULL;
 	      pre_expr lhsexpr = get_or_alloc_expr_for_name (lhs);
 	      pre_expr sprimeexpr;
 
-	      if (gimple_assign_single_p (stmt))
-		rhs = gimple_assign_rhs1 (stmt);
-
 	      sprimeexpr = bitmap_find_leader (AVAIL_OUT (b),
 					       get_expr_value_id (lhsexpr),
 					       NULL);
@@ -4332,10 +4344,9 @@ eliminate (void)
 	     dead.  */
 	  else if (gimple_assign_single_p (stmt)
 		   && !is_gimple_reg (gimple_assign_lhs (stmt))
-		   && (TREE_CODE (gimple_assign_rhs1 (stmt)) == SSA_NAME
-		       || is_gimple_min_invariant (gimple_assign_rhs1 (stmt))))
+		   && (TREE_CODE (rhs) == SSA_NAME
+		       || is_gimple_min_invariant (rhs)))
 	    {
-	      tree rhs = gimple_assign_rhs1 (stmt);
 	      tree val;
 	      val = vn_reference_lookup (gimple_assign_lhs (stmt),
 					 gimple_vuse (stmt), VN_WALK, NULL);