[PATCH][4.2] Work around PR31136

[Richard Guenther]

This patch works around the fact that FRE does constant propagation
without folding the result.  This may cause us to call fold_unary
with (<unnamed-unsigned:6>)(<unnamed-unsigned:4>) 31:6 which is folded
incorrectly by fold_convert_const which we ask to convert
31:6 to (<unnamed-unsigned:6>).  The patch fixes that by asking it to
fold (<unnamed-unsigned:4>) 31:6 instead - a no-op patch in case we'd
have folded arguments to fold_unary.

Bootstrapped and tested on x86_64-unknown-linux-gnu, applied to the
mainline and the 4.2 branch.

Richard.

2007-04-21  Richard Guenther  <rguenther@suse.de>

	PR middle-end/31136
	* fold-const.c (fold_unary): Call fold_convert_const on the
	original tree.

	* gcc.c-torture/execute/pr31136.c: New testcase.

Index: fold-const.c
===================================================================
*** fold-const.c	(revision 124018)
--- fold-const.c	(working copy)
*************** fold_unary (enum tree_code code, tree ty
*** 7734,7740 ****
  	    return fold_build1 (BIT_NOT_EXPR, type, fold_convert (type, tem));
  	}
  
!       tem = fold_convert_const (code, type, arg0);
        return tem ? tem : NULL_TREE;
  
      case VIEW_CONVERT_EXPR:
--- 7734,7740 ----
  	    return fold_build1 (BIT_NOT_EXPR, type, fold_convert (type, tem));
  	}
  
!       tem = fold_convert_const (code, type, op0);
        return tem ? tem : NULL_TREE;
  
      case VIEW_CONVERT_EXPR:
Index: testsuite/gcc.c-torture/execute/pr31136.c
===================================================================
*** testsuite/gcc.c-torture/execute/pr31136.c	(revision 0)
--- testsuite/gcc.c-torture/execute/pr31136.c	(revision 0)
***************
*** 0 ****
--- 1,17 ----
+ extern void abort (void);
+ 
+ struct S {
+   unsigned b4:4;
+   unsigned b6:6;
+ } s;
+ 
+ int main()
+ {
+   s.b6 = 31;
+   s.b4 = s.b6;
+   s.b6 = s.b4;
+   if (s.b6 != 15)
+     abort ();
+   return 0;
+ }
+