[PATCH] Fix points-to analysis wrt union accesses, PR59287

[Richard Biener <rguenther at suse dot de>]

When looking at the code generated for wide-int idioms on the branch
again I noticed that we fail to eliminate quite some dead stores
(on GIMPLE, eventually we recover from that at RTL level - I didn't
check).  This is caused by some old special-casing of union
accesses during points-to analysis which eventually causes
'anything' to escape nearly everywhere in GCC (meh...).  It turns
out the ancient special-case for union is no longer necessary
as we now properly transfer pointers through all sorts of weird
obfuscation.

Thus, fixed with the following - bootstrapped and tested on 
x86_64-unknown-linux-gnu and applied to trunk.

Richard.

2013-11-26  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/59287
	* tree-ssa-structalias.c (get_constraint_for_component_ref):
	Remove no longer necessary special-casing of union accesses.

	* gcc.dg/tree-ssa/alias-29.c: New testcase.

Index: gcc/tree-ssa-structalias.c
===================================================================
*** gcc/tree-ssa-structalias.c	(revision 205344)
--- gcc/tree-ssa-structalias.c	(working copy)
*************** get_constraint_for_component_ref (tree t
*** 3163,3191 ****
        return;
      }
  
-   /* Handle type-punning through unions.  If we are extracting a pointer
-      from a union via a possibly type-punning access that pointer
-      points to anything, similar to a conversion of an integer to
-      a pointer.  */
-   if (!lhs_p)
-     {
-       tree u;
-       for (u = t;
- 	   TREE_CODE (u) == COMPONENT_REF || TREE_CODE (u) == ARRAY_REF;
- 	   u = TREE_OPERAND (u, 0))
- 	if (TREE_CODE (u) == COMPONENT_REF
- 	    && TREE_CODE (TREE_TYPE (TREE_OPERAND (u, 0))) == UNION_TYPE)
- 	  {
- 	    struct constraint_expr temp;
- 
- 	    temp.offset = 0;
- 	    temp.var = anything_id;
- 	    temp.type = ADDRESSOF;
- 	    results->safe_push (temp);
- 	    return;
- 	  }
-     }
- 
    t = get_ref_base_and_extent (t, &bitpos, &bitsize, &bitmaxsize);
  
    /* Pretend to take the address of the base, we'll take care of
--- 3163,3168 ----
Index: gcc/testsuite/gcc.dg/tree-ssa/alias-29.c
===================================================================
*** gcc/testsuite/gcc.dg/tree-ssa/alias-29.c	(revision 0)
--- gcc/testsuite/gcc.dg/tree-ssa/alias-29.c	(working copy)
***************
*** 0 ****
--- 1,27 ----
+ /* { dg-do compile } */
+ /* { dg-options "-O -fdump-tree-optimized" } */
+ 
+ union X {
+     int i;
+     void *p;
+ };
+ void bar (int);
+ 
+ int * __attribute__((noinline,noclone))
+ baz (int *p) { return p; }
+ 
+ void foo (union X *x)
+ {
+   struct Y { int i; } ystruct = {};
+   ystruct.i = * baz (&ystruct.i);
+   bar (x->i);
+ }
+ 
+ /* DSE and then DCE should be able to remove all uses of ystruct.
+    Formerly the union access for the parameter to bar let 'anything'
+    escape which made the call to bar possibly use ystruct and thus
+    prevent the store to ystruct.i from being eliminated.  The call to
+    baz makes sure that ystruct has its address taken.  */
+ 
+ /* { dg-final { scan-tree-dump-not "ystruct" "optimized" } } */
+ /* { dg-final { cleanup-tree-dump "optimized" } } */