* tree-ssa-alias.c (count_ptr_derefs): Do not consider

        &PTR->FLD a dereference of PTR.
        * tree-ssa-structalias.c (update_alias_info): Consider &PTR->FLD
        a potential dereference of PTR.

testsuite/ChangeLog

        * gcc.dg/tree-ssa/20050719-1.c: New test.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@102283 138bc75d-0d04-0410-961f-82ee72b054a4

1 files changed, 37 insertions, 2 deletions

diff --git a/gcc/tree-ssa-structalias.c b/gcc/tree-ssa-structalias.c
index 867c6a76364..2a10a8d3d11 100644
--- a/gcc/tree-ssa-structalias.c
+++ b/gcc/tree-ssa-structalias.c
@@ -2555,7 +2555,7 @@ update_alias_info (tree stmt, struct alias_info *ai)
       tree op, var;
       var_ann_t v_ann;
       struct ptr_info_def *pi;
-      bool is_store;
+      bool is_store, is_potential_deref;
       unsigned num_uses, num_derefs;
 
       op = USE_FROM_PTR (use_p);
@@ -2612,7 +2612,42 @@ update_alias_info (tree stmt, struct alias_info *ai)
 	 is an escape point, whether OP escapes.  */
       count_uses_and_derefs (op, stmt, &num_uses, &num_derefs, &is_store);
 
-      if (num_derefs > 0)
+      /* Handle a corner case involving address expressions of the
+	 form '&PTR->FLD'.  The problem with these expressions is that
+	 they do not represent a dereference of PTR.  However, if some
+	 other transformation propagates them into an INDIRECT_REF
+	 expression, we end up with '*(&PTR->FLD)' which is folded
+	 into 'PTR->FLD'.
+
+	 So, if the original code had no other dereferences of PTR,
+	 the aliaser will not create memory tags for it, and when
+	 &PTR->FLD gets propagated to INDIRECT_REF expressions, the
+	 memory operations will receive no V_MAY_DEF/VUSE operands.
+
+	 One solution would be to have count_uses_and_derefs consider
+	 &PTR->FLD a dereference of PTR.  But that is wrong, since it
+	 is not really a dereference but an offset calculation.
+
+	 What we do here is to recognize these special ADDR_EXPR
+	 nodes.  Since these expressions are never GIMPLE values (they
+	 are not GIMPLE invariants), they can only appear on the RHS
+	 of an assignment and their base address is always an
+	 INDIRECT_REF expression.  */
+      is_potential_deref = false;
+      if (TREE_CODE (stmt) == MODIFY_EXPR
+	  && TREE_CODE (TREE_OPERAND (stmt, 1)) == ADDR_EXPR
+	  && !is_gimple_val (TREE_OPERAND (stmt, 1)))
+	{
+	  /* If the RHS if of the form &PTR->FLD and PTR == OP, then
+	     this represents a potential dereference of PTR.  */
+	  tree rhs = TREE_OPERAND (stmt, 1);
+	  tree base = get_base_address (TREE_OPERAND (rhs, 0));
+	  if (TREE_CODE (base) == INDIRECT_REF
+	      && TREE_OPERAND (base, 0) == op)
+	    is_potential_deref = true;
+	}
+
+      if (num_derefs > 0 || is_potential_deref)
 	{
 	  /* Mark OP as dereferenced.  In a subsequent pass,
 	     dereferenced pointers that point to a set of