PR 41952 -- simple folding rule

[Xinliang David Li]

Hi, the patch is to used to delete redundant comparisons and branches
resulting from inlining. While it does not address any particular bug
in 4.5, it is safe to be considered.

Tested: bootstrap and regression tested: x86_64/linux (with 4.5).

Ok to get in?

Thanks,

David

[fold.patch]

---

Index: fold-const.c
===================================================================
--- fold-const.c (revision 153643)
+++ fold-const.c (working copy)
@@ -9572,6 +9572,26 @@ fold_comparison (location_t loc, enum tr
       return fold_build2_loc (loc, cmp_code, type, variable1, const2);
     }
 
+  /* Fold comparison using aliasing guarantee:
+     &local_var != parameter_default
+     &local_var == parameter_default
+
+     A local variable can never be pointed to by
+     the default SSA name of an incoming parameter.  */
+
+  if (TREE_CODE (arg0) == ADDR_EXPR
+      && TREE_CODE (TREE_OPERAND (arg0, 0)) == VAR_DECL
+      && auto_var_in_fn_p (TREE_OPERAND (arg0, 0), current_function_decl)
+      && TREE_CODE (arg1) == SSA_NAME
+      && TREE_CODE (SSA_NAME_VAR (arg1)) == PARM_DECL
+      && SSA_NAME_IS_DEFAULT_DEF (arg1))
+    {
+      if (code == NE_EXPR)
+        return constant_boolean_node (1, type);
+      else if (code == EQ_EXPR)
+        return constant_boolean_node (0, type);
+    }
+
   tem = maybe_canonicalize_comparison (loc, code, type, op0, op1);
   if (tem)
     return tem;
Index: testsuite/g++.dg/tree-ssa/fold-compare.C
===================================================================
--- testsuite/g++.dg/tree-ssa/fold-compare.C (revision 0)
+++ testsuite/g++.dg/tree-ssa/fold-compare.C (revision 0)
@@ -0,0 +1,27 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+struct ExtentsBase {
+ ExtentsBase() : startx_(), endx_() { }
+ ExtentsBase(const ExtentsBase &b) {
+  *this = b;
+ }
+
+ const ExtentsBase & operator=(const ExtentsBase &b) {
+  if (this != &b) {
+    startx_ = b.startx_;
+  }
+  return *this;
+ }
+
+ int startx_;
+  int endx_;
+};
+
+int f(const ExtentsBase &e1) {
+ ExtentsBase my_extents = e1;
+ return my_extents.startx_;
+}
+
+/* { dg-final { scan-tree-dump-not "&my_extents" "optimized" } } */
+/* { dg-final { cleanup-tree-dump "optimized" } } */

---