Re: C++ PATCH for c++/70744 (wrong-code with x ?: y extension)

[Marek Polacek <polacek at redhat dot com>]

On Fri, Apr 22, 2016 at 09:43:31AM -0400, Jason Merrill wrote:
> On 04/22/2016 07:50 AM, Marek Polacek wrote:
> >This PR shows that we generate wrong code with the x ?: y extension in case the
> >first operand contains either predecrement or preincrement.  The problem is
> >that we don't emit SAVE_EXPR, thus the operand is evaluated twice, which it
> >should not be.
> >
> >While ++i or --i can be lvalues in C++, i++ or i-- can not.  The code in
> >build_conditional_expr_1 has:
> >  4635       /* Make sure that lvalues remain lvalues.  See g++.oliva/ext1.C.  */
> >  4636       if (real_lvalue_p (arg1))
> >  4637         arg2 = arg1 = stabilize_reference (arg1);
> >  4638       else
> >  4639         arg2 = arg1 = save_expr (arg1);
> >so for ++i/--i we call stabilize_reference, but that doesn't know anything
> >about PREINCREMENT_EXPR or PREDECREMENT_EXPR and just returns the same
> >expression, so SAVE_EXPR isn't created.
> >
> >I think one fix would be to teach stabilize_reference what to do with those,
> >similarly to how we handle COMPOUND_EXPR there.
> 
> Your change will turn the expression into an rvalue, so that isn't enough.

Oops.

> We need to turn the expression into some sort of _REF before passing it to
> stabilize_reference, perhaps by factoring out the cast-to-reference code
> from force_paren_expr.  This should probably be part of a
> cp_stabilize_reference function that replaces all uses of
> stabilize_reference in the front end.

Thanks, this magic seems to work.  So something like the following?  I didn't
put the cast-to-reference code into its separate function because it didn't
seem convenient, but I can work on that, too, if you prefer.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2016-04-22  Marek Polacek  <polacek@redhat.com>

	PR c++/70744
	* call.c (build_conditional_expr_1): Call cp_stabilize_reference
	instead of stabilize_reference.
	(build_over_call): Likewise.
	* cp-tree.h (cp_stabilize_reference): Declare.
	* tree.c (cp_stabilize_reference): New function.
	* typeck.c (cp_build_unary_op): Call cp_stabilize_reference instead of
	stabilize_reference.
	(unary_complex_lvalue): Likewise.
	(cp_build_modify_expr): Likewise.

	* g++.dg/ext/cond2.C: New test.

diff --git gcc/cp/call.c gcc/cp/call.c
index 11f2d42..476e806 100644
--- gcc/cp/call.c
+++ gcc/cp/call.c
@@ -4634,7 +4634,7 @@ build_conditional_expr_1 (location_t loc, tree arg1, tree arg2, tree arg3,
 
       /* Make sure that lvalues remain lvalues.  See g++.oliva/ext1.C.  */
       if (real_lvalue_p (arg1))
-	arg2 = arg1 = stabilize_reference (arg1);
+	arg2 = arg1 = cp_stabilize_reference (arg1);
       else
 	arg2 = arg1 = save_expr (arg1);
     }
@@ -7644,8 +7644,9 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
 	       || (TREE_CODE (arg) == TARGET_EXPR
 		   && !unsafe_copy_elision_p (fa, arg)))
 	{
-	  tree to = stabilize_reference (cp_build_indirect_ref (fa, RO_NULL,
-								complain));
+	  tree to = cp_stabilize_reference (cp_build_indirect_ref (fa,
+								   RO_NULL,
+								   complain));
 
 	  val = build2 (INIT_EXPR, DECL_CONTEXT (fn), to, arg);
 	  return val;
@@ -7655,7 +7656,7 @@ build_over_call (struct z_candidate *cand, int flags, tsubst_flags_t complain)
 	   && trivial_fn_p (fn)
 	   && !DECL_DELETED_FN (fn))
     {
-      tree to = stabilize_reference
+      tree to = cp_stabilize_reference
 	(cp_build_indirect_ref (argarray[0], RO_NULL, complain));
       tree type = TREE_TYPE (to);
       tree as_base = CLASSTYPE_AS_BASE (type);
diff --git gcc/cp/cp-tree.h gcc/cp/cp-tree.h
index ec92718..0e46ae1 100644
--- gcc/cp/cp-tree.h
+++ gcc/cp/cp-tree.h
@@ -6494,6 +6494,7 @@ extern cp_lvalue_kind real_lvalue_p		(const_tree);
 extern cp_lvalue_kind lvalue_kind		(const_tree);
 extern bool lvalue_or_rvalue_with_address_p	(const_tree);
 extern bool xvalue_p	                        (const_tree);
+extern tree cp_stabilize_reference		(tree);
 extern bool builtin_valid_in_constant_expr_p    (const_tree);
 extern tree build_min				(enum tree_code, tree, ...);
 extern tree build_min_nt_loc			(location_t, enum tree_code,
diff --git gcc/cp/tree.c gcc/cp/tree.c
index 112c8c7..44e3893 100644
--- gcc/cp/tree.c
+++ gcc/cp/tree.c
@@ -296,6 +296,31 @@ xvalue_p (const_tree ref)
   return (lvalue_kind (ref) == clk_rvalueref);
 }
 
+/* C++-specific version of stabilize_reference.  For preincrement and
+   predecrement expressions we need to turn the expression into some
+   sort of _REF before passing it to stabilize_reference.  */
+
+tree
+cp_stabilize_reference (tree ref)
+{
+  if (TREE_CODE (ref) == PREINCREMENT_EXPR
+      || TREE_CODE (ref) == PREDECREMENT_EXPR)
+    {
+      cp_lvalue_kind kind = lvalue_kind (ref);
+      if ((kind & ~clk_class) != clk_none)
+	{
+	  tree type = unlowered_expr_type (ref);
+	  bool rval = !!(kind & clk_rvalueref);
+	  type = cp_build_reference_type (type, rval);
+	  /* This inhibits warnings in, eg, cxx_mark_addressable
+	     (c++/60955).  */
+	  warning_sentinel s (extra_warnings);
+	  ref = build_static_cast (type, ref, tf_error);
+	}
+    }
+  return stabilize_reference (ref);
+}
+
 /* Test whether DECL is a builtin that may appear in a
    constant-expression. */
 
diff --git gcc/cp/typeck.c gcc/cp/typeck.c
index cef5604..7e12009 100644
--- gcc/cp/typeck.c
+++ gcc/cp/typeck.c
@@ -5912,7 +5912,7 @@ cp_build_unary_op (enum tree_code code, tree xarg, int noconvert,
 	{
 	  tree real, imag;
 
-	  arg = stabilize_reference (arg);
+	  arg = cp_stabilize_reference (arg);
 	  real = cp_build_unary_op (REALPART_EXPR, arg, 1, complain);
 	  imag = cp_build_unary_op (IMAGPART_EXPR, arg, 1, complain);
 	  real = cp_build_unary_op (code, real, 1, complain);
@@ -6112,7 +6112,7 @@ unary_complex_lvalue (enum tree_code code, tree arg)
       tree lvalue = TREE_OPERAND (arg, 0);
       if (TREE_SIDE_EFFECTS (lvalue))
 	{
-	  lvalue = stabilize_reference (lvalue);
+	  lvalue = cp_stabilize_reference (lvalue);
 	  arg = build2 (TREE_CODE (arg), TREE_TYPE (arg),
 			lvalue, TREE_OPERAND (arg, 1));
 	}
@@ -7496,7 +7496,7 @@ cp_build_modify_expr (tree lhs, enum tree_code modifycode, tree rhs,
     case PREINCREMENT_EXPR:
       if (TREE_SIDE_EFFECTS (TREE_OPERAND (lhs, 0)))
 	lhs = build2 (TREE_CODE (lhs), TREE_TYPE (lhs),
-		      stabilize_reference (TREE_OPERAND (lhs, 0)),
+		      cp_stabilize_reference (TREE_OPERAND (lhs, 0)),
 		      TREE_OPERAND (lhs, 1));
       newrhs = cp_build_modify_expr (TREE_OPERAND (lhs, 0),
 				     modifycode, rhs, complain);
@@ -7516,7 +7516,7 @@ cp_build_modify_expr (tree lhs, enum tree_code modifycode, tree rhs,
     case MODIFY_EXPR:
       if (TREE_SIDE_EFFECTS (TREE_OPERAND (lhs, 0)))
 	lhs = build2 (TREE_CODE (lhs), TREE_TYPE (lhs),
-		      stabilize_reference (TREE_OPERAND (lhs, 0)),
+		      cp_stabilize_reference (TREE_OPERAND (lhs, 0)),
 		      TREE_OPERAND (lhs, 1));
       newrhs = cp_build_modify_expr (TREE_OPERAND (lhs, 0), modifycode, rhs,
 				     complain);
@@ -7665,7 +7665,7 @@ cp_build_modify_expr (tree lhs, enum tree_code modifycode, tree rhs,
 	     not intervene between the lvalue-to-rvalue conversion and the
 	     side effect associated with any single compound assignment
 	     operator. -- end note ]  */
-	  lhs = stabilize_reference (lhs);
+	  lhs = cp_stabilize_reference (lhs);
 	  rhs = rvalue (rhs);
 	  rhs = stabilize_expr (rhs, &init);
 	  newrhs = cp_build_binary_op (input_location,
diff --git gcc/testsuite/g++.dg/ext/cond2.C gcc/testsuite/g++.dg/ext/cond2.C
index e69de29..d9f1d59 100644
--- gcc/testsuite/g++.dg/ext/cond2.C
+++ gcc/testsuite/g++.dg/ext/cond2.C
@@ -0,0 +1,28 @@
+// PR c++/70744
+// { dg-do run }
+// { dg-options "" }
+
+static void
+fn1 (void)
+{
+  int x = 2;
+  ++x ? : 42;
+  if (x != 3)
+    __builtin_abort ();
+  --x ? : 42;
+  if (x != 2)
+    __builtin_abort ();
+  x++ ? : 42;
+  if (x != 3)
+    __builtin_abort ();
+  x-- ? : 42;
+  if (x != 2)
+    __builtin_abort ();
+}
+
+int
+main ()
+{
+  fn1 ();
+  return 0;
+}

	Marek