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C++ PATCH for core 1148 (move on return of function parameter)
- From: Jason Merrill <jason at redhat dot com>
- To: gcc-patches List <gcc-patches at gcc dot gnu dot org>
- Date: Wed, 16 Mar 2011 16:16:58 -0400
- Subject: C++ PATCH for core 1148 (move on return of function parameter)
The resolution of core issue 1148 clarifies that even though it isn't
safe to elide the copy from a function parameter to its return value, we
can change it to a move.
Tested x86_64-pc-linux-gnu, applied to trunk.
commit 3122c2fd2d08a4ac78c2954dda8330e59295e23f
Author: Jason Merrill <jason@redhat.com>
Date: Wed Mar 16 15:42:13 2011 -0400
Core 1148
* typeck.c (check_return_expr): Fix conditions for setting
LOOKUP_PREFER_RVALUE.
diff --git a/gcc/cp/typeck.c b/gcc/cp/typeck.c
index 0e8a6d7..03aa49e 100644
--- a/gcc/cp/typeck.c
+++ b/gcc/cp/typeck.c
@@ -7760,12 +7760,19 @@ check_return_expr (tree retval, bool *no_warning)
/* Under C++0x [12.8/16 class.copy], a returned lvalue is sometimes
treated as an rvalue for the purposes of overload resolution to
- favor move constructors over copy constructors. */
- if ((cxx_dialect != cxx98)
- && named_return_value_okay_p
- /* The variable must not have the `volatile' qualifier. */
- && !CP_TYPE_VOLATILE_P (TREE_TYPE (retval))
- /* The return type must be a class type. */
+ favor move constructors over copy constructors.
+
+ Note that these conditions are similar to, but not as strict as,
+ the conditions for the named return value optimization. */
+ if ((cxx_dialect != cxx98)
+ && (TREE_CODE (retval) == VAR_DECL
+ || TREE_CODE (retval) == PARM_DECL)
+ && DECL_CONTEXT (retval) == current_function_decl
+ && !TREE_STATIC (retval)
+ && same_type_p ((TYPE_MAIN_VARIANT (TREE_TYPE (retval))),
+ (TYPE_MAIN_VARIANT
+ (TREE_TYPE (TREE_TYPE (current_function_decl)))))
+ /* This is only interesting for class type. */
&& CLASS_TYPE_P (TREE_TYPE (TREE_TYPE (current_function_decl))))
flags = flags | LOOKUP_PREFER_RVALUE;
diff --git a/gcc/testsuite/g++.dg/cpp0x/elision2.C b/gcc/testsuite/g++.dg/cpp0x/elision2.C
new file mode 100644
index 0000000..216b1b5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/elision2.C
@@ -0,0 +1,13 @@
+// Core 1148: should be able to move from value parameter on return
+// { dg-options -std=c++0x }
+
+struct A
+{
+ A(const A&) = delete;
+ A(A&&);
+};
+
+A f (A a)
+{
+ return a;
+}