C++ PATCH for P0135, C++17 guaranteed copy elision
Jason Merrill
jason@redhat.com
Wed Oct 5 23:22:00 GMT 2016
On Wed, Oct 5, 2016 at 7:15 PM, Pedro Alves <palves@redhat.com> wrote:
> On 10/05/2016 11:57 PM, Jason Merrill wrote:
>> --- a/gcc/c-family/c-opts.c
>> +++ b/gcc/c-family/c-opts.c
>> @@ -1579,6 +1579,7 @@ set_std_cxx1z (int iso)
>> + flag_elide_constructors = 2;
Oops, that was an earlier version of the patch. Here's what I checked
in, which doesn't touch flag_elide_constructors.
> Does -fno-elide-constructors have any effect in C++17 mode?
> Should it perhaps be an error, or ignored?
It does have an effect: it avoids open-coding trivial copies. I
suppose I should update the documentation.
Jason
-------------- next part --------------
commit d47a992158cc6f8a6d84f365e7d4f88ee567b640
Author: jason <jason@138bc75d-0d04-0410-961f-82ee72b054a4>
Date: Wed Oct 5 22:59:02 2016 +0000
Implement P0135R1, Guaranteed copy elision.
* cvt.c (ocp_convert): Don't re-copy a TARGET_EXPR in C++17.
git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@240820 138bc75d-0d04-0410-961f-82ee72b054a4
diff --git a/gcc/cp/cvt.c b/gcc/cp/cvt.c
index 2f5f15a..ecc8ef8 100644
--- a/gcc/cp/cvt.c
+++ b/gcc/cp/cvt.c
@@ -693,8 +693,11 @@ ocp_convert (tree type, tree expr, int convtype, int flags,
if (error_operand_p (e))
return error_mark_node;
- if (MAYBE_CLASS_TYPE_P (type) && (convtype & CONV_FORCE_TEMP))
- /* We need a new temporary; don't take this shortcut. */;
+ if (MAYBE_CLASS_TYPE_P (type) && (convtype & CONV_FORCE_TEMP)
+ && !(cxx_dialect >= cxx1z
+ && TREE_CODE (e) == TARGET_EXPR))
+ /* We need a new temporary; don't take this shortcut. But in C++17, don't
+ force a temporary if we already have one. */;
else if (same_type_ignoring_top_level_qualifiers_p (type, TREE_TYPE (e)))
{
if (same_type_p (type, TREE_TYPE (e)))
diff --git a/gcc/testsuite/g++.dg/cpp1z/elide1.C b/gcc/testsuite/g++.dg/cpp1z/elide1.C
new file mode 100644
index 0000000..a0538bb
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/elide1.C
@@ -0,0 +1,16 @@
+// { dg-options -std=c++1z }
+
+struct A
+{
+ A();
+ A(const A&) = delete;
+};
+
+bool b;
+A a = A();
+A a1 = b ? A() : A();
+A a2 = (42, A());
+
+A f();
+A a3 = f();
+A a4 = b ? A() : f();
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