Re: C++ PATCH for P0135, C++17 guaranteed copy elision

[Jason Merrill <jason@redhat.com>]

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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

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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();