[cxx-abi-dev] Non-trivial move constructor

[Jonathan Wakely]

On 22/06/16 12:59 -0700, Reid Kleckner wrote:
>This bug still isn't fixed in Clang. It's
>https://llvm.org/bugs/show_bug.cgi?id=19668. You should probably go ahead
>and update the document.

It's probably also the cause of
https://llvm.org/bugs/show_bug.cgi?id=23034
which I've been asked about (because it involves the libstdc++
std::tuple).

Is the current status that Clang is still believed to require a
change, and that G++ is doing the right thing already?



>On Wed, Feb 24, 2016 at 10:41 PM, John McCall <rjmccall at apple.com> wrote:
>
>> > On Feb 24, 2016, at 1:14 PM, Richard Smith <richardsmith at googlers.com>
>> wrote:
>> > On 24 February 2016 at 12:56, John McCall <rjmccall at apple.com> wrote:
>> >>> On Feb 24, 2016, at 11:43 AM, Richard Smith <richardsmith at googlers.com>
>> wrote:
>> >>> On 24 February 2016 at 05:54, Jason Merrill <jason at redhat.com> wrote:
>> >>>> On 02/24/2016 05:51 AM, Marc Glisse wrote:
>> >>>>>
>> >>>>> in 3.1.1, we use "In the special case where the parameter type has a
>> >>>>> non-trivial copy constructor or destructor" to force passing by
>> >>>>> reference. It seems that for C++11, this should also include move
>> >>>>> constructors, for the same reasons.
>> >>>>
>> >>>>
>> >>>> We talked about adding move constructors to that sentence years ago.
>> Did it
>> >>>> never make it into the spec?
>> >>>
>> >>> Looks like it didn't. The rule we ended up with was:
>> >>>
>> >>> "[Pass an object of class type by value if] every copy constructor and
>> >>> move constructor is deleted or trivial and at least one of them is not
>> >>> deleted, and the destructor is trivial.”
>> >>>
>> >>>
>> >>> However, this seems overly-cautious to me; it would seem sufficient
>> >>> for there to be at least one copy or move constructor that is trivial
>> >>> and not deleted, and a trivial destructor. It's not really
>> >>> particularly plausible for there to be a trivial copy and a
>> >>> non-trivial move or vice versa, but it *is* plausible for there to be
>> >>> two non-deleted copy constructors -- a trivial one, and one that takes
>> >>> a const volatile reference -- and in that case, passing through
>> >>> registers seems completely reasonable. How about changing the rule in
>> >>> 3.1.1 bullet 1 to:
>> >>>
>> >>> "In the special case where the parameter type does not have both a
>> >>> trivial destructor and at least one trivial copy or move constructor
>> >>> that is not deleted, the caller must allocate space for a temporary
>> >>> copy, and pass the resulting copy by reference (below). Specifically
>> >>> […]"
>> >>
>> >> I agree with your proposal in theory, but I’m concerned about changing
>> >> the ABI at this point.  We *are* talking about the language standard
>> that was
>> >> released six years ago, and an area of that standard that was
>> theoretically
>> >> fully implemented by compilers several years before that.
>> >>
>> >> Do we understand the scope of the ABI disagreement between GCC and
>> Clang here?
>> >> What do other compilers do?
>> >
>> > Clang's rule is the one in the ABI: a class is passed indirectly if it
>> > has a non-trivial destructor or a non-trivial copy constructor. This
>> > rule definitely needs some adjustment, because it's not meaningful to
>> > ask whether an implicitly-deleted function is trivial.
>>
>> That sounds like it’s on us to fix.  Do GCC and other compilers correctly
>> implement the rule that we agreed on?  If so, I’ll go ahead and apply
>> the change to the ABI document, and we should fix this in clang.
>>
>> John.
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>>

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