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

[Richard Smith]

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.