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

[Reid Kleckner]

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.

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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