[llvm] r215478 - APInt: Make self-move-assignment a no-op to fix stage3 clang-cl

[David Blaikie]

Do they generate ones that no-op on self-move? Or did they fix their
stable_sort (& other algorithms) so as not to rely on self-move?

On Thu, Aug 14, 2014 at 5:58 PM, David Majnemer
<david.majnemer at gmail.com> wrote:
> On Thu, Aug 14, 2014 at 3:27 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>
>> On Thu, Aug 14, 2014 at 2:42 PM, Reid Kleckner <rnk at google.com> wrote:
>> > On Tue, Aug 12, 2014 at 3:29 PM, David Blaikie <dblaikie at gmail.com>
>> > wrote:
>> >>
>> >> On Tue, Aug 12, 2014 at 3:01 PM, Reid Kleckner <reid at kleckner.net>
>> >> wrote:
>> >> > Author: rnk
>> >> > Date: Tue Aug 12 17:01:39 2014
>> >> > New Revision: 215478
>> >> >
>> >> > URL: http://llvm.org/viewvc/llvm-project?rev=215478&view=rev
>> >> > Log:
>> >> > APInt: Make self-move-assignment a no-op to fix stage3 clang-cl
>> >> >
>> >> > It's not clear what the semantics of a self-move should be.  The
>> >> > consensus appears to be that a self-move should leave the object in a
>> >> > moved-from state, which is what our existing move assignment operator
>> >> > does.
>> >> >
>> >> > However, the MSVC 2013 STL will perform self-moves in some cases.  In
>> >> > particular, when doing a std::stable_sort of an already sorted APSInt
>> >> > vector of an appropriate size, one of the merge steps will self-move
>> >> > half of the elements.
>> >> >
>> >> > We don't notice this when building with MSVC, because MSVC will not
>> >> > synthesize the move assignment operator for APSInt.  Presumably
>> >>
>> >> I'm assuming you're going to look into this a bit further, though?
>> >> (other than just 'presumably') both to understand when/how long we
>> >> need this workaround in place, and whether we need to do anything to
>> >> Clang for MSVC compatibility here?
>> >
>> >
>> > David Majnemer says that VS 2013 can't generate implicit move
>> > assignments at
>> > all, so I just gave up too early and rationalized it.
>>
>> Oh, right, yes.
>>
>> >
>> >>
>> >> > MSVC
>> >> > does this because APInt, the base class, has user-declared special
>> >> > members that implicitly delete move special members.  Instead, MSVC
>> >> > selects the copy-assign operator, which defends against
>> >> > self-assignment.
>> >> > Clang, on the other hand, selects the move-assign operator, and we
>> >> > get
>> >> > garbage APInts.
>> >>
>> >> I'm curious how Clang synthesizes a move assignment operator if the
>> >> base class's move assignment is implicitly deleted. Any idea what's
>> >> going on there?
>> >
>> >
>> > The base class (APInt) has a user-defined move assignment, which is the
>> > one
>> > that I'm modifying here. We reach it in Clang through an implicit APSInt
>> > move assignment, but MSVC doesn't generate that and doesn't reach it.
>>
>> Yep, that all makes sense.
>>
>> Reckon we need to disable generation of implicit move members in MSVC
>> compatibility mode?
>
>
> Both the November 2013 CTP and VS "14" generate move members; personally, I
> think our behavior here is fine.
>
>>
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