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

[Reid Kleckner]

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.


> > 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.
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