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

[David Majnemer]

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.


> _______________________________________________
> llvm-commits mailing list
> llvm-commits at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20140814/cbb7ad72/attachment.html>