[llvm-dev] AtomicExpandPass and branch weighting

[Kyle Butt via llvm-dev]

On Wed, Dec 14, 2016 at 1:34 PM, James Knight <jyknight at google.com> wrote:

> Seems reasonable.
>
> I'd note additionally that on some architectures, that the success block
> *must* be the fallthrough case (that is to say: you must not have any taken
> branches between the load-linked and store-conditional) in order to have an
> architectural guarantee that two such loops on different CPUs won't
> livelock against eachother.
>

Do we have a way to *require* that 2 blocks be laid out consecutively? I
don't think that we do. A hint is better than nothing, but not a guarantee.


>
>
> On Dec 12, 2016, at 12:30 PM, Kyle Butt via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> > I'm working on a change to the layout algorithm, and I noted that
> test/CodeGen/ARM/cmpxchg-weak.ll was affected.
> >
> > Normally, that would be fine, but I noted that the layout changed the
> fallthrough from the success case to the failure case. I was surprised to
> see that the success case isn't annotated with a branch weight by
> AtomicExpandPass.cpp
> >
> > Would it make sense to annotate the success case as more likely when we
> expand the intrinsic to help guarantee that the success case remains the
> fallthrough? Even a 2:1 or 3:2 weighting would correct the layout issue I
> noted.
> >
> > Thanks,
> > Kyle.
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
>
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