[llvm-dev] RFC: Removal of noduplicate attribute

[Doerfert, Johannes via llvm-dev]

On 10/27, Aaron Ballman via llvm-dev wrote:
> On Fri, Oct 25, 2019 at 7:37 PM Arsenault, Matthew via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > Hi,
> >
> > Can the noduplicate attribute be deleted? It was originally added to
> > satisfy the constraints of OpenCL barriers, but it didn’t really
> > solve this problem. Convergent is strictly better and more
> > accurately captures the real restriction. Is anyone still using
> > noduplicate or really needs the semantics of noduplicate? There are
> > a number of passes that now respect convergent, but do not try to
> > handle noduplicate properly. There is a maintenance cost to keeping
> > it around, and does require work to keep it working as advertised.
> 
> The noduplicate attribute is user-facing and is not tied to OpenCL
> (there are no constraints for it, anyway), so I would be suspicious of
> removing it just because it doesn't satisfy OpenCL needs -- users may
> be using it for other purposes. It may be reasonable to deprecate it
> if there is a better user-facing option for users to use and
> eventually remove the attribute when we think it's safe to do so, but
> I think it's a bit early to talk about removal.

These are good points. I think the first question should be: Do we know
of any active users of this attribute right now? If not, deprecation
seems like something we could do, e.g., through a warning in clang and
in the middle-end to ensure other front-ends are aware of it as well.

> > A related question is if it’s OK to recycle the attribute encoding
> > bit, or if this will be a problematic bitcode compatibility change.

I doubt it is all that helpful to do so but potentially problematic. We
can support a variable number now and we already crossed the critical
threshold so I would, for now, argue against reusing old bits.

Cheers,
  Johannes
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