[PATCH] D69498: IR: Invert convergent attribute handling

[Mehdi AMINI via Phabricator via cfe-commits]

mehdi_amini added a comment.

In D69498#1727080 <https://reviews.llvm.org/D69498#1727080>, @jdoerfert wrote:

> Let me quote @arsenm here because this is so important: "Basically no frontend has gotten this right, including clang and non-clang frontends." For me, that statement alone is reason to change the status quo.


I am not convinced by this: this seems overly simplistic. The fact that convergent wasn't rolled out properly in frontend can also been as an artifact of the past. There is a logical gap between this and concluding that the only solution is to change the default. For instance someone mentioned a pass that could be inserted as the very beginning of the pipeline by any GPU compiler and add the convergent attribute everywhere.

>> As of the frontend, it seems to me that this is just about structuring the code-path for function emission to define the right set of default attribute. It isn't clear to me why a refactoring of the frontend isn't a better course of actions here.
> 
> Whatever we do, there will be consequences for current and future front-ends. At the end of the day, the underlying question we need to answer here is: Do we want to have "optimization with opt-in soundness" or "soundness with opt-in optimizations", and I would choose the latter any time.

This view seems appropriate to me only if you consider a GPU (or SIMT) compiler alone. Another view (which seems to be what @rjmccall has) is that SIMT/GPU is a marginal use-case and "soundness" is already existing for most LLVM use-cases.


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