[PATCH] D85603: IR: Add convergence control operand bundle and intrinsics

[Anastasia Stulova via Phabricator via llvm-commits]

Anastasia added a comment.

In D85603#2685900 <https://reviews.llvm.org/D85603#2685900>, @nhaehnle wrote:

>> To address this there was an attempt to invert the behavior of convergent attribute in this patch (https://reviews.llvm.org/D69498) then the frontend wouldn't need to generate the attribute everywhere and the optimizer wouldn't need to undo what frontend does. The change in this review doesn't address (2) as far as I can see - it seems it only generalized old convergent semantics to cover the cases with non-uniform CF. I am not clear yet about the details of how and what frontend should generate in IR for this new logic but it looks more complex than before. And if we have to stick to the conservative approach of assuming everything is convergent as it is now this might complicate and slow down the parsing. So I am just checking whether addressing (2) is still feasible with the new approach or it is not a direction we can/should go?
>
> This is a good point. Generally, HLL need to be more conscious about what they actually expect convergent operations to do :) I tend to be optimistic: I mentioned on D85609 <https://reviews.llvm.org/D85609> a proposal I presented in the context of Khronos. The important point from there is that every statement of the HLL would be (possibly implicitly) annotated with its "canonical convergence token" using very simple rules. This only really falls flat if you have `goto` jumping into the middle of a loop (or Duff's device etc.). I don't know how efficiently e.g. the Clang frontend can decide whether such constructs exist or not.

I see. Technically this sounds feasible to add i.e. we could insert a custom AST visitor to detect the pattern after the AST is parsed or perhaps the detection can be done during the parsing itself. The only question is how many of such patterns exist considering the variety HL language constructs and how this will impact the parsing time, etc. I would say prototyping this could be a good starting point. However what happens when such pattern are detected? Do we generate IR slightly differently?


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