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

[Nicolai Hähnle via Phabricator via llvm-commits]

nhaehnle added a comment.

Hi @jholewinski, sorry for missing your comment earlier. It's been a while! I still need to work through the rest of the comments here, but there's a pretty crucial point here that seems to have been missed:

In D85603#2422804 <https://reviews.llvm.org/D85603#2422804>, @jholewinski wrote:

> Things are less clear when you consider odd looping structures, for example:
>
>   entry:
>       entry_token = llvm.experimental.convergence.anchor();
>       if (cond1) goto head1;
>       else goto head2;
>   
>   head1:
>       head1_loop_token = llvm.experimental.convergence.loop() [ "convergencectrl"(entry_token) ]
>       cond2 = ...;
>       if cond2 goto tail1;
>       else goto tail2;
>   
>   head2:
>       head2_loop_token = llvm.experimental.convergence.loop() [ "convergencectrl"(entry_token) ]
>       break_cond = ...
>       if break_cond goto exit;
>       else goto head2b;
>   
>   head2b:
>       cond3 = ...;
>       if cond3 goto tail2;
>       else goto tail1;
>   
>   tail1:
>       cond4 = subgroupOp(...);      // What does this anchor to?
>       if cond4 goto head1;
>       else goto head2;
>   
>   tail2:
>       cond5 = subgroupOp(...);      // What does this anchor to?
>       if cond5 goto head2;
>       else goto head1;
>   
>   exit:
>       ...

Regardless of the question about `subgroupOp`, this example is not valid IR: it breaks the static rule that "Every cycle in the CFG that contains two different uses of a convergence token T must also contain the definition of T."Specifically, there are two uses of `entry_token`, in `head1` and `head2`, and a cycle `head1 -> tail1 -> head2 -> head2b -> tail1 -> head1` that goes through both of them without going through the definition of `entry_token`.

Roughly speaking, an irreducible loop can contain at most one loop intrinsic that refers to a token from outside the irreducible loop.


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https://reviews.llvm.org/D85603