[Openmp-commits] [PATCH] D109468: [OpenMP][FIX] Be more deliberate about invalidating the AAKernelInfo state
Johannes Doerfert via Phabricator via Openmp-commits
openmp-commits at lists.llvm.org
Fri Sep 10 11:29:51 PDT 2021
jdoerfert added inline comments.
Comment at: llvm/lib/Transforms/IPO/OpenMPOpt.cpp:3848
> jdoerfert wrote:
> > tianshilei1992 wrote:
> > > jdoerfert wrote:
> > > > tianshilei1992 wrote:
> > > > > IIRC, `ReachedUnknownParallelRegions` will be invalidated on insertion. With this change, this AA indicates optimistic fixed point?
> > > > Yes. But optimistic fixpoint != everything is optimistic. It will move the known state to the assumed state. Given that we already invalidated ReachedUnknownParallelRegions before, it won't "un-invalidate" it, ever.
> > > I'm fine as long as this is consistent. But in other hand, I feel it might be better to invalid the AA as well if it is using another state that has already been invalidated.
> > I don't follow. If an AA (X) uses another one (Y) and Y has an invalid state, X should not use the state of Y, ever. That is not a question nor is it changed here in any way. What is changed here is the fact that we do not need to give up on all information provided by AAKernelInfoCallSite if we see an __kmpc_omp_task call. We cannot argue SPMD and we cannot know all unknown parallel regions but we can certainly still know ReachingKernels (among other things). Indicating a pessimistic fixpoint here will give up on all information including stuff that we can use without problem.
> `indicatePessimisticFixpoint` invalidates the AA (X), which is `AAKernelInfoCallSite` here, as well. When another AA (Y) uses its member, such as `ReachingKernels`, if we only check the state of Y, it's gonna break. If we directly check the member instead, then we are good. That's what I mean by "consistent".
> But yeah, I agree now that we could use partial information. However, if it is fixed point, no matter optimistic or pessimistic, it will not be updated, right? What if the partial information we want to use gets updated? Is it possible?
A pessimistic fixpoint on AAKernelInfo should (almost) never be necessary. This patch removes (almost) all occurrences. A pessimistic fixpoint on AAKernelInfo will invalidate all information in the state, basically reverting everything to the previously known state. Since we track various things deduced in different ways, there is basically no good reason to invalidate everything at once. One could argue we should not track everything in a single state but that is a different discussion.
In your example, if X is invalidated it also invalidates reaching kernels. So Y will realize reaching kernels of X are unusable and deal with it accordingly (probably give up). However, the fact that this is a task runtime call did not impact the reaching kernels so there was no reason to invalidate it in the first place.
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