[PATCH] D119078: [LAA, LV] Add initial support for pointer-diff memory checks.

Thu May 26 04:09:42 PDT 2022

bjope added inline comments.

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Comment at: llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:3090
+  // across all iterations.
+  if (!Legal->getLAI()->getRuntimePointerChecking()->getDiffChecks()) {
+    //  We currently don't use LoopVersioning for the actual loop cloning but we
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Hi @fhahn and @dmgreen (and well, anyone else who might happen to read this).

I've been trying to understand some regressions that we've seen downstream after this patch.
It seems to be related to this part, where the new memchecks somehow are weaker(?) in the sense that we can't deduce NoAlias across all iterations (well, that is how I've interpreted the diff here).

What we've seen happening for a couple of benchmarks is that in our OOT backend some vectorized loops aren't software pipelined any longer. And the SWP scheduler is bailing out since noalias isn't guaranteed. No SWP => quite huge regressions. No idea if this could be a problem for other targets as well.

So far I haven't figured out what to do downstream in this situation.

Maybe we should look into the SWP scheduler to see if it can deduce "no alias" in some other way (I'm not sure, but I figure SWP isn't requiring no overlap across all iterations, but depending on how much pipelining it might require no overlap across iteration N and N+1 etc.).

Maybe we should add some heuristic already in the LoopVectorizer to not use the new kind of memory checks when we think that it would block SWP (an initial heuristic would probably be to use the old kind of checks for out target).
Here I'm not quite sure about the plans in-tree for this. Are the new memory checks supposed to replace the old checks in the future?

If anyone has some insight/ideas here, then I'd be happy to read your comments on this.

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D119078/new/

https://reviews.llvm.org/D119078