[PATCH] D134161: [MemorySSA] Reset location size if IsGuaranteedLoopInvariant after phi tranlation

[Nikita Popov via Phabricator via llvm-commits]

nikic added inline comments.


================
Comment at: llvm/include/llvm/Analysis/MemorySSA.h:1273
+            CurrentPair.second =
+                CurrentPair.second.getWithNewSize(Location.Size);
+
----------------
StephenFan wrote:
> nikic wrote:
> > The logic here looks somewhat convoluted -- isn't this equivalent to doing the translation first and then doing the invariance check afterwards (dropping the size adjustment before phi translation entirely)?
> Only if the phi translator gets a value different from the original one, line 1271 would be executed. if drop the size adjustment before phi translation entirely, in the case that `Location.Ptr` is not guaranteed loop invariant and can not be phi translated, the location size of `Location.Ptr` would not be adjusted to beforeOrAfterPointer. I hope I understood your question correctly and answered it clearly. :)
Maybe it's easier to explain with code, this is what I had in mind:
```
  void fillInCurrentPair() {
    CurrentPair.first = *DefIterator;
    CurrentPair.second = Location;
    if (WalkingPhi && Location.Ptr) {
      PHITransAddr Translator(
          const_cast<Value *>(Location.Ptr),
          OriginalAccess->getBlock()->getModule()->getDataLayout(), nullptr);

      if (!Translator.PHITranslateValue(OriginalAccess->getBlock(),
                                        DefIterator.getPhiArgBlock(), DT, true))
        CurrentPair.second =
            CurrentPair.second.getWithNewPtr(Translator.getAddr());

      // Mark size as unknown, if the location is not guaranteed to be
      // loop-invariant for any possible loop in the function. Setting the size
      // to unknown guarantees that any memory accesses that access locations
      // after the pointer are considered as clobbers, which is important to
      // catch loop carried dependences.
      if (!IsGuaranteedLoopInvariant(CurrentPair.second.Ptr))
        CurrentPair.second =
            Location.getWithNewSize(LocationSize::beforeOrAfterPointer());
    }
  }
```


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https://reviews.llvm.org/D134161