[PATCH] D34181: MachineInstr: Reason locally about some memory objects before going to AA.

[Geoff Berry via Phabricator via llvm-commits]

gberry added inline comments.


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Comment at: lib/CodeGen/MachineInstr.cpp:1664
+  int64_t WidthB = MMOb->getSize();
+  bool SameVal = MMOa->getValue() == MMOb->getValue();
+  if (!SameVal) {
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bmakam wrote:
> gberry wrote:
> > I don't think this is right since getValue() will return nullptr for any PseudoSourceValue, so SameVal will always be true for any two PseudoSourceValues, even if they are different.
> I am not sure I fully understand your comment, getValue will not return nullptr here because of the earlier condition that returns true (i.e. mayAlias) if it was nullptr.
I missed that check before.  Given that check is there, the code below is useless since an MMO can only have a Value or a PseudoValue, so it will never change SameVal.

I think there are three interesting cases (at least) here:
- pseudo vs. pseudo: i believe these can only alias if they are equal (and even then an overlap check can find cases that don't alias)
- pseudo vs. value: can never alias if pseudo is not mayAlias (perhaps a stronger check is possible)
- value vs. value: do overlap checking if no AA is present, otherwise just let AA do it  


https://reviews.llvm.org/D34181