[PATCH] D88778: [MemCpyOpt] Fix MemorySSA preservation

[Nikita Popov via Phabricator via llvm-commits]

nikic added inline comments.


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Comment at: llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp:547
+  MemoryDef *PDef = MSSAU
+      ? cast<MemoryDef>(MSSAU->getMemorySSA()->getMemoryAccess(P)) : nullptr;
   for (auto *I : llvm::reverse(ToLift)) {
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asbirlea wrote:
> Is it possible for the cast here to fail?
> P is known to modify the Load location a the callsite. Is it possible this is true for specific cases not modeled as MemoryDefs in MemorySSA (e.g. when using a non-standard aa pipeline, and basicaa is disabled, certain accesses are seen as "can access/modify" but are ignored in the MemorySSA modeling - see MemorySSA.cpp:1766).
I wasn't able to come up with a case that fails based on some quick experiments -- anything I tried would get foiled by the above safety checks without basicaa. However, it doesn't cost us anything to be conservative here, so I changed this to use a dyn_cast_or_null.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D88778/new/

https://reviews.llvm.org/D88778