[PATCH] D26659: [MemorySSA] Invoke the right getModRefInfo check in `instructionClobbersQuery` when UseLoc is passed.

[bryant via llvm-commits]

bryant created this revision.
bryant added reviewers: dberlin, george.burgess.iv, hfinkel.
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bryant set the repository for this revision to rL LLVM.

Calls to `instructionClobbersQuery` are meant to answer two kinds of queries:
Given two memory instructions X and Y,

- Does X clobber Y?
- Does X clobber Y with respect to a MemoryLocation UseLoc?

For queries of the first kind, a null MemoryLocation is passed, and it makes
sense for `getModRefInfo(Instruction *, ImmutableCallSite)` to be called.
For the second kind, however, UseLoc would be a non-null and the above call to
`getModRefInfo` would be unnecessarily conservative; it would be sufficient to
check `getModRefInfo(Instruction *, const MemoryLocation &)`.

An example:

  call void @llvm.memcpy(i8* %dest, i8* %src, i64 128, ...)   ; MemoryDef *M0
  call void @llvm.memcpy(i8* %dest, i8* %src, i64 128, ...)   ; MemoryDef *M1

The call

  instructionClobbersQuery(M0, MemoryLocation::getForSource(M1->getMemoryInst()),
                           M1->getMemoryInst(), AA)

would return true -- that M0 does clobber M1's source. But this can't possibly
be the case since overlapping memcpy operands are UB.


Repository:
  rL LLVM

https://reviews.llvm.org/D26659

Files:
  lib/Transforms/Utils/MemorySSA.cpp


Index: lib/Transforms/Utils/MemorySSA.cpp
===================================================================
--- lib/Transforms/Utils/MemorySSA.cpp
+++ lib/Transforms/Utils/MemorySSA.cpp
@@ -235,7 +235,7 @@
   }
 
   ImmutableCallSite UseCS(UseInst);
-  if (UseCS) {
+  if (UseCS && UseLoc == MemoryLocation()) {
     ModRefInfo I = AA.getModRefInfo(DefInst, UseCS);
     return I != MRI_NoModRef;
   }


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