MemCpyOpt does not preserve MemDep, unforunately. See PR 36965, 36964, 36940, 36944, 36943, and 36941 Testcases coming.

[• dberlin]

Summary

There are a number of causes here:

1. Same as pr36063, change in dependence type from local to nonlocal,

or vice versa, can't be invalidated in the cache.

2. MemDep does not do transitive cache invalidation, so

loads that would now point to a store/memcpy/etc that it has inserted,
will still point to the wrong place.

3. An unknown cause i'm still tracking down that is causing preservation failure

even when memcpyopt does nothing!

The TL;DR is that anything that moves stores across blocks probably should not be preserving memdep
(or is most likely broken right now), a least until it can be fixed.

[• dberlin]

Updating D45238: MemCpyOpt does not preserve MemDep, unforunately. See PR 36965, 36964, 36940, 36944, 36943, and 36941

Testcases coming.

[davide]

yes, I've been following this (and I agree with you we should really disable the preservation).
Do we happen to have some tests lying around we can use? I think Zhendong generated many, but I haven't really checked whether they can be used.

[davide]

Looks like there was a race condition. This LGTM with the test cases added.

lib/Transforms/Scalar/MemCpyOptimizer.cpp

private:
   void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.setPreservesCFG();
     AU.addRequired<AssumptionCacheTracker>();
     AU.addRequired<DominatorTreeWrapperPass>();
     AU.addRequired<MemoryDependenceWrapperPass>();
     AU.addRequired<AAResultsWrapperPass>();
     AU.addRequired<TargetLibraryInfoWrapperPass>();
     AU.addPreserved<GlobalsAAWrapperPass>();
-    AU.addPreserved<MemoryDependenceWrapperPass>();
   }
 };
 
 } // end anonymous namespace
 
 char MemCpyOptLegacyPass::ID = 0;
 
 /// The public interface to this file...
PreservedAnalyses MemCpyOptPass::run(Function &F, FunctionAnalysisManager &AM) {
   bool MadeChange = runImpl(F, &MD, &TLI, LookupAliasAnalysis,
                             LookupAssumptionCache, LookupDomTree);
   if (!MadeChange)
     return PreservedAnalyses::all();
 
   PreservedAnalyses PA;
   PA.preserveSet<CFGAnalyses>();
   PA.preserve<GlobalsAA>();
-  PA.preserve<MemoryDependenceAnalysis>();
   return PA;
 }
 
 bool MemCpyOptPass::runImpl(
     Function &F, MemoryDependenceResults *MD_, TargetLibraryInfo *TLI_,
     std::function<AliasAnalysis &()> LookupAliasAnalysis_,
     std::function<AssumptionCache &()> LookupAssumptionCache_,
     std::function<DominatorTree &()> LookupDomTree_) {