[PATCH] D15124: Use @llvm.invariant.start/end intrinsics to extend the meaning of basic AA's pointsToConstantMemory(), for GVN-based load elimination purposes [Local objects only]

[Larisse Voufo via llvm-commits]

lvoufo added a comment.

In http://reviews.llvm.org/D15124#307279, @dberlin wrote:

> "(2) -gvn already requires -basicaa. So, -basicaa would be redundant in
>  '-basicaa -gvn'. I could add it for clarity if you prefer.
>  "
>  -gvn does not require -basicaa.
>  You've stated this a few times, and i'm not sure why you think this.
>
> <from GVN>:
>
>   void getAnalysisUsage(AnalysisUsage &AU) const override {
>         AU.addRequired<AssumptionCacheTracker>();
>         AU.addRequired<DominatorTreeWrapperPass>();
>         AU.addRequired<TargetLibraryInfoWrapperPass>();
>         if (!NoLoads)
>           AU.addRequired<MemoryDependenceAnalysis>();
>         AU.addRequired<AAResultsWrapperPass>();
>   
>
> }
>
> GVN requires AC, DominatorTree, and TLI all the time.
>  It uses AA results, which is "whatever aa you have enabled".
>  It requires memorydependence if you want to optimize loads.
>  Memory dependence, in turn, does not require basicaa either:
>
> void MemoryDependenceAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
>
>     AU.setPreservesAll();
>     AU.addRequired<AssumptionCacheTracker>();
>     AU.addRequiredTransitive<AAResultsWrapperPass>();
>     AU.addRequiredTransitive<TargetLibraryInfoWrapperPass>();
>   }
>   
>   
>
> No passes *require* basicaa, because basicaa is an optional pass providing
>  AA results.


Point taken. I may be misusing "require" here, but I sure understand what is going on better now thanks to this.
To clarify my statements and for added clarity from your points above, I have 2 questions:

1 - What is one to make of the dependence on `BasicAAWrapperPass` in the following?

  INITIALIZE_PASS_BEGIN(AAResultsWrapperPass, "aa",
                        "Function Alias Analysis Results", false, true)
  INITIALIZE_PASS_DEPENDENCY(BasicAAWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(CFLAAWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(ExternalAAWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(GlobalsAAWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(ObjCARCAAWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(SCEVAAWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(ScopedNoAliasAAWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(TypeBasedAAWrapperPass)
  INITIALIZE_PASS_END(AAResultsWrapperPass, "aa",
                      "Function Alias Analysis Results", false, true)

So far as I understand, this *will* initialize a `BaiscAAWrapperPass` pass which, when run (either via `run()` or `runOnFunction()`), will build a `BasicAAResult` object.

2 - There does not seem to be a difference between `opt -gvn` and `opt -basicaa -gvn`. In fact, running `opt -gvn -debug-pass=Arguments` and  `opt -basicaa -gvn -debug-pass=Arguments` both return the following, where -basicaa is automatically added.

  Pass Arguments:  -targetlibinfo -tti -assumption-cache-tracker -invariant-info-marker -basicaa -domtree -aa -memdep -gvn -verify




http://reviews.llvm.org/D15124