[LLVMdev] Memory Dependence Analysis: getNonLocalDependency()

[Prakash Prabhu]

Hi,

I have a question about the memory dependence analysis. I am trying to use
it to selectively enumerate a set of pairs of (load, store) instructions for
every function by calling getNonLocalDependency() on the
MemoryDependenceAnalysis. This populates a DenseMap<BasicBlock*, Value*>. I
just looked up an usage of this in GVN.cpp:

MD->getNonLocalDependency(C, deps);

for (DenseMap<BasicBlock*, Value*>::iterator I = deps.begin(),
           E = deps.end(); I != E; ++I) {
        if (I->second == MemoryDependenceAnalysis::None) {
         ....
        } else if (I->second != MemoryDependenceAnalysis::NonLocal) {
            ....
          } else {
             ....
          }
        }
      }

>From the documentation, I got to know that MD can return the set of
inter-block dependencies for any instruction (which is great!).
My question is: what does None, Local and NonLocal (special markers defined
in MemoryDependenceAnalysis.cpp) mean in this context ?
Given a store statement, my objective is to identify other load statements
that it is dependent on ? I tried this code and it seems to run, but I
wanted to make sure I am not missing anything (as a consequence of not
knowing the semantics of None, Local and NonLocal) :

    for (DenseMap<BasicBlock*, Value*>::iterator B = depMap.begin(),
                                E = depMap.end(); B != E; B++)
    {
        if (B->second == MemoryDependenceAnalysis::None ||     B->second ==
MemoryDependenceAnalysis::NonLocal)
            continue;
        Value* depV = B->second;
        if (isa<LoadInst>(depV))
        {
            ...
         }
     }

Thanks in advance for any pointers!

- Prakash
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