[LLVMdev] writing an alias analysis pass?

[Matthew O'Connor]

On Sat, Apr 26, 2014 at 4:59 PM, Nick Lewycky <nicholas at mxc.ca> wrote:

> Matthew O'Connor wrote:
>
>> Hi,
>>
>> I'm attempting to do some alias analysis & other memory inspection. I've
>> written a pointless AliasAnalysis pass (that says everything must alias)
>> to attempt to verify that my pass is getting picked up & run by opt.
>>
>> I run opt with: opt -load ~/Applications/llvm/lib/MustAA.so -must-aa
>> -aa-eval -debug < trace0.ll
>>
>> I see my pass being initialized, but never being called (I see only may
>> alias results).
>>
>> Any ideas as to what to do to debug this? Or what I'm missing? I've read
>> through http://llvm.org/docs/AliasAnalysis.html and don't see anything
>> that I'm missing.
>>
>> Thanks,
>> Matthew
>>
>> P.S. Here's the full source code of my pass:
>>
>> #define DEBUG_TYPE "must-aa"
>> #include "llvm/Pass.h"
>> #include "llvm/Analysis/AliasAnalysis.h"
>> #include "llvm/Support/raw_ostream.h"
>> #include "llvm/Support/Debug.h"
>> using namespace llvm;
>>
>> namespace {
>> struct EverythingMustAlias : public ImmutablePass, public AliasAnalysis {
>>
>
> Because you have multiple inheritance on a Pass, you'll need this:
>
>     /// getAdjustedAnalysisPointer - This method is used when a pass
> implements
>     /// an analysis interface through multiple inheritance.  If needed, it
>     /// should override this to adjust the this pointer as needed for the
>     /// specified pass info.
>     void *getAdjustedAnalysisPointer(const void *ID) override {
>       if (ID == &AliasAnalysis::ID)
>         return (AliasAnalysis*)this;
>       return this;
>     }
>
> There may be another issue beyond that, I haven't tried it.
>
> Nick
>

I overrode Pass's member function:
    virtual void *getAdjustedAnalysisPointer(AnalysisID ID);

I'm not seeing getAdjustedAnalysisPointer being called.

Any other ideas?


>
>     static char ID;
>>    EverythingMustAlias() : ImmutablePass(ID) {}
>>
>>    virtual void initializePass() {
>>      DEBUG(dbgs() << "Initializing everything-must-alias\n");
>>      InitializeAliasAnalysis(this);
>>    }
>>
>>    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
>>      AliasAnalysis::getAnalysisUsage(AU);
>>      AU.setPreservesAll();
>>    }
>>
>>    virtual AliasResult alias(const Location &LocA, const Location &LocB) {
>>      DEBUG(dbgs() << "Everything must alias!\n");
>>      return AliasAnalysis::MustAlias;
>>    }
>> };
>> }
>>
>> namespace llvm {
>> void initializeEverythingMustAliasPass(PassRegistry &Registry);
>> }
>>
>> char EverythingMustAlias::ID = 0;
>> static RegisterPass<EverythingMustAlias> A("must-aa", "Everything must
>> alias");
>> INITIALIZE_AG_PASS(EverythingMustAlias, AliasAnalysis, "must-aa",
>> "Everything must alias", false, true, false)
>>
>>
>>
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>>
>
>
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