[LLVMdev] Using an alias analysis pass

[Félix Cloutier]

Hello LLVMdev,

I’m using LLVM to do static analysis exclusively (without any code generation). To implement this analysis, I’m using multiple address spaces to disambiguate the purpose of the pointed memory. Since address spaces never alias in my model, I set on to implement an alias analysis pass that would exactly provide this information, as I’m seeing a couple of otherwise dead store that won’t go away. However, I’m struggling to get it to work.

Four years ago, Tobias Grosser implemented such a pass <http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20111010/129635.html>, so I based mine off his. And one year ago, Matthew O’Connor ran into a similar issue <http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-April/072427.html>, and I tried to use the solution that worked for him. The result code is this:

> namespace
> {
> 	struct AddressSpaceAliasAnalysis : public FunctionPass, public AliasAnalysis
> 	{
> 		static char ID;
> 		AddressSpaceAliasAnalysis() : FunctionPass(ID) {
> 		}
> 		
> 		virtual bool runOnFunction(Function& f) override
> 		{
> 			InitializeAliasAnalysis(this, &f.getParent()->getDataLayout());
> 			return false;
> 		}
> 		
> 		virtual void getAnalysisUsage(AnalysisUsage &AU) const override
> 		{
> 			AliasAnalysis::getAnalysisUsage(AU);
> 		}
> 		
> 		virtual AliasResult alias(const Location &LocA, const Location &LocB) override
> 		{
> 			const Value *V1 = LocA.Ptr;
> 			const Value *V2 = LocB.Ptr;
> 			
> 			const PointerType *PT1 = dyn_cast<const PointerType>(V1->getType());
> 			const PointerType *PT2 = dyn_cast<const PointerType>(V2->getType());
> 			
> 			// The logic here is very simple: pointers to two different address spaces
> 			// cannot alias.
> 			if (PT1 != nullptr && PT2 != nullptr)
> 			{
> 				if (PT1->getAddressSpace() != PT2->getAddressSpace())
> 				{
> 					return NoAlias;
> 				}
> 			}
> 			
> 			return AliasAnalysis::alias(LocA, LocB);
> 		}
> 		
> 		virtual void *getAdjustedAnalysisPointer(AnalysisID PI) override
> 		{
> 			if (PI == &AliasAnalysis::ID)
> 				return (AliasAnalysis*)this;
> 			return this;
> 		}
> 	};
> }
> 
> // Register this pass...
> char AddressSpaceAliasAnalysis::ID = 0;
> 
> static RegisterPass<AddressSpaceAliasAnalysis> aasa("asaa", "NoAlias for pointers in different address spaces", false, true);
> static RegisterAnalysisGroup<AliasAnalysis> aag(aasa);
> 
> FunctionPass* createAddressSpaceAliasAnalysisPass() {
> 	return new AddressSpaceAliasAnalysis();
> }


I made it a FunctionPass instead of an ImmutablePass because InitializeAliasAnalysis now needs a DataLayout argument, and I wasn’t sure how to get that one from an ImmutablePass.

The problem: I can add the pass to a `legacy::PassManager` object, and the `runOnFunction` method is called, but `alias` never is, and neither is `getAdjustedAnalysisPointer`.

I use a `PassManagerBuilder` object to populate a `legacy::PassManager`, which I then run on my module. I tried two different approaches to insert my AA pass.

The first was to add the AA pass before calling `populateModulePassManager`, since the AA passes are the first ones to be added, and this would put mine first. This didn’t help: `runOnFunction` was called, but `alias` never was. Additionally, now that I moved from `INITIALIZE_AG_PASS` to `RegisterPass<T>`, trying this causes an assert to trigger with the message "No default implementation found for analysis group!"

The second was to use the `EP_ModuleOptimizerEarly` extension point, since it is called right after the other AA passes are added. This makes my pass the last AA pass to be added. Same result: `runOnFunction` was called, but `alias` never was.

So what am I missing? How can I use my alias analysis pass?

Félix

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