Question

I am trying to determine for certain Load instructions from my pass their corresponding Alloca instructions (that can be in other previous blocks). The chain can be something like : TargetLoad(var) -> other stores/loads that use var (or dependencies on var) -> alloca(var). , linked on several basic blocks. Do you know how can I do it?

I tried to use the methods from DependenceAnalysis and MemoryDependenceAnalysis, but the results were not correct. For instance, MemoryDependenceAnalysis::getDependency should be good with option “Def”, but works only for stores, not for loads. Also I have a segfault when trying to use MemoryDependenceAnalysis::getNonLocalPointerDependency or MemoryDependenceAnalysis::getPointerDependencyFrom . When I try to check my result using MemDepResult::getDef(), the result for Load instructions is the same instruction ! So its depending on itself, that being weird since it is using a variable that is previously defined in the code.

The alternative of making the intersection for identifying common parts between all the variables used by target_load_instructions and all the allocated variables is not an option. Because there might be something like : alloca(a) ... c=a*b+4 .... load(c).

It seems also that DependenceAnalysis::depends() is not ok for my pass. The next line of code is only for reference: if(DA.depends(allocaInstrArray[i],loadInstrArray[j],true)) is always false. And it should be true in several cases. I think I am not using it correctly.

However, I made the assumption that maybe depends() does not work for Alloca. So I checked the dependencies among all Load instructions kept in an array. Some results are not based on the loaded variable as they should. For example: LOAD %3 = load i32* %c, align 4 IS DEPENDENT ON %1 = load i32* %j, align 4. As you can see, one is loading c and one is loading j. In my Test.cpp target code there is no dependence between j and c. Maybe the dependence is not based on variables/memory locations used?

Thank you for any suggestion !

Answer 1

First, use getOperand(0) or getOperand(1) of the ICMP instructions. If there is isa<LoadInst> valid, then cast them to LoadInst. getPointerOperand() will get the Value* that is the actual variable which is searched.
Second, do the same procedure between Load instructions and Alloca instructions. getOperand(0) applied on Load gives the corresponding Alloca instruction.
Finally, link the two results together, by checking the dependencies.