Count number of arrays

问题:

How can the total number of arrays be counted in a C program ?

The array declarations in LLVM IR correspond to alloca type of operation. So

int a[10]; 

corresponds to

%a = alloca [10 x i32], align 4

in LLVM IR.

But I also noticed that

 int j = 0;

also corresponds to an alloca instruction

 %j = alloca i32, align 4

So how to count the number of alloca instructions that correspond only to arrays ?

EDIT:

  for (Function::iterator i = F.begin(), e = F.end(); i != e; ++i)
  {
      for (BasicBlock::iterator ii =(*i).begin(), ii_e = (*i).end(); ii != ii_e; ++ii) 
      {
           Instruction *n = dyn_cast<Instruction>(&*ii);
           for( int num = 0; num < n->getNumOperands(); ++num)  
            if(isa<ArrayType>(n->getOperand(num)->getType()))
        {
              // doesn't work
          errs()<<"yayayayay Array\n";
        }
       }
   }

回答1:

AllocaInst has public method isArrayAllocation(). You can use it to count the number of alloca instructions that correspond only to arrays.

for (Function::iterator BB = F.begin(), BE = F.end(); BB != BE; ++BB)
  for (BasicBlock::iterator II = BB->begin(), IE = BB->end(); II != IE; ++II)
    if (AllocaInst *AI = dyn_cast<AllocaInst>(II))
      if (AI->isArrayAllocation())
        errs() << "Alloca of array is found!\n";

回答2:

Open the LLVM demo page and compile following code

int main(int argc, char **argv) {
  int a[10];
  return 0;
}

to the LLVM C++ API calls.

This is how a created:

AllocaInst* ptr_a = new AllocaInst(ArrayTy_6, "a", label_9);

where ArrayTy_6 is:

ArrayType* ArrayTy_6 = ArrayType::get(IntegerType::get(mod->getContext(), 32), 10);

So, to find out if alloca instruction you are looking at is defining array, just do isa<ArrayType>() on it's first argument.

See LLVM docs for more info.