How to create LLVM Array type using AllocaInst?

Question

I want to create LLVM ArrayType on stack so I wanted to use AllocaInst (Type *Ty, Value *ArraySize=nullptr, const Twine &Name="", Instruction *InsertBefore=nullptr). The problem is I don't understand this interface. I guessed that Ty will be something like ArrayType::get(I.getType(), 4), but what should I give for ArraySize. Furthermore, it requires Value*, so it confused me a lot.

Either I misunderstood llvm alloc or I need to provide a llvm constant as a value for array size. If I have to give the constant, isn't it little bit redundant since ArrayType contains the numElement as an information.

As an example line of code, the way I am trying to:

AllocaInst* arr_alloc = new AllocaInst(ArrayType::get(I.getType(), num)
                                       /*, What is this parameter for?*/,
                                       "",
                                       funcEntry.getFirstInsertionPt());

Answer 1

I is the type of the array elements such as:

Type* I = IntegerType::getInt32Ty(module->getContext());

Then you can create an ArrayType of num elements:

ArrayType* arrayType = ArrayType::get(I, num);

This type can be used in an AllocInstr as following:

AllocaInst* arr_alloc = new AllocaInst(
    arrayType, "myarray" , funcEntry
//             ~~~~~~~~~
// -> custom variable name in the LLVM IR which can be omitted,
//    LLVM will create a random name then such as %2.
);

This example will yield the following LLVM IR instruction:

%myarray = alloca [10 x i32]

EDIT Also it seems like you are allowed to pass a variable array size to the AllocInstr as following:

Type* I = IntegerType::getInt32Ty(module->getContext());
auto num = 10;
auto funcEntry = label_entry;

ArrayType* arrayType = ArrayType::get(I, num);

AllocaInst* variable = new AllocaInst(
  I, "array_size", funcEntry
);

new StoreInst(ConstantInt::get(I, APInt(32, 10)), variable, funcEntry);

auto load = new LoadInst(variable, "loader", funcEntry);

AllocaInst* arr_alloc = new AllocaInst(
  I, load, "my_array", funcEntry
);