How to dynamically allocate and access an array using LLVM API? - llvm

Question

I'd like to generate LLVM IR for a C++ code like this:
T *arr = (T*) malloc(sizeof(T) * 100);
arr[0] = somevalue;
Here's my code:
llvm::Type* element_type = /... some T type .../;
llvm::Type* int32type = llvm::Type::getInt32Ty(context);
auto element_size = llvm::ConstantInt::get(int32type, data_layout.getTypeAllocSize(element_type));
auto array_size = llvm::ConstantInt::get(int32type, 100);
// malloc:
auto malloc_inst = llvm::CallInst::CreateMalloc(
ir_builder.GetInsertBlock(),
element_type->getPointerTo(), // T*
element_type, // T
element_size, // sizeof(T)
array_size, // 100
nullptr,
"")
ir_builder.Insert(malloc_inst);
// cast to T*:
auto arr = ir_builder.CreatePointerCast(malloc_inst, element_type->getPointerTo());
// arr[0] = somevalue:
llvm::Value *value = /... some value of type T .../
auto element_ptr = ir_builder.CreateInBoundsGEP(
arr, {
llvm::ConstantInt::get(llvm::Type::getInt32Ty(context), 0),
llvm::ConstantInt::get(llvm::Type::getInt32Ty(context), 0)
});
ir_builder.CreateStore(value, element_ptr)
However, when running generated IR I'm getting:
Assertion failed: (Ty && "Invalid GetElementPtrInst indices for type!"), function checkGEPType, file /usr/local/opt/llvm/include/llvm/IR/Instructions.h, line 847
I tried different combinations of types passed to the GEP instruction, but it's obvious I'm missing something basic.
Can someone point me to a working example for this?

Answer 1

There were two issues:
First, to access element array there's no need to pass two indices into GEP instruction.
Second, 5th argument of CreateMalloc is casted to a pointer for some reason here. After doing the multiplication of element_size * array_size by myself everything worked.
Also, explicit pointer cast (from i8* to T*) was not needed as it happens automatically.
Here's the final working code:
llvm::Type* element_type = /... some T type .../;
llvm::Type* int32type = llvm::Type::getInt32Ty(context);
auto element_size = llvm::ConstantInt::get(int32type, data_layout.getTypeAllocSize(element_type));
auto array_size = llvm::ConstantInt::get(int32type, 100);
auto alloc_size = llvm::ConstantExpr::getMul(element_size, array_size);
// malloc:
auto arr = llvm::CallInst::CreateMalloc(
ir_builder.GetInsertBlock(),
element_type->getPointerTo(), // T*
element_type, // T
alloc_size, // sizeof(T) * 100
nullptr,
nullptr,
"")
ir_builder.Insert(arr);
// arr[0] = somevalue:
llvm::Value *value = /... some value of type T .../
auto element_ptr = ir_builder.CreateInBoundsGEP(
arr, llvm::ConstantInt::get(llvm::Type::getInt32Ty(context), 0));
ir_builder.CreateStore(value, element_ptr)