CODEWITHSUNDEEP
c++cllvm
Mantas
Mantas

Reputation: 1223

LoadInst and StoreInst Values and addresses LLVM

I have a file print.c, which has two functions:

void printLoad(...) {
  // print address and value of memory location from which value
  printf("address=... value=...", ...); 
}

void printStore(...) {
  // print address and value of memory location from which value 
}

I have an LLVM pass which iterates over the instructions and adds CallInst instruction either printLoad or printStore (depending on the instruction type) after the current one (load/store inst).

In order to call this printStore or printLoad I need to add appropriate arguments to CallInst::Create function, which are the address and the value of the memory location.

This is an example of what I want to achieve:

define void @mains() #0 {
  %1 = alloca i32, align 4
  %2 = alloca i32, align 4
  store i32 0, i32* %1, align 4
  store i32 5, i32* %1, align 4
  store i32 2, i32* %2, align 4
  store i32 4, i32* %2, align 4
  %3 = load i32, i32* %2, align 4
  %4 = add nsw i32 %3, 5
  store i32 %4, i32* %1, align 4
  ret void
}

The output should be:
  store instruction: 
    address=...   // address of %1
    value=0
  ...
  ...
  ...
  load instruction:
    address=...  // address of %2
    value=4
  store instruction:
    address=...  // address of %1
    value=9

Progress so far:

I am able to get the addresses of the operands using getPointerOperand() on LoadInst/StoreInst.

I can also get the value of StoreInst in the first 4 store instructions by casting the operand to ConstantInt, but I don't know how to extract the value in the last StoreInst. Is it even possible?

EDITED:

Using 

void printLoad(int32_t p)

and

Constant *hookLoadFunc = M.getOrInsertFunction("printLoad", Type::getVoidTy(M.getContext()), Type::getInt32Ty(M.getContext()));

.

  %1 = alloca i32, align 4
  %2 = alloca i32, align 4
  %3 = alloca i32, align 4
  store i32 0, i32* %1, align 4
  call void @printStore(i32 0)
  store i32 0, i32* %2, align 4
  call void @printStore(i32 0)
  store i32 5, i32* %2, align 4
  call void @printStore(i32 5)
  store i32 2, i32* %3, align 4
  call void @printStore(i32 2)
  store i32 4, i32* %3, align 4
  call void @printStore(i32 4)
  %4 = load i32, i32* %3, align 4
  %5 = add nsw i32 %4, 5
  store i32 %5, i32* %2, align 4
  call void @printStore(i32 %5)
  ret i32 0
  %2 = alloca i32, align 4
  store i32 %0, i32* %2, align 4
  call void @printStore(i32 %0)
  %3 = load i32, i32* %2, align 4
  %4 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([22 x i8], [22 x i8]* @.str, i32 0, i32 0), i32 %3)
  ret void
  %2 = alloca i32, align 4
  store i32 %0, i32* %2, align 4
  call void @printStore(i32 %0)
  %3 = load i32, i32* %2, align 4
  %4 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([22 x i8], [22 x i8]* @.str.1, i32 0, i32 0), i32 %3)
  ret void

This causes Segmentation fault: 11 when run.

SOLVED:

Figured out that I had infinity loop (due to recursion). printStore actually uses load/store instructions, thus creating another call to printStore and so on.

Upvotes: 3

Views: 5300

Answers (1)

sbabbi
sbabbi

Reputation: 11181

Assuming that you have an llvm::Function that represents printLoad() and printStore():

llvm::Function * print_load = ....
llvm::Function * print_store = ...

You can emit a CallInst for each LoadInst and StoreInst.

For LoadInst:

LoadInst * some_load = ...
Value * address_of_load = some_load->getOperand(0);
Value * print_load_arguments[] = { address_of_load, some_load };

// Insert a CallInst just after the load.
CallInst::Create(print_load, print_load_arguments )->insertAfter( some_load );

Remember that in llvm the value loaded by the LoadInst is the same thing as the LoadInst itself.

For StoreInst:

StoreInst * some_store = ...
Value * value_to_store = some_store->getOperand(0);
Value * address_of_store = some_store->getOperand(1);
Value * print_store_arguments[] = { address_of_store, value_to_store };

// Insert a CallInst just after the store.
CallInst::Create(print_store, print_store_arguments)->insertAfter(some_store);

This will work if all the types match. Otherwise, you have to insert BitCast instructions just before calling printStore() or printLoad().

Upvotes: 6

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