How can I update global variable value in LLVM IR using IRBuilder? - c++

Question

I want to update value of global variable in LLVM IR.
I created new global variable in ModulePass:
bool runOnModule(llvm::Module &M) {
IRBuilder<> Builder(M.getContext());
Instruction *I = &*inst_begin(M.getFunction("main"));
Builder.SetInsertPoint(I);
M.getOrInsertGlobal("globalKey", Builder.getInt64Ty());
GlobalVariable* gVar = M.getNamedGlobal("globalKey");
gVar->setLinkage(GlobalValue::InternalLinkage);
gVar->setAlignment(Align(8));
gVar->setInitializer(Builder.getInt64(0));
gVar->setConstant(false);
for (Function &F : M.functions()) {
InstructionVisitor visitor(DL, getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F));
for (Instruction &I : instructions(F)) {
visitor.visit(I);
}
}
return true;
}
Later in InstructionVisitor I try to increment globalKey on each allocation and print it using printf function:
Instruction* InstructionVisitor::print(Instruction* I, const char* text, Value* arg1, Value* arg2, Value* arg3, Value* arg4) {
Function* printfFn = I->getModule()->getFunction("printf");
if (printfFn) {
IRBuilder<> Builder(I->getContext());
Builder.SetInsertPoint(I->getNextNode());
Value* convertedText = Builder.CreateGlobalStringPtr(text);
std::vector <Value *> params;
params.push_back(convertedText);
if (arg1)
params.push_back(arg1);
if (arg2)
params.push_back(arg2);
if (arg3)
params.push_back(arg3);
if (arg4)
params.push_back(arg4);
return Builder.CreateCall(printfFn, params);
}
return I;
}
Instruction* InstructionVisitor::incrementGlobalKey(Instruction* I) {
IRBuilder<> Builder(I->getContext());
Builder.SetInsertPoint(I->getNextNode());
GlobalVariable* key = I->getModule()->getNamedGlobal("globalKey");
if (key) {
LoadInst* load = Builder.CreateLoad(key);
Value* inc = Builder.CreateAdd(load, Builder.getInt64(1));
StoreInst* store = Builder.CreateStore(inc, key);
return store;
}
return I;
}
void InstructionVisitor::visitCallInst(CallInst &CI) {
if (isAllocationFn(&CI, &TLI)) {
Value* allocatedAddress = &CI;
Instruction* I = &CI;
Value* allocatedSize = I->getOperand(0);
Instruction* next = incrementGlobalKey(I);
GlobalVariable* key = I->getModule()->getNamedGlobal("globalKey");
const char* message = "Allocated address: 0x%p, size: %d, key: 0x%x\n";
print(next, message, allocatedAddress, allocatedSize, key->getOperand(0));
}
}
I print that global variable during execution of instrumented code (using injected printf call). I access it's value by key->getOperand(0) (as shown above), but it's unchanged. I'm using ORC JIT based on this tutorial: https://llvm.org/docs/tutorial/BuildingAJIT2.html and I run ModulePass from optimizeModule function from this tutorial.
IR, souce code that I'm instrumenting and program output can be found here:
https://pastebin.com/JbDR2Wug
Does anyone know how to make it work? I will be grateful for help!

Answer 1

After #droptop helpful comment I changed my code to actually load the global variable's value using load instruction. It works fine now. Updated code is shown below if anyone need it:
Instruction* InstructionVisitor::getGlobalValue(Instruction* I, StringRef Name) {
IRBuilder<> Builder(I->getContext());
Builder.SetInsertPoint(I->getNextNode());
GlobalVariable* key = I->getModule()->getNamedGlobal(Name);
if (key) {
LoadInst* load = Builder.CreateLoad(key);
return load;
}
return nullptr;
}
void InstructionVisitor::visitCallInst(CallInst &CI) {
if (isAllocationFn(&CI, &TLI)) {
Value* allocatedAddress = &CI;
Instruction* I = &CI;
Value* allocatedSize = I->getOperand(0);
Instruction* next = incrementGlobalKey(I, allocatedAddress, allocatedSize);
Instruction* loadKey = getGlobalValue(next, "globalKey"); //here
const char* message = "Allocated address: 0x%p, size: %d, key: %lld\n";
next = print(loadKey, message, allocatedAddress, allocatedSize, loadKey);
}
}