[llvm-dev] Fwd: I cannot change value of global variable in LLVM IR using IRBuilder

[Emma Luciano via llvm-dev]

I don't think it's the same problem as you described. By printing I meant
calling printf function and passing my global variable as one of the
arguments.

My code:

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;
}

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;
}

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: %lld\n";
print(next, message, allocatedAddress, allocatedSize, key->getOperand(0));
}
}

Code that I'm executing (source code) is:
int main()
{
char* buffer;
int size = 101;
buffer = new char[size];
printf("CHECK address: 0x%p, size: %d\n", buffer, size);
TestClass* test = new TestClass(1, 2);
printf("(CHECK address: 0x%p, size: %llu\n", test, sizeof(TestClass));

delete buffer;
delete test;
}

And program output after instrumentation:

Allocated address: 0x000001CD326C1B20, size: 101, key: 0
CHECK address: 0x000001CD326C1B20, size: 101
Allocated address: 0x000001CD326C1B90, size: 8, key: 0
CHECK address: 0x000001CD326C1B90, size: 8

Regards,
Emma Luciano

śr., 3 cze 2020 o 11:21 Eli Friedman <efriedma at quicinc.com> napisał(a):

> Suppose I have the following program:
>
>
>
> $ cat test.c
>
> int a = 0;
>
> int main() { a = 1; };
>
>
>
> Then I compile and run it:
>
> $ clang test.c
>
> $ ./a.out
>
>
>
> Then I print the contents of test.c again:
>
>
>
> $ cat test.c
>
> int a = 0;
>
> int main() { a = 1; }
>
>
>
> Why does it say “int a = 0;”?  The program set it to 1, no?
>
>
>
> As far as I can tell, this is equivalent to your question… except it’s
> less obvious because the “source code” is an LLVM IR module in memory.
>
>
>
> Maybe you’re looking for ExecutionSession::lookup?  The same way the
> tutorial computes the address of “main”, you can compute the runtime
> address of any other externally visible symbol.
>
>
>
> See also https://llvm.org/docs/ORCv2.html ,
> https://www.youtube.com/watch?v=hILdR8XRvdQ .
>
>
>
> -Eli
>
>
>
> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> *On Behalf Of *Emma
> Luciano via llvm-dev
> *Sent:* Wednesday, June 3, 2020 1:35 AM
> *To:* llvm-dev at lists.llvm.org
> *Subject:* [EXT] [llvm-dev] Fwd: I cannot change value of global variable
> in LLVM IR using IRBuilder
>
>
>
> Hi Everyone,
>
> I'm quite new to LLVM and 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 update it like that:
>
> 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);
>
> }
>
> I print that global variable during execution of instrumented code. I
> access it's value by key->getOperand(0), 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.
>
> In IR it looks like that:
>
>   %2 = load i64, i64* @globalKey
>
>   %3 = add i64 %2, 1
>
>   store i64 %3, i64* @globalKey
>
> I tried updating value of global variable, which was present in source
> code that I'm instrumenting. It didn't work either.
>
> I created corresponding stackoverflow topic:
> https://stackoverflow.com/questions/62142574/how-can-i-update-global-variable-value-in-llvm-ir-using-irbuilder?fbclid=IwAR2X7uGhUjm_SN2UcxWYqaRQW2-cs6iPbMx2_tbKubeyYUkZ_pS6rkFfY9I
>
> I will be really grateful for help.
>
> Regards,
>
> Emma Luciano
>
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