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

[Eli Friedman via llvm-dev]

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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