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

Wed Jun 3 11:56:12 PDT 2020

I'm with Eli on this "GlobalVariable" is not the actual global
variable in memory - it's the IR representation of the global variable
before it is compiled to machine code & allocated in the running
process - it always has the same value/isn't affected by the execution
of loads and stores at runtime.

On Wed, Jun 3, 2020 at 2:53 AM Emma Luciano via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> 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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