LLVM Pass to insert an external function call to LLVM bitcode - c

Question

I am writing an LLVM pass to instrument a C source program. I want to insert a function call before each branch instruction which calls an external function like this :
void print(int x){
printf("x = %d\n", x);
return;
}
I want to link this external function to C source code using llvm-link tool and then instrument the code using opt tool.
Ths pass that I have implemented is like this:
#include "llvm/Pass.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Function.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/IR/IRBuilder.h"
#include <vector>
using namespace llvm;
namespace{
struct ir_instrumentation : public ModulePass{
static char ID;
Function *monitor;
ir_instrumentation() : ModulePass(ID) {}
virtual bool runOnModule(Module &M)
{
std::vector<Type *> args;
args.push_back(Type::getInt32Ty(M.getContext()));
ArrayRef<Type*> argsRef(args);
FunctionType *FT = FunctionType::get(Type::getVoidTy(M.getContext()), args, false);
Constant* myFunc = M.getOrInsertFunction("print", FT, NULL);
minitor = cast<Function>(myFunc);
for(Module::iterator F = M.begin(), E = M.end(); F!= E; ++F)
{
for(Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
{
for(BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE; ++BI)
{
if(isa<BranchInst>(&(*BI)) )
{
errs() << "found a branch instruction!\n";
ArrayRef< Value* > arguments(ConstantInt::get(Type::getInt32Ty(M.getContext()), 5, true));
Instruction *newInst = CallInst::Create(monitor, arguments, "");
BB->getInstList().insert(BI, newInst);
errs() << "Inserted the function!\n";
}
}
}
}
return true;
}
};
char ir_instrumentation::ID = 0;
static RegisterPass<ir_instrumentation> X("ir-instrumentation", "LLVM IR Instrumentation Pass");
}
LLVM is configured and built fine with this pass but when I use opt, I get this error :
opt: /llvm/lib/IR/Type.cpp:281:
llvm::FunctionType::FunctionType(llvm::Type*, llvm::ArrayRefllvm::Type*, bool):
Assertion `isValidReturnType(Result) && "invalid return type for function"' failed.
I think the problem is something like mismatching between the function type that I have declared and the external function (like the context).
LLVM version: LLVM version 7.0.0svn
Until now I have not solved the problem.
Thanks

Answer 1

I could finally solve this problem and successfully instrument LLVM bitcode. After a lot of trouble with function getOrInsertFunction, I found out it is not really necessary to use this method in my case. I just simply changed my pass to this:
#include "llvm/Pass.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Function.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/IR/IRBuilder.h"
#include <vector>
using namespace llvm;
namespace{
struct ir_instrumentation : public ModulePass{
static char ID;
Function *monitor;
ir_instrumentation() : ModulePass(ID) {}
virtual bool runOnModule(Module &M)
{
errs() << "====----- Entered Module " << M.getName() << ".\n";
int counter = 0;
for(Module::iterator F = M.begin(), E = M.end(); F!= E; ++F)
{
errs() << "Function name: " << F->getName() << ".\n";
if(F->getName() == "print"){
monitor = cast<Function>(F);
continue;
}
for(Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
{
for(BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE; ++BI)
{
if(isa<BranchInst>(&(*BI)) )
{
errs() << "found a brach instruction!\n";
ArrayRef< Value* > arguments(ConstantInt::get(Type::getInt32Ty(M.getContext()), counter, true));
counter++;
Instruction *newInst = CallInst::Create(monitor, arguments, "");
BB->getInstList().insert(BI, newInst);
errs() << "Inserted the function!\n";
}
}
}
}
return true;
}
};
char ir_instrumentation::ID = 0;
static RegisterPass<ir_instrumentation> X("ir-instrumentation", "LLVM IR Instrumentation Pass");
}
As I know the name of the external function, I can simply find it through iterating over all functions of module and then use it in the desired way.
Obviously the problem was caused by calling module->getOrInsertFunction and the function type. My experience says that this method is more useful when you want to insert a new function and declare the prototype of your own function. Using it to get an existing function is challenging (e.g. setting the right prototype, ...)
Thanks