在运行期间在LLVM中记录分支指令的结果

Question

I am trying to obtain the outcomes of branch instructions by writing a LLVM pass so I can figure out the edge frequencies of a CFG.

I have read multiple tutorial files and the LLVM documentation. The solution I have tried is to identify branch instructions, insert a call to a log function that displays the branch source and destination.

I have written a Function pass that looks like this:

for (auto &BB : F) {
    for (auto &I: BB) {
        if (auto* op = dyn_cast<BranchInst>(&I)) {
            IRBuilder<> builder(op);
            builder.SetInsertPoint(&BB, builder.GetInsertPoint());

            Value* condition;
            Value* false_dst;
            Value* true_dst;
            if (op->isConditional()) {
                condition = op->getOperand(0);
                false_dst = op->getOperand(1);
                true_dst = op->getOperand(2);
            } else {
                condition = builder.getInt32(1);
                false_dst = op->getOperand(0);
                true_dst = op->getOperand(0);
            }

            Value* args[] = {condition, false_dst, true_dst};
            builder.CreateCall(log_func, args);
        }
    }
 }

I have been trying multiple solution for around 10 hours now and none of them have worked. I keep running into this error:

void llvm::CallInst::init(llvm::FunctionType*, llvm::Value*, llvm::ArrayRef<llvm::Value*>, llvm::ArrayRef<llvm::OperandBundleDefT<llvm::Value*> >, const llvm::Twine&): Assertion `(Args.size() == FTy->getNumParams() || (FTy->isVarArg() && Args.size() > FTy->getNumParams())) && "Calling a function with bad signature!"' failed.

It seems to only be doing this with branch instructions, since I have tested with binary operators, and it works fine.

Answer 1

Run your code under debugger and evaluate log_func->dump() expression. This will give you an idea what arguments of what types this function expects.

I suspect, the problem is with last 2 arguments. These are basic block labels, which can't be passed as arguments of call instruction. You may want to create string constants with BB names and pass them instead.