llvm builder adding terminator to basic block

Question

If I have a basic block declared within function F like so:

BasicBlock* loopSetup = BasicBlock::Create(F.getContext(), "loop_setup", &F);

How would I add a terminator instruction to it? If I just do:

IRBuilder<> builder(loopSetup);
builder.SetInsertPoint(loopSetup);
BranchInst *end = builder.CreateBr(loopStart);

Then I get this error on running the pass with opt:

Basic Block in function 'main' does not have terminator!
label %loop_setup

The terminator is clearly not set as well since loopSetup->getTerminator(); returns 0.

Answer 1

The code you provided, with some changes, seems to work fine..

#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/Support/raw_ostream.h"
#include <vector>
#include <memory>
#include <iostream>

using namespace llvm;

int main() {
    LLVMContext context;

    std::unique_ptr<Module> TheModule = std::make_unique<Module>(std::string("mod"), context);

    FunctionType *FT = FunctionType::get(Type::getVoidTy(context), std::vector<Type*>(), false);

    Function *F = Function::Create(FT, Function::ExternalLinkage, "main", TheModule.get());

    BasicBlock* loopSetup = BasicBlock::Create(context, "loop_setup", F);

    IRBuilder<> builder(loopSetup);
    builder.SetInsertPoint(loopSetup);
    BranchInst *end = builder.CreateBr(loopSetup);

    TheModule->print(errs(), nullptr);
}

This creates an endless loop. Calling loopSetup->getTerminator()->print(errs(), false) shows that the br used is the terminator of the block.

The IR output seems fine too

define void @main() {
loop_setup:
  br label %loop_setup
}

The problem might be with loopStart you used, as you didn't show how you created it.