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//===- UnifyFunctionExitNodes.cpp - Make all functions have a single exit -===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This pass is used to ensure that functions have at most one return and one
// unreachable instruction in them.
#include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Type.h"
#include "llvm/Transforms/Utils.h"
using namespace llvm;
namespace {
bool unifyUnreachableBlocks(Function &F) {
std::vector<BasicBlock *> UnreachableBlocks;
for (BasicBlock &I : F)
if (isa<UnreachableInst>(I.getTerminator()))
if (UnreachableBlocks.size() <= 1)
return false;
BasicBlock *UnreachableBlock =
BasicBlock::Create(F.getContext(), "UnifiedUnreachableBlock", &F);
new UnreachableInst(F.getContext(), UnreachableBlock);
for (BasicBlock *BB : UnreachableBlocks) {
BB->back().eraseFromParent(); // Remove the unreachable inst.
BranchInst::Create(UnreachableBlock, BB);
return true;
bool unifyReturnBlocks(Function &F) {
std::vector<BasicBlock *> ReturningBlocks;
for (BasicBlock &I : F)
if (isa<ReturnInst>(I.getTerminator()))
if (ReturningBlocks.size() <= 1)
return false;
// Insert a new basic block into the function, add PHI nodes (if the function
// returns values), and convert all of the return instructions into
// unconditional branches.
BasicBlock *NewRetBlock = BasicBlock::Create(F.getContext(),
"UnifiedReturnBlock", &F);
PHINode *PN = nullptr;
if (F.getReturnType()->isVoidTy()) {
ReturnInst::Create(F.getContext(), nullptr, NewRetBlock);
} else {
// If the function doesn't return void... add a PHI node to the block...
PN = PHINode::Create(F.getReturnType(), ReturningBlocks.size(),
PN->insertInto(NewRetBlock, NewRetBlock->end());
ReturnInst::Create(F.getContext(), PN, NewRetBlock);
// Loop over all of the blocks, replacing the return instruction with an
// unconditional branch.
for (BasicBlock *BB : ReturningBlocks) {
// Add an incoming element to the PHI node for every return instruction that
// is merging into this new block...
if (PN)
PN->addIncoming(BB->getTerminator()->getOperand(0), BB);
BB->back().eraseFromParent(); // Remove the return insn
BranchInst::Create(NewRetBlock, BB);
return true;
} // namespace
PreservedAnalyses UnifyFunctionExitNodesPass::run(Function &F,
FunctionAnalysisManager &AM) {
bool Changed = false;
Changed |= unifyUnreachableBlocks(F);
Changed |= unifyReturnBlocks(F);
return Changed ? PreservedAnalyses() : PreservedAnalyses::all();