95 lines · cpp
1//===- UnifyFunctionExitNodes.cpp - Make all functions have a single exit -===//2//3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.4// See https://llvm.org/LICENSE.txt for license information.5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception6//7//===----------------------------------------------------------------------===//8//9// This pass is used to ensure that functions have at most one return and one10// unreachable instruction in them.11//12//===----------------------------------------------------------------------===//13 14#include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h"15#include "llvm/IR/BasicBlock.h"16#include "llvm/IR/Function.h"17#include "llvm/IR/Instructions.h"18#include "llvm/IR/Type.h"19using namespace llvm;20 21namespace {22 23bool unifyUnreachableBlocks(Function &F) {24 std::vector<BasicBlock *> UnreachableBlocks;25 26 for (BasicBlock &I : F)27 if (isa<UnreachableInst>(I.getTerminator()))28 UnreachableBlocks.push_back(&I);29 30 if (UnreachableBlocks.size() <= 1)31 return false;32 33 BasicBlock *UnreachableBlock =34 BasicBlock::Create(F.getContext(), "UnifiedUnreachableBlock", &F);35 new UnreachableInst(F.getContext(), UnreachableBlock);36 37 for (BasicBlock *BB : UnreachableBlocks) {38 BB->back().eraseFromParent(); // Remove the unreachable inst.39 BranchInst::Create(UnreachableBlock, BB);40 }41 42 return true;43}44 45bool unifyReturnBlocks(Function &F) {46 std::vector<BasicBlock *> ReturningBlocks;47 48 for (BasicBlock &I : F)49 if (isa<ReturnInst>(I.getTerminator()))50 ReturningBlocks.push_back(&I);51 52 if (ReturningBlocks.size() <= 1)53 return false;54 55 // Insert a new basic block into the function, add PHI nodes (if the function56 // returns values), and convert all of the return instructions into57 // unconditional branches.58 BasicBlock *NewRetBlock = BasicBlock::Create(F.getContext(),59 "UnifiedReturnBlock", &F);60 61 PHINode *PN = nullptr;62 if (F.getReturnType()->isVoidTy()) {63 ReturnInst::Create(F.getContext(), nullptr, NewRetBlock);64 } else {65 // If the function doesn't return void... add a PHI node to the block...66 PN = PHINode::Create(F.getReturnType(), ReturningBlocks.size(),67 "UnifiedRetVal");68 PN->insertInto(NewRetBlock, NewRetBlock->end());69 ReturnInst::Create(F.getContext(), PN, NewRetBlock);70 }71 72 // Loop over all of the blocks, replacing the return instruction with an73 // unconditional branch.74 for (BasicBlock *BB : ReturningBlocks) {75 // Add an incoming element to the PHI node for every return instruction that76 // is merging into this new block...77 if (PN)78 PN->addIncoming(BB->getTerminator()->getOperand(0), BB);79 80 BB->back().eraseFromParent(); // Remove the return insn81 BranchInst::Create(NewRetBlock, BB);82 }83 84 return true;85}86} // namespace87 88PreservedAnalyses UnifyFunctionExitNodesPass::run(Function &F,89 FunctionAnalysisManager &AM) {90 bool Changed = false;91 Changed |= unifyUnreachableBlocks(F);92 Changed |= unifyReturnBlocks(F);93 return Changed ? PreservedAnalyses() : PreservedAnalyses::all();94}95