UnifyFunctionExitNodes.cpp

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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 https://llvm.org/LICENSE.txt 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
// instruction in them.  Additionally, it keeps track of which node is the new
// exit node of the CFG.  If there are no exit nodes in the CFG, the getExitNode
// method will return a null pointer.
//
//===----------------------------------------------------------------------===//

#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;

char UnifyFunctionExitNodes::ID = 0;
INITIALIZE_PASS(UnifyFunctionExitNodes, "mergereturn",
                "Unify function exit nodes", false, false)

Pass *llvm::createUnifyFunctionExitNodesPass() {
  return new UnifyFunctionExitNodes();
}

void UnifyFunctionExitNodes::getAnalysisUsage(AnalysisUsage &AU) const{
  // We preserve the non-critical-edgeness property
  AU.addPreservedID(BreakCriticalEdgesID);
  // This is a cluster of orthogonal Transforms
  AU.addPreservedID(LowerSwitchID);
}

// UnifyAllExitNodes - Unify all exit nodes of the CFG by creating a new
// BasicBlock, and converting all returns to unconditional branches to this
// new basic block.  The singular exit node is returned.
//
// If there are no return stmts in the Function, a null pointer is returned.
//
bool UnifyFunctionExitNodes::runOnFunction(Function &F) {
  // Loop over all of the blocks in a function, tracking all of the blocks that
  // return.
  //
  std::vector<BasicBlock*> ReturningBlocks;
  std::vector<BasicBlock*> UnreachableBlocks;
  for (BasicBlock &I : F)
    if (isa<ReturnInst>(I.getTerminator()))
      ReturningBlocks.push_back(&I);
    else if (isa<UnreachableInst>(I.getTerminator()))
      UnreachableBlocks.push_back(&I);

  // Then unreachable blocks.
  if (UnreachableBlocks.empty()) {
    UnreachableBlock = nullptr;
  } else if (UnreachableBlocks.size() == 1) {
    UnreachableBlock = UnreachableBlocks.front();
  } else {
    UnreachableBlock = BasicBlock::Create(F.getContext(),
                                          "UnifiedUnreachableBlock", &F);
    new UnreachableInst(F.getContext(), UnreachableBlock);

    for (BasicBlock *BB : UnreachableBlocks) {
      BB->getInstList().pop_back();  // Remove the unreachable inst.
      BranchInst::Create(UnreachableBlock, BB);
    }
  }

  // Now handle return blocks.
  if (ReturningBlocks.empty()) {
    ReturnBlock = nullptr;
    return false;                          // No blocks return
  } else if (ReturningBlocks.size() == 1) {
    ReturnBlock = ReturningBlocks.front(); // Already has a single return block
    return false;
  }

  // Otherwise, we need to insert a new basic block into the function, add a 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(),
                         "UnifiedRetVal");
    NewRetBlock->getInstList().push_back(PN);
    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->getInstList().pop_back();  // Remove the return insn
    BranchInst::Create(NewRetBlock, BB);
  }
  ReturnBlock = NewRetBlock;
  return true;
}