UnifyFunctionExitNodes.cpp

//===- UnifyFunctionExitNodes.cpp - Make all functions have a single exit -===//
// 
//                     The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
// 
//===----------------------------------------------------------------------===//
//
// 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/Transforms/Scalar.h"
#include "llvm/BasicBlock.h"
#include "llvm/Function.h"
#include "llvm/iTerminators.h"
#include "llvm/iPHINode.h"
#include "llvm/Type.h"
using namespace llvm;

static RegisterOpt<UnifyFunctionExitNodes>
X("mergereturn", "Unify function exit nodes");

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

void UnifyFunctionExitNodes::getAnalysisUsage(AnalysisUsage &AU) const{
  // We preserve the non-critical-edgeness property
  AU.addPreservedID(BreakCriticalEdgesID);
}

// 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*> UnwindingBlocks;
  for(Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
    if (isa<ReturnInst>(I->getTerminator()))
      ReturningBlocks.push_back(I);
    else if (isa<UnwindInst>(I->getTerminator()))
      UnwindingBlocks.push_back(I);

  // Handle unwinding blocks first...
  if (UnwindingBlocks.empty()) {
    UnwindBlock = 0;
  } else if (UnwindingBlocks.size() == 1) {
    UnwindBlock = UnwindingBlocks.front();
  } else {
    UnwindBlock = new BasicBlock("UnifiedUnwindBlock", &F);
    new UnwindInst(UnwindBlock);

    for (std::vector<BasicBlock*>::iterator I = UnwindingBlocks.begin(), 
           E = UnwindingBlocks.end(); I != E; ++I) {
      BasicBlock *BB = *I;
      BB->getInstList().pop_back();  // Remove the return insn
      new BranchInst(UnwindBlock, 0, 0, BB);
    }
  }

  // Now handle return blocks...
  if (ReturningBlocks.empty()) {
    ReturnBlock = 0;
    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
  // node (if the function returns a value), and convert all of the return 
  // instructions into unconditional branches.
  //
  BasicBlock *NewRetBlock = new BasicBlock("UnifiedReturnBlock", &F);

  PHINode *PN = 0;
  if (F.getReturnType() != Type::VoidTy) {
    // If the function doesn't return void... add a PHI node to the block...
    PN = new PHINode(F.getReturnType(), "UnifiedRetVal");
    NewRetBlock->getInstList().push_back(PN);
  }
  new ReturnInst(PN, NewRetBlock);

  // Loop over all of the blocks, replacing the return instruction with an
  // unconditional branch.
  //
  for (std::vector<BasicBlock*>::iterator I = ReturningBlocks.begin(), 
         E = ReturningBlocks.end(); I != E; ++I) {
    BasicBlock *BB = *I;

    // 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
    new BranchInst(NewRetBlock, BB);
  }
  ReturnBlock = NewRetBlock;
  return true;
}