path: root/lib/Transforms/Utils/SimplifyCFG.cpp

//===- SimplifyCFG.cpp - Code to perform CFG simplification ---------------===//
// 
//                     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.
// 
//===----------------------------------------------------------------------===//
//
// Peephole optimize the CFG.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "simplifycfg"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Constants.h"
#include "llvm/Instructions.h"
#include "llvm/Type.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/Debug.h"
#include <algorithm>
#include <functional>
#include <set>

using namespace llvm;

// PropagatePredecessorsForPHIs - This gets "Succ" ready to have the
// predecessors from "BB".  This is a little tricky because "Succ" has PHI
// nodes, which need to have extra slots added to them to hold the merge edges
// from BB's predecessors, and BB itself might have had PHI nodes in it.  This
// function returns true (failure) if the Succ BB already has a predecessor that
// is a predecessor of BB and incoming PHI arguments would not be discernible.
//
// Assumption: Succ is the single successor for BB.
//
static bool PropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
  assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");

  if (!isa<PHINode>(Succ->front()))
    return false;  // We can make the transformation, no problem.

  // If there is more than one predecessor, and there are PHI nodes in
  // the successor, then we need to add incoming edges for the PHI nodes
  //
  const std::vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));

  // Check to see if one of the predecessors of BB is already a predecessor of
  // Succ.  If so, we cannot do the transformation if there are any PHI nodes
  // with incompatible values coming in from the two edges!
  //
  for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ); PI != PE; ++PI)
    if (find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end()) {
      // Loop over all of the PHI nodes checking to see if there are
      // incompatible values coming in.
      for (BasicBlock::iterator I = Succ->begin();
           PHINode *PN = dyn_cast<PHINode>(I); ++I) {
        // Loop up the entries in the PHI node for BB and for *PI if the values
        // coming in are non-equal, we cannot merge these two blocks (instead we
        // should insert a conditional move or something, then merge the
        // blocks).
        int Idx1 = PN->getBasicBlockIndex(BB);
        int Idx2 = PN->getBasicBlockIndex(*PI);
        assert(Idx1 != -1 && Idx2 != -1 &&
               "Didn't have entries for my predecessors??");
        if (PN->getIncomingValue(Idx1) != PN->getIncomingValue(Idx2))
          return true;  // Values are not equal...
      }
    }

  // Loop over all of the PHI nodes in the successor BB.
  for (BasicBlock::iterator I = Succ->begin();
       PHINode *PN = dyn_cast<PHINode>(I); ++I) {
    Value *OldVal = PN->removeIncomingValue(BB, false);
    assert(OldVal && "No entry in PHI for Pred BB!");

    // If this incoming value is one of the PHI nodes in BB, the new entries in
    // the PHI node are the entries from the old PHI.
    if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) {
      PHINode *OldValPN = cast<PHINode>(OldVal);
      for (unsigned i = 0, e = OldValPN->getNumIncomingValues(); i != e; ++i)
        PN->addIncoming(OldValPN->getIncomingValue(i),
                        OldValPN->getIncomingBlock(i));
    } else {
      for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(), 
             End = BBPreds.end(); PredI != End; ++PredI) {
        // Add an incoming value for each of the new incoming values...
        PN->addIncoming(OldVal, *PredI);
      }
    }
  }
  return false;
}

/// GetIfCondition - Given a basic block (BB) with two predecessors (and
/// presumably PHI nodes in it), check to see if the merge at this block is due
/// to an "if condition".  If so, return the boolean condition that determines
/// which entry into BB will be taken.  Also, return by references the block
/// that will be entered from if the condition is true, and the block that will
/// be entered if the condition is false.
/// 
///
static Value *GetIfCondition(BasicBlock *BB,