JumpThreading.cpp

//===- JumpThreading.cpp - Thread control through conditional blocks ------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the Jump Threading pass.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "jump-threading"
#include "llvm/Transforms/Scalar.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/Pass.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
using namespace llvm;

STATISTIC(NumThreads, "Number of jumps threaded");
STATISTIC(NumFolds,   "Number of terminators folded");

static cl::opt<unsigned>
Threshold("jump-threading-threshold", 
          cl::desc("Max block size to duplicate for jump threading"),
          cl::init(6), cl::Hidden);

namespace {
  /// This pass performs 'jump threading', which looks at blocks that have
  /// multiple predecessors and multiple successors.  If one or more of the
  /// predecessors of the block can be proven to always jump to one of the
  /// successors, we forward the edge from the predecessor to the successor by
  /// duplicating the contents of this block.
  ///
  /// An example of when this can occur is code like this:
  ///
  ///   if () { ...
  ///     X = 4;
  ///   }
  ///   if (X < 3) {
  ///
  /// In this case, the unconditional branch at the end of the first if can be
  /// revectored to the false side of the second if.
  ///
  class VISIBILITY_HIDDEN JumpThreading : public FunctionPass {
  public:
    static char ID; // Pass identification
    JumpThreading() : FunctionPass((intptr_t)&ID) {}

    bool runOnFunction(Function &F);
    bool ThreadBlock(BasicBlock *BB);
    void ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BasicBlock *SuccBB);
  };
  char JumpThreading::ID = 0;
  RegisterPass<JumpThreading> X("jump-threading", "Jump Threading");
}

// Public interface to the Jump Threading pass
FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); }

/// runOnFunction - Top level algorithm.
///
bool JumpThreading::runOnFunction(Function &F) {
  DOUT << "Jump threading on function '" << F.getNameStart() << "'\n";
  
  bool AnotherIteration = true, EverChanged = false;
  while (AnotherIteration) {
    AnotherIteration = false;
    bool Changed = false;
    for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
      while (ThreadBlock(I))
        Changed = true;
    AnotherIteration = Changed;
    EverChanged |= Changed;
  }
  return EverChanged;
}

/// getJumpThreadDuplicationCost - Return the cost of duplicating this block to
/// thread across it.
static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) {
  BasicBlock::const_iterator I = BB->begin();
  /// Ignore PHI nodes, these will be flattened when duplication happens.
  while (isa<PHINode>(*I)) ++I;

  // Sum up the cost of each instruction until we get to the terminator.  Don't
  // include the terminator because the copy won't include it.
  unsigned Size = 0;
  for (; !isa<TerminatorInst>(I); ++I) {
    // Debugger intrinsics don't incur code size.
    if (isa<DbgInfoIntrinsic>(I)) continue;
    
    // If this is a pointer->pointer bitcast, it is free.
    if (isa<BitCastInst>(I) && isa<PointerType>(I->getType()))
      continue;
    
    // All other instructions count for at least one unit.
    ++Size;
    
    // Calls are more expensive.  If they are non-intrinsic calls, we model them
    // as having cost of 4.  If they are a non-vector intrinsic, we model them
    // as having cost of 2 total, and if they are a vector intrinsic, we model
    // them as having cost 1.
    if (const CallInst *CI = dyn_cast<CallInst>(I)) {
      if (!isa<IntrinsicInst>(CI))
        Size += 3;
      else if (isa<VectorType>(CI->getType()))
        Size += 1;
    }
  }
  
  // Threading through a switch statement is particularly profitable.  If this
  // block ends in a switch, decrease its cost to make it more likely to happen.
  if (isa<SwitchInst>(I))
    Size = Size > 6 ? Size-6 : 0;
  
  return Size;
}


/// ThreadBlock - If there are any predecessors whose control can be threaded
/// through to a successor, transform them now.
bool JumpThreading::ThreadBlock(BasicBlock *BB) {
  // See if this block ends with a branch of switch.  If so, see if the
  // condition is a phi node.  If so, and if an entry of the phi node is a
  // constant, we can thread the block.
  Value *Condition;
  if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
    // Can't thread an unconditional jump.
    if (BI->isUnconditional()) return false;
    Condition = BI->getCondition();
  } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator()))
    Condition = SI->getCondition();
  else
    return false; // Must be an invoke.
  
  // If the terminator of this block is branching on a constant, simplify the
  // terminator to an unconditional branch.  This can occur do to threading in
  // other blocks.
  if (isa<ConstantInt>(Condition)) {
    DOUT << "  In block '" << BB->getNameStart()
         << "' folding terminator: " << *BB->getTerminator();
    ++NumFolds;
    ConstantFoldTerminator(BB);
    return true;
  }
  
  // If there is only a single predecessor of this block, nothing to fold.
  if (BB->getSinglePredecessor())
    return false;

  // See if this is a phi node in the current block.
  PHINode *PN = dyn_cast<PHINode>(Condition);
  if (!PN || PN->getParent() != BB) return false;
  
  // See if the phi node has any constant values.  If so, we can determine where
  // the corresponding predecessor will branch.
  unsigned PredNo = ~0U;
  ConstantInt *PredCst = 0;
  for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
    if ((PredCst = dyn_cast<ConstantInt>(PN->getIncomingValue(i)))) {
      PredNo = i;
      break;
    }
  }
  
  // If no incoming value has a constant, we don't know the destination of any
  // predecessors.
  if (PredNo == ~0U)
    return false;
  
  // See if the cost of duplicating this block is low enough.
  unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
  if (JumpThreadCost > Threshold) {
    DOUT << "  Not threading BB '" << BB->getNameStart()
         << "' - Cost is too high: " << JumpThreadCost << "\n";
    return false;
  }
  
  // If so, we can actually do this threading.  Figure out which predecessor and
  // which successor we are threading for.
  BasicBlock *PredBB = PN->getIncomingBlock(PredNo);
  BasicBlock *SuccBB;
  if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
    SuccBB = BI->getSuccessor(PredCst == ConstantInt::getFalse());
  else {
    SwitchInst *SI = cast<SwitchInst>(BB->getTerminator());
    SuccBB = SI->getSuccessor(SI->findCaseValue(PredCst));
  }
  
  // TODO: If there are multiple preds with the same incoming value for the PHI,
  // factor them together so we get one thread block for the whole group.  This
  // is important for things like "phi i1 [true, true, false, true, x]" where
  // we only need to clone the block for the true blocks once.
  
  DOUT << "  Threading edge from '" << PredBB->getNameStart() << "' to '"
       << SuccBB->getNameStart() << "' with cost: " << JumpThreadCost
       << ", across block:\n    "
       << *BB;
       
  ThreadEdge(BB, PredBB, SuccBB);
  ++NumThreads;
  return true;
}

/// ThreadEdge - We have decided that it is safe and profitable to thread an
/// edge from PredBB to SuccBB across BB.  Transform the IR to reflect this
/// change.
void JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, 
                               BasicBlock *SuccBB) {

  // Jump Threading can not update SSA properties correctly if the values
  // defined in the duplicated block are used outside of the block itself.  For
  // this reason, we spill all values that are used outside of BB to the stack.
  for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I)
    if (I->isUsedOutsideOfBlock(BB)) {
      // We found a use of I outside of BB.  Create a new stack slot to
      // break this inter-block usage pattern.
      DemoteRegToStack(*I);
    }
 
  // We are going to have to map operands from the original BB block to the new
  // copy of the block 'NewBB'.  If there are PHI nodes in BB, evaluate them to
  // account for entry from PredBB.
  DenseMap<Instruction*, Value*> ValueMapping;
  
  BasicBlock *NewBB =
    BasicBlock::Create(BB->getName()+".thread", BB->getParent(), BB);
  NewBB->moveAfter(PredBB);
  
  BasicBlock::iterator BI = BB->begin();
  for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
    ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
  
  // Clone the non-phi instructions of BB into NewBB, keeping track of the
  // mapping and using it to remap operands in the cloned instructions.
  for (; !isa<TerminatorInst>(BI); ++BI) {
    Instruction *New = BI->clone();
    New->setName(BI->getNameStart());
    NewBB->getInstList().push_back(New);
    ValueMapping[BI] = New;
   
    // Remap operands to patch up intra-block references.
    for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
      if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i)))
        if (Value *Remapped = ValueMapping[Inst])
          New->setOperand(i, Remapped);
  }
  
  // We didn't copy the terminator from BB over to NewBB, because there is now
  // an unconditional jump to SuccBB.  Insert the unconditional jump.
  BranchInst::Create(SuccBB, NewBB);
  
  // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
  // PHI nodes for NewBB now.
  for (BasicBlock::iterator PNI = SuccBB->begin(); isa<PHINode>(PNI); ++PNI) {
    PHINode *PN = cast<PHINode>(PNI);
    // Ok, we have a PHI node.  Figure out what the incoming value was for the
    // DestBlock.
    Value *IV = PN->getIncomingValueForBlock(BB);
    
    // Remap the value if necessary.
    if (Instruction *Inst = dyn_cast<Instruction>(IV))
      if (Value *MappedIV = ValueMapping[Inst])
        IV = MappedIV;
    PN->addIncoming(IV, NewBB);
  }
  
  // Finally, NewBB is good to go.  Update the terminator of PredBB to jump to
  // NewBB instead of BB.  This eliminates predecessors from BB, which requires
  // us to simplify any PHI nodes in BB.
  TerminatorInst *PredTerm = PredBB->getTerminator();
  for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
    if (PredTerm->getSuccessor(i) == BB) {
      BB->removePredecessor(PredBB);
      PredTerm->setSuccessor(i, NewBB);
    }
}