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/LLVMContext.h"
#include "llvm/Pass.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Target/TargetData.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ValueHandle.h"
#include "llvm/Support/raw_ostream.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 {
    TargetData *TD;
#ifdef NDEBUG
    SmallPtrSet<BasicBlock*, 16> LoopHeaders;
#else
    SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders;
#endif
  public:
    static char ID; // Pass identification
    JumpThreading() : FunctionPass(&ID) {}

    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
    }

    bool runOnFunction(Function &F);
    void FindLoopHeaders(Function &F);
    
    bool ProcessBlock(BasicBlock *BB);
    bool ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BasicBlock *SuccBB,
                    unsigned JumpThreadCost);
    BasicBlock *FactorCommonPHIPreds(PHINode *PN, Value *Val);
    bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
    bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);

    bool ProcessJumpOnPHI(PHINode *PN);
    bool ProcessBranchOnLogical(Value *V, BasicBlock *BB, bool isAnd);
    bool ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB);
    
    bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
  };
}

char JumpThreading::ID = 0;
static 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) {
  DEBUG(errs() << "Jump threading on function '" << F.getName() << "'\n");
  TD = getAnalysisIfAvailable<TargetData>();
  
  FindLoopHeaders(F);
  
  bool AnotherIteration = true, EverChanged = false;
  while (AnotherIteration) {
    AnotherIteration = false;
    bool Changed = false;
    for (Function::iterator I = F.begin(), E = F.end(); I != E;) {
      BasicBlock *BB = I;
      while (ProcessBlock(BB))
        Changed = true;
      
      ++I;
      
      // If the block is trivially dead, zap it.  This eliminates the successor
      // edges which simplifies the CFG.
      if (pred_begin(BB) == pred_end(BB) &&
          BB != &BB->getParent()->getEntryBlock()) {
        DEBUG(errs() << "  JT: Deleting dead block '" << BB->getName()
              << "' with terminator: " << *BB->getTerminator());
        LoopHeaders.erase(BB);
        DeleteDeadBlock(BB);
        Changed = true;
      }
    }
    AnotherIteration = Changed;
    EverChanged |= Changed;
  }
  
  LoopHeaders.clear();
  return EverChanged;
}

/// FindLoopHeaders - We do not want jump threading to turn proper loop
/// structures into irreducible loops.  Doing this breaks up the loop nesting
/// hierarchy and pessimizes later transformations.  To prevent this from
/// happening, we first have to find the loop headers.  Here we approximate this
/// by finding targets of backedges in the CFG.
///
/// Note that there definitely are cases when we want to allow threading of
/// edges across a loop header.  For example, threading a jump from outside the
/// loop (the preheader) to an exit block of the loop is definitely profitable.
/// It is also almost always profitable to thread backedges from within the loop
/// to exit blocks, and is often profitable to thread backedges to other blocks
/// within the loop (forming a nested loop).  This simple analysis is not rich
/// enough to track all of these properties and keep it up-to-date as the CFG
/// mutates, so we don't allow any of these transformations.
///
void JumpThreading::FindLoopHeaders(Function &F) {
  SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges;
  FindFunctionBackedges(F, Edges);
  
  for (unsigned i = 0, e = Edges.size(); i != e; ++i)
    LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second));
}


/// FactorCommonPHIPreds - If there are multiple preds with the same incoming
/// value for the PHI, factor them together so we get one block to thread 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.
///
BasicBlock *JumpThreading::FactorCommonPHIPreds(PHINode *PN, Value *Val) {
  SmallVector<BasicBlock*, 16> CommonPreds;
  for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
    if (PN->getIncomingValue(i) == Val)
      CommonPreds.push_back(PN->getIncomingBlock(i));
  
  if (CommonPreds.size() == 1)
    return CommonPreds[0];
    
  DEBUG(errs() << "  Factoring out " << CommonPreds.size()
        << " common predecessors.\n");
  return SplitBlockPredecessors(PN->getParent(),
                                &CommonPreds[0], CommonPreds.size(),
                                ".thr_comm", this);
}
  

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

  // 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;