BranchFolding.cpp

//===-- BranchFolding.cpp - Fold machine code branch instructions ---------===//
//
//                     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 forwards branches to unconditional branches to make them branch
// directly to the target block.  This pass often results in dead MBB's, which
// it then removes.
//
// Note that this pass must be run after register allocation, it cannot handle
// SSA form.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "branchfolding"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineJumpTableInfo.h"
#include "llvm/CodeGen/RegisterScavenging.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/MRegisterInfo.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include <algorithm>
using namespace llvm;

STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
STATISTIC(NumBranchOpts, "Number of branches optimized");
STATISTIC(NumTailMerge , "Number of block tails merged");
static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge", 
                              cl::init(cl::BOU_UNSET), cl::Hidden);
namespace {
  // Throttle for huge numbers of predecessors (compile speed problems)
  cl::opt<unsigned>
  TailMergeThreshold("tail-merge-threshold", 
            cl::desc("Max number of predecessors to consider tail merging"),
            cl::init(100), cl::Hidden);

  struct BranchFolder : public MachineFunctionPass {
    static char ID;
    BranchFolder(bool defaultEnableTailMerge) : 
        MachineFunctionPass((intptr_t)&ID) {
          switch (FlagEnableTailMerge) {
          case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break;
          case cl::BOU_TRUE: EnableTailMerge = true; break;
          case cl::BOU_FALSE: EnableTailMerge = false; break;
          }
    }

    virtual bool runOnMachineFunction(MachineFunction &MF);
    virtual const char *getPassName() const { return "Control Flow Optimizer"; }
    const TargetInstrInfo *TII;
    MachineModuleInfo *MMI;
    bool MadeChange;
  private:
    // Tail Merging.
    bool EnableTailMerge;
    bool TailMergeBlocks(MachineFunction &MF);
    bool TryMergeBlocks(MachineBasicBlock* SuccBB,
                        MachineBasicBlock* PredBB);
    void ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
                                 MachineBasicBlock *NewDest);
    MachineBasicBlock *SplitMBBAt(MachineBasicBlock &CurMBB,
                                  MachineBasicBlock::iterator BBI1);

    std::vector<std::pair<unsigned,MachineBasicBlock*> > MergePotentials;
    const MRegisterInfo *RegInfo;
    RegScavenger *RS;
    // Branch optzn.
    bool OptimizeBranches(MachineFunction &MF);
    void OptimizeBlock(MachineBasicBlock *MBB);
    void RemoveDeadBlock(MachineBasicBlock *MBB);
    
    bool CanFallThrough(MachineBasicBlock *CurBB);
    bool CanFallThrough(MachineBasicBlock *CurBB, bool BranchUnAnalyzable,
                        MachineBasicBlock *TBB, MachineBasicBlock *FBB,
                        const std::vector<MachineOperand> &Cond);
  };
  char BranchFolder::ID = 0;
}

static bool CorrectExtraCFGEdges(MachineBasicBlock &MBB, 
                                 MachineBasicBlock *DestA,
                                 MachineBasicBlock *DestB,
                                 bool isCond, 
                                 MachineFunction::iterator FallThru);

FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) { 
      return new BranchFolder(DefaultEnableTailMerge); }

/// RemoveDeadBlock - Remove the specified dead machine basic block from the
/// function, updating the CFG.
void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) {
  assert(MBB->pred_empty() && "MBB must be dead!");
  DOUT << "\nRemoving MBB: " << *MBB;
  
  MachineFunction *MF = MBB->getParent();
  // drop all successors.
  while (!MBB->succ_empty())
    MBB->removeSuccessor(MBB->succ_end()-1);
  
  // If there is DWARF info to active, check to see if there are any LABEL
  // records in the basic block.  If so, unregister them from MachineModuleInfo.
  if (MMI && !MBB->empty()) {
    for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
         I != E; ++I) {
      if ((unsigned)I->getOpcode() == TargetInstrInfo::LABEL) {
        // The label ID # is always operand #0, an immediate.
        MMI->InvalidateLabel(I->getOperand(0).getImm());
      }
    }
  }
  
  // Remove the block.
  MF->getBasicBlockList().erase(MBB);
}

bool BranchFolder::runOnMachineFunction(MachineFunction &MF) {
  TII = MF.getTarget().getInstrInfo();
  if (!TII) return false;

  // Fix CFG.  The later algorithms expect it to be right.
  bool EverMadeChange = false;
  for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) {
    MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0;
    std::vector<MachineOperand> Cond;
    if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond))
      EverMadeChange |= CorrectExtraCFGEdges(*MBB, TBB, FBB, 
                        !Cond.empty(), next(I));
  }

  RegInfo = MF.getTarget().getRegisterInfo();
  RS = RegInfo->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL;

  MMI = getAnalysisToUpdate<MachineModuleInfo>();

  bool MadeChangeThisIteration = true;
  while (MadeChangeThisIteration) {
    MadeChangeThisIteration = false;
    MadeChangeThisIteration |= TailMergeBlocks(MF);
    MadeChangeThisIteration |= OptimizeBranches(MF);
    EverMadeChange |= MadeChangeThisIteration;
  }

  // See if any jump tables have become mergable or dead as the code generator
  // did its thing.
  MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
  const std::vector<MachineJumpTableEntry> &JTs = JTI->getJumpTables();
  if (!JTs.empty()) {
    // Figure out how these jump tables should be merged.
    std::vector<unsigned> JTMapping;
    JTMapping.reserve(JTs.size());
    
    // We always keep the 0th jump table.
    JTMapping.push_back(0);

    // Scan the jump tables, seeing if there are any duplicates.  Note that this
    // is N^2, which should be fixed someday.
    for (unsigned i = 1, e = JTs.size(); i != e; ++i)
      JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs));
    
    // If a jump table was merge with another one, walk the function rewriting
    // references to jump tables to reference the new JT ID's.  Keep track of
    // whether we see a jump table idx, if not, we can delete the JT.
    std::vector<bool> JTIsLive;
    JTIsLive.resize(JTs.size());
    for (MachineFunction::iterator BB = MF.begin(), E = MF.end();
         BB != E; ++BB) {
      for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end();
           I != E; ++I)
        for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) {
          MachineOperand &Op = I->getOperand(op);
          if (!Op.isJumpTableIndex()) continue;
          unsigned NewIdx = JTMapping[Op.getJumpTableIndex()];
          Op.setJumpTableIndex(NewIdx);

          // Remember that this JT is live.
          JTIsLive[NewIdx] = true;
        }
    }
   
    // Finally, remove dead jump tables.  This happens either because the
    // indirect jump was unreachable (and thus deleted) or because the jump
    // table was merged with some other one.
    for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i)
      if (!JTIsLive[i]) {
        JTI->RemoveJumpTable(i);
        EverMadeChange = true;
      }
  }
  
  delete RS;