path: root/lib/CodeGen/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.
//
//===----------------------------------------------------------------------===//

#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/MachineDebugInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineJumpTableInfo.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
using namespace llvm;

static Statistic<> NumDeadBlocks("branchfold", "Number of dead blocks removed");
static Statistic<> NumBranchOpts("branchfold", "Number of branches optimized");
static Statistic<> NumTailMerge ("branchfold", "Number of block tails merged");
static cl::opt<bool> EnableTailMerge("enable-tail-merge", cl::init(false));

namespace {
  struct BranchFolder : public MachineFunctionPass {
    virtual bool runOnMachineFunction(MachineFunction &MF);
    virtual const char *getPassName() const { return "Control Flow Optimizer"; }
    const TargetInstrInfo *TII;
    MachineDebugInfo *MDI;
    bool MadeChange;
  private:
    // Tail Merging.
    bool TailMergeBlocks(MachineFunction &MF);
    void ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
                                 MachineBasicBlock *NewDest);

    // Branch optzn.
    bool OptimizeBranches(MachineFunction &MF);
    void OptimizeBlock(MachineFunction::iterator MBB);
    void RemoveDeadBlock(MachineBasicBlock *MBB);
  };
}

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

/// 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!");
  
  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 DWARF_LABEL
  // records in the basic block.  If so, unregister them from MachineDebugInfo.
  if (MDI && !MBB->empty()) {
    unsigned DWARF_LABELOpc = TII->getDWARF_LABELOpcode();
    assert(DWARF_LABELOpc &&
           "Target supports dwarf but didn't implement getDWARF_LABELOpcode!");
    
    for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
         I != E; ++I) {
      if ((unsigned)I->getOpcode() == DWARF_LABELOpc) {
        // The label ID # is always operand #0, an immediate.
        MDI->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;

  MDI = getAnalysisToUpdate<MachineDebugInfo>();
  
  bool EverMadeChange = false;
  bool MadeChangeThisIteration = true;
  while (MadeChangeThisIteration) {
    MadeChangeThisIteration = false;
    MadeChangeThisIteration |= TailMergeBlocks(MF);
    MadeChangeThisIteration |= OptimizeBranches(MF);
    EverMadeChange |= MadeChangeThisIteration;
  }

  return EverMadeChange;
}

//===----------------------------------------------------------------------===//
//  Tail Merging of Blocks
//===----------------------------------------------------------------------===//

/// HashMachineInstr - Compute a hash value for MI and its operands.
static unsigned HashMachineInstr(const MachineInstr *MI) {
  unsigned Hash = MI->getOpcode();
  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
    const MachineOperand &Op = MI->getOperand(i);
    
    // Merge in bits from the operand if easy.
    unsigned OperandHash = 0;
    switch (Op.getType()) {
    case MachineOperand::MO_Register:          OperandHash = Op.getReg(); break;
    case MachineOperand::MO_Immediate:         OperandHash = Op.getImm(); break;
    case MachineOperand::MO_MachineBasicBlock:
      OperandHash = Op.getMachineBasicBlock()->getNumber();
      break;
    case MachineOperand::MO_FrameIndex: OperandHash = Op.getFrameIndex(); break;
    case MachineOperand::MO_ConstantPoolIndex:
      OperandHash = Op.getConstantPoolIndex();
      break;
    case MachineOperand::MO_JumpTableIndex:
      OperandHash = Op.getJumpTableIndex();
      break;
    case MachineOperand::MO_GlobalAddress:
    case MachineOperand::MO_ExternalSymbol:
      // Global address / external symbol are too hard, don't bother, but do
      // pull in the offset.
      OperandHash = Op.getOffset();
      break;
    default: break;
    }
    
    Hash += ((OperandHash << 3) | Op.getType()) << (i&31);
  }
  return Hash;
}

/// HashEndOfMBB - Hash the last two instructions in the MBB.  We hash two
/// instructions, because cross-jumping only saves code when at least two
/// instructions are removed (since a branch must be inserted).
static unsigned HashEndOfMBB(const MachineBasicBlock *MBB) {
  MachineBasicBlock::const_iterator I = MBB->end();
  if (I == MBB->begin())
    return 0;   // Empty MBB.
  
  --I;
  unsigned Hash = HashMachineInstr(I);
    
  if (I == MBB->begin())
    return Hash;   // Single instr MBB.
  
  --I;
  // Hash in the second-to-last instruction.
  Hash ^= HashMachineInstr(I) << 2;
  return Hash;
}

/// ComputeCommonTailLength - Given two machine basic blocks, compute the number
/// of instructions they actually have in common together at their end.  Return
/// iterators for the first shared instruction in each block.
static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1,
                                        MachineBasicBlock *MBB2,
                                        MachineBasicBlock::iterator &I1,
                                        MachineBasicBlock::iterator &I2) {
  I1 = MBB1->end();
  I2 = MBB2->end();
  
  unsigned TailLen = 0;
  while (I1 != MBB1->begin() && I2 != MBB2->begin()) {
    --I1; --I2;
    if (!I1->isIdenticalTo(I2)) {
      ++I1; ++I2;
      break;
    }
    ++TailLen;
  }
  return TailLen;
}

/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything
/// after it, replacing it with an unconditional branch to NewDest.  This
/// returns true if OldInst's block is modified, false if NewDest is modified.
void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
                                           MachineBasicBlock *NewDest) {
  MachineBasicBlock *OldBB = OldInst->getParent();
  
  // Remove all the old successors of OldBB from the CFG.
  while (!OldBB->succ_empty())
    OldBB->removeSuccessor(OldBB->succ_begin());
  
  // Remove all the dead instructions from the end of OldBB.
  OldBB->erase(OldInst, OldBB->end());

  // If OldBB isn't immediately before OldBB, insert a branch to it.
  if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest))
    TII->InsertBranch(*OldBB, NewDest, 0, std::vector<MachineOperand>());
  OldBB->addSuccessor(NewDest);
  ++NumTailMerge;
}

bool BranchFolder::TailMergeBlocks(MachineFunction &MF