path: root/lib/CodeGen/BranchFolding.cpp

//===-- BranchFolding.cpp - Fold machine code branch instructions ---------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file 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/TargetRegisterInfo.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/ADT/SmallSet.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);
// Throttle for huge numbers of predecessors (compile speed problems)
static cl::opt<unsigned>
TailMergeThreshold("tail-merge-threshold", 
          cl::desc("Max number of predecessors to consider tail merging"),
          cl::init(150), cl::Hidden);

namespace {
  struct VISIBILITY_HIDDEN BranchFolder : public MachineFunctionPass {
    static char ID;
    explicit BranchFolder(bool defaultEnableTailMerge) : 
      MachineFunctionPass(&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);
    unsigned ComputeSameTails(unsigned CurHash, unsigned minCommonTailLength);
    void RemoveBlocksWithHash(unsigned CurHash, MachineBasicBlock* SuccBB,
                                                MachineBasicBlock* PredBB);
    unsigned CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
                                       unsigned maxCommonTailLength);

    typedef std::pair<unsigned