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.
//
//===----------------------------------------------------------------------===//

#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;
    explicit 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;
}

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