path: root/lib/CodeGen/IfConversion.cpp

//===-- IfConversion.cpp - Machine code if conversion pass. ---------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file was developed by the Evan Cheng and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the machine instruction level if-conversion pass.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "ifconversion"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetLowering.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;

STATISTIC(NumIfConvBBs, "Number of if-converted blocks");

namespace {
  class IfConverter : public MachineFunctionPass {
    enum BBICKind {
      ICInvalid,       // BB data invalid.
      ICNotClassfied,  // BB data valid, but not classified.
      ICTriangle,      // BB is part of a triangle sub-CFG.
      ICDiamond,       // BB is part of a diamond sub-CFG.
      ICTriangleEntry, // BB is entry of a triangle sub-CFG.
      ICDiamondEntry   // BB is entry of a diamond sub-CFG.
    };

    /// BBInfo - One per MachineBasicBlock, this is used to cache the result
    /// if-conversion feasibility analysis. This includes results from
    /// TargetInstrInfo::AnalyzeBranch() (i.e. TBB, FBB, and Cond), and its
    /// classification, and common tail block of its successors (if it's a
    /// diamond shape).
    struct BBInfo {
      BBICKind Kind;
      MachineBasicBlock *BB;
      MachineBasicBlock *TrueBB;
      MachineBasicBlock *FalseBB;
      MachineBasicBlock *TailBB;
      std::vector<MachineOperand> Cond;
      unsigned Size;
      BBInfo() : Kind(ICInvalid), BB(0), TrueBB(0), FalseBB(0), TailBB(0), Size(0) {}
    };

    /// BBAnalysis - Results of if-conversion feasibility analysis indexed by
    /// basic block number.
    std::vector<BBInfo> BBAnalysis;

    const TargetLowering *TLI;
    const TargetInstrInfo *TII;
    bool MadeChange;
  public:
    static char ID;
    IfConverter() : MachineFunctionPass((intptr_t)&ID) {}

    virtual bool runOnMachineFunction(MachineFunction &MF);
    virtual const char *getPassName() const { return "If converter"; }

  private:
    void AnalyzeBlock(MachineBasicBlock *BB);
    void InitialFunctionAnalysis(MachineFunction &MF,
                                 std::vector<int> &Candidates);
    bool IfConvertDiamond(BBInfo &BBI);
    bool IfConvertTriangle(BBInfo &BBI);
    bool isBlockPredicable(MachineBasicBlock *BB) const;
    void PredicateBlock(MachineBasicBlock *BB,
                        std::vector<MachineOperand> &Cond,
                        bool IgnoreTerm = false);
    void MergeBlocks(BBInfo &TrueBBI, BBInfo &FalseBBI);
  };
  char IfConverter::ID = 0;
}

FunctionPass *llvm::createIfConverterPass() { return new IfConverter(); }

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

  MF.RenumberBlocks();
  unsigned NumBBs = MF.getNumBlockIDs();
  BBAnalysis.resize(NumBBs);

  std::vector<int> Candidates;
  // Do an intial analysis for each basic block and finding all the potential
  // candidates to perform if-convesion.
  InitialFunctionAnalysis(MF, Candidates);

  MadeChange = false;
  for (unsigned i = 0, e = Candidates.size(); i != e; ++i) {
    BBInfo &BBI = BBAnalysis[Candidates[i]];
    switch (BBI.Kind) {
    default: assert(false && "Unexpected!");
      break;
    case ICTriangleEntry:
      MadeChange |= IfConvertTriangle(BBI);
      break;
    case ICDiamondEntry:
      MadeChange |= IfConvertDiamond(BBI);
      break;
    }
  }

  BBAnalysis.clear();

  return MadeChange;
}

static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB,
                                         MachineBasicBlock *TrueBB) {
  for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
         E = BB->succ_end(); SI != E; ++SI) {
    MachineBasicBlock *SuccBB = *SI;
    if (SuccBB != TrueBB)
      return SuccBB;
  }
  return NULL;
}

void IfConverter::AnalyzeBlock(MachineBasicBlock *BB) {
  BBInfo &BBI = BBAnalysis[BB->getNumber()];

  if (BBI.Kind != ICInvalid)
    return;  // Always analyzed.
  BBI.BB = BB;
  BBI.Size = std::distance(BB->begin(), BB->end());

  // Look for 'root' of a simple (non-nested) triangle or diamond.
  BBI.Kind = ICNotClassfied;
  if (TII->AnalyzeBranch(*BB, BBI.TrueBB, BBI.FalseBB, BBI.Cond)
      || !BBI.TrueBB || BBI.Cond.size() == 0)
    return;

  // Not a candidate if 'true' block has another predecessor.
  // FIXME: Use or'd predicate or predicated cmp.
  if (BBI.TrueBB->pred_size() > 1)
    return;

  // Not a candidate if 'true' block is going to be if-converted.
  AnalyzeBlock(BBI.TrueBB);
  BBInfo &TrueBBI = BBAnalysis[BBI.TrueBB->getNumber()];
  if (TrueBBI.Kind != ICNotClassfied)
    return;

  // TODO: Only handle very simple cases for now.
  if (TrueBBI.FalseBB || TrueBBI.Cond.size())
    return;

  // No false branch. This BB must end with a conditional branch and a
  // fallthrough.
  if (!BBI.FalseBB)
    BBI.FalseBB = findFalseBlock(BB, BBI.TrueBB);  
  assert(BBI.FalseBB && "Expected to find the fallthrough block!");

  // Not a candidate if 'false' block has another predecessor.
  // FIXME: Invert condition and swap 'true' / 'false' blocks?
  if (BBI.FalseBB->pred_size() > 1)
    return;

  // Not a candidate if 'false' block is going to be if-converted.
  AnalyzeBlock(BBI.FalseBB);
  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
  if (FalseBBI.Kind != ICNotClassfied)
    return;

  // TODO: Only handle very simple cases for now.
  if (FalseBBI.FalseBB || FalseBBI.Cond.size())
    return;

  if (TrueBBI.TrueBB && TrueBBI.TrueBB == BBI.FalseBB) {
    // Triangle:
    //   EBB
    //   | \_
    //   |  |
    //   | TBB
    //   |  /
    //   FBB
    BBI.Kind = ICTriangleEntry;
    TrueBBI.Kind = FalseBBI.Kind = ICTriangle;
  } else if (TrueBBI.TrueBB == FalseBBI.TrueBB) {
    // Diamond:
    //   EBB
    //   / \_
    //  |   |
    // TBB FBB
    //   \ /
    //  TailBB
    // Note MBB can be empty in case both TBB and FBB are return blocks.
    BBI.Kind = ICDiamondEntry;
    TrueBBI.Kind = FalseBBI.Kind = ICDiamond;
    BBI.TailBB = TrueBBI.TrueBB;
  }
  return;
}

/// InitialFunctionAnalysis - Analyze all blocks and find entries for all
/// if-conversion candidates.
void IfConverter::InitialFunctionAnalysis(MachineFunction &MF,
                                          std::vector<int> &Candidates) {
  for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
    MachineBasicBlock *BB = I;
    AnalyzeBlock(BB);
    BBInfo &BBI = BBAnalysis[BB->getNumber()];
    if (BBI.Kind == ICTriangleEntry || BBI.Kind == ICDiamondEntry)
      Candidates.push_back(BB->getNumber());
  }
}

/// TransferPreds - Transfer all the predecessors of FromBB to ToBB.
///
static void TransferPreds(MachineBasicBlock *ToBB, MachineBasicBlock *FromBB) {
   std::vector<MachineBasicBlock*> Preds(FromBB->pred_begin(),
                                         FromBB->pred_end());
    for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
      MachineBasicBlock *Pred = Preds[i];
      Pred->removeSuccessor(FromBB);
      if (!Pred->isSuccessor(