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/DepthFirstIterator.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.
      ICEarlyExit,     // BB is entry of an early-exit sub-CFG.
      ICTriangle,      // BB is entry of a triangle sub-CFG.
      ICDiamond,       // BB is entry of a diamond sub-CFG.
      ICChild,         // BB is part of the sub-CFG that'll be predicated.
      ICDead           // BB has been converted and merged, it's now dead.
    };

    /// 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), its size, whether it's predicable, and whether any
    /// instruction can clobber the 'would-be' predicate.
    struct BBInfo {
      BBICKind Kind;
      unsigned Size;
      bool isPredicable;
      bool ClobbersPred;
      bool hasEarlyExit;
      MachineBasicBlock *BB;
      MachineBasicBlock *TrueBB;
      MachineBasicBlock *FalseBB;
      MachineBasicBlock *TailBB;
      std::vector<MachineOperand> Cond;
      BBInfo() : Kind(ICInvalid), Size(0), isPredicable(false),
                 ClobbersPred(false), hasEarlyExit(false),
                 BB(0), TrueBB(0), FalseBB(0), TailBB(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 StructuralAnalysis(MachineBasicBlock *BB);
    void FeasibilityAnalysis(BBInfo &BBI);
    void InitialFunctionAnalysis(MachineFunction &MF,
                                 std::vector<BBInfo*> &Candidates);
    bool IfConvertEarlyExit(BBInfo &BBI);
    bool IfConvertTriangle(BBInfo &BBI);
    bool IfConvertDiamond(BBInfo &BBI);
    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<BBInfo*> 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 = *Candidates[i];
    switch (BBI.Kind) {
    default: assert(false && "Unexpected!");
      break;
    case ICEarlyExit:
      MadeChange |= IfConvertEarlyExit(BBI);
      break;
    case ICTriangle:
      MadeChange |= IfConvertTriangle(BBI);
      break;
    case ICDiamond:
      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;
}

/// StructuralAnalysis - Analyze the structure of the sub-CFG starting from
/// the specified block. Record its successors and whether it looks like an
/// if-conversion candidate.
void IfConverter::StructuralAnalysis(MachineBasicBlock *BB) {
  BBInfo &BBI = BBAnalysis[BB->getNumber()];

  if (BBI.Kind != ICInvalid)
    return;  // Already 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;
  bool CanAnalyze = !TII->AnalyzeBranch(*BB, BBI.TrueBB, BBI.FalseBB, BBI.Cond);
  // Does it end with a return, indirect jump, or jumptable branch?
  BBI.hasEarlyExit = TII->BlockHasNoFallThrough(*BB) && !BBI.TrueBB;
  if (!CanAnalyze || !BBI.TrueBB || BBI.Cond.size() == 0)
    return;

  // Not a candidate if 'true' block is going to be if-converted.
  StructuralAnalysis(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 is going to be if-converted.
  StructuralAnalysis(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;

  unsigned TrueNumPreds  = BBI.TrueBB->pred_size();
  unsigned FalseNumPreds = BBI.FalseBB->pred_size();
  if ((TrueBBI.hasEarlyExit && TrueNumPreds <= 1) &&
      !(FalseBBI.hasEarlyExit && FalseNumPreds <=1)) {
    BBI.Kind = ICEarlyExit;
    TrueBBI.Kind = ICChild;
  } else if (!(TrueBBI.hasEarlyExit && TrueNumPreds <= 1) &&
             (FalseBBI.hasEarlyExit && FalseNumPreds <=1)) {
    BBI.Kind = ICEarlyExit;
    FalseBBI.Kind = ICChild;
  } else if (TrueBBI.TrueBB