IfConversion.cpp revision 86cbfea33bc736482cbf6c4ed9a01aefa704d097 
1//===-- IfConversion.cpp - Machine code if conversion pass. ---------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file was developed by the Evan Cheng and is distributed under
6// the University of Illinois Open Source License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This file implements the machine instruction level if-conversion pass.
11//
12//===----------------------------------------------------------------------===//
13
14#define DEBUG_TYPE "ifconversion"
15#include "llvm/CodeGen/Passes.h"
16#include "llvm/CodeGen/MachineModuleInfo.h"
17#include "llvm/CodeGen/MachineFunctionPass.h"
18#include "llvm/Target/TargetInstrInfo.h"
19#include "llvm/Target/TargetLowering.h"
20#include "llvm/Target/TargetMachine.h"
21#include "llvm/Support/Debug.h"
22#include "llvm/ADT/Statistic.h"
23using namespace llvm;
24
25STATISTIC(NumIfConvBBs, "Number of if-converted blocks");
26
27namespace {
28  class IfConverter : public MachineFunctionPass {
29    enum BBICKind {
30      ICInvalid,       // BB data invalid.
31      ICNotClassfied,  // BB data valid, but not classified.
32      ICTriangle,      // BB is part of a triangle sub-CFG.
33      ICDiamond,       // BB is part of a diamond sub-CFG.
34      ICTriangleEntry, // BB is entry of a triangle sub-CFG.
35      ICDiamondEntry   // BB is entry of a diamond sub-CFG.
36    };
37
38    /// BBInfo - One per MachineBasicBlock, this is used to cache the result
39    /// if-conversion feasibility analysis. This includes results from
40    /// TargetInstrInfo::AnalyzeBranch() (i.e. TBB, FBB, and Cond), and its
41    /// classification, and common tail block of its successors (if it's a
42    /// diamond shape).
43    struct BBInfo {
44      BBICKind Kind;
45      MachineBasicBlock *BB;
46      MachineBasicBlock *TrueBB;
47      MachineBasicBlock *FalseBB;
48      MachineBasicBlock *TailBB;
49      std::vector<MachineOperand> Cond;
50      unsigned Size;
51      BBInfo() : Kind(ICInvalid), BB(0), TrueBB(0), FalseBB(0), TailBB(0), Size(0) {}
52    };
53
54    /// BBAnalysis - Results of if-conversion feasibility analysis indexed by
55    /// basic block number.
56    std::vector<BBInfo> BBAnalysis;
57
58    const TargetLowering *TLI;
59    const TargetInstrInfo *TII;
60    bool MadeChange;
61  public:
62    static char ID;
63    IfConverter() : MachineFunctionPass((intptr_t)&ID) {}
64
65    virtual bool runOnMachineFunction(MachineFunction &MF);
66    virtual const char *getPassName() const { return "If converter"; }
67
68  private:
69    void AnalyzeBlock(MachineBasicBlock *BB);
70    void InitialFunctionAnalysis(MachineFunction &MF,
71                                 std::vector<int> &Candidates);
72    bool IfConvertDiamond(BBInfo &BBI);
73    bool IfConvertTriangle(BBInfo &BBI);
74    bool isBlockPredicable(MachineBasicBlock *BB) const;
75    void PredicateBlock(MachineBasicBlock *BB,
76                        std::vector<MachineOperand> &Cond,
77                        bool IgnoreTerm = false);
78    void MergeBlocks(BBInfo &TrueBBI, BBInfo &FalseBBI);
79  };
80  char IfConverter::ID = 0;
81}
82
83FunctionPass *llvm::createIfConverterPass() { return new IfConverter(); }
84
85bool IfConverter::runOnMachineFunction(MachineFunction &MF) {
86  TLI = MF.getTarget().getTargetLowering();
87  TII = MF.getTarget().getInstrInfo();
88  if (!TII) return false;
89
90  MadeChange = false;
91
92  MF.RenumberBlocks();
93  unsigned NumBBs = MF.getNumBlockIDs();
94  BBAnalysis.resize(NumBBs);
95
96  std::vector<int> Candidates;
97  // Do an intial analysis for each basic block and finding all the potential
98  // candidates to perform if-convesion.
99  InitialFunctionAnalysis(MF, Candidates);
100
101  for (unsigned i = 0, e = Candidates.size(); i != e; ++i) {
102    BBInfo &BBI = BBAnalysis[Candidates[i]];
103    switch (BBI.Kind) {
104    default: assert(false && "Unexpected!");
105      break;
106    case ICTriangleEntry:
107      MadeChange |= IfConvertTriangle(BBI);
108      break;
109    case ICDiamondEntry:
110      MadeChange |= IfConvertDiamond(BBI);
111      break;
112    }
113  }
114  return MadeChange;
115}
116
117static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB,
118                                         MachineBasicBlock *TrueBB) {
119  for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
120         E = BB->succ_end(); SI != E; ++SI) {
121    MachineBasicBlock *SuccBB = *SI;
122    if (SuccBB != TrueBB)
123      return SuccBB;
124  }
125  return NULL;
126}
127
128void IfConverter::AnalyzeBlock(MachineBasicBlock *BB) {
129  BBInfo &BBI = BBAnalysis[BB->getNumber()];
130
131  if (BBI.Kind != ICInvalid)
132    return;  // Always analyzed.
133  BBI.BB = BB;
134  BBI.Size = std::distance(BB->begin(), BB->end());
135
136  // Look for 'root' of a simple (non-nested) triangle or diamond.
137  BBI.Kind = ICNotClassfied;
138  if (TII->AnalyzeBranch(*BB, BBI.TrueBB, BBI.FalseBB, BBI.Cond)
139      || !BBI.TrueBB || BBI.Cond.size() == 0)
140    return;
141
142  // Not a candidate if 'true' block has another predecessor.
143  // FIXME: Use or'd predicate or predicated cmp.
144  if (BBI.TrueBB->pred_size() > 1)
145    return;
146
147  // Not a candidate if 'true' block is going to be if-converted.
148  AnalyzeBlock(BBI.TrueBB);
149  BBInfo &TrueBBI = BBAnalysis[BBI.TrueBB->getNumber()];
150  if (TrueBBI.Kind != ICNotClassfied)
151    return;
152
153  // No false branch. This BB must end with a conditional branch and a
154  // fallthrough.
155  if (!BBI.FalseBB)
156    BBI.FalseBB = findFalseBlock(BB, BBI.TrueBB);
157  assert(BBI.FalseBB && "Expected to find the fallthrough block!");
158
159  // Not a candidate if 'false' block has another predecessor.
160  // FIXME: Invert condition and swap 'true' / 'false' blocks?
161  if (BBI.FalseBB->pred_size() > 1)
162    return;
163
164  // Not a candidate if 'false' block is going to be if-converted.
165  AnalyzeBlock(BBI.FalseBB);
166  BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
167  if (FalseBBI.Kind != ICNotClassfied)
168    return;
169
170  // TODO: Only handle very simple cases for now.
171  if (TrueBBI.FalseBB || FalseBBI.FalseBB ||
172      TrueBBI.Cond.size() || FalseBBI.Cond.size())
173    return;
174
175  if (TrueBBI.TrueBB && TrueBBI.TrueBB == BBI.FalseBB) {
176    // Triangle:
177    //   EBB
178    //   | \_
179    //   |  |
180    //   | TBB
181    //   |  /
182    //   FBB
183    BBI.Kind = ICTriangleEntry;
184    TrueBBI.Kind = FalseBBI.Kind = ICTriangle;
185  } else if (TrueBBI.TrueBB == FalseBBI.TrueBB) {
186    // Diamond:
187    //   EBB
188    //   / \_
189    //  |   |
190    // TBB FBB
191    //   \ /
192    //  TailBB
193    // Note MBB can be empty in case both TBB and FBB are return blocks.
194    BBI.Kind = ICDiamondEntry;
195    TrueBBI.Kind = FalseBBI.Kind = ICDiamond;
196    BBI.TailBB = TrueBBI.TrueBB;
197  }
198  return;
199}
200
201/// InitialFunctionAnalysis - Analyze all blocks and find entries for all
202/// if-conversion candidates.
203void IfConverter::InitialFunctionAnalysis(MachineFunction &MF,
204                                          std::vector<int> &Candidates) {
205  for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
206    MachineBasicBlock *BB = I;
207    AnalyzeBlock(BB);
208    BBInfo &BBI = BBAnalysis[BB->getNumber()];
209    if (BBI.Kind == ICTriangleEntry || BBI.Kind == ICDiamondEntry)
210      Candidates.push_back(BB->getNumber());
211  }
212}
213
214/// TransferPreds - Transfer all the predecessors of FromBB to ToBB.
215///
216static void TransferPreds(MachineBasicBlock *ToBB, MachineBasicBlock *FromBB) {
217   std::vector<MachineBasicBlock*> Preds(FromBB->pred_begin(),
218                                         FromBB->pred_end());
219    for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
220      MachineBasicBlock *Pred = Preds[i];
221      Pred->removeSuccessor(FromBB);
222      if (!Pred->isSuccessor(ToBB))
223        Pred->addSuccessor(ToBB);
224    }
225}
226
227/// TransferSuccs - Transfer all the successors of FromBB to ToBB.
228///
229static void TransferSuccs(MachineBasicBlock *ToBB, MachineBasicBlock *FromBB) {
230   std::vector<MachineBasicBlock*> Succs(FromBB->succ_begin(),
231                                         FromBB->succ_end());
232    for (unsigned i = 0, e = Succs.size(); i != e; ++i) {
233      MachineBasicBlock *Succ = Succs[i];
234      FromBB->removeSuccessor(Succ);
235      if (!ToBB->isSuccessor(Succ))
236        ToBB->addSuccessor(Succ);
237    }
238}
239
240/// IfConvertTriangle - If convert a triangle sub-CFG.
241///
242bool IfConverter::IfConvertTriangle(BBInfo &BBI) {
243  if (isBlockPredicable(BBI.TrueBB)) {
244    BBInfo &TrueBBI = BBAnalysis[BBI.TrueBB->getNumber()];
245    BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
246
247    // Predicate the 'true' block after removing its branch.
248    TrueBBI.Size -= TII->RemoveBranch(*BBI.TrueBB);
249    PredicateBlock(BBI.TrueBB, BBI.Cond);
250
251    // Join the 'true' and 'false' blocks by copying the instructions
252    // from the 'false' block to the 'true' block.
253    BBI.TrueBB->removeSuccessor(BBI.FalseBB);
254    MergeBlocks(TrueBBI, FalseBBI);
255
256    // Now merge the entry of the triangle with the true block.
257    BBI.Size -= TII->RemoveBranch(*BBI.BB);
258    MergeBlocks(BBI, TrueBBI);
259
260    // Update block info.
261    TrueBBI.Kind = ICInvalid;
262    FalseBBI.Kind = ICInvalid;
263
264    // FIXME: Must maintain LiveIns.
265    NumIfConvBBs++;
266    return true;
267  }
268  return false;
269}
270
271/// IfConvertDiamond - If convert a diamond sub-CFG.
272///
273bool IfConverter::IfConvertDiamond(BBInfo &BBI) {
274  if (isBlockPredicable(BBI.TrueBB) && isBlockPredicable(BBI.FalseBB)) {
275    std::vector<MachineInstr*> Dups;
276    if (!BBI.TailBB) {
277      // No common merge block. Check if the terminators (e.g. return) are
278      // the same or predicable.
279      MachineBasicBlock::iterator TT = BBI.TrueBB->getFirstTerminator();
280      MachineBasicBlock::iterator FT = BBI.FalseBB->getFirstTerminator();
281      while (TT != BBI.TrueBB->end() && FT != BBI.FalseBB->end()) {
282        if (TT->isIdenticalTo(FT))
283          Dups.push_back(TT);  // Will erase these later.
284        else if (!TT->isPredicable() && !FT->isPredicable())
285          return false; // Can't if-convert. Abort!
286        ++TT;
287        ++FT;
288      }
289      // One of the two pathes have more terminators, make sure they are all
290      // predicable.
291      while (TT != BBI.TrueBB->end())
292        if (!TT->isPredicable())
293          return false; // Can't if-convert. Abort!
294      while (FT != BBI.FalseBB->end())
295        if (!FT->isPredicable())
296          return false; // Can't if-convert. Abort!
297    }
298
299    BBInfo &TrueBBI = BBAnalysis[BBI.TrueBB->getNumber()];
300    BBInfo &FalseBBI = BBAnalysis[BBI.FalseBB->getNumber()];
301
302    // Remove the duplicated instructions from the 'true' block.
303    for (unsigned i = 0, e = Dups.size(); i != e; ++i) {
304      Dups[i]->eraseFromParent();
305      --TrueBBI.Size;
306    }
307
308    // Predicate the 'true' block after removing its branch.
309    TrueBBI.Size -= TII->RemoveBranch(*BBI.TrueBB);
310    PredicateBlock(BBI.TrueBB, BBI.Cond);
311
312    // Predicate the 'false' block.
313    std::vector<MachineOperand> NewCond(BBI.Cond);
314    TII->ReverseBranchCondition(NewCond);
315    PredicateBlock(BBI.FalseBB, NewCond, true);
316
317    // Merge the 'true' and 'false' blocks by copying the instructions
318    // from the 'false' block to the 'true' block.
319    MergeBlocks(TrueBBI, FalseBBI);
320
321    // Remove the conditional branch from entry to the blocks.
322    BBI.Size -= TII->RemoveBranch(*BBI.BB);
323
324    // Merge the combined block into the entry of the diamond if the entry
325    // block is the only predecessor. Otherwise, insert an unconditional
326    // branch.
327    BBInfo *CvtBBI = &TrueBBI;
328    if (BBI.TrueBB->pred_size() == 1) {
329      BBI.BB->removeSuccessor(BBI.TrueBB);
330      MergeBlocks(BBI, TrueBBI);
331      CvtBBI = &BBI;
332    } else {
333      std::vector<MachineOperand> NoCond;
334      TII->InsertBranch(*BBI.BB, BBI.TrueBB, NULL, NoCond);
335    }
336
337    // If the if-converted block fallthrough into the tail block, then
338    // fold the tail block in as well.
339    if (BBI.TailBB && CvtBBI->BB->succ_size() == 1) {
340      CvtBBI->Size -= TII->RemoveBranch(*CvtBBI->BB);
341      CvtBBI->BB->removeSuccessor(BBI.TailBB);
342      BBInfo TailBBI = BBAnalysis[BBI.TailBB->getNumber()];
343      MergeBlocks(*CvtBBI, TailBBI);
344      TailBBI.Kind = ICInvalid;
345    }
346
347    // Update block info.
348    TrueBBI.Kind = ICInvalid;
349    FalseBBI.Kind = ICInvalid;
350
351    // FIXME: Must maintain LiveIns.
352    NumIfConvBBs += 2;
353    return true;
354  }
355  return false;
356}
357
358/// isBlockPredicable - Returns true if the block is predicable. In most
359/// cases, that means all the instructions in the block has M_PREDICABLE flag.
360/// It assume all the terminator instructions can be converted or deleted.
361bool IfConverter::isBlockPredicable(MachineBasicBlock *BB) const {
362  const BBInfo &BBI = BBAnalysis[BB->getNumber()];
363  if (BBI.Size == 0 || BBI.Size > TLI->getIfCvtBlockSizeLimit())
364    return false;
365
366  for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end();
367       I != E; ++I) {
368    if (TII->isTerminatorInstr(I->getOpcode()))
369      continue;
370    if (!I->isPredicable())
371      return false;
372  }
373  return true;
374}
375
376/// PredicateBlock - Predicate every instruction in the block with the specified
377/// condition. If IgnoreTerm is true, skip over all terminator instructions.
378void IfConverter::PredicateBlock(MachineBasicBlock *BB,
379                                 std::vector<MachineOperand> &Cond,
380                                 bool IgnoreTerm) {
381  for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end();
382       I != E; ++I) {
383    if (IgnoreTerm && TII->isTerminatorInstr(I->getOpcode()))
384      continue;
385    if (!TII->PredicateInstruction(&*I, Cond)) {
386      cerr << "Unable to predication " << *I << "!\n";
387      abort();
388    }
389  }
390}
391
392/// MergeBlocks - Move all instructions from FromBB to the end of ToBB.
393///
394void IfConverter::MergeBlocks(BBInfo &ToBBI, BBInfo &FromBBI) {
395  ToBBI.BB->splice(ToBBI.BB->end(),
396                   FromBBI.BB, FromBBI.BB->begin(), FromBBI.BB->end());
397  TransferPreds(ToBBI.BB, FromBBI.BB);
398  TransferSuccs(ToBBI.BB, FromBBI.BB);
399  ToBBI.Size += FromBBI.Size;
400  FromBBI.Size = 0;
401}
402