TailDuplication.cpp revision 3858225afc2a7f1027dce34dc72de5686b636c2d 
1//===-- TailDuplication.cpp - Duplicate blocks into predecessors' tails ---===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This pass duplicates basic blocks ending in unconditional branches into
11// the tails of their predecessors.
12//
13//===----------------------------------------------------------------------===//
14
15#define DEBUG_TYPE "tailduplication"
16#include "llvm/Function.h"
17#include "llvm/CodeGen/Passes.h"
18#include "llvm/CodeGen/MachineModuleInfo.h"
19#include "llvm/CodeGen/MachineFunctionPass.h"
20#include "llvm/Target/TargetInstrInfo.h"
21#include "llvm/Support/CommandLine.h"
22#include "llvm/Support/Debug.h"
23#include "llvm/Support/raw_ostream.h"
24#include "llvm/ADT/SmallSet.h"
25#include "llvm/ADT/SetVector.h"
26#include "llvm/ADT/Statistic.h"
27using namespace llvm;
28
29STATISTIC(NumTailDups  , "Number of tail duplicated blocks");
30STATISTIC(NumInstrDups , "Additional instructions due to tail duplication");
31STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
32
33// Heuristic for tail duplication.
34static cl::opt<unsigned>
35TailDuplicateSize("tail-dup-size",
36                  cl::desc("Maximum instructions to consider tail duplicating"),
37                  cl::init(2), cl::Hidden);
38
39namespace {
40  /// TailDuplicatePass - Perform tail duplication.
41  class TailDuplicatePass : public MachineFunctionPass {
42    const TargetInstrInfo *TII;
43    MachineModuleInfo *MMI;
44
45  public:
46    static char ID;
47    explicit TailDuplicatePass() : MachineFunctionPass(&ID) {}
48
49    virtual bool runOnMachineFunction(MachineFunction &MF);
50    virtual const char *getPassName() const { return "Tail Duplication"; }
51
52  private:
53    bool TailDuplicateBlocks(MachineFunction &MF);
54    bool TailDuplicate(MachineBasicBlock *TailBB, MachineFunction &MF);
55    void RemoveDeadBlock(MachineBasicBlock *MBB);
56  };
57
58  char TailDuplicatePass::ID = 0;
59}
60
61FunctionPass *llvm::createTailDuplicatePass() {
62  return new TailDuplicatePass();
63}
64
65bool TailDuplicatePass::runOnMachineFunction(MachineFunction &MF) {
66  TII = MF.getTarget().getInstrInfo();
67  MMI = getAnalysisIfAvailable<MachineModuleInfo>();
68
69  bool MadeChange = false;
70  bool MadeChangeThisIteration = true;
71  while (MadeChangeThisIteration) {
72    MadeChangeThisIteration = false;
73    MadeChangeThisIteration |= TailDuplicateBlocks(MF);
74    MadeChange |= MadeChangeThisIteration;
75  }
76
77  return MadeChange;
78}
79
80/// TailDuplicateBlocks - Look for small blocks that are unconditionally
81/// branched to and do not fall through. Tail-duplicate their instructions
82/// into their predecessors to eliminate (dynamic) branches.
83bool TailDuplicatePass::TailDuplicateBlocks(MachineFunction &MF) {
84  bool MadeChange = false;
85
86  for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
87    MachineBasicBlock *MBB = I++;
88
89    // Only duplicate blocks that end with unconditional branches.
90    if (MBB->canFallThrough())
91      continue;
92
93    MadeChange |= TailDuplicate(MBB, MF);
94
95    // If it is dead, remove it.
96    if (MBB->pred_empty()) {
97      NumInstrDups -= MBB->size();
98      RemoveDeadBlock(MBB);
99      MadeChange = true;
100      ++NumDeadBlocks;
101    }
102  }
103  return MadeChange;
104}
105
106/// TailDuplicate - If it is profitable, duplicate TailBB's contents in each
107/// of its predecessors.
108bool TailDuplicatePass::TailDuplicate(MachineBasicBlock *TailBB,
109                                        MachineFunction &MF) {
110  // Don't try to tail-duplicate single-block loops.
111  if (TailBB->isSuccessor(TailBB))
112    return false;
113
114  // Set the limit on the number of instructions to duplicate, with a default
115  // of one less than the tail-merge threshold. When optimizing for size,
116  // duplicate only one, because one branch instruction can be eliminated to
117  // compensate for the duplication.
118  unsigned MaxDuplicateCount;
119  if (!TailBB->empty() && TailBB->back().getDesc().isIndirectBranch())
120    // If the target has hardware branch prediction that can handle indirect
121    // branches, duplicating them can often make them predictable when there
122    // are common paths through the code.  The limit needs to be high enough
123    // to allow undoing the effects of tail merging.
124    MaxDuplicateCount = 20;
125  else if (MF.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
126    MaxDuplicateCount = 1;
127  else
128    MaxDuplicateCount = TailDuplicateSize;
129
130  // Check the instructions in the block to determine whether tail-duplication
131  // is invalid or unlikely to be profitable.
132  unsigned i = 0;
133  bool HasCall = false;
134  for (MachineBasicBlock::iterator I = TailBB->begin();
135       I != TailBB->end(); ++I, ++i) {
136    // Non-duplicable things shouldn't be tail-duplicated.
137    if (I->getDesc().isNotDuplicable()) return false;
138    // Don't duplicate more than the threshold.
139    if (i == MaxDuplicateCount) return false;
140    // Remember if we saw a call.
141    if (I->getDesc().isCall()) HasCall = true;
142  }
143  // Heuristically, don't tail-duplicate calls if it would expand code size,
144  // as it's less likely to be worth the extra cost.
145  if (i > 1 && HasCall)
146    return false;
147
148  // Iterate through all the unique predecessors and tail-duplicate this
149  // block into them, if possible. Copying the list ahead of time also
150  // avoids trouble with the predecessor list reallocating.
151  bool Changed = false;
152  SmallSetVector<MachineBasicBlock *, 8> Preds(TailBB->pred_begin(),
153                                               TailBB->pred_end());
154  for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
155       PE = Preds.end(); PI != PE; ++PI) {
156    MachineBasicBlock *PredBB = *PI;
157
158    assert(TailBB != PredBB &&
159           "Single-block loop should have been rejected earlier!");
160    if (PredBB->succ_size() > 1) continue;
161
162    MachineBasicBlock *PredTBB, *PredFBB;
163    SmallVector<MachineOperand, 4> PredCond;
164    if (TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
165      continue;
166    if (!PredCond.empty())
167      continue;
168    // EH edges are ignored by AnalyzeBranch.
169    if (PredBB->succ_size() != 1)
170      continue;
171    // Don't duplicate into a fall-through predecessor (at least for now).
172    if (PredBB->isLayoutSuccessor(TailBB) && PredBB->canFallThrough())
173      continue;
174
175    DEBUG(errs() << "\nTail-duplicating into PredBB: " << *PredBB
176                 << "From Succ: " << *TailBB);
177
178    // Remove PredBB's unconditional branch.
179    TII->RemoveBranch(*PredBB);
180    // Clone the contents of TailBB into PredBB.
181    for (MachineBasicBlock::iterator I = TailBB->begin(), E = TailBB->end();
182         I != E; ++I) {
183      MachineInstr *NewMI = MF.CloneMachineInstr(I);
184      PredBB->insert(PredBB->end(), NewMI);
185    }
186    NumInstrDups += TailBB->size() - 1; // subtract one for removed branch
187
188    // Update the CFG.
189    PredBB->removeSuccessor(PredBB->succ_begin());
190    assert(PredBB->succ_empty() &&
191           "TailDuplicate called on block with multiple successors!");
192    for (MachineBasicBlock::succ_iterator I = TailBB->succ_begin(),
193         E = TailBB->succ_end(); I != E; ++I)
194       PredBB->addSuccessor(*I);
195
196    Changed = true;
197    ++NumTailDups;
198  }
199
200  // If TailBB was duplicated into all its predecessors except for the prior
201  // block, which falls through unconditionally, move the contents of this
202  // block into the prior block.
203  MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(TailBB));
204  MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
205  SmallVector<MachineOperand, 4> PriorCond;
206  bool PriorUnAnalyzable =
207    TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
208  // This has to check PrevBB->succ_size() because EH edges are ignored by
209  // AnalyzeBranch.
210  if (!PriorUnAnalyzable && PriorCond.empty() && !PriorTBB &&
211      TailBB->pred_size() == 1 && PrevBB.succ_size() == 1 &&
212      !TailBB->hasAddressTaken()) {
213    DEBUG(errs() << "\nMerging into block: " << PrevBB
214          << "From MBB: " << *TailBB);
215    PrevBB.splice(PrevBB.end(), TailBB, TailBB->begin(), TailBB->end());
216    PrevBB.removeSuccessor(PrevBB.succ_begin());;
217    assert(PrevBB.succ_empty());
218    PrevBB.transferSuccessors(TailBB);
219    Changed = true;
220  }
221
222  return Changed;
223}
224
225/// RemoveDeadBlock - Remove the specified dead machine basic block from the
226/// function, updating the CFG.
227void TailDuplicatePass::RemoveDeadBlock(MachineBasicBlock *MBB) {
228  assert(MBB->pred_empty() && "MBB must be dead!");
229  DEBUG(errs() << "\nRemoving MBB: " << *MBB);
230
231  // Remove all successors.
232  while (!MBB->succ_empty())
233    MBB->removeSuccessor(MBB->succ_end()-1);
234
235  // If there are any labels in the basic block, unregister them from
236  // MachineModuleInfo.
237  if (MMI && !MBB->empty()) {
238    for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
239         I != E; ++I) {
240      if (I->isLabel())
241        // The label ID # is always operand #0, an immediate.
242        MMI->InvalidateLabel(I->getOperand(0).getImm());
243    }
244  }
245
246  // Remove the block.
247  MBB->eraseFromParent();
248}
249
250