TailDuplication.cpp revision 810ced7daebe78a8d84b94fac07e320a02cecb71 
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  /// TailDuplicationPass - Perform tail duplication.
41  class TailDuplicationPass : public MachineFunctionPass {
42    const TargetInstrInfo *TII;
43    MachineModuleInfo *MMI;
44
45  public:
46    static char ID;
47    explicit TailDuplicationPass() : 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 TailDuplicationPass::ID = 0;
59}
60
61FunctionPass *llvm::createTailDuplicationPass() {
62  return new TailDuplicationPass();
63}
64
65bool TailDuplicationPass::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 TailDuplicationPass::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 TailDuplicationPass::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 (MF.getFunction()->hasFnAttr(Attribute::OptimizeForSize))
120    MaxDuplicateCount = 1;
121  else if (TII->isProfitableToDuplicateIndirectBranch() &&
122           !TailBB->empty() && TailBB->back().getDesc().isIndirectBranch())
123    // If the target has hardware branch prediction that can handle indirect
124    // branches, duplicating them can often make them predictable when there
125    // are common paths through the code.  The limit needs to be high enough
126    // to allow undoing the effects of tail merging.
127    MaxDuplicateCount = 20;
128  else
129    MaxDuplicateCount = TailDuplicateSize;
130
131  // Check the instructions in the block to determine whether tail-duplication
132  // is invalid or unlikely to be profitable.
133  unsigned i = 0;
134  bool HasCall = false;
135  for (MachineBasicBlock::iterator I = TailBB->begin();
136       I != TailBB->end(); ++I, ++i) {
137    // Non-duplicable things shouldn't be tail-duplicated.
138    if (I->getDesc().isNotDuplicable()) return false;
139    // Don't duplicate more than the threshold.
140    if (i == MaxDuplicateCount) return false;
141    // Remember if we saw a call.
142    if (I->getDesc().isCall()) HasCall = true;
143  }
144  // Heuristically, don't tail-duplicate calls if it would expand code size,
145  // as it's less likely to be worth the extra cost.
146  if (i > 1 && HasCall)
147    return false;
148
149  // Iterate through all the unique predecessors and tail-duplicate this
150  // block into them, if possible. Copying the list ahead of time also
151  // avoids trouble with the predecessor list reallocating.
152  bool Changed = false;
153  SmallSetVector<MachineBasicBlock *, 8> Preds(TailBB->pred_begin(),
154                                               TailBB->pred_end());
155  for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
156       PE = Preds.end(); PI != PE; ++PI) {
157    MachineBasicBlock *PredBB = *PI;
158
159    assert(TailBB != PredBB &&
160           "Single-block loop should have been rejected earlier!");
161    if (PredBB->succ_size() > 1) continue;
162
163    MachineBasicBlock *PredTBB, *PredFBB;
164    SmallVector<MachineOperand, 4> PredCond;
165    if (TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
166      continue;
167    if (!PredCond.empty())
168      continue;
169    // EH edges are ignored by AnalyzeBranch.
170    if (PredBB->succ_size() != 1)
171      continue;
172    // Don't duplicate into a fall-through predecessor (at least for now).
173    if (PredBB->isLayoutSuccessor(TailBB) && PredBB->canFallThrough())
174      continue;
175
176    DEBUG(errs() << "\nTail-duplicating into PredBB: " << *PredBB
177                 << "From Succ: " << *TailBB);
178
179    // Remove PredBB's unconditional branch.
180    TII->RemoveBranch(*PredBB);
181    // Clone the contents of TailBB into PredBB.
182    for (MachineBasicBlock::iterator I = TailBB->begin(), E = TailBB->end();
183         I != E; ++I) {
184      MachineInstr *NewMI = MF.CloneMachineInstr(I);
185      PredBB->insert(PredBB->end(), NewMI);
186    }
187    NumInstrDups += TailBB->size() - 1; // subtract one for removed branch
188
189    // Update the CFG.
190    PredBB->removeSuccessor(PredBB->succ_begin());
191    assert(PredBB->succ_empty() &&
192           "TailDuplicate called on block with multiple successors!");
193    for (MachineBasicBlock::succ_iterator I = TailBB->succ_begin(),
194         E = TailBB->succ_end(); I != E; ++I)
195       PredBB->addSuccessor(*I);
196
197    Changed = true;
198    ++NumTailDups;
199  }
200
201  // If TailBB was duplicated into all its predecessors except for the prior
202  // block, which falls through unconditionally, move the contents of this
203  // block into the prior block.
204  MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(TailBB));
205  MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
206  SmallVector<MachineOperand, 4> PriorCond;
207  bool PriorUnAnalyzable =
208    TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
209  // This has to check PrevBB->succ_size() because EH edges are ignored by
210  // AnalyzeBranch.
211  if (!PriorUnAnalyzable && PriorCond.empty() && !PriorTBB &&
212      TailBB->pred_size() == 1 && PrevBB.succ_size() == 1 &&
213      !TailBB->hasAddressTaken()) {
214    DEBUG(errs() << "\nMerging into block: " << PrevBB
215          << "From MBB: " << *TailBB);
216    PrevBB.splice(PrevBB.end(), TailBB, TailBB->begin(), TailBB->end());
217    PrevBB.removeSuccessor(PrevBB.succ_begin());;
218    assert(PrevBB.succ_empty());
219    PrevBB.transferSuccessors(TailBB);
220    Changed = true;
221  }
222
223  return Changed;
224}
225
226/// RemoveDeadBlock - Remove the specified dead machine basic block from the
227/// function, updating the CFG.
228void TailDuplicationPass::RemoveDeadBlock(MachineBasicBlock *MBB) {
229  assert(MBB->pred_empty() && "MBB must be dead!");
230  DEBUG(errs() << "\nRemoving MBB: " << *MBB);
231
232  // Remove all successors.
233  while (!MBB->succ_empty())
234    MBB->removeSuccessor(MBB->succ_end()-1);
235
236  // If there are any labels in the basic block, unregister them from
237  // MachineModuleInfo.
238  if (MMI && !MBB->empty()) {
239    for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
240         I != E; ++I) {
241      if (I->isLabel())
242        // The label ID # is always operand #0, an immediate.
243        MMI->InvalidateLabel(I->getOperand(0).getImm());
244    }
245  }
246
247  // Remove the block.
248  MBB->eraseFromParent();
249}
250
251