1 //===-- TailDuplication.cpp - Duplicate blocks into predecessors' tails ---===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass duplicates basic blocks ending in unconditional branches into
11 // the tails of their predecessors.
13 //===----------------------------------------------------------------------===//
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"
29 STATISTIC(NumTailDups , "Number of tail duplicated blocks");
30 STATISTIC(NumInstrDups , "Additional instructions due to tail duplication");
31 STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
33 // Heuristic for tail duplication.
34 static cl::opt<unsigned>
35 TailDuplicateSize("tail-dup-size",
36 cl::desc("Maximum instructions to consider tail duplicating"),
37 cl::init(2), cl::Hidden);
40 /// TailDuplicatePass - Perform tail duplication.
41 class TailDuplicatePass : public MachineFunctionPass {
42 const TargetInstrInfo *TII;
43 MachineModuleInfo *MMI;
47 explicit TailDuplicatePass() : MachineFunctionPass(&ID) {}
49 virtual bool runOnMachineFunction(MachineFunction &MF);
50 virtual const char *getPassName() const { return "Tail Duplication"; }
53 bool TailDuplicateBlocks(MachineFunction &MF);
54 bool TailDuplicate(MachineBasicBlock *TailBB, MachineFunction &MF);
55 void RemoveDeadBlock(MachineBasicBlock *MBB);
58 char TailDuplicatePass::ID = 0;
61 FunctionPass *llvm::createTailDuplicatePass() {
62 return new TailDuplicatePass();
65 bool TailDuplicatePass::runOnMachineFunction(MachineFunction &MF) {
66 TII = MF.getTarget().getInstrInfo();
67 MMI = getAnalysisIfAvailable<MachineModuleInfo>();
69 bool MadeChange = false;
70 bool MadeChangeThisIteration = true;
71 while (MadeChangeThisIteration) {
72 MadeChangeThisIteration = false;
73 MadeChangeThisIteration |= TailDuplicateBlocks(MF);
74 MadeChange |= MadeChangeThisIteration;
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.
83 bool TailDuplicatePass::TailDuplicateBlocks(MachineFunction &MF) {
84 bool MadeChange = false;
86 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
87 MachineBasicBlock *MBB = I++;
89 // Only duplicate blocks that end with unconditional branches.
90 if (MBB->canFallThrough())
93 MadeChange |= TailDuplicate(MBB, MF);
95 // If it is dead, remove it.
96 if (MBB->pred_empty()) {
97 NumInstrDups -= MBB->size();
106 /// TailDuplicate - If it is profitable, duplicate TailBB's contents in each
107 /// of its predecessors.
108 bool TailDuplicatePass::TailDuplicate(MachineBasicBlock *TailBB,
109 MachineFunction &MF) {
110 // Don't try to tail-duplicate single-block loops.
111 if (TailBB->isSuccessor(TailBB))
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 (!TailBB->empty() && TailBB->back().getDesc().isIndirectBranch())
122 // If the target has hardware branch prediction that can handle indirect
123 // branches, duplicating them can often make them predictable when there
124 // are common paths through the code. The limit needs to be high enough
125 // to allow undoing the effects of tail merging.
126 MaxDuplicateCount = 20;
128 MaxDuplicateCount = TailDuplicateSize;
130 // Check the instructions in the block to determine whether tail-duplication
131 // is invalid or unlikely to be profitable.
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;
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)
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(),
154 for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
155 PE = Preds.end(); PI != PE; ++PI) {
156 MachineBasicBlock *PredBB = *PI;
158 assert(TailBB != PredBB &&
159 "Single-block loop should have been rejected earlier!");
160 if (PredBB->succ_size() > 1) continue;
162 MachineBasicBlock *PredTBB, *PredFBB;
163 SmallVector<MachineOperand, 4> PredCond;
164 if (TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
166 if (!PredCond.empty())
168 // EH edges are ignored by AnalyzeBranch.
169 if (PredBB->succ_size() != 1)
171 // Don't duplicate into a fall-through predecessor (at least for now).
172 if (PredBB->isLayoutSuccessor(TailBB) && PredBB->canFallThrough())
175 DEBUG(errs() << "\nTail-duplicating into PredBB: " << *PredBB
176 << "From Succ: " << *TailBB);
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();
183 MachineInstr *NewMI = MF.CloneMachineInstr(I);
184 PredBB->insert(PredBB->end(), NewMI);
186 NumInstrDups += TailBB->size() - 1; // subtract one for removed branch
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);
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
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);
225 /// RemoveDeadBlock - Remove the specified dead machine basic block from the
226 /// function, updating the CFG.
227 void TailDuplicatePass::RemoveDeadBlock(MachineBasicBlock *MBB) {
228 assert(MBB->pred_empty() && "MBB must be dead!");
229 DEBUG(errs() << "\nRemoving MBB: " << *MBB);
231 // Remove all successors.
232 while (!MBB->succ_empty())
233 MBB->removeSuccessor(MBB->succ_end()-1);
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();
241 // The label ID # is always operand #0, an immediate.
242 MMI->InvalidateLabel(I->getOperand(0).getImm());
247 MBB->eraseFromParent();