1//===-- BranchFolding.cpp - Fold machine code branch instructions ---------===//
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 forwards branches to unconditional branches to make them branch
11// directly to the target block. This pass often results in dead MBB's, which
12// it then removes.
13//
14// Note that this pass must be run after register allocation, it cannot handle
15// SSA form.
16//
17//===----------------------------------------------------------------------===//
18
19#define DEBUG_TYPE "branchfolding"
20#include "BranchFolding.h"
21#include "llvm/Function.h"
22#include "llvm/CodeGen/Passes.h"
23#include "llvm/CodeGen/MachineModuleInfo.h"
24#include "llvm/CodeGen/MachineFunctionPass.h"
25#include "llvm/CodeGen/MachineJumpTableInfo.h"
26#include "llvm/CodeGen/RegisterScavenging.h"
27#include "llvm/Target/TargetInstrInfo.h"
28#include "llvm/Target/TargetMachine.h"
29#include "llvm/Target/TargetRegisterInfo.h"
30#include "llvm/Support/CommandLine.h"
31#include "llvm/Support/Debug.h"
32#include "llvm/Support/ErrorHandling.h"
33#include "llvm/Support/raw_ostream.h"
34#include "llvm/ADT/SmallSet.h"
35#include "llvm/ADT/SetVector.h"
36#include "llvm/ADT/Statistic.h"
37#include "llvm/ADT/STLExtras.h"
38#include <algorithm>
39using namespace llvm;
40
41STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
42STATISTIC(NumBranchOpts, "Number of branches optimized");
43STATISTIC(NumTailMerge , "Number of block tails merged");
| 1//===-- BranchFolding.cpp - Fold machine code branch instructions ---------===//
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 forwards branches to unconditional branches to make them branch
11// directly to the target block. This pass often results in dead MBB's, which
12// it then removes.
13//
14// Note that this pass must be run after register allocation, it cannot handle
15// SSA form.
16//
17//===----------------------------------------------------------------------===//
18
19#define DEBUG_TYPE "branchfolding"
20#include "BranchFolding.h"
21#include "llvm/Function.h"
22#include "llvm/CodeGen/Passes.h"
23#include "llvm/CodeGen/MachineModuleInfo.h"
24#include "llvm/CodeGen/MachineFunctionPass.h"
25#include "llvm/CodeGen/MachineJumpTableInfo.h"
26#include "llvm/CodeGen/RegisterScavenging.h"
27#include "llvm/Target/TargetInstrInfo.h"
28#include "llvm/Target/TargetMachine.h"
29#include "llvm/Target/TargetRegisterInfo.h"
30#include "llvm/Support/CommandLine.h"
31#include "llvm/Support/Debug.h"
32#include "llvm/Support/ErrorHandling.h"
33#include "llvm/Support/raw_ostream.h"
34#include "llvm/ADT/SmallSet.h"
35#include "llvm/ADT/SetVector.h"
36#include "llvm/ADT/Statistic.h"
37#include "llvm/ADT/STLExtras.h"
38#include <algorithm>
39using namespace llvm;
40
41STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
42STATISTIC(NumBranchOpts, "Number of branches optimized");
43STATISTIC(NumTailMerge , "Number of block tails merged");
|
| 44STATISTIC(NumTailDups , "Number of tail duplicated blocks");
45STATISTIC(NumInstrDups , "Additional instructions due to tail duplication");
46
|
44static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge",
45 cl::init(cl::BOU_UNSET), cl::Hidden);
| 47static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge",
48 cl::init(cl::BOU_UNSET), cl::Hidden);
|
| 49
|
46// Throttle for huge numbers of predecessors (compile speed problems)
47static cl::opt<unsigned>
48TailMergeThreshold("tail-merge-threshold",
49 cl::desc("Max number of predecessors to consider tail merging"),
50 cl::init(150), cl::Hidden);
51
52// Heuristic for tail merging (and, inversely, tail duplication).
53// TODO: This should be replaced with a target query.
54static cl::opt<unsigned>
55TailMergeSize("tail-merge-size",
56 cl::desc("Min number of instructions to consider tail merging"),
57 cl::init(3), cl::Hidden);
58
59namespace {
60 /// BranchFolderPass - Wrap branch folder in a machine function pass.
61 class BranchFolderPass : public MachineFunctionPass,
62 public BranchFolder {
63 public:
64 static char ID;
65 explicit BranchFolderPass(bool defaultEnableTailMerge)
66 : MachineFunctionPass(&ID), BranchFolder(defaultEnableTailMerge) {}
67
68 virtual bool runOnMachineFunction(MachineFunction &MF);
69 virtual const char *getPassName() const { return "Control Flow Optimizer"; }
70 };
71}
72
73char BranchFolderPass::ID = 0;
74
75FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) {
76 return new BranchFolderPass(DefaultEnableTailMerge);
77}
78
79bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) {
80 return OptimizeFunction(MF,
81 MF.getTarget().getInstrInfo(),
82 MF.getTarget().getRegisterInfo(),
83 getAnalysisIfAvailable<MachineModuleInfo>());
84}
85
86
87BranchFolder::BranchFolder(bool defaultEnableTailMerge) {
88 switch (FlagEnableTailMerge) {
89 case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break;
90 case cl::BOU_TRUE: EnableTailMerge = true; break;
91 case cl::BOU_FALSE: EnableTailMerge = false; break;
92 }
93}
94
95/// RemoveDeadBlock - Remove the specified dead machine basic block from the
96/// function, updating the CFG.
97void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) {
98 assert(MBB->pred_empty() && "MBB must be dead!");
99 DEBUG(errs() << "\nRemoving MBB: " << *MBB);
100
101 MachineFunction *MF = MBB->getParent();
102 // drop all successors.
103 while (!MBB->succ_empty())
104 MBB->removeSuccessor(MBB->succ_end()-1);
105
106 // If there are any labels in the basic block, unregister them from
107 // MachineModuleInfo.
108 if (MMI && !MBB->empty()) {
109 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
110 I != E; ++I) {
111 if (I->isLabel())
112 // The label ID # is always operand #0, an immediate.
113 MMI->InvalidateLabel(I->getOperand(0).getImm());
114 }
115 }
116
117 // Remove the block.
118 MF->erase(MBB);
119}
120
121/// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def
122/// followed by terminators, and if the implicitly defined registers are not
123/// used by the terminators, remove those implicit_def's. e.g.
124/// BB1:
125/// r0 = implicit_def
126/// r1 = implicit_def
127/// br
128/// This block can be optimized away later if the implicit instructions are
129/// removed.
130bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) {
131 SmallSet<unsigned, 4> ImpDefRegs;
132 MachineBasicBlock::iterator I = MBB->begin();
133 while (I != MBB->end()) {
134 if (I->getOpcode() != TargetInstrInfo::IMPLICIT_DEF)
135 break;
136 unsigned Reg = I->getOperand(0).getReg();
137 ImpDefRegs.insert(Reg);
138 for (const unsigned *SubRegs = TRI->getSubRegisters(Reg);
139 unsigned SubReg = *SubRegs; ++SubRegs)
140 ImpDefRegs.insert(SubReg);
141 ++I;
142 }
143 if (ImpDefRegs.empty())
144 return false;
145
146 MachineBasicBlock::iterator FirstTerm = I;
147 while (I != MBB->end()) {
148 if (!TII->isUnpredicatedTerminator(I))
149 return false;
150 // See if it uses any of the implicitly defined registers.
151 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
152 MachineOperand &MO = I->getOperand(i);
153 if (!MO.isReg() || !MO.isUse())
154 continue;
155 unsigned Reg = MO.getReg();
156 if (ImpDefRegs.count(Reg))
157 return false;
158 }
159 ++I;
160 }
161
162 I = MBB->begin();
163 while (I != FirstTerm) {
164 MachineInstr *ImpDefMI = &*I;
165 ++I;
166 MBB->erase(ImpDefMI);
167 }
168
169 return true;
170}
171
172/// OptimizeFunction - Perhaps branch folding, tail merging and other
173/// CFG optimizations on the given function.
174bool BranchFolder::OptimizeFunction(MachineFunction &MF,
175 const TargetInstrInfo *tii,
176 const TargetRegisterInfo *tri,
177 MachineModuleInfo *mmi) {
178 if (!tii) return false;
179
180 TII = tii;
181 TRI = tri;
182 MMI = mmi;
183
184 RS = TRI->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL;
185
186 // Fix CFG. The later algorithms expect it to be right.
187 bool MadeChange = false;
188 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) {
189 MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0;
190 SmallVector<MachineOperand, 4> Cond;
191 if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true))
192 MadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty());
193 MadeChange |= OptimizeImpDefsBlock(MBB);
194 }
195
| 50// Throttle for huge numbers of predecessors (compile speed problems)
51static cl::opt<unsigned>
52TailMergeThreshold("tail-merge-threshold",
53 cl::desc("Max number of predecessors to consider tail merging"),
54 cl::init(150), cl::Hidden);
55
56// Heuristic for tail merging (and, inversely, tail duplication).
57// TODO: This should be replaced with a target query.
58static cl::opt<unsigned>
59TailMergeSize("tail-merge-size",
60 cl::desc("Min number of instructions to consider tail merging"),
61 cl::init(3), cl::Hidden);
62
63namespace {
64 /// BranchFolderPass - Wrap branch folder in a machine function pass.
65 class BranchFolderPass : public MachineFunctionPass,
66 public BranchFolder {
67 public:
68 static char ID;
69 explicit BranchFolderPass(bool defaultEnableTailMerge)
70 : MachineFunctionPass(&ID), BranchFolder(defaultEnableTailMerge) {}
71
72 virtual bool runOnMachineFunction(MachineFunction &MF);
73 virtual const char *getPassName() const { return "Control Flow Optimizer"; }
74 };
75}
76
77char BranchFolderPass::ID = 0;
78
79FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) {
80 return new BranchFolderPass(DefaultEnableTailMerge);
81}
82
83bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) {
84 return OptimizeFunction(MF,
85 MF.getTarget().getInstrInfo(),
86 MF.getTarget().getRegisterInfo(),
87 getAnalysisIfAvailable<MachineModuleInfo>());
88}
89
90
91BranchFolder::BranchFolder(bool defaultEnableTailMerge) {
92 switch (FlagEnableTailMerge) {
93 case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break;
94 case cl::BOU_TRUE: EnableTailMerge = true; break;
95 case cl::BOU_FALSE: EnableTailMerge = false; break;
96 }
97}
98
99/// RemoveDeadBlock - Remove the specified dead machine basic block from the
100/// function, updating the CFG.
101void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) {
102 assert(MBB->pred_empty() && "MBB must be dead!");
103 DEBUG(errs() << "\nRemoving MBB: " << *MBB);
104
105 MachineFunction *MF = MBB->getParent();
106 // drop all successors.
107 while (!MBB->succ_empty())
108 MBB->removeSuccessor(MBB->succ_end()-1);
109
110 // If there are any labels in the basic block, unregister them from
111 // MachineModuleInfo.
112 if (MMI && !MBB->empty()) {
113 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
114 I != E; ++I) {
115 if (I->isLabel())
116 // The label ID # is always operand #0, an immediate.
117 MMI->InvalidateLabel(I->getOperand(0).getImm());
118 }
119 }
120
121 // Remove the block.
122 MF->erase(MBB);
123}
124
125/// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def
126/// followed by terminators, and if the implicitly defined registers are not
127/// used by the terminators, remove those implicit_def's. e.g.
128/// BB1:
129/// r0 = implicit_def
130/// r1 = implicit_def
131/// br
132/// This block can be optimized away later if the implicit instructions are
133/// removed.
134bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) {
135 SmallSet<unsigned, 4> ImpDefRegs;
136 MachineBasicBlock::iterator I = MBB->begin();
137 while (I != MBB->end()) {
138 if (I->getOpcode() != TargetInstrInfo::IMPLICIT_DEF)
139 break;
140 unsigned Reg = I->getOperand(0).getReg();
141 ImpDefRegs.insert(Reg);
142 for (const unsigned *SubRegs = TRI->getSubRegisters(Reg);
143 unsigned SubReg = *SubRegs; ++SubRegs)
144 ImpDefRegs.insert(SubReg);
145 ++I;
146 }
147 if (ImpDefRegs.empty())
148 return false;
149
150 MachineBasicBlock::iterator FirstTerm = I;
151 while (I != MBB->end()) {
152 if (!TII->isUnpredicatedTerminator(I))
153 return false;
154 // See if it uses any of the implicitly defined registers.
155 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
156 MachineOperand &MO = I->getOperand(i);
157 if (!MO.isReg() || !MO.isUse())
158 continue;
159 unsigned Reg = MO.getReg();
160 if (ImpDefRegs.count(Reg))
161 return false;
162 }
163 ++I;
164 }
165
166 I = MBB->begin();
167 while (I != FirstTerm) {
168 MachineInstr *ImpDefMI = &*I;
169 ++I;
170 MBB->erase(ImpDefMI);
171 }
172
173 return true;
174}
175
176/// OptimizeFunction - Perhaps branch folding, tail merging and other
177/// CFG optimizations on the given function.
178bool BranchFolder::OptimizeFunction(MachineFunction &MF,
179 const TargetInstrInfo *tii,
180 const TargetRegisterInfo *tri,
181 MachineModuleInfo *mmi) {
182 if (!tii) return false;
183
184 TII = tii;
185 TRI = tri;
186 MMI = mmi;
187
188 RS = TRI->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL;
189
190 // Fix CFG. The later algorithms expect it to be right.
191 bool MadeChange = false;
192 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) {
193 MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0;
194 SmallVector<MachineOperand, 4> Cond;
195 if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true))
196 MadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty());
197 MadeChange |= OptimizeImpDefsBlock(MBB);
198 }
199
|
196
| |
197 bool MadeChangeThisIteration = true;
198 while (MadeChangeThisIteration) {
199 MadeChangeThisIteration = false;
200 MadeChangeThisIteration |= TailMergeBlocks(MF);
201 MadeChangeThisIteration |= OptimizeBranches(MF);
202 MadeChange |= MadeChangeThisIteration;
203 }
204
| 200 bool MadeChangeThisIteration = true;
201 while (MadeChangeThisIteration) {
202 MadeChangeThisIteration = false;
203 MadeChangeThisIteration |= TailMergeBlocks(MF);
204 MadeChangeThisIteration |= OptimizeBranches(MF);
205 MadeChange |= MadeChangeThisIteration;
206 }
207
|
205 // Do tail duplication once after tail merging is done. Otherwise it is
| 208 // Do tail duplication after tail merging is done. Otherwise it is
|
206 // tough to avoid situations where tail duplication and tail merging undo
207 // each other's transformations ad infinitum.
| 209 // tough to avoid situations where tail duplication and tail merging undo
210 // each other's transformations ad infinitum.
|
208 MadeChange |= TailDuplicateBlocks(MF);
| 211 MadeChangeThisIteration = true;
212 while (MadeChangeThisIteration) {
213 MadeChangeThisIteration = false;
214 MadeChangeThisIteration |= TailDuplicateBlocks(MF);
215 MadeChange |= MadeChangeThisIteration;
216 }
|
209
210 // See if any jump tables have become mergable or dead as the code generator
211 // did its thing.
212 MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
213 const std::vector<MachineJumpTableEntry> &JTs = JTI->getJumpTables();
214 if (!JTs.empty()) {
215 // Figure out how these jump tables should be merged.
216 std::vector<unsigned> JTMapping;
217 JTMapping.reserve(JTs.size());
218
219 // We always keep the 0th jump table.
220 JTMapping.push_back(0);
221
222 // Scan the jump tables, seeing if there are any duplicates. Note that this
223 // is N^2, which should be fixed someday.
224 for (unsigned i = 1, e = JTs.size(); i != e; ++i) {
225 if (JTs[i].MBBs.empty())
226 JTMapping.push_back(i);
227 else
228 JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs));
229 }
230
231 // If a jump table was merge with another one, walk the function rewriting
232 // references to jump tables to reference the new JT ID's. Keep track of
233 // whether we see a jump table idx, if not, we can delete the JT.
234 BitVector JTIsLive(JTs.size());
235 for (MachineFunction::iterator BB = MF.begin(), E = MF.end();
236 BB != E; ++BB) {
237 for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end();
238 I != E; ++I)
239 for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) {
240 MachineOperand &Op = I->getOperand(op);
241 if (!Op.isJTI()) continue;
242 unsigned NewIdx = JTMapping[Op.getIndex()];
243 Op.setIndex(NewIdx);
244
245 // Remember that this JT is live.
246 JTIsLive.set(NewIdx);
247 }
248 }
249
250 // Finally, remove dead jump tables. This happens either because the
251 // indirect jump was unreachable (and thus deleted) or because the jump
252 // table was merged with some other one.
253 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i)
254 if (!JTIsLive.test(i)) {
255 JTI->RemoveJumpTable(i);
256 MadeChange = true;
257 }
258 }
259
260 delete RS;
261 return MadeChange;
262}
263
264//===----------------------------------------------------------------------===//
265// Tail Merging of Blocks
266//===----------------------------------------------------------------------===//
267
268/// HashMachineInstr - Compute a hash value for MI and its operands.
269static unsigned HashMachineInstr(const MachineInstr *MI) {
270 unsigned Hash = MI->getOpcode();
271 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
272 const MachineOperand &Op = MI->getOperand(i);
273
274 // Merge in bits from the operand if easy.
275 unsigned OperandHash = 0;
276 switch (Op.getType()) {
277 case MachineOperand::MO_Register: OperandHash = Op.getReg(); break;
278 case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break;
279 case MachineOperand::MO_MachineBasicBlock:
280 OperandHash = Op.getMBB()->getNumber();
281 break;
282 case MachineOperand::MO_FrameIndex:
283 case MachineOperand::MO_ConstantPoolIndex:
284 case MachineOperand::MO_JumpTableIndex:
285 OperandHash = Op.getIndex();
286 break;
287 case MachineOperand::MO_GlobalAddress:
288 case MachineOperand::MO_ExternalSymbol:
289 // Global address / external symbol are too hard, don't bother, but do
290 // pull in the offset.
291 OperandHash = Op.getOffset();
292 break;
293 default: break;
294 }
295
296 Hash += ((OperandHash << 3) | Op.getType()) << (i&31);
297 }
298 return Hash;
299}
300
301/// HashEndOfMBB - Hash the last few instructions in the MBB. For blocks
302/// with no successors, we hash two instructions, because cross-jumping
303/// only saves code when at least two instructions are removed (since a
304/// branch must be inserted). For blocks with a successor, one of the
305/// two blocks to be tail-merged will end with a branch already, so
306/// it gains to cross-jump even for one instruction.
307static unsigned HashEndOfMBB(const MachineBasicBlock *MBB,
308 unsigned minCommonTailLength) {
309 MachineBasicBlock::const_iterator I = MBB->end();
310 if (I == MBB->begin())
311 return 0; // Empty MBB.
312
313 --I;
314 unsigned Hash = HashMachineInstr(I);
315
316 if (I == MBB->begin() || minCommonTailLength == 1)
317 return Hash; // Single instr MBB.
318
319 --I;
320 // Hash in the second-to-last instruction.
321 Hash ^= HashMachineInstr(I) << 2;
322 return Hash;
323}
324
325/// ComputeCommonTailLength - Given two machine basic blocks, compute the number
326/// of instructions they actually have in common together at their end. Return
327/// iterators for the first shared instruction in each block.
328static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1,
329 MachineBasicBlock *MBB2,
330 MachineBasicBlock::iterator &I1,
331 MachineBasicBlock::iterator &I2) {
332 I1 = MBB1->end();
333 I2 = MBB2->end();
334
335 unsigned TailLen = 0;
336 while (I1 != MBB1->begin() && I2 != MBB2->begin()) {
337 --I1; --I2;
338 if (!I1->isIdenticalTo(I2) ||
339 // FIXME: This check is dubious. It's used to get around a problem where
340 // people incorrectly expect inline asm directives to remain in the same
341 // relative order. This is untenable because normal compiler
342 // optimizations (like this one) may reorder and/or merge these
343 // directives.
344 I1->getOpcode() == TargetInstrInfo::INLINEASM) {
345 ++I1; ++I2;
346 break;
347 }
348 ++TailLen;
349 }
350 return TailLen;
351}
352
353/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything
354/// after it, replacing it with an unconditional branch to NewDest. This
355/// returns true if OldInst's block is modified, false if NewDest is modified.
356void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
357 MachineBasicBlock *NewDest) {
358 MachineBasicBlock *OldBB = OldInst->getParent();
359
360 // Remove all the old successors of OldBB from the CFG.
361 while (!OldBB->succ_empty())
362 OldBB->removeSuccessor(OldBB->succ_begin());
363
364 // Remove all the dead instructions from the end of OldBB.
365 OldBB->erase(OldInst, OldBB->end());
366
367 // If OldBB isn't immediately before OldBB, insert a branch to it.
368 if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest))
369 TII->InsertBranch(*OldBB, NewDest, 0, SmallVector<MachineOperand, 0>());
370 OldBB->addSuccessor(NewDest);
371 ++NumTailMerge;
372}
373
374/// SplitMBBAt - Given a machine basic block and an iterator into it, split the
375/// MBB so that the part before the iterator falls into the part starting at the
376/// iterator. This returns the new MBB.
377MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB,
378 MachineBasicBlock::iterator BBI1) {
379 MachineFunction &MF = *CurMBB.getParent();
380
381 // Create the fall-through block.
382 MachineFunction::iterator MBBI = &CurMBB;
383 MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(CurMBB.getBasicBlock());
384 CurMBB.getParent()->insert(++MBBI, NewMBB);
385
386 // Move all the successors of this block to the specified block.
387 NewMBB->transferSuccessors(&CurMBB);
388
389 // Add an edge from CurMBB to NewMBB for the fall-through.
390 CurMBB.addSuccessor(NewMBB);
391
392 // Splice the code over.
393 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end());
394
395 // For targets that use the register scavenger, we must maintain LiveIns.
396 if (RS) {
397 RS->enterBasicBlock(&CurMBB);
398 if (!CurMBB.empty())
399 RS->forward(prior(CurMBB.end()));
400 BitVector RegsLiveAtExit(TRI->getNumRegs());
401 RS->getRegsUsed(RegsLiveAtExit, false);
402 for (unsigned int i = 0, e = TRI->getNumRegs(); i != e; i++)
403 if (RegsLiveAtExit[i])
404 NewMBB->addLiveIn(i);
405 }
406
407 return NewMBB;
408}
409
410/// EstimateRuntime - Make a rough estimate for how long it will take to run
411/// the specified code.
412static unsigned EstimateRuntime(MachineBasicBlock::iterator I,
413 MachineBasicBlock::iterator E) {
414 unsigned Time = 0;
415 for (; I != E; ++I) {
416 const TargetInstrDesc &TID = I->getDesc();
417 if (TID.isCall())
418 Time += 10;
419 else if (TID.mayLoad() || TID.mayStore())
420 Time += 2;
421 else
422 ++Time;
423 }
424 return Time;
425}
426
427// CurMBB needs to add an unconditional branch to SuccMBB (we removed these
428// branches temporarily for tail merging). In the case where CurMBB ends
429// with a conditional branch to the next block, optimize by reversing the
430// test and conditionally branching to SuccMBB instead.
431static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB,
432 const TargetInstrInfo *TII) {
433 MachineFunction *MF = CurMBB->getParent();
434 MachineFunction::iterator I = next(MachineFunction::iterator(CurMBB));
435 MachineBasicBlock *TBB = 0, *FBB = 0;
436 SmallVector<MachineOperand, 4> Cond;
437 if (I != MF->end() &&
438 !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond, true)) {
439 MachineBasicBlock *NextBB = I;
440 if (TBB == NextBB && !Cond.empty() && !FBB) {
441 if (!TII->ReverseBranchCondition(Cond)) {
442 TII->RemoveBranch(*CurMBB);
443 TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond);
444 return;
445 }
446 }
447 }
448 TII->InsertBranch(*CurMBB, SuccBB, NULL, SmallVector<MachineOperand, 0>());
449}
450
451bool
452BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const {
453 if (getHash() < o.getHash())
454 return true;
455 else if (getHash() > o.getHash())
456 return false;
457 else if (getBlock()->getNumber() < o.getBlock()->getNumber())
458 return true;
459 else if (getBlock()->getNumber() > o.getBlock()->getNumber())
460 return false;
461 else {
462 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing
463 // an object with itself.
464#ifndef _GLIBCXX_DEBUG
465 llvm_unreachable("Predecessor appears twice");
466#endif
467 return false;
468 }
469}
470
471/// CountTerminators - Count the number of terminators in the given
472/// block and set I to the position of the first non-terminator, if there
473/// is one, or MBB->end() otherwise.
474static unsigned CountTerminators(MachineBasicBlock *MBB,
475 MachineBasicBlock::iterator &I) {
476 I = MBB->end();
477 unsigned NumTerms = 0;
478 for (;;) {
479 if (I == MBB->begin()) {
480 I = MBB->end();
481 break;
482 }
483 --I;
484 if (!I->getDesc().isTerminator()) break;
485 ++NumTerms;
486 }
487 return NumTerms;
488}
489
490/// ProfitableToMerge - Check if two machine basic blocks have a common tail
491/// and decide if it would be profitable to merge those tails. Return the
492/// length of the common tail and iterators to the first common instruction
493/// in each block.
494static bool ProfitableToMerge(MachineBasicBlock *MBB1,
495 MachineBasicBlock *MBB2,
496 unsigned minCommonTailLength,
497 unsigned &CommonTailLen,
498 MachineBasicBlock::iterator &I1,
499 MachineBasicBlock::iterator &I2,
500 MachineBasicBlock *SuccBB,
501 MachineBasicBlock *PredBB) {
502 CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2);
503 MachineFunction *MF = MBB1->getParent();
504
505 if (CommonTailLen == 0)
506 return false;
507
508 // It's almost always profitable to merge any number of non-terminator
509 // instructions with the block that falls through into the common successor.
510 if (MBB1 == PredBB || MBB2 == PredBB) {
511 MachineBasicBlock::iterator I;
512 unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I);
513 if (CommonTailLen > NumTerms)
514 return true;
515 }
516
517 // If one of the blocks can be completely merged and happens to be in
518 // a position where the other could fall through into it, merge any number
519 // of instructions, because it can be done without a branch.
520 // TODO: If the blocks are not adjacent, move one of them so that they are?
521 if (MBB1->isLayoutSuccessor(MBB2) && I2 == MBB2->begin())
522 return true;
523 if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin())
524 return true;
525
526 // If both blocks have an unconditional branch temporarily stripped out,
527 // count that as an additional common instruction for the following
528 // heuristics.
529 unsigned EffectiveTailLen = CommonTailLen;
530 if (SuccBB && MBB1 != PredBB && MBB2 != PredBB &&
531 !MBB1->back().getDesc().isBarrier() &&
532 !MBB2->back().getDesc().isBarrier())
533 ++EffectiveTailLen;
534
535 // Check if the common tail is long enough to be worthwhile.
536 if (EffectiveTailLen >= minCommonTailLength)
537 return true;
538
539 // If we are optimizing for code size, 2 instructions in common is enough if
540 // we don't have to split a block. At worst we will be introducing 1 new
541 // branch instruction, which is likely to be smaller than the 2
542 // instructions that would be deleted in the merge.
543 if (EffectiveTailLen >= 2 &&
544 MF->getFunction()->hasFnAttr(Attribute::OptimizeForSize) &&
545 (I1 == MBB1->begin() || I2 == MBB2->begin()))
546 return true;
547
548 return false;
549}
550
551/// ComputeSameTails - Look through all the blocks in MergePotentials that have
552/// hash CurHash (guaranteed to match the last element). Build the vector
553/// SameTails of all those that have the (same) largest number of instructions
554/// in common of any pair of these blocks. SameTails entries contain an
555/// iterator into MergePotentials (from which the MachineBasicBlock can be
556/// found) and a MachineBasicBlock::iterator into that MBB indicating the
557/// instruction where the matching code sequence begins.
558/// Order of elements in SameTails is the reverse of the order in which
559/// those blocks appear in MergePotentials (where they are not necessarily
560/// consecutive).
561unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
562 unsigned minCommonTailLength,
563 MachineBasicBlock *SuccBB,
564 MachineBasicBlock *PredBB) {
565 unsigned maxCommonTailLength = 0U;
566 SameTails.clear();
567 MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
568 MPIterator HighestMPIter = prior(MergePotentials.end());
569 for (MPIterator CurMPIter = prior(MergePotentials.end()),
570 B = MergePotentials.begin();
571 CurMPIter != B && CurMPIter->getHash() == CurHash;
572 --CurMPIter) {
573 for (MPIterator I = prior(CurMPIter); I->getHash() == CurHash ; --I) {
574 unsigned CommonTailLen;
575 if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(),
576 minCommonTailLength,
577 CommonTailLen, TrialBBI1, TrialBBI2,
578 SuccBB, PredBB)) {
579 if (CommonTailLen > maxCommonTailLength) {
580 SameTails.clear();
581 maxCommonTailLength = CommonTailLen;
582 HighestMPIter = CurMPIter;
583 SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1));
584 }
585 if (HighestMPIter == CurMPIter &&
586 CommonTailLen == maxCommonTailLength)
587 SameTails.push_back(SameTailElt(I, TrialBBI2));
588 }
589 if (I == B)
590 break;
591 }
592 }
593 return maxCommonTailLength;
594}
595
596/// RemoveBlocksWithHash - Remove all blocks with hash CurHash from
597/// MergePotentials, restoring branches at ends of blocks as appropriate.
598void BranchFolder::RemoveBlocksWithHash(unsigned CurHash,
599 MachineBasicBlock *SuccBB,
600 MachineBasicBlock *PredBB) {
601 MPIterator CurMPIter, B;
602 for (CurMPIter = prior(MergePotentials.end()), B = MergePotentials.begin();
603 CurMPIter->getHash() == CurHash;
604 --CurMPIter) {
605 // Put the unconditional branch back, if we need one.
606 MachineBasicBlock *CurMBB = CurMPIter->getBlock();
607 if (SuccBB && CurMBB != PredBB)
608 FixTail(CurMBB, SuccBB, TII);
609 if (CurMPIter == B)
610 break;
611 }
612 if (CurMPIter->getHash() != CurHash)
613 CurMPIter++;
614 MergePotentials.erase(CurMPIter, MergePotentials.end());
615}
616
617/// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist
618/// only of the common tail. Create a block that does by splitting one.
619unsigned BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
620 unsigned maxCommonTailLength) {
621 unsigned commonTailIndex = 0;
622 unsigned TimeEstimate = ~0U;
623 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
624 // Use PredBB if possible; that doesn't require a new branch.
625 if (SameTails[i].getBlock() == PredBB) {
626 commonTailIndex = i;
627 break;
628 }
629 // Otherwise, make a (fairly bogus) choice based on estimate of
630 // how long it will take the various blocks to execute.
631 unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(),
632 SameTails[i].getTailStartPos());
633 if (t <= TimeEstimate) {
634 TimeEstimate = t;
635 commonTailIndex = i;
636 }
637 }
638
639 MachineBasicBlock::iterator BBI =
640 SameTails[commonTailIndex].getTailStartPos();
641 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
642
643 DEBUG(errs() << "\nSplitting BB#" << MBB->getNumber() << ", size "
644 << maxCommonTailLength);
645
646 MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI);
647 SameTails[commonTailIndex].setBlock(newMBB);
648 SameTails[commonTailIndex].setTailStartPos(newMBB->begin());
649
650 // If we split PredBB, newMBB is the new predecessor.
651 if (PredBB == MBB)
652 PredBB = newMBB;
653
654 return commonTailIndex;
655}
656
657// See if any of the blocks in MergePotentials (which all have a common single
658// successor, or all have no successor) can be tail-merged. If there is a
659// successor, any blocks in MergePotentials that are not tail-merged and
660// are not immediately before Succ must have an unconditional branch to
661// Succ added (but the predecessor/successor lists need no adjustment).
662// The lone predecessor of Succ that falls through into Succ,
663// if any, is given in PredBB.
664
665bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
666 MachineBasicBlock *PredBB) {
667 bool MadeChange = false;
668
669 // Except for the special cases below, tail-merge if there are at least
670 // this many instructions in common.
671 unsigned minCommonTailLength = TailMergeSize;
672
673 DEBUG(errs() << "\nTryTailMergeBlocks: ";
674 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
675 errs() << "BB#" << MergePotentials[i].getBlock()->getNumber()
676 << (i == e-1 ? "" : ", ");
677 errs() << "\n";
678 if (SuccBB) {
679 errs() << " with successor BB#" << SuccBB->getNumber() << '\n';
680 if (PredBB)
681 errs() << " which has fall-through from BB#"
682 << PredBB->getNumber() << "\n";
683 }
684 errs() << "Looking for common tails of at least "
685 << minCommonTailLength << " instruction"
686 << (minCommonTailLength == 1 ? "" : "s") << '\n';
687 );
688
689 // Sort by hash value so that blocks with identical end sequences sort
690 // together.
691 std::stable_sort(MergePotentials.begin(), MergePotentials.end());
692
693 // Walk through equivalence sets looking for actual exact matches.
694 while (MergePotentials.size() > 1) {
695 unsigned CurHash = MergePotentials.back().getHash();
696
697 // Build SameTails, identifying the set of blocks with this hash code
698 // and with the maximum number of instructions in common.
699 unsigned maxCommonTailLength = ComputeSameTails(CurHash,
700 minCommonTailLength,
701 SuccBB, PredBB);
702
703 // If we didn't find any pair that has at least minCommonTailLength
704 // instructions in common, remove all blocks with this hash code and retry.
705 if (SameTails.empty()) {
706 RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
707 continue;
708 }
709
710 // If one of the blocks is the entire common tail (and not the entry
711 // block, which we can't jump to), we can treat all blocks with this same
712 // tail at once. Use PredBB if that is one of the possibilities, as that
713 // will not introduce any extra branches.
714 MachineBasicBlock *EntryBB = MergePotentials.begin()->getBlock()->
715 getParent()->begin();
716 unsigned commonTailIndex = SameTails.size();
717 // If there are two blocks, check to see if one can be made to fall through
718 // into the other.
719 if (SameTails.size() == 2 &&
720 SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) &&
721 SameTails[1].tailIsWholeBlock())
722 commonTailIndex = 1;
723 else if (SameTails.size() == 2 &&
724 SameTails[1].getBlock()->isLayoutSuccessor(
725 SameTails[0].getBlock()) &&
726 SameTails[0].tailIsWholeBlock())
727 commonTailIndex = 0;
728 else {
729 // Otherwise just pick one, favoring the fall-through predecessor if
730 // there is one.
731 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
732 MachineBasicBlock *MBB = SameTails[i].getBlock();
733 if (MBB == EntryBB && SameTails[i].tailIsWholeBlock())
734 continue;
735 if (MBB == PredBB) {
736 commonTailIndex = i;
737 break;
738 }
739 if (SameTails[i].tailIsWholeBlock())
740 commonTailIndex = i;
741 }
742 }
743
744 if (commonTailIndex == SameTails.size() ||
745 (SameTails[commonTailIndex].getBlock() == PredBB &&
746 !SameTails[commonTailIndex].tailIsWholeBlock())) {
747 // None of the blocks consist entirely of the common tail.
748 // Split a block so that one does.
749 commonTailIndex = CreateCommonTailOnlyBlock(PredBB, maxCommonTailLength);
750 }
751
752 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
753 // MBB is common tail. Adjust all other BB's to jump to this one.
754 // Traversal must be forwards so erases work.
755 DEBUG(errs() << "\nUsing common tail in BB#" << MBB->getNumber()
756 << " for ");
757 for (unsigned int i=0, e = SameTails.size(); i != e; ++i) {
758 if (commonTailIndex == i)
759 continue;
760 DEBUG(errs() << "BB#" << SameTails[i].getBlock()->getNumber()
761 << (i == e-1 ? "" : ", "));
762 // Hack the end off BB i, making it jump to BB commonTailIndex instead.
763 ReplaceTailWithBranchTo(SameTails[i].getTailStartPos(), MBB);
764 // BB i is no longer a predecessor of SuccBB; remove it from the worklist.
765 MergePotentials.erase(SameTails[i].getMPIter());
766 }
767 DEBUG(errs() << "\n");
768 // We leave commonTailIndex in the worklist in case there are other blocks
769 // that match it with a smaller number of instructions.
770 MadeChange = true;
771 }
772 return MadeChange;
773}
774
775bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
776
777 if (!EnableTailMerge) return false;
778
779 bool MadeChange = false;
780
781 // First find blocks with no successors.
782 MergePotentials.clear();
783 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
784 if (I->succ_empty())
785 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(I, 2U), I));
786 }
787
788 // See if we can do any tail merging on those.
789 if (MergePotentials.size() < TailMergeThreshold &&
790 MergePotentials.size() >= 2)
791 MadeChange |= TryTailMergeBlocks(NULL, NULL);
792
793 // Look at blocks (IBB) with multiple predecessors (PBB).
794 // We change each predecessor to a canonical form, by
795 // (1) temporarily removing any unconditional branch from the predecessor
796 // to IBB, and
797 // (2) alter conditional branches so they branch to the other block
798 // not IBB; this may require adding back an unconditional branch to IBB
799 // later, where there wasn't one coming in. E.g.
800 // Bcc IBB
801 // fallthrough to QBB
802 // here becomes
803 // Bncc QBB
804 // with a conceptual B to IBB after that, which never actually exists.
805 // With those changes, we see whether the predecessors' tails match,
806 // and merge them if so. We change things out of canonical form and
807 // back to the way they were later in the process. (OptimizeBranches
808 // would undo some of this, but we can't use it, because we'd get into
809 // a compile-time infinite loop repeatedly doing and undoing the same
810 // transformations.)
811
812 for (MachineFunction::iterator I = next(MF.begin()), E = MF.end();
813 I != E; ++I) {
814 if (I->pred_size() >= 2 && I->pred_size() < TailMergeThreshold) {
815 SmallPtrSet<MachineBasicBlock *, 8> UniquePreds;
816 MachineBasicBlock *IBB = I;
817 MachineBasicBlock *PredBB = prior(I);
818 MergePotentials.clear();
819 for (MachineBasicBlock::pred_iterator P = I->pred_begin(),
820 E2 = I->pred_end();
821 P != E2; ++P) {
822 MachineBasicBlock *PBB = *P;
823 // Skip blocks that loop to themselves, can't tail merge these.
824 if (PBB == IBB)
825 continue;
826 // Visit each predecessor only once.
827 if (!UniquePreds.insert(PBB))
828 continue;
829 MachineBasicBlock *TBB = 0, *FBB = 0;
830 SmallVector<MachineOperand, 4> Cond;
831 if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond, true)) {
832 // Failing case: IBB is the target of a cbr, and
833 // we cannot reverse the branch.
834 SmallVector<MachineOperand, 4> NewCond(Cond);
835 if (!Cond.empty() && TBB == IBB) {
836 if (TII->ReverseBranchCondition(NewCond))
837 continue;
838 // This is the QBB case described above
839 if (!FBB)
840 FBB = next(MachineFunction::iterator(PBB));
841 }
842 // Failing case: the only way IBB can be reached from PBB is via
843 // exception handling. Happens for landing pads. Would be nice
844 // to have a bit in the edge so we didn't have to do all this.
845 if (IBB->isLandingPad()) {
846 MachineFunction::iterator IP = PBB; IP++;
847 MachineBasicBlock *PredNextBB = NULL;
848 if (IP != MF.end())
849 PredNextBB = IP;
850 if (TBB == NULL) {
851 if (IBB != PredNextBB) // fallthrough
852 continue;
853 } else if (FBB) {
854 if (TBB != IBB && FBB != IBB) // cbr then ubr
855 continue;
856 } else if (Cond.empty()) {
857 if (TBB != IBB) // ubr
858 continue;
859 } else {
860 if (TBB != IBB && IBB != PredNextBB) // cbr
861 continue;
862 }
863 }
864 // Remove the unconditional branch at the end, if any.
865 if (TBB && (Cond.empty() || FBB)) {
866 TII->RemoveBranch(*PBB);
867 if (!Cond.empty())
868 // reinsert conditional branch only, for now
869 TII->InsertBranch(*PBB, (TBB == IBB) ? FBB : TBB, 0, NewCond);
870 }
871 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(PBB, 1U),
872 *P));
873 }
874 }
875 if (MergePotentials.size() >= 2)
876 MadeChange |= TryTailMergeBlocks(IBB, PredBB);
877 // Reinsert an unconditional branch if needed.
878 // The 1 below can occur as a result of removing blocks in TryTailMergeBlocks.
879 PredBB = prior(I); // this may have been changed in TryTailMergeBlocks
880 if (MergePotentials.size() == 1 &&
881 MergePotentials.begin()->getBlock() != PredBB)
882 FixTail(MergePotentials.begin()->getBlock(), IBB, TII);
883 }
884 }
885 return MadeChange;
886}
887
888//===----------------------------------------------------------------------===//
889// Branch Optimization
890//===----------------------------------------------------------------------===//
891
892bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
893 bool MadeChange = false;
894
895 // Make sure blocks are numbered in order
896 MF.RenumberBlocks();
897
898 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
899 MachineBasicBlock *MBB = I++;
900 MadeChange |= OptimizeBlock(MBB);
901
902 // If it is dead, remove it.
903 if (MBB->pred_empty()) {
904 RemoveDeadBlock(MBB);
905 MadeChange = true;
906 ++NumDeadBlocks;
907 }
908 }
909 return MadeChange;
910}
911
912
913/// CanFallThrough - Return true if the specified block (with the specified
914/// branch condition) can implicitly transfer control to the block after it by
915/// falling off the end of it. This should return false if it can reach the
916/// block after it, but it uses an explicit branch to do so (e.g. a table jump).
917///
918/// True is a conservative answer.
919///
920bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB,
921 bool BranchUnAnalyzable,
922 MachineBasicBlock *TBB,
923 MachineBasicBlock *FBB,
924 const SmallVectorImpl<MachineOperand> &Cond) {
925 MachineFunction::iterator Fallthrough = CurBB;
926 ++Fallthrough;
927 // If FallthroughBlock is off the end of the function, it can't fall through.
928 if (Fallthrough == CurBB->getParent()->end())
929 return false;
930
931 // If FallthroughBlock isn't a successor of CurBB, no fallthrough is possible.
932 if (!CurBB->isSuccessor(Fallthrough))
933 return false;
934
935 // If we couldn't analyze the branch, examine the last instruction.
936 // If the block doesn't end in a known control barrier, assume fallthrough
937 // is possible. The isPredicable check is needed because this code can be
938 // called during IfConversion, where an instruction which is normally a
939 // Barrier is predicated and thus no longer an actual control barrier. This
940 // is over-conservative though, because if an instruction isn't actually
941 // predicated we could still treat it like a barrier.
942 if (BranchUnAnalyzable)
943 return CurBB->empty() || !CurBB->back().getDesc().isBarrier() ||
944 CurBB->back().getDesc().isPredicable();
945
946 // If there is no branch, control always falls through.
947 if (TBB == 0) return true;
948
949 // If there is some explicit branch to the fallthrough block, it can obviously
950 // reach, even though the branch should get folded to fall through implicitly.
951 if (MachineFunction::iterator(TBB) == Fallthrough ||
952 MachineFunction::iterator(FBB) == Fallthrough)
953 return true;
954
955 // If it's an unconditional branch to some block not the fall through, it
956 // doesn't fall through.
957 if (Cond.empty()) return false;
958
959 // Otherwise, if it is conditional and has no explicit false block, it falls
960 // through.
961 return FBB == 0;
962}
963
964/// CanFallThrough - Return true if the specified can implicitly transfer
965/// control to the block after it by falling off the end of it. This should
966/// return false if it can reach the block after it, but it uses an explicit
967/// branch to do so (e.g. a table jump).
968///
969/// True is a conservative answer.
970///
971bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) {
972 MachineBasicBlock *TBB = 0, *FBB = 0;
973 SmallVector<MachineOperand, 4> Cond;
974 bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond, true);
975 return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond);
976}
977
978/// IsBetterFallthrough - Return true if it would be clearly better to
979/// fall-through to MBB1 than to fall through into MBB2. This has to return
980/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will
981/// result in infinite loops.
982static bool IsBetterFallthrough(MachineBasicBlock *MBB1,
983 MachineBasicBlock *MBB2) {
984 // Right now, we use a simple heuristic. If MBB2 ends with a call, and
985 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to
986 // optimize branches that branch to either a return block or an assert block
987 // into a fallthrough to the return.
988 if (MBB1->empty() || MBB2->empty()) return false;
989
990 // If there is a clear successor ordering we make sure that one block
991 // will fall through to the next
992 if (MBB1->isSuccessor(MBB2)) return true;
993 if (MBB2->isSuccessor(MBB1)) return false;
994
995 MachineInstr *MBB1I = --MBB1->end();
996 MachineInstr *MBB2I = --MBB2->end();
997 return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall();
998}
999
1000/// TailDuplicateBlocks - Look for small blocks that are unconditionally
1001/// branched to and do not fall through. Tail-duplicate their instructions
1002/// into their predecessors to eliminate (dynamic) branches.
1003bool BranchFolder::TailDuplicateBlocks(MachineFunction &MF) {
1004 bool MadeChange = false;
1005
| 217
218 // See if any jump tables have become mergable or dead as the code generator
219 // did its thing.
220 MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
221 const std::vector<MachineJumpTableEntry> &JTs = JTI->getJumpTables();
222 if (!JTs.empty()) {
223 // Figure out how these jump tables should be merged.
224 std::vector<unsigned> JTMapping;
225 JTMapping.reserve(JTs.size());
226
227 // We always keep the 0th jump table.
228 JTMapping.push_back(0);
229
230 // Scan the jump tables, seeing if there are any duplicates. Note that this
231 // is N^2, which should be fixed someday.
232 for (unsigned i = 1, e = JTs.size(); i != e; ++i) {
233 if (JTs[i].MBBs.empty())
234 JTMapping.push_back(i);
235 else
236 JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs));
237 }
238
239 // If a jump table was merge with another one, walk the function rewriting
240 // references to jump tables to reference the new JT ID's. Keep track of
241 // whether we see a jump table idx, if not, we can delete the JT.
242 BitVector JTIsLive(JTs.size());
243 for (MachineFunction::iterator BB = MF.begin(), E = MF.end();
244 BB != E; ++BB) {
245 for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end();
246 I != E; ++I)
247 for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) {
248 MachineOperand &Op = I->getOperand(op);
249 if (!Op.isJTI()) continue;
250 unsigned NewIdx = JTMapping[Op.getIndex()];
251 Op.setIndex(NewIdx);
252
253 // Remember that this JT is live.
254 JTIsLive.set(NewIdx);
255 }
256 }
257
258 // Finally, remove dead jump tables. This happens either because the
259 // indirect jump was unreachable (and thus deleted) or because the jump
260 // table was merged with some other one.
261 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i)
262 if (!JTIsLive.test(i)) {
263 JTI->RemoveJumpTable(i);
264 MadeChange = true;
265 }
266 }
267
268 delete RS;
269 return MadeChange;
270}
271
272//===----------------------------------------------------------------------===//
273// Tail Merging of Blocks
274//===----------------------------------------------------------------------===//
275
276/// HashMachineInstr - Compute a hash value for MI and its operands.
277static unsigned HashMachineInstr(const MachineInstr *MI) {
278 unsigned Hash = MI->getOpcode();
279 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
280 const MachineOperand &Op = MI->getOperand(i);
281
282 // Merge in bits from the operand if easy.
283 unsigned OperandHash = 0;
284 switch (Op.getType()) {
285 case MachineOperand::MO_Register: OperandHash = Op.getReg(); break;
286 case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break;
287 case MachineOperand::MO_MachineBasicBlock:
288 OperandHash = Op.getMBB()->getNumber();
289 break;
290 case MachineOperand::MO_FrameIndex:
291 case MachineOperand::MO_ConstantPoolIndex:
292 case MachineOperand::MO_JumpTableIndex:
293 OperandHash = Op.getIndex();
294 break;
295 case MachineOperand::MO_GlobalAddress:
296 case MachineOperand::MO_ExternalSymbol:
297 // Global address / external symbol are too hard, don't bother, but do
298 // pull in the offset.
299 OperandHash = Op.getOffset();
300 break;
301 default: break;
302 }
303
304 Hash += ((OperandHash << 3) | Op.getType()) << (i&31);
305 }
306 return Hash;
307}
308
309/// HashEndOfMBB - Hash the last few instructions in the MBB. For blocks
310/// with no successors, we hash two instructions, because cross-jumping
311/// only saves code when at least two instructions are removed (since a
312/// branch must be inserted). For blocks with a successor, one of the
313/// two blocks to be tail-merged will end with a branch already, so
314/// it gains to cross-jump even for one instruction.
315static unsigned HashEndOfMBB(const MachineBasicBlock *MBB,
316 unsigned minCommonTailLength) {
317 MachineBasicBlock::const_iterator I = MBB->end();
318 if (I == MBB->begin())
319 return 0; // Empty MBB.
320
321 --I;
322 unsigned Hash = HashMachineInstr(I);
323
324 if (I == MBB->begin() || minCommonTailLength == 1)
325 return Hash; // Single instr MBB.
326
327 --I;
328 // Hash in the second-to-last instruction.
329 Hash ^= HashMachineInstr(I) << 2;
330 return Hash;
331}
332
333/// ComputeCommonTailLength - Given two machine basic blocks, compute the number
334/// of instructions they actually have in common together at their end. Return
335/// iterators for the first shared instruction in each block.
336static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1,
337 MachineBasicBlock *MBB2,
338 MachineBasicBlock::iterator &I1,
339 MachineBasicBlock::iterator &I2) {
340 I1 = MBB1->end();
341 I2 = MBB2->end();
342
343 unsigned TailLen = 0;
344 while (I1 != MBB1->begin() && I2 != MBB2->begin()) {
345 --I1; --I2;
346 if (!I1->isIdenticalTo(I2) ||
347 // FIXME: This check is dubious. It's used to get around a problem where
348 // people incorrectly expect inline asm directives to remain in the same
349 // relative order. This is untenable because normal compiler
350 // optimizations (like this one) may reorder and/or merge these
351 // directives.
352 I1->getOpcode() == TargetInstrInfo::INLINEASM) {
353 ++I1; ++I2;
354 break;
355 }
356 ++TailLen;
357 }
358 return TailLen;
359}
360
361/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything
362/// after it, replacing it with an unconditional branch to NewDest. This
363/// returns true if OldInst's block is modified, false if NewDest is modified.
364void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
365 MachineBasicBlock *NewDest) {
366 MachineBasicBlock *OldBB = OldInst->getParent();
367
368 // Remove all the old successors of OldBB from the CFG.
369 while (!OldBB->succ_empty())
370 OldBB->removeSuccessor(OldBB->succ_begin());
371
372 // Remove all the dead instructions from the end of OldBB.
373 OldBB->erase(OldInst, OldBB->end());
374
375 // If OldBB isn't immediately before OldBB, insert a branch to it.
376 if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest))
377 TII->InsertBranch(*OldBB, NewDest, 0, SmallVector<MachineOperand, 0>());
378 OldBB->addSuccessor(NewDest);
379 ++NumTailMerge;
380}
381
382/// SplitMBBAt - Given a machine basic block and an iterator into it, split the
383/// MBB so that the part before the iterator falls into the part starting at the
384/// iterator. This returns the new MBB.
385MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB,
386 MachineBasicBlock::iterator BBI1) {
387 MachineFunction &MF = *CurMBB.getParent();
388
389 // Create the fall-through block.
390 MachineFunction::iterator MBBI = &CurMBB;
391 MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(CurMBB.getBasicBlock());
392 CurMBB.getParent()->insert(++MBBI, NewMBB);
393
394 // Move all the successors of this block to the specified block.
395 NewMBB->transferSuccessors(&CurMBB);
396
397 // Add an edge from CurMBB to NewMBB for the fall-through.
398 CurMBB.addSuccessor(NewMBB);
399
400 // Splice the code over.
401 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end());
402
403 // For targets that use the register scavenger, we must maintain LiveIns.
404 if (RS) {
405 RS->enterBasicBlock(&CurMBB);
406 if (!CurMBB.empty())
407 RS->forward(prior(CurMBB.end()));
408 BitVector RegsLiveAtExit(TRI->getNumRegs());
409 RS->getRegsUsed(RegsLiveAtExit, false);
410 for (unsigned int i = 0, e = TRI->getNumRegs(); i != e; i++)
411 if (RegsLiveAtExit[i])
412 NewMBB->addLiveIn(i);
413 }
414
415 return NewMBB;
416}
417
418/// EstimateRuntime - Make a rough estimate for how long it will take to run
419/// the specified code.
420static unsigned EstimateRuntime(MachineBasicBlock::iterator I,
421 MachineBasicBlock::iterator E) {
422 unsigned Time = 0;
423 for (; I != E; ++I) {
424 const TargetInstrDesc &TID = I->getDesc();
425 if (TID.isCall())
426 Time += 10;
427 else if (TID.mayLoad() || TID.mayStore())
428 Time += 2;
429 else
430 ++Time;
431 }
432 return Time;
433}
434
435// CurMBB needs to add an unconditional branch to SuccMBB (we removed these
436// branches temporarily for tail merging). In the case where CurMBB ends
437// with a conditional branch to the next block, optimize by reversing the
438// test and conditionally branching to SuccMBB instead.
439static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB,
440 const TargetInstrInfo *TII) {
441 MachineFunction *MF = CurMBB->getParent();
442 MachineFunction::iterator I = next(MachineFunction::iterator(CurMBB));
443 MachineBasicBlock *TBB = 0, *FBB = 0;
444 SmallVector<MachineOperand, 4> Cond;
445 if (I != MF->end() &&
446 !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond, true)) {
447 MachineBasicBlock *NextBB = I;
448 if (TBB == NextBB && !Cond.empty() && !FBB) {
449 if (!TII->ReverseBranchCondition(Cond)) {
450 TII->RemoveBranch(*CurMBB);
451 TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond);
452 return;
453 }
454 }
455 }
456 TII->InsertBranch(*CurMBB, SuccBB, NULL, SmallVector<MachineOperand, 0>());
457}
458
459bool
460BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const {
461 if (getHash() < o.getHash())
462 return true;
463 else if (getHash() > o.getHash())
464 return false;
465 else if (getBlock()->getNumber() < o.getBlock()->getNumber())
466 return true;
467 else if (getBlock()->getNumber() > o.getBlock()->getNumber())
468 return false;
469 else {
470 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing
471 // an object with itself.
472#ifndef _GLIBCXX_DEBUG
473 llvm_unreachable("Predecessor appears twice");
474#endif
475 return false;
476 }
477}
478
479/// CountTerminators - Count the number of terminators in the given
480/// block and set I to the position of the first non-terminator, if there
481/// is one, or MBB->end() otherwise.
482static unsigned CountTerminators(MachineBasicBlock *MBB,
483 MachineBasicBlock::iterator &I) {
484 I = MBB->end();
485 unsigned NumTerms = 0;
486 for (;;) {
487 if (I == MBB->begin()) {
488 I = MBB->end();
489 break;
490 }
491 --I;
492 if (!I->getDesc().isTerminator()) break;
493 ++NumTerms;
494 }
495 return NumTerms;
496}
497
498/// ProfitableToMerge - Check if two machine basic blocks have a common tail
499/// and decide if it would be profitable to merge those tails. Return the
500/// length of the common tail and iterators to the first common instruction
501/// in each block.
502static bool ProfitableToMerge(MachineBasicBlock *MBB1,
503 MachineBasicBlock *MBB2,
504 unsigned minCommonTailLength,
505 unsigned &CommonTailLen,
506 MachineBasicBlock::iterator &I1,
507 MachineBasicBlock::iterator &I2,
508 MachineBasicBlock *SuccBB,
509 MachineBasicBlock *PredBB) {
510 CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2);
511 MachineFunction *MF = MBB1->getParent();
512
513 if (CommonTailLen == 0)
514 return false;
515
516 // It's almost always profitable to merge any number of non-terminator
517 // instructions with the block that falls through into the common successor.
518 if (MBB1 == PredBB || MBB2 == PredBB) {
519 MachineBasicBlock::iterator I;
520 unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I);
521 if (CommonTailLen > NumTerms)
522 return true;
523 }
524
525 // If one of the blocks can be completely merged and happens to be in
526 // a position where the other could fall through into it, merge any number
527 // of instructions, because it can be done without a branch.
528 // TODO: If the blocks are not adjacent, move one of them so that they are?
529 if (MBB1->isLayoutSuccessor(MBB2) && I2 == MBB2->begin())
530 return true;
531 if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin())
532 return true;
533
534 // If both blocks have an unconditional branch temporarily stripped out,
535 // count that as an additional common instruction for the following
536 // heuristics.
537 unsigned EffectiveTailLen = CommonTailLen;
538 if (SuccBB && MBB1 != PredBB && MBB2 != PredBB &&
539 !MBB1->back().getDesc().isBarrier() &&
540 !MBB2->back().getDesc().isBarrier())
541 ++EffectiveTailLen;
542
543 // Check if the common tail is long enough to be worthwhile.
544 if (EffectiveTailLen >= minCommonTailLength)
545 return true;
546
547 // If we are optimizing for code size, 2 instructions in common is enough if
548 // we don't have to split a block. At worst we will be introducing 1 new
549 // branch instruction, which is likely to be smaller than the 2
550 // instructions that would be deleted in the merge.
551 if (EffectiveTailLen >= 2 &&
552 MF->getFunction()->hasFnAttr(Attribute::OptimizeForSize) &&
553 (I1 == MBB1->begin() || I2 == MBB2->begin()))
554 return true;
555
556 return false;
557}
558
559/// ComputeSameTails - Look through all the blocks in MergePotentials that have
560/// hash CurHash (guaranteed to match the last element). Build the vector
561/// SameTails of all those that have the (same) largest number of instructions
562/// in common of any pair of these blocks. SameTails entries contain an
563/// iterator into MergePotentials (from which the MachineBasicBlock can be
564/// found) and a MachineBasicBlock::iterator into that MBB indicating the
565/// instruction where the matching code sequence begins.
566/// Order of elements in SameTails is the reverse of the order in which
567/// those blocks appear in MergePotentials (where they are not necessarily
568/// consecutive).
569unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
570 unsigned minCommonTailLength,
571 MachineBasicBlock *SuccBB,
572 MachineBasicBlock *PredBB) {
573 unsigned maxCommonTailLength = 0U;
574 SameTails.clear();
575 MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
576 MPIterator HighestMPIter = prior(MergePotentials.end());
577 for (MPIterator CurMPIter = prior(MergePotentials.end()),
578 B = MergePotentials.begin();
579 CurMPIter != B && CurMPIter->getHash() == CurHash;
580 --CurMPIter) {
581 for (MPIterator I = prior(CurMPIter); I->getHash() == CurHash ; --I) {
582 unsigned CommonTailLen;
583 if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(),
584 minCommonTailLength,
585 CommonTailLen, TrialBBI1, TrialBBI2,
586 SuccBB, PredBB)) {
587 if (CommonTailLen > maxCommonTailLength) {
588 SameTails.clear();
589 maxCommonTailLength = CommonTailLen;
590 HighestMPIter = CurMPIter;
591 SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1));
592 }
593 if (HighestMPIter == CurMPIter &&
594 CommonTailLen == maxCommonTailLength)
595 SameTails.push_back(SameTailElt(I, TrialBBI2));
596 }
597 if (I == B)
598 break;
599 }
600 }
601 return maxCommonTailLength;
602}
603
604/// RemoveBlocksWithHash - Remove all blocks with hash CurHash from
605/// MergePotentials, restoring branches at ends of blocks as appropriate.
606void BranchFolder::RemoveBlocksWithHash(unsigned CurHash,
607 MachineBasicBlock *SuccBB,
608 MachineBasicBlock *PredBB) {
609 MPIterator CurMPIter, B;
610 for (CurMPIter = prior(MergePotentials.end()), B = MergePotentials.begin();
611 CurMPIter->getHash() == CurHash;
612 --CurMPIter) {
613 // Put the unconditional branch back, if we need one.
614 MachineBasicBlock *CurMBB = CurMPIter->getBlock();
615 if (SuccBB && CurMBB != PredBB)
616 FixTail(CurMBB, SuccBB, TII);
617 if (CurMPIter == B)
618 break;
619 }
620 if (CurMPIter->getHash() != CurHash)
621 CurMPIter++;
622 MergePotentials.erase(CurMPIter, MergePotentials.end());
623}
624
625/// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist
626/// only of the common tail. Create a block that does by splitting one.
627unsigned BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
628 unsigned maxCommonTailLength) {
629 unsigned commonTailIndex = 0;
630 unsigned TimeEstimate = ~0U;
631 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
632 // Use PredBB if possible; that doesn't require a new branch.
633 if (SameTails[i].getBlock() == PredBB) {
634 commonTailIndex = i;
635 break;
636 }
637 // Otherwise, make a (fairly bogus) choice based on estimate of
638 // how long it will take the various blocks to execute.
639 unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(),
640 SameTails[i].getTailStartPos());
641 if (t <= TimeEstimate) {
642 TimeEstimate = t;
643 commonTailIndex = i;
644 }
645 }
646
647 MachineBasicBlock::iterator BBI =
648 SameTails[commonTailIndex].getTailStartPos();
649 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
650
651 DEBUG(errs() << "\nSplitting BB#" << MBB->getNumber() << ", size "
652 << maxCommonTailLength);
653
654 MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI);
655 SameTails[commonTailIndex].setBlock(newMBB);
656 SameTails[commonTailIndex].setTailStartPos(newMBB->begin());
657
658 // If we split PredBB, newMBB is the new predecessor.
659 if (PredBB == MBB)
660 PredBB = newMBB;
661
662 return commonTailIndex;
663}
664
665// See if any of the blocks in MergePotentials (which all have a common single
666// successor, or all have no successor) can be tail-merged. If there is a
667// successor, any blocks in MergePotentials that are not tail-merged and
668// are not immediately before Succ must have an unconditional branch to
669// Succ added (but the predecessor/successor lists need no adjustment).
670// The lone predecessor of Succ that falls through into Succ,
671// if any, is given in PredBB.
672
673bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
674 MachineBasicBlock *PredBB) {
675 bool MadeChange = false;
676
677 // Except for the special cases below, tail-merge if there are at least
678 // this many instructions in common.
679 unsigned minCommonTailLength = TailMergeSize;
680
681 DEBUG(errs() << "\nTryTailMergeBlocks: ";
682 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
683 errs() << "BB#" << MergePotentials[i].getBlock()->getNumber()
684 << (i == e-1 ? "" : ", ");
685 errs() << "\n";
686 if (SuccBB) {
687 errs() << " with successor BB#" << SuccBB->getNumber() << '\n';
688 if (PredBB)
689 errs() << " which has fall-through from BB#"
690 << PredBB->getNumber() << "\n";
691 }
692 errs() << "Looking for common tails of at least "
693 << minCommonTailLength << " instruction"
694 << (minCommonTailLength == 1 ? "" : "s") << '\n';
695 );
696
697 // Sort by hash value so that blocks with identical end sequences sort
698 // together.
699 std::stable_sort(MergePotentials.begin(), MergePotentials.end());
700
701 // Walk through equivalence sets looking for actual exact matches.
702 while (MergePotentials.size() > 1) {
703 unsigned CurHash = MergePotentials.back().getHash();
704
705 // Build SameTails, identifying the set of blocks with this hash code
706 // and with the maximum number of instructions in common.
707 unsigned maxCommonTailLength = ComputeSameTails(CurHash,
708 minCommonTailLength,
709 SuccBB, PredBB);
710
711 // If we didn't find any pair that has at least minCommonTailLength
712 // instructions in common, remove all blocks with this hash code and retry.
713 if (SameTails.empty()) {
714 RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
715 continue;
716 }
717
718 // If one of the blocks is the entire common tail (and not the entry
719 // block, which we can't jump to), we can treat all blocks with this same
720 // tail at once. Use PredBB if that is one of the possibilities, as that
721 // will not introduce any extra branches.
722 MachineBasicBlock *EntryBB = MergePotentials.begin()->getBlock()->
723 getParent()->begin();
724 unsigned commonTailIndex = SameTails.size();
725 // If there are two blocks, check to see if one can be made to fall through
726 // into the other.
727 if (SameTails.size() == 2 &&
728 SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) &&
729 SameTails[1].tailIsWholeBlock())
730 commonTailIndex = 1;
731 else if (SameTails.size() == 2 &&
732 SameTails[1].getBlock()->isLayoutSuccessor(
733 SameTails[0].getBlock()) &&
734 SameTails[0].tailIsWholeBlock())
735 commonTailIndex = 0;
736 else {
737 // Otherwise just pick one, favoring the fall-through predecessor if
738 // there is one.
739 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
740 MachineBasicBlock *MBB = SameTails[i].getBlock();
741 if (MBB == EntryBB && SameTails[i].tailIsWholeBlock())
742 continue;
743 if (MBB == PredBB) {
744 commonTailIndex = i;
745 break;
746 }
747 if (SameTails[i].tailIsWholeBlock())
748 commonTailIndex = i;
749 }
750 }
751
752 if (commonTailIndex == SameTails.size() ||
753 (SameTails[commonTailIndex].getBlock() == PredBB &&
754 !SameTails[commonTailIndex].tailIsWholeBlock())) {
755 // None of the blocks consist entirely of the common tail.
756 // Split a block so that one does.
757 commonTailIndex = CreateCommonTailOnlyBlock(PredBB, maxCommonTailLength);
758 }
759
760 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
761 // MBB is common tail. Adjust all other BB's to jump to this one.
762 // Traversal must be forwards so erases work.
763 DEBUG(errs() << "\nUsing common tail in BB#" << MBB->getNumber()
764 << " for ");
765 for (unsigned int i=0, e = SameTails.size(); i != e; ++i) {
766 if (commonTailIndex == i)
767 continue;
768 DEBUG(errs() << "BB#" << SameTails[i].getBlock()->getNumber()
769 << (i == e-1 ? "" : ", "));
770 // Hack the end off BB i, making it jump to BB commonTailIndex instead.
771 ReplaceTailWithBranchTo(SameTails[i].getTailStartPos(), MBB);
772 // BB i is no longer a predecessor of SuccBB; remove it from the worklist.
773 MergePotentials.erase(SameTails[i].getMPIter());
774 }
775 DEBUG(errs() << "\n");
776 // We leave commonTailIndex in the worklist in case there are other blocks
777 // that match it with a smaller number of instructions.
778 MadeChange = true;
779 }
780 return MadeChange;
781}
782
783bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
784
785 if (!EnableTailMerge) return false;
786
787 bool MadeChange = false;
788
789 // First find blocks with no successors.
790 MergePotentials.clear();
791 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
792 if (I->succ_empty())
793 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(I, 2U), I));
794 }
795
796 // See if we can do any tail merging on those.
797 if (MergePotentials.size() < TailMergeThreshold &&
798 MergePotentials.size() >= 2)
799 MadeChange |= TryTailMergeBlocks(NULL, NULL);
800
801 // Look at blocks (IBB) with multiple predecessors (PBB).
802 // We change each predecessor to a canonical form, by
803 // (1) temporarily removing any unconditional branch from the predecessor
804 // to IBB, and
805 // (2) alter conditional branches so they branch to the other block
806 // not IBB; this may require adding back an unconditional branch to IBB
807 // later, where there wasn't one coming in. E.g.
808 // Bcc IBB
809 // fallthrough to QBB
810 // here becomes
811 // Bncc QBB
812 // with a conceptual B to IBB after that, which never actually exists.
813 // With those changes, we see whether the predecessors' tails match,
814 // and merge them if so. We change things out of canonical form and
815 // back to the way they were later in the process. (OptimizeBranches
816 // would undo some of this, but we can't use it, because we'd get into
817 // a compile-time infinite loop repeatedly doing and undoing the same
818 // transformations.)
819
820 for (MachineFunction::iterator I = next(MF.begin()), E = MF.end();
821 I != E; ++I) {
822 if (I->pred_size() >= 2 && I->pred_size() < TailMergeThreshold) {
823 SmallPtrSet<MachineBasicBlock *, 8> UniquePreds;
824 MachineBasicBlock *IBB = I;
825 MachineBasicBlock *PredBB = prior(I);
826 MergePotentials.clear();
827 for (MachineBasicBlock::pred_iterator P = I->pred_begin(),
828 E2 = I->pred_end();
829 P != E2; ++P) {
830 MachineBasicBlock *PBB = *P;
831 // Skip blocks that loop to themselves, can't tail merge these.
832 if (PBB == IBB)
833 continue;
834 // Visit each predecessor only once.
835 if (!UniquePreds.insert(PBB))
836 continue;
837 MachineBasicBlock *TBB = 0, *FBB = 0;
838 SmallVector<MachineOperand, 4> Cond;
839 if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond, true)) {
840 // Failing case: IBB is the target of a cbr, and
841 // we cannot reverse the branch.
842 SmallVector<MachineOperand, 4> NewCond(Cond);
843 if (!Cond.empty() && TBB == IBB) {
844 if (TII->ReverseBranchCondition(NewCond))
845 continue;
846 // This is the QBB case described above
847 if (!FBB)
848 FBB = next(MachineFunction::iterator(PBB));
849 }
850 // Failing case: the only way IBB can be reached from PBB is via
851 // exception handling. Happens for landing pads. Would be nice
852 // to have a bit in the edge so we didn't have to do all this.
853 if (IBB->isLandingPad()) {
854 MachineFunction::iterator IP = PBB; IP++;
855 MachineBasicBlock *PredNextBB = NULL;
856 if (IP != MF.end())
857 PredNextBB = IP;
858 if (TBB == NULL) {
859 if (IBB != PredNextBB) // fallthrough
860 continue;
861 } else if (FBB) {
862 if (TBB != IBB && FBB != IBB) // cbr then ubr
863 continue;
864 } else if (Cond.empty()) {
865 if (TBB != IBB) // ubr
866 continue;
867 } else {
868 if (TBB != IBB && IBB != PredNextBB) // cbr
869 continue;
870 }
871 }
872 // Remove the unconditional branch at the end, if any.
873 if (TBB && (Cond.empty() || FBB)) {
874 TII->RemoveBranch(*PBB);
875 if (!Cond.empty())
876 // reinsert conditional branch only, for now
877 TII->InsertBranch(*PBB, (TBB == IBB) ? FBB : TBB, 0, NewCond);
878 }
879 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(PBB, 1U),
880 *P));
881 }
882 }
883 if (MergePotentials.size() >= 2)
884 MadeChange |= TryTailMergeBlocks(IBB, PredBB);
885 // Reinsert an unconditional branch if needed.
886 // The 1 below can occur as a result of removing blocks in TryTailMergeBlocks.
887 PredBB = prior(I); // this may have been changed in TryTailMergeBlocks
888 if (MergePotentials.size() == 1 &&
889 MergePotentials.begin()->getBlock() != PredBB)
890 FixTail(MergePotentials.begin()->getBlock(), IBB, TII);
891 }
892 }
893 return MadeChange;
894}
895
896//===----------------------------------------------------------------------===//
897// Branch Optimization
898//===----------------------------------------------------------------------===//
899
900bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
901 bool MadeChange = false;
902
903 // Make sure blocks are numbered in order
904 MF.RenumberBlocks();
905
906 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
907 MachineBasicBlock *MBB = I++;
908 MadeChange |= OptimizeBlock(MBB);
909
910 // If it is dead, remove it.
911 if (MBB->pred_empty()) {
912 RemoveDeadBlock(MBB);
913 MadeChange = true;
914 ++NumDeadBlocks;
915 }
916 }
917 return MadeChange;
918}
919
920
921/// CanFallThrough - Return true if the specified block (with the specified
922/// branch condition) can implicitly transfer control to the block after it by
923/// falling off the end of it. This should return false if it can reach the
924/// block after it, but it uses an explicit branch to do so (e.g. a table jump).
925///
926/// True is a conservative answer.
927///
928bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB,
929 bool BranchUnAnalyzable,
930 MachineBasicBlock *TBB,
931 MachineBasicBlock *FBB,
932 const SmallVectorImpl<MachineOperand> &Cond) {
933 MachineFunction::iterator Fallthrough = CurBB;
934 ++Fallthrough;
935 // If FallthroughBlock is off the end of the function, it can't fall through.
936 if (Fallthrough == CurBB->getParent()->end())
937 return false;
938
939 // If FallthroughBlock isn't a successor of CurBB, no fallthrough is possible.
940 if (!CurBB->isSuccessor(Fallthrough))
941 return false;
942
943 // If we couldn't analyze the branch, examine the last instruction.
944 // If the block doesn't end in a known control barrier, assume fallthrough
945 // is possible. The isPredicable check is needed because this code can be
946 // called during IfConversion, where an instruction which is normally a
947 // Barrier is predicated and thus no longer an actual control barrier. This
948 // is over-conservative though, because if an instruction isn't actually
949 // predicated we could still treat it like a barrier.
950 if (BranchUnAnalyzable)
951 return CurBB->empty() || !CurBB->back().getDesc().isBarrier() ||
952 CurBB->back().getDesc().isPredicable();
953
954 // If there is no branch, control always falls through.
955 if (TBB == 0) return true;
956
957 // If there is some explicit branch to the fallthrough block, it can obviously
958 // reach, even though the branch should get folded to fall through implicitly.
959 if (MachineFunction::iterator(TBB) == Fallthrough ||
960 MachineFunction::iterator(FBB) == Fallthrough)
961 return true;
962
963 // If it's an unconditional branch to some block not the fall through, it
964 // doesn't fall through.
965 if (Cond.empty()) return false;
966
967 // Otherwise, if it is conditional and has no explicit false block, it falls
968 // through.
969 return FBB == 0;
970}
971
972/// CanFallThrough - Return true if the specified can implicitly transfer
973/// control to the block after it by falling off the end of it. This should
974/// return false if it can reach the block after it, but it uses an explicit
975/// branch to do so (e.g. a table jump).
976///
977/// True is a conservative answer.
978///
979bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) {
980 MachineBasicBlock *TBB = 0, *FBB = 0;
981 SmallVector<MachineOperand, 4> Cond;
982 bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond, true);
983 return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond);
984}
985
986/// IsBetterFallthrough - Return true if it would be clearly better to
987/// fall-through to MBB1 than to fall through into MBB2. This has to return
988/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will
989/// result in infinite loops.
990static bool IsBetterFallthrough(MachineBasicBlock *MBB1,
991 MachineBasicBlock *MBB2) {
992 // Right now, we use a simple heuristic. If MBB2 ends with a call, and
993 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to
994 // optimize branches that branch to either a return block or an assert block
995 // into a fallthrough to the return.
996 if (MBB1->empty() || MBB2->empty()) return false;
997
998 // If there is a clear successor ordering we make sure that one block
999 // will fall through to the next
1000 if (MBB1->isSuccessor(MBB2)) return true;
1001 if (MBB2->isSuccessor(MBB1)) return false;
1002
1003 MachineInstr *MBB1I = --MBB1->end();
1004 MachineInstr *MBB2I = --MBB2->end();
1005 return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall();
1006}
1007
1008/// TailDuplicateBlocks - Look for small blocks that are unconditionally
1009/// branched to and do not fall through. Tail-duplicate their instructions
1010/// into their predecessors to eliminate (dynamic) branches.
1011bool BranchFolder::TailDuplicateBlocks(MachineFunction &MF) {
1012 bool MadeChange = false;
1013
|
1006 // Make sure blocks are numbered in order
1007 MF.RenumberBlocks();
1008
| |
1009 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
1010 MachineBasicBlock *MBB = I++;
1011
1012 // Only duplicate blocks that end with unconditional branches.
1013 if (CanFallThrough(MBB))
1014 continue;
1015
1016 MadeChange |= TailDuplicate(MBB, MF);
1017
1018 // If it is dead, remove it.
1019 if (MBB->pred_empty()) {
| 1014 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
1015 MachineBasicBlock *MBB = I++;
1016
1017 // Only duplicate blocks that end with unconditional branches.
1018 if (CanFallThrough(MBB))
1019 continue;
1020
1021 MadeChange |= TailDuplicate(MBB, MF);
1022
1023 // If it is dead, remove it.
1024 if (MBB->pred_empty()) {
|
| 1025 NumInstrDups -= MBB->size();
|
1020 RemoveDeadBlock(MBB);
1021 MadeChange = true;
1022 ++NumDeadBlocks;
1023 }
1024 }
1025 return MadeChange;
1026}
1027
1028/// TailDuplicate - If it is profitable, duplicate TailBB's contents in each
1029/// of its predecessors.
1030bool BranchFolder::TailDuplicate(MachineBasicBlock *TailBB,
1031 MachineFunction &MF) {
1032 // Don't try to tail-duplicate single-block loops.
1033 if (TailBB->isSuccessor(TailBB))
1034 return false;
1035
1036 // Set the limit on the number of instructions to duplicate, with a default
1037 // of one less than the tail-merge threshold. When optimizing for size,
1038 // duplicate only one, because one branch instruction can be eliminated to
1039 // compensate for the duplication.
1040 unsigned MaxDuplicateCount =
1041 MF.getFunction()->hasFnAttr(Attribute::OptimizeForSize) ?
1042 1 : TII->TailDuplicationLimit(*TailBB, TailMergeSize - 1);
1043
1044 // Check the instructions in the block to determine whether tail-duplication
1045 // is invalid or unlikely to be profitable.
1046 unsigned i = 0;
1047 bool HasCall = false;
1048 for (MachineBasicBlock::iterator I = TailBB->begin();
1049 I != TailBB->end(); ++I, ++i) {
1050 // Non-duplicable things shouldn't be tail-duplicated.
1051 if (I->getDesc().isNotDuplicable()) return false;
1052 // Don't duplicate more than the threshold.
1053 if (i == MaxDuplicateCount) return false;
1054 // Remember if we saw a call.
1055 if (I->getDesc().isCall()) HasCall = true;
1056 }
1057 // Heuristically, don't tail-duplicate calls if it would expand code size,
1058 // as it's less likely to be worth the extra cost.
1059 if (i > 1 && HasCall)
1060 return false;
1061
1062 // Iterate through all the unique predecessors and tail-duplicate this
1063 // block into them, if possible. Copying the list ahead of time also
1064 // avoids trouble with the predecessor list reallocating.
1065 bool Changed = false;
1066 SmallSetVector<MachineBasicBlock *, 8> Preds(TailBB->pred_begin(),
1067 TailBB->pred_end());
1068 for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
1069 PE = Preds.end(); PI != PE; ++PI) {
1070 MachineBasicBlock *PredBB = *PI;
1071
1072 assert(TailBB != PredBB &&
1073 "Single-block loop should have been rejected earlier!");
1074 if (PredBB->succ_size() > 1) continue;
1075
1076 MachineBasicBlock *PredTBB, *PredFBB;
1077 SmallVector<MachineOperand, 4> PredCond;
1078 if (TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
1079 continue;
1080 if (!PredCond.empty())
1081 continue;
1082 // EH edges are ignored by AnalyzeBranch.
1083 if (PredBB->succ_size() != 1)
1084 continue;
1085 // Don't duplicate into a fall-through predecessor (at least for now).
1086 if (PredBB->isLayoutSuccessor(TailBB) && CanFallThrough(PredBB))
1087 continue;
1088
1089 DEBUG(errs() << "\nTail-duplicating into PredBB: " << *PredBB
1090 << "From Succ: " << *TailBB);
1091
1092 // Remove PredBB's unconditional branch.
1093 TII->RemoveBranch(*PredBB);
1094 // Clone the contents of TailBB into PredBB.
1095 for (MachineBasicBlock::iterator I = TailBB->begin(), E = TailBB->end();
1096 I != E; ++I) {
1097 MachineInstr *NewMI = MF.CloneMachineInstr(I);
1098 PredBB->insert(PredBB->end(), NewMI);
1099 }
| 1026 RemoveDeadBlock(MBB);
1027 MadeChange = true;
1028 ++NumDeadBlocks;
1029 }
1030 }
1031 return MadeChange;
1032}
1033
1034/// TailDuplicate - If it is profitable, duplicate TailBB's contents in each
1035/// of its predecessors.
1036bool BranchFolder::TailDuplicate(MachineBasicBlock *TailBB,
1037 MachineFunction &MF) {
1038 // Don't try to tail-duplicate single-block loops.
1039 if (TailBB->isSuccessor(TailBB))
1040 return false;
1041
1042 // Set the limit on the number of instructions to duplicate, with a default
1043 // of one less than the tail-merge threshold. When optimizing for size,
1044 // duplicate only one, because one branch instruction can be eliminated to
1045 // compensate for the duplication.
1046 unsigned MaxDuplicateCount =
1047 MF.getFunction()->hasFnAttr(Attribute::OptimizeForSize) ?
1048 1 : TII->TailDuplicationLimit(*TailBB, TailMergeSize - 1);
1049
1050 // Check the instructions in the block to determine whether tail-duplication
1051 // is invalid or unlikely to be profitable.
1052 unsigned i = 0;
1053 bool HasCall = false;
1054 for (MachineBasicBlock::iterator I = TailBB->begin();
1055 I != TailBB->end(); ++I, ++i) {
1056 // Non-duplicable things shouldn't be tail-duplicated.
1057 if (I->getDesc().isNotDuplicable()) return false;
1058 // Don't duplicate more than the threshold.
1059 if (i == MaxDuplicateCount) return false;
1060 // Remember if we saw a call.
1061 if (I->getDesc().isCall()) HasCall = true;
1062 }
1063 // Heuristically, don't tail-duplicate calls if it would expand code size,
1064 // as it's less likely to be worth the extra cost.
1065 if (i > 1 && HasCall)
1066 return false;
1067
1068 // Iterate through all the unique predecessors and tail-duplicate this
1069 // block into them, if possible. Copying the list ahead of time also
1070 // avoids trouble with the predecessor list reallocating.
1071 bool Changed = false;
1072 SmallSetVector<MachineBasicBlock *, 8> Preds(TailBB->pred_begin(),
1073 TailBB->pred_end());
1074 for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(),
1075 PE = Preds.end(); PI != PE; ++PI) {
1076 MachineBasicBlock *PredBB = *PI;
1077
1078 assert(TailBB != PredBB &&
1079 "Single-block loop should have been rejected earlier!");
1080 if (PredBB->succ_size() > 1) continue;
1081
1082 MachineBasicBlock *PredTBB, *PredFBB;
1083 SmallVector<MachineOperand, 4> PredCond;
1084 if (TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true))
1085 continue;
1086 if (!PredCond.empty())
1087 continue;
1088 // EH edges are ignored by AnalyzeBranch.
1089 if (PredBB->succ_size() != 1)
1090 continue;
1091 // Don't duplicate into a fall-through predecessor (at least for now).
1092 if (PredBB->isLayoutSuccessor(TailBB) && CanFallThrough(PredBB))
1093 continue;
1094
1095 DEBUG(errs() << "\nTail-duplicating into PredBB: " << *PredBB
1096 << "From Succ: " << *TailBB);
1097
1098 // Remove PredBB's unconditional branch.
1099 TII->RemoveBranch(*PredBB);
1100 // Clone the contents of TailBB into PredBB.
1101 for (MachineBasicBlock::iterator I = TailBB->begin(), E = TailBB->end();
1102 I != E; ++I) {
1103 MachineInstr *NewMI = MF.CloneMachineInstr(I);
1104 PredBB->insert(PredBB->end(), NewMI);
1105 }
|
| 1106 NumInstrDups += TailBB->size() - 1; // subtract one for removed branch
|
1100
1101 // Update the CFG.
1102 PredBB->removeSuccessor(PredBB->succ_begin());
1103 assert(PredBB->succ_empty() &&
1104 "TailDuplicate called on block with multiple successors!");
1105 for (MachineBasicBlock::succ_iterator I = TailBB->succ_begin(),
1106 E = TailBB->succ_end(); I != E; ++I)
1107 PredBB->addSuccessor(*I);
1108
1109 Changed = true;
| 1107
1108 // Update the CFG.
1109 PredBB->removeSuccessor(PredBB->succ_begin());
1110 assert(PredBB->succ_empty() &&
1111 "TailDuplicate called on block with multiple successors!");
1112 for (MachineBasicBlock::succ_iterator I = TailBB->succ_begin(),
1113 E = TailBB->succ_end(); I != E; ++I)
1114 PredBB->addSuccessor(*I);
1115
1116 Changed = true;
|
| 1117 ++NumTailDups;
|
1110 }
1111
1112 // If TailBB was duplicated into all its predecessors except for the prior
1113 // block, which falls through unconditionally, move the contents of this
1114 // block into the prior block.
1115 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(TailBB));
1116 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
1117 SmallVector<MachineOperand, 4> PriorCond;
1118 bool PriorUnAnalyzable =
1119 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
1120 // This has to check PrevBB->succ_size() because EH edges are ignored by
1121 // AnalyzeBranch.
1122 if (!PriorUnAnalyzable && PriorCond.empty() && !PriorTBB &&
1123 TailBB->pred_size() == 1 && PrevBB.succ_size() == 1 &&
1124 !TailBB->hasAddressTaken()) {
1125 DEBUG(errs() << "\nMerging into block: " << PrevBB
1126 << "From MBB: " << *TailBB);
1127 PrevBB.splice(PrevBB.end(), TailBB, TailBB->begin(), TailBB->end());
1128 PrevBB.removeSuccessor(PrevBB.succ_begin());;
1129 assert(PrevBB.succ_empty());
1130 PrevBB.transferSuccessors(TailBB);
1131 Changed = true;
1132 }
1133
1134 return Changed;
1135}
1136
1137/// OptimizeBlock - Analyze and optimize control flow related to the specified
1138/// block. This is never called on the entry block.
1139bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
1140 bool MadeChange = false;
1141 MachineFunction &MF = *MBB->getParent();
1142ReoptimizeBlock:
1143
1144 MachineFunction::iterator FallThrough = MBB;
1145 ++FallThrough;
1146
1147 // If this block is empty, make everyone use its fall-through, not the block
1148 // explicitly. Landing pads should not do this since the landing-pad table
1149 // points to this block. Blocks with their addresses taken shouldn't be
1150 // optimized away.
1151 if (MBB->empty() && !MBB->isLandingPad() && !MBB->hasAddressTaken()) {
1152 // Dead block? Leave for cleanup later.
1153 if (MBB->pred_empty()) return MadeChange;
1154
1155 if (FallThrough == MF.end()) {
1156 // TODO: Simplify preds to not branch here if possible!
1157 } else {
1158 // Rewrite all predecessors of the old block to go to the fallthrough
1159 // instead.
1160 while (!MBB->pred_empty()) {
1161 MachineBasicBlock *Pred = *(MBB->pred_end()-1);
1162 Pred->ReplaceUsesOfBlockWith(MBB, FallThrough);
1163 }
1164 // If MBB was the target of a jump table, update jump tables to go to the
1165 // fallthrough instead.
1166 MF.getJumpTableInfo()->ReplaceMBBInJumpTables(MBB, FallThrough);
1167 MadeChange = true;
1168 }
1169 return MadeChange;
1170 }
1171
1172 // Check to see if we can simplify the terminator of the block before this
1173 // one.
1174 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB));
1175
1176 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
1177 SmallVector<MachineOperand, 4> PriorCond;
1178 bool PriorUnAnalyzable =
1179 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
1180 if (!PriorUnAnalyzable) {
1181 // If the CFG for the prior block has extra edges, remove them.
1182 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB,
1183 !PriorCond.empty());
1184
1185 // If the previous branch is conditional and both conditions go to the same
1186 // destination, remove the branch, replacing it with an unconditional one or
1187 // a fall-through.
1188 if (PriorTBB && PriorTBB == PriorFBB) {
1189 TII->RemoveBranch(PrevBB);
1190 PriorCond.clear();
1191 if (PriorTBB != MBB)
1192 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond);
1193 MadeChange = true;
1194 ++NumBranchOpts;
1195 goto ReoptimizeBlock;
1196 }
1197
1198 // If the previous block unconditionally falls through to this block and
1199 // this block has no other predecessors, move the contents of this block
1200 // into the prior block. This doesn't usually happen when SimplifyCFG
1201 // has been used, but it can happen if tail merging splits a fall-through
1202 // predecessor of a block.
1203 // This has to check PrevBB->succ_size() because EH edges are ignored by
1204 // AnalyzeBranch.
1205 if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 &&
1206 PrevBB.succ_size() == 1 &&
1207 !MBB->hasAddressTaken()) {
1208 DEBUG(errs() << "\nMerging into block: " << PrevBB
1209 << "From MBB: " << *MBB);
1210 PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end());
1211 PrevBB.removeSuccessor(PrevBB.succ_begin());;
1212 assert(PrevBB.succ_empty());
1213 PrevBB.transferSuccessors(MBB);
1214 MadeChange = true;
1215 return MadeChange;
1216 }
1217
1218 // If the previous branch *only* branches to *this* block (conditional or
1219 // not) remove the branch.
1220 if (PriorTBB == MBB && PriorFBB == 0) {
1221 TII->RemoveBranch(PrevBB);
1222 MadeChange = true;
1223 ++NumBranchOpts;
1224 goto ReoptimizeBlock;
1225 }
1226
1227 // If the prior block branches somewhere else on the condition and here if
1228 // the condition is false, remove the uncond second branch.
1229 if (PriorFBB == MBB) {
1230 TII->RemoveBranch(PrevBB);
1231 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond);
1232 MadeChange = true;
1233 ++NumBranchOpts;
1234 goto ReoptimizeBlock;
1235 }
1236
1237 // If the prior block branches here on true and somewhere else on false, and
1238 // if the branch condition is reversible, reverse the branch to create a
1239 // fall-through.
1240 if (PriorTBB == MBB) {
1241 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1242 if (!TII->ReverseBranchCondition(NewPriorCond)) {
1243 TII->RemoveBranch(PrevBB);
1244 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond);
1245 MadeChange = true;
1246 ++NumBranchOpts;
1247 goto ReoptimizeBlock;
1248 }
1249 }
1250
1251 // If this block has no successors (e.g. it is a return block or ends with
1252 // a call to a no-return function like abort or __cxa_throw) and if the pred
1253 // falls through into this block, and if it would otherwise fall through
1254 // into the block after this, move this block to the end of the function.
1255 //
1256 // We consider it more likely that execution will stay in the function (e.g.
1257 // due to loops) than it is to exit it. This asserts in loops etc, moving
1258 // the assert condition out of the loop body.
1259 if (MBB->succ_empty() && !PriorCond.empty() && PriorFBB == 0 &&
1260 MachineFunction::iterator(PriorTBB) == FallThrough &&
1261 !CanFallThrough(MBB)) {
1262 bool DoTransform = true;
1263
1264 // We have to be careful that the succs of PredBB aren't both no-successor
1265 // blocks. If neither have successors and if PredBB is the second from
1266 // last block in the function, we'd just keep swapping the two blocks for
1267 // last. Only do the swap if one is clearly better to fall through than
1268 // the other.
1269 if (FallThrough == --MF.end() &&
1270 !IsBetterFallthrough(PriorTBB, MBB))
1271 DoTransform = false;
1272
1273 // We don't want to do this transformation if we have control flow like:
1274 // br cond BB2
1275 // BB1:
1276 // ..
1277 // jmp BBX
1278 // BB2:
1279 // ..
1280 // ret
1281 //
1282 // In this case, we could actually be moving the return block *into* a
1283 // loop!
1284 if (DoTransform && !MBB->succ_empty() &&
1285 (!CanFallThrough(PriorTBB) || PriorTBB->empty()))
1286 DoTransform = false;
1287
1288
1289 if (DoTransform) {
1290 // Reverse the branch so we will fall through on the previous true cond.
1291 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1292 if (!TII->ReverseBranchCondition(NewPriorCond)) {
1293 DEBUG(errs() << "\nMoving MBB: " << *MBB
1294 << "To make fallthrough to: " << *PriorTBB << "\n");
1295
1296 TII->RemoveBranch(PrevBB);
1297 TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond);
1298
1299 // Move this block to the end of the function.
1300 MBB->moveAfter(--MF.end());
1301 MadeChange = true;
1302 ++NumBranchOpts;
1303 return MadeChange;
1304 }
1305 }
1306 }
1307 }
1308
1309 // Analyze the branch in the current block.
1310 MachineBasicBlock *CurTBB = 0, *CurFBB = 0;
1311 SmallVector<MachineOperand, 4> CurCond;
1312 bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true);
1313 if (!CurUnAnalyzable) {
1314 // If the CFG for the prior block has extra edges, remove them.
1315 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty());
1316
1317 // If this is a two-way branch, and the FBB branches to this block, reverse
1318 // the condition so the single-basic-block loop is faster. Instead of:
1319 // Loop: xxx; jcc Out; jmp Loop
1320 // we want:
1321 // Loop: xxx; jncc Loop; jmp Out
1322 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) {
1323 SmallVector<MachineOperand, 4> NewCond(CurCond);
1324 if (!TII->ReverseBranchCondition(NewCond)) {
1325 TII->RemoveBranch(*MBB);
1326 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond);
1327 MadeChange = true;
1328 ++NumBranchOpts;
1329 goto ReoptimizeBlock;
1330 }
1331 }
1332
1333 // If this branch is the only thing in its block, see if we can forward
1334 // other blocks across it.
1335 if (CurTBB && CurCond.empty() && CurFBB == 0 &&
1336 MBB->begin()->getDesc().isBranch() && CurTBB != MBB &&
1337 !MBB->hasAddressTaken()) {
1338 // This block may contain just an unconditional branch. Because there can
1339 // be 'non-branch terminators' in the block, try removing the branch and
1340 // then seeing if the block is empty.
1341 TII->RemoveBranch(*MBB);
1342
1343 // If this block is just an unconditional branch to CurTBB, we can
1344 // usually completely eliminate the block. The only case we cannot
1345 // completely eliminate the block is when the block before this one
1346 // falls through into MBB and we can't understand the prior block's branch
1347 // condition.
1348 if (MBB->empty()) {
1349 bool PredHasNoFallThrough = TII->BlockHasNoFallThrough(PrevBB);
1350 if (PredHasNoFallThrough || !PriorUnAnalyzable ||
1351 !PrevBB.isSuccessor(MBB)) {
1352 // If the prior block falls through into us, turn it into an
1353 // explicit branch to us to make updates simpler.
1354 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) &&
1355 PriorTBB != MBB && PriorFBB != MBB) {
1356 if (PriorTBB == 0) {
1357 assert(PriorCond.empty() && PriorFBB == 0 &&
1358 "Bad branch analysis");
1359 PriorTBB = MBB;
1360 } else {
1361 assert(PriorFBB == 0 && "Machine CFG out of date!");
1362 PriorFBB = MBB;
1363 }
1364 TII->RemoveBranch(PrevBB);
1365 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond);
1366 }
1367
1368 // Iterate through all the predecessors, revectoring each in-turn.
1369 size_t PI = 0;
1370 bool DidChange = false;
1371 bool HasBranchToSelf = false;
1372 while(PI != MBB->pred_size()) {
1373 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI);
1374 if (PMBB == MBB) {
1375 // If this block has an uncond branch to itself, leave it.
1376 ++PI;
1377 HasBranchToSelf = true;
1378 } else {
1379 DidChange = true;
1380 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB);
1381 // If this change resulted in PMBB ending in a conditional
1382 // branch where both conditions go to the same destination,
1383 // change this to an unconditional branch (and fix the CFG).
1384 MachineBasicBlock *NewCurTBB = 0, *NewCurFBB = 0;
1385 SmallVector<MachineOperand, 4> NewCurCond;
1386 bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB,
1387 NewCurFBB, NewCurCond, true);
1388 if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) {
1389 TII->RemoveBranch(*PMBB);
1390 NewCurCond.clear();
1391 TII->InsertBranch(*PMBB, NewCurTBB, 0, NewCurCond);
1392 MadeChange = true;
1393 ++NumBranchOpts;
1394 PMBB->CorrectExtraCFGEdges(NewCurTBB, 0, false);
1395 }
1396 }
1397 }
1398
1399 // Change any jumptables to go to the new MBB.
1400 MF.getJumpTableInfo()->ReplaceMBBInJumpTables(MBB, CurTBB);
1401 if (DidChange) {
1402 ++NumBranchOpts;
1403 MadeChange = true;
1404 if (!HasBranchToSelf) return MadeChange;
1405 }
1406 }
1407 }
1408
1409 // Add the branch back if the block is more than just an uncond branch.
1410 TII->InsertBranch(*MBB, CurTBB, 0, CurCond);
1411 }
1412 }
1413
1414 // If the prior block doesn't fall through into this block, and if this
1415 // block doesn't fall through into some other block, see if we can find a
1416 // place to move this block where a fall-through will happen.
1417 if (!CanFallThrough(&PrevBB, PriorUnAnalyzable,
1418 PriorTBB, PriorFBB, PriorCond)) {
1419
1420 // Now we know that there was no fall-through into this block, check to
1421 // see if it has a fall-through into its successor.
1422 bool CurFallsThru = CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB,
1423 CurCond);
1424
1425 if (!MBB->isLandingPad()) {
1426 // Check all the predecessors of this block. If one of them has no fall
1427 // throughs, move this block right after it.
1428 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
1429 E = MBB->pred_end(); PI != E; ++PI) {
1430 // Analyze the branch at the end of the pred.
1431 MachineBasicBlock *PredBB = *PI;
1432 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough;
1433 MachineBasicBlock *PredTBB, *PredFBB;
1434 SmallVector<MachineOperand, 4> PredCond;
1435 if (PredBB != MBB && !CanFallThrough(PredBB) &&
1436 !TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true)
1437 && (!CurFallsThru || !CurTBB || !CurFBB)
1438 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
1439 // If the current block doesn't fall through, just move it.
1440 // If the current block can fall through and does not end with a
1441 // conditional branch, we need to append an unconditional jump to
1442 // the (current) next block. To avoid a possible compile-time
1443 // infinite loop, move blocks only backward in this case.
1444 // Also, if there are already 2 branches here, we cannot add a third;
1445 // this means we have the case
1446 // Bcc next
1447 // B elsewhere
1448 // next:
1449 if (CurFallsThru) {
1450 MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB));
1451 CurCond.clear();
1452 TII->InsertBranch(*MBB, NextBB, 0, CurCond);
1453 }
1454 MBB->moveAfter(PredBB);
1455 MadeChange = true;
1456 goto ReoptimizeBlock;
1457 }
1458 }
1459 }
1460
1461 if (!CurFallsThru) {
1462 // Check all successors to see if we can move this block before it.
1463 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
1464 E = MBB->succ_end(); SI != E; ++SI) {
1465 // Analyze the branch at the end of the block before the succ.
1466 MachineBasicBlock *SuccBB = *SI;
1467 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev;
1468
1469 // If this block doesn't already fall-through to that successor, and if
1470 // the succ doesn't already have a block that can fall through into it,
1471 // and if the successor isn't an EH destination, we can arrange for the
1472 // fallthrough to happen.
1473 if (SuccBB != MBB && &*SuccPrev != MBB &&
1474 !CanFallThrough(SuccPrev) && !CurUnAnalyzable &&
1475 !SuccBB->isLandingPad()) {
1476 MBB->moveBefore(SuccBB);
1477 MadeChange = true;
1478 goto ReoptimizeBlock;
1479 }
1480 }
1481
1482 // Okay, there is no really great place to put this block. If, however,
1483 // the block before this one would be a fall-through if this block were
1484 // removed, move this block to the end of the function.
1485 MachineBasicBlock *PrevTBB, *PrevFBB;
1486 SmallVector<MachineOperand, 4> PrevCond;
1487 if (FallThrough != MF.end() &&
1488 !TII->AnalyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) &&
1489 PrevBB.isSuccessor(FallThrough)) {
1490 MBB->moveAfter(--MF.end());
1491 MadeChange = true;
1492 return MadeChange;
1493 }
1494 }
1495 }
1496
1497 return MadeChange;
1498}
| 1118 }
1119
1120 // If TailBB was duplicated into all its predecessors except for the prior
1121 // block, which falls through unconditionally, move the contents of this
1122 // block into the prior block.
1123 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(TailBB));
1124 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
1125 SmallVector<MachineOperand, 4> PriorCond;
1126 bool PriorUnAnalyzable =
1127 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
1128 // This has to check PrevBB->succ_size() because EH edges are ignored by
1129 // AnalyzeBranch.
1130 if (!PriorUnAnalyzable && PriorCond.empty() && !PriorTBB &&
1131 TailBB->pred_size() == 1 && PrevBB.succ_size() == 1 &&
1132 !TailBB->hasAddressTaken()) {
1133 DEBUG(errs() << "\nMerging into block: " << PrevBB
1134 << "From MBB: " << *TailBB);
1135 PrevBB.splice(PrevBB.end(), TailBB, TailBB->begin(), TailBB->end());
1136 PrevBB.removeSuccessor(PrevBB.succ_begin());;
1137 assert(PrevBB.succ_empty());
1138 PrevBB.transferSuccessors(TailBB);
1139 Changed = true;
1140 }
1141
1142 return Changed;
1143}
1144
1145/// OptimizeBlock - Analyze and optimize control flow related to the specified
1146/// block. This is never called on the entry block.
1147bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
1148 bool MadeChange = false;
1149 MachineFunction &MF = *MBB->getParent();
1150ReoptimizeBlock:
1151
1152 MachineFunction::iterator FallThrough = MBB;
1153 ++FallThrough;
1154
1155 // If this block is empty, make everyone use its fall-through, not the block
1156 // explicitly. Landing pads should not do this since the landing-pad table
1157 // points to this block. Blocks with their addresses taken shouldn't be
1158 // optimized away.
1159 if (MBB->empty() && !MBB->isLandingPad() && !MBB->hasAddressTaken()) {
1160 // Dead block? Leave for cleanup later.
1161 if (MBB->pred_empty()) return MadeChange;
1162
1163 if (FallThrough == MF.end()) {
1164 // TODO: Simplify preds to not branch here if possible!
1165 } else {
1166 // Rewrite all predecessors of the old block to go to the fallthrough
1167 // instead.
1168 while (!MBB->pred_empty()) {
1169 MachineBasicBlock *Pred = *(MBB->pred_end()-1);
1170 Pred->ReplaceUsesOfBlockWith(MBB, FallThrough);
1171 }
1172 // If MBB was the target of a jump table, update jump tables to go to the
1173 // fallthrough instead.
1174 MF.getJumpTableInfo()->ReplaceMBBInJumpTables(MBB, FallThrough);
1175 MadeChange = true;
1176 }
1177 return MadeChange;
1178 }
1179
1180 // Check to see if we can simplify the terminator of the block before this
1181 // one.
1182 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB));
1183
1184 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
1185 SmallVector<MachineOperand, 4> PriorCond;
1186 bool PriorUnAnalyzable =
1187 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
1188 if (!PriorUnAnalyzable) {
1189 // If the CFG for the prior block has extra edges, remove them.
1190 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB,
1191 !PriorCond.empty());
1192
1193 // If the previous branch is conditional and both conditions go to the same
1194 // destination, remove the branch, replacing it with an unconditional one or
1195 // a fall-through.
1196 if (PriorTBB && PriorTBB == PriorFBB) {
1197 TII->RemoveBranch(PrevBB);
1198 PriorCond.clear();
1199 if (PriorTBB != MBB)
1200 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond);
1201 MadeChange = true;
1202 ++NumBranchOpts;
1203 goto ReoptimizeBlock;
1204 }
1205
1206 // If the previous block unconditionally falls through to this block and
1207 // this block has no other predecessors, move the contents of this block
1208 // into the prior block. This doesn't usually happen when SimplifyCFG
1209 // has been used, but it can happen if tail merging splits a fall-through
1210 // predecessor of a block.
1211 // This has to check PrevBB->succ_size() because EH edges are ignored by
1212 // AnalyzeBranch.
1213 if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 &&
1214 PrevBB.succ_size() == 1 &&
1215 !MBB->hasAddressTaken()) {
1216 DEBUG(errs() << "\nMerging into block: " << PrevBB
1217 << "From MBB: " << *MBB);
1218 PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end());
1219 PrevBB.removeSuccessor(PrevBB.succ_begin());;
1220 assert(PrevBB.succ_empty());
1221 PrevBB.transferSuccessors(MBB);
1222 MadeChange = true;
1223 return MadeChange;
1224 }
1225
1226 // If the previous branch *only* branches to *this* block (conditional or
1227 // not) remove the branch.
1228 if (PriorTBB == MBB && PriorFBB == 0) {
1229 TII->RemoveBranch(PrevBB);
1230 MadeChange = true;
1231 ++NumBranchOpts;
1232 goto ReoptimizeBlock;
1233 }
1234
1235 // If the prior block branches somewhere else on the condition and here if
1236 // the condition is false, remove the uncond second branch.
1237 if (PriorFBB == MBB) {
1238 TII->RemoveBranch(PrevBB);
1239 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond);
1240 MadeChange = true;
1241 ++NumBranchOpts;
1242 goto ReoptimizeBlock;
1243 }
1244
1245 // If the prior block branches here on true and somewhere else on false, and
1246 // if the branch condition is reversible, reverse the branch to create a
1247 // fall-through.
1248 if (PriorTBB == MBB) {
1249 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1250 if (!TII->ReverseBranchCondition(NewPriorCond)) {
1251 TII->RemoveBranch(PrevBB);
1252 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond);
1253 MadeChange = true;
1254 ++NumBranchOpts;
1255 goto ReoptimizeBlock;
1256 }
1257 }
1258
1259 // If this block has no successors (e.g. it is a return block or ends with
1260 // a call to a no-return function like abort or __cxa_throw) and if the pred
1261 // falls through into this block, and if it would otherwise fall through
1262 // into the block after this, move this block to the end of the function.
1263 //
1264 // We consider it more likely that execution will stay in the function (e.g.
1265 // due to loops) than it is to exit it. This asserts in loops etc, moving
1266 // the assert condition out of the loop body.
1267 if (MBB->succ_empty() && !PriorCond.empty() && PriorFBB == 0 &&
1268 MachineFunction::iterator(PriorTBB) == FallThrough &&
1269 !CanFallThrough(MBB)) {
1270 bool DoTransform = true;
1271
1272 // We have to be careful that the succs of PredBB aren't both no-successor
1273 // blocks. If neither have successors and if PredBB is the second from
1274 // last block in the function, we'd just keep swapping the two blocks for
1275 // last. Only do the swap if one is clearly better to fall through than
1276 // the other.
1277 if (FallThrough == --MF.end() &&
1278 !IsBetterFallthrough(PriorTBB, MBB))
1279 DoTransform = false;
1280
1281 // We don't want to do this transformation if we have control flow like:
1282 // br cond BB2
1283 // BB1:
1284 // ..
1285 // jmp BBX
1286 // BB2:
1287 // ..
1288 // ret
1289 //
1290 // In this case, we could actually be moving the return block *into* a
1291 // loop!
1292 if (DoTransform && !MBB->succ_empty() &&
1293 (!CanFallThrough(PriorTBB) || PriorTBB->empty()))
1294 DoTransform = false;
1295
1296
1297 if (DoTransform) {
1298 // Reverse the branch so we will fall through on the previous true cond.
1299 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1300 if (!TII->ReverseBranchCondition(NewPriorCond)) {
1301 DEBUG(errs() << "\nMoving MBB: " << *MBB
1302 << "To make fallthrough to: " << *PriorTBB << "\n");
1303
1304 TII->RemoveBranch(PrevBB);
1305 TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond);
1306
1307 // Move this block to the end of the function.
1308 MBB->moveAfter(--MF.end());
1309 MadeChange = true;
1310 ++NumBranchOpts;
1311 return MadeChange;
1312 }
1313 }
1314 }
1315 }
1316
1317 // Analyze the branch in the current block.
1318 MachineBasicBlock *CurTBB = 0, *CurFBB = 0;
1319 SmallVector<MachineOperand, 4> CurCond;
1320 bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true);
1321 if (!CurUnAnalyzable) {
1322 // If the CFG for the prior block has extra edges, remove them.
1323 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty());
1324
1325 // If this is a two-way branch, and the FBB branches to this block, reverse
1326 // the condition so the single-basic-block loop is faster. Instead of:
1327 // Loop: xxx; jcc Out; jmp Loop
1328 // we want:
1329 // Loop: xxx; jncc Loop; jmp Out
1330 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) {
1331 SmallVector<MachineOperand, 4> NewCond(CurCond);
1332 if (!TII->ReverseBranchCondition(NewCond)) {
1333 TII->RemoveBranch(*MBB);
1334 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond);
1335 MadeChange = true;
1336 ++NumBranchOpts;
1337 goto ReoptimizeBlock;
1338 }
1339 }
1340
1341 // If this branch is the only thing in its block, see if we can forward
1342 // other blocks across it.
1343 if (CurTBB && CurCond.empty() && CurFBB == 0 &&
1344 MBB->begin()->getDesc().isBranch() && CurTBB != MBB &&
1345 !MBB->hasAddressTaken()) {
1346 // This block may contain just an unconditional branch. Because there can
1347 // be 'non-branch terminators' in the block, try removing the branch and
1348 // then seeing if the block is empty.
1349 TII->RemoveBranch(*MBB);
1350
1351 // If this block is just an unconditional branch to CurTBB, we can
1352 // usually completely eliminate the block. The only case we cannot
1353 // completely eliminate the block is when the block before this one
1354 // falls through into MBB and we can't understand the prior block's branch
1355 // condition.
1356 if (MBB->empty()) {
1357 bool PredHasNoFallThrough = TII->BlockHasNoFallThrough(PrevBB);
1358 if (PredHasNoFallThrough || !PriorUnAnalyzable ||
1359 !PrevBB.isSuccessor(MBB)) {
1360 // If the prior block falls through into us, turn it into an
1361 // explicit branch to us to make updates simpler.
1362 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) &&
1363 PriorTBB != MBB && PriorFBB != MBB) {
1364 if (PriorTBB == 0) {
1365 assert(PriorCond.empty() && PriorFBB == 0 &&
1366 "Bad branch analysis");
1367 PriorTBB = MBB;
1368 } else {
1369 assert(PriorFBB == 0 && "Machine CFG out of date!");
1370 PriorFBB = MBB;
1371 }
1372 TII->RemoveBranch(PrevBB);
1373 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond);
1374 }
1375
1376 // Iterate through all the predecessors, revectoring each in-turn.
1377 size_t PI = 0;
1378 bool DidChange = false;
1379 bool HasBranchToSelf = false;
1380 while(PI != MBB->pred_size()) {
1381 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI);
1382 if (PMBB == MBB) {
1383 // If this block has an uncond branch to itself, leave it.
1384 ++PI;
1385 HasBranchToSelf = true;
1386 } else {
1387 DidChange = true;
1388 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB);
1389 // If this change resulted in PMBB ending in a conditional
1390 // branch where both conditions go to the same destination,
1391 // change this to an unconditional branch (and fix the CFG).
1392 MachineBasicBlock *NewCurTBB = 0, *NewCurFBB = 0;
1393 SmallVector<MachineOperand, 4> NewCurCond;
1394 bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB,
1395 NewCurFBB, NewCurCond, true);
1396 if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) {
1397 TII->RemoveBranch(*PMBB);
1398 NewCurCond.clear();
1399 TII->InsertBranch(*PMBB, NewCurTBB, 0, NewCurCond);
1400 MadeChange = true;
1401 ++NumBranchOpts;
1402 PMBB->CorrectExtraCFGEdges(NewCurTBB, 0, false);
1403 }
1404 }
1405 }
1406
1407 // Change any jumptables to go to the new MBB.
1408 MF.getJumpTableInfo()->ReplaceMBBInJumpTables(MBB, CurTBB);
1409 if (DidChange) {
1410 ++NumBranchOpts;
1411 MadeChange = true;
1412 if (!HasBranchToSelf) return MadeChange;
1413 }
1414 }
1415 }
1416
1417 // Add the branch back if the block is more than just an uncond branch.
1418 TII->InsertBranch(*MBB, CurTBB, 0, CurCond);
1419 }
1420 }
1421
1422 // If the prior block doesn't fall through into this block, and if this
1423 // block doesn't fall through into some other block, see if we can find a
1424 // place to move this block where a fall-through will happen.
1425 if (!CanFallThrough(&PrevBB, PriorUnAnalyzable,
1426 PriorTBB, PriorFBB, PriorCond)) {
1427
1428 // Now we know that there was no fall-through into this block, check to
1429 // see if it has a fall-through into its successor.
1430 bool CurFallsThru = CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB,
1431 CurCond);
1432
1433 if (!MBB->isLandingPad()) {
1434 // Check all the predecessors of this block. If one of them has no fall
1435 // throughs, move this block right after it.
1436 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
1437 E = MBB->pred_end(); PI != E; ++PI) {
1438 // Analyze the branch at the end of the pred.
1439 MachineBasicBlock *PredBB = *PI;
1440 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough;
1441 MachineBasicBlock *PredTBB, *PredFBB;
1442 SmallVector<MachineOperand, 4> PredCond;
1443 if (PredBB != MBB && !CanFallThrough(PredBB) &&
1444 !TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true)
1445 && (!CurFallsThru || !CurTBB || !CurFBB)
1446 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
1447 // If the current block doesn't fall through, just move it.
1448 // If the current block can fall through and does not end with a
1449 // conditional branch, we need to append an unconditional jump to
1450 // the (current) next block. To avoid a possible compile-time
1451 // infinite loop, move blocks only backward in this case.
1452 // Also, if there are already 2 branches here, we cannot add a third;
1453 // this means we have the case
1454 // Bcc next
1455 // B elsewhere
1456 // next:
1457 if (CurFallsThru) {
1458 MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB));
1459 CurCond.clear();
1460 TII->InsertBranch(*MBB, NextBB, 0, CurCond);
1461 }
1462 MBB->moveAfter(PredBB);
1463 MadeChange = true;
1464 goto ReoptimizeBlock;
1465 }
1466 }
1467 }
1468
1469 if (!CurFallsThru) {
1470 // Check all successors to see if we can move this block before it.
1471 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
1472 E = MBB->succ_end(); SI != E; ++SI) {
1473 // Analyze the branch at the end of the block before the succ.
1474 MachineBasicBlock *SuccBB = *SI;
1475 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev;
1476
1477 // If this block doesn't already fall-through to that successor, and if
1478 // the succ doesn't already have a block that can fall through into it,
1479 // and if the successor isn't an EH destination, we can arrange for the
1480 // fallthrough to happen.
1481 if (SuccBB != MBB && &*SuccPrev != MBB &&
1482 !CanFallThrough(SuccPrev) && !CurUnAnalyzable &&
1483 !SuccBB->isLandingPad()) {
1484 MBB->moveBefore(SuccBB);
1485 MadeChange = true;
1486 goto ReoptimizeBlock;
1487 }
1488 }
1489
1490 // Okay, there is no really great place to put this block. If, however,
1491 // the block before this one would be a fall-through if this block were
1492 // removed, move this block to the end of the function.
1493 MachineBasicBlock *PrevTBB, *PrevFBB;
1494 SmallVector<MachineOperand, 4> PrevCond;
1495 if (FallThrough != MF.end() &&
1496 !TII->AnalyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) &&
1497 PrevBB.isSuccessor(FallThrough)) {
1498 MBB->moveAfter(--MF.end());
1499 MadeChange = true;
1500 return MadeChange;
1501 }
1502 }
1503 }
1504
1505 return MadeChange;
1506}
|