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