1	//===- BranchFolding.cpp - Fold machine code branch instructions ----------===//
2	//
3	// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4	// See https://llvm.org/LICENSE.txt for license information.
5	// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6	//
7	//===----------------------------------------------------------------------===//
8	//
9	// This pass forwards branches to unconditional branches to make them branch
10	// directly to the target block. This pass often results in dead MBB's, which
11	// it then removes.
12	//
13	// Note that this pass must be run after register allocation, it cannot handle
14	// SSA form. It also must handle virtual registers for targets that emit virtual
15	// ISA (e.g. NVPTX).
16	//
17	//===----------------------------------------------------------------------===//
18
19	#include "BranchFolding.h"
20	#include "llvm/ADT/BitVector.h"
21	#include "llvm/ADT/STLExtras.h"
22	#include "llvm/ADT/SmallSet.h"
23	#include "llvm/ADT/SmallVector.h"
24	#include "llvm/ADT/Statistic.h"
25	#include "llvm/Analysis/ProfileSummaryInfo.h"
26	#include "llvm/CodeGen/Analysis.h"
27	#include "llvm/CodeGen/MBFIWrapper.h"
28	#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
29	#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
30	#include "llvm/CodeGen/MachineFunction.h"
31	#include "llvm/CodeGen/MachineFunctionPass.h"
32	#include "llvm/CodeGen/MachineInstr.h"
33	#include "llvm/CodeGen/MachineInstrBuilder.h"
34	#include "llvm/CodeGen/MachineJumpTableInfo.h"
35	#include "llvm/CodeGen/MachineLoopInfo.h"
36	#include "llvm/CodeGen/MachineOperand.h"
37	#include "llvm/CodeGen/MachineRegisterInfo.h"
38	#include "llvm/CodeGen/MachineSizeOpts.h"
39	#include "llvm/CodeGen/TargetInstrInfo.h"
40	#include "llvm/CodeGen/TargetOpcodes.h"
41	#include "llvm/CodeGen/TargetPassConfig.h"
42	#include "llvm/CodeGen/TargetRegisterInfo.h"
43	#include "llvm/CodeGen/TargetSubtargetInfo.h"
44	#include "llvm/IR/DebugInfoMetadata.h"
45	#include "llvm/IR/DebugLoc.h"
46	#include "llvm/IR/Function.h"
47	#include "llvm/InitializePasses.h"
48	#include "llvm/MC/LaneBitmask.h"
49	#include "llvm/MC/MCRegisterInfo.h"
50	#include "llvm/Pass.h"
51	#include "llvm/Support/BlockFrequency.h"
52	#include "llvm/Support/BranchProbability.h"
53	#include "llvm/Support/CommandLine.h"
54	#include "llvm/Support/Debug.h"
55	#include "llvm/Support/ErrorHandling.h"
56	#include "llvm/Support/raw_ostream.h"
57	#include "llvm/Target/TargetMachine.h"
58	#include <cassert>
59	#include <cstddef>
60	#include <iterator>
61	#include <numeric>
62
63	using namespace llvm;
64
65	#define DEBUG_TYPE "branch-folder"
66
67	STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
68	STATISTIC(NumBranchOpts, "Number of branches optimized");
69	STATISTIC(NumTailMerge , "Number of block tails merged");
70	STATISTIC(NumHoist , "Number of times common instructions are hoisted");
71	STATISTIC(NumTailCalls, "Number of tail calls optimized");
72
73	static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge",
74	cl::init(cl::BOU_UNSET), cl::Hidden);
75
76	// Throttle for huge numbers of predecessors (compile speed problems)
77	static cl::opt<unsigned>
78	TailMergeThreshold("tail-merge-threshold",
79	cl::desc("Max number of predecessors to consider tail merging"),
80	cl::init(150), cl::Hidden);
81
82	// Heuristic for tail merging (and, inversely, tail duplication).
83	// TODO: This should be replaced with a target query.
84	static cl::opt<unsigned>
85	TailMergeSize("tail-merge-size",
86	cl::desc("Min number of instructions to consider tail merging"),
87	cl::init(3), cl::Hidden);
88
89	namespace {
90
91	/// BranchFolderPass - Wrap branch folder in a machine function pass.
92	class BranchFolderPass : public MachineFunctionPass {
93	public:
94	static char ID;
95
96	explicit BranchFolderPass(): MachineFunctionPass(ID) {}
97
98	bool runOnMachineFunction(MachineFunction &MF) override;
99
100	void getAnalysisUsage(AnalysisUsage &AU) const override {
101	AU.addRequired<MachineBlockFrequencyInfo>();
102	AU.addRequired<MachineBranchProbabilityInfo>();
103	AU.addRequired<ProfileSummaryInfoWrapperPass>();
104	AU.addRequired<TargetPassConfig>();
105	MachineFunctionPass::getAnalysisUsage(AU);
106	}
107
108	MachineFunctionProperties getRequiredProperties() const override {
109	return MachineFunctionProperties().set(
110	MachineFunctionProperties::Property::NoPHIs);
111	}
112	};
113
114	} // end anonymous namespace
115
116	char BranchFolderPass::ID = 0;
117
118	char &llvm::BranchFolderPassID = BranchFolderPass::ID;
119
120	INITIALIZE_PASS(BranchFolderPass, DEBUG_TYPE,
121	"Control Flow Optimizer", false, false)
122
123	bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) {
124	if (skipFunction(MF.getFunction()))
125	return false;
126
127	TargetPassConfig *PassConfig = &getAnalysis<TargetPassConfig>();
128	// TailMerge can create jump into if branches that make CFG irreducible for
129	// HW that requires structurized CFG.
130	bool EnableTailMerge = !MF.getTarget().requiresStructuredCFG() &&
131	PassConfig->getEnableTailMerge();
132	MBFIWrapper MBBFreqInfo(
133	getAnalysis<MachineBlockFrequencyInfo>());
134	BranchFolder Folder(EnableTailMerge, /*CommonHoist=*/true, MBBFreqInfo,
135	getAnalysis<MachineBranchProbabilityInfo>(),
136	&getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI());
137	return Folder.OptimizeFunction(MF, MF.getSubtarget().getInstrInfo(),
138	MF.getSubtarget().getRegisterInfo());
139	}
140
141	BranchFolder::BranchFolder(bool DefaultEnableTailMerge, bool CommonHoist,
142	MBFIWrapper &FreqInfo,
143	const MachineBranchProbabilityInfo &ProbInfo,
144	ProfileSummaryInfo *PSI, unsigned MinTailLength)
145	: EnableHoistCommonCode(CommonHoist), MinCommonTailLength(MinTailLength),
146	MBBFreqInfo(FreqInfo), MBPI(ProbInfo), PSI(PSI) {
147	if (MinCommonTailLength == 0)
148	MinCommonTailLength = TailMergeSize;
149	switch (FlagEnableTailMerge) {
150	case cl::BOU_UNSET:
151	EnableTailMerge = DefaultEnableTailMerge;
152	break;
153	case cl::BOU_TRUE: EnableTailMerge = true; break;
154	case cl::BOU_FALSE: EnableTailMerge = false; break;
155	}
156	}
157
158	void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) {
159	assert(MBB->pred_empty() && "MBB must be dead!");
160	LLVM_DEBUG(dbgs() << "\nRemoving MBB: " << *MBB);
161
162	MachineFunction *MF = MBB->getParent();
163	// drop all successors.
164	while (!MBB->succ_empty())
165	MBB->removeSuccessor(MBB->succ_end()-1);
166
167	// Avoid matching if this pointer gets reused.
168	TriedMerging.erase(MBB);
169
170	// Update call site info.
171	for (const MachineInstr &MI : *MBB)
172	if (MI.shouldUpdateCallSiteInfo())
173	MF->eraseCallSiteInfo(&MI);
174
175	// Remove the block.
176	MF->erase(MBB);
177	EHScopeMembership.erase(MBB);
178	if (MLI)
179	MLI->removeBlock(MBB);
180	}
181
182	bool BranchFolder::OptimizeFunction(MachineFunction &MF,
183	const TargetInstrInfo *tii,
184	const TargetRegisterInfo *tri,
185	MachineLoopInfo *mli, bool AfterPlacement) {
186	if (!tii) return false;
187
188	TriedMerging.clear();
189
190	MachineRegisterInfo &MRI = MF.getRegInfo();
191	AfterBlockPlacement = AfterPlacement;
192	TII = tii;
193	TRI = tri;
194	MLI = mli;
195	this->MRI = &MRI;
196
197	UpdateLiveIns = MRI.tracksLiveness() && TRI->trackLivenessAfterRegAlloc(MF);
198	if (!UpdateLiveIns)
199	MRI.invalidateLiveness();
200
201	bool MadeChange = false;
202
203	// Recalculate EH scope membership.
204	EHScopeMembership = getEHScopeMembership(MF);
205
206	bool MadeChangeThisIteration = true;
207	while (MadeChangeThisIteration) {
208	MadeChangeThisIteration = TailMergeBlocks(MF);
209	// No need to clean up if tail merging does not change anything after the
210	// block placement.
211	if (!AfterBlockPlacement || MadeChangeThisIteration)
212	MadeChangeThisIteration |= OptimizeBranches(MF);
213	if (EnableHoistCommonCode)
214	MadeChangeThisIteration |= HoistCommonCode(MF);
215	MadeChange |= MadeChangeThisIteration;
216	}
217
218	// See if any jump tables have become dead as the code generator
219	// did its thing.
220	MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
221	if (!JTI)
222	return MadeChange;
223
224	// Walk the function to find jump tables that are live.
225	BitVector JTIsLive(JTI->getJumpTables().size());
226	for (const MachineBasicBlock &BB : MF) {
227	for (const MachineInstr &I : BB)
228	for (const MachineOperand &Op : I.operands()) {
229	if (!Op.isJTI()) continue;
230
231	// Remember that this JT is live.
232	JTIsLive.set(Op.getIndex());
233	}
234	}
235
236	// Finally, remove dead jump tables. This happens when the
237	// indirect jump was unreachable (and thus deleted).
238	for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i)
239	if (!JTIsLive.test(i)) {
240	JTI->RemoveJumpTable(i);
241	MadeChange = true;
242	}
243
244	return MadeChange;
245	}
246
247	//===----------------------------------------------------------------------===//
248	// Tail Merging of Blocks
249	//===----------------------------------------------------------------------===//
250
251	/// HashMachineInstr - Compute a hash value for MI and its operands.
252	static unsigned HashMachineInstr(const MachineInstr &MI) {
253	unsigned Hash = MI.getOpcode();
254	for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
255	const MachineOperand &Op = MI.getOperand(i);
256
257	// Merge in bits from the operand if easy. We can't use MachineOperand's
258	// hash_code here because it's not deterministic and we sort by hash value
259	// later.
260	unsigned OperandHash = 0;
261	switch (Op.getType()) {
262	case MachineOperand::MO_Register:
263	OperandHash = Op.getReg();
264	break;
265	case MachineOperand::MO_Immediate:
266	OperandHash = Op.getImm();
267	break;
268	case MachineOperand::MO_MachineBasicBlock:
269	OperandHash = Op.getMBB()->getNumber();
270	break;
271	case MachineOperand::MO_FrameIndex:
272	case MachineOperand::MO_ConstantPoolIndex:
273	case MachineOperand::MO_JumpTableIndex:
274	OperandHash = Op.getIndex();
275	break;
276	case MachineOperand::MO_GlobalAddress:
277	case MachineOperand::MO_ExternalSymbol:
278	// Global address / external symbol are too hard, don't bother, but do
279	// pull in the offset.
280	OperandHash = Op.getOffset();
281	break;
282	default:
283	break;
284	}
285
286	Hash += ((OperandHash << 3) | Op.getType()) << (i & 31);
287	}
288	return Hash;
289	}
290
291	/// HashEndOfMBB - Hash the last instruction in the MBB.
292	static unsigned HashEndOfMBB(const MachineBasicBlock &MBB) {
293	MachineBasicBlock::const_iterator I = MBB.getLastNonDebugInstr(false);
294	if (I == MBB.end())
295	return 0;
296
297	return HashMachineInstr(*I);
298	}
299
300	/// Whether MI should be counted as an instruction when calculating common tail.
301	static bool countsAsInstruction(const MachineInstr &MI) {
302	return !(MI.isDebugInstr() || MI.isCFIInstruction());
303	}
304
305	/// Iterate backwards from the given iterator \p I, towards the beginning of the
306	/// block. If a MI satisfying 'countsAsInstruction' is found, return an iterator
307	/// pointing to that MI. If no such MI is found, return the end iterator.
308	static MachineBasicBlock::iterator
309	skipBackwardPastNonInstructions(MachineBasicBlock::iterator I,
310	MachineBasicBlock *MBB) {
311	while (I != MBB->begin()) {
312	--I;
313	if (countsAsInstruction(*I))
314	return I;
315	}
316	return MBB->end();
317	}
318
319	/// Given two machine basic blocks, return the number of instructions they
320	/// actually have in common together at their end. If a common tail is found (at
321	/// least by one instruction), then iterators for the first shared instruction
322	/// in each block are returned as well.
323	///
324	/// Non-instructions according to countsAsInstruction are ignored.
325	static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1,
326	MachineBasicBlock *MBB2,
327	MachineBasicBlock::iterator &I1,
328	MachineBasicBlock::iterator &I2) {
329	MachineBasicBlock::iterator MBBI1 = MBB1->end();
330	MachineBasicBlock::iterator MBBI2 = MBB2->end();
331
332	unsigned TailLen = 0;
333	while (true) {
334	MBBI1 = skipBackwardPastNonInstructions(MBBI1, MBB1);
335	MBBI2 = skipBackwardPastNonInstructions(MBBI2, MBB2);
336	if (MBBI1 == MBB1->end() || MBBI2 == MBB2->end())
337	break;
338	if (!MBBI1->isIdenticalTo(*MBBI2) ||
339	// FIXME: This check is dubious. It's used to get around a problem where
340	// people incorrectly expect inline asm directives to remain in the same
341	// relative order. This is untenable because normal compiler
342	// optimizations (like this one) may reorder and/or merge these
343	// directives.
344	MBBI1->isInlineAsm()) {
345	break;
346	}
347	if (MBBI1->getFlag(MachineInstr::NoMerge) ||
348	MBBI2->getFlag(MachineInstr::NoMerge))
349	break;
350	++TailLen;
351	I1 = MBBI1;
352	I2 = MBBI2;
353	}
354
355	return TailLen;
356	}
357
358	void BranchFolder::replaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
359	MachineBasicBlock &NewDest) {
360	if (UpdateLiveIns) {
361	// OldInst should always point to an instruction.
362	MachineBasicBlock &OldMBB = *OldInst->getParent();
363	LiveRegs.clear();
364	LiveRegs.addLiveOuts(OldMBB);
365	// Move backward to the place where will insert the jump.
366	MachineBasicBlock::iterator I = OldMBB.end();
367	do {
368	--I;
369	LiveRegs.stepBackward(*I);
370	} while (I != OldInst);
371
372	// Merging the tails may have switched some undef operand to non-undef ones.
373	// Add IMPLICIT_DEFS into OldMBB as necessary to have a definition of the
374	// register.
375	for (MachineBasicBlock::RegisterMaskPair P : NewDest.liveins()) {
376	// We computed the liveins with computeLiveIn earlier and should only see
377	// full registers:
378	assert(P.LaneMask == LaneBitmask::getAll() &&
379	"Can only handle full register.");
380	MCPhysReg Reg = P.PhysReg;
381	if (!LiveRegs.available(*MRI, Reg))
382	continue;
383	DebugLoc DL;
384	BuildMI(OldMBB, OldInst, DL, TII->get(TargetOpcode::IMPLICIT_DEF), Reg);
385	}
386	}
387
388	TII->ReplaceTailWithBranchTo(OldInst, &NewDest);
389	++NumTailMerge;
390	}
391
392	MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB,
393	MachineBasicBlock::iterator BBI1,
394	const BasicBlock *BB) {
395	if (!TII->isLegalToSplitMBBAt(CurMBB, BBI1))
396	return nullptr;
397
398	MachineFunction &MF = *CurMBB.getParent();
399
400	// Create the fall-through block.
401	MachineFunction::iterator MBBI = CurMBB.getIterator();
402	MachineBasicBlock *NewMBB = MF.CreateMachineBasicBlock(BB);
403	CurMBB.getParent()->insert(++MBBI, NewMBB);
404
405	// Move all the successors of this block to the specified block.
406	NewMBB->transferSuccessors(&CurMBB);
407
408	// Add an edge from CurMBB to NewMBB for the fall-through.
409	CurMBB.addSuccessor(NewMBB);
410
411	// Splice the code over.
412	NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end());
413
414	// NewMBB belongs to the same loop as CurMBB.
415	if (MLI)
416	if (MachineLoop *ML = MLI->getLoopFor(&CurMBB))
417	ML->addBasicBlockToLoop(NewMBB, MLI->getBase());
418
419	// NewMBB inherits CurMBB's block frequency.
420	MBBFreqInfo.setBlockFreq(NewMBB, MBBFreqInfo.getBlockFreq(&CurMBB));
421
422	if (UpdateLiveIns)
423	computeAndAddLiveIns(LiveRegs, *NewMBB);
424
425	// Add the new block to the EH scope.
426	const auto &EHScopeI = EHScopeMembership.find(&CurMBB);
427	if (EHScopeI != EHScopeMembership.end()) {
428	auto n = EHScopeI->second;
429	EHScopeMembership[NewMBB] = n;
430	}
431
432	return NewMBB;
433	}
434
435	/// EstimateRuntime - Make a rough estimate for how long it will take to run
436	/// the specified code.
437	static unsigned EstimateRuntime(MachineBasicBlock::iterator I,
438	MachineBasicBlock::iterator E) {
439	unsigned Time = 0;
440	for (; I != E; ++I) {
441	if (!countsAsInstruction(*I))
442	continue;
443	if (I->isCall())
444	Time += 10;
445	else if (I->mayLoadOrStore())
446	Time += 2;
447	else
448	++Time;
449	}
450	return Time;
451	}
452
453	// CurMBB needs to add an unconditional branch to SuccMBB (we removed these
454	// branches temporarily for tail merging). In the case where CurMBB ends
455	// with a conditional branch to the next block, optimize by reversing the
456	// test and conditionally branching to SuccMBB instead.
457	static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB,
458	const TargetInstrInfo *TII) {
459	MachineFunction *MF = CurMBB->getParent();
460	MachineFunction::iterator I = std::next(MachineFunction::iterator(CurMBB));
461	MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
462	SmallVector<MachineOperand, 4> Cond;
463	DebugLoc dl = CurMBB->findBranchDebugLoc();
464	if (I != MF->end() && !TII->analyzeBranch(*CurMBB, TBB, FBB, Cond, true)) {
465	MachineBasicBlock *NextBB = &*I;
466	if (TBB == NextBB && !Cond.empty() && !FBB) {
467	if (!TII->reverseBranchCondition(Cond)) {
468	TII->removeBranch(*CurMBB);
469	TII->insertBranch(*CurMBB, SuccBB, nullptr, Cond, dl);
470	return;
471	}
472	}
473	}
474	TII->insertBranch(*CurMBB, SuccBB, nullptr,
475	SmallVector<MachineOperand, 0>(), dl);
476	}
477
478	bool
479	BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const {
480	if (getHash() < o.getHash())
481	return true;
482	if (getHash() > o.getHash())
483	return false;
484	if (getBlock()->getNumber() < o.getBlock()->getNumber())
485	return true;
486	if (getBlock()->getNumber() > o.getBlock()->getNumber())
487	return false;
488	// _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing
489	// an object with itself.
490	#ifndef _GLIBCXX_DEBUG
491	llvm_unreachable("Predecessor appears twice");
492	#else
493	return false;
494	#endif
495	}
496
497	/// CountTerminators - Count the number of terminators in the given
498	/// block and set I to the position of the first non-terminator, if there
499	/// is one, or MBB->end() otherwise.
500	static unsigned CountTerminators(MachineBasicBlock *MBB,
501	MachineBasicBlock::iterator &I) {
502	I = MBB->end();
503	unsigned NumTerms = 0;
504	while (true) {
505	if (I == MBB->begin()) {
506	I = MBB->end();
507	break;
508	}
509	--I;
510	if (!I->isTerminator()) break;
511	++NumTerms;
512	}
513	return NumTerms;
514	}
515
516	/// A no successor, non-return block probably ends in unreachable and is cold.
517	/// Also consider a block that ends in an indirect branch to be a return block,
518	/// since many targets use plain indirect branches to return.
519	static bool blockEndsInUnreachable(const MachineBasicBlock *MBB) {
520	if (!MBB->succ_empty())
521	return false;
522	if (MBB->empty())
523	return true;
524	return !(MBB->back().isReturn() || MBB->back().isIndirectBranch());
525	}
526
527	/// ProfitableToMerge - Check if two machine basic blocks have a common tail
528	/// and decide if it would be profitable to merge those tails. Return the
529	/// length of the common tail and iterators to the first common instruction
530	/// in each block.
531	/// MBB1, MBB2 The blocks to check
532	/// MinCommonTailLength Minimum size of tail block to be merged.
533	/// CommonTailLen Out parameter to record the size of the shared tail between
534	/// MBB1 and MBB2
535	/// I1, I2 Iterator references that will be changed to point to the first
536	/// instruction in the common tail shared by MBB1,MBB2
537	/// SuccBB A common successor of MBB1, MBB2 which are in a canonical form
538	/// relative to SuccBB
539	/// PredBB The layout predecessor of SuccBB, if any.
540	/// EHScopeMembership map from block to EH scope #.
541	/// AfterPlacement True if we are merging blocks after layout. Stricter
542	/// thresholds apply to prevent undoing tail-duplication.
543	static bool
544	ProfitableToMerge(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2,
545	unsigned MinCommonTailLength, unsigned &CommonTailLen,
546	MachineBasicBlock::iterator &I1,
547	MachineBasicBlock::iterator &I2, MachineBasicBlock *SuccBB,
548	MachineBasicBlock *PredBB,
549	DenseMap<const MachineBasicBlock *, int> &EHScopeMembership,
550	bool AfterPlacement,
551	MBFIWrapper &MBBFreqInfo,
552	ProfileSummaryInfo *PSI) {
553	// It is never profitable to tail-merge blocks from two different EH scopes.
554	if (!EHScopeMembership.empty()) {
555	auto EHScope1 = EHScopeMembership.find(MBB1);
556	assert(EHScope1 != EHScopeMembership.end());
557	auto EHScope2 = EHScopeMembership.find(MBB2);
558	assert(EHScope2 != EHScopeMembership.end());
559	if (EHScope1->second != EHScope2->second)
560	return false;
561	}
562
563	CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2);
564	if (CommonTailLen == 0)
565	return false;
566	LLVM_DEBUG(dbgs() << "Common tail length of " << printMBBReference(*MBB1)
567	<< " and " << printMBBReference(*MBB2) << " is "
568	<< CommonTailLen << '\n');
569
570	// Move the iterators to the beginning of the MBB if we only got debug
571	// instructions before the tail. This is to avoid splitting a block when we
572	// only got debug instructions before the tail (to be invariant on -g).
573	if (skipDebugInstructionsForward(MBB1->begin(), MBB1->end(), false) == I1)
574	I1 = MBB1->begin();
575	if (skipDebugInstructionsForward(MBB2->begin(), MBB2->end(), false) == I2)
576	I2 = MBB2->begin();
577
578	bool FullBlockTail1 = I1 == MBB1->begin();
579	bool FullBlockTail2 = I2 == MBB2->begin();
580
581	// It's almost always profitable to merge any number of non-terminator
582	// instructions with the block that falls through into the common successor.
583	// This is true only for a single successor. For multiple successors, we are
584	// trading a conditional branch for an unconditional one.
585	// TODO: Re-visit successor size for non-layout tail merging.
586	if ((MBB1 == PredBB || MBB2 == PredBB) &&
587	(!AfterPlacement || MBB1->succ_size() == 1)) {
588	MachineBasicBlock::iterator I;
589	unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I);
590	if (CommonTailLen > NumTerms)
591	return true;
592	}
593
594	// If these are identical non-return blocks with no successors, merge them.
595	// Such blocks are typically cold calls to noreturn functions like abort, and
596	// are unlikely to become a fallthrough target after machine block placement.
597	// Tail merging these blocks is unlikely to create additional unconditional
598	// branches, and will reduce the size of this cold code.
599	if (FullBlockTail1 && FullBlockTail2 &&
600	blockEndsInUnreachable(MBB1) && blockEndsInUnreachable(MBB2))
601	return true;
602
603	// If one of the blocks can be completely merged and happens to be in
604	// a position where the other could fall through into it, merge any number
605	// of instructions, because it can be done without a branch.
606	// TODO: If the blocks are not adjacent, move one of them so that they are?
607	if (MBB1->isLayoutSuccessor(MBB2) && FullBlockTail2)
608	return true;
609	if (MBB2->isLayoutSuccessor(MBB1) && FullBlockTail1)
610	return true;
611
612	// If both blocks are identical and end in a branch, merge them unless they
613	// both have a fallthrough predecessor and successor.
614	// We can only do this after block placement because it depends on whether
615	// there are fallthroughs, and we don't know until after layout.
616	if (AfterPlacement && FullBlockTail1 && FullBlockTail2) {
617	auto BothFallThrough = [](MachineBasicBlock *MBB) {
618	if (!MBB->succ_empty() && !MBB->canFallThrough())
619	return false;
620	MachineFunction::iterator I(MBB);
621	MachineFunction *MF = MBB->getParent();
622	return (MBB != &*MF->begin()) && std::prev(I)->canFallThrough();
623	};
624	if (!BothFallThrough(MBB1) || !BothFallThrough(MBB2))
625	return true;
626	}
627
628	// If both blocks have an unconditional branch temporarily stripped out,
629	// count that as an additional common instruction for the following
630	// heuristics. This heuristic is only accurate for single-succ blocks, so to
631	// make sure that during layout merging and duplicating don't crash, we check
632	// for that when merging during layout.
633	unsigned EffectiveTailLen = CommonTailLen;
634	if (SuccBB && MBB1 != PredBB && MBB2 != PredBB &&
635	(MBB1->succ_size() == 1 || !AfterPlacement) &&
636	!MBB1->back().isBarrier() &&
637	!MBB2->back().isBarrier())
638	++EffectiveTailLen;
639
640	// Check if the common tail is long enough to be worthwhile.
641	if (EffectiveTailLen >= MinCommonTailLength)
642	return true;
643
644	// If we are optimizing for code size, 2 instructions in common is enough if
645	// we don't have to split a block. At worst we will be introducing 1 new
646	// branch instruction, which is likely to be smaller than the 2
647	// instructions that would be deleted in the merge.
648	MachineFunction *MF = MBB1->getParent();
649	bool OptForSize =
650	MF->getFunction().hasOptSize() ||
651	(llvm::shouldOptimizeForSize(MBB1, PSI, &MBBFreqInfo) &&
652	llvm::shouldOptimizeForSize(MBB2, PSI, &MBBFreqInfo));
653	return EffectiveTailLen >= 2 && OptForSize &&
654	(FullBlockTail1 || FullBlockTail2);
655	}
656
657	unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
658	unsigned MinCommonTailLength,
659	MachineBasicBlock *SuccBB,
660	MachineBasicBlock *PredBB) {
661	unsigned maxCommonTailLength = 0U;
662	SameTails.clear();
663	MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
664	MPIterator HighestMPIter = std::prev(MergePotentials.end());
665	for (MPIterator CurMPIter = std::prev(MergePotentials.end()),
666	B = MergePotentials.begin();
667	CurMPIter != B && CurMPIter->getHash() == CurHash; --CurMPIter) {
668	for (MPIterator I = std::prev(CurMPIter); I->getHash() == CurHash; --I) {
669	unsigned CommonTailLen;
670	if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(),
671	MinCommonTailLength,
672	CommonTailLen, TrialBBI1, TrialBBI2,
673	SuccBB, PredBB,
674	EHScopeMembership,
675	AfterBlockPlacement, MBBFreqInfo, PSI)) {
676	if (CommonTailLen > maxCommonTailLength) {
677	SameTails.clear();
678	maxCommonTailLength = CommonTailLen;
679	HighestMPIter = CurMPIter;
680	SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1));
681	}
682	if (HighestMPIter == CurMPIter &&
683	CommonTailLen == maxCommonTailLength)
684	SameTails.push_back(SameTailElt(I, TrialBBI2));
685	}
686	if (I == B)
687	break;
688	}
689	}
690	return maxCommonTailLength;
691	}
692
693	void BranchFolder::RemoveBlocksWithHash(unsigned CurHash,
694	MachineBasicBlock *SuccBB,
695	MachineBasicBlock *PredBB) {
696	MPIterator CurMPIter, B;
697	for (CurMPIter = std::prev(MergePotentials.end()),
698	B = MergePotentials.begin();
699	CurMPIter->getHash() == CurHash; --CurMPIter) {
700	// Put the unconditional branch back, if we need one.
701	MachineBasicBlock *CurMBB = CurMPIter->getBlock();
702	if (SuccBB && CurMBB != PredBB)
703	FixTail(CurMBB, SuccBB, TII);
704	if (CurMPIter == B)
705	break;
706	}
707	if (CurMPIter->getHash() != CurHash)
708	CurMPIter++;
709	MergePotentials.erase(CurMPIter, MergePotentials.end());
710	}
711
712	bool BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
713	MachineBasicBlock *SuccBB,
714	unsigned maxCommonTailLength,
715	unsigned &commonTailIndex) {
716	commonTailIndex = 0;
717	unsigned TimeEstimate = ~0U;
718	for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
719	// Use PredBB if possible; that doesn't require a new branch.
720	if (SameTails[i].getBlock() == PredBB) {
721	commonTailIndex = i;
722	break;
723	}
724	// Otherwise, make a (fairly bogus) choice based on estimate of
725	// how long it will take the various blocks to execute.
726	unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(),
727	SameTails[i].getTailStartPos());
728	if (t <= TimeEstimate) {
729	TimeEstimate = t;
730	commonTailIndex = i;
731	}
732	}
733
734	MachineBasicBlock::iterator BBI =
735	SameTails[commonTailIndex].getTailStartPos();
736	MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
737
738	LLVM_DEBUG(dbgs() << "\nSplitting " << printMBBReference(*MBB) << ", size "
739	<< maxCommonTailLength);
740
741	// If the split block unconditionally falls-thru to SuccBB, it will be
742	// merged. In control flow terms it should then take SuccBB's name. e.g. If
743	// SuccBB is an inner loop, the common tail is still part of the inner loop.
744	const BasicBlock *BB = (SuccBB && MBB->succ_size() == 1) ?
745	SuccBB->getBasicBlock() : MBB->getBasicBlock();
746	MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI, BB);
747	if (!newMBB) {
748	LLVM_DEBUG(dbgs() << "... failed!");
749	return false;
750	}
751
752	SameTails[commonTailIndex].setBlock(newMBB);
753	SameTails[commonTailIndex].setTailStartPos(newMBB->begin());
754
755	// If we split PredBB, newMBB is the new predecessor.
756	if (PredBB == MBB)
757	PredBB = newMBB;
758
759	return true;
760	}
761
762	static void
763	mergeOperations(MachineBasicBlock::iterator MBBIStartPos,
764	MachineBasicBlock &MBBCommon) {
765	MachineBasicBlock *MBB = MBBIStartPos->getParent();
766	// Note CommonTailLen does not necessarily matches the size of
767	// the common BB nor all its instructions because of debug
768	// instructions differences.
769	unsigned CommonTailLen = 0;
770	for (auto E = MBB->end(); MBBIStartPos != E; ++MBBIStartPos)
771	++CommonTailLen;
772
773	MachineBasicBlock::reverse_iterator MBBI = MBB->rbegin();
774	MachineBasicBlock::reverse_iterator MBBIE = MBB->rend();
775	MachineBasicBlock::reverse_iterator MBBICommon = MBBCommon.rbegin();
776	MachineBasicBlock::reverse_iterator MBBIECommon = MBBCommon.rend();
777
778	while (CommonTailLen--) {
779	assert(MBBI != MBBIE && "Reached BB end within common tail length!");
780	(void)MBBIE;
781
782	if (!countsAsInstruction(*MBBI)) {
783	++MBBI;
784	continue;
785	}
786
787	while ((MBBICommon != MBBIECommon) && !countsAsInstruction(*MBBICommon))
788	++MBBICommon;
789
790	assert(MBBICommon != MBBIECommon &&
791	"Reached BB end within common tail length!");
792	assert(MBBICommon->isIdenticalTo(*MBBI) && "Expected matching MIIs!");
793
794	// Merge MMOs from memory operations in the common block.
795	if (MBBICommon->mayLoadOrStore())
796	MBBICommon->cloneMergedMemRefs(*MBB->getParent(), {&*MBBICommon, &*MBBI});
797	// Drop undef flags if they aren't present in all merged instructions.
798	for (unsigned I = 0, E = MBBICommon->getNumOperands(); I != E; ++I) {
799	MachineOperand &MO = MBBICommon->getOperand(I);
800	if (MO.isReg() && MO.isUndef()) {
801	const MachineOperand &OtherMO = MBBI->getOperand(I);
802	if (!OtherMO.isUndef())
803	MO.setIsUndef(false);
804	}
805	}
806
807	++MBBI;
808	++MBBICommon;
809	}
810	}
811
812	void BranchFolder::mergeCommonTails(unsigned commonTailIndex) {
813	MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
814
815	std::vector<MachineBasicBlock::iterator> NextCommonInsts(SameTails.size());
816	for (unsigned int i = 0 ; i != SameTails.size() ; ++i) {
817	if (i != commonTailIndex) {
818	NextCommonInsts[i] = SameTails[i].getTailStartPos();
819	mergeOperations(SameTails[i].getTailStartPos(), *MBB);
820	} else {
821	assert(SameTails[i].getTailStartPos() == MBB->begin() &&
822	"MBB is not a common tail only block");
823	}
824	}
825
826	for (auto &MI : *MBB) {
827	if (!countsAsInstruction(MI))
828	continue;
829	DebugLoc DL = MI.getDebugLoc();
830	for (unsigned int i = 0 ; i < NextCommonInsts.size() ; i++) {
831	if (i == commonTailIndex)
832	continue;
833
834	auto &Pos = NextCommonInsts[i];
835	assert(Pos != SameTails[i].getBlock()->end() &&
836	"Reached BB end within common tail");
837	while (!countsAsInstruction(*Pos)) {
838	++Pos;
839	assert(Pos != SameTails[i].getBlock()->end() &&
840	"Reached BB end within common tail");
841	}
842	assert(MI.isIdenticalTo(*Pos) && "Expected matching MIIs!");
843	DL = DILocation::getMergedLocation(DL, Pos->getDebugLoc());
844	NextCommonInsts[i] = ++Pos;
845	}
846	MI.setDebugLoc(DL);
847	}
848
849	if (UpdateLiveIns) {
850	LivePhysRegs NewLiveIns(*TRI);
851	computeLiveIns(NewLiveIns, *MBB);
852	LiveRegs.init(*TRI);
853
854	// The flag merging may lead to some register uses no longer using the
855	// <undef> flag, add IMPLICIT_DEFs in the predecessors as necessary.
856	for (MachineBasicBlock *Pred : MBB->predecessors()) {
857	LiveRegs.clear();
858	LiveRegs.addLiveOuts(*Pred);
859	MachineBasicBlock::iterator InsertBefore = Pred->getFirstTerminator();
860	for (Register Reg : NewLiveIns) {
861	if (!LiveRegs.available(*MRI, Reg))
862	continue;
863	DebugLoc DL;
864	BuildMI(*Pred, InsertBefore, DL, TII->get(TargetOpcode::IMPLICIT_DEF),
865	Reg);
866	}
867	}
868
869	MBB->clearLiveIns();
870	addLiveIns(*MBB, NewLiveIns);
871	}
872	}
873
874	// See if any of the blocks in MergePotentials (which all have SuccBB as a
875	// successor, or all have no successor if it is null) can be tail-merged.
876	// If there is a successor, any blocks in MergePotentials that are not
877	// tail-merged and are not immediately before Succ must have an unconditional
878	// branch to Succ added (but the predecessor/successor lists need no
879	// adjustment). The lone predecessor of Succ that falls through into Succ,
880	// if any, is given in PredBB.
881	// MinCommonTailLength - Except for the special cases below, tail-merge if
882	// there are at least this many instructions in common.
883	bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
884	MachineBasicBlock *PredBB,
885	unsigned MinCommonTailLength) {
886	bool MadeChange = false;
887
888	LLVM_DEBUG(
889	dbgs() << "\nTryTailMergeBlocks: ";
890	for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i) dbgs()
891	<< printMBBReference(*MergePotentials[i].getBlock())
892	<< (i == e - 1 ? "" : ", ");
893	dbgs() << "\n"; if (SuccBB) {
894	dbgs() << " with successor " << printMBBReference(*SuccBB) << '\n';
895	if (PredBB)
896	dbgs() << " which has fall-through from "
897	<< printMBBReference(*PredBB) << "\n";
898	} dbgs() << "Looking for common tails of at least "
899	<< MinCommonTailLength << " instruction"
900	<< (MinCommonTailLength == 1 ? "" : "s") << '\n';);
901
902	// Sort by hash value so that blocks with identical end sequences sort
903	// together.
904	array_pod_sort(MergePotentials.begin(), MergePotentials.end());
905
906	// Walk through equivalence sets looking for actual exact matches.
907	while (MergePotentials.size() > 1) {
908	unsigned CurHash = MergePotentials.back().getHash();
909
910	// Build SameTails, identifying the set of blocks with this hash code
911	// and with the maximum number of instructions in common.
912	unsigned maxCommonTailLength = ComputeSameTails(CurHash,
913	MinCommonTailLength,
914	SuccBB, PredBB);
915
916	// If we didn't find any pair that has at least MinCommonTailLength
917	// instructions in common, remove all blocks with this hash code and retry.
918	if (SameTails.empty()) {
919	RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
920	continue;
921	}
922
923	// If one of the blocks is the entire common tail (and is not the entry
924	// block/an EH pad, which we can't jump to), we can treat all blocks with
925	// this same tail at once. Use PredBB if that is one of the possibilities,
926	// as that will not introduce any extra branches.
927	MachineBasicBlock *EntryBB =
928	&MergePotentials.front().getBlock()->getParent()->front();
929	unsigned commonTailIndex = SameTails.size();
930	// If there are two blocks, check to see if one can be made to fall through
931	// into the other.
932	if (SameTails.size() == 2 &&
933	SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) &&
934	SameTails[1].tailIsWholeBlock() && !SameTails[1].getBlock()->isEHPad())
935	commonTailIndex = 1;
936	else if (SameTails.size() == 2 &&
937	SameTails[1].getBlock()->isLayoutSuccessor(
938	SameTails[0].getBlock()) &&
939	SameTails[0].tailIsWholeBlock() &&
940	!SameTails[0].getBlock()->isEHPad())
941	commonTailIndex = 0;
942	else {
943	// Otherwise just pick one, favoring the fall-through predecessor if
944	// there is one.
945	for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
946	MachineBasicBlock *MBB = SameTails[i].getBlock();
947	if ((MBB == EntryBB || MBB->isEHPad()) &&
948	SameTails[i].tailIsWholeBlock())
949	continue;
950	if (MBB == PredBB) {
951	commonTailIndex = i;
952	break;
953	}
954	if (SameTails[i].tailIsWholeBlock())
955	commonTailIndex = i;
956	}
957	}
958
959	if (commonTailIndex == SameTails.size() ||
960	(SameTails[commonTailIndex].getBlock() == PredBB &&
961	!SameTails[commonTailIndex].tailIsWholeBlock())) {
962	// None of the blocks consist entirely of the common tail.
963	// Split a block so that one does.
964	if (!CreateCommonTailOnlyBlock(PredBB, SuccBB,
965	maxCommonTailLength, commonTailIndex)) {
966	RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
967	continue;
968	}
969	}
970
971	MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
972
973	// Recompute common tail MBB's edge weights and block frequency.
974	setCommonTailEdgeWeights(*MBB);
975
976	// Merge debug locations, MMOs and undef flags across identical instructions
977	// for common tail.
978	mergeCommonTails(commonTailIndex);
979
980	// MBB is common tail. Adjust all other BB's to jump to this one.
981	// Traversal must be forwards so erases work.
982	LLVM_DEBUG(dbgs() << "\nUsing common tail in " << printMBBReference(*MBB)
983	<< " for ");
984	for (unsigned int i=0, e = SameTails.size(); i != e; ++i) {
985	if (commonTailIndex == i)
986	continue;
987	LLVM_DEBUG(dbgs() << printMBBReference(*SameTails[i].getBlock())
988	<< (i == e - 1 ? "" : ", "));
989	// Hack the end off BB i, making it jump to BB commonTailIndex instead.
990	replaceTailWithBranchTo(SameTails[i].getTailStartPos(), *MBB);
991	// BB i is no longer a predecessor of SuccBB; remove it from the worklist.
992	MergePotentials.erase(SameTails[i].getMPIter());
993	}
994	LLVM_DEBUG(dbgs() << "\n");
995	// We leave commonTailIndex in the worklist in case there are other blocks
996	// that match it with a smaller number of instructions.
997	MadeChange = true;
998	}
999	return MadeChange;
1000	}
1001
1002	bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
1003	bool MadeChange = false;
1004	if (!EnableTailMerge)
1005	return MadeChange;
1006
1007	// First find blocks with no successors.
1008	// Block placement may create new tail merging opportunities for these blocks.
1009	MergePotentials.clear();
1010	for (MachineBasicBlock &MBB : MF) {
1011	if (MergePotentials.size() == TailMergeThreshold)
1012	break;
1013	if (!TriedMerging.count(&MBB) && MBB.succ_empty())
1014	MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(MBB), &MBB));
1015	}
1016
1017	// If this is a large problem, avoid visiting the same basic blocks
1018	// multiple times.
1019	if (MergePotentials.size() == TailMergeThreshold)
1020	for (const MergePotentialsElt &Elt : MergePotentials)
1021	TriedMerging.insert(Elt.getBlock());
1022
1023	// See if we can do any tail merging on those.
1024	if (MergePotentials.size() >= 2)
1025	MadeChange |= TryTailMergeBlocks(nullptr, nullptr, MinCommonTailLength);
1026
1027	// Look at blocks (IBB) with multiple predecessors (PBB).
1028	// We change each predecessor to a canonical form, by
1029	// (1) temporarily removing any unconditional branch from the predecessor
1030	// to IBB, and
1031	// (2) alter conditional branches so they branch to the other block
1032	// not IBB; this may require adding back an unconditional branch to IBB
1033	// later, where there wasn't one coming in. E.g.
1034	// Bcc IBB
1035	// fallthrough to QBB
1036	// here becomes
1037	// Bncc QBB
1038	// with a conceptual B to IBB after that, which never actually exists.
1039	// With those changes, we see whether the predecessors' tails match,
1040	// and merge them if so. We change things out of canonical form and
1041	// back to the way they were later in the process. (OptimizeBranches
1042	// would undo some of this, but we can't use it, because we'd get into
1043	// a compile-time infinite loop repeatedly doing and undoing the same
1044	// transformations.)
1045
1046	for (MachineFunction::iterator I = std::next(MF.begin()), E = MF.end();
1047	I != E; ++I) {
1048	if (I->pred_size() < 2) continue;
1049	SmallPtrSet<MachineBasicBlock *, 8> UniquePreds;
1050	MachineBasicBlock *IBB = &*I;
1051	MachineBasicBlock *PredBB = &*std::prev(I);
1052	MergePotentials.clear();
1053	MachineLoop *ML;
1054
1055	// Bail if merging after placement and IBB is the loop header because
1056	// -- If merging predecessors that belong to the same loop as IBB, the
1057	// common tail of merged predecessors may become the loop top if block
1058	// placement is called again and the predecessors may branch to this common
1059	// tail and require more branches. This can be relaxed if
1060	// MachineBlockPlacement::findBestLoopTop is more flexible.
1061	// --If merging predecessors that do not belong to the same loop as IBB, the
1062	// loop info of IBB's loop and the other loops may be affected. Calling the
1063	// block placement again may make big change to the layout and eliminate the
1064	// reason to do tail merging here.
1065	if (AfterBlockPlacement && MLI) {
1066	ML = MLI->getLoopFor(IBB);
1067	if (ML && IBB == ML->getHeader())
1068	continue;
1069	}
1070
1071	for (MachineBasicBlock *PBB : I->predecessors()) {
1072	if (MergePotentials.size() == TailMergeThreshold)
1073	break;
1074
1075	if (TriedMerging.count(PBB))
1076	continue;
1077
1078	// Skip blocks that loop to themselves, can't tail merge these.
1079	if (PBB == IBB)
1080	continue;
1081
1082	// Visit each predecessor only once.
1083	if (!UniquePreds.insert(PBB).second)
1084	continue;
1085
1086	// Skip blocks which may jump to a landing pad or jump from an asm blob.
1087	// Can't tail merge these.
1088	if (PBB->hasEHPadSuccessor() || PBB->mayHaveInlineAsmBr())
1089	continue;
1090
1091	// After block placement, only consider predecessors that belong to the
1092	// same loop as IBB. The reason is the same as above when skipping loop
1093	// header.
1094	if (AfterBlockPlacement && MLI)
1095	if (ML != MLI->getLoopFor(PBB))
1096	continue;
1097
1098	MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
1099	SmallVector<MachineOperand, 4> Cond;
1100	if (!TII->analyzeBranch(*PBB, TBB, FBB, Cond, true)) {
1101	// Failing case: IBB is the target of a cbr, and we cannot reverse the
1102	// branch.
1103	SmallVector<MachineOperand, 4> NewCond(Cond);
1104	if (!Cond.empty() && TBB == IBB) {
1105	if (TII->reverseBranchCondition(NewCond))
1106	continue;
1107	// This is the QBB case described above
1108	if (!FBB) {
1109	auto Next = ++PBB->getIterator();
1110	if (Next != MF.end())
1111	FBB = &*Next;
1112	}
1113	}
1114
1115	// Remove the unconditional branch at the end, if any.
1116	if (TBB && (Cond.empty() || FBB)) {
1117	DebugLoc dl = PBB->findBranchDebugLoc();
1118	TII->removeBranch(*PBB);
1119	if (!Cond.empty())
1120	// reinsert conditional branch only, for now
1121	TII->insertBranch(*PBB, (TBB == IBB) ? FBB : TBB, nullptr,
1122	NewCond, dl);
1123	}
1124
1125	MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(*PBB), PBB));
1126	}
1127	}
1128
1129	// If this is a large problem, avoid visiting the same basic blocks multiple
1130	// times.
1131	if (MergePotentials.size() == TailMergeThreshold)
1132	for (MergePotentialsElt &Elt : MergePotentials)
1133	TriedMerging.insert(Elt.getBlock());
1134
1135	if (MergePotentials.size() >= 2)
1136	MadeChange |= TryTailMergeBlocks(IBB, PredBB, MinCommonTailLength);
1137
1138	// Reinsert an unconditional branch if needed. The 1 below can occur as a
1139	// result of removing blocks in TryTailMergeBlocks.
1140	PredBB = &*std::prev(I); // this may have been changed in TryTailMergeBlocks
1141	if (MergePotentials.size() == 1 &&
1142	MergePotentials.begin()->getBlock() != PredBB)
1143	FixTail(MergePotentials.begin()->getBlock(), IBB, TII);
1144	}
1145
1146	return MadeChange;
1147	}
1148
1149	void BranchFolder::setCommonTailEdgeWeights(MachineBasicBlock &TailMBB) {
1150	SmallVector<BlockFrequency, 2> EdgeFreqLs(TailMBB.succ_size());
1151	BlockFrequency AccumulatedMBBFreq;
1152
1153	// Aggregate edge frequency of successor edge j:
1154	// edgeFreq(j) = sum (freq(bb) * edgeProb(bb, j)),
1155	// where bb is a basic block that is in SameTails.
1156	for (const auto &Src : SameTails) {
1157	const MachineBasicBlock *SrcMBB = Src.getBlock();
1158	BlockFrequency BlockFreq = MBBFreqInfo.getBlockFreq(SrcMBB);
1159	AccumulatedMBBFreq += BlockFreq;
1160
1161	// It is not necessary to recompute edge weights if TailBB has less than two
1162	// successors.
1163	if (TailMBB.succ_size() <= 1)
1164	continue;
1165
1166	auto EdgeFreq = EdgeFreqLs.begin();
1167
1168	for (auto SuccI = TailMBB.succ_begin(), SuccE = TailMBB.succ_end();
1169	SuccI != SuccE; ++SuccI, ++EdgeFreq)
1170	*EdgeFreq += BlockFreq * MBPI.getEdgeProbability(SrcMBB, *SuccI);
1171	}
1172
1173	MBBFreqInfo.setBlockFreq(&TailMBB, AccumulatedMBBFreq);
1174
1175	if (TailMBB.succ_size() <= 1)
1176	return;
1177
1178	auto SumEdgeFreq =
1179	std::accumulate(EdgeFreqLs.begin(), EdgeFreqLs.end(), BlockFrequency(0))
1180	.getFrequency();
1181	auto EdgeFreq = EdgeFreqLs.begin();
1182
1183	if (SumEdgeFreq > 0) {
1184	for (auto SuccI = TailMBB.succ_begin(), SuccE = TailMBB.succ_end();
1185	SuccI != SuccE; ++SuccI, ++EdgeFreq) {
1186	auto Prob = BranchProbability::getBranchProbability(
1187	EdgeFreq->getFrequency(), SumEdgeFreq);
1188	TailMBB.setSuccProbability(SuccI, Prob);
1189	}
1190	}
1191	}
1192
1193	//===----------------------------------------------------------------------===//
1194	// Branch Optimization
1195	//===----------------------------------------------------------------------===//
1196
1197	bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
1198	bool MadeChange = false;
1199
1200	// Make sure blocks are numbered in order
1201	MF.RenumberBlocks();
1202	// Renumbering blocks alters EH scope membership, recalculate it.
1203	EHScopeMembership = getEHScopeMembership(MF);
1204
1205	for (MachineBasicBlock &MBB :
1206	llvm::make_early_inc_range(llvm::drop_begin(MF))) {
1207	MadeChange |= OptimizeBlock(&MBB);
1208
1209	// If it is dead, remove it.
1210	if (MBB.pred_empty()) {
1211	RemoveDeadBlock(&MBB);
1212	MadeChange = true;
1213	++NumDeadBlocks;
1214	}
1215	}
1216
1217	return MadeChange;
1218	}
1219
1220	// Blocks should be considered empty if they contain only debug info;
1221	// else the debug info would affect codegen.
1222	static bool IsEmptyBlock(MachineBasicBlock *MBB) {
1223	return MBB->getFirstNonDebugInstr(true) == MBB->end();
1224	}
1225
1226	// Blocks with only debug info and branches should be considered the same
1227	// as blocks with only branches.
1228	static bool IsBranchOnlyBlock(MachineBasicBlock *MBB) {
1229	MachineBasicBlock::iterator I = MBB->getFirstNonDebugInstr();
1230	assert(I != MBB->end() && "empty block!");
1231	return I->isBranch();
1232	}
1233
1234	/// IsBetterFallthrough - Return true if it would be clearly better to
1235	/// fall-through to MBB1 than to fall through into MBB2. This has to return
1236	/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will
1237	/// result in infinite loops.
1238	static bool IsBetterFallthrough(MachineBasicBlock *MBB1,
1239	MachineBasicBlock *MBB2) {
1240	assert(MBB1 && MBB2 && "Unknown MachineBasicBlock");
1241
1242	// Right now, we use a simple heuristic. If MBB2 ends with a call, and
1243	// MBB1 doesn't, we prefer to fall through into MBB1. This allows us to
1244	// optimize branches that branch to either a return block or an assert block
1245	// into a fallthrough to the return.
1246	MachineBasicBlock::iterator MBB1I = MBB1->getLastNonDebugInstr();
1247	MachineBasicBlock::iterator MBB2I = MBB2->getLastNonDebugInstr();
1248	if (MBB1I == MBB1->end() || MBB2I == MBB2->end())
1249	return false;
1250
1251	// If there is a clear successor ordering we make sure that one block
1252	// will fall through to the next
1253	if (MBB1->isSuccessor(MBB2)) return true;
1254	if (MBB2->isSuccessor(MBB1)) return false;
1255
1256	return MBB2I->isCall() && !MBB1I->isCall();
1257	}
1258
1259	/// getBranchDebugLoc - Find and return, if any, the DebugLoc of the branch
1260	/// instructions on the block.
1261	static DebugLoc getBranchDebugLoc(MachineBasicBlock &MBB) {
1262	MachineBasicBlock::iterator I = MBB.getLastNonDebugInstr();
1263	if (I != MBB.end() && I->isBranch())
1264	return I->getDebugLoc();
1265	return DebugLoc();
1266	}
1267
1268	static void copyDebugInfoToPredecessor(const TargetInstrInfo *TII,
1269	MachineBasicBlock &MBB,
1270	MachineBasicBlock &PredMBB) {
1271	auto InsertBefore = PredMBB.getFirstTerminator();
1272	for (MachineInstr &MI : MBB.instrs())
1273	if (MI.isDebugInstr()) {
1274	TII->duplicate(PredMBB, InsertBefore, MI);
1275	LLVM_DEBUG(dbgs() << "Copied debug entity from empty block to pred: "
1276	<< MI);
1277	}
1278	}
1279
1280	static void copyDebugInfoToSuccessor(const TargetInstrInfo *TII,
1281	MachineBasicBlock &MBB,
1282	MachineBasicBlock &SuccMBB) {
1283	auto InsertBefore = SuccMBB.SkipPHIsAndLabels(SuccMBB.begin());
1284	for (MachineInstr &MI : MBB.instrs())
1285	if (MI.isDebugInstr()) {
1286	TII->duplicate(SuccMBB, InsertBefore, MI);
1287	LLVM_DEBUG(dbgs() << "Copied debug entity from empty block to succ: "
1288	<< MI);
1289	}
1290	}
1291
1292	// Try to salvage DBG_VALUE instructions from an otherwise empty block. If such
1293	// a basic block is removed we would lose the debug information unless we have
1294	// copied the information to a predecessor/successor.
1295	//
1296	// TODO: This function only handles some simple cases. An alternative would be
1297	// to run a heavier analysis, such as the LiveDebugValues pass, before we do
1298	// branch folding.
1299	static void salvageDebugInfoFromEmptyBlock(const TargetInstrInfo *TII,
1300	MachineBasicBlock &MBB) {
1301	assert(IsEmptyBlock(&MBB) && "Expected an empty block (except debug info).");
1302	// If this MBB is the only predecessor of a successor it is legal to copy
1303	// DBG_VALUE instructions to the beginning of the successor.
1304	for (MachineBasicBlock *SuccBB : MBB.successors())
1305	if (SuccBB->pred_size() == 1)
1306	copyDebugInfoToSuccessor(TII, MBB, *SuccBB);
1307	// If this MBB is the only successor of a predecessor it is legal to copy the
1308	// DBG_VALUE instructions to the end of the predecessor (just before the
1309	// terminators, assuming that the terminator isn't affecting the DBG_VALUE).
1310	for (MachineBasicBlock *PredBB : MBB.predecessors())
1311	if (PredBB->succ_size() == 1)
1312	copyDebugInfoToPredecessor(TII, MBB, *PredBB);
1313	}
1314
1315	bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
1316	bool MadeChange = false;
1317	MachineFunction &MF = *MBB->getParent();
1318	ReoptimizeBlock:
1319
1320	MachineFunction::iterator FallThrough = MBB->getIterator();
1321	++FallThrough;
1322
1323	// Make sure MBB and FallThrough belong to the same EH scope.
1324	bool SameEHScope = true;
1325	if (!EHScopeMembership.empty() && FallThrough != MF.end()) {
1326	auto MBBEHScope = EHScopeMembership.find(MBB);
1327	assert(MBBEHScope != EHScopeMembership.end());
1328	auto FallThroughEHScope = EHScopeMembership.find(&*FallThrough);
1329	assert(FallThroughEHScope != EHScopeMembership.end());
1330	SameEHScope = MBBEHScope->second == FallThroughEHScope->second;
1331	}
1332
1333	// Analyze the branch in the current block. As a side-effect, this may cause
1334	// the block to become empty.
1335	MachineBasicBlock *CurTBB = nullptr, *CurFBB = nullptr;
1336	SmallVector<MachineOperand, 4> CurCond;
1337	bool CurUnAnalyzable =
1338	TII->analyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true);
1339
1340	// If this block is empty, make everyone use its fall-through, not the block
1341	// explicitly. Landing pads should not do this since the landing-pad table
1342	// points to this block. Blocks with their addresses taken shouldn't be
1343	// optimized away.
1344	if (IsEmptyBlock(MBB) && !MBB->isEHPad() && !MBB->hasAddressTaken() &&
1345	SameEHScope) {
1346	salvageDebugInfoFromEmptyBlock(TII, *MBB);
1347	// Dead block? Leave for cleanup later.
1348	if (MBB->pred_empty()) return MadeChange;
1349
1350	if (FallThrough == MF.end()) {
1351	// TODO: Simplify preds to not branch here if possible!
1352	} else if (FallThrough->isEHPad()) {
1353	// Don't rewrite to a landing pad fallthough. That could lead to the case
1354	// where a BB jumps to more than one landing pad.
1355	// TODO: Is it ever worth rewriting predecessors which don't already
1356	// jump to a landing pad, and so can safely jump to the fallthrough?
1357	} else if (MBB->isSuccessor(&*FallThrough)) {
1358	// Rewrite all predecessors of the old block to go to the fallthrough
1359	// instead.
1360	while (!MBB->pred_empty()) {
1361	MachineBasicBlock *Pred = *(MBB->pred_end()-1);
1362	Pred->ReplaceUsesOfBlockWith(MBB, &*FallThrough);
1363	}
1364	// If MBB was the target of a jump table, update jump tables to go to the
1365	// fallthrough instead.
1366	if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
1367	MJTI->ReplaceMBBInJumpTables(MBB, &*FallThrough);
1368	MadeChange = true;
1369	}
1370	return MadeChange;
1371	}
1372
1373	// Check to see if we can simplify the terminator of the block before this
1374	// one.
1375	MachineBasicBlock &PrevBB = *std::prev(MachineFunction::iterator(MBB));
1376
1377	MachineBasicBlock *PriorTBB = nullptr, *PriorFBB = nullptr;
1378	SmallVector<MachineOperand, 4> PriorCond;
1379	bool PriorUnAnalyzable =
1380	TII->analyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
1381	if (!PriorUnAnalyzable) {
1382	// If the previous branch is conditional and both conditions go to the same
1383	// destination, remove the branch, replacing it with an unconditional one or
1384	// a fall-through.
1385	if (PriorTBB && PriorTBB == PriorFBB) {
1386	DebugLoc dl = getBranchDebugLoc(PrevBB);
1387	TII->removeBranch(PrevBB);
1388	PriorCond.clear();
1389	if (PriorTBB != MBB)
1390	TII->insertBranch(PrevBB, PriorTBB, nullptr, PriorCond, dl);
1391	MadeChange = true;
1392	++NumBranchOpts;
1393	goto ReoptimizeBlock;
1394	}
1395
1396	// If the previous block unconditionally falls through to this block and
1397	// this block has no other predecessors, move the contents of this block
1398	// into the prior block. This doesn't usually happen when SimplifyCFG
1399	// has been used, but it can happen if tail merging splits a fall-through
1400	// predecessor of a block.
1401	// This has to check PrevBB->succ_size() because EH edges are ignored by
1402	// analyzeBranch.
1403	if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 &&
1404	PrevBB.succ_size() == 1 &&
1405	!MBB->hasAddressTaken() && !MBB->isEHPad()) {
1406	LLVM_DEBUG(dbgs() << "\nMerging into block: " << PrevBB
1407	<< "From MBB: " << *MBB);
1408	// Remove redundant DBG_VALUEs first.
1409	if (!PrevBB.empty()) {
1410	MachineBasicBlock::iterator PrevBBIter = PrevBB.end();
1411	--PrevBBIter;
1412	MachineBasicBlock::iterator MBBIter = MBB->begin();
1413	// Check if DBG_VALUE at the end of PrevBB is identical to the
1414	// DBG_VALUE at the beginning of MBB.
1415	while (PrevBBIter != PrevBB.begin() && MBBIter != MBB->end()
1416	&& PrevBBIter->isDebugInstr() && MBBIter->isDebugInstr()) {
1417	if (!MBBIter->isIdenticalTo(*PrevBBIter))
1418	break;
1419	MachineInstr &DuplicateDbg = *MBBIter;
1420	++MBBIter; -- PrevBBIter;
1421	DuplicateDbg.eraseFromParent();
1422	}
1423	}
1424	PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end());
1425	PrevBB.removeSuccessor(PrevBB.succ_begin());
1426	assert(PrevBB.succ_empty());
1427	PrevBB.transferSuccessors(MBB);
1428	MadeChange = true;
1429	return MadeChange;
1430	}
1431
1432	// If the previous branch *only* branches to *this* block (conditional or
1433	// not) remove the branch.
1434	if (PriorTBB == MBB && !PriorFBB) {
1435	TII->removeBranch(PrevBB);
1436	MadeChange = true;
1437	++NumBranchOpts;
1438	goto ReoptimizeBlock;
1439	}
1440
1441	// If the prior block branches somewhere else on the condition and here if
1442	// the condition is false, remove the uncond second branch.
1443	if (PriorFBB == MBB) {
1444	DebugLoc dl = getBranchDebugLoc(PrevBB);
1445	TII->removeBranch(PrevBB);
1446	TII->insertBranch(PrevBB, PriorTBB, nullptr, PriorCond, dl);
1447	MadeChange = true;
1448	++NumBranchOpts;
1449	goto ReoptimizeBlock;
1450	}
1451
1452	// If the prior block branches here on true and somewhere else on false, and
1453	// if the branch condition is reversible, reverse the branch to create a
1454	// fall-through.
1455	if (PriorTBB == MBB) {
1456	SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1457	if (!TII->reverseBranchCondition(NewPriorCond)) {
1458	DebugLoc dl = getBranchDebugLoc(PrevBB);
1459	TII->removeBranch(PrevBB);
1460	TII->insertBranch(PrevBB, PriorFBB, nullptr, NewPriorCond, dl);
1461	MadeChange = true;
1462	++NumBranchOpts;
1463	goto ReoptimizeBlock;
1464	}
1465	}
1466
1467	// If this block has no successors (e.g. it is a return block or ends with
1468	// a call to a no-return function like abort or __cxa_throw) and if the pred
1469	// falls through into this block, and if it would otherwise fall through
1470	// into the block after this, move this block to the end of the function.
1471	//
1472	// We consider it more likely that execution will stay in the function (e.g.
1473	// due to loops) than it is to exit it. This asserts in loops etc, moving
1474	// the assert condition out of the loop body.
1475	if (MBB->succ_empty() && !PriorCond.empty() && !PriorFBB &&
1476	MachineFunction::iterator(PriorTBB) == FallThrough &&
1477	!MBB->canFallThrough()) {
1478	bool DoTransform = true;
1479
1480	// We have to be careful that the succs of PredBB aren't both no-successor
1481	// blocks. If neither have successors and if PredBB is the second from
1482	// last block in the function, we'd just keep swapping the two blocks for
1483	// last. Only do the swap if one is clearly better to fall through than
1484	// the other.
1485	if (FallThrough == --MF.end() &&
1486	!IsBetterFallthrough(PriorTBB, MBB))
1487	DoTransform = false;
1488
1489	if (DoTransform) {
1490	// Reverse the branch so we will fall through on the previous true cond.
1491	SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1492	if (!TII->reverseBranchCondition(NewPriorCond)) {
1493	LLVM_DEBUG(dbgs() << "\nMoving MBB: " << *MBB
1494	<< "To make fallthrough to: " << *PriorTBB << "\n");
1495
1496	DebugLoc dl = getBranchDebugLoc(PrevBB);
1497	TII->removeBranch(PrevBB);
1498	TII->insertBranch(PrevBB, MBB, nullptr, NewPriorCond, dl);
1499
1500	// Move this block to the end of the function.
1501	MBB->moveAfter(&MF.back());
1502	MadeChange = true;
1503	++NumBranchOpts;
1504	return MadeChange;
1505	}
1506	}
1507	}
1508	}
1509
1510	bool OptForSize =
1511	MF.getFunction().hasOptSize() ||
1512	llvm::shouldOptimizeForSize(MBB, PSI, &MBBFreqInfo);
1513	if (!IsEmptyBlock(MBB) && MBB->pred_size() == 1 && OptForSize) {
1514	// Changing "Jcc foo; foo: jmp bar;" into "Jcc bar;" might change the branch
1515	// direction, thereby defeating careful block placement and regressing
1516	// performance. Therefore, only consider this for optsize functions.
1517	MachineInstr &TailCall = *MBB->getFirstNonDebugInstr();
1518	if (TII->isUnconditionalTailCall(TailCall)) {
1519	MachineBasicBlock *Pred = *MBB->pred_begin();
1520	MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
1521	SmallVector<MachineOperand, 4> PredCond;
1522	bool PredAnalyzable =
1523	!TII->analyzeBranch(*Pred, PredTBB, PredFBB, PredCond, true);
1524
1525	if (PredAnalyzable && !PredCond.empty() && PredTBB == MBB &&
1526	PredTBB != PredFBB) {
1527	// The predecessor has a conditional branch to this block which consists
1528	// of only a tail call. Try to fold the tail call into the conditional
1529	// branch.
1530	if (TII->canMakeTailCallConditional(PredCond, TailCall)) {
1531	// TODO: It would be nice if analyzeBranch() could provide a pointer
1532	// to the branch instruction so replaceBranchWithTailCall() doesn't
1533	// have to search for it.
1534	TII->replaceBranchWithTailCall(*Pred, PredCond, TailCall);
1535	++NumTailCalls;
1536	Pred->removeSuccessor(MBB);
1537	MadeChange = true;
1538	return MadeChange;
1539	}
1540	}
1541	// If the predecessor is falling through to this block, we could reverse
1542	// the branch condition and fold the tail call into that. However, after
1543	// that we might have to re-arrange the CFG to fall through to the other
1544	// block and there is a high risk of regressing code size rather than
1545	// improving it.
1546	}
1547	}
1548
1549	if (!CurUnAnalyzable) {
1550	// If this is a two-way branch, and the FBB branches to this block, reverse
1551	// the condition so the single-basic-block loop is faster. Instead of:
1552	// Loop: xxx; jcc Out; jmp Loop
1553	// we want:
1554	// Loop: xxx; jncc Loop; jmp Out
1555	if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) {
1556	SmallVector<MachineOperand, 4> NewCond(CurCond);
1557	if (!TII->reverseBranchCondition(NewCond)) {
1558	DebugLoc dl = getBranchDebugLoc(*MBB);
1559	TII->removeBranch(*MBB);
1560	TII->insertBranch(*MBB, CurFBB, CurTBB, NewCond, dl);
1561	MadeChange = true;
1562	++NumBranchOpts;
1563	goto ReoptimizeBlock;
1564	}
1565	}
1566
1567	// If this branch is the only thing in its block, see if we can forward
1568	// other blocks across it.
1569	if (CurTBB && CurCond.empty() && !CurFBB &&
1570	IsBranchOnlyBlock(MBB) && CurTBB != MBB &&
1571	!MBB->hasAddressTaken() && !MBB->isEHPad()) {
1572	DebugLoc dl = getBranchDebugLoc(*MBB);
1573	// This block may contain just an unconditional branch. Because there can
1574	// be 'non-branch terminators' in the block, try removing the branch and
1575	// then seeing if the block is empty.
1576	TII->removeBranch(*MBB);
1577	// If the only things remaining in the block are debug info, remove these
1578	// as well, so this will behave the same as an empty block in non-debug
1579	// mode.
1580	if (IsEmptyBlock(MBB)) {
1581	// Make the block empty, losing the debug info (we could probably
1582	// improve this in some cases.)
1583	MBB->erase(MBB->begin(), MBB->end());
1584	}
1585	// If this block is just an unconditional branch to CurTBB, we can
1586	// usually completely eliminate the block. The only case we cannot
1587	// completely eliminate the block is when the block before this one
1588	// falls through into MBB and we can't understand the prior block's branch
1589	// condition.
1590	if (MBB->empty()) {
1591	bool PredHasNoFallThrough = !PrevBB.canFallThrough();
1592	if (PredHasNoFallThrough || !PriorUnAnalyzable ||
1593	!PrevBB.isSuccessor(MBB)) {
1594	// If the prior block falls through into us, turn it into an
1595	// explicit branch to us to make updates simpler.
1596	if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) &&
1597	PriorTBB != MBB && PriorFBB != MBB) {
1598	if (!PriorTBB) {
1599	assert(PriorCond.empty() && !PriorFBB &&
1600	"Bad branch analysis");
1601	PriorTBB = MBB;
1602	} else {
1603	assert(!PriorFBB && "Machine CFG out of date!");
1604	PriorFBB = MBB;
1605	}
1606	DebugLoc pdl = getBranchDebugLoc(PrevBB);
1607	TII->removeBranch(PrevBB);
1608	TII->insertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, pdl);
1609	}
1610
1611	// Iterate through all the predecessors, revectoring each in-turn.
1612	size_t PI = 0;
1613	bool DidChange = false;
1614	bool HasBranchToSelf = false;
1615	while(PI != MBB->pred_size()) {
1616	MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI);
1617	if (PMBB == MBB) {
1618	// If this block has an uncond branch to itself, leave it.
1619	++PI;
1620	HasBranchToSelf = true;
1621	} else {
1622	DidChange = true;
1623	PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB);
1624	// If this change resulted in PMBB ending in a conditional
1625	// branch where both conditions go to the same destination,
1626	// change this to an unconditional branch.
1627	MachineBasicBlock *NewCurTBB = nullptr, *NewCurFBB = nullptr;
1628	SmallVector<MachineOperand, 4> NewCurCond;
1629	bool NewCurUnAnalyzable = TII->analyzeBranch(
1630	*PMBB, NewCurTBB, NewCurFBB, NewCurCond, true);
1631	if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) {
1632	DebugLoc pdl = getBranchDebugLoc(*PMBB);
1633	TII->removeBranch(*PMBB);
1634	NewCurCond.clear();
1635	TII->insertBranch(*PMBB, NewCurTBB, nullptr, NewCurCond, pdl);
1636	MadeChange = true;
1637	++NumBranchOpts;
1638	}
1639	}
1640	}
1641
1642	// Change any jumptables to go to the new MBB.
1643	if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
1644	MJTI->ReplaceMBBInJumpTables(MBB, CurTBB);
1645	if (DidChange) {
1646	++NumBranchOpts;
1647	MadeChange = true;
1648	if (!HasBranchToSelf) return MadeChange;
1649	}
1650	}
1651	}
1652
1653	// Add the branch back if the block is more than just an uncond branch.
1654	TII->insertBranch(*MBB, CurTBB, nullptr, CurCond, dl);
1655	}
1656	}
1657
1658	// If the prior block doesn't fall through into this block, and if this
1659	// block doesn't fall through into some other block, see if we can find a
1660	// place to move this block where a fall-through will happen.
1661	if (!PrevBB.canFallThrough()) {
1662	// Now we know that there was no fall-through into this block, check to
1663	// see if it has a fall-through into its successor.
1664	bool CurFallsThru = MBB->canFallThrough();
1665
1666	if (!MBB->isEHPad()) {
1667	// Check all the predecessors of this block. If one of them has no fall
1668	// throughs, and analyzeBranch thinks it _could_ fallthrough to this
1669	// block, move this block right after it.
1670	for (MachineBasicBlock *PredBB : MBB->predecessors()) {
1671	// Analyze the branch at the end of the pred.
1672	MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
1673	SmallVector<MachineOperand, 4> PredCond;
1674	if (PredBB != MBB && !PredBB->canFallThrough() &&
1675	!TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true) &&
1676	(PredTBB == MBB || PredFBB == MBB) &&
1677	(!CurFallsThru || !CurTBB || !CurFBB) &&
1678	(!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
1679	// If the current block doesn't fall through, just move it.
1680	// If the current block can fall through and does not end with a
1681	// conditional branch, we need to append an unconditional jump to
1682	// the (current) next block. To avoid a possible compile-time
1683	// infinite loop, move blocks only backward in this case.
1684	// Also, if there are already 2 branches here, we cannot add a third;
1685	// this means we have the case
1686	// Bcc next
1687	// B elsewhere
1688	// next:
1689	if (CurFallsThru) {
1690	MachineBasicBlock *NextBB = &*std::next(MBB->getIterator());
1691	CurCond.clear();
1692	TII->insertBranch(*MBB, NextBB, nullptr, CurCond, DebugLoc());
1693	}
1694	MBB->moveAfter(PredBB);
1695	MadeChange = true;
1696	goto ReoptimizeBlock;
1697	}
1698	}
1699	}
1700
1701	if (!CurFallsThru) {
1702	// Check analyzable branch-successors to see if we can move this block
1703	// before one.
1704	if (!CurUnAnalyzable) {
1705	for (MachineBasicBlock *SuccBB : {CurFBB, CurTBB}) {
1706	if (!SuccBB)
1707	continue;
1708	// Analyze the branch at the end of the block before the succ.
1709	MachineFunction::iterator SuccPrev = --SuccBB->getIterator();
1710
1711	// If this block doesn't already fall-through to that successor, and
1712	// if the succ doesn't already have a block that can fall through into
1713	// it, we can arrange for the fallthrough to happen.
1714	if (SuccBB != MBB && &*SuccPrev != MBB &&
1715	!SuccPrev->canFallThrough()) {
1716	MBB->moveBefore(SuccBB);
1717	MadeChange = true;
1718	goto ReoptimizeBlock;
1719	}
1720	}
1721	}
1722
1723	// Okay, there is no really great place to put this block. If, however,
1724	// the block before this one would be a fall-through if this block were
1725	// removed, move this block to the end of the function. There is no real
1726	// advantage in "falling through" to an EH block, so we don't want to
1727	// perform this transformation for that case.
1728	//
1729	// Also, Windows EH introduced the possibility of an arbitrary number of
1730	// successors to a given block. The analyzeBranch call does not consider
1731	// exception handling and so we can get in a state where a block
1732	// containing a call is followed by multiple EH blocks that would be
1733	// rotated infinitely at the end of the function if the transformation
1734	// below were performed for EH "FallThrough" blocks. Therefore, even if
1735	// that appears not to be happening anymore, we should assume that it is
1736	// possible and not remove the "!FallThrough()->isEHPad" condition below.
1737	MachineBasicBlock *PrevTBB = nullptr, *PrevFBB = nullptr;
1738	SmallVector<MachineOperand, 4> PrevCond;
1739	if (FallThrough != MF.end() &&
1740	!FallThrough->isEHPad() &&
1741	!TII->analyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) &&
1742	PrevBB.isSuccessor(&*FallThrough)) {
1743	MBB->moveAfter(&MF.back());
1744	MadeChange = true;
1745	return MadeChange;
1746	}
1747	}
1748	}
1749
1750	return MadeChange;
1751	}
1752
1753	//===----------------------------------------------------------------------===//
1754	// Hoist Common Code
1755	//===----------------------------------------------------------------------===//
1756
1757	bool BranchFolder::HoistCommonCode(MachineFunction &MF) {
1758	bool MadeChange = false;
1759	for (MachineBasicBlock &MBB : llvm::make_early_inc_range(MF))
1760	MadeChange |= HoistCommonCodeInSuccs(&MBB);
1761
1762	return MadeChange;
1763	}
1764
1765	/// findFalseBlock - BB has a fallthrough. Find its 'false' successor given
1766	/// its 'true' successor.
1767	static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB,
1768	MachineBasicBlock *TrueBB) {
1769	for (MachineBasicBlock *SuccBB : BB->successors())
1770	if (SuccBB != TrueBB)
1771	return SuccBB;
1772	return nullptr;
1773	}
1774
1775	template <class Container>
1776	static void addRegAndItsAliases(Register Reg, const TargetRegisterInfo *TRI,
1777	Container &Set) {
1778	if (Reg.isPhysical()) {
1779	for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
1780	Set.insert(*AI);
1781	} else {
1782	Set.insert(Reg);
1783	}
1784	}
1785
1786	/// findHoistingInsertPosAndDeps - Find the location to move common instructions
1787	/// in successors to. The location is usually just before the terminator,
1788	/// however if the terminator is a conditional branch and its previous
1789	/// instruction is the flag setting instruction, the previous instruction is
1790	/// the preferred location. This function also gathers uses and defs of the
1791	/// instructions from the insertion point to the end of the block. The data is
1792	/// used by HoistCommonCodeInSuccs to ensure safety.
1793	static
1794	MachineBasicBlock::iterator findHoistingInsertPosAndDeps(MachineBasicBlock *MBB,
1795	const TargetInstrInfo *TII,
1796	const TargetRegisterInfo *TRI,
1797	SmallSet<Register, 4> &Uses,
1798	SmallSet<Register, 4> &Defs) {
1799	MachineBasicBlock::iterator Loc = MBB->getFirstTerminator();
1800	if (!TII->isUnpredicatedTerminator(*Loc))
1801	return MBB->end();
1802
1803	for (const MachineOperand &MO : Loc->operands()) {
1804	if (!MO.isReg())
1805	continue;
1806	Register Reg = MO.getReg();
1807	if (!Reg)
1808	continue;
1809	if (MO.isUse()) {
1810	addRegAndItsAliases(Reg, TRI, Uses);
1811	} else {
1812	if (!MO.isDead())
1813	// Don't try to hoist code in the rare case the terminator defines a
1814	// register that is later used.
1815	return MBB->end();
1816
1817	// If the terminator defines a register, make sure we don't hoist
1818	// the instruction whose def might be clobbered by the terminator.
1819	addRegAndItsAliases(Reg, TRI, Defs);
1820	}
1821	}
1822
1823	if (Uses.empty())
1824	return Loc;
1825	// If the terminator is the only instruction in the block and Uses is not
1826	// empty (or we would have returned above), we can still safely hoist
1827	// instructions just before the terminator as long as the Defs/Uses are not
1828	// violated (which is checked in HoistCommonCodeInSuccs).
1829	if (Loc == MBB->begin())
1830	return Loc;
1831
1832	// The terminator is probably a conditional branch, try not to separate the
1833	// branch from condition setting instruction.
1834	MachineBasicBlock::iterator PI = prev_nodbg(Loc, MBB->begin());
1835
1836	bool IsDef = false;
1837	for (const MachineOperand &MO : PI->operands()) {
1838	// If PI has a regmask operand, it is probably a call. Separate away.
1839	if (MO.isRegMask())
1840	return Loc;
1841	if (!MO.isReg() || MO.isUse())
1842	continue;
1843	Register Reg = MO.getReg();
1844	if (!Reg)
1845	continue;
1846	if (Uses.count(Reg)) {
1847	IsDef = true;
1848	break;
1849	}
1850	}
1851	if (!IsDef)
1852	// The condition setting instruction is not just before the conditional
1853	// branch.
1854	return Loc;
1855
1856	// Be conservative, don't insert instruction above something that may have
1857	// side-effects. And since it's potentially bad to separate flag setting
1858	// instruction from the conditional branch, just abort the optimization
1859	// completely.
1860	// Also avoid moving code above predicated instruction since it's hard to
1861	// reason about register liveness with predicated instruction.
1862	bool DontMoveAcrossStore = true;
1863	if (!PI->isSafeToMove(nullptr, DontMoveAcrossStore) || TII->isPredicated(*PI))
1864	return MBB->end();
1865
1866	// Find out what registers are live. Note this routine is ignoring other live
1867	// registers which are only used by instructions in successor blocks.
1868	for (const MachineOperand &MO : PI->operands()) {
1869	if (!MO.isReg())
1870	continue;
1871	Register Reg = MO.getReg();
1872	if (!Reg)
1873	continue;
1874	if (MO.isUse()) {
1875	addRegAndItsAliases(Reg, TRI, Uses);
1876	} else {
1877	if (Uses.erase(Reg)) {
1878	if (Register::isPhysicalRegister(Reg)) {
1879	for (MCSubRegIterator SubRegs(Reg, TRI); SubRegs.isValid(); ++SubRegs)
1880	Uses.erase(*SubRegs); // Use sub-registers to be conservative
1881	}
1882	}
1883	addRegAndItsAliases(Reg, TRI, Defs);
1884	}
1885	}
1886
1887	return PI;
1888	}
1889
1890	bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
1891	MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
1892	SmallVector<MachineOperand, 4> Cond;
1893	if (TII->analyzeBranch(*MBB, TBB, FBB, Cond, true) || !TBB || Cond.empty())
1894	return false;
1895
1896	if (!FBB) FBB = findFalseBlock(MBB, TBB);
1897	if (!FBB)
1898	// Malformed bcc? True and false blocks are the same?
1899	return false;
1900
1901	// Restrict the optimization to cases where MBB is the only predecessor,
1902	// it is an obvious win.
1903	if (TBB->pred_size() > 1 || FBB->pred_size() > 1)
1904	return false;
1905
1906	// Find a suitable position to hoist the common instructions to. Also figure
1907	// out which registers are used or defined by instructions from the insertion
1908	// point to the end of the block.
1909	SmallSet<Register, 4> Uses, Defs;
1910	MachineBasicBlock::iterator Loc =
1911	findHoistingInsertPosAndDeps(MBB, TII, TRI, Uses, Defs);
1912	if (Loc == MBB->end())
1913	return false;
1914
1915	bool HasDups = false;
1916	SmallSet<Register, 4> ActiveDefsSet, AllDefsSet;
1917	MachineBasicBlock::iterator TIB = TBB->begin();
1918	MachineBasicBlock::iterator FIB = FBB->begin();
1919	MachineBasicBlock::iterator TIE = TBB->end();
1920	MachineBasicBlock::iterator FIE = FBB->end();
1921	while (TIB != TIE && FIB != FIE) {
1922	// Skip dbg_value instructions. These do not count.
1923	TIB = skipDebugInstructionsForward(TIB, TIE, false);
1924	FIB = skipDebugInstructionsForward(FIB, FIE, false);
1925	if (TIB == TIE || FIB == FIE)
1926	break;
1927
1928	if (!TIB->isIdenticalTo(*FIB, MachineInstr::CheckKillDead))
1929	break;
1930
1931	if (TII->isPredicated(*TIB))
1932	// Hard to reason about register liveness with predicated instruction.
1933	break;
1934
1935	bool IsSafe = true;
1936	for (MachineOperand &MO : TIB->operands()) {
1937	// Don't attempt to hoist instructions with register masks.
1938	if (MO.isRegMask()) {
1939	IsSafe = false;
1940	break;
1941	}
1942	if (!MO.isReg())
1943	continue;
1944	Register Reg = MO.getReg();
1945	if (!Reg)
1946	continue;
1947	if (MO.isDef()) {
1948	if (Uses.count(Reg)) {
1949	// Avoid clobbering a register that's used by the instruction at
1950	// the point of insertion.
1951	IsSafe = false;
1952	break;
1953	}
1954
1955	if (Defs.count(Reg) && !MO.isDead()) {
1956	// Don't hoist the instruction if the def would be clobber by the
1957	// instruction at the point insertion. FIXME: This is overly
1958	// conservative. It should be possible to hoist the instructions
1959	// in BB2 in the following example:
1960	// BB1:
1961	// r1, eflag = op1 r2, r3
1962	// brcc eflag
1963	//
1964	// BB2:
1965	// r1 = op2, ...
1966	// = op3, killed r1
1967	IsSafe = false;
1968	break;
1969	}
1970	} else if (!ActiveDefsSet.count(Reg)) {
1971	if (Defs.count(Reg)) {
1972	// Use is defined by the instruction at the point of insertion.
1973	IsSafe = false;
1974	break;
1975	}
1976
1977	if (MO.isKill() && Uses.count(Reg))
1978	// Kills a register that's read by the instruction at the point of
1979	// insertion. Remove the kill marker.
1980	MO.setIsKill(false);
1981	}
1982	}
1983	if (!IsSafe)
1984	break;
1985
1986	bool DontMoveAcrossStore = true;
1987	if (!TIB->isSafeToMove(nullptr, DontMoveAcrossStore))
1988	break;
1989
1990	// Remove kills from ActiveDefsSet, these registers had short live ranges.
1991	for (const MachineOperand &MO : TIB->operands()) {
1992	if (!MO.isReg() || !MO.isUse() || !MO.isKill())
1993	continue;
1994	Register Reg = MO.getReg();
1995	if (!Reg)
1996	continue;
1997	if (!AllDefsSet.count(Reg)) {
1998	continue;
1999	}
2000	if (Reg.isPhysical()) {
2001	for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
2002	ActiveDefsSet.erase(*AI);
2003	} else {
2004	ActiveDefsSet.erase(Reg);
2005	}
2006	}
2007
2008	// Track local defs so we can update liveins.
2009	for (const MachineOperand &MO : TIB->operands()) {
2010	if (!MO.isReg() || !MO.isDef() || MO.isDead())
2011	continue;
2012	Register Reg = MO.getReg();
2013	if (!Reg || Reg.isVirtual())
2014	continue;
2015	addRegAndItsAliases(Reg, TRI, ActiveDefsSet);
2016	addRegAndItsAliases(Reg, TRI, AllDefsSet);
2017	}
2018
2019	HasDups = true;
2020	++TIB;
2021	++FIB;
2022	}
2023
2024	if (!HasDups)
2025	return false;
2026
2027	MBB->splice(Loc, TBB, TBB->begin(), TIB);
2028	FBB->erase(FBB->begin(), FIB);
2029
2030	if (UpdateLiveIns) {
2031	recomputeLiveIns(*TBB);
2032	recomputeLiveIns(*FBB);
2033	}
2034
2035	++NumHoist;
2036	return true;
2037	}
2038