1 //===-- BranchFolding.cpp - Fold machine code branch instructions ---------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass forwards branches to unconditional branches to make them branch
11 // directly to the target block. This pass often results in dead MBB's, which
14 // Note that this pass must be run after register allocation, it cannot handle
15 // SSA form. It also must handle virtual registers for targets that emit virtual
18 //===----------------------------------------------------------------------===//
20 #include "BranchFolding.h"
21 #include "llvm/ADT/STLExtras.h"
22 #include "llvm/ADT/SmallSet.h"
23 #include "llvm/ADT/Statistic.h"
24 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
25 #include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
26 #include "llvm/CodeGen/MachineFunctionPass.h"
27 #include "llvm/CodeGen/MachineJumpTableInfo.h"
28 #include "llvm/CodeGen/MachineMemOperand.h"
29 #include "llvm/CodeGen/MachineModuleInfo.h"
30 #include "llvm/CodeGen/MachineRegisterInfo.h"
31 #include "llvm/CodeGen/Passes.h"
32 #include "llvm/CodeGen/RegisterScavenging.h"
33 #include "llvm/IR/Function.h"
34 #include "llvm/Support/CommandLine.h"
35 #include "llvm/Support/Debug.h"
36 #include "llvm/Support/ErrorHandling.h"
37 #include "llvm/Support/raw_ostream.h"
38 #include "llvm/Target/TargetInstrInfo.h"
39 #include "llvm/Target/TargetRegisterInfo.h"
40 #include "llvm/Target/TargetSubtargetInfo.h"
44 #define DEBUG_TYPE "branchfolding"
46 STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
47 STATISTIC(NumBranchOpts, "Number of branches optimized");
48 STATISTIC(NumTailMerge , "Number of block tails merged");
49 STATISTIC(NumHoist , "Number of times common instructions are hoisted");
51 static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge",
52 cl::init(cl::BOU_UNSET), cl::Hidden);
54 // Throttle for huge numbers of predecessors (compile speed problems)
55 static cl::opt<unsigned>
56 TailMergeThreshold("tail-merge-threshold",
57 cl::desc("Max number of predecessors to consider tail merging"),
58 cl::init(150), cl::Hidden);
60 // Heuristic for tail merging (and, inversely, tail duplication).
61 // TODO: This should be replaced with a target query.
62 static cl::opt<unsigned>
63 TailMergeSize("tail-merge-size",
64 cl::desc("Min number of instructions to consider tail merging"),
65 cl::init(3), cl::Hidden);
68 /// BranchFolderPass - Wrap branch folder in a machine function pass.
69 class BranchFolderPass : public MachineFunctionPass {
72 explicit BranchFolderPass(): MachineFunctionPass(ID) {}
74 bool runOnMachineFunction(MachineFunction &MF) override;
76 void getAnalysisUsage(AnalysisUsage &AU) const override {
77 AU.addRequired<MachineBlockFrequencyInfo>();
78 AU.addRequired<MachineBranchProbabilityInfo>();
79 AU.addRequired<TargetPassConfig>();
80 MachineFunctionPass::getAnalysisUsage(AU);
85 char BranchFolderPass::ID = 0;
86 char &llvm::BranchFolderPassID = BranchFolderPass::ID;
88 INITIALIZE_PASS(BranchFolderPass, "branch-folder",
89 "Control Flow Optimizer", false, false)
91 bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) {
92 if (skipOptnoneFunction(*MF.getFunction()))
95 TargetPassConfig *PassConfig = &getAnalysis<TargetPassConfig>();
96 // TailMerge can create jump into if branches that make CFG irreducible for
97 // HW that requires structurized CFG.
98 bool EnableTailMerge = !MF.getTarget().requiresStructuredCFG() &&
99 PassConfig->getEnableTailMerge();
100 BranchFolder Folder(EnableTailMerge, /*CommonHoist=*/true,
101 getAnalysis<MachineBlockFrequencyInfo>(),
102 getAnalysis<MachineBranchProbabilityInfo>());
103 return Folder.OptimizeFunction(MF, MF.getSubtarget().getInstrInfo(),
104 MF.getSubtarget().getRegisterInfo(),
105 getAnalysisIfAvailable<MachineModuleInfo>());
108 BranchFolder::BranchFolder(bool defaultEnableTailMerge, bool CommonHoist,
109 const MachineBlockFrequencyInfo &FreqInfo,
110 const MachineBranchProbabilityInfo &ProbInfo)
111 : EnableHoistCommonCode(CommonHoist), MBBFreqInfo(FreqInfo),
113 switch (FlagEnableTailMerge) {
114 case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break;
115 case cl::BOU_TRUE: EnableTailMerge = true; break;
116 case cl::BOU_FALSE: EnableTailMerge = false; break;
120 /// RemoveDeadBlock - Remove the specified dead machine basic block from the
121 /// function, updating the CFG.
122 void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) {
123 assert(MBB->pred_empty() && "MBB must be dead!");
124 DEBUG(dbgs() << "\nRemoving MBB: " << *MBB);
126 MachineFunction *MF = MBB->getParent();
127 // drop all successors.
128 while (!MBB->succ_empty())
129 MBB->removeSuccessor(MBB->succ_end()-1);
131 // Avoid matching if this pointer gets reused.
132 TriedMerging.erase(MBB);
138 /// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def
139 /// followed by terminators, and if the implicitly defined registers are not
140 /// used by the terminators, remove those implicit_def's. e.g.
142 /// r0 = implicit_def
143 /// r1 = implicit_def
145 /// This block can be optimized away later if the implicit instructions are
147 bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) {
148 SmallSet<unsigned, 4> ImpDefRegs;
149 MachineBasicBlock::iterator I = MBB->begin();
150 while (I != MBB->end()) {
151 if (!I->isImplicitDef())
153 unsigned Reg = I->getOperand(0).getReg();
154 if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
155 for (MCSubRegIterator SubRegs(Reg, TRI, /*IncludeSelf=*/true);
156 SubRegs.isValid(); ++SubRegs)
157 ImpDefRegs.insert(*SubRegs);
159 ImpDefRegs.insert(Reg);
163 if (ImpDefRegs.empty())
166 MachineBasicBlock::iterator FirstTerm = I;
167 while (I != MBB->end()) {
168 if (!TII->isUnpredicatedTerminator(I))
170 // See if it uses any of the implicitly defined registers.
171 for (const MachineOperand &MO : I->operands()) {
172 if (!MO.isReg() || !MO.isUse())
174 unsigned Reg = MO.getReg();
175 if (ImpDefRegs.count(Reg))
182 while (I != FirstTerm) {
183 MachineInstr *ImpDefMI = &*I;
185 MBB->erase(ImpDefMI);
191 /// OptimizeFunction - Perhaps branch folding, tail merging and other
192 /// CFG optimizations on the given function.
193 bool BranchFolder::OptimizeFunction(MachineFunction &MF,
194 const TargetInstrInfo *tii,
195 const TargetRegisterInfo *tri,
196 MachineModuleInfo *mmi) {
197 if (!tii) return false;
199 TriedMerging.clear();
206 // Use a RegScavenger to help update liveness when required.
207 MachineRegisterInfo &MRI = MF.getRegInfo();
208 if (MRI.tracksLiveness() && TRI->trackLivenessAfterRegAlloc(MF))
209 RS = new RegScavenger();
211 MRI.invalidateLiveness();
213 // Fix CFG. The later algorithms expect it to be right.
214 bool MadeChange = false;
215 for (MachineBasicBlock &MBB : MF) {
216 MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
217 SmallVector<MachineOperand, 4> Cond;
218 if (!TII->AnalyzeBranch(MBB, TBB, FBB, Cond, true))
219 MadeChange |= MBB.CorrectExtraCFGEdges(TBB, FBB, !Cond.empty());
220 MadeChange |= OptimizeImpDefsBlock(&MBB);
223 bool MadeChangeThisIteration = true;
224 while (MadeChangeThisIteration) {
225 MadeChangeThisIteration = TailMergeBlocks(MF);
226 MadeChangeThisIteration |= OptimizeBranches(MF);
227 if (EnableHoistCommonCode)
228 MadeChangeThisIteration |= HoistCommonCode(MF);
229 MadeChange |= MadeChangeThisIteration;
232 // See if any jump tables have become dead as the code generator
234 MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
240 // Walk the function to find jump tables that are live.
241 BitVector JTIsLive(JTI->getJumpTables().size());
242 for (const MachineBasicBlock &BB : MF) {
243 for (const MachineInstr &I : BB)
244 for (const MachineOperand &Op : I.operands()) {
245 if (!Op.isJTI()) continue;
247 // Remember that this JT is live.
248 JTIsLive.set(Op.getIndex());
252 // Finally, remove dead jump tables. This happens when the
253 // indirect jump was unreachable (and thus deleted).
254 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i)
255 if (!JTIsLive.test(i)) {
256 JTI->RemoveJumpTable(i);
264 //===----------------------------------------------------------------------===//
265 // Tail Merging of Blocks
266 //===----------------------------------------------------------------------===//
268 /// HashMachineInstr - Compute a hash value for MI and its operands.
269 static unsigned HashMachineInstr(const MachineInstr *MI) {
270 unsigned Hash = MI->getOpcode();
271 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
272 const MachineOperand &Op = MI->getOperand(i);
274 // Merge in bits from the operand if easy. We can't use MachineOperand's
275 // hash_code here because it's not deterministic and we sort by hash value
277 unsigned OperandHash = 0;
278 switch (Op.getType()) {
279 case MachineOperand::MO_Register:
280 OperandHash = Op.getReg();
282 case MachineOperand::MO_Immediate:
283 OperandHash = Op.getImm();
285 case MachineOperand::MO_MachineBasicBlock:
286 OperandHash = Op.getMBB()->getNumber();
288 case MachineOperand::MO_FrameIndex:
289 case MachineOperand::MO_ConstantPoolIndex:
290 case MachineOperand::MO_JumpTableIndex:
291 OperandHash = Op.getIndex();
293 case MachineOperand::MO_GlobalAddress:
294 case MachineOperand::MO_ExternalSymbol:
295 // Global address / external symbol are too hard, don't bother, but do
296 // pull in the offset.
297 OperandHash = Op.getOffset();
303 Hash += ((OperandHash << 3) | Op.getType()) << (i & 31);
308 /// HashEndOfMBB - Hash the last instruction in the MBB.
309 static unsigned HashEndOfMBB(const MachineBasicBlock *MBB) {
310 MachineBasicBlock::const_iterator I = MBB->getLastNonDebugInstr();
314 return HashMachineInstr(I);
317 /// ComputeCommonTailLength - Given two machine basic blocks, compute the number
318 /// of instructions they actually have in common together at their end. Return
319 /// iterators for the first shared instruction in each block.
320 static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1,
321 MachineBasicBlock *MBB2,
322 MachineBasicBlock::iterator &I1,
323 MachineBasicBlock::iterator &I2) {
327 unsigned TailLen = 0;
328 while (I1 != MBB1->begin() && I2 != MBB2->begin()) {
330 // Skip debugging pseudos; necessary to avoid changing the code.
331 while (I1->isDebugValue()) {
332 if (I1==MBB1->begin()) {
333 while (I2->isDebugValue()) {
334 if (I2==MBB2->begin())
335 // I1==DBG at begin; I2==DBG at begin
340 // I1==DBG at begin; I2==non-DBG, or first of DBGs not at begin
345 // I1==first (untested) non-DBG preceding known match
346 while (I2->isDebugValue()) {
347 if (I2==MBB2->begin()) {
349 // I1==non-DBG, or first of DBGs not at begin; I2==DBG at begin
354 // I1, I2==first (untested) non-DBGs preceding known match
355 if (!I1->isIdenticalTo(I2) ||
356 // FIXME: This check is dubious. It's used to get around a problem where
357 // people incorrectly expect inline asm directives to remain in the same
358 // relative order. This is untenable because normal compiler
359 // optimizations (like this one) may reorder and/or merge these
367 // Back past possible debugging pseudos at beginning of block. This matters
368 // when one block differs from the other only by whether debugging pseudos
369 // are present at the beginning. (This way, the various checks later for
370 // I1==MBB1->begin() work as expected.)
371 if (I1 == MBB1->begin() && I2 != MBB2->begin()) {
373 while (I2->isDebugValue()) {
374 if (I2 == MBB2->begin())
380 if (I2 == MBB2->begin() && I1 != MBB1->begin()) {
382 while (I1->isDebugValue()) {
383 if (I1 == MBB1->begin())
392 void BranchFolder::MaintainLiveIns(MachineBasicBlock *CurMBB,
393 MachineBasicBlock *NewMBB) {
395 RS->enterBasicBlock(CurMBB);
396 if (!CurMBB->empty())
397 RS->forward(std::prev(CurMBB->end()));
398 for (unsigned int i = 1, e = TRI->getNumRegs(); i != e; i++)
399 if (RS->isRegUsed(i, false))
400 NewMBB->addLiveIn(i);
404 /// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything
405 /// after it, replacing it with an unconditional branch to NewDest.
406 void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
407 MachineBasicBlock *NewDest) {
408 MachineBasicBlock *CurMBB = OldInst->getParent();
410 TII->ReplaceTailWithBranchTo(OldInst, NewDest);
412 // For targets that use the register scavenger, we must maintain LiveIns.
413 MaintainLiveIns(CurMBB, NewDest);
418 /// SplitMBBAt - Given a machine basic block and an iterator into it, split the
419 /// MBB so that the part before the iterator falls into the part starting at the
420 /// iterator. This returns the new MBB.
421 MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB,
422 MachineBasicBlock::iterator BBI1,
423 const BasicBlock *BB) {
424 if (!TII->isLegalToSplitMBBAt(CurMBB, BBI1))
427 MachineFunction &MF = *CurMBB.getParent();
429 // Create the fall-through block.
430 MachineFunction::iterator MBBI = &CurMBB;
431 MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(BB);
432 CurMBB.getParent()->insert(++MBBI, NewMBB);
434 // Move all the successors of this block to the specified block.
435 NewMBB->transferSuccessors(&CurMBB);
437 // Add an edge from CurMBB to NewMBB for the fall-through.
438 CurMBB.addSuccessor(NewMBB);
440 // Splice the code over.
441 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end());
443 // NewMBB inherits CurMBB's block frequency.
444 MBBFreqInfo.setBlockFreq(NewMBB, MBBFreqInfo.getBlockFreq(&CurMBB));
446 // For targets that use the register scavenger, we must maintain LiveIns.
447 MaintainLiveIns(&CurMBB, NewMBB);
452 /// EstimateRuntime - Make a rough estimate for how long it will take to run
453 /// the specified code.
454 static unsigned EstimateRuntime(MachineBasicBlock::iterator I,
455 MachineBasicBlock::iterator E) {
457 for (; I != E; ++I) {
458 if (I->isDebugValue())
462 else if (I->mayLoad() || I->mayStore())
470 // CurMBB needs to add an unconditional branch to SuccMBB (we removed these
471 // branches temporarily for tail merging). In the case where CurMBB ends
472 // with a conditional branch to the next block, optimize by reversing the
473 // test and conditionally branching to SuccMBB instead.
474 static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB,
475 const TargetInstrInfo *TII) {
476 MachineFunction *MF = CurMBB->getParent();
477 MachineFunction::iterator I = std::next(MachineFunction::iterator(CurMBB));
478 MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
479 SmallVector<MachineOperand, 4> Cond;
480 DebugLoc dl; // FIXME: this is nowhere
481 if (I != MF->end() &&
482 !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond, true)) {
483 MachineBasicBlock *NextBB = I;
484 if (TBB == NextBB && !Cond.empty() && !FBB) {
485 if (!TII->ReverseBranchCondition(Cond)) {
486 TII->RemoveBranch(*CurMBB);
487 TII->InsertBranch(*CurMBB, SuccBB, nullptr, Cond, dl);
492 TII->InsertBranch(*CurMBB, SuccBB, nullptr,
493 SmallVector<MachineOperand, 0>(), dl);
497 BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const {
498 if (getHash() < o.getHash())
500 if (getHash() > o.getHash())
502 if (getBlock()->getNumber() < o.getBlock()->getNumber())
504 if (getBlock()->getNumber() > o.getBlock()->getNumber())
506 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing
507 // an object with itself.
508 #ifndef _GLIBCXX_DEBUG
509 llvm_unreachable("Predecessor appears twice");
516 BranchFolder::MBFIWrapper::getBlockFreq(const MachineBasicBlock *MBB) const {
517 auto I = MergedBBFreq.find(MBB);
519 if (I != MergedBBFreq.end())
522 return MBFI.getBlockFreq(MBB);
525 void BranchFolder::MBFIWrapper::setBlockFreq(const MachineBasicBlock *MBB,
527 MergedBBFreq[MBB] = F;
530 /// CountTerminators - Count the number of terminators in the given
531 /// block and set I to the position of the first non-terminator, if there
532 /// is one, or MBB->end() otherwise.
533 static unsigned CountTerminators(MachineBasicBlock *MBB,
534 MachineBasicBlock::iterator &I) {
536 unsigned NumTerms = 0;
538 if (I == MBB->begin()) {
543 if (!I->isTerminator()) break;
549 /// ProfitableToMerge - Check if two machine basic blocks have a common tail
550 /// and decide if it would be profitable to merge those tails. Return the
551 /// length of the common tail and iterators to the first common instruction
553 static bool ProfitableToMerge(MachineBasicBlock *MBB1,
554 MachineBasicBlock *MBB2,
555 unsigned minCommonTailLength,
556 unsigned &CommonTailLen,
557 MachineBasicBlock::iterator &I1,
558 MachineBasicBlock::iterator &I2,
559 MachineBasicBlock *SuccBB,
560 MachineBasicBlock *PredBB) {
561 CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2);
562 if (CommonTailLen == 0)
564 DEBUG(dbgs() << "Common tail length of BB#" << MBB1->getNumber()
565 << " and BB#" << MBB2->getNumber() << " is " << CommonTailLen
568 // It's almost always profitable to merge any number of non-terminator
569 // instructions with the block that falls through into the common successor.
570 if (MBB1 == PredBB || MBB2 == PredBB) {
571 MachineBasicBlock::iterator I;
572 unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I);
573 if (CommonTailLen > NumTerms)
577 // If one of the blocks can be completely merged and happens to be in
578 // a position where the other could fall through into it, merge any number
579 // of instructions, because it can be done without a branch.
580 // TODO: If the blocks are not adjacent, move one of them so that they are?
581 if (MBB1->isLayoutSuccessor(MBB2) && I2 == MBB2->begin())
583 if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin())
586 // If both blocks have an unconditional branch temporarily stripped out,
587 // count that as an additional common instruction for the following
589 unsigned EffectiveTailLen = CommonTailLen;
590 if (SuccBB && MBB1 != PredBB && MBB2 != PredBB &&
591 !MBB1->back().isBarrier() &&
592 !MBB2->back().isBarrier())
595 // Check if the common tail is long enough to be worthwhile.
596 if (EffectiveTailLen >= minCommonTailLength)
599 // If we are optimizing for code size, 2 instructions in common is enough if
600 // we don't have to split a block. At worst we will be introducing 1 new
601 // branch instruction, which is likely to be smaller than the 2
602 // instructions that would be deleted in the merge.
603 MachineFunction *MF = MBB1->getParent();
604 if (EffectiveTailLen >= 2 && MF->getFunction()->optForSize() &&
605 (I1 == MBB1->begin() || I2 == MBB2->begin()))
611 /// ComputeSameTails - Look through all the blocks in MergePotentials that have
612 /// hash CurHash (guaranteed to match the last element). Build the vector
613 /// SameTails of all those that have the (same) largest number of instructions
614 /// in common of any pair of these blocks. SameTails entries contain an
615 /// iterator into MergePotentials (from which the MachineBasicBlock can be
616 /// found) and a MachineBasicBlock::iterator into that MBB indicating the
617 /// instruction where the matching code sequence begins.
618 /// Order of elements in SameTails is the reverse of the order in which
619 /// those blocks appear in MergePotentials (where they are not necessarily
621 unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
622 unsigned minCommonTailLength,
623 MachineBasicBlock *SuccBB,
624 MachineBasicBlock *PredBB) {
625 unsigned maxCommonTailLength = 0U;
627 MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
628 MPIterator HighestMPIter = std::prev(MergePotentials.end());
629 for (MPIterator CurMPIter = std::prev(MergePotentials.end()),
630 B = MergePotentials.begin();
631 CurMPIter != B && CurMPIter->getHash() == CurHash; --CurMPIter) {
632 for (MPIterator I = std::prev(CurMPIter); I->getHash() == CurHash; --I) {
633 unsigned CommonTailLen;
634 if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(),
636 CommonTailLen, TrialBBI1, TrialBBI2,
638 if (CommonTailLen > maxCommonTailLength) {
640 maxCommonTailLength = CommonTailLen;
641 HighestMPIter = CurMPIter;
642 SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1));
644 if (HighestMPIter == CurMPIter &&
645 CommonTailLen == maxCommonTailLength)
646 SameTails.push_back(SameTailElt(I, TrialBBI2));
652 return maxCommonTailLength;
655 /// RemoveBlocksWithHash - Remove all blocks with hash CurHash from
656 /// MergePotentials, restoring branches at ends of blocks as appropriate.
657 void BranchFolder::RemoveBlocksWithHash(unsigned CurHash,
658 MachineBasicBlock *SuccBB,
659 MachineBasicBlock *PredBB) {
660 MPIterator CurMPIter, B;
661 for (CurMPIter = std::prev(MergePotentials.end()),
662 B = MergePotentials.begin();
663 CurMPIter->getHash() == CurHash; --CurMPIter) {
664 // Put the unconditional branch back, if we need one.
665 MachineBasicBlock *CurMBB = CurMPIter->getBlock();
666 if (SuccBB && CurMBB != PredBB)
667 FixTail(CurMBB, SuccBB, TII);
671 if (CurMPIter->getHash() != CurHash)
673 MergePotentials.erase(CurMPIter, MergePotentials.end());
676 /// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist
677 /// only of the common tail. Create a block that does by splitting one.
678 bool BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
679 MachineBasicBlock *SuccBB,
680 unsigned maxCommonTailLength,
681 unsigned &commonTailIndex) {
683 unsigned TimeEstimate = ~0U;
684 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
685 // Use PredBB if possible; that doesn't require a new branch.
686 if (SameTails[i].getBlock() == PredBB) {
690 // Otherwise, make a (fairly bogus) choice based on estimate of
691 // how long it will take the various blocks to execute.
692 unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(),
693 SameTails[i].getTailStartPos());
694 if (t <= TimeEstimate) {
700 MachineBasicBlock::iterator BBI =
701 SameTails[commonTailIndex].getTailStartPos();
702 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
704 // If the common tail includes any debug info we will take it pretty
705 // randomly from one of the inputs. Might be better to remove it?
706 DEBUG(dbgs() << "\nSplitting BB#" << MBB->getNumber() << ", size "
707 << maxCommonTailLength);
709 // If the split block unconditionally falls-thru to SuccBB, it will be
710 // merged. In control flow terms it should then take SuccBB's name. e.g. If
711 // SuccBB is an inner loop, the common tail is still part of the inner loop.
712 const BasicBlock *BB = (SuccBB && MBB->succ_size() == 1) ?
713 SuccBB->getBasicBlock() : MBB->getBasicBlock();
714 MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI, BB);
716 DEBUG(dbgs() << "... failed!");
720 SameTails[commonTailIndex].setBlock(newMBB);
721 SameTails[commonTailIndex].setTailStartPos(newMBB->begin());
723 // If we split PredBB, newMBB is the new predecessor.
730 static bool hasIdenticalMMOs(const MachineInstr *MI1, const MachineInstr *MI2) {
731 auto I1 = MI1->memoperands_begin(), E1 = MI1->memoperands_end();
732 auto I2 = MI2->memoperands_begin(), E2 = MI2->memoperands_end();
733 if ((E1 - I1) != (E2 - I2))
735 for (; I1 != E1; ++I1, ++I2) {
743 removeMMOsFromMemoryOperations(MachineBasicBlock::iterator MBBIStartPos,
744 MachineBasicBlock &MBBCommon) {
745 // Remove MMOs from memory operations in the common block
746 // when they do not match the ones from the block being tail-merged.
747 // This ensures later passes conservatively compute dependencies.
748 MachineBasicBlock *MBB = MBBIStartPos->getParent();
749 // Note CommonTailLen does not necessarily matches the size of
750 // the common BB nor all its instructions because of debug
751 // instructions differences.
752 unsigned CommonTailLen = 0;
753 for (auto E = MBB->end(); MBBIStartPos != E; ++MBBIStartPos)
756 MachineBasicBlock::reverse_iterator MBBI = MBB->rbegin();
757 MachineBasicBlock::reverse_iterator MBBIE = MBB->rend();
758 MachineBasicBlock::reverse_iterator MBBICommon = MBBCommon.rbegin();
759 MachineBasicBlock::reverse_iterator MBBIECommon = MBBCommon.rend();
761 while (CommonTailLen--) {
762 assert(MBBI != MBBIE && "Reached BB end within common tail length!");
765 if (MBBI->isDebugValue()) {
770 while ((MBBICommon != MBBIECommon) && MBBICommon->isDebugValue())
773 assert(MBBICommon != MBBIECommon &&
774 "Reached BB end within common tail length!");
775 assert(MBBICommon->isIdenticalTo(&*MBBI) && "Expected matching MIIs!");
777 if (MBBICommon->mayLoad() || MBBICommon->mayStore())
778 if (!hasIdenticalMMOs(&*MBBI, &*MBBICommon))
779 MBBICommon->clearMemRefs();
786 // See if any of the blocks in MergePotentials (which all have a common single
787 // successor, or all have no successor) can be tail-merged. If there is a
788 // successor, any blocks in MergePotentials that are not tail-merged and
789 // are not immediately before Succ must have an unconditional branch to
790 // Succ added (but the predecessor/successor lists need no adjustment).
791 // The lone predecessor of Succ that falls through into Succ,
792 // if any, is given in PredBB.
794 bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
795 MachineBasicBlock *PredBB) {
796 bool MadeChange = false;
798 // Except for the special cases below, tail-merge if there are at least
799 // this many instructions in common.
800 unsigned minCommonTailLength = TailMergeSize;
802 DEBUG(dbgs() << "\nTryTailMergeBlocks: ";
803 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
804 dbgs() << "BB#" << MergePotentials[i].getBlock()->getNumber()
805 << (i == e-1 ? "" : ", ");
808 dbgs() << " with successor BB#" << SuccBB->getNumber() << '\n';
810 dbgs() << " which has fall-through from BB#"
811 << PredBB->getNumber() << "\n";
813 dbgs() << "Looking for common tails of at least "
814 << minCommonTailLength << " instruction"
815 << (minCommonTailLength == 1 ? "" : "s") << '\n';
818 // Sort by hash value so that blocks with identical end sequences sort
820 array_pod_sort(MergePotentials.begin(), MergePotentials.end());
822 // Walk through equivalence sets looking for actual exact matches.
823 while (MergePotentials.size() > 1) {
824 unsigned CurHash = MergePotentials.back().getHash();
826 // Build SameTails, identifying the set of blocks with this hash code
827 // and with the maximum number of instructions in common.
828 unsigned maxCommonTailLength = ComputeSameTails(CurHash,
832 // If we didn't find any pair that has at least minCommonTailLength
833 // instructions in common, remove all blocks with this hash code and retry.
834 if (SameTails.empty()) {
835 RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
839 // If one of the blocks is the entire common tail (and not the entry
840 // block, which we can't jump to), we can treat all blocks with this same
841 // tail at once. Use PredBB if that is one of the possibilities, as that
842 // will not introduce any extra branches.
843 MachineBasicBlock *EntryBB = MergePotentials.begin()->getBlock()->
844 getParent()->begin();
845 unsigned commonTailIndex = SameTails.size();
846 // If there are two blocks, check to see if one can be made to fall through
848 if (SameTails.size() == 2 &&
849 SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) &&
850 SameTails[1].tailIsWholeBlock())
852 else if (SameTails.size() == 2 &&
853 SameTails[1].getBlock()->isLayoutSuccessor(
854 SameTails[0].getBlock()) &&
855 SameTails[0].tailIsWholeBlock())
858 // Otherwise just pick one, favoring the fall-through predecessor if
860 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
861 MachineBasicBlock *MBB = SameTails[i].getBlock();
862 if (MBB == EntryBB && SameTails[i].tailIsWholeBlock())
868 if (SameTails[i].tailIsWholeBlock())
873 if (commonTailIndex == SameTails.size() ||
874 (SameTails[commonTailIndex].getBlock() == PredBB &&
875 !SameTails[commonTailIndex].tailIsWholeBlock())) {
876 // None of the blocks consist entirely of the common tail.
877 // Split a block so that one does.
878 if (!CreateCommonTailOnlyBlock(PredBB, SuccBB,
879 maxCommonTailLength, commonTailIndex)) {
880 RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
885 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
887 // Recompute commont tail MBB's edge weights and block frequency.
888 setCommonTailEdgeWeights(*MBB);
890 // MBB is common tail. Adjust all other BB's to jump to this one.
891 // Traversal must be forwards so erases work.
892 DEBUG(dbgs() << "\nUsing common tail in BB#" << MBB->getNumber()
894 for (unsigned int i=0, e = SameTails.size(); i != e; ++i) {
895 if (commonTailIndex == i)
897 DEBUG(dbgs() << "BB#" << SameTails[i].getBlock()->getNumber()
898 << (i == e-1 ? "" : ", "));
899 // Remove MMOs from memory operations as needed.
900 removeMMOsFromMemoryOperations(SameTails[i].getTailStartPos(), *MBB);
901 // Hack the end off BB i, making it jump to BB commonTailIndex instead.
902 ReplaceTailWithBranchTo(SameTails[i].getTailStartPos(), MBB);
903 // BB i is no longer a predecessor of SuccBB; remove it from the worklist.
904 MergePotentials.erase(SameTails[i].getMPIter());
906 DEBUG(dbgs() << "\n");
907 // We leave commonTailIndex in the worklist in case there are other blocks
908 // that match it with a smaller number of instructions.
914 bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
915 bool MadeChange = false;
916 if (!EnableTailMerge) return MadeChange;
918 // First find blocks with no successors.
919 MergePotentials.clear();
920 for (MachineBasicBlock &MBB : MF) {
921 if (MergePotentials.size() == TailMergeThreshold)
923 if (!TriedMerging.count(&MBB) && MBB.succ_empty())
924 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(&MBB), &MBB));
927 // If this is a large problem, avoid visiting the same basic blocks
929 if (MergePotentials.size() == TailMergeThreshold)
930 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
931 TriedMerging.insert(MergePotentials[i].getBlock());
933 // See if we can do any tail merging on those.
934 if (MergePotentials.size() >= 2)
935 MadeChange |= TryTailMergeBlocks(nullptr, nullptr);
937 // Look at blocks (IBB) with multiple predecessors (PBB).
938 // We change each predecessor to a canonical form, by
939 // (1) temporarily removing any unconditional branch from the predecessor
941 // (2) alter conditional branches so they branch to the other block
942 // not IBB; this may require adding back an unconditional branch to IBB
943 // later, where there wasn't one coming in. E.g.
945 // fallthrough to QBB
948 // with a conceptual B to IBB after that, which never actually exists.
949 // With those changes, we see whether the predecessors' tails match,
950 // and merge them if so. We change things out of canonical form and
951 // back to the way they were later in the process. (OptimizeBranches
952 // would undo some of this, but we can't use it, because we'd get into
953 // a compile-time infinite loop repeatedly doing and undoing the same
956 for (MachineFunction::iterator I = std::next(MF.begin()), E = MF.end();
958 if (I->pred_size() < 2) continue;
959 SmallPtrSet<MachineBasicBlock *, 8> UniquePreds;
960 MachineBasicBlock *IBB = I;
961 MachineBasicBlock *PredBB = std::prev(I);
962 MergePotentials.clear();
963 for (MachineBasicBlock *PBB : I->predecessors()) {
964 if (MergePotentials.size() == TailMergeThreshold)
967 if (TriedMerging.count(PBB))
970 // Skip blocks that loop to themselves, can't tail merge these.
974 // Visit each predecessor only once.
975 if (!UniquePreds.insert(PBB).second)
978 // Skip blocks which may jump to a landing pad. Can't tail merge these.
979 if (PBB->hasEHPadSuccessor())
982 MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
983 SmallVector<MachineOperand, 4> Cond;
984 if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond, true)) {
985 // Failing case: IBB is the target of a cbr, and we cannot reverse the
987 SmallVector<MachineOperand, 4> NewCond(Cond);
988 if (!Cond.empty() && TBB == IBB) {
989 if (TII->ReverseBranchCondition(NewCond))
991 // This is the QBB case described above
993 FBB = std::next(MachineFunction::iterator(PBB));
996 // Failing case: the only way IBB can be reached from PBB is via
997 // exception handling. Happens for landing pads. Would be nice to have
998 // a bit in the edge so we didn't have to do all this.
999 if (IBB->isEHPad()) {
1000 MachineFunction::iterator IP = PBB; IP++;
1001 MachineBasicBlock *PredNextBB = nullptr;
1005 if (IBB != PredNextBB) // fallthrough
1008 if (TBB != IBB && FBB != IBB) // cbr then ubr
1010 } else if (Cond.empty()) {
1011 if (TBB != IBB) // ubr
1014 if (TBB != IBB && IBB != PredNextBB) // cbr
1019 // Remove the unconditional branch at the end, if any.
1020 if (TBB && (Cond.empty() || FBB)) {
1021 DebugLoc dl; // FIXME: this is nowhere
1022 TII->RemoveBranch(*PBB);
1024 // reinsert conditional branch only, for now
1025 TII->InsertBranch(*PBB, (TBB == IBB) ? FBB : TBB, nullptr,
1029 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(PBB), PBB));
1033 // If this is a large problem, avoid visiting the same basic blocks multiple
1035 if (MergePotentials.size() == TailMergeThreshold)
1036 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
1037 TriedMerging.insert(MergePotentials[i].getBlock());
1039 if (MergePotentials.size() >= 2)
1040 MadeChange |= TryTailMergeBlocks(IBB, PredBB);
1042 // Reinsert an unconditional branch if needed. The 1 below can occur as a
1043 // result of removing blocks in TryTailMergeBlocks.
1044 PredBB = std::prev(I); // this may have been changed in TryTailMergeBlocks
1045 if (MergePotentials.size() == 1 &&
1046 MergePotentials.begin()->getBlock() != PredBB)
1047 FixTail(MergePotentials.begin()->getBlock(), IBB, TII);
1053 void BranchFolder::setCommonTailEdgeWeights(MachineBasicBlock &TailMBB) {
1054 SmallVector<BlockFrequency, 2> EdgeFreqLs(TailMBB.succ_size());
1055 BlockFrequency AccumulatedMBBFreq;
1057 // Aggregate edge frequency of successor edge j:
1058 // edgeFreq(j) = sum (freq(bb) * edgeProb(bb, j)),
1059 // where bb is a basic block that is in SameTails.
1060 for (const auto &Src : SameTails) {
1061 const MachineBasicBlock *SrcMBB = Src.getBlock();
1062 BlockFrequency BlockFreq = MBBFreqInfo.getBlockFreq(SrcMBB);
1063 AccumulatedMBBFreq += BlockFreq;
1065 // It is not necessary to recompute edge weights if TailBB has less than two
1067 if (TailMBB.succ_size() <= 1)
1070 auto EdgeFreq = EdgeFreqLs.begin();
1072 for (auto SuccI = TailMBB.succ_begin(), SuccE = TailMBB.succ_end();
1073 SuccI != SuccE; ++SuccI, ++EdgeFreq)
1074 *EdgeFreq += BlockFreq * MBPI.getEdgeProbability(SrcMBB, *SuccI);
1077 MBBFreqInfo.setBlockFreq(&TailMBB, AccumulatedMBBFreq);
1079 if (TailMBB.succ_size() <= 1)
1082 auto MaxEdgeFreq = *std::max_element(EdgeFreqLs.begin(), EdgeFreqLs.end());
1083 uint64_t Scale = MaxEdgeFreq.getFrequency() / UINT32_MAX + 1;
1084 auto EdgeFreq = EdgeFreqLs.begin();
1086 for (auto SuccI = TailMBB.succ_begin(), SuccE = TailMBB.succ_end();
1087 SuccI != SuccE; ++SuccI, ++EdgeFreq)
1088 TailMBB.setSuccWeight(SuccI, EdgeFreq->getFrequency() / Scale);
1091 //===----------------------------------------------------------------------===//
1092 // Branch Optimization
1093 //===----------------------------------------------------------------------===//
1095 bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
1096 bool MadeChange = false;
1098 // Make sure blocks are numbered in order
1099 MF.RenumberBlocks();
1101 for (MachineFunction::iterator I = std::next(MF.begin()), E = MF.end();
1103 MachineBasicBlock *MBB = I++;
1104 MadeChange |= OptimizeBlock(MBB);
1106 // If it is dead, remove it.
1107 if (MBB->pred_empty()) {
1108 RemoveDeadBlock(MBB);
1116 // Blocks should be considered empty if they contain only debug info;
1117 // else the debug info would affect codegen.
1118 static bool IsEmptyBlock(MachineBasicBlock *MBB) {
1119 return MBB->getFirstNonDebugInstr() == MBB->end();
1122 // Blocks with only debug info and branches should be considered the same
1123 // as blocks with only branches.
1124 static bool IsBranchOnlyBlock(MachineBasicBlock *MBB) {
1125 MachineBasicBlock::iterator I = MBB->getFirstNonDebugInstr();
1126 assert(I != MBB->end() && "empty block!");
1127 return I->isBranch();
1130 /// IsBetterFallthrough - Return true if it would be clearly better to
1131 /// fall-through to MBB1 than to fall through into MBB2. This has to return
1132 /// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will
1133 /// result in infinite loops.
1134 static bool IsBetterFallthrough(MachineBasicBlock *MBB1,
1135 MachineBasicBlock *MBB2) {
1136 // Right now, we use a simple heuristic. If MBB2 ends with a call, and
1137 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to
1138 // optimize branches that branch to either a return block or an assert block
1139 // into a fallthrough to the return.
1140 MachineBasicBlock::iterator MBB1I = MBB1->getLastNonDebugInstr();
1141 MachineBasicBlock::iterator MBB2I = MBB2->getLastNonDebugInstr();
1142 if (MBB1I == MBB1->end() || MBB2I == MBB2->end())
1145 // If there is a clear successor ordering we make sure that one block
1146 // will fall through to the next
1147 if (MBB1->isSuccessor(MBB2)) return true;
1148 if (MBB2->isSuccessor(MBB1)) return false;
1150 return MBB2I->isCall() && !MBB1I->isCall();
1153 /// getBranchDebugLoc - Find and return, if any, the DebugLoc of the branch
1154 /// instructions on the block.
1155 static DebugLoc getBranchDebugLoc(MachineBasicBlock &MBB) {
1156 MachineBasicBlock::iterator I = MBB.getLastNonDebugInstr();
1157 if (I != MBB.end() && I->isBranch())
1158 return I->getDebugLoc();
1162 /// OptimizeBlock - Analyze and optimize control flow related to the specified
1163 /// block. This is never called on the entry block.
1164 bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
1165 bool MadeChange = false;
1166 MachineFunction &MF = *MBB->getParent();
1169 MachineFunction::iterator FallThrough = MBB;
1172 // If this block is empty, make everyone use its fall-through, not the block
1173 // explicitly. Landing pads should not do this since the landing-pad table
1174 // points to this block. Blocks with their addresses taken shouldn't be
1176 if (IsEmptyBlock(MBB) && !MBB->isEHPad() && !MBB->hasAddressTaken()) {
1177 // Dead block? Leave for cleanup later.
1178 if (MBB->pred_empty()) return MadeChange;
1180 if (FallThrough == MF.end()) {
1181 // TODO: Simplify preds to not branch here if possible!
1182 } else if (FallThrough->isEHPad()) {
1183 // Don't rewrite to a landing pad fallthough. That could lead to the case
1184 // where a BB jumps to more than one landing pad.
1185 // TODO: Is it ever worth rewriting predecessors which don't already
1186 // jump to a landing pad, and so can safely jump to the fallthrough?
1188 // Rewrite all predecessors of the old block to go to the fallthrough
1190 while (!MBB->pred_empty()) {
1191 MachineBasicBlock *Pred = *(MBB->pred_end()-1);
1192 Pred->ReplaceUsesOfBlockWith(MBB, FallThrough);
1194 // If MBB was the target of a jump table, update jump tables to go to the
1195 // fallthrough instead.
1196 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
1197 MJTI->ReplaceMBBInJumpTables(MBB, FallThrough);
1203 // Check to see if we can simplify the terminator of the block before this
1205 MachineBasicBlock &PrevBB = *std::prev(MachineFunction::iterator(MBB));
1207 MachineBasicBlock *PriorTBB = nullptr, *PriorFBB = nullptr;
1208 SmallVector<MachineOperand, 4> PriorCond;
1209 bool PriorUnAnalyzable =
1210 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
1211 if (!PriorUnAnalyzable) {
1212 // If the CFG for the prior block has extra edges, remove them.
1213 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB,
1214 !PriorCond.empty());
1216 // If the previous branch is conditional and both conditions go to the same
1217 // destination, remove the branch, replacing it with an unconditional one or
1219 if (PriorTBB && PriorTBB == PriorFBB) {
1220 DebugLoc dl = getBranchDebugLoc(PrevBB);
1221 TII->RemoveBranch(PrevBB);
1223 if (PriorTBB != MBB)
1224 TII->InsertBranch(PrevBB, PriorTBB, nullptr, PriorCond, dl);
1227 goto ReoptimizeBlock;
1230 // If the previous block unconditionally falls through to this block and
1231 // this block has no other predecessors, move the contents of this block
1232 // into the prior block. This doesn't usually happen when SimplifyCFG
1233 // has been used, but it can happen if tail merging splits a fall-through
1234 // predecessor of a block.
1235 // This has to check PrevBB->succ_size() because EH edges are ignored by
1237 if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 &&
1238 PrevBB.succ_size() == 1 &&
1239 !MBB->hasAddressTaken() && !MBB->isEHPad()) {
1240 DEBUG(dbgs() << "\nMerging into block: " << PrevBB
1241 << "From MBB: " << *MBB);
1242 // Remove redundant DBG_VALUEs first.
1243 if (PrevBB.begin() != PrevBB.end()) {
1244 MachineBasicBlock::iterator PrevBBIter = PrevBB.end();
1246 MachineBasicBlock::iterator MBBIter = MBB->begin();
1247 // Check if DBG_VALUE at the end of PrevBB is identical to the
1248 // DBG_VALUE at the beginning of MBB.
1249 while (PrevBBIter != PrevBB.begin() && MBBIter != MBB->end()
1250 && PrevBBIter->isDebugValue() && MBBIter->isDebugValue()) {
1251 if (!MBBIter->isIdenticalTo(PrevBBIter))
1253 MachineInstr *DuplicateDbg = MBBIter;
1254 ++MBBIter; -- PrevBBIter;
1255 DuplicateDbg->eraseFromParent();
1258 PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end());
1259 PrevBB.removeSuccessor(PrevBB.succ_begin());
1260 assert(PrevBB.succ_empty());
1261 PrevBB.transferSuccessors(MBB);
1266 // If the previous branch *only* branches to *this* block (conditional or
1267 // not) remove the branch.
1268 if (PriorTBB == MBB && !PriorFBB) {
1269 TII->RemoveBranch(PrevBB);
1272 goto ReoptimizeBlock;
1275 // If the prior block branches somewhere else on the condition and here if
1276 // the condition is false, remove the uncond second branch.
1277 if (PriorFBB == MBB) {
1278 DebugLoc dl = getBranchDebugLoc(PrevBB);
1279 TII->RemoveBranch(PrevBB);
1280 TII->InsertBranch(PrevBB, PriorTBB, nullptr, PriorCond, dl);
1283 goto ReoptimizeBlock;
1286 // If the prior block branches here on true and somewhere else on false, and
1287 // if the branch condition is reversible, reverse the branch to create a
1289 if (PriorTBB == MBB) {
1290 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1291 if (!TII->ReverseBranchCondition(NewPriorCond)) {
1292 DebugLoc dl = getBranchDebugLoc(PrevBB);
1293 TII->RemoveBranch(PrevBB);
1294 TII->InsertBranch(PrevBB, PriorFBB, nullptr, NewPriorCond, dl);
1297 goto ReoptimizeBlock;
1301 // If this block has no successors (e.g. it is a return block or ends with
1302 // a call to a no-return function like abort or __cxa_throw) and if the pred
1303 // falls through into this block, and if it would otherwise fall through
1304 // into the block after this, move this block to the end of the function.
1306 // We consider it more likely that execution will stay in the function (e.g.
1307 // due to loops) than it is to exit it. This asserts in loops etc, moving
1308 // the assert condition out of the loop body.
1309 if (MBB->succ_empty() && !PriorCond.empty() && !PriorFBB &&
1310 MachineFunction::iterator(PriorTBB) == FallThrough &&
1311 !MBB->canFallThrough()) {
1312 bool DoTransform = true;
1314 // We have to be careful that the succs of PredBB aren't both no-successor
1315 // blocks. If neither have successors and if PredBB is the second from
1316 // last block in the function, we'd just keep swapping the two blocks for
1317 // last. Only do the swap if one is clearly better to fall through than
1319 if (FallThrough == --MF.end() &&
1320 !IsBetterFallthrough(PriorTBB, MBB))
1321 DoTransform = false;
1324 // Reverse the branch so we will fall through on the previous true cond.
1325 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1326 if (!TII->ReverseBranchCondition(NewPriorCond)) {
1327 DEBUG(dbgs() << "\nMoving MBB: " << *MBB
1328 << "To make fallthrough to: " << *PriorTBB << "\n");
1330 DebugLoc dl = getBranchDebugLoc(PrevBB);
1331 TII->RemoveBranch(PrevBB);
1332 TII->InsertBranch(PrevBB, MBB, nullptr, NewPriorCond, dl);
1334 // Move this block to the end of the function.
1335 MBB->moveAfter(--MF.end());
1344 // Analyze the branch in the current block.
1345 MachineBasicBlock *CurTBB = nullptr, *CurFBB = nullptr;
1346 SmallVector<MachineOperand, 4> CurCond;
1347 bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true);
1348 if (!CurUnAnalyzable) {
1349 // If the CFG for the prior block has extra edges, remove them.
1350 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty());
1352 // If this is a two-way branch, and the FBB branches to this block, reverse
1353 // the condition so the single-basic-block loop is faster. Instead of:
1354 // Loop: xxx; jcc Out; jmp Loop
1356 // Loop: xxx; jncc Loop; jmp Out
1357 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) {
1358 SmallVector<MachineOperand, 4> NewCond(CurCond);
1359 if (!TII->ReverseBranchCondition(NewCond)) {
1360 DebugLoc dl = getBranchDebugLoc(*MBB);
1361 TII->RemoveBranch(*MBB);
1362 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond, dl);
1365 goto ReoptimizeBlock;
1369 // If this branch is the only thing in its block, see if we can forward
1370 // other blocks across it.
1371 if (CurTBB && CurCond.empty() && !CurFBB &&
1372 IsBranchOnlyBlock(MBB) && CurTBB != MBB &&
1373 !MBB->hasAddressTaken()) {
1374 DebugLoc dl = getBranchDebugLoc(*MBB);
1375 // This block may contain just an unconditional branch. Because there can
1376 // be 'non-branch terminators' in the block, try removing the branch and
1377 // then seeing if the block is empty.
1378 TII->RemoveBranch(*MBB);
1379 // If the only things remaining in the block are debug info, remove these
1380 // as well, so this will behave the same as an empty block in non-debug
1382 if (IsEmptyBlock(MBB)) {
1383 // Make the block empty, losing the debug info (we could probably
1384 // improve this in some cases.)
1385 MBB->erase(MBB->begin(), MBB->end());
1387 // If this block is just an unconditional branch to CurTBB, we can
1388 // usually completely eliminate the block. The only case we cannot
1389 // completely eliminate the block is when the block before this one
1390 // falls through into MBB and we can't understand the prior block's branch
1393 bool PredHasNoFallThrough = !PrevBB.canFallThrough();
1394 if (PredHasNoFallThrough || !PriorUnAnalyzable ||
1395 !PrevBB.isSuccessor(MBB)) {
1396 // If the prior block falls through into us, turn it into an
1397 // explicit branch to us to make updates simpler.
1398 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) &&
1399 PriorTBB != MBB && PriorFBB != MBB) {
1401 assert(PriorCond.empty() && !PriorFBB &&
1402 "Bad branch analysis");
1405 assert(!PriorFBB && "Machine CFG out of date!");
1408 DebugLoc pdl = getBranchDebugLoc(PrevBB);
1409 TII->RemoveBranch(PrevBB);
1410 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, pdl);
1413 // Iterate through all the predecessors, revectoring each in-turn.
1415 bool DidChange = false;
1416 bool HasBranchToSelf = false;
1417 while(PI != MBB->pred_size()) {
1418 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI);
1420 // If this block has an uncond branch to itself, leave it.
1422 HasBranchToSelf = true;
1425 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB);
1426 // If this change resulted in PMBB ending in a conditional
1427 // branch where both conditions go to the same destination,
1428 // change this to an unconditional branch (and fix the CFG).
1429 MachineBasicBlock *NewCurTBB = nullptr, *NewCurFBB = nullptr;
1430 SmallVector<MachineOperand, 4> NewCurCond;
1431 bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB,
1432 NewCurFBB, NewCurCond, true);
1433 if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) {
1434 DebugLoc pdl = getBranchDebugLoc(*PMBB);
1435 TII->RemoveBranch(*PMBB);
1437 TII->InsertBranch(*PMBB, NewCurTBB, nullptr, NewCurCond, pdl);
1440 PMBB->CorrectExtraCFGEdges(NewCurTBB, nullptr, false);
1445 // Change any jumptables to go to the new MBB.
1446 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
1447 MJTI->ReplaceMBBInJumpTables(MBB, CurTBB);
1451 if (!HasBranchToSelf) return MadeChange;
1456 // Add the branch back if the block is more than just an uncond branch.
1457 TII->InsertBranch(*MBB, CurTBB, nullptr, CurCond, dl);
1461 // If the prior block doesn't fall through into this block, and if this
1462 // block doesn't fall through into some other block, see if we can find a
1463 // place to move this block where a fall-through will happen.
1464 if (!PrevBB.canFallThrough()) {
1466 // Now we know that there was no fall-through into this block, check to
1467 // see if it has a fall-through into its successor.
1468 bool CurFallsThru = MBB->canFallThrough();
1470 if (!MBB->isEHPad()) {
1471 // Check all the predecessors of this block. If one of them has no fall
1472 // throughs, move this block right after it.
1473 for (MachineBasicBlock *PredBB : MBB->predecessors()) {
1474 // Analyze the branch at the end of the pred.
1475 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough;
1476 MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
1477 SmallVector<MachineOperand, 4> PredCond;
1478 if (PredBB != MBB && !PredBB->canFallThrough() &&
1479 !TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true)
1480 && (!CurFallsThru || !CurTBB || !CurFBB)
1481 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
1482 // If the current block doesn't fall through, just move it.
1483 // If the current block can fall through and does not end with a
1484 // conditional branch, we need to append an unconditional jump to
1485 // the (current) next block. To avoid a possible compile-time
1486 // infinite loop, move blocks only backward in this case.
1487 // Also, if there are already 2 branches here, we cannot add a third;
1488 // this means we have the case
1493 MachineBasicBlock *NextBB =
1494 std::next(MachineFunction::iterator(MBB));
1496 TII->InsertBranch(*MBB, NextBB, nullptr, CurCond, DebugLoc());
1498 MBB->moveAfter(PredBB);
1500 goto ReoptimizeBlock;
1505 if (!CurFallsThru) {
1506 // Check all successors to see if we can move this block before it.
1507 for (MachineBasicBlock *SuccBB : MBB->successors()) {
1508 // Analyze the branch at the end of the block before the succ.
1509 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev;
1511 // If this block doesn't already fall-through to that successor, and if
1512 // the succ doesn't already have a block that can fall through into it,
1513 // and if the successor isn't an EH destination, we can arrange for the
1514 // fallthrough to happen.
1515 if (SuccBB != MBB && &*SuccPrev != MBB &&
1516 !SuccPrev->canFallThrough() && !CurUnAnalyzable &&
1517 !SuccBB->isEHPad()) {
1518 MBB->moveBefore(SuccBB);
1520 goto ReoptimizeBlock;
1524 // Okay, there is no really great place to put this block. If, however,
1525 // the block before this one would be a fall-through if this block were
1526 // removed, move this block to the end of the function.
1527 MachineBasicBlock *PrevTBB = nullptr, *PrevFBB = nullptr;
1528 SmallVector<MachineOperand, 4> PrevCond;
1529 if (FallThrough != MF.end() &&
1530 !TII->AnalyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) &&
1531 PrevBB.isSuccessor(FallThrough)) {
1532 MBB->moveAfter(--MF.end());
1542 //===----------------------------------------------------------------------===//
1543 // Hoist Common Code
1544 //===----------------------------------------------------------------------===//
1546 /// HoistCommonCode - Hoist common instruction sequences at the start of basic
1547 /// blocks to their common predecessor.
1548 bool BranchFolder::HoistCommonCode(MachineFunction &MF) {
1549 bool MadeChange = false;
1550 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ) {
1551 MachineBasicBlock *MBB = I++;
1552 MadeChange |= HoistCommonCodeInSuccs(MBB);
1558 /// findFalseBlock - BB has a fallthrough. Find its 'false' successor given
1559 /// its 'true' successor.
1560 static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB,
1561 MachineBasicBlock *TrueBB) {
1562 for (MachineBasicBlock *SuccBB : BB->successors())
1563 if (SuccBB != TrueBB)
1568 template <class Container>
1569 static void addRegAndItsAliases(unsigned Reg, const TargetRegisterInfo *TRI,
1571 if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
1572 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
1579 /// findHoistingInsertPosAndDeps - Find the location to move common instructions
1580 /// in successors to. The location is usually just before the terminator,
1581 /// however if the terminator is a conditional branch and its previous
1582 /// instruction is the flag setting instruction, the previous instruction is
1583 /// the preferred location. This function also gathers uses and defs of the
1584 /// instructions from the insertion point to the end of the block. The data is
1585 /// used by HoistCommonCodeInSuccs to ensure safety.
1587 MachineBasicBlock::iterator findHoistingInsertPosAndDeps(MachineBasicBlock *MBB,
1588 const TargetInstrInfo *TII,
1589 const TargetRegisterInfo *TRI,
1590 SmallSet<unsigned,4> &Uses,
1591 SmallSet<unsigned,4> &Defs) {
1592 MachineBasicBlock::iterator Loc = MBB->getFirstTerminator();
1593 if (!TII->isUnpredicatedTerminator(Loc))
1596 for (const MachineOperand &MO : Loc->operands()) {
1599 unsigned Reg = MO.getReg();
1603 addRegAndItsAliases(Reg, TRI, Uses);
1606 // Don't try to hoist code in the rare case the terminator defines a
1607 // register that is later used.
1610 // If the terminator defines a register, make sure we don't hoist
1611 // the instruction whose def might be clobbered by the terminator.
1612 addRegAndItsAliases(Reg, TRI, Defs);
1618 if (Loc == MBB->begin())
1621 // The terminator is probably a conditional branch, try not to separate the
1622 // branch from condition setting instruction.
1623 MachineBasicBlock::iterator PI = Loc;
1625 while (PI != MBB->begin() && PI->isDebugValue())
1629 for (const MachineOperand &MO : PI->operands()) {
1630 // If PI has a regmask operand, it is probably a call. Separate away.
1633 if (!MO.isReg() || MO.isUse())
1635 unsigned Reg = MO.getReg();
1638 if (Uses.count(Reg)) {
1644 // The condition setting instruction is not just before the conditional
1648 // Be conservative, don't insert instruction above something that may have
1649 // side-effects. And since it's potentially bad to separate flag setting
1650 // instruction from the conditional branch, just abort the optimization
1652 // Also avoid moving code above predicated instruction since it's hard to
1653 // reason about register liveness with predicated instruction.
1654 bool DontMoveAcrossStore = true;
1655 if (!PI->isSafeToMove(nullptr, DontMoveAcrossStore) || TII->isPredicated(PI))
1659 // Find out what registers are live. Note this routine is ignoring other live
1660 // registers which are only used by instructions in successor blocks.
1661 for (const MachineOperand &MO : PI->operands()) {
1664 unsigned Reg = MO.getReg();
1668 addRegAndItsAliases(Reg, TRI, Uses);
1670 if (Uses.erase(Reg)) {
1671 if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
1672 for (MCSubRegIterator SubRegs(Reg, TRI); SubRegs.isValid(); ++SubRegs)
1673 Uses.erase(*SubRegs); // Use sub-registers to be conservative
1676 addRegAndItsAliases(Reg, TRI, Defs);
1683 /// HoistCommonCodeInSuccs - If the successors of MBB has common instruction
1684 /// sequence at the start of the function, move the instructions before MBB
1685 /// terminator if it's legal.
1686 bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
1687 MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
1688 SmallVector<MachineOperand, 4> Cond;
1689 if (TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true) || !TBB || Cond.empty())
1692 if (!FBB) FBB = findFalseBlock(MBB, TBB);
1694 // Malformed bcc? True and false blocks are the same?
1697 // Restrict the optimization to cases where MBB is the only predecessor,
1698 // it is an obvious win.
1699 if (TBB->pred_size() > 1 || FBB->pred_size() > 1)
1702 // Find a suitable position to hoist the common instructions to. Also figure
1703 // out which registers are used or defined by instructions from the insertion
1704 // point to the end of the block.
1705 SmallSet<unsigned, 4> Uses, Defs;
1706 MachineBasicBlock::iterator Loc =
1707 findHoistingInsertPosAndDeps(MBB, TII, TRI, Uses, Defs);
1708 if (Loc == MBB->end())
1711 bool HasDups = false;
1712 SmallVector<unsigned, 4> LocalDefs;
1713 SmallSet<unsigned, 4> LocalDefsSet;
1714 MachineBasicBlock::iterator TIB = TBB->begin();
1715 MachineBasicBlock::iterator FIB = FBB->begin();
1716 MachineBasicBlock::iterator TIE = TBB->end();
1717 MachineBasicBlock::iterator FIE = FBB->end();
1718 while (TIB != TIE && FIB != FIE) {
1719 // Skip dbg_value instructions. These do not count.
1720 if (TIB->isDebugValue()) {
1721 while (TIB != TIE && TIB->isDebugValue())
1726 if (FIB->isDebugValue()) {
1727 while (FIB != FIE && FIB->isDebugValue())
1732 if (!TIB->isIdenticalTo(FIB, MachineInstr::CheckKillDead))
1735 if (TII->isPredicated(TIB))
1736 // Hard to reason about register liveness with predicated instruction.
1740 for (MachineOperand &MO : TIB->operands()) {
1741 // Don't attempt to hoist instructions with register masks.
1742 if (MO.isRegMask()) {
1748 unsigned Reg = MO.getReg();
1752 if (Uses.count(Reg)) {
1753 // Avoid clobbering a register that's used by the instruction at
1754 // the point of insertion.
1759 if (Defs.count(Reg) && !MO.isDead()) {
1760 // Don't hoist the instruction if the def would be clobber by the
1761 // instruction at the point insertion. FIXME: This is overly
1762 // conservative. It should be possible to hoist the instructions
1763 // in BB2 in the following example:
1765 // r1, eflag = op1 r2, r3
1774 } else if (!LocalDefsSet.count(Reg)) {
1775 if (Defs.count(Reg)) {
1776 // Use is defined by the instruction at the point of insertion.
1781 if (MO.isKill() && Uses.count(Reg))
1782 // Kills a register that's read by the instruction at the point of
1783 // insertion. Remove the kill marker.
1784 MO.setIsKill(false);
1790 bool DontMoveAcrossStore = true;
1791 if (!TIB->isSafeToMove(nullptr, DontMoveAcrossStore))
1794 // Remove kills from LocalDefsSet, these registers had short live ranges.
1795 for (const MachineOperand &MO : TIB->operands()) {
1796 if (!MO.isReg() || !MO.isUse() || !MO.isKill())
1798 unsigned Reg = MO.getReg();
1799 if (!Reg || !LocalDefsSet.count(Reg))
1801 if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
1802 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
1803 LocalDefsSet.erase(*AI);
1805 LocalDefsSet.erase(Reg);
1809 // Track local defs so we can update liveins.
1810 for (const MachineOperand &MO : TIB->operands()) {
1811 if (!MO.isReg() || !MO.isDef() || MO.isDead())
1813 unsigned Reg = MO.getReg();
1816 LocalDefs.push_back(Reg);
1817 addRegAndItsAliases(Reg, TRI, LocalDefsSet);
1828 MBB->splice(Loc, TBB, TBB->begin(), TIB);
1829 FBB->erase(FBB->begin(), FIB);
1832 for (unsigned i = 0, e = LocalDefs.size(); i != e; ++i) {
1833 unsigned Def = LocalDefs[i];
1834 if (LocalDefsSet.count(Def)) {
1835 TBB->addLiveIn(Def);
1836 FBB->addLiveIn(Def);