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
17 //===----------------------------------------------------------------------===//
19 #define DEBUG_TYPE "branchfolding"
20 #include "BranchFolding.h"
21 #include "llvm/Function.h"
22 #include "llvm/CodeGen/Passes.h"
23 #include "llvm/CodeGen/MachineModuleInfo.h"
24 #include "llvm/CodeGen/MachineFunctionPass.h"
25 #include "llvm/CodeGen/MachineJumpTableInfo.h"
26 #include "llvm/CodeGen/RegisterScavenging.h"
27 #include "llvm/Target/TargetInstrInfo.h"
28 #include "llvm/Target/TargetMachine.h"
29 #include "llvm/Target/TargetRegisterInfo.h"
30 #include "llvm/Support/CommandLine.h"
31 #include "llvm/Support/Debug.h"
32 #include "llvm/Support/ErrorHandling.h"
33 #include "llvm/Support/raw_ostream.h"
34 #include "llvm/ADT/SmallSet.h"
35 #include "llvm/ADT/SetVector.h"
36 #include "llvm/ADT/Statistic.h"
37 #include "llvm/ADT/STLExtras.h"
41 STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
42 STATISTIC(NumBranchOpts, "Number of branches optimized");
43 STATISTIC(NumTailMerge , "Number of block tails merged");
45 static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge",
46 cl::init(cl::BOU_UNSET), cl::Hidden);
48 // Throttle for huge numbers of predecessors (compile speed problems)
49 static cl::opt<unsigned>
50 TailMergeThreshold("tail-merge-threshold",
51 cl::desc("Max number of predecessors to consider tail merging"),
52 cl::init(150), cl::Hidden);
54 // Heuristic for tail merging (and, inversely, tail duplication).
55 // TODO: This should be replaced with a target query.
56 static cl::opt<unsigned>
57 TailMergeSize("tail-merge-size",
58 cl::desc("Min number of instructions to consider tail merging"),
59 cl::init(3), cl::Hidden);
62 /// BranchFolderPass - Wrap branch folder in a machine function pass.
63 class BranchFolderPass : public MachineFunctionPass,
67 explicit BranchFolderPass(bool defaultEnableTailMerge)
68 : MachineFunctionPass(&ID), BranchFolder(defaultEnableTailMerge) {}
70 virtual bool runOnMachineFunction(MachineFunction &MF);
71 virtual const char *getPassName() const { return "Control Flow Optimizer"; }
75 char BranchFolderPass::ID = 0;
77 FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) {
78 return new BranchFolderPass(DefaultEnableTailMerge);
81 bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) {
82 return OptimizeFunction(MF,
83 MF.getTarget().getInstrInfo(),
84 MF.getTarget().getRegisterInfo(),
85 getAnalysisIfAvailable<MachineModuleInfo>());
89 BranchFolder::BranchFolder(bool defaultEnableTailMerge) {
90 switch (FlagEnableTailMerge) {
91 case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break;
92 case cl::BOU_TRUE: EnableTailMerge = true; break;
93 case cl::BOU_FALSE: EnableTailMerge = false; break;
97 /// RemoveDeadBlock - Remove the specified dead machine basic block from the
98 /// function, updating the CFG.
99 void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) {
100 assert(MBB->pred_empty() && "MBB must be dead!");
101 DEBUG(dbgs() << "\nRemoving MBB: " << *MBB);
103 MachineFunction *MF = MBB->getParent();
104 // drop all successors.
105 while (!MBB->succ_empty())
106 MBB->removeSuccessor(MBB->succ_end()-1);
112 /// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def
113 /// followed by terminators, and if the implicitly defined registers are not
114 /// used by the terminators, remove those implicit_def's. e.g.
116 /// r0 = implicit_def
117 /// r1 = implicit_def
119 /// This block can be optimized away later if the implicit instructions are
121 bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) {
122 SmallSet<unsigned, 4> ImpDefRegs;
123 MachineBasicBlock::iterator I = MBB->begin();
124 while (I != MBB->end()) {
125 if (!I->isImplicitDef())
127 unsigned Reg = I->getOperand(0).getReg();
128 ImpDefRegs.insert(Reg);
129 for (const unsigned *SubRegs = TRI->getSubRegisters(Reg);
130 unsigned SubReg = *SubRegs; ++SubRegs)
131 ImpDefRegs.insert(SubReg);
134 if (ImpDefRegs.empty())
137 MachineBasicBlock::iterator FirstTerm = I;
138 while (I != MBB->end()) {
139 if (!TII->isUnpredicatedTerminator(I))
141 // See if it uses any of the implicitly defined registers.
142 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
143 MachineOperand &MO = I->getOperand(i);
144 if (!MO.isReg() || !MO.isUse())
146 unsigned Reg = MO.getReg();
147 if (ImpDefRegs.count(Reg))
154 while (I != FirstTerm) {
155 MachineInstr *ImpDefMI = &*I;
157 MBB->erase(ImpDefMI);
163 /// OptimizeFunction - Perhaps branch folding, tail merging and other
164 /// CFG optimizations on the given function.
165 bool BranchFolder::OptimizeFunction(MachineFunction &MF,
166 const TargetInstrInfo *tii,
167 const TargetRegisterInfo *tri,
168 MachineModuleInfo *mmi) {
169 if (!tii) return false;
175 RS = TRI->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL;
177 // Fix CFG. The later algorithms expect it to be right.
178 bool MadeChange = false;
179 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) {
180 MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0;
181 SmallVector<MachineOperand, 4> Cond;
182 if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true))
183 MadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty());
184 MadeChange |= OptimizeImpDefsBlock(MBB);
187 bool MadeChangeThisIteration = true;
188 while (MadeChangeThisIteration) {
189 MadeChangeThisIteration = false;
190 MadeChangeThisIteration |= TailMergeBlocks(MF);
191 MadeChangeThisIteration |= OptimizeBranches(MF);
192 MadeChange |= MadeChangeThisIteration;
195 // See if any jump tables have become dead as the code generator
197 MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
203 // Walk the function to find jump tables that are live.
204 BitVector JTIsLive(JTI->getJumpTables().size());
205 for (MachineFunction::iterator BB = MF.begin(), E = MF.end();
207 for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end();
209 for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) {
210 MachineOperand &Op = I->getOperand(op);
211 if (!Op.isJTI()) continue;
213 // Remember that this JT is live.
214 JTIsLive.set(Op.getIndex());
218 // Finally, remove dead jump tables. This happens when the
219 // indirect jump was unreachable (and thus deleted).
220 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i)
221 if (!JTIsLive.test(i)) {
222 JTI->RemoveJumpTable(i);
230 //===----------------------------------------------------------------------===//
231 // Tail Merging of Blocks
232 //===----------------------------------------------------------------------===//
234 /// HashMachineInstr - Compute a hash value for MI and its operands.
235 static unsigned HashMachineInstr(const MachineInstr *MI) {
236 unsigned Hash = MI->getOpcode();
237 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
238 const MachineOperand &Op = MI->getOperand(i);
240 // Merge in bits from the operand if easy.
241 unsigned OperandHash = 0;
242 switch (Op.getType()) {
243 case MachineOperand::MO_Register: OperandHash = Op.getReg(); break;
244 case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break;
245 case MachineOperand::MO_MachineBasicBlock:
246 OperandHash = Op.getMBB()->getNumber();
248 case MachineOperand::MO_FrameIndex:
249 case MachineOperand::MO_ConstantPoolIndex:
250 case MachineOperand::MO_JumpTableIndex:
251 OperandHash = Op.getIndex();
253 case MachineOperand::MO_GlobalAddress:
254 case MachineOperand::MO_ExternalSymbol:
255 // Global address / external symbol are too hard, don't bother, but do
256 // pull in the offset.
257 OperandHash = Op.getOffset();
262 Hash += ((OperandHash << 3) | Op.getType()) << (i&31);
267 /// HashEndOfMBB - Hash the last instruction in the MBB.
268 static unsigned HashEndOfMBB(const MachineBasicBlock *MBB) {
269 MachineBasicBlock::const_iterator I = MBB->end();
270 if (I == MBB->begin())
271 return 0; // Empty MBB.
274 // Skip debug info so it will not affect codegen.
275 while (I->isDebugValue()) {
277 return 0; // MBB empty except for debug info.
281 return HashMachineInstr(I);
284 /// ComputeCommonTailLength - Given two machine basic blocks, compute the number
285 /// of instructions they actually have in common together at their end. Return
286 /// iterators for the first shared instruction in each block.
287 static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1,
288 MachineBasicBlock *MBB2,
289 MachineBasicBlock::iterator &I1,
290 MachineBasicBlock::iterator &I2) {
294 unsigned TailLen = 0;
295 while (I1 != MBB1->begin() && I2 != MBB2->begin()) {
297 // Skip debugging pseudos; necessary to avoid changing the code.
298 while (I1->isDebugValue()) {
299 if (I1==MBB1->begin()) {
300 while (I2->isDebugValue()) {
301 if (I2==MBB2->begin())
302 // I1==DBG at begin; I2==DBG at begin
307 // I1==DBG at begin; I2==non-DBG, or first of DBGs not at begin
312 // I1==first (untested) non-DBG preceding known match
313 while (I2->isDebugValue()) {
314 if (I2==MBB2->begin()) {
316 // I1==non-DBG, or first of DBGs not at begin; I2==DBG at begin
321 // I1, I2==first (untested) non-DBGs preceding known match
322 if (!I1->isIdenticalTo(I2) ||
323 // FIXME: This check is dubious. It's used to get around a problem where
324 // people incorrectly expect inline asm directives to remain in the same
325 // relative order. This is untenable because normal compiler
326 // optimizations (like this one) may reorder and/or merge these
334 // Back past possible debugging pseudos at beginning of block. This matters
335 // when one block differs from the other only by whether debugging pseudos
336 // are present at the beginning. (This way, the various checks later for
337 // I1==MBB1->begin() work as expected.)
338 if (I1 == MBB1->begin() && I2 != MBB2->begin()) {
340 while (I2->isDebugValue()) {
341 if (I2 == MBB2->begin()) {
348 if (I2 == MBB2->begin() && I1 != MBB1->begin()) {
350 while (I1->isDebugValue()) {
351 if (I1 == MBB1->begin())
360 /// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything
361 /// after it, replacing it with an unconditional branch to NewDest.
362 void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
363 MachineBasicBlock *NewDest) {
364 TII->ReplaceTailWithBranchTo(OldInst, NewDest);
368 /// SplitMBBAt - Given a machine basic block and an iterator into it, split the
369 /// MBB so that the part before the iterator falls into the part starting at the
370 /// iterator. This returns the new MBB.
371 MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB,
372 MachineBasicBlock::iterator BBI1) {
373 MachineFunction &MF = *CurMBB.getParent();
375 // Create the fall-through block.
376 MachineFunction::iterator MBBI = &CurMBB;
377 MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(CurMBB.getBasicBlock());
378 CurMBB.getParent()->insert(++MBBI, NewMBB);
380 // Move all the successors of this block to the specified block.
381 NewMBB->transferSuccessors(&CurMBB);
383 // Add an edge from CurMBB to NewMBB for the fall-through.
384 CurMBB.addSuccessor(NewMBB);
386 // Splice the code over.
387 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end());
389 // For targets that use the register scavenger, we must maintain LiveIns.
391 RS->enterBasicBlock(&CurMBB);
393 RS->forward(prior(CurMBB.end()));
394 BitVector RegsLiveAtExit(TRI->getNumRegs());
395 RS->getRegsUsed(RegsLiveAtExit, false);
396 for (unsigned int i = 0, e = TRI->getNumRegs(); i != e; i++)
397 if (RegsLiveAtExit[i])
398 NewMBB->addLiveIn(i);
404 /// EstimateRuntime - Make a rough estimate for how long it will take to run
405 /// the specified code.
406 static unsigned EstimateRuntime(MachineBasicBlock::iterator I,
407 MachineBasicBlock::iterator E) {
409 for (; I != E; ++I) {
410 if (I->isDebugValue())
412 const TargetInstrDesc &TID = I->getDesc();
415 else if (TID.mayLoad() || TID.mayStore())
423 // CurMBB needs to add an unconditional branch to SuccMBB (we removed these
424 // branches temporarily for tail merging). In the case where CurMBB ends
425 // with a conditional branch to the next block, optimize by reversing the
426 // test and conditionally branching to SuccMBB instead.
427 static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB,
428 const TargetInstrInfo *TII) {
429 MachineFunction *MF = CurMBB->getParent();
430 MachineFunction::iterator I = llvm::next(MachineFunction::iterator(CurMBB));
431 MachineBasicBlock *TBB = 0, *FBB = 0;
432 SmallVector<MachineOperand, 4> Cond;
433 DebugLoc dl; // FIXME: this is nowhere
434 if (I != MF->end() &&
435 !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond, true)) {
436 MachineBasicBlock *NextBB = I;
437 if (TBB == NextBB && !Cond.empty() && !FBB) {
438 if (!TII->ReverseBranchCondition(Cond)) {
439 TII->RemoveBranch(*CurMBB);
440 TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond, dl);
445 TII->InsertBranch(*CurMBB, SuccBB, NULL,
446 SmallVector<MachineOperand, 0>(), dl);
450 BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const {
451 if (getHash() < o.getHash())
453 else if (getHash() > o.getHash())
455 else if (getBlock()->getNumber() < o.getBlock()->getNumber())
457 else if (getBlock()->getNumber() > o.getBlock()->getNumber())
460 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing
461 // an object with itself.
462 #ifndef _GLIBCXX_DEBUG
463 llvm_unreachable("Predecessor appears twice");
469 /// CountTerminators - Count the number of terminators in the given
470 /// block and set I to the position of the first non-terminator, if there
471 /// is one, or MBB->end() otherwise.
472 static unsigned CountTerminators(MachineBasicBlock *MBB,
473 MachineBasicBlock::iterator &I) {
475 unsigned NumTerms = 0;
477 if (I == MBB->begin()) {
482 if (!I->getDesc().isTerminator()) break;
488 /// ProfitableToMerge - Check if two machine basic blocks have a common tail
489 /// and decide if it would be profitable to merge those tails. Return the
490 /// length of the common tail and iterators to the first common instruction
492 static bool ProfitableToMerge(MachineBasicBlock *MBB1,
493 MachineBasicBlock *MBB2,
494 unsigned minCommonTailLength,
495 unsigned &CommonTailLen,
496 MachineBasicBlock::iterator &I1,
497 MachineBasicBlock::iterator &I2,
498 MachineBasicBlock *SuccBB,
499 MachineBasicBlock *PredBB) {
500 CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2);
501 MachineFunction *MF = MBB1->getParent();
503 if (CommonTailLen == 0)
506 // It's almost always profitable to merge any number of non-terminator
507 // instructions with the block that falls through into the common successor.
508 if (MBB1 == PredBB || MBB2 == PredBB) {
509 MachineBasicBlock::iterator I;
510 unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I);
511 if (CommonTailLen > NumTerms)
515 // If one of the blocks can be completely merged and happens to be in
516 // a position where the other could fall through into it, merge any number
517 // of instructions, because it can be done without a branch.
518 // TODO: If the blocks are not adjacent, move one of them so that they are?
519 if (MBB1->isLayoutSuccessor(MBB2) && I2 == MBB2->begin())
521 if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin())
524 // If both blocks have an unconditional branch temporarily stripped out,
525 // count that as an additional common instruction for the following
527 unsigned EffectiveTailLen = CommonTailLen;
528 if (SuccBB && MBB1 != PredBB && MBB2 != PredBB &&
529 !MBB1->back().getDesc().isBarrier() &&
530 !MBB2->back().getDesc().isBarrier())
533 // Check if the common tail is long enough to be worthwhile.
534 if (EffectiveTailLen >= minCommonTailLength)
537 // If we are optimizing for code size, 2 instructions in common is enough if
538 // we don't have to split a block. At worst we will be introducing 1 new
539 // branch instruction, which is likely to be smaller than the 2
540 // instructions that would be deleted in the merge.
541 if (EffectiveTailLen >= 2 &&
542 MF->getFunction()->hasFnAttr(Attribute::OptimizeForSize) &&
543 (I1 == MBB1->begin() || I2 == MBB2->begin()))
549 /// ComputeSameTails - Look through all the blocks in MergePotentials that have
550 /// hash CurHash (guaranteed to match the last element). Build the vector
551 /// SameTails of all those that have the (same) largest number of instructions
552 /// in common of any pair of these blocks. SameTails entries contain an
553 /// iterator into MergePotentials (from which the MachineBasicBlock can be
554 /// found) and a MachineBasicBlock::iterator into that MBB indicating the
555 /// instruction where the matching code sequence begins.
556 /// Order of elements in SameTails is the reverse of the order in which
557 /// those blocks appear in MergePotentials (where they are not necessarily
559 unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
560 unsigned minCommonTailLength,
561 MachineBasicBlock *SuccBB,
562 MachineBasicBlock *PredBB) {
563 unsigned maxCommonTailLength = 0U;
565 MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
566 MPIterator HighestMPIter = prior(MergePotentials.end());
567 for (MPIterator CurMPIter = prior(MergePotentials.end()),
568 B = MergePotentials.begin();
569 CurMPIter != B && CurMPIter->getHash() == CurHash;
571 for (MPIterator I = prior(CurMPIter); I->getHash() == CurHash ; --I) {
572 unsigned CommonTailLen;
573 if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(),
575 CommonTailLen, TrialBBI1, TrialBBI2,
577 if (CommonTailLen > maxCommonTailLength) {
579 maxCommonTailLength = CommonTailLen;
580 HighestMPIter = CurMPIter;
581 SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1));
583 if (HighestMPIter == CurMPIter &&
584 CommonTailLen == maxCommonTailLength)
585 SameTails.push_back(SameTailElt(I, TrialBBI2));
591 return maxCommonTailLength;
594 /// RemoveBlocksWithHash - Remove all blocks with hash CurHash from
595 /// MergePotentials, restoring branches at ends of blocks as appropriate.
596 void BranchFolder::RemoveBlocksWithHash(unsigned CurHash,
597 MachineBasicBlock *SuccBB,
598 MachineBasicBlock *PredBB) {
599 MPIterator CurMPIter, B;
600 for (CurMPIter = prior(MergePotentials.end()), B = MergePotentials.begin();
601 CurMPIter->getHash() == CurHash;
603 // Put the unconditional branch back, if we need one.
604 MachineBasicBlock *CurMBB = CurMPIter->getBlock();
605 if (SuccBB && CurMBB != PredBB)
606 FixTail(CurMBB, SuccBB, TII);
610 if (CurMPIter->getHash() != CurHash)
612 MergePotentials.erase(CurMPIter, MergePotentials.end());
615 /// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist
616 /// only of the common tail. Create a block that does by splitting one.
617 unsigned BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
618 unsigned maxCommonTailLength) {
619 unsigned commonTailIndex = 0;
620 unsigned TimeEstimate = ~0U;
621 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
622 // Use PredBB if possible; that doesn't require a new branch.
623 if (SameTails[i].getBlock() == PredBB) {
627 // Otherwise, make a (fairly bogus) choice based on estimate of
628 // how long it will take the various blocks to execute.
629 unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(),
630 SameTails[i].getTailStartPos());
631 if (t <= TimeEstimate) {
637 MachineBasicBlock::iterator BBI =
638 SameTails[commonTailIndex].getTailStartPos();
639 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
641 // If the common tail includes any debug info we will take it pretty
642 // randomly from one of the inputs. Might be better to remove it?
643 DEBUG(dbgs() << "\nSplitting BB#" << MBB->getNumber() << ", size "
644 << maxCommonTailLength);
646 MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI);
647 SameTails[commonTailIndex].setBlock(newMBB);
648 SameTails[commonTailIndex].setTailStartPos(newMBB->begin());
650 // If we split PredBB, newMBB is the new predecessor.
654 return commonTailIndex;
657 // See if any of the blocks in MergePotentials (which all have a common single
658 // successor, or all have no successor) can be tail-merged. If there is a
659 // successor, any blocks in MergePotentials that are not tail-merged and
660 // are not immediately before Succ must have an unconditional branch to
661 // Succ added (but the predecessor/successor lists need no adjustment).
662 // The lone predecessor of Succ that falls through into Succ,
663 // if any, is given in PredBB.
665 bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
666 MachineBasicBlock *PredBB) {
667 bool MadeChange = false;
669 // Except for the special cases below, tail-merge if there are at least
670 // this many instructions in common.
671 unsigned minCommonTailLength = TailMergeSize;
673 DEBUG(dbgs() << "\nTryTailMergeBlocks: ";
674 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
675 dbgs() << "BB#" << MergePotentials[i].getBlock()->getNumber()
676 << (i == e-1 ? "" : ", ");
679 dbgs() << " with successor BB#" << SuccBB->getNumber() << '\n';
681 dbgs() << " which has fall-through from BB#"
682 << PredBB->getNumber() << "\n";
684 dbgs() << "Looking for common tails of at least "
685 << minCommonTailLength << " instruction"
686 << (minCommonTailLength == 1 ? "" : "s") << '\n';
689 // Sort by hash value so that blocks with identical end sequences sort
691 std::stable_sort(MergePotentials.begin(), MergePotentials.end());
693 // Walk through equivalence sets looking for actual exact matches.
694 while (MergePotentials.size() > 1) {
695 unsigned CurHash = MergePotentials.back().getHash();
697 // Build SameTails, identifying the set of blocks with this hash code
698 // and with the maximum number of instructions in common.
699 unsigned maxCommonTailLength = ComputeSameTails(CurHash,
703 // If we didn't find any pair that has at least minCommonTailLength
704 // instructions in common, remove all blocks with this hash code and retry.
705 if (SameTails.empty()) {
706 RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
710 // If one of the blocks is the entire common tail (and not the entry
711 // block, which we can't jump to), we can treat all blocks with this same
712 // tail at once. Use PredBB if that is one of the possibilities, as that
713 // will not introduce any extra branches.
714 MachineBasicBlock *EntryBB = MergePotentials.begin()->getBlock()->
715 getParent()->begin();
716 unsigned commonTailIndex = SameTails.size();
717 // If there are two blocks, check to see if one can be made to fall through
719 if (SameTails.size() == 2 &&
720 SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) &&
721 SameTails[1].tailIsWholeBlock())
723 else if (SameTails.size() == 2 &&
724 SameTails[1].getBlock()->isLayoutSuccessor(
725 SameTails[0].getBlock()) &&
726 SameTails[0].tailIsWholeBlock())
729 // Otherwise just pick one, favoring the fall-through predecessor if
731 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
732 MachineBasicBlock *MBB = SameTails[i].getBlock();
733 if (MBB == EntryBB && SameTails[i].tailIsWholeBlock())
739 if (SameTails[i].tailIsWholeBlock())
744 if (commonTailIndex == SameTails.size() ||
745 (SameTails[commonTailIndex].getBlock() == PredBB &&
746 !SameTails[commonTailIndex].tailIsWholeBlock())) {
747 // None of the blocks consist entirely of the common tail.
748 // Split a block so that one does.
749 commonTailIndex = CreateCommonTailOnlyBlock(PredBB, maxCommonTailLength);
752 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
753 // MBB is common tail. Adjust all other BB's to jump to this one.
754 // Traversal must be forwards so erases work.
755 DEBUG(dbgs() << "\nUsing common tail in BB#" << MBB->getNumber()
757 for (unsigned int i=0, e = SameTails.size(); i != e; ++i) {
758 if (commonTailIndex == i)
760 DEBUG(dbgs() << "BB#" << SameTails[i].getBlock()->getNumber()
761 << (i == e-1 ? "" : ", "));
762 // Hack the end off BB i, making it jump to BB commonTailIndex instead.
763 ReplaceTailWithBranchTo(SameTails[i].getTailStartPos(), MBB);
764 // BB i is no longer a predecessor of SuccBB; remove it from the worklist.
765 MergePotentials.erase(SameTails[i].getMPIter());
767 DEBUG(dbgs() << "\n");
768 // We leave commonTailIndex in the worklist in case there are other blocks
769 // that match it with a smaller number of instructions.
775 bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
777 if (!EnableTailMerge) return false;
779 bool MadeChange = false;
781 // First find blocks with no successors.
782 MergePotentials.clear();
783 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
785 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(I), I));
788 // See if we can do any tail merging on those.
789 if (MergePotentials.size() < TailMergeThreshold &&
790 MergePotentials.size() >= 2)
791 MadeChange |= TryTailMergeBlocks(NULL, NULL);
793 // Look at blocks (IBB) with multiple predecessors (PBB).
794 // We change each predecessor to a canonical form, by
795 // (1) temporarily removing any unconditional branch from the predecessor
797 // (2) alter conditional branches so they branch to the other block
798 // not IBB; this may require adding back an unconditional branch to IBB
799 // later, where there wasn't one coming in. E.g.
801 // fallthrough to QBB
804 // with a conceptual B to IBB after that, which never actually exists.
805 // With those changes, we see whether the predecessors' tails match,
806 // and merge them if so. We change things out of canonical form and
807 // back to the way they were later in the process. (OptimizeBranches
808 // would undo some of this, but we can't use it, because we'd get into
809 // a compile-time infinite loop repeatedly doing and undoing the same
812 for (MachineFunction::iterator I = llvm::next(MF.begin()), E = MF.end();
814 if (I->pred_size() >= 2 && I->pred_size() < TailMergeThreshold) {
815 SmallPtrSet<MachineBasicBlock *, 8> UniquePreds;
816 MachineBasicBlock *IBB = I;
817 MachineBasicBlock *PredBB = prior(I);
818 MergePotentials.clear();
819 for (MachineBasicBlock::pred_iterator P = I->pred_begin(),
822 MachineBasicBlock *PBB = *P;
823 // Skip blocks that loop to themselves, can't tail merge these.
826 // Visit each predecessor only once.
827 if (!UniquePreds.insert(PBB))
829 MachineBasicBlock *TBB = 0, *FBB = 0;
830 SmallVector<MachineOperand, 4> Cond;
831 if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond, true)) {
832 // Failing case: IBB is the target of a cbr, and
833 // we cannot reverse the branch.
834 SmallVector<MachineOperand, 4> NewCond(Cond);
835 if (!Cond.empty() && TBB == IBB) {
836 if (TII->ReverseBranchCondition(NewCond))
838 // This is the QBB case described above
840 FBB = llvm::next(MachineFunction::iterator(PBB));
842 // Failing case: the only way IBB can be reached from PBB is via
843 // exception handling. Happens for landing pads. Would be nice
844 // to have a bit in the edge so we didn't have to do all this.
845 if (IBB->isLandingPad()) {
846 MachineFunction::iterator IP = PBB; IP++;
847 MachineBasicBlock *PredNextBB = NULL;
851 if (IBB != PredNextBB) // fallthrough
854 if (TBB != IBB && FBB != IBB) // cbr then ubr
856 } else if (Cond.empty()) {
857 if (TBB != IBB) // ubr
860 if (TBB != IBB && IBB != PredNextBB) // cbr
864 // Remove the unconditional branch at the end, if any.
865 if (TBB && (Cond.empty() || FBB)) {
866 DebugLoc dl; // FIXME: this is nowhere
867 TII->RemoveBranch(*PBB);
869 // reinsert conditional branch only, for now
870 TII->InsertBranch(*PBB, (TBB == IBB) ? FBB : TBB, 0, NewCond, dl);
872 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(PBB), *P));
875 if (MergePotentials.size() >= 2)
876 MadeChange |= TryTailMergeBlocks(IBB, PredBB);
877 // Reinsert an unconditional branch if needed.
878 // The 1 below can occur as a result of removing blocks in TryTailMergeBlocks.
879 PredBB = prior(I); // this may have been changed in TryTailMergeBlocks
880 if (MergePotentials.size() == 1 &&
881 MergePotentials.begin()->getBlock() != PredBB)
882 FixTail(MergePotentials.begin()->getBlock(), IBB, TII);
888 //===----------------------------------------------------------------------===//
889 // Branch Optimization
890 //===----------------------------------------------------------------------===//
892 bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
893 bool MadeChange = false;
895 // Make sure blocks are numbered in order
898 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
899 MachineBasicBlock *MBB = I++;
900 MadeChange |= OptimizeBlock(MBB);
902 // If it is dead, remove it.
903 if (MBB->pred_empty()) {
904 RemoveDeadBlock(MBB);
912 // Blocks should be considered empty if they contain only debug info;
913 // else the debug info would affect codegen.
914 static bool IsEmptyBlock(MachineBasicBlock *MBB) {
917 for (MachineBasicBlock::iterator MBBI = MBB->begin(), MBBE = MBB->end();
918 MBBI!=MBBE; ++MBBI) {
919 if (!MBBI->isDebugValue())
925 // Blocks with only debug info and branches should be considered the same
926 // as blocks with only branches.
927 static bool IsBranchOnlyBlock(MachineBasicBlock *MBB) {
928 MachineBasicBlock::iterator MBBI, MBBE;
929 for (MBBI = MBB->begin(), MBBE = MBB->end(); MBBI!=MBBE; ++MBBI) {
930 if (!MBBI->isDebugValue())
933 return (MBBI->getDesc().isBranch());
936 /// IsBetterFallthrough - Return true if it would be clearly better to
937 /// fall-through to MBB1 than to fall through into MBB2. This has to return
938 /// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will
939 /// result in infinite loops.
940 static bool IsBetterFallthrough(MachineBasicBlock *MBB1,
941 MachineBasicBlock *MBB2) {
942 // Right now, we use a simple heuristic. If MBB2 ends with a call, and
943 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to
944 // optimize branches that branch to either a return block or an assert block
945 // into a fallthrough to the return.
946 if (IsEmptyBlock(MBB1) || IsEmptyBlock(MBB2)) return false;
948 // If there is a clear successor ordering we make sure that one block
949 // will fall through to the next
950 if (MBB1->isSuccessor(MBB2)) return true;
951 if (MBB2->isSuccessor(MBB1)) return false;
953 // Neither block consists entirely of debug info (per IsEmptyBlock check),
954 // so we needn't test for falling off the beginning here.
955 MachineBasicBlock::iterator MBB1I = --MBB1->end();
956 while (MBB1I->isDebugValue())
958 MachineBasicBlock::iterator MBB2I = --MBB2->end();
959 while (MBB2I->isDebugValue())
961 return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall();
964 /// OptimizeBlock - Analyze and optimize control flow related to the specified
965 /// block. This is never called on the entry block.
966 bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
967 bool MadeChange = false;
968 MachineFunction &MF = *MBB->getParent();
969 DebugLoc dl; // FIXME: this is nowhere
972 MachineFunction::iterator FallThrough = MBB;
975 // If this block is empty, make everyone use its fall-through, not the block
976 // explicitly. Landing pads should not do this since the landing-pad table
977 // points to this block. Blocks with their addresses taken shouldn't be
979 if (IsEmptyBlock(MBB) && !MBB->isLandingPad() && !MBB->hasAddressTaken()) {
980 // Dead block? Leave for cleanup later.
981 if (MBB->pred_empty()) return MadeChange;
983 if (FallThrough == MF.end()) {
984 // TODO: Simplify preds to not branch here if possible!
986 // Rewrite all predecessors of the old block to go to the fallthrough
988 while (!MBB->pred_empty()) {
989 MachineBasicBlock *Pred = *(MBB->pred_end()-1);
990 Pred->ReplaceUsesOfBlockWith(MBB, FallThrough);
992 // If MBB was the target of a jump table, update jump tables to go to the
993 // fallthrough instead.
994 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
995 MJTI->ReplaceMBBInJumpTables(MBB, FallThrough);
1001 // Check to see if we can simplify the terminator of the block before this
1003 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB));
1005 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
1006 SmallVector<MachineOperand, 4> PriorCond;
1007 bool PriorUnAnalyzable =
1008 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
1009 if (!PriorUnAnalyzable) {
1010 // If the CFG for the prior block has extra edges, remove them.
1011 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB,
1012 !PriorCond.empty());
1014 // If the previous branch is conditional and both conditions go to the same
1015 // destination, remove the branch, replacing it with an unconditional one or
1017 if (PriorTBB && PriorTBB == PriorFBB) {
1018 TII->RemoveBranch(PrevBB);
1020 if (PriorTBB != MBB)
1021 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond, dl);
1024 goto ReoptimizeBlock;
1027 // If the previous block unconditionally falls through to this block and
1028 // this block has no other predecessors, move the contents of this block
1029 // into the prior block. This doesn't usually happen when SimplifyCFG
1030 // has been used, but it can happen if tail merging splits a fall-through
1031 // predecessor of a block.
1032 // This has to check PrevBB->succ_size() because EH edges are ignored by
1034 if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 &&
1035 PrevBB.succ_size() == 1 &&
1036 !MBB->hasAddressTaken()) {
1037 DEBUG(dbgs() << "\nMerging into block: " << PrevBB
1038 << "From MBB: " << *MBB);
1039 PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end());
1040 PrevBB.removeSuccessor(PrevBB.succ_begin());;
1041 assert(PrevBB.succ_empty());
1042 PrevBB.transferSuccessors(MBB);
1047 // If the previous branch *only* branches to *this* block (conditional or
1048 // not) remove the branch.
1049 if (PriorTBB == MBB && PriorFBB == 0) {
1050 TII->RemoveBranch(PrevBB);
1053 goto ReoptimizeBlock;
1056 // If the prior block branches somewhere else on the condition and here if
1057 // the condition is false, remove the uncond second branch.
1058 if (PriorFBB == MBB) {
1059 TII->RemoveBranch(PrevBB);
1060 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond, dl);
1063 goto ReoptimizeBlock;
1066 // If the prior block branches here on true and somewhere else on false, and
1067 // if the branch condition is reversible, reverse the branch to create a
1069 if (PriorTBB == MBB) {
1070 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1071 if (!TII->ReverseBranchCondition(NewPriorCond)) {
1072 TII->RemoveBranch(PrevBB);
1073 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond, dl);
1076 goto ReoptimizeBlock;
1080 // If this block has no successors (e.g. it is a return block or ends with
1081 // a call to a no-return function like abort or __cxa_throw) and if the pred
1082 // falls through into this block, and if it would otherwise fall through
1083 // into the block after this, move this block to the end of the function.
1085 // We consider it more likely that execution will stay in the function (e.g.
1086 // due to loops) than it is to exit it. This asserts in loops etc, moving
1087 // the assert condition out of the loop body.
1088 if (MBB->succ_empty() && !PriorCond.empty() && PriorFBB == 0 &&
1089 MachineFunction::iterator(PriorTBB) == FallThrough &&
1090 !MBB->canFallThrough()) {
1091 bool DoTransform = true;
1093 // We have to be careful that the succs of PredBB aren't both no-successor
1094 // blocks. If neither have successors and if PredBB is the second from
1095 // last block in the function, we'd just keep swapping the two blocks for
1096 // last. Only do the swap if one is clearly better to fall through than
1098 if (FallThrough == --MF.end() &&
1099 !IsBetterFallthrough(PriorTBB, MBB))
1100 DoTransform = false;
1103 // Reverse the branch so we will fall through on the previous true cond.
1104 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1105 if (!TII->ReverseBranchCondition(NewPriorCond)) {
1106 DEBUG(dbgs() << "\nMoving MBB: " << *MBB
1107 << "To make fallthrough to: " << *PriorTBB << "\n");
1109 TII->RemoveBranch(PrevBB);
1110 TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond, dl);
1112 // Move this block to the end of the function.
1113 MBB->moveAfter(--MF.end());
1122 // Analyze the branch in the current block.
1123 MachineBasicBlock *CurTBB = 0, *CurFBB = 0;
1124 SmallVector<MachineOperand, 4> CurCond;
1125 bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true);
1126 if (!CurUnAnalyzable) {
1127 // If the CFG for the prior block has extra edges, remove them.
1128 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty());
1130 // If this is a two-way branch, and the FBB branches to this block, reverse
1131 // the condition so the single-basic-block loop is faster. Instead of:
1132 // Loop: xxx; jcc Out; jmp Loop
1134 // Loop: xxx; jncc Loop; jmp Out
1135 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) {
1136 SmallVector<MachineOperand, 4> NewCond(CurCond);
1137 if (!TII->ReverseBranchCondition(NewCond)) {
1138 TII->RemoveBranch(*MBB);
1139 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond, dl);
1142 goto ReoptimizeBlock;
1146 // If this branch is the only thing in its block, see if we can forward
1147 // other blocks across it.
1148 if (CurTBB && CurCond.empty() && CurFBB == 0 &&
1149 IsBranchOnlyBlock(MBB) && CurTBB != MBB &&
1150 !MBB->hasAddressTaken()) {
1151 // This block may contain just an unconditional branch. Because there can
1152 // be 'non-branch terminators' in the block, try removing the branch and
1153 // then seeing if the block is empty.
1154 TII->RemoveBranch(*MBB);
1155 // If the only things remaining in the block are debug info, remove these
1156 // as well, so this will behave the same as an empty block in non-debug
1158 if (!MBB->empty()) {
1159 bool NonDebugInfoFound = false;
1160 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
1162 if (!I->isDebugValue()) {
1163 NonDebugInfoFound = true;
1167 if (!NonDebugInfoFound)
1168 // Make the block empty, losing the debug info (we could probably
1169 // improve this in some cases.)
1170 MBB->erase(MBB->begin(), MBB->end());
1172 // If this block is just an unconditional branch to CurTBB, we can
1173 // usually completely eliminate the block. The only case we cannot
1174 // completely eliminate the block is when the block before this one
1175 // falls through into MBB and we can't understand the prior block's branch
1178 bool PredHasNoFallThrough = !PrevBB.canFallThrough();
1179 if (PredHasNoFallThrough || !PriorUnAnalyzable ||
1180 !PrevBB.isSuccessor(MBB)) {
1181 // If the prior block falls through into us, turn it into an
1182 // explicit branch to us to make updates simpler.
1183 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) &&
1184 PriorTBB != MBB && PriorFBB != MBB) {
1185 if (PriorTBB == 0) {
1186 assert(PriorCond.empty() && PriorFBB == 0 &&
1187 "Bad branch analysis");
1190 assert(PriorFBB == 0 && "Machine CFG out of date!");
1193 TII->RemoveBranch(PrevBB);
1194 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, dl);
1197 // Iterate through all the predecessors, revectoring each in-turn.
1199 bool DidChange = false;
1200 bool HasBranchToSelf = false;
1201 while(PI != MBB->pred_size()) {
1202 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI);
1204 // If this block has an uncond branch to itself, leave it.
1206 HasBranchToSelf = true;
1209 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB);
1210 // If this change resulted in PMBB ending in a conditional
1211 // branch where both conditions go to the same destination,
1212 // change this to an unconditional branch (and fix the CFG).
1213 MachineBasicBlock *NewCurTBB = 0, *NewCurFBB = 0;
1214 SmallVector<MachineOperand, 4> NewCurCond;
1215 bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB,
1216 NewCurFBB, NewCurCond, true);
1217 if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) {
1218 TII->RemoveBranch(*PMBB);
1220 TII->InsertBranch(*PMBB, NewCurTBB, 0, NewCurCond, dl);
1223 PMBB->CorrectExtraCFGEdges(NewCurTBB, 0, false);
1228 // Change any jumptables to go to the new MBB.
1229 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
1230 MJTI->ReplaceMBBInJumpTables(MBB, CurTBB);
1234 if (!HasBranchToSelf) return MadeChange;
1239 // Add the branch back if the block is more than just an uncond branch.
1240 TII->InsertBranch(*MBB, CurTBB, 0, CurCond, dl);
1244 // If the prior block doesn't fall through into this block, and if this
1245 // block doesn't fall through into some other block, see if we can find a
1246 // place to move this block where a fall-through will happen.
1247 if (!PrevBB.canFallThrough()) {
1249 // Now we know that there was no fall-through into this block, check to
1250 // see if it has a fall-through into its successor.
1251 bool CurFallsThru = MBB->canFallThrough();
1253 if (!MBB->isLandingPad()) {
1254 // Check all the predecessors of this block. If one of them has no fall
1255 // throughs, move this block right after it.
1256 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
1257 E = MBB->pred_end(); PI != E; ++PI) {
1258 // Analyze the branch at the end of the pred.
1259 MachineBasicBlock *PredBB = *PI;
1260 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough;
1261 MachineBasicBlock *PredTBB = 0, *PredFBB = 0;
1262 SmallVector<MachineOperand, 4> PredCond;
1263 if (PredBB != MBB && !PredBB->canFallThrough() &&
1264 !TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true)
1265 && (!CurFallsThru || !CurTBB || !CurFBB)
1266 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
1267 // If the current block doesn't fall through, just move it.
1268 // If the current block can fall through and does not end with a
1269 // conditional branch, we need to append an unconditional jump to
1270 // the (current) next block. To avoid a possible compile-time
1271 // infinite loop, move blocks only backward in this case.
1272 // Also, if there are already 2 branches here, we cannot add a third;
1273 // this means we have the case
1278 MachineBasicBlock *NextBB = llvm::next(MachineFunction::iterator(MBB));
1280 TII->InsertBranch(*MBB, NextBB, 0, CurCond, dl);
1282 MBB->moveAfter(PredBB);
1284 goto ReoptimizeBlock;
1289 if (!CurFallsThru) {
1290 // Check all successors to see if we can move this block before it.
1291 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
1292 E = MBB->succ_end(); SI != E; ++SI) {
1293 // Analyze the branch at the end of the block before the succ.
1294 MachineBasicBlock *SuccBB = *SI;
1295 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev;
1297 // If this block doesn't already fall-through to that successor, and if
1298 // the succ doesn't already have a block that can fall through into it,
1299 // and if the successor isn't an EH destination, we can arrange for the
1300 // fallthrough to happen.
1301 if (SuccBB != MBB && &*SuccPrev != MBB &&
1302 !SuccPrev->canFallThrough() && !CurUnAnalyzable &&
1303 !SuccBB->isLandingPad()) {
1304 MBB->moveBefore(SuccBB);
1306 goto ReoptimizeBlock;
1310 // Okay, there is no really great place to put this block. If, however,
1311 // the block before this one would be a fall-through if this block were
1312 // removed, move this block to the end of the function.
1313 MachineBasicBlock *PrevTBB = 0, *PrevFBB = 0;
1314 SmallVector<MachineOperand, 4> PrevCond;
1315 if (FallThrough != MF.end() &&
1316 !TII->AnalyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) &&
1317 PrevBB.isSuccessor(FallThrough)) {
1318 MBB->moveAfter(--MF.end());