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/Statistic.h"
36 #include "llvm/ADT/STLExtras.h"
40 STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
41 STATISTIC(NumBranchOpts, "Number of branches optimized");
42 STATISTIC(NumTailMerge , "Number of block tails merged");
43 static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge",
44 cl::init(cl::BOU_UNSET), cl::Hidden);
45 // Throttle for huge numbers of predecessors (compile speed problems)
46 static cl::opt<unsigned>
47 TailMergeThreshold("tail-merge-threshold",
48 cl::desc("Max number of predecessors to consider tail merging"),
49 cl::init(150), cl::Hidden);
52 char BranchFolderPass::ID = 0;
54 FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) {
55 return new BranchFolderPass(DefaultEnableTailMerge);
58 bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) {
59 return OptimizeFunction(MF,
60 MF.getTarget().getInstrInfo(),
61 MF.getTarget().getRegisterInfo(),
62 getAnalysisIfAvailable<MachineModuleInfo>());
67 BranchFolder::BranchFolder(bool defaultEnableTailMerge) {
68 switch (FlagEnableTailMerge) {
69 case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break;
70 case cl::BOU_TRUE: EnableTailMerge = true; break;
71 case cl::BOU_FALSE: EnableTailMerge = false; break;
75 /// RemoveDeadBlock - Remove the specified dead machine basic block from the
76 /// function, updating the CFG.
77 void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) {
78 assert(MBB->pred_empty() && "MBB must be dead!");
79 DEBUG(errs() << "\nRemoving MBB: " << *MBB);
81 MachineFunction *MF = MBB->getParent();
82 // drop all successors.
83 while (!MBB->succ_empty())
84 MBB->removeSuccessor(MBB->succ_end()-1);
86 // If there are any labels in the basic block, unregister them from
88 if (MMI && !MBB->empty()) {
89 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
92 // The label ID # is always operand #0, an immediate.
93 MMI->InvalidateLabel(I->getOperand(0).getImm());
101 /// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def
102 /// followed by terminators, and if the implicitly defined registers are not
103 /// used by the terminators, remove those implicit_def's. e.g.
105 /// r0 = implicit_def
106 /// r1 = implicit_def
108 /// This block can be optimized away later if the implicit instructions are
110 bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) {
111 SmallSet<unsigned, 4> ImpDefRegs;
112 MachineBasicBlock::iterator I = MBB->begin();
113 while (I != MBB->end()) {
114 if (I->getOpcode() != TargetInstrInfo::IMPLICIT_DEF)
116 unsigned Reg = I->getOperand(0).getReg();
117 ImpDefRegs.insert(Reg);
118 for (const unsigned *SubRegs = TRI->getSubRegisters(Reg);
119 unsigned SubReg = *SubRegs; ++SubRegs)
120 ImpDefRegs.insert(SubReg);
123 if (ImpDefRegs.empty())
126 MachineBasicBlock::iterator FirstTerm = I;
127 while (I != MBB->end()) {
128 if (!TII->isUnpredicatedTerminator(I))
130 // See if it uses any of the implicitly defined registers.
131 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
132 MachineOperand &MO = I->getOperand(i);
133 if (!MO.isReg() || !MO.isUse())
135 unsigned Reg = MO.getReg();
136 if (ImpDefRegs.count(Reg))
143 while (I != FirstTerm) {
144 MachineInstr *ImpDefMI = &*I;
146 MBB->erase(ImpDefMI);
152 /// OptimizeFunction - Perhaps branch folding, tail merging and other
153 /// CFG optimizations on the given function.
154 bool BranchFolder::OptimizeFunction(MachineFunction &MF,
155 const TargetInstrInfo *tii,
156 const TargetRegisterInfo *tri,
157 MachineModuleInfo *mmi) {
158 if (!tii) return false;
164 RS = TRI->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL;
166 // Fix CFG. The later algorithms expect it to be right.
167 bool MadeChange = false;
168 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) {
169 MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0;
170 SmallVector<MachineOperand, 4> Cond;
171 if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true))
172 MadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty());
173 MadeChange |= OptimizeImpDefsBlock(MBB);
177 bool MadeChangeThisIteration = true;
178 while (MadeChangeThisIteration) {
179 MadeChangeThisIteration = false;
180 MadeChangeThisIteration |= TailMergeBlocks(MF);
181 MadeChangeThisIteration |= OptimizeBranches(MF);
182 MadeChange |= MadeChangeThisIteration;
185 // See if any jump tables have become mergable or dead as the code generator
187 MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
188 const std::vector<MachineJumpTableEntry> &JTs = JTI->getJumpTables();
190 // Figure out how these jump tables should be merged.
191 std::vector<unsigned> JTMapping;
192 JTMapping.reserve(JTs.size());
194 // We always keep the 0th jump table.
195 JTMapping.push_back(0);
197 // Scan the jump tables, seeing if there are any duplicates. Note that this
198 // is N^2, which should be fixed someday.
199 for (unsigned i = 1, e = JTs.size(); i != e; ++i) {
200 if (JTs[i].MBBs.empty())
201 JTMapping.push_back(i);
203 JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs));
206 // If a jump table was merge with another one, walk the function rewriting
207 // references to jump tables to reference the new JT ID's. Keep track of
208 // whether we see a jump table idx, if not, we can delete the JT.
209 BitVector JTIsLive(JTs.size());
210 for (MachineFunction::iterator BB = MF.begin(), E = MF.end();
212 for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end();
214 for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) {
215 MachineOperand &Op = I->getOperand(op);
216 if (!Op.isJTI()) continue;
217 unsigned NewIdx = JTMapping[Op.getIndex()];
220 // Remember that this JT is live.
221 JTIsLive.set(NewIdx);
225 // Finally, remove dead jump tables. This happens either because the
226 // indirect jump was unreachable (and thus deleted) or because the jump
227 // table was merged with some other one.
228 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i)
229 if (!JTIsLive.test(i)) {
230 JTI->RemoveJumpTable(i);
239 //===----------------------------------------------------------------------===//
240 // Tail Merging of Blocks
241 //===----------------------------------------------------------------------===//
243 /// HashMachineInstr - Compute a hash value for MI and its operands.
244 static unsigned HashMachineInstr(const MachineInstr *MI) {
245 unsigned Hash = MI->getOpcode();
246 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
247 const MachineOperand &Op = MI->getOperand(i);
249 // Merge in bits from the operand if easy.
250 unsigned OperandHash = 0;
251 switch (Op.getType()) {
252 case MachineOperand::MO_Register: OperandHash = Op.getReg(); break;
253 case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break;
254 case MachineOperand::MO_MachineBasicBlock:
255 OperandHash = Op.getMBB()->getNumber();
257 case MachineOperand::MO_FrameIndex:
258 case MachineOperand::MO_ConstantPoolIndex:
259 case MachineOperand::MO_JumpTableIndex:
260 OperandHash = Op.getIndex();
262 case MachineOperand::MO_GlobalAddress:
263 case MachineOperand::MO_ExternalSymbol:
264 // Global address / external symbol are too hard, don't bother, but do
265 // pull in the offset.
266 OperandHash = Op.getOffset();
271 Hash += ((OperandHash << 3) | Op.getType()) << (i&31);
276 /// HashEndOfMBB - Hash the last few instructions in the MBB. For blocks
277 /// with no successors, we hash two instructions, because cross-jumping
278 /// only saves code when at least two instructions are removed (since a
279 /// branch must be inserted). For blocks with a successor, one of the
280 /// two blocks to be tail-merged will end with a branch already, so
281 /// it gains to cross-jump even for one instruction.
283 static unsigned HashEndOfMBB(const MachineBasicBlock *MBB,
284 unsigned minCommonTailLength) {
285 MachineBasicBlock::const_iterator I = MBB->end();
286 if (I == MBB->begin())
287 return 0; // Empty MBB.
290 unsigned Hash = HashMachineInstr(I);
292 if (I == MBB->begin() || minCommonTailLength == 1)
293 return Hash; // Single instr MBB.
296 // Hash in the second-to-last instruction.
297 Hash ^= HashMachineInstr(I) << 2;
301 /// ComputeCommonTailLength - Given two machine basic blocks, compute the number
302 /// of instructions they actually have in common together at their end. Return
303 /// iterators for the first shared instruction in each block.
304 static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1,
305 MachineBasicBlock *MBB2,
306 MachineBasicBlock::iterator &I1,
307 MachineBasicBlock::iterator &I2) {
311 unsigned TailLen = 0;
312 while (I1 != MBB1->begin() && I2 != MBB2->begin()) {
314 if (!I1->isIdenticalTo(I2) ||
315 // FIXME: This check is dubious. It's used to get around a problem where
316 // people incorrectly expect inline asm directives to remain in the same
317 // relative order. This is untenable because normal compiler
318 // optimizations (like this one) may reorder and/or merge these
320 I1->getOpcode() == TargetInstrInfo::INLINEASM) {
329 /// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything
330 /// after it, replacing it with an unconditional branch to NewDest. This
331 /// returns true if OldInst's block is modified, false if NewDest is modified.
332 void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
333 MachineBasicBlock *NewDest) {
334 MachineBasicBlock *OldBB = OldInst->getParent();
336 // Remove all the old successors of OldBB from the CFG.
337 while (!OldBB->succ_empty())
338 OldBB->removeSuccessor(OldBB->succ_begin());
340 // Remove all the dead instructions from the end of OldBB.
341 OldBB->erase(OldInst, OldBB->end());
343 // If OldBB isn't immediately before OldBB, insert a branch to it.
344 if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest))
345 TII->InsertBranch(*OldBB, NewDest, 0, SmallVector<MachineOperand, 0>());
346 OldBB->addSuccessor(NewDest);
350 /// SplitMBBAt - Given a machine basic block and an iterator into it, split the
351 /// MBB so that the part before the iterator falls into the part starting at the
352 /// iterator. This returns the new MBB.
353 MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB,
354 MachineBasicBlock::iterator BBI1) {
355 MachineFunction &MF = *CurMBB.getParent();
357 // Create the fall-through block.
358 MachineFunction::iterator MBBI = &CurMBB;
359 MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(CurMBB.getBasicBlock());
360 CurMBB.getParent()->insert(++MBBI, NewMBB);
362 // Move all the successors of this block to the specified block.
363 NewMBB->transferSuccessors(&CurMBB);
365 // Add an edge from CurMBB to NewMBB for the fall-through.
366 CurMBB.addSuccessor(NewMBB);
368 // Splice the code over.
369 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end());
371 // For targets that use the register scavenger, we must maintain LiveIns.
373 RS->enterBasicBlock(&CurMBB);
375 RS->forward(prior(CurMBB.end()));
376 BitVector RegsLiveAtExit(TRI->getNumRegs());
377 RS->getRegsUsed(RegsLiveAtExit, false);
378 for (unsigned int i=0, e=TRI->getNumRegs(); i!=e; i++)
379 if (RegsLiveAtExit[i])
380 NewMBB->addLiveIn(i);
386 /// EstimateRuntime - Make a rough estimate for how long it will take to run
387 /// the specified code.
388 static unsigned EstimateRuntime(MachineBasicBlock::iterator I,
389 MachineBasicBlock::iterator E) {
391 for (; I != E; ++I) {
392 const TargetInstrDesc &TID = I->getDesc();
395 else if (TID.mayLoad() || TID.mayStore())
403 // CurMBB needs to add an unconditional branch to SuccMBB (we removed these
404 // branches temporarily for tail merging). In the case where CurMBB ends
405 // with a conditional branch to the next block, optimize by reversing the
406 // test and conditionally branching to SuccMBB instead.
408 static void FixTail(MachineBasicBlock* CurMBB, MachineBasicBlock *SuccBB,
409 const TargetInstrInfo *TII) {
410 MachineFunction *MF = CurMBB->getParent();
411 MachineFunction::iterator I = next(MachineFunction::iterator(CurMBB));
412 MachineBasicBlock *TBB = 0, *FBB = 0;
413 SmallVector<MachineOperand, 4> Cond;
414 if (I != MF->end() &&
415 !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond, true)) {
416 MachineBasicBlock *NextBB = I;
417 if (TBB == NextBB && !Cond.empty() && !FBB) {
418 if (!TII->ReverseBranchCondition(Cond)) {
419 TII->RemoveBranch(*CurMBB);
420 TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond);
425 TII->InsertBranch(*CurMBB, SuccBB, NULL, SmallVector<MachineOperand, 0>());
428 static bool MergeCompare(const std::pair<unsigned,MachineBasicBlock*> &p,
429 const std::pair<unsigned,MachineBasicBlock*> &q) {
430 if (p.first < q.first)
432 else if (p.first > q.first)
434 else if (p.second->getNumber() < q.second->getNumber())
436 else if (p.second->getNumber() > q.second->getNumber())
439 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing
440 // an object with itself.
441 #ifndef _GLIBCXX_DEBUG
442 llvm_unreachable("Predecessor appears twice");
448 /// ComputeSameTails - Look through all the blocks in MergePotentials that have
449 /// hash CurHash (guaranteed to match the last element). Build the vector
450 /// SameTails of all those that have the (same) largest number of instructions
451 /// in common of any pair of these blocks. SameTails entries contain an
452 /// iterator into MergePotentials (from which the MachineBasicBlock can be
453 /// found) and a MachineBasicBlock::iterator into that MBB indicating the
454 /// instruction where the matching code sequence begins.
455 /// Order of elements in SameTails is the reverse of the order in which
456 /// those blocks appear in MergePotentials (where they are not necessarily
458 unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
459 unsigned minCommonTailLength) {
460 unsigned maxCommonTailLength = 0U;
462 MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
463 MPIterator HighestMPIter = prior(MergePotentials.end());
464 for (MPIterator CurMPIter = prior(MergePotentials.end()),
465 B = MergePotentials.begin();
466 CurMPIter!=B && CurMPIter->first==CurHash;
468 for (MPIterator I = prior(CurMPIter); I->first==CurHash ; --I) {
469 unsigned CommonTailLen = ComputeCommonTailLength(CurMPIter->second,
471 TrialBBI1, TrialBBI2);
472 // If we will have to split a block, there should be at least
473 // minCommonTailLength instructions in common. Otherwise, if we are
474 // optimizing for code size, 1 instruction in common is enough. At
475 // worst we will be replacing a fallthrough into the common tail with a
476 // branch, which at worst breaks even with falling through into the
477 // duplicated common tail. We will always pick a block we do not have
478 // to split as the common tail if there is one. (Empty blocks will get
479 // forwarded and need not be considered.)
480 MachineFunction *MF = CurMPIter->second->getParent();
481 if (CommonTailLen >= minCommonTailLength ||
482 (CommonTailLen > 0 &&
483 MF->getFunction()->hasFnAttr(Attribute::OptimizeForSize) &&
484 (TrialBBI1 == CurMPIter->second->begin() ||
485 TrialBBI2 == I->second->begin()))) {
486 if (CommonTailLen > maxCommonTailLength) {
488 maxCommonTailLength = CommonTailLen;
489 HighestMPIter = CurMPIter;
490 SameTails.push_back(std::make_pair(CurMPIter, TrialBBI1));
492 if (HighestMPIter == CurMPIter &&
493 CommonTailLen == maxCommonTailLength)
494 SameTails.push_back(std::make_pair(I, TrialBBI2));
500 return maxCommonTailLength;
503 /// RemoveBlocksWithHash - Remove all blocks with hash CurHash from
504 /// MergePotentials, restoring branches at ends of blocks as appropriate.
505 void BranchFolder::RemoveBlocksWithHash(unsigned CurHash,
506 MachineBasicBlock* SuccBB,
507 MachineBasicBlock* PredBB) {
508 MPIterator CurMPIter, B;
509 for (CurMPIter = prior(MergePotentials.end()), B = MergePotentials.begin();
510 CurMPIter->first==CurHash;
512 // Put the unconditional branch back, if we need one.
513 MachineBasicBlock *CurMBB = CurMPIter->second;
514 if (SuccBB && CurMBB != PredBB)
515 FixTail(CurMBB, SuccBB, TII);
519 if (CurMPIter->first!=CurHash)
521 MergePotentials.erase(CurMPIter, MergePotentials.end());
524 /// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist
525 /// only of the common tail. Create a block that does by splitting one.
526 unsigned BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
527 unsigned maxCommonTailLength) {
528 unsigned i, commonTailIndex;
529 unsigned TimeEstimate = ~0U;
530 for (i=0, commonTailIndex=0; i<SameTails.size(); i++) {
531 // Use PredBB if possible; that doesn't require a new branch.
532 if (SameTails[i].first->second==PredBB) {
536 // Otherwise, make a (fairly bogus) choice based on estimate of
537 // how long it will take the various blocks to execute.
538 unsigned t = EstimateRuntime(SameTails[i].first->second->begin(),
539 SameTails[i].second);
540 if (t<=TimeEstimate) {
546 MachineBasicBlock::iterator BBI = SameTails[commonTailIndex].second;
547 MachineBasicBlock *MBB = SameTails[commonTailIndex].first->second;
549 DEBUG(errs() << "\nSplitting " << MBB->getNumber() << ", size "
550 << maxCommonTailLength);
552 MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI);
553 SameTails[commonTailIndex].first->second = newMBB;
554 SameTails[commonTailIndex].second = newMBB->begin();
555 // If we split PredBB, newMBB is the new predecessor.
559 return commonTailIndex;
562 // See if any of the blocks in MergePotentials (which all have a common single
563 // successor, or all have no successor) can be tail-merged. If there is a
564 // successor, any blocks in MergePotentials that are not tail-merged and
565 // are not immediately before Succ must have an unconditional branch to
566 // Succ added (but the predecessor/successor lists need no adjustment).
567 // The lone predecessor of Succ that falls through into Succ,
568 // if any, is given in PredBB.
570 bool BranchFolder::TryMergeBlocks(MachineBasicBlock *SuccBB,
571 MachineBasicBlock* PredBB) {
572 bool MadeChange = false;
574 // It doesn't make sense to save a single instruction since tail merging
576 // FIXME: Ask the target to provide the threshold?
577 unsigned minCommonTailLength = (SuccBB ? 1 : 2) + 1;
579 DEBUG(errs() << "\nTryMergeBlocks " << MergePotentials.size() << '\n');
581 // Sort by hash value so that blocks with identical end sequences sort
583 std::stable_sort(MergePotentials.begin(), MergePotentials.end(),MergeCompare);
585 // Walk through equivalence sets looking for actual exact matches.
586 while (MergePotentials.size() > 1) {
587 unsigned CurHash = prior(MergePotentials.end())->first;
589 // Build SameTails, identifying the set of blocks with this hash code
590 // and with the maximum number of instructions in common.
591 unsigned maxCommonTailLength = ComputeSameTails(CurHash,
592 minCommonTailLength);
594 // If we didn't find any pair that has at least minCommonTailLength
595 // instructions in common, remove all blocks with this hash code and retry.
596 if (SameTails.empty()) {
597 RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
601 // If one of the blocks is the entire common tail (and not the entry
602 // block, which we can't jump to), we can treat all blocks with this same
603 // tail at once. Use PredBB if that is one of the possibilities, as that
604 // will not introduce any extra branches.
605 MachineBasicBlock *EntryBB = MergePotentials.begin()->second->
606 getParent()->begin();
607 unsigned int commonTailIndex, i;
608 for (commonTailIndex=SameTails.size(), i=0; i<SameTails.size(); i++) {
609 MachineBasicBlock *MBB = SameTails[i].first->second;
610 if (MBB->begin() == SameTails[i].second && MBB != EntryBB) {
617 if (commonTailIndex==SameTails.size()) {
618 // None of the blocks consist entirely of the common tail.
619 // Split a block so that one does.
620 commonTailIndex = CreateCommonTailOnlyBlock(PredBB, maxCommonTailLength);
623 MachineBasicBlock *MBB = SameTails[commonTailIndex].first->second;
624 // MBB is common tail. Adjust all other BB's to jump to this one.
625 // Traversal must be forwards so erases work.
626 DEBUG(errs() << "\nUsing common tail " << MBB->getNumber() << " for ");
627 for (unsigned int i=0; i<SameTails.size(); ++i) {
628 if (commonTailIndex==i)
630 DEBUG(errs() << SameTails[i].first->second->getNumber() << ",");
631 // Hack the end off BB i, making it jump to BB commonTailIndex instead.
632 ReplaceTailWithBranchTo(SameTails[i].second, MBB);
633 // BB i is no longer a predecessor of SuccBB; remove it from the worklist.
634 MergePotentials.erase(SameTails[i].first);
636 DEBUG(errs() << "\n");
637 // We leave commonTailIndex in the worklist in case there are other blocks
638 // that match it with a smaller number of instructions.
644 bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
646 if (!EnableTailMerge) return false;
648 bool MadeChange = false;
650 // First find blocks with no successors.
651 MergePotentials.clear();
652 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
654 MergePotentials.push_back(std::make_pair(HashEndOfMBB(I, 2U), I));
656 // See if we can do any tail merging on those.
657 if (MergePotentials.size() < TailMergeThreshold &&
658 MergePotentials.size() >= 2)
659 MadeChange |= TryMergeBlocks(NULL, NULL);
661 // Look at blocks (IBB) with multiple predecessors (PBB).
662 // We change each predecessor to a canonical form, by
663 // (1) temporarily removing any unconditional branch from the predecessor
665 // (2) alter conditional branches so they branch to the other block
666 // not IBB; this may require adding back an unconditional branch to IBB
667 // later, where there wasn't one coming in. E.g.
669 // fallthrough to QBB
672 // with a conceptual B to IBB after that, which never actually exists.
673 // With those changes, we see whether the predecessors' tails match,
674 // and merge them if so. We change things out of canonical form and
675 // back to the way they were later in the process. (OptimizeBranches
676 // would undo some of this, but we can't use it, because we'd get into
677 // a compile-time infinite loop repeatedly doing and undoing the same
680 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
681 if (I->pred_size() >= 2 && I->pred_size() < TailMergeThreshold) {
682 SmallPtrSet<MachineBasicBlock *, 8> UniquePreds;
683 MachineBasicBlock *IBB = I;
684 MachineBasicBlock *PredBB = prior(I);
685 MergePotentials.clear();
686 for (MachineBasicBlock::pred_iterator P = I->pred_begin(),
689 MachineBasicBlock* PBB = *P;
690 // Skip blocks that loop to themselves, can't tail merge these.
693 // Visit each predecessor only once.
694 if (!UniquePreds.insert(PBB))
696 MachineBasicBlock *TBB = 0, *FBB = 0;
697 SmallVector<MachineOperand, 4> Cond;
698 if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond, true)) {
699 // Failing case: IBB is the target of a cbr, and
700 // we cannot reverse the branch.
701 SmallVector<MachineOperand, 4> NewCond(Cond);
702 if (!Cond.empty() && TBB==IBB) {
703 if (TII->ReverseBranchCondition(NewCond))
705 // This is the QBB case described above
707 FBB = next(MachineFunction::iterator(PBB));
709 // Failing case: the only way IBB can be reached from PBB is via
710 // exception handling. Happens for landing pads. Would be nice
711 // to have a bit in the edge so we didn't have to do all this.
712 if (IBB->isLandingPad()) {
713 MachineFunction::iterator IP = PBB; IP++;
714 MachineBasicBlock* PredNextBB = NULL;
718 if (IBB!=PredNextBB) // fallthrough
721 if (TBB!=IBB && FBB!=IBB) // cbr then ubr
723 } else if (Cond.empty()) {
727 if (TBB!=IBB && IBB!=PredNextBB) // cbr
731 // Remove the unconditional branch at the end, if any.
732 if (TBB && (Cond.empty() || FBB)) {
733 TII->RemoveBranch(*PBB);
735 // reinsert conditional branch only, for now
736 TII->InsertBranch(*PBB, (TBB==IBB) ? FBB : TBB, 0, NewCond);
738 MergePotentials.push_back(std::make_pair(HashEndOfMBB(PBB, 1U), *P));
741 if (MergePotentials.size() >= 2)
742 MadeChange |= TryMergeBlocks(I, PredBB);
743 // Reinsert an unconditional branch if needed.
744 // The 1 below can occur as a result of removing blocks in TryMergeBlocks.
745 PredBB = prior(I); // this may have been changed in TryMergeBlocks
746 if (MergePotentials.size()==1 &&
747 MergePotentials.begin()->second != PredBB)
748 FixTail(MergePotentials.begin()->second, I, TII);
754 //===----------------------------------------------------------------------===//
755 // Branch Optimization
756 //===----------------------------------------------------------------------===//
758 bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
759 bool MadeChange = false;
761 // Make sure blocks are numbered in order
764 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
765 MachineBasicBlock *MBB = I++;
766 MadeChange |= OptimizeBlock(MBB);
768 // If it is dead, remove it.
769 if (MBB->pred_empty()) {
770 RemoveDeadBlock(MBB);
779 /// CanFallThrough - Return true if the specified block (with the specified
780 /// branch condition) can implicitly transfer control to the block after it by
781 /// falling off the end of it. This should return false if it can reach the
782 /// block after it, but it uses an explicit branch to do so (e.g. a table jump).
784 /// True is a conservative answer.
786 bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB,
787 bool BranchUnAnalyzable,
788 MachineBasicBlock *TBB,
789 MachineBasicBlock *FBB,
790 const SmallVectorImpl<MachineOperand> &Cond) {
791 MachineFunction::iterator Fallthrough = CurBB;
793 // If FallthroughBlock is off the end of the function, it can't fall through.
794 if (Fallthrough == CurBB->getParent()->end())
797 // If FallthroughBlock isn't a successor of CurBB, no fallthrough is possible.
798 if (!CurBB->isSuccessor(Fallthrough))
801 // If we couldn't analyze the branch, assume it could fall through.
802 if (BranchUnAnalyzable) return true;
804 // If there is no branch, control always falls through.
805 if (TBB == 0) return true;
807 // If there is some explicit branch to the fallthrough block, it can obviously
808 // reach, even though the branch should get folded to fall through implicitly.
809 if (MachineFunction::iterator(TBB) == Fallthrough ||
810 MachineFunction::iterator(FBB) == Fallthrough)
813 // If it's an unconditional branch to some block not the fall through, it
814 // doesn't fall through.
815 if (Cond.empty()) return false;
817 // Otherwise, if it is conditional and has no explicit false block, it falls
822 /// CanFallThrough - Return true if the specified can implicitly transfer
823 /// control to the block after it by falling off the end of it. This should
824 /// return false if it can reach the block after it, but it uses an explicit
825 /// branch to do so (e.g. a table jump).
827 /// True is a conservative answer.
829 bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) {
830 MachineBasicBlock *TBB = 0, *FBB = 0;
831 SmallVector<MachineOperand, 4> Cond;
832 bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond, true);
833 return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond);
836 /// IsBetterFallthrough - Return true if it would be clearly better to
837 /// fall-through to MBB1 than to fall through into MBB2. This has to return
838 /// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will
839 /// result in infinite loops.
840 static bool IsBetterFallthrough(MachineBasicBlock *MBB1,
841 MachineBasicBlock *MBB2) {
842 // Right now, we use a simple heuristic. If MBB2 ends with a call, and
843 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to
844 // optimize branches that branch to either a return block or an assert block
845 // into a fallthrough to the return.
846 if (MBB1->empty() || MBB2->empty()) return false;
848 // If there is a clear successor ordering we make sure that one block
849 // will fall through to the next
850 if (MBB1->isSuccessor(MBB2)) return true;
851 if (MBB2->isSuccessor(MBB1)) return false;
853 MachineInstr *MBB1I = --MBB1->end();
854 MachineInstr *MBB2I = --MBB2->end();
855 return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall();
858 /// OptimizeBlock - Analyze and optimize control flow related to the specified
859 /// block. This is never called on the entry block.
860 bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
861 bool MadeChange = false;
863 MachineFunction::iterator FallThrough = MBB;
866 // If this block is empty, make everyone use its fall-through, not the block
867 // explicitly. Landing pads should not do this since the landing-pad table
868 // points to this block.
869 if (MBB->empty() && !MBB->isLandingPad()) {
870 // Dead block? Leave for cleanup later.
871 if (MBB->pred_empty()) return MadeChange;
873 if (FallThrough == MBB->getParent()->end()) {
874 // TODO: Simplify preds to not branch here if possible!
876 // Rewrite all predecessors of the old block to go to the fallthrough
878 while (!MBB->pred_empty()) {
879 MachineBasicBlock *Pred = *(MBB->pred_end()-1);
880 Pred->ReplaceUsesOfBlockWith(MBB, FallThrough);
882 // If MBB was the target of a jump table, update jump tables to go to the
883 // fallthrough instead.
884 MBB->getParent()->getJumpTableInfo()->
885 ReplaceMBBInJumpTables(MBB, FallThrough);
891 // Check to see if we can simplify the terminator of the block before this
893 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB));
895 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
896 SmallVector<MachineOperand, 4> PriorCond;
897 bool PriorUnAnalyzable =
898 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
899 if (!PriorUnAnalyzable) {
900 // If the CFG for the prior block has extra edges, remove them.
901 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB,
904 // If the previous branch is conditional and both conditions go to the same
905 // destination, remove the branch, replacing it with an unconditional one or
907 if (PriorTBB && PriorTBB == PriorFBB) {
908 TII->RemoveBranch(PrevBB);
911 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond);
914 return OptimizeBlock(MBB);
917 // If the previous branch *only* branches to *this* block (conditional or
918 // not) remove the branch.
919 if (PriorTBB == MBB && PriorFBB == 0) {
920 TII->RemoveBranch(PrevBB);
923 return OptimizeBlock(MBB);
926 // If the prior block branches somewhere else on the condition and here if
927 // the condition is false, remove the uncond second branch.
928 if (PriorFBB == MBB) {
929 TII->RemoveBranch(PrevBB);
930 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond);
933 return OptimizeBlock(MBB);
936 // If the prior block branches here on true and somewhere else on false, and
937 // if the branch condition is reversible, reverse the branch to create a
939 if (PriorTBB == MBB) {
940 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
941 if (!TII->ReverseBranchCondition(NewPriorCond)) {
942 TII->RemoveBranch(PrevBB);
943 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond);
946 return OptimizeBlock(MBB);
950 // If this block has no successors (e.g. it is a return block or ends with
951 // a call to a no-return function like abort or __cxa_throw) and if the pred
952 // falls through into this block, and if it would otherwise fall through
953 // into the block after this, move this block to the end of the function.
955 // We consider it more likely that execution will stay in the function (e.g.
956 // due to loops) than it is to exit it. This asserts in loops etc, moving
957 // the assert condition out of the loop body.
958 if (MBB->succ_empty() && !PriorCond.empty() && PriorFBB == 0 &&
959 MachineFunction::iterator(PriorTBB) == FallThrough &&
960 !CanFallThrough(MBB)) {
961 bool DoTransform = true;
963 // We have to be careful that the succs of PredBB aren't both no-successor
964 // blocks. If neither have successors and if PredBB is the second from
965 // last block in the function, we'd just keep swapping the two blocks for
966 // last. Only do the swap if one is clearly better to fall through than
968 if (FallThrough == --MBB->getParent()->end() &&
969 !IsBetterFallthrough(PriorTBB, MBB))
972 // We don't want to do this transformation if we have control flow like:
981 // In this case, we could actually be moving the return block *into* a
983 if (DoTransform && !MBB->succ_empty() &&
984 (!CanFallThrough(PriorTBB) || PriorTBB->empty()))
989 // Reverse the branch so we will fall through on the previous true cond.
990 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
991 if (!TII->ReverseBranchCondition(NewPriorCond)) {
992 DEBUG(errs() << "\nMoving MBB: " << *MBB
993 << "To make fallthrough to: " << *PriorTBB << "\n");
995 TII->RemoveBranch(PrevBB);
996 TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond);
998 // Move this block to the end of the function.
999 MBB->moveAfter(--MBB->getParent()->end());
1008 // Analyze the branch in the current block.
1009 MachineBasicBlock *CurTBB = 0, *CurFBB = 0;
1010 SmallVector<MachineOperand, 4> CurCond;
1011 bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true);
1012 if (!CurUnAnalyzable) {
1013 // If the CFG for the prior block has extra edges, remove them.
1014 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty());
1016 // If this is a two-way branch, and the FBB branches to this block, reverse
1017 // the condition so the single-basic-block loop is faster. Instead of:
1018 // Loop: xxx; jcc Out; jmp Loop
1020 // Loop: xxx; jncc Loop; jmp Out
1021 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) {
1022 SmallVector<MachineOperand, 4> NewCond(CurCond);
1023 if (!TII->ReverseBranchCondition(NewCond)) {
1024 TII->RemoveBranch(*MBB);
1025 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond);
1028 return OptimizeBlock(MBB);
1033 // If this branch is the only thing in its block, see if we can forward
1034 // other blocks across it.
1035 if (CurTBB && CurCond.empty() && CurFBB == 0 &&
1036 MBB->begin()->getDesc().isBranch() && CurTBB != MBB) {
1037 // This block may contain just an unconditional branch. Because there can
1038 // be 'non-branch terminators' in the block, try removing the branch and
1039 // then seeing if the block is empty.
1040 TII->RemoveBranch(*MBB);
1042 // If this block is just an unconditional branch to CurTBB, we can
1043 // usually completely eliminate the block. The only case we cannot
1044 // completely eliminate the block is when the block before this one
1045 // falls through into MBB and we can't understand the prior block's branch
1048 bool PredHasNoFallThrough = TII->BlockHasNoFallThrough(PrevBB);
1049 if (PredHasNoFallThrough || !PriorUnAnalyzable ||
1050 !PrevBB.isSuccessor(MBB)) {
1051 // If the prior block falls through into us, turn it into an
1052 // explicit branch to us to make updates simpler.
1053 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) &&
1054 PriorTBB != MBB && PriorFBB != MBB) {
1055 if (PriorTBB == 0) {
1056 assert(PriorCond.empty() && PriorFBB == 0 &&
1057 "Bad branch analysis");
1060 assert(PriorFBB == 0 && "Machine CFG out of date!");
1063 TII->RemoveBranch(PrevBB);
1064 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond);
1067 // Iterate through all the predecessors, revectoring each in-turn.
1069 bool DidChange = false;
1070 bool HasBranchToSelf = false;
1071 while(PI != MBB->pred_size()) {
1072 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI);
1074 // If this block has an uncond branch to itself, leave it.
1076 HasBranchToSelf = true;
1079 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB);
1080 // If this change resulted in PMBB ending in a conditional
1081 // branch where both conditions go to the same destination,
1082 // change this to an unconditional branch (and fix the CFG).
1083 MachineBasicBlock *NewCurTBB = 0, *NewCurFBB = 0;
1084 SmallVector<MachineOperand, 4> NewCurCond;
1085 bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB,
1086 NewCurFBB, NewCurCond, true);
1087 if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) {
1088 TII->RemoveBranch(*PMBB);
1090 TII->InsertBranch(*PMBB, NewCurTBB, 0, NewCurCond);
1093 PMBB->CorrectExtraCFGEdges(NewCurTBB, NewCurFBB, false);
1098 // Change any jumptables to go to the new MBB.
1099 MBB->getParent()->getJumpTableInfo()->
1100 ReplaceMBBInJumpTables(MBB, CurTBB);
1104 if (!HasBranchToSelf) return MadeChange;
1109 // Add the branch back if the block is more than just an uncond branch.
1110 TII->InsertBranch(*MBB, CurTBB, 0, CurCond);
1114 // If the prior block doesn't fall through into this block, and if this
1115 // block doesn't fall through into some other block, see if we can find a
1116 // place to move this block where a fall-through will happen.
1117 if (!CanFallThrough(&PrevBB, PriorUnAnalyzable,
1118 PriorTBB, PriorFBB, PriorCond)) {
1119 // Now we know that there was no fall-through into this block, check to
1120 // see if it has a fall-through into its successor.
1121 bool CurFallsThru = CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB,
1124 if (!MBB->isLandingPad()) {
1125 // Check all the predecessors of this block. If one of them has no fall
1126 // throughs, move this block right after it.
1127 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
1128 E = MBB->pred_end(); PI != E; ++PI) {
1129 // Analyze the branch at the end of the pred.
1130 MachineBasicBlock *PredBB = *PI;
1131 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough;
1132 if (PredBB != MBB && !CanFallThrough(PredBB)
1133 && (!CurFallsThru || !CurTBB || !CurFBB)
1134 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
1135 // If the current block doesn't fall through, just move it.
1136 // If the current block can fall through and does not end with a
1137 // conditional branch, we need to append an unconditional jump to
1138 // the (current) next block. To avoid a possible compile-time
1139 // infinite loop, move blocks only backward in this case.
1140 // Also, if there are already 2 branches here, we cannot add a third;
1141 // this means we have the case
1146 MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB));
1148 TII->InsertBranch(*MBB, NextBB, 0, CurCond);
1150 MBB->moveAfter(PredBB);
1152 return OptimizeBlock(MBB);
1157 if (!CurFallsThru) {
1158 // Check all successors to see if we can move this block before it.
1159 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
1160 E = MBB->succ_end(); SI != E; ++SI) {
1161 // Analyze the branch at the end of the block before the succ.
1162 MachineBasicBlock *SuccBB = *SI;
1163 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev;
1164 std::vector<MachineOperand> SuccPrevCond;
1166 // If this block doesn't already fall-through to that successor, and if
1167 // the succ doesn't already have a block that can fall through into it,
1168 // and if the successor isn't an EH destination, we can arrange for the
1169 // fallthrough to happen.
1170 if (SuccBB != MBB && !CanFallThrough(SuccPrev) &&
1171 !SuccBB->isLandingPad()) {
1172 MBB->moveBefore(SuccBB);
1174 return OptimizeBlock(MBB);
1178 // Okay, there is no really great place to put this block. If, however,
1179 // the block before this one would be a fall-through if this block were
1180 // removed, move this block to the end of the function.
1181 if (FallThrough != MBB->getParent()->end() &&
1182 PrevBB.isSuccessor(FallThrough)) {
1183 MBB->moveAfter(--MBB->getParent()->end());