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 #include "BranchFolding.h"
20 #include "llvm/ADT/STLExtras.h"
21 #include "llvm/ADT/SmallSet.h"
22 #include "llvm/ADT/Statistic.h"
23 #include "llvm/CodeGen/MachineFunctionPass.h"
24 #include "llvm/CodeGen/MachineJumpTableInfo.h"
25 #include "llvm/CodeGen/MachineModuleInfo.h"
26 #include "llvm/CodeGen/MachineRegisterInfo.h"
27 #include "llvm/CodeGen/Passes.h"
28 #include "llvm/CodeGen/RegisterScavenging.h"
29 #include "llvm/IR/Function.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/Target/TargetInstrInfo.h"
35 #include "llvm/Target/TargetMachine.h"
36 #include "llvm/Target/TargetRegisterInfo.h"
37 #include "llvm/Target/TargetSubtargetInfo.h"
41 #define DEBUG_TYPE "branchfolding"
43 STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
44 STATISTIC(NumBranchOpts, "Number of branches optimized");
45 STATISTIC(NumTailMerge , "Number of block tails merged");
46 STATISTIC(NumHoist , "Number of times common instructions are hoisted");
48 static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge",
49 cl::init(cl::BOU_UNSET), cl::Hidden);
51 // Throttle for huge numbers of predecessors (compile speed problems)
52 static cl::opt<unsigned>
53 TailMergeThreshold("tail-merge-threshold",
54 cl::desc("Max number of predecessors to consider tail merging"),
55 cl::init(150), cl::Hidden);
57 // Heuristic for tail merging (and, inversely, tail duplication).
58 // TODO: This should be replaced with a target query.
59 static cl::opt<unsigned>
60 TailMergeSize("tail-merge-size",
61 cl::desc("Min number of instructions to consider tail merging"),
62 cl::init(3), cl::Hidden);
65 /// BranchFolderPass - Wrap branch folder in a machine function pass.
66 class BranchFolderPass : public MachineFunctionPass {
69 explicit BranchFolderPass(): MachineFunctionPass(ID) {}
71 bool runOnMachineFunction(MachineFunction &MF) override;
73 void getAnalysisUsage(AnalysisUsage &AU) const override {
74 AU.addRequired<TargetPassConfig>();
75 MachineFunctionPass::getAnalysisUsage(AU);
80 char BranchFolderPass::ID = 0;
81 char &llvm::BranchFolderPassID = BranchFolderPass::ID;
83 INITIALIZE_PASS(BranchFolderPass, "branch-folder",
84 "Control Flow Optimizer", false, false)
86 bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) {
87 if (skipOptnoneFunction(*MF.getFunction()))
90 TargetPassConfig *PassConfig = &getAnalysis<TargetPassConfig>();
91 // TailMerge can create jump into if branches that make CFG irreducible for
92 // HW that requires structurized CFG.
93 bool EnableTailMerge = !MF.getTarget().requiresStructuredCFG() &&
94 PassConfig->getEnableTailMerge();
95 BranchFolder Folder(EnableTailMerge, /*CommonHoist=*/true);
96 return Folder.OptimizeFunction(
97 MF, MF.getTarget().getSubtargetImpl()->getInstrInfo(),
98 MF.getTarget().getSubtargetImpl()->getRegisterInfo(),
99 getAnalysisIfAvailable<MachineModuleInfo>());
103 BranchFolder::BranchFolder(bool defaultEnableTailMerge, bool CommonHoist) {
104 switch (FlagEnableTailMerge) {
105 case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break;
106 case cl::BOU_TRUE: EnableTailMerge = true; break;
107 case cl::BOU_FALSE: EnableTailMerge = false; break;
110 EnableHoistCommonCode = CommonHoist;
113 /// RemoveDeadBlock - Remove the specified dead machine basic block from the
114 /// function, updating the CFG.
115 void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) {
116 assert(MBB->pred_empty() && "MBB must be dead!");
117 DEBUG(dbgs() << "\nRemoving MBB: " << *MBB);
119 MachineFunction *MF = MBB->getParent();
120 // drop all successors.
121 while (!MBB->succ_empty())
122 MBB->removeSuccessor(MBB->succ_end()-1);
124 // Avoid matching if this pointer gets reused.
125 TriedMerging.erase(MBB);
131 /// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def
132 /// followed by terminators, and if the implicitly defined registers are not
133 /// used by the terminators, remove those implicit_def's. e.g.
135 /// r0 = implicit_def
136 /// r1 = implicit_def
138 /// This block can be optimized away later if the implicit instructions are
140 bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) {
141 SmallSet<unsigned, 4> ImpDefRegs;
142 MachineBasicBlock::iterator I = MBB->begin();
143 while (I != MBB->end()) {
144 if (!I->isImplicitDef())
146 unsigned Reg = I->getOperand(0).getReg();
147 for (MCSubRegIterator SubRegs(Reg, TRI, /*IncludeSelf=*/true);
148 SubRegs.isValid(); ++SubRegs)
149 ImpDefRegs.insert(*SubRegs);
152 if (ImpDefRegs.empty())
155 MachineBasicBlock::iterator FirstTerm = I;
156 while (I != MBB->end()) {
157 if (!TII->isUnpredicatedTerminator(I))
159 // See if it uses any of the implicitly defined registers.
160 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
161 MachineOperand &MO = I->getOperand(i);
162 if (!MO.isReg() || !MO.isUse())
164 unsigned Reg = MO.getReg();
165 if (ImpDefRegs.count(Reg))
172 while (I != FirstTerm) {
173 MachineInstr *ImpDefMI = &*I;
175 MBB->erase(ImpDefMI);
181 /// OptimizeFunction - Perhaps branch folding, tail merging and other
182 /// CFG optimizations on the given function.
183 bool BranchFolder::OptimizeFunction(MachineFunction &MF,
184 const TargetInstrInfo *tii,
185 const TargetRegisterInfo *tri,
186 MachineModuleInfo *mmi) {
187 if (!tii) return false;
189 TriedMerging.clear();
196 // Use a RegScavenger to help update liveness when required.
197 MachineRegisterInfo &MRI = MF.getRegInfo();
198 if (MRI.tracksLiveness() && TRI->trackLivenessAfterRegAlloc(MF))
199 RS = new RegScavenger();
201 MRI.invalidateLiveness();
203 // Fix CFG. The later algorithms expect it to be right.
204 bool MadeChange = false;
205 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) {
206 MachineBasicBlock *MBB = I, *TBB = nullptr, *FBB = nullptr;
207 SmallVector<MachineOperand, 4> Cond;
208 if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true))
209 MadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty());
210 MadeChange |= OptimizeImpDefsBlock(MBB);
213 bool MadeChangeThisIteration = true;
214 while (MadeChangeThisIteration) {
215 MadeChangeThisIteration = TailMergeBlocks(MF);
216 MadeChangeThisIteration |= OptimizeBranches(MF);
217 if (EnableHoistCommonCode)
218 MadeChangeThisIteration |= HoistCommonCode(MF);
219 MadeChange |= MadeChangeThisIteration;
222 // See if any jump tables have become dead as the code generator
224 MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
230 // Walk the function to find jump tables that are live.
231 BitVector JTIsLive(JTI->getJumpTables().size());
232 for (MachineFunction::iterator BB = MF.begin(), E = MF.end();
234 for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end();
236 for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) {
237 MachineOperand &Op = I->getOperand(op);
238 if (!Op.isJTI()) continue;
240 // Remember that this JT is live.
241 JTIsLive.set(Op.getIndex());
245 // Finally, remove dead jump tables. This happens when the
246 // indirect jump was unreachable (and thus deleted).
247 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i)
248 if (!JTIsLive.test(i)) {
249 JTI->RemoveJumpTable(i);
257 //===----------------------------------------------------------------------===//
258 // Tail Merging of Blocks
259 //===----------------------------------------------------------------------===//
261 /// HashMachineInstr - Compute a hash value for MI and its operands.
262 static unsigned HashMachineInstr(const MachineInstr *MI) {
263 unsigned Hash = MI->getOpcode();
264 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
265 const MachineOperand &Op = MI->getOperand(i);
267 // Merge in bits from the operand if easy.
268 unsigned OperandHash = 0;
269 switch (Op.getType()) {
270 case MachineOperand::MO_Register: OperandHash = Op.getReg(); break;
271 case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break;
272 case MachineOperand::MO_MachineBasicBlock:
273 OperandHash = Op.getMBB()->getNumber();
275 case MachineOperand::MO_FrameIndex:
276 case MachineOperand::MO_ConstantPoolIndex:
277 case MachineOperand::MO_JumpTableIndex:
278 OperandHash = Op.getIndex();
280 case MachineOperand::MO_GlobalAddress:
281 case MachineOperand::MO_ExternalSymbol:
282 // Global address / external symbol are too hard, don't bother, but do
283 // pull in the offset.
284 OperandHash = Op.getOffset();
289 Hash += ((OperandHash << 3) | Op.getType()) << (i&31);
294 /// HashEndOfMBB - Hash the last instruction in the MBB.
295 static unsigned HashEndOfMBB(const MachineBasicBlock *MBB) {
296 MachineBasicBlock::const_iterator I = MBB->end();
297 if (I == MBB->begin())
298 return 0; // Empty MBB.
301 // Skip debug info so it will not affect codegen.
302 while (I->isDebugValue()) {
304 return 0; // MBB empty except for debug info.
308 return HashMachineInstr(I);
311 /// ComputeCommonTailLength - Given two machine basic blocks, compute the number
312 /// of instructions they actually have in common together at their end. Return
313 /// iterators for the first shared instruction in each block.
314 static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1,
315 MachineBasicBlock *MBB2,
316 MachineBasicBlock::iterator &I1,
317 MachineBasicBlock::iterator &I2) {
321 unsigned TailLen = 0;
322 while (I1 != MBB1->begin() && I2 != MBB2->begin()) {
324 // Skip debugging pseudos; necessary to avoid changing the code.
325 while (I1->isDebugValue()) {
326 if (I1==MBB1->begin()) {
327 while (I2->isDebugValue()) {
328 if (I2==MBB2->begin())
329 // I1==DBG at begin; I2==DBG at begin
334 // I1==DBG at begin; I2==non-DBG, or first of DBGs not at begin
339 // I1==first (untested) non-DBG preceding known match
340 while (I2->isDebugValue()) {
341 if (I2==MBB2->begin()) {
343 // I1==non-DBG, or first of DBGs not at begin; I2==DBG at begin
348 // I1, I2==first (untested) non-DBGs preceding known match
349 if (!I1->isIdenticalTo(I2) ||
350 // FIXME: This check is dubious. It's used to get around a problem where
351 // people incorrectly expect inline asm directives to remain in the same
352 // relative order. This is untenable because normal compiler
353 // optimizations (like this one) may reorder and/or merge these
361 // Back past possible debugging pseudos at beginning of block. This matters
362 // when one block differs from the other only by whether debugging pseudos
363 // are present at the beginning. (This way, the various checks later for
364 // I1==MBB1->begin() work as expected.)
365 if (I1 == MBB1->begin() && I2 != MBB2->begin()) {
367 while (I2->isDebugValue()) {
368 if (I2 == MBB2->begin())
374 if (I2 == MBB2->begin() && I1 != MBB1->begin()) {
376 while (I1->isDebugValue()) {
377 if (I1 == MBB1->begin())
386 void BranchFolder::MaintainLiveIns(MachineBasicBlock *CurMBB,
387 MachineBasicBlock *NewMBB) {
389 RS->enterBasicBlock(CurMBB);
390 if (!CurMBB->empty())
391 RS->forward(std::prev(CurMBB->end()));
392 BitVector RegsLiveAtExit(TRI->getNumRegs());
393 RS->getRegsUsed(RegsLiveAtExit, false);
394 for (unsigned int i = 0, e = TRI->getNumRegs(); i != e; i++)
395 if (RegsLiveAtExit[i])
396 NewMBB->addLiveIn(i);
400 /// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything
401 /// after it, replacing it with an unconditional branch to NewDest.
402 void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
403 MachineBasicBlock *NewDest) {
404 MachineBasicBlock *CurMBB = OldInst->getParent();
406 TII->ReplaceTailWithBranchTo(OldInst, NewDest);
408 // For targets that use the register scavenger, we must maintain LiveIns.
409 MaintainLiveIns(CurMBB, NewDest);
414 /// SplitMBBAt - Given a machine basic block and an iterator into it, split the
415 /// MBB so that the part before the iterator falls into the part starting at the
416 /// iterator. This returns the new MBB.
417 MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB,
418 MachineBasicBlock::iterator BBI1,
419 const BasicBlock *BB) {
420 if (!TII->isLegalToSplitMBBAt(CurMBB, BBI1))
423 MachineFunction &MF = *CurMBB.getParent();
425 // Create the fall-through block.
426 MachineFunction::iterator MBBI = &CurMBB;
427 MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(BB);
428 CurMBB.getParent()->insert(++MBBI, NewMBB);
430 // Move all the successors of this block to the specified block.
431 NewMBB->transferSuccessors(&CurMBB);
433 // Add an edge from CurMBB to NewMBB for the fall-through.
434 CurMBB.addSuccessor(NewMBB);
436 // Splice the code over.
437 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end());
439 // For targets that use the register scavenger, we must maintain LiveIns.
440 MaintainLiveIns(&CurMBB, NewMBB);
445 /// EstimateRuntime - Make a rough estimate for how long it will take to run
446 /// the specified code.
447 static unsigned EstimateRuntime(MachineBasicBlock::iterator I,
448 MachineBasicBlock::iterator E) {
450 for (; I != E; ++I) {
451 if (I->isDebugValue())
455 else if (I->mayLoad() || I->mayStore())
463 // CurMBB needs to add an unconditional branch to SuccMBB (we removed these
464 // branches temporarily for tail merging). In the case where CurMBB ends
465 // with a conditional branch to the next block, optimize by reversing the
466 // test and conditionally branching to SuccMBB instead.
467 static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB,
468 const TargetInstrInfo *TII) {
469 MachineFunction *MF = CurMBB->getParent();
470 MachineFunction::iterator I = std::next(MachineFunction::iterator(CurMBB));
471 MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
472 SmallVector<MachineOperand, 4> Cond;
473 DebugLoc dl; // FIXME: this is nowhere
474 if (I != MF->end() &&
475 !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond, true)) {
476 MachineBasicBlock *NextBB = I;
477 if (TBB == NextBB && !Cond.empty() && !FBB) {
478 if (!TII->ReverseBranchCondition(Cond)) {
479 TII->RemoveBranch(*CurMBB);
480 TII->InsertBranch(*CurMBB, SuccBB, nullptr, Cond, dl);
485 TII->InsertBranch(*CurMBB, SuccBB, nullptr,
486 SmallVector<MachineOperand, 0>(), dl);
490 BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const {
491 if (getHash() < o.getHash())
493 if (getHash() > o.getHash())
495 if (getBlock()->getNumber() < o.getBlock()->getNumber())
497 if (getBlock()->getNumber() > o.getBlock()->getNumber())
499 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing
500 // an object with itself.
501 #ifndef _GLIBCXX_DEBUG
502 llvm_unreachable("Predecessor appears twice");
508 /// CountTerminators - Count the number of terminators in the given
509 /// block and set I to the position of the first non-terminator, if there
510 /// is one, or MBB->end() otherwise.
511 static unsigned CountTerminators(MachineBasicBlock *MBB,
512 MachineBasicBlock::iterator &I) {
514 unsigned NumTerms = 0;
516 if (I == MBB->begin()) {
521 if (!I->isTerminator()) break;
527 /// ProfitableToMerge - Check if two machine basic blocks have a common tail
528 /// and decide if it would be profitable to merge those tails. Return the
529 /// length of the common tail and iterators to the first common instruction
531 static bool ProfitableToMerge(MachineBasicBlock *MBB1,
532 MachineBasicBlock *MBB2,
533 unsigned minCommonTailLength,
534 unsigned &CommonTailLen,
535 MachineBasicBlock::iterator &I1,
536 MachineBasicBlock::iterator &I2,
537 MachineBasicBlock *SuccBB,
538 MachineBasicBlock *PredBB) {
539 CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2);
540 if (CommonTailLen == 0)
542 DEBUG(dbgs() << "Common tail length of BB#" << MBB1->getNumber()
543 << " and BB#" << MBB2->getNumber() << " is " << CommonTailLen
546 // It's almost always profitable to merge any number of non-terminator
547 // instructions with the block that falls through into the common successor.
548 if (MBB1 == PredBB || MBB2 == PredBB) {
549 MachineBasicBlock::iterator I;
550 unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I);
551 if (CommonTailLen > NumTerms)
555 // If one of the blocks can be completely merged and happens to be in
556 // a position where the other could fall through into it, merge any number
557 // of instructions, because it can be done without a branch.
558 // TODO: If the blocks are not adjacent, move one of them so that they are?
559 if (MBB1->isLayoutSuccessor(MBB2) && I2 == MBB2->begin())
561 if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin())
564 // If both blocks have an unconditional branch temporarily stripped out,
565 // count that as an additional common instruction for the following
567 unsigned EffectiveTailLen = CommonTailLen;
568 if (SuccBB && MBB1 != PredBB && MBB2 != PredBB &&
569 !MBB1->back().isBarrier() &&
570 !MBB2->back().isBarrier())
573 // Check if the common tail is long enough to be worthwhile.
574 if (EffectiveTailLen >= minCommonTailLength)
577 // If we are optimizing for code size, 2 instructions in common is enough if
578 // we don't have to split a block. At worst we will be introducing 1 new
579 // branch instruction, which is likely to be smaller than the 2
580 // instructions that would be deleted in the merge.
581 MachineFunction *MF = MBB1->getParent();
582 if (EffectiveTailLen >= 2 &&
583 MF->getFunction()->getAttributes().
584 hasAttribute(AttributeSet::FunctionIndex, Attribute::OptimizeForSize) &&
585 (I1 == MBB1->begin() || I2 == MBB2->begin()))
591 /// ComputeSameTails - Look through all the blocks in MergePotentials that have
592 /// hash CurHash (guaranteed to match the last element). Build the vector
593 /// SameTails of all those that have the (same) largest number of instructions
594 /// in common of any pair of these blocks. SameTails entries contain an
595 /// iterator into MergePotentials (from which the MachineBasicBlock can be
596 /// found) and a MachineBasicBlock::iterator into that MBB indicating the
597 /// instruction where the matching code sequence begins.
598 /// Order of elements in SameTails is the reverse of the order in which
599 /// those blocks appear in MergePotentials (where they are not necessarily
601 unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
602 unsigned minCommonTailLength,
603 MachineBasicBlock *SuccBB,
604 MachineBasicBlock *PredBB) {
605 unsigned maxCommonTailLength = 0U;
607 MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
608 MPIterator HighestMPIter = std::prev(MergePotentials.end());
609 for (MPIterator CurMPIter = std::prev(MergePotentials.end()),
610 B = MergePotentials.begin();
611 CurMPIter != B && CurMPIter->getHash() == CurHash; --CurMPIter) {
612 for (MPIterator I = std::prev(CurMPIter); I->getHash() == CurHash; --I) {
613 unsigned CommonTailLen;
614 if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(),
616 CommonTailLen, TrialBBI1, TrialBBI2,
618 if (CommonTailLen > maxCommonTailLength) {
620 maxCommonTailLength = CommonTailLen;
621 HighestMPIter = CurMPIter;
622 SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1));
624 if (HighestMPIter == CurMPIter &&
625 CommonTailLen == maxCommonTailLength)
626 SameTails.push_back(SameTailElt(I, TrialBBI2));
632 return maxCommonTailLength;
635 /// RemoveBlocksWithHash - Remove all blocks with hash CurHash from
636 /// MergePotentials, restoring branches at ends of blocks as appropriate.
637 void BranchFolder::RemoveBlocksWithHash(unsigned CurHash,
638 MachineBasicBlock *SuccBB,
639 MachineBasicBlock *PredBB) {
640 MPIterator CurMPIter, B;
641 for (CurMPIter = std::prev(MergePotentials.end()),
642 B = MergePotentials.begin();
643 CurMPIter->getHash() == CurHash; --CurMPIter) {
644 // Put the unconditional branch back, if we need one.
645 MachineBasicBlock *CurMBB = CurMPIter->getBlock();
646 if (SuccBB && CurMBB != PredBB)
647 FixTail(CurMBB, SuccBB, TII);
651 if (CurMPIter->getHash() != CurHash)
653 MergePotentials.erase(CurMPIter, MergePotentials.end());
656 /// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist
657 /// only of the common tail. Create a block that does by splitting one.
658 bool BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
659 MachineBasicBlock *SuccBB,
660 unsigned maxCommonTailLength,
661 unsigned &commonTailIndex) {
663 unsigned TimeEstimate = ~0U;
664 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
665 // Use PredBB if possible; that doesn't require a new branch.
666 if (SameTails[i].getBlock() == PredBB) {
670 // Otherwise, make a (fairly bogus) choice based on estimate of
671 // how long it will take the various blocks to execute.
672 unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(),
673 SameTails[i].getTailStartPos());
674 if (t <= TimeEstimate) {
680 MachineBasicBlock::iterator BBI =
681 SameTails[commonTailIndex].getTailStartPos();
682 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
684 // If the common tail includes any debug info we will take it pretty
685 // randomly from one of the inputs. Might be better to remove it?
686 DEBUG(dbgs() << "\nSplitting BB#" << MBB->getNumber() << ", size "
687 << maxCommonTailLength);
689 // If the split block unconditionally falls-thru to SuccBB, it will be
690 // merged. In control flow terms it should then take SuccBB's name. e.g. If
691 // SuccBB is an inner loop, the common tail is still part of the inner loop.
692 const BasicBlock *BB = (SuccBB && MBB->succ_size() == 1) ?
693 SuccBB->getBasicBlock() : MBB->getBasicBlock();
694 MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI, BB);
696 DEBUG(dbgs() << "... failed!");
700 SameTails[commonTailIndex].setBlock(newMBB);
701 SameTails[commonTailIndex].setTailStartPos(newMBB->begin());
703 // If we split PredBB, newMBB is the new predecessor.
710 // See if any of the blocks in MergePotentials (which all have a common single
711 // successor, or all have no successor) can be tail-merged. If there is a
712 // successor, any blocks in MergePotentials that are not tail-merged and
713 // are not immediately before Succ must have an unconditional branch to
714 // Succ added (but the predecessor/successor lists need no adjustment).
715 // The lone predecessor of Succ that falls through into Succ,
716 // if any, is given in PredBB.
718 bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
719 MachineBasicBlock *PredBB) {
720 bool MadeChange = false;
722 // Except for the special cases below, tail-merge if there are at least
723 // this many instructions in common.
724 unsigned minCommonTailLength = TailMergeSize;
726 DEBUG(dbgs() << "\nTryTailMergeBlocks: ";
727 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
728 dbgs() << "BB#" << MergePotentials[i].getBlock()->getNumber()
729 << (i == e-1 ? "" : ", ");
732 dbgs() << " with successor BB#" << SuccBB->getNumber() << '\n';
734 dbgs() << " which has fall-through from BB#"
735 << PredBB->getNumber() << "\n";
737 dbgs() << "Looking for common tails of at least "
738 << minCommonTailLength << " instruction"
739 << (minCommonTailLength == 1 ? "" : "s") << '\n';
742 // Sort by hash value so that blocks with identical end sequences sort
744 std::stable_sort(MergePotentials.begin(), MergePotentials.end());
746 // Walk through equivalence sets looking for actual exact matches.
747 while (MergePotentials.size() > 1) {
748 unsigned CurHash = MergePotentials.back().getHash();
750 // Build SameTails, identifying the set of blocks with this hash code
751 // and with the maximum number of instructions in common.
752 unsigned maxCommonTailLength = ComputeSameTails(CurHash,
756 // If we didn't find any pair that has at least minCommonTailLength
757 // instructions in common, remove all blocks with this hash code and retry.
758 if (SameTails.empty()) {
759 RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
763 // If one of the blocks is the entire common tail (and not the entry
764 // block, which we can't jump to), we can treat all blocks with this same
765 // tail at once. Use PredBB if that is one of the possibilities, as that
766 // will not introduce any extra branches.
767 MachineBasicBlock *EntryBB = MergePotentials.begin()->getBlock()->
768 getParent()->begin();
769 unsigned commonTailIndex = SameTails.size();
770 // If there are two blocks, check to see if one can be made to fall through
772 if (SameTails.size() == 2 &&
773 SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) &&
774 SameTails[1].tailIsWholeBlock())
776 else if (SameTails.size() == 2 &&
777 SameTails[1].getBlock()->isLayoutSuccessor(
778 SameTails[0].getBlock()) &&
779 SameTails[0].tailIsWholeBlock())
782 // Otherwise just pick one, favoring the fall-through predecessor if
784 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
785 MachineBasicBlock *MBB = SameTails[i].getBlock();
786 if (MBB == EntryBB && SameTails[i].tailIsWholeBlock())
792 if (SameTails[i].tailIsWholeBlock())
797 if (commonTailIndex == SameTails.size() ||
798 (SameTails[commonTailIndex].getBlock() == PredBB &&
799 !SameTails[commonTailIndex].tailIsWholeBlock())) {
800 // None of the blocks consist entirely of the common tail.
801 // Split a block so that one does.
802 if (!CreateCommonTailOnlyBlock(PredBB, SuccBB,
803 maxCommonTailLength, commonTailIndex)) {
804 RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
809 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
810 // MBB is common tail. Adjust all other BB's to jump to this one.
811 // Traversal must be forwards so erases work.
812 DEBUG(dbgs() << "\nUsing common tail in BB#" << MBB->getNumber()
814 for (unsigned int i=0, e = SameTails.size(); i != e; ++i) {
815 if (commonTailIndex == i)
817 DEBUG(dbgs() << "BB#" << SameTails[i].getBlock()->getNumber()
818 << (i == e-1 ? "" : ", "));
819 // Hack the end off BB i, making it jump to BB commonTailIndex instead.
820 ReplaceTailWithBranchTo(SameTails[i].getTailStartPos(), MBB);
821 // BB i is no longer a predecessor of SuccBB; remove it from the worklist.
822 MergePotentials.erase(SameTails[i].getMPIter());
824 DEBUG(dbgs() << "\n");
825 // We leave commonTailIndex in the worklist in case there are other blocks
826 // that match it with a smaller number of instructions.
832 bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
833 bool MadeChange = false;
834 if (!EnableTailMerge) return MadeChange;
836 // First find blocks with no successors.
837 MergePotentials.clear();
838 for (MachineFunction::iterator I = MF.begin(), E = MF.end();
839 I != E && MergePotentials.size() < TailMergeThreshold; ++I) {
840 if (TriedMerging.count(I))
843 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(I), I));
846 // If this is a large problem, avoid visiting the same basic blocks
848 if (MergePotentials.size() == TailMergeThreshold)
849 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
850 TriedMerging.insert(MergePotentials[i].getBlock());
852 // See if we can do any tail merging on those.
853 if (MergePotentials.size() >= 2)
854 MadeChange |= TryTailMergeBlocks(nullptr, nullptr);
856 // Look at blocks (IBB) with multiple predecessors (PBB).
857 // We change each predecessor to a canonical form, by
858 // (1) temporarily removing any unconditional branch from the predecessor
860 // (2) alter conditional branches so they branch to the other block
861 // not IBB; this may require adding back an unconditional branch to IBB
862 // later, where there wasn't one coming in. E.g.
864 // fallthrough to QBB
867 // with a conceptual B to IBB after that, which never actually exists.
868 // With those changes, we see whether the predecessors' tails match,
869 // and merge them if so. We change things out of canonical form and
870 // back to the way they were later in the process. (OptimizeBranches
871 // would undo some of this, but we can't use it, because we'd get into
872 // a compile-time infinite loop repeatedly doing and undoing the same
875 for (MachineFunction::iterator I = std::next(MF.begin()), E = MF.end();
877 if (I->pred_size() < 2) continue;
878 SmallPtrSet<MachineBasicBlock *, 8> UniquePreds;
879 MachineBasicBlock *IBB = I;
880 MachineBasicBlock *PredBB = std::prev(I);
881 MergePotentials.clear();
882 for (MachineBasicBlock::pred_iterator P = I->pred_begin(),
884 P != E2 && MergePotentials.size() < TailMergeThreshold; ++P) {
885 MachineBasicBlock *PBB = *P;
886 if (TriedMerging.count(PBB))
889 // Skip blocks that loop to themselves, can't tail merge these.
893 // Visit each predecessor only once.
894 if (!UniquePreds.insert(PBB))
897 // Skip blocks which may jump to a landing pad. Can't tail merge these.
898 if (PBB->getLandingPadSuccessor())
901 MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
902 SmallVector<MachineOperand, 4> Cond;
903 if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond, true)) {
904 // Failing case: IBB is the target of a cbr, and we cannot reverse the
906 SmallVector<MachineOperand, 4> NewCond(Cond);
907 if (!Cond.empty() && TBB == IBB) {
908 if (TII->ReverseBranchCondition(NewCond))
910 // This is the QBB case described above
912 FBB = std::next(MachineFunction::iterator(PBB));
915 // Failing case: the only way IBB can be reached from PBB is via
916 // exception handling. Happens for landing pads. Would be nice to have
917 // a bit in the edge so we didn't have to do all this.
918 if (IBB->isLandingPad()) {
919 MachineFunction::iterator IP = PBB; IP++;
920 MachineBasicBlock *PredNextBB = nullptr;
924 if (IBB != PredNextBB) // fallthrough
927 if (TBB != IBB && FBB != IBB) // cbr then ubr
929 } else if (Cond.empty()) {
930 if (TBB != IBB) // ubr
933 if (TBB != IBB && IBB != PredNextBB) // cbr
938 // Remove the unconditional branch at the end, if any.
939 if (TBB && (Cond.empty() || FBB)) {
940 DebugLoc dl; // FIXME: this is nowhere
941 TII->RemoveBranch(*PBB);
943 // reinsert conditional branch only, for now
944 TII->InsertBranch(*PBB, (TBB == IBB) ? FBB : TBB, nullptr,
948 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(PBB), *P));
952 // If this is a large problem, avoid visiting the same basic blocks multiple
954 if (MergePotentials.size() == TailMergeThreshold)
955 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
956 TriedMerging.insert(MergePotentials[i].getBlock());
958 if (MergePotentials.size() >= 2)
959 MadeChange |= TryTailMergeBlocks(IBB, PredBB);
961 // Reinsert an unconditional branch if needed. The 1 below can occur as a
962 // result of removing blocks in TryTailMergeBlocks.
963 PredBB = std::prev(I); // this may have been changed in TryTailMergeBlocks
964 if (MergePotentials.size() == 1 &&
965 MergePotentials.begin()->getBlock() != PredBB)
966 FixTail(MergePotentials.begin()->getBlock(), IBB, TII);
972 //===----------------------------------------------------------------------===//
973 // Branch Optimization
974 //===----------------------------------------------------------------------===//
976 bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
977 bool MadeChange = false;
979 // Make sure blocks are numbered in order
982 for (MachineFunction::iterator I = std::next(MF.begin()), E = MF.end();
984 MachineBasicBlock *MBB = I++;
985 MadeChange |= OptimizeBlock(MBB);
987 // If it is dead, remove it.
988 if (MBB->pred_empty()) {
989 RemoveDeadBlock(MBB);
997 // Blocks should be considered empty if they contain only debug info;
998 // else the debug info would affect codegen.
999 static bool IsEmptyBlock(MachineBasicBlock *MBB) {
1002 for (MachineBasicBlock::iterator MBBI = MBB->begin(), MBBE = MBB->end();
1003 MBBI!=MBBE; ++MBBI) {
1004 if (!MBBI->isDebugValue())
1010 // Blocks with only debug info and branches should be considered the same
1011 // as blocks with only branches.
1012 static bool IsBranchOnlyBlock(MachineBasicBlock *MBB) {
1013 MachineBasicBlock::iterator MBBI, MBBE;
1014 for (MBBI = MBB->begin(), MBBE = MBB->end(); MBBI!=MBBE; ++MBBI) {
1015 if (!MBBI->isDebugValue())
1018 return (MBBI->isBranch());
1021 /// IsBetterFallthrough - Return true if it would be clearly better to
1022 /// fall-through to MBB1 than to fall through into MBB2. This has to return
1023 /// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will
1024 /// result in infinite loops.
1025 static bool IsBetterFallthrough(MachineBasicBlock *MBB1,
1026 MachineBasicBlock *MBB2) {
1027 // Right now, we use a simple heuristic. If MBB2 ends with a call, and
1028 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to
1029 // optimize branches that branch to either a return block or an assert block
1030 // into a fallthrough to the return.
1031 if (IsEmptyBlock(MBB1) || IsEmptyBlock(MBB2)) return false;
1033 // If there is a clear successor ordering we make sure that one block
1034 // will fall through to the next
1035 if (MBB1->isSuccessor(MBB2)) return true;
1036 if (MBB2->isSuccessor(MBB1)) return false;
1038 // Neither block consists entirely of debug info (per IsEmptyBlock check),
1039 // so we needn't test for falling off the beginning here.
1040 MachineBasicBlock::iterator MBB1I = --MBB1->end();
1041 while (MBB1I->isDebugValue())
1043 MachineBasicBlock::iterator MBB2I = --MBB2->end();
1044 while (MBB2I->isDebugValue())
1046 return MBB2I->isCall() && !MBB1I->isCall();
1049 /// getBranchDebugLoc - Find and return, if any, the DebugLoc of the branch
1050 /// instructions on the block. Always use the DebugLoc of the first
1051 /// branching instruction found unless its absent, in which case use the
1052 /// DebugLoc of the second if present.
1053 static DebugLoc getBranchDebugLoc(MachineBasicBlock &MBB) {
1054 MachineBasicBlock::iterator I = MBB.end();
1055 if (I == MBB.begin())
1058 while (I->isDebugValue() && I != MBB.begin())
1061 return I->getDebugLoc();
1065 /// OptimizeBlock - Analyze and optimize control flow related to the specified
1066 /// block. This is never called on the entry block.
1067 bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
1068 bool MadeChange = false;
1069 MachineFunction &MF = *MBB->getParent();
1072 MachineFunction::iterator FallThrough = MBB;
1075 // If this block is empty, make everyone use its fall-through, not the block
1076 // explicitly. Landing pads should not do this since the landing-pad table
1077 // points to this block. Blocks with their addresses taken shouldn't be
1079 if (IsEmptyBlock(MBB) && !MBB->isLandingPad() && !MBB->hasAddressTaken()) {
1080 // Dead block? Leave for cleanup later.
1081 if (MBB->pred_empty()) return MadeChange;
1083 if (FallThrough == MF.end()) {
1084 // TODO: Simplify preds to not branch here if possible!
1086 // Rewrite all predecessors of the old block to go to the fallthrough
1088 while (!MBB->pred_empty()) {
1089 MachineBasicBlock *Pred = *(MBB->pred_end()-1);
1090 Pred->ReplaceUsesOfBlockWith(MBB, FallThrough);
1092 // If MBB was the target of a jump table, update jump tables to go to the
1093 // fallthrough instead.
1094 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
1095 MJTI->ReplaceMBBInJumpTables(MBB, FallThrough);
1101 // Check to see if we can simplify the terminator of the block before this
1103 MachineBasicBlock &PrevBB = *std::prev(MachineFunction::iterator(MBB));
1105 MachineBasicBlock *PriorTBB = nullptr, *PriorFBB = nullptr;
1106 SmallVector<MachineOperand, 4> PriorCond;
1107 bool PriorUnAnalyzable =
1108 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
1109 if (!PriorUnAnalyzable) {
1110 // If the CFG for the prior block has extra edges, remove them.
1111 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB,
1112 !PriorCond.empty());
1114 // If the previous branch is conditional and both conditions go to the same
1115 // destination, remove the branch, replacing it with an unconditional one or
1117 if (PriorTBB && PriorTBB == PriorFBB) {
1118 DebugLoc dl = getBranchDebugLoc(PrevBB);
1119 TII->RemoveBranch(PrevBB);
1121 if (PriorTBB != MBB)
1122 TII->InsertBranch(PrevBB, PriorTBB, nullptr, PriorCond, dl);
1125 goto ReoptimizeBlock;
1128 // If the previous block unconditionally falls through to this block and
1129 // this block has no other predecessors, move the contents of this block
1130 // into the prior block. This doesn't usually happen when SimplifyCFG
1131 // has been used, but it can happen if tail merging splits a fall-through
1132 // predecessor of a block.
1133 // This has to check PrevBB->succ_size() because EH edges are ignored by
1135 if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 &&
1136 PrevBB.succ_size() == 1 &&
1137 !MBB->hasAddressTaken() && !MBB->isLandingPad()) {
1138 DEBUG(dbgs() << "\nMerging into block: " << PrevBB
1139 << "From MBB: " << *MBB);
1140 // Remove redundant DBG_VALUEs first.
1141 if (PrevBB.begin() != PrevBB.end()) {
1142 MachineBasicBlock::iterator PrevBBIter = PrevBB.end();
1144 MachineBasicBlock::iterator MBBIter = MBB->begin();
1145 // Check if DBG_VALUE at the end of PrevBB is identical to the
1146 // DBG_VALUE at the beginning of MBB.
1147 while (PrevBBIter != PrevBB.begin() && MBBIter != MBB->end()
1148 && PrevBBIter->isDebugValue() && MBBIter->isDebugValue()) {
1149 if (!MBBIter->isIdenticalTo(PrevBBIter))
1151 MachineInstr *DuplicateDbg = MBBIter;
1152 ++MBBIter; -- PrevBBIter;
1153 DuplicateDbg->eraseFromParent();
1156 PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end());
1157 PrevBB.removeSuccessor(PrevBB.succ_begin());
1158 assert(PrevBB.succ_empty());
1159 PrevBB.transferSuccessors(MBB);
1164 // If the previous branch *only* branches to *this* block (conditional or
1165 // not) remove the branch.
1166 if (PriorTBB == MBB && !PriorFBB) {
1167 TII->RemoveBranch(PrevBB);
1170 goto ReoptimizeBlock;
1173 // If the prior block branches somewhere else on the condition and here if
1174 // the condition is false, remove the uncond second branch.
1175 if (PriorFBB == MBB) {
1176 DebugLoc dl = getBranchDebugLoc(PrevBB);
1177 TII->RemoveBranch(PrevBB);
1178 TII->InsertBranch(PrevBB, PriorTBB, nullptr, PriorCond, dl);
1181 goto ReoptimizeBlock;
1184 // If the prior block branches here on true and somewhere else on false, and
1185 // if the branch condition is reversible, reverse the branch to create a
1187 if (PriorTBB == MBB) {
1188 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1189 if (!TII->ReverseBranchCondition(NewPriorCond)) {
1190 DebugLoc dl = getBranchDebugLoc(PrevBB);
1191 TII->RemoveBranch(PrevBB);
1192 TII->InsertBranch(PrevBB, PriorFBB, nullptr, NewPriorCond, dl);
1195 goto ReoptimizeBlock;
1199 // If this block has no successors (e.g. it is a return block or ends with
1200 // a call to a no-return function like abort or __cxa_throw) and if the pred
1201 // falls through into this block, and if it would otherwise fall through
1202 // into the block after this, move this block to the end of the function.
1204 // We consider it more likely that execution will stay in the function (e.g.
1205 // due to loops) than it is to exit it. This asserts in loops etc, moving
1206 // the assert condition out of the loop body.
1207 if (MBB->succ_empty() && !PriorCond.empty() && !PriorFBB &&
1208 MachineFunction::iterator(PriorTBB) == FallThrough &&
1209 !MBB->canFallThrough()) {
1210 bool DoTransform = true;
1212 // We have to be careful that the succs of PredBB aren't both no-successor
1213 // blocks. If neither have successors and if PredBB is the second from
1214 // last block in the function, we'd just keep swapping the two blocks for
1215 // last. Only do the swap if one is clearly better to fall through than
1217 if (FallThrough == --MF.end() &&
1218 !IsBetterFallthrough(PriorTBB, MBB))
1219 DoTransform = false;
1222 // Reverse the branch so we will fall through on the previous true cond.
1223 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1224 if (!TII->ReverseBranchCondition(NewPriorCond)) {
1225 DEBUG(dbgs() << "\nMoving MBB: " << *MBB
1226 << "To make fallthrough to: " << *PriorTBB << "\n");
1228 DebugLoc dl = getBranchDebugLoc(PrevBB);
1229 TII->RemoveBranch(PrevBB);
1230 TII->InsertBranch(PrevBB, MBB, nullptr, NewPriorCond, dl);
1232 // Move this block to the end of the function.
1233 MBB->moveAfter(--MF.end());
1242 // Analyze the branch in the current block.
1243 MachineBasicBlock *CurTBB = nullptr, *CurFBB = nullptr;
1244 SmallVector<MachineOperand, 4> CurCond;
1245 bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true);
1246 if (!CurUnAnalyzable) {
1247 // If the CFG for the prior block has extra edges, remove them.
1248 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty());
1250 // If this is a two-way branch, and the FBB branches to this block, reverse
1251 // the condition so the single-basic-block loop is faster. Instead of:
1252 // Loop: xxx; jcc Out; jmp Loop
1254 // Loop: xxx; jncc Loop; jmp Out
1255 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) {
1256 SmallVector<MachineOperand, 4> NewCond(CurCond);
1257 if (!TII->ReverseBranchCondition(NewCond)) {
1258 DebugLoc dl = getBranchDebugLoc(*MBB);
1259 TII->RemoveBranch(*MBB);
1260 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond, dl);
1263 goto ReoptimizeBlock;
1267 // If this branch is the only thing in its block, see if we can forward
1268 // other blocks across it.
1269 if (CurTBB && CurCond.empty() && !CurFBB &&
1270 IsBranchOnlyBlock(MBB) && CurTBB != MBB &&
1271 !MBB->hasAddressTaken()) {
1272 DebugLoc dl = getBranchDebugLoc(*MBB);
1273 // This block may contain just an unconditional branch. Because there can
1274 // be 'non-branch terminators' in the block, try removing the branch and
1275 // then seeing if the block is empty.
1276 TII->RemoveBranch(*MBB);
1277 // If the only things remaining in the block are debug info, remove these
1278 // as well, so this will behave the same as an empty block in non-debug
1280 if (!MBB->empty()) {
1281 bool NonDebugInfoFound = false;
1282 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
1284 if (!I->isDebugValue()) {
1285 NonDebugInfoFound = true;
1289 if (!NonDebugInfoFound)
1290 // Make the block empty, losing the debug info (we could probably
1291 // improve this in some cases.)
1292 MBB->erase(MBB->begin(), MBB->end());
1294 // If this block is just an unconditional branch to CurTBB, we can
1295 // usually completely eliminate the block. The only case we cannot
1296 // completely eliminate the block is when the block before this one
1297 // falls through into MBB and we can't understand the prior block's branch
1300 bool PredHasNoFallThrough = !PrevBB.canFallThrough();
1301 if (PredHasNoFallThrough || !PriorUnAnalyzable ||
1302 !PrevBB.isSuccessor(MBB)) {
1303 // If the prior block falls through into us, turn it into an
1304 // explicit branch to us to make updates simpler.
1305 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) &&
1306 PriorTBB != MBB && PriorFBB != MBB) {
1308 assert(PriorCond.empty() && !PriorFBB &&
1309 "Bad branch analysis");
1312 assert(!PriorFBB && "Machine CFG out of date!");
1315 DebugLoc pdl = getBranchDebugLoc(PrevBB);
1316 TII->RemoveBranch(PrevBB);
1317 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, pdl);
1320 // Iterate through all the predecessors, revectoring each in-turn.
1322 bool DidChange = false;
1323 bool HasBranchToSelf = false;
1324 while(PI != MBB->pred_size()) {
1325 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI);
1327 // If this block has an uncond branch to itself, leave it.
1329 HasBranchToSelf = true;
1332 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB);
1333 // If this change resulted in PMBB ending in a conditional
1334 // branch where both conditions go to the same destination,
1335 // change this to an unconditional branch (and fix the CFG).
1336 MachineBasicBlock *NewCurTBB = nullptr, *NewCurFBB = nullptr;
1337 SmallVector<MachineOperand, 4> NewCurCond;
1338 bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB,
1339 NewCurFBB, NewCurCond, true);
1340 if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) {
1341 DebugLoc pdl = getBranchDebugLoc(*PMBB);
1342 TII->RemoveBranch(*PMBB);
1344 TII->InsertBranch(*PMBB, NewCurTBB, nullptr, NewCurCond, pdl);
1347 PMBB->CorrectExtraCFGEdges(NewCurTBB, nullptr, false);
1352 // Change any jumptables to go to the new MBB.
1353 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
1354 MJTI->ReplaceMBBInJumpTables(MBB, CurTBB);
1358 if (!HasBranchToSelf) return MadeChange;
1363 // Add the branch back if the block is more than just an uncond branch.
1364 TII->InsertBranch(*MBB, CurTBB, nullptr, CurCond, dl);
1368 // If the prior block doesn't fall through into this block, and if this
1369 // block doesn't fall through into some other block, see if we can find a
1370 // place to move this block where a fall-through will happen.
1371 if (!PrevBB.canFallThrough()) {
1373 // Now we know that there was no fall-through into this block, check to
1374 // see if it has a fall-through into its successor.
1375 bool CurFallsThru = MBB->canFallThrough();
1377 if (!MBB->isLandingPad()) {
1378 // Check all the predecessors of this block. If one of them has no fall
1379 // throughs, move this block right after it.
1380 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
1381 E = MBB->pred_end(); PI != E; ++PI) {
1382 // Analyze the branch at the end of the pred.
1383 MachineBasicBlock *PredBB = *PI;
1384 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough;
1385 MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
1386 SmallVector<MachineOperand, 4> PredCond;
1387 if (PredBB != MBB && !PredBB->canFallThrough() &&
1388 !TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true)
1389 && (!CurFallsThru || !CurTBB || !CurFBB)
1390 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
1391 // If the current block doesn't fall through, just move it.
1392 // If the current block can fall through and does not end with a
1393 // conditional branch, we need to append an unconditional jump to
1394 // the (current) next block. To avoid a possible compile-time
1395 // infinite loop, move blocks only backward in this case.
1396 // Also, if there are already 2 branches here, we cannot add a third;
1397 // this means we have the case
1402 MachineBasicBlock *NextBB =
1403 std::next(MachineFunction::iterator(MBB));
1405 TII->InsertBranch(*MBB, NextBB, nullptr, CurCond, DebugLoc());
1407 MBB->moveAfter(PredBB);
1409 goto ReoptimizeBlock;
1414 if (!CurFallsThru) {
1415 // Check all successors to see if we can move this block before it.
1416 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
1417 E = MBB->succ_end(); SI != E; ++SI) {
1418 // Analyze the branch at the end of the block before the succ.
1419 MachineBasicBlock *SuccBB = *SI;
1420 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev;
1422 // If this block doesn't already fall-through to that successor, and if
1423 // the succ doesn't already have a block that can fall through into it,
1424 // and if the successor isn't an EH destination, we can arrange for the
1425 // fallthrough to happen.
1426 if (SuccBB != MBB && &*SuccPrev != MBB &&
1427 !SuccPrev->canFallThrough() && !CurUnAnalyzable &&
1428 !SuccBB->isLandingPad()) {
1429 MBB->moveBefore(SuccBB);
1431 goto ReoptimizeBlock;
1435 // Okay, there is no really great place to put this block. If, however,
1436 // the block before this one would be a fall-through if this block were
1437 // removed, move this block to the end of the function.
1438 MachineBasicBlock *PrevTBB = nullptr, *PrevFBB = nullptr;
1439 SmallVector<MachineOperand, 4> PrevCond;
1440 if (FallThrough != MF.end() &&
1441 !TII->AnalyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) &&
1442 PrevBB.isSuccessor(FallThrough)) {
1443 MBB->moveAfter(--MF.end());
1453 //===----------------------------------------------------------------------===//
1454 // Hoist Common Code
1455 //===----------------------------------------------------------------------===//
1457 /// HoistCommonCode - Hoist common instruction sequences at the start of basic
1458 /// blocks to their common predecessor.
1459 bool BranchFolder::HoistCommonCode(MachineFunction &MF) {
1460 bool MadeChange = false;
1461 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ) {
1462 MachineBasicBlock *MBB = I++;
1463 MadeChange |= HoistCommonCodeInSuccs(MBB);
1469 /// findFalseBlock - BB has a fallthrough. Find its 'false' successor given
1470 /// its 'true' successor.
1471 static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB,
1472 MachineBasicBlock *TrueBB) {
1473 for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
1474 E = BB->succ_end(); SI != E; ++SI) {
1475 MachineBasicBlock *SuccBB = *SI;
1476 if (SuccBB != TrueBB)
1482 /// findHoistingInsertPosAndDeps - Find the location to move common instructions
1483 /// in successors to. The location is usually just before the terminator,
1484 /// however if the terminator is a conditional branch and its previous
1485 /// instruction is the flag setting instruction, the previous instruction is
1486 /// the preferred location. This function also gathers uses and defs of the
1487 /// instructions from the insertion point to the end of the block. The data is
1488 /// used by HoistCommonCodeInSuccs to ensure safety.
1490 MachineBasicBlock::iterator findHoistingInsertPosAndDeps(MachineBasicBlock *MBB,
1491 const TargetInstrInfo *TII,
1492 const TargetRegisterInfo *TRI,
1493 SmallSet<unsigned,4> &Uses,
1494 SmallSet<unsigned,4> &Defs) {
1495 MachineBasicBlock::iterator Loc = MBB->getFirstTerminator();
1496 if (!TII->isUnpredicatedTerminator(Loc))
1499 for (unsigned i = 0, e = Loc->getNumOperands(); i != e; ++i) {
1500 const MachineOperand &MO = Loc->getOperand(i);
1503 unsigned Reg = MO.getReg();
1507 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
1511 // Don't try to hoist code in the rare case the terminator defines a
1512 // register that is later used.
1515 // If the terminator defines a register, make sure we don't hoist
1516 // the instruction whose def might be clobbered by the terminator.
1517 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
1524 if (Loc == MBB->begin())
1527 // The terminator is probably a conditional branch, try not to separate the
1528 // branch from condition setting instruction.
1529 MachineBasicBlock::iterator PI = Loc;
1531 while (PI != MBB->begin() && PI->isDebugValue())
1535 for (unsigned i = 0, e = PI->getNumOperands(); !IsDef && i != e; ++i) {
1536 const MachineOperand &MO = PI->getOperand(i);
1537 // If PI has a regmask operand, it is probably a call. Separate away.
1540 if (!MO.isReg() || MO.isUse())
1542 unsigned Reg = MO.getReg();
1545 if (Uses.count(Reg))
1549 // The condition setting instruction is not just before the conditional
1553 // Be conservative, don't insert instruction above something that may have
1554 // side-effects. And since it's potentially bad to separate flag setting
1555 // instruction from the conditional branch, just abort the optimization
1557 // Also avoid moving code above predicated instruction since it's hard to
1558 // reason about register liveness with predicated instruction.
1559 bool DontMoveAcrossStore = true;
1560 if (!PI->isSafeToMove(TII, nullptr, DontMoveAcrossStore) ||
1561 TII->isPredicated(PI))
1565 // Find out what registers are live. Note this routine is ignoring other live
1566 // registers which are only used by instructions in successor blocks.
1567 for (unsigned i = 0, e = PI->getNumOperands(); i != e; ++i) {
1568 const MachineOperand &MO = PI->getOperand(i);
1571 unsigned Reg = MO.getReg();
1575 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
1578 if (Uses.erase(Reg)) {
1579 for (MCSubRegIterator SubRegs(Reg, TRI); SubRegs.isValid(); ++SubRegs)
1580 Uses.erase(*SubRegs); // Use sub-registers to be conservative
1582 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
1590 /// HoistCommonCodeInSuccs - If the successors of MBB has common instruction
1591 /// sequence at the start of the function, move the instructions before MBB
1592 /// terminator if it's legal.
1593 bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
1594 MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
1595 SmallVector<MachineOperand, 4> Cond;
1596 if (TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true) || !TBB || Cond.empty())
1599 if (!FBB) FBB = findFalseBlock(MBB, TBB);
1601 // Malformed bcc? True and false blocks are the same?
1604 // Restrict the optimization to cases where MBB is the only predecessor,
1605 // it is an obvious win.
1606 if (TBB->pred_size() > 1 || FBB->pred_size() > 1)
1609 // Find a suitable position to hoist the common instructions to. Also figure
1610 // out which registers are used or defined by instructions from the insertion
1611 // point to the end of the block.
1612 SmallSet<unsigned, 4> Uses, Defs;
1613 MachineBasicBlock::iterator Loc =
1614 findHoistingInsertPosAndDeps(MBB, TII, TRI, Uses, Defs);
1615 if (Loc == MBB->end())
1618 bool HasDups = false;
1619 SmallVector<unsigned, 4> LocalDefs;
1620 SmallSet<unsigned, 4> LocalDefsSet;
1621 MachineBasicBlock::iterator TIB = TBB->begin();
1622 MachineBasicBlock::iterator FIB = FBB->begin();
1623 MachineBasicBlock::iterator TIE = TBB->end();
1624 MachineBasicBlock::iterator FIE = FBB->end();
1625 while (TIB != TIE && FIB != FIE) {
1626 // Skip dbg_value instructions. These do not count.
1627 if (TIB->isDebugValue()) {
1628 while (TIB != TIE && TIB->isDebugValue())
1633 if (FIB->isDebugValue()) {
1634 while (FIB != FIE && FIB->isDebugValue())
1639 if (!TIB->isIdenticalTo(FIB, MachineInstr::CheckKillDead))
1642 if (TII->isPredicated(TIB))
1643 // Hard to reason about register liveness with predicated instruction.
1647 for (unsigned i = 0, e = TIB->getNumOperands(); i != e; ++i) {
1648 MachineOperand &MO = TIB->getOperand(i);
1649 // Don't attempt to hoist instructions with register masks.
1650 if (MO.isRegMask()) {
1656 unsigned Reg = MO.getReg();
1660 if (Uses.count(Reg)) {
1661 // Avoid clobbering a register that's used by the instruction at
1662 // the point of insertion.
1667 if (Defs.count(Reg) && !MO.isDead()) {
1668 // Don't hoist the instruction if the def would be clobber by the
1669 // instruction at the point insertion. FIXME: This is overly
1670 // conservative. It should be possible to hoist the instructions
1671 // in BB2 in the following example:
1673 // r1, eflag = op1 r2, r3
1682 } else if (!LocalDefsSet.count(Reg)) {
1683 if (Defs.count(Reg)) {
1684 // Use is defined by the instruction at the point of insertion.
1689 if (MO.isKill() && Uses.count(Reg))
1690 // Kills a register that's read by the instruction at the point of
1691 // insertion. Remove the kill marker.
1692 MO.setIsKill(false);
1698 bool DontMoveAcrossStore = true;
1699 if (!TIB->isSafeToMove(TII, nullptr, DontMoveAcrossStore))
1702 // Remove kills from LocalDefsSet, these registers had short live ranges.
1703 for (unsigned i = 0, e = TIB->getNumOperands(); i != e; ++i) {
1704 MachineOperand &MO = TIB->getOperand(i);
1705 if (!MO.isReg() || !MO.isUse() || !MO.isKill())
1707 unsigned Reg = MO.getReg();
1708 if (!Reg || !LocalDefsSet.count(Reg))
1710 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
1711 LocalDefsSet.erase(*AI);
1714 // Track local defs so we can update liveins.
1715 for (unsigned i = 0, e = TIB->getNumOperands(); i != e; ++i) {
1716 MachineOperand &MO = TIB->getOperand(i);
1717 if (!MO.isReg() || !MO.isDef() || MO.isDead())
1719 unsigned Reg = MO.getReg();
1722 LocalDefs.push_back(Reg);
1723 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
1724 LocalDefsSet.insert(*AI);
1735 MBB->splice(Loc, TBB, TBB->begin(), TIB);
1736 FBB->erase(FBB->begin(), FIB);
1739 for (unsigned i = 0, e = LocalDefs.size(); i != e; ++i) {
1740 unsigned Def = LocalDefs[i];
1741 if (LocalDefsSet.count(Def)) {
1742 TBB->addLiveIn(Def);
1743 FBB->addLiveIn(Def);