1 //===-- BranchFolding.cpp - Fold machine code branch instructions ---------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass forwards branches to unconditional branches to make them branch
11 // directly to the target block. This pass often results in dead MBB's, which
14 // Note that this pass must be run after register allocation, it cannot handle
17 //===----------------------------------------------------------------------===//
19 #define DEBUG_TYPE "branchfolding"
20 #include "BranchFolding.h"
21 #include "llvm/Function.h"
22 #include "llvm/CodeGen/Passes.h"
23 #include "llvm/CodeGen/MachineModuleInfo.h"
24 #include "llvm/CodeGen/MachineFunctionPass.h"
25 #include "llvm/CodeGen/MachineJumpTableInfo.h"
26 #include "llvm/CodeGen/RegisterScavenging.h"
27 #include "llvm/Target/TargetInstrInfo.h"
28 #include "llvm/Target/TargetMachine.h"
29 #include "llvm/Target/TargetRegisterInfo.h"
30 #include "llvm/Support/CommandLine.h"
31 #include "llvm/Support/Debug.h"
32 #include "llvm/Support/ErrorHandling.h"
33 #include "llvm/Support/raw_ostream.h"
34 #include "llvm/ADT/SmallSet.h"
35 #include "llvm/ADT/SetVector.h"
36 #include "llvm/ADT/Statistic.h"
37 #include "llvm/ADT/STLExtras.h"
41 STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
42 STATISTIC(NumBranchOpts, "Number of branches optimized");
43 STATISTIC(NumTailMerge , "Number of block tails merged");
44 STATISTIC(NumHoist , "Number of times common instructions are hoisted");
46 static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge",
47 cl::init(cl::BOU_UNSET), cl::Hidden);
49 // Throttle for huge numbers of predecessors (compile speed problems)
50 static cl::opt<unsigned>
51 TailMergeThreshold("tail-merge-threshold",
52 cl::desc("Max number of predecessors to consider tail merging"),
53 cl::init(150), cl::Hidden);
55 // Heuristic for tail merging (and, inversely, tail duplication).
56 // TODO: This should be replaced with a target query.
57 static cl::opt<unsigned>
58 TailMergeSize("tail-merge-size",
59 cl::desc("Min number of instructions to consider tail merging"),
60 cl::init(3), cl::Hidden);
63 /// BranchFolderPass - Wrap branch folder in a machine function pass.
64 class BranchFolderPass : public MachineFunctionPass,
68 explicit BranchFolderPass(bool defaultEnableTailMerge)
69 : MachineFunctionPass(ID), BranchFolder(defaultEnableTailMerge, true) {}
71 virtual bool runOnMachineFunction(MachineFunction &MF);
72 virtual const char *getPassName() const { return "Control Flow Optimizer"; }
76 char BranchFolderPass::ID = 0;
78 FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) {
79 return new BranchFolderPass(DefaultEnableTailMerge);
82 bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) {
83 return OptimizeFunction(MF,
84 MF.getTarget().getInstrInfo(),
85 MF.getTarget().getRegisterInfo(),
86 getAnalysisIfAvailable<MachineModuleInfo>());
90 BranchFolder::BranchFolder(bool defaultEnableTailMerge, bool CommonHoist) {
91 switch (FlagEnableTailMerge) {
92 case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break;
93 case cl::BOU_TRUE: EnableTailMerge = true; break;
94 case cl::BOU_FALSE: EnableTailMerge = false; break;
97 EnableHoistCommonCode = CommonHoist;
100 /// RemoveDeadBlock - Remove the specified dead machine basic block from the
101 /// function, updating the CFG.
102 void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) {
103 assert(MBB->pred_empty() && "MBB must be dead!");
104 DEBUG(dbgs() << "\nRemoving MBB: " << *MBB);
106 MachineFunction *MF = MBB->getParent();
107 // drop all successors.
108 while (!MBB->succ_empty())
109 MBB->removeSuccessor(MBB->succ_end()-1);
115 /// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def
116 /// followed by terminators, and if the implicitly defined registers are not
117 /// used by the terminators, remove those implicit_def's. e.g.
119 /// r0 = implicit_def
120 /// r1 = implicit_def
122 /// This block can be optimized away later if the implicit instructions are
124 bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) {
125 SmallSet<unsigned, 4> ImpDefRegs;
126 MachineBasicBlock::iterator I = MBB->begin();
127 while (I != MBB->end()) {
128 if (!I->isImplicitDef())
130 unsigned Reg = I->getOperand(0).getReg();
131 ImpDefRegs.insert(Reg);
132 for (const unsigned *SubRegs = TRI->getSubRegisters(Reg);
133 unsigned SubReg = *SubRegs; ++SubRegs)
134 ImpDefRegs.insert(SubReg);
137 if (ImpDefRegs.empty())
140 MachineBasicBlock::iterator FirstTerm = I;
141 while (I != MBB->end()) {
142 if (!TII->isUnpredicatedTerminator(I))
144 // See if it uses any of the implicitly defined registers.
145 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
146 MachineOperand &MO = I->getOperand(i);
147 if (!MO.isReg() || !MO.isUse())
149 unsigned Reg = MO.getReg();
150 if (ImpDefRegs.count(Reg))
157 while (I != FirstTerm) {
158 MachineInstr *ImpDefMI = &*I;
160 MBB->erase(ImpDefMI);
166 /// OptimizeFunction - Perhaps branch folding, tail merging and other
167 /// CFG optimizations on the given function.
168 bool BranchFolder::OptimizeFunction(MachineFunction &MF,
169 const TargetInstrInfo *tii,
170 const TargetRegisterInfo *tri,
171 MachineModuleInfo *mmi) {
172 if (!tii) return false;
178 RS = TRI->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL;
180 // Fix CFG. The later algorithms expect it to be right.
181 bool MadeChange = false;
182 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) {
183 MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0;
184 SmallVector<MachineOperand, 4> Cond;
185 if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true))
186 MadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty());
187 MadeChange |= OptimizeImpDefsBlock(MBB);
190 bool MadeChangeThisIteration = true;
191 while (MadeChangeThisIteration) {
192 MadeChangeThisIteration = TailMergeBlocks(MF);
193 MadeChangeThisIteration |= OptimizeBranches(MF);
194 if (EnableHoistCommonCode)
195 MadeChangeThisIteration |= HoistCommonCode(MF);
196 MadeChange |= MadeChangeThisIteration;
199 // See if any jump tables have become dead as the code generator
201 MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
207 // Walk the function to find jump tables that are live.
208 BitVector JTIsLive(JTI->getJumpTables().size());
209 for (MachineFunction::iterator BB = MF.begin(), E = MF.end();
211 for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end();
213 for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) {
214 MachineOperand &Op = I->getOperand(op);
215 if (!Op.isJTI()) continue;
217 // Remember that this JT is live.
218 JTIsLive.set(Op.getIndex());
222 // Finally, remove dead jump tables. This happens when the
223 // indirect jump was unreachable (and thus deleted).
224 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i)
225 if (!JTIsLive.test(i)) {
226 JTI->RemoveJumpTable(i);
234 //===----------------------------------------------------------------------===//
235 // Tail Merging of Blocks
236 //===----------------------------------------------------------------------===//
238 /// HashMachineInstr - Compute a hash value for MI and its operands.
239 static unsigned HashMachineInstr(const MachineInstr *MI) {
240 unsigned Hash = MI->getOpcode();
241 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
242 const MachineOperand &Op = MI->getOperand(i);
244 // Merge in bits from the operand if easy.
245 unsigned OperandHash = 0;
246 switch (Op.getType()) {
247 case MachineOperand::MO_Register: OperandHash = Op.getReg(); break;
248 case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break;
249 case MachineOperand::MO_MachineBasicBlock:
250 OperandHash = Op.getMBB()->getNumber();
252 case MachineOperand::MO_FrameIndex:
253 case MachineOperand::MO_ConstantPoolIndex:
254 case MachineOperand::MO_JumpTableIndex:
255 OperandHash = Op.getIndex();
257 case MachineOperand::MO_GlobalAddress:
258 case MachineOperand::MO_ExternalSymbol:
259 // Global address / external symbol are too hard, don't bother, but do
260 // pull in the offset.
261 OperandHash = Op.getOffset();
266 Hash += ((OperandHash << 3) | Op.getType()) << (i&31);
271 /// HashEndOfMBB - Hash the last instruction in the MBB.
272 static unsigned HashEndOfMBB(const MachineBasicBlock *MBB) {
273 MachineBasicBlock::const_iterator I = MBB->end();
274 if (I == MBB->begin())
275 return 0; // Empty MBB.
278 // Skip debug info so it will not affect codegen.
279 while (I->isDebugValue()) {
281 return 0; // MBB empty except for debug info.
285 return HashMachineInstr(I);
288 /// ComputeCommonTailLength - Given two machine basic blocks, compute the number
289 /// of instructions they actually have in common together at their end. Return
290 /// iterators for the first shared instruction in each block.
291 static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1,
292 MachineBasicBlock *MBB2,
293 MachineBasicBlock::iterator &I1,
294 MachineBasicBlock::iterator &I2) {
298 unsigned TailLen = 0;
299 while (I1 != MBB1->begin() && I2 != MBB2->begin()) {
301 // Skip debugging pseudos; necessary to avoid changing the code.
302 while (I1->isDebugValue()) {
303 if (I1==MBB1->begin()) {
304 while (I2->isDebugValue()) {
305 if (I2==MBB2->begin())
306 // I1==DBG at begin; I2==DBG at begin
311 // I1==DBG at begin; I2==non-DBG, or first of DBGs not at begin
316 // I1==first (untested) non-DBG preceding known match
317 while (I2->isDebugValue()) {
318 if (I2==MBB2->begin()) {
320 // I1==non-DBG, or first of DBGs not at begin; I2==DBG at begin
325 // I1, I2==first (untested) non-DBGs preceding known match
326 if (!I1->isIdenticalTo(I2) ||
327 // FIXME: This check is dubious. It's used to get around a problem where
328 // people incorrectly expect inline asm directives to remain in the same
329 // relative order. This is untenable because normal compiler
330 // optimizations (like this one) may reorder and/or merge these
338 // Back past possible debugging pseudos at beginning of block. This matters
339 // when one block differs from the other only by whether debugging pseudos
340 // are present at the beginning. (This way, the various checks later for
341 // I1==MBB1->begin() work as expected.)
342 if (I1 == MBB1->begin() && I2 != MBB2->begin()) {
344 while (I2->isDebugValue()) {
345 if (I2 == MBB2->begin()) {
352 if (I2 == MBB2->begin() && I1 != MBB1->begin()) {
354 while (I1->isDebugValue()) {
355 if (I1 == MBB1->begin())
364 /// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything
365 /// after it, replacing it with an unconditional branch to NewDest.
366 void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
367 MachineBasicBlock *NewDest) {
368 TII->ReplaceTailWithBranchTo(OldInst, NewDest);
372 /// SplitMBBAt - Given a machine basic block and an iterator into it, split the
373 /// MBB so that the part before the iterator falls into the part starting at the
374 /// iterator. This returns the new MBB.
375 MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB,
376 MachineBasicBlock::iterator BBI1) {
377 if (!TII->isLegalToSplitMBBAt(CurMBB, BBI1))
380 MachineFunction &MF = *CurMBB.getParent();
382 // Create the fall-through block.
383 MachineFunction::iterator MBBI = &CurMBB;
384 MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(CurMBB.getBasicBlock());
385 CurMBB.getParent()->insert(++MBBI, NewMBB);
387 // Move all the successors of this block to the specified block.
388 NewMBB->transferSuccessors(&CurMBB);
390 // Add an edge from CurMBB to NewMBB for the fall-through.
391 CurMBB.addSuccessor(NewMBB);
393 // Splice the code over.
394 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end());
396 // For targets that use the register scavenger, we must maintain LiveIns.
398 RS->enterBasicBlock(&CurMBB);
400 RS->forward(prior(CurMBB.end()));
401 BitVector RegsLiveAtExit(TRI->getNumRegs());
402 RS->getRegsUsed(RegsLiveAtExit, false);
403 for (unsigned int i = 0, e = TRI->getNumRegs(); i != e; i++)
404 if (RegsLiveAtExit[i])
405 NewMBB->addLiveIn(i);
411 /// EstimateRuntime - Make a rough estimate for how long it will take to run
412 /// the specified code.
413 static unsigned EstimateRuntime(MachineBasicBlock::iterator I,
414 MachineBasicBlock::iterator E) {
416 for (; I != E; ++I) {
417 if (I->isDebugValue())
419 const TargetInstrDesc &TID = I->getDesc();
422 else if (TID.mayLoad() || TID.mayStore())
430 // CurMBB needs to add an unconditional branch to SuccMBB (we removed these
431 // branches temporarily for tail merging). In the case where CurMBB ends
432 // with a conditional branch to the next block, optimize by reversing the
433 // test and conditionally branching to SuccMBB instead.
434 static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB,
435 const TargetInstrInfo *TII) {
436 MachineFunction *MF = CurMBB->getParent();
437 MachineFunction::iterator I = llvm::next(MachineFunction::iterator(CurMBB));
438 MachineBasicBlock *TBB = 0, *FBB = 0;
439 SmallVector<MachineOperand, 4> Cond;
440 DebugLoc dl; // FIXME: this is nowhere
441 if (I != MF->end() &&
442 !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond, true)) {
443 MachineBasicBlock *NextBB = I;
444 if (TBB == NextBB && !Cond.empty() && !FBB) {
445 if (!TII->ReverseBranchCondition(Cond)) {
446 TII->RemoveBranch(*CurMBB);
447 TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond, dl);
452 TII->InsertBranch(*CurMBB, SuccBB, NULL,
453 SmallVector<MachineOperand, 0>(), dl);
457 BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const {
458 if (getHash() < o.getHash())
460 else if (getHash() > o.getHash())
462 else if (getBlock()->getNumber() < o.getBlock()->getNumber())
464 else if (getBlock()->getNumber() > o.getBlock()->getNumber())
467 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing
468 // an object with itself.
469 #ifndef _GLIBCXX_DEBUG
470 llvm_unreachable("Predecessor appears twice");
476 /// CountTerminators - Count the number of terminators in the given
477 /// block and set I to the position of the first non-terminator, if there
478 /// is one, or MBB->end() otherwise.
479 static unsigned CountTerminators(MachineBasicBlock *MBB,
480 MachineBasicBlock::iterator &I) {
482 unsigned NumTerms = 0;
484 if (I == MBB->begin()) {
489 if (!I->getDesc().isTerminator()) break;
495 /// ProfitableToMerge - Check if two machine basic blocks have a common tail
496 /// and decide if it would be profitable to merge those tails. Return the
497 /// length of the common tail and iterators to the first common instruction
499 static bool ProfitableToMerge(MachineBasicBlock *MBB1,
500 MachineBasicBlock *MBB2,
501 unsigned minCommonTailLength,
502 unsigned &CommonTailLen,
503 MachineBasicBlock::iterator &I1,
504 MachineBasicBlock::iterator &I2,
505 MachineBasicBlock *SuccBB,
506 MachineBasicBlock *PredBB) {
507 CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2);
508 if (CommonTailLen == 0)
510 DEBUG(dbgs() << "Common tail length of BB#" << MBB1->getNumber()
511 << " and BB#" << MBB2->getNumber() << " is " << CommonTailLen
514 // It's almost always profitable to merge any number of non-terminator
515 // instructions with the block that falls through into the common successor.
516 if (MBB1 == PredBB || MBB2 == PredBB) {
517 MachineBasicBlock::iterator I;
518 unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I);
519 if (CommonTailLen > NumTerms)
523 // If one of the blocks can be completely merged and happens to be in
524 // a position where the other could fall through into it, merge any number
525 // of instructions, because it can be done without a branch.
526 // TODO: If the blocks are not adjacent, move one of them so that they are?
527 if (MBB1->isLayoutSuccessor(MBB2) && I2 == MBB2->begin())
529 if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin())
532 // If both blocks have an unconditional branch temporarily stripped out,
533 // count that as an additional common instruction for the following
535 unsigned EffectiveTailLen = CommonTailLen;
536 if (SuccBB && MBB1 != PredBB && MBB2 != PredBB &&
537 !MBB1->back().getDesc().isBarrier() &&
538 !MBB2->back().getDesc().isBarrier())
541 // Check if the common tail is long enough to be worthwhile.
542 if (EffectiveTailLen >= minCommonTailLength)
545 // If we are optimizing for code size, 2 instructions in common is enough if
546 // we don't have to split a block. At worst we will be introducing 1 new
547 // branch instruction, which is likely to be smaller than the 2
548 // instructions that would be deleted in the merge.
549 MachineFunction *MF = MBB1->getParent();
550 if (EffectiveTailLen >= 2 &&
551 MF->getFunction()->hasFnAttr(Attribute::OptimizeForSize) &&
552 (I1 == MBB1->begin() || I2 == MBB2->begin()))
558 /// ComputeSameTails - Look through all the blocks in MergePotentials that have
559 /// hash CurHash (guaranteed to match the last element). Build the vector
560 /// SameTails of all those that have the (same) largest number of instructions
561 /// in common of any pair of these blocks. SameTails entries contain an
562 /// iterator into MergePotentials (from which the MachineBasicBlock can be
563 /// found) and a MachineBasicBlock::iterator into that MBB indicating the
564 /// instruction where the matching code sequence begins.
565 /// Order of elements in SameTails is the reverse of the order in which
566 /// those blocks appear in MergePotentials (where they are not necessarily
568 unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
569 unsigned minCommonTailLength,
570 MachineBasicBlock *SuccBB,
571 MachineBasicBlock *PredBB) {
572 unsigned maxCommonTailLength = 0U;
574 MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
575 MPIterator HighestMPIter = prior(MergePotentials.end());
576 for (MPIterator CurMPIter = prior(MergePotentials.end()),
577 B = MergePotentials.begin();
578 CurMPIter != B && CurMPIter->getHash() == CurHash;
580 for (MPIterator I = prior(CurMPIter); I->getHash() == CurHash ; --I) {
581 unsigned CommonTailLen;
582 if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(),
584 CommonTailLen, TrialBBI1, TrialBBI2,
586 if (CommonTailLen > maxCommonTailLength) {
588 maxCommonTailLength = CommonTailLen;
589 HighestMPIter = CurMPIter;
590 SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1));
592 if (HighestMPIter == CurMPIter &&
593 CommonTailLen == maxCommonTailLength)
594 SameTails.push_back(SameTailElt(I, TrialBBI2));
600 return maxCommonTailLength;
603 /// RemoveBlocksWithHash - Remove all blocks with hash CurHash from
604 /// MergePotentials, restoring branches at ends of blocks as appropriate.
605 void BranchFolder::RemoveBlocksWithHash(unsigned CurHash,
606 MachineBasicBlock *SuccBB,
607 MachineBasicBlock *PredBB) {
608 MPIterator CurMPIter, B;
609 for (CurMPIter = prior(MergePotentials.end()), B = MergePotentials.begin();
610 CurMPIter->getHash() == CurHash;
612 // Put the unconditional branch back, if we need one.
613 MachineBasicBlock *CurMBB = CurMPIter->getBlock();
614 if (SuccBB && CurMBB != PredBB)
615 FixTail(CurMBB, SuccBB, TII);
619 if (CurMPIter->getHash() != CurHash)
621 MergePotentials.erase(CurMPIter, MergePotentials.end());
624 /// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist
625 /// only of the common tail. Create a block that does by splitting one.
626 bool BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
627 unsigned maxCommonTailLength,
628 unsigned &commonTailIndex) {
630 unsigned TimeEstimate = ~0U;
631 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
632 // Use PredBB if possible; that doesn't require a new branch.
633 if (SameTails[i].getBlock() == PredBB) {
637 // Otherwise, make a (fairly bogus) choice based on estimate of
638 // how long it will take the various blocks to execute.
639 unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(),
640 SameTails[i].getTailStartPos());
641 if (t <= TimeEstimate) {
647 MachineBasicBlock::iterator BBI =
648 SameTails[commonTailIndex].getTailStartPos();
649 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
651 // If the common tail includes any debug info we will take it pretty
652 // randomly from one of the inputs. Might be better to remove it?
653 DEBUG(dbgs() << "\nSplitting BB#" << MBB->getNumber() << ", size "
654 << maxCommonTailLength);
656 MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI);
658 DEBUG(dbgs() << "... failed!");
662 SameTails[commonTailIndex].setBlock(newMBB);
663 SameTails[commonTailIndex].setTailStartPos(newMBB->begin());
665 // If we split PredBB, newMBB is the new predecessor.
672 // See if any of the blocks in MergePotentials (which all have a common single
673 // successor, or all have no successor) can be tail-merged. If there is a
674 // successor, any blocks in MergePotentials that are not tail-merged and
675 // are not immediately before Succ must have an unconditional branch to
676 // Succ added (but the predecessor/successor lists need no adjustment).
677 // The lone predecessor of Succ that falls through into Succ,
678 // if any, is given in PredBB.
680 bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
681 MachineBasicBlock *PredBB) {
682 bool MadeChange = false;
684 // Except for the special cases below, tail-merge if there are at least
685 // this many instructions in common.
686 unsigned minCommonTailLength = TailMergeSize;
688 DEBUG(dbgs() << "\nTryTailMergeBlocks: ";
689 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
690 dbgs() << "BB#" << MergePotentials[i].getBlock()->getNumber()
691 << (i == e-1 ? "" : ", ");
694 dbgs() << " with successor BB#" << SuccBB->getNumber() << '\n';
696 dbgs() << " which has fall-through from BB#"
697 << PredBB->getNumber() << "\n";
699 dbgs() << "Looking for common tails of at least "
700 << minCommonTailLength << " instruction"
701 << (minCommonTailLength == 1 ? "" : "s") << '\n';
704 // Sort by hash value so that blocks with identical end sequences sort
706 std::stable_sort(MergePotentials.begin(), MergePotentials.end());
708 // Walk through equivalence sets looking for actual exact matches.
709 while (MergePotentials.size() > 1) {
710 unsigned CurHash = MergePotentials.back().getHash();
712 // Build SameTails, identifying the set of blocks with this hash code
713 // and with the maximum number of instructions in common.
714 unsigned maxCommonTailLength = ComputeSameTails(CurHash,
718 // If we didn't find any pair that has at least minCommonTailLength
719 // instructions in common, remove all blocks with this hash code and retry.
720 if (SameTails.empty()) {
721 RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
725 // If one of the blocks is the entire common tail (and not the entry
726 // block, which we can't jump to), we can treat all blocks with this same
727 // tail at once. Use PredBB if that is one of the possibilities, as that
728 // will not introduce any extra branches.
729 MachineBasicBlock *EntryBB = MergePotentials.begin()->getBlock()->
730 getParent()->begin();
731 unsigned commonTailIndex = SameTails.size();
732 // If there are two blocks, check to see if one can be made to fall through
734 if (SameTails.size() == 2 &&
735 SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) &&
736 SameTails[1].tailIsWholeBlock())
738 else if (SameTails.size() == 2 &&
739 SameTails[1].getBlock()->isLayoutSuccessor(
740 SameTails[0].getBlock()) &&
741 SameTails[0].tailIsWholeBlock())
744 // Otherwise just pick one, favoring the fall-through predecessor if
746 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
747 MachineBasicBlock *MBB = SameTails[i].getBlock();
748 if (MBB == EntryBB && SameTails[i].tailIsWholeBlock())
754 if (SameTails[i].tailIsWholeBlock())
759 if (commonTailIndex == SameTails.size() ||
760 (SameTails[commonTailIndex].getBlock() == PredBB &&
761 !SameTails[commonTailIndex].tailIsWholeBlock())) {
762 // None of the blocks consist entirely of the common tail.
763 // Split a block so that one does.
764 if (!CreateCommonTailOnlyBlock(PredBB,
765 maxCommonTailLength, commonTailIndex)) {
766 RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
771 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
772 // MBB is common tail. Adjust all other BB's to jump to this one.
773 // Traversal must be forwards so erases work.
774 DEBUG(dbgs() << "\nUsing common tail in BB#" << MBB->getNumber()
776 for (unsigned int i=0, e = SameTails.size(); i != e; ++i) {
777 if (commonTailIndex == i)
779 DEBUG(dbgs() << "BB#" << SameTails[i].getBlock()->getNumber()
780 << (i == e-1 ? "" : ", "));
781 // Hack the end off BB i, making it jump to BB commonTailIndex instead.
782 ReplaceTailWithBranchTo(SameTails[i].getTailStartPos(), MBB);
783 // BB i is no longer a predecessor of SuccBB; remove it from the worklist.
784 MergePotentials.erase(SameTails[i].getMPIter());
786 DEBUG(dbgs() << "\n");
787 // We leave commonTailIndex in the worklist in case there are other blocks
788 // that match it with a smaller number of instructions.
794 bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
796 if (!EnableTailMerge) return false;
798 bool MadeChange = false;
800 // First find blocks with no successors.
801 MergePotentials.clear();
802 for (MachineFunction::iterator I = MF.begin(), E = MF.end();
803 I != E && MergePotentials.size() < TailMergeThreshold; ++I) {
804 if (TriedMerging.count(I))
807 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(I), I));
810 // If this is a large problem, avoid visiting the same basic blocks
812 if (MergePotentials.size() == TailMergeThreshold)
813 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
814 TriedMerging.insert(MergePotentials[i].getBlock());
816 // See if we can do any tail merging on those.
817 if (MergePotentials.size() >= 2)
818 MadeChange |= TryTailMergeBlocks(NULL, NULL);
820 // Look at blocks (IBB) with multiple predecessors (PBB).
821 // We change each predecessor to a canonical form, by
822 // (1) temporarily removing any unconditional branch from the predecessor
824 // (2) alter conditional branches so they branch to the other block
825 // not IBB; this may require adding back an unconditional branch to IBB
826 // later, where there wasn't one coming in. E.g.
828 // fallthrough to QBB
831 // with a conceptual B to IBB after that, which never actually exists.
832 // With those changes, we see whether the predecessors' tails match,
833 // and merge them if so. We change things out of canonical form and
834 // back to the way they were later in the process. (OptimizeBranches
835 // would undo some of this, but we can't use it, because we'd get into
836 // a compile-time infinite loop repeatedly doing and undoing the same
839 for (MachineFunction::iterator I = llvm::next(MF.begin()), E = MF.end();
841 if (I->pred_size() >= 2) {
842 SmallPtrSet<MachineBasicBlock *, 8> UniquePreds;
843 MachineBasicBlock *IBB = I;
844 MachineBasicBlock *PredBB = prior(I);
845 MergePotentials.clear();
846 for (MachineBasicBlock::pred_iterator P = I->pred_begin(),
848 P != E2 && MergePotentials.size() < TailMergeThreshold; ++P) {
849 MachineBasicBlock *PBB = *P;
850 if (TriedMerging.count(PBB))
852 // Skip blocks that loop to themselves, can't tail merge these.
855 // Visit each predecessor only once.
856 if (!UniquePreds.insert(PBB))
858 MachineBasicBlock *TBB = 0, *FBB = 0;
859 SmallVector<MachineOperand, 4> Cond;
860 if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond, true)) {
861 // Failing case: IBB is the target of a cbr, and
862 // we cannot reverse the branch.
863 SmallVector<MachineOperand, 4> NewCond(Cond);
864 if (!Cond.empty() && TBB == IBB) {
865 if (TII->ReverseBranchCondition(NewCond))
867 // This is the QBB case described above
869 FBB = llvm::next(MachineFunction::iterator(PBB));
871 // Failing case: the only way IBB can be reached from PBB is via
872 // exception handling. Happens for landing pads. Would be nice
873 // to have a bit in the edge so we didn't have to do all this.
874 if (IBB->isLandingPad()) {
875 MachineFunction::iterator IP = PBB; IP++;
876 MachineBasicBlock *PredNextBB = NULL;
880 if (IBB != PredNextBB) // fallthrough
883 if (TBB != IBB && FBB != IBB) // cbr then ubr
885 } else if (Cond.empty()) {
886 if (TBB != IBB) // ubr
889 if (TBB != IBB && IBB != PredNextBB) // cbr
893 // Remove the unconditional branch at the end, if any.
894 if (TBB && (Cond.empty() || FBB)) {
895 DebugLoc dl; // FIXME: this is nowhere
896 TII->RemoveBranch(*PBB);
898 // reinsert conditional branch only, for now
899 TII->InsertBranch(*PBB, (TBB == IBB) ? FBB : TBB, 0, NewCond, dl);
901 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(PBB), *P));
904 // If this is a large problem, avoid visiting the same basic blocks
906 if (MergePotentials.size() == TailMergeThreshold)
907 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
908 TriedMerging.insert(MergePotentials[i].getBlock());
910 if (MergePotentials.size() >= 2)
911 MadeChange |= TryTailMergeBlocks(IBB, PredBB);
912 // Reinsert an unconditional branch if needed.
913 // The 1 below can occur as a result of removing blocks in TryTailMergeBlocks.
914 PredBB = prior(I); // this may have been changed in TryTailMergeBlocks
915 if (MergePotentials.size() == 1 &&
916 MergePotentials.begin()->getBlock() != PredBB)
917 FixTail(MergePotentials.begin()->getBlock(), IBB, TII);
923 //===----------------------------------------------------------------------===//
924 // Branch Optimization
925 //===----------------------------------------------------------------------===//
927 bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
928 bool MadeChange = false;
930 // Make sure blocks are numbered in order
933 for (MachineFunction::iterator I = llvm::next(MF.begin()), E = MF.end();
935 MachineBasicBlock *MBB = I++;
936 MadeChange |= OptimizeBlock(MBB);
938 // If it is dead, remove it.
939 if (MBB->pred_empty()) {
940 RemoveDeadBlock(MBB);
948 // Blocks should be considered empty if they contain only debug info;
949 // else the debug info would affect codegen.
950 static bool IsEmptyBlock(MachineBasicBlock *MBB) {
953 for (MachineBasicBlock::iterator MBBI = MBB->begin(), MBBE = MBB->end();
954 MBBI!=MBBE; ++MBBI) {
955 if (!MBBI->isDebugValue())
961 // Blocks with only debug info and branches should be considered the same
962 // as blocks with only branches.
963 static bool IsBranchOnlyBlock(MachineBasicBlock *MBB) {
964 MachineBasicBlock::iterator MBBI, MBBE;
965 for (MBBI = MBB->begin(), MBBE = MBB->end(); MBBI!=MBBE; ++MBBI) {
966 if (!MBBI->isDebugValue())
969 return (MBBI->getDesc().isBranch());
972 /// IsBetterFallthrough - Return true if it would be clearly better to
973 /// fall-through to MBB1 than to fall through into MBB2. This has to return
974 /// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will
975 /// result in infinite loops.
976 static bool IsBetterFallthrough(MachineBasicBlock *MBB1,
977 MachineBasicBlock *MBB2) {
978 // Right now, we use a simple heuristic. If MBB2 ends with a call, and
979 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to
980 // optimize branches that branch to either a return block or an assert block
981 // into a fallthrough to the return.
982 if (IsEmptyBlock(MBB1) || IsEmptyBlock(MBB2)) return false;
984 // If there is a clear successor ordering we make sure that one block
985 // will fall through to the next
986 if (MBB1->isSuccessor(MBB2)) return true;
987 if (MBB2->isSuccessor(MBB1)) return false;
989 // Neither block consists entirely of debug info (per IsEmptyBlock check),
990 // so we needn't test for falling off the beginning here.
991 MachineBasicBlock::iterator MBB1I = --MBB1->end();
992 while (MBB1I->isDebugValue())
994 MachineBasicBlock::iterator MBB2I = --MBB2->end();
995 while (MBB2I->isDebugValue())
997 return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall();
1000 /// OptimizeBlock - Analyze and optimize control flow related to the specified
1001 /// block. This is never called on the entry block.
1002 bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
1003 bool MadeChange = false;
1004 MachineFunction &MF = *MBB->getParent();
1005 DebugLoc dl; // FIXME: this is nowhere
1008 MachineFunction::iterator FallThrough = MBB;
1011 // If this block is empty, make everyone use its fall-through, not the block
1012 // explicitly. Landing pads should not do this since the landing-pad table
1013 // points to this block. Blocks with their addresses taken shouldn't be
1015 if (IsEmptyBlock(MBB) && !MBB->isLandingPad() && !MBB->hasAddressTaken()) {
1016 // Dead block? Leave for cleanup later.
1017 if (MBB->pred_empty()) return MadeChange;
1019 if (FallThrough == MF.end()) {
1020 // TODO: Simplify preds to not branch here if possible!
1022 // Rewrite all predecessors of the old block to go to the fallthrough
1024 while (!MBB->pred_empty()) {
1025 MachineBasicBlock *Pred = *(MBB->pred_end()-1);
1026 Pred->ReplaceUsesOfBlockWith(MBB, FallThrough);
1028 // If MBB was the target of a jump table, update jump tables to go to the
1029 // fallthrough instead.
1030 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
1031 MJTI->ReplaceMBBInJumpTables(MBB, FallThrough);
1037 // Check to see if we can simplify the terminator of the block before this
1039 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB));
1041 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
1042 SmallVector<MachineOperand, 4> PriorCond;
1043 bool PriorUnAnalyzable =
1044 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
1045 if (!PriorUnAnalyzable) {
1046 // If the CFG for the prior block has extra edges, remove them.
1047 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB,
1048 !PriorCond.empty());
1050 // If the previous branch is conditional and both conditions go to the same
1051 // destination, remove the branch, replacing it with an unconditional one or
1053 if (PriorTBB && PriorTBB == PriorFBB) {
1054 TII->RemoveBranch(PrevBB);
1056 if (PriorTBB != MBB)
1057 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond, dl);
1060 goto ReoptimizeBlock;
1063 // If the previous block unconditionally falls through to this block and
1064 // this block has no other predecessors, move the contents of this block
1065 // into the prior block. This doesn't usually happen when SimplifyCFG
1066 // has been used, but it can happen if tail merging splits a fall-through
1067 // predecessor of a block.
1068 // This has to check PrevBB->succ_size() because EH edges are ignored by
1070 if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 &&
1071 PrevBB.succ_size() == 1 &&
1072 !MBB->hasAddressTaken() && !MBB->isLandingPad()) {
1073 DEBUG(dbgs() << "\nMerging into block: " << PrevBB
1074 << "From MBB: " << *MBB);
1075 // Remove redundant DBG_VALUEs first.
1076 if (PrevBB.begin() != PrevBB.end()) {
1077 MachineBasicBlock::iterator PrevBBIter = PrevBB.end();
1079 MachineBasicBlock::iterator MBBIter = MBB->begin();
1080 // Check if DBG_VALUE at the end of PrevBB is identical to the
1081 // DBG_VALUE at the beginning of MBB.
1082 while (PrevBBIter != PrevBB.begin() && MBBIter != MBB->end()
1083 && PrevBBIter->isDebugValue() && MBBIter->isDebugValue()) {
1084 if (!MBBIter->isIdenticalTo(PrevBBIter))
1086 MachineInstr *DuplicateDbg = MBBIter;
1087 ++MBBIter; -- PrevBBIter;
1088 DuplicateDbg->eraseFromParent();
1091 PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end());
1092 PrevBB.removeSuccessor(PrevBB.succ_begin());;
1093 assert(PrevBB.succ_empty());
1094 PrevBB.transferSuccessors(MBB);
1099 // If the previous branch *only* branches to *this* block (conditional or
1100 // not) remove the branch.
1101 if (PriorTBB == MBB && PriorFBB == 0) {
1102 TII->RemoveBranch(PrevBB);
1105 goto ReoptimizeBlock;
1108 // If the prior block branches somewhere else on the condition and here if
1109 // the condition is false, remove the uncond second branch.
1110 if (PriorFBB == MBB) {
1111 TII->RemoveBranch(PrevBB);
1112 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond, dl);
1115 goto ReoptimizeBlock;
1118 // If the prior block branches here on true and somewhere else on false, and
1119 // if the branch condition is reversible, reverse the branch to create a
1121 if (PriorTBB == MBB) {
1122 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1123 if (!TII->ReverseBranchCondition(NewPriorCond)) {
1124 TII->RemoveBranch(PrevBB);
1125 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond, dl);
1128 goto ReoptimizeBlock;
1132 // If this block has no successors (e.g. it is a return block or ends with
1133 // a call to a no-return function like abort or __cxa_throw) and if the pred
1134 // falls through into this block, and if it would otherwise fall through
1135 // into the block after this, move this block to the end of the function.
1137 // We consider it more likely that execution will stay in the function (e.g.
1138 // due to loops) than it is to exit it. This asserts in loops etc, moving
1139 // the assert condition out of the loop body.
1140 if (MBB->succ_empty() && !PriorCond.empty() && PriorFBB == 0 &&
1141 MachineFunction::iterator(PriorTBB) == FallThrough &&
1142 !MBB->canFallThrough()) {
1143 bool DoTransform = true;
1145 // We have to be careful that the succs of PredBB aren't both no-successor
1146 // blocks. If neither have successors and if PredBB is the second from
1147 // last block in the function, we'd just keep swapping the two blocks for
1148 // last. Only do the swap if one is clearly better to fall through than
1150 if (FallThrough == --MF.end() &&
1151 !IsBetterFallthrough(PriorTBB, MBB))
1152 DoTransform = false;
1155 // Reverse the branch so we will fall through on the previous true cond.
1156 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1157 if (!TII->ReverseBranchCondition(NewPriorCond)) {
1158 DEBUG(dbgs() << "\nMoving MBB: " << *MBB
1159 << "To make fallthrough to: " << *PriorTBB << "\n");
1161 TII->RemoveBranch(PrevBB);
1162 TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond, dl);
1164 // Move this block to the end of the function.
1165 MBB->moveAfter(--MF.end());
1174 // Analyze the branch in the current block.
1175 MachineBasicBlock *CurTBB = 0, *CurFBB = 0;
1176 SmallVector<MachineOperand, 4> CurCond;
1177 bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true);
1178 if (!CurUnAnalyzable) {
1179 // If the CFG for the prior block has extra edges, remove them.
1180 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty());
1182 // If this is a two-way branch, and the FBB branches to this block, reverse
1183 // the condition so the single-basic-block loop is faster. Instead of:
1184 // Loop: xxx; jcc Out; jmp Loop
1186 // Loop: xxx; jncc Loop; jmp Out
1187 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) {
1188 SmallVector<MachineOperand, 4> NewCond(CurCond);
1189 if (!TII->ReverseBranchCondition(NewCond)) {
1190 TII->RemoveBranch(*MBB);
1191 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond, dl);
1194 goto ReoptimizeBlock;
1198 // If this branch is the only thing in its block, see if we can forward
1199 // other blocks across it.
1200 if (CurTBB && CurCond.empty() && CurFBB == 0 &&
1201 IsBranchOnlyBlock(MBB) && CurTBB != MBB &&
1202 !MBB->hasAddressTaken()) {
1203 // This block may contain just an unconditional branch. Because there can
1204 // be 'non-branch terminators' in the block, try removing the branch and
1205 // then seeing if the block is empty.
1206 TII->RemoveBranch(*MBB);
1207 // If the only things remaining in the block are debug info, remove these
1208 // as well, so this will behave the same as an empty block in non-debug
1210 if (!MBB->empty()) {
1211 bool NonDebugInfoFound = false;
1212 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
1214 if (!I->isDebugValue()) {
1215 NonDebugInfoFound = true;
1219 if (!NonDebugInfoFound)
1220 // Make the block empty, losing the debug info (we could probably
1221 // improve this in some cases.)
1222 MBB->erase(MBB->begin(), MBB->end());
1224 // If this block is just an unconditional branch to CurTBB, we can
1225 // usually completely eliminate the block. The only case we cannot
1226 // completely eliminate the block is when the block before this one
1227 // falls through into MBB and we can't understand the prior block's branch
1230 bool PredHasNoFallThrough = !PrevBB.canFallThrough();
1231 if (PredHasNoFallThrough || !PriorUnAnalyzable ||
1232 !PrevBB.isSuccessor(MBB)) {
1233 // If the prior block falls through into us, turn it into an
1234 // explicit branch to us to make updates simpler.
1235 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) &&
1236 PriorTBB != MBB && PriorFBB != MBB) {
1237 if (PriorTBB == 0) {
1238 assert(PriorCond.empty() && PriorFBB == 0 &&
1239 "Bad branch analysis");
1242 assert(PriorFBB == 0 && "Machine CFG out of date!");
1245 TII->RemoveBranch(PrevBB);
1246 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, dl);
1249 // Iterate through all the predecessors, revectoring each in-turn.
1251 bool DidChange = false;
1252 bool HasBranchToSelf = false;
1253 while(PI != MBB->pred_size()) {
1254 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI);
1256 // If this block has an uncond branch to itself, leave it.
1258 HasBranchToSelf = true;
1261 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB);
1262 // If this change resulted in PMBB ending in a conditional
1263 // branch where both conditions go to the same destination,
1264 // change this to an unconditional branch (and fix the CFG).
1265 MachineBasicBlock *NewCurTBB = 0, *NewCurFBB = 0;
1266 SmallVector<MachineOperand, 4> NewCurCond;
1267 bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB,
1268 NewCurFBB, NewCurCond, true);
1269 if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) {
1270 TII->RemoveBranch(*PMBB);
1272 TII->InsertBranch(*PMBB, NewCurTBB, 0, NewCurCond, dl);
1275 PMBB->CorrectExtraCFGEdges(NewCurTBB, 0, false);
1280 // Change any jumptables to go to the new MBB.
1281 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
1282 MJTI->ReplaceMBBInJumpTables(MBB, CurTBB);
1286 if (!HasBranchToSelf) return MadeChange;
1291 // Add the branch back if the block is more than just an uncond branch.
1292 TII->InsertBranch(*MBB, CurTBB, 0, CurCond, dl);
1296 // If the prior block doesn't fall through into this block, and if this
1297 // block doesn't fall through into some other block, see if we can find a
1298 // place to move this block where a fall-through will happen.
1299 if (!PrevBB.canFallThrough()) {
1301 // Now we know that there was no fall-through into this block, check to
1302 // see if it has a fall-through into its successor.
1303 bool CurFallsThru = MBB->canFallThrough();
1305 if (!MBB->isLandingPad()) {
1306 // Check all the predecessors of this block. If one of them has no fall
1307 // throughs, move this block right after it.
1308 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
1309 E = MBB->pred_end(); PI != E; ++PI) {
1310 // Analyze the branch at the end of the pred.
1311 MachineBasicBlock *PredBB = *PI;
1312 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough;
1313 MachineBasicBlock *PredTBB = 0, *PredFBB = 0;
1314 SmallVector<MachineOperand, 4> PredCond;
1315 if (PredBB != MBB && !PredBB->canFallThrough() &&
1316 !TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true)
1317 && (!CurFallsThru || !CurTBB || !CurFBB)
1318 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
1319 // If the current block doesn't fall through, just move it.
1320 // If the current block can fall through and does not end with a
1321 // conditional branch, we need to append an unconditional jump to
1322 // the (current) next block. To avoid a possible compile-time
1323 // infinite loop, move blocks only backward in this case.
1324 // Also, if there are already 2 branches here, we cannot add a third;
1325 // this means we have the case
1330 MachineBasicBlock *NextBB = llvm::next(MachineFunction::iterator(MBB));
1332 TII->InsertBranch(*MBB, NextBB, 0, CurCond, dl);
1334 MBB->moveAfter(PredBB);
1336 goto ReoptimizeBlock;
1341 if (!CurFallsThru) {
1342 // Check all successors to see if we can move this block before it.
1343 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
1344 E = MBB->succ_end(); SI != E; ++SI) {
1345 // Analyze the branch at the end of the block before the succ.
1346 MachineBasicBlock *SuccBB = *SI;
1347 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev;
1349 // If this block doesn't already fall-through to that successor, and if
1350 // the succ doesn't already have a block that can fall through into it,
1351 // and if the successor isn't an EH destination, we can arrange for the
1352 // fallthrough to happen.
1353 if (SuccBB != MBB && &*SuccPrev != MBB &&
1354 !SuccPrev->canFallThrough() && !CurUnAnalyzable &&
1355 !SuccBB->isLandingPad()) {
1356 MBB->moveBefore(SuccBB);
1358 goto ReoptimizeBlock;
1362 // Okay, there is no really great place to put this block. If, however,
1363 // the block before this one would be a fall-through if this block were
1364 // removed, move this block to the end of the function.
1365 MachineBasicBlock *PrevTBB = 0, *PrevFBB = 0;
1366 SmallVector<MachineOperand, 4> PrevCond;
1367 if (FallThrough != MF.end() &&
1368 !TII->AnalyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) &&
1369 PrevBB.isSuccessor(FallThrough)) {
1370 MBB->moveAfter(--MF.end());
1380 //===----------------------------------------------------------------------===//
1381 // Hoist Common Code
1382 //===----------------------------------------------------------------------===//
1384 /// HoistCommonCode - Hoist common instruction sequences at the start of basic
1385 /// blocks to their common predecessor.
1386 bool BranchFolder::HoistCommonCode(MachineFunction &MF) {
1387 bool MadeChange = false;
1388 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ) {
1389 MachineBasicBlock *MBB = I++;
1390 MadeChange |= HoistCommonCodeInSuccs(MBB);
1396 /// findFalseBlock - BB has a fallthrough. Find its 'false' successor given
1397 /// its 'true' successor.
1398 static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB,
1399 MachineBasicBlock *TrueBB) {
1400 for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
1401 E = BB->succ_end(); SI != E; ++SI) {
1402 MachineBasicBlock *SuccBB = *SI;
1403 if (SuccBB != TrueBB)
1409 /// findHoistingInsertPosAndDeps - Find the location to move common instructions
1410 /// in successors to. The location is ususally just before the terminator,
1411 /// however if the terminator is a conditional branch and its previous
1412 /// instruction is the flag setting instruction, the previous instruction is
1413 /// the preferred location. This function also gathers uses and defs of the
1414 /// instructions from the insertion point to the end of the block. The data is
1415 /// used by HoistCommonCodeInSuccs to ensure safety.
1417 MachineBasicBlock::iterator findHoistingInsertPosAndDeps(MachineBasicBlock *MBB,
1418 const TargetInstrInfo *TII,
1419 const TargetRegisterInfo *TRI,
1420 SmallSet<unsigned,4> &Uses,
1421 SmallSet<unsigned,4> &Defs) {
1422 MachineBasicBlock::iterator Loc = MBB->getFirstTerminator();
1423 if (!TII->isUnpredicatedTerminator(Loc))
1426 for (unsigned i = 0, e = Loc->getNumOperands(); i != e; ++i) {
1427 const MachineOperand &MO = Loc->getOperand(i);
1430 unsigned Reg = MO.getReg();
1435 for (const unsigned *AS = TRI->getAliasSet(Reg); *AS; ++AS)
1437 } else if (!MO.isDead())
1438 // Don't try to hoist code in the rare case the terminator defines a
1439 // register that is later used.
1445 if (Loc == MBB->begin())
1448 // The terminator is probably a conditional branch, try not to separate the
1449 // branch from condition setting instruction.
1450 MachineBasicBlock::iterator PI = Loc;
1452 while (PI != MBB->begin() && Loc->isDebugValue())
1456 for (unsigned i = 0, e = PI->getNumOperands(); !IsDef && i != e; ++i) {
1457 const MachineOperand &MO = PI->getOperand(i);
1458 if (!MO.isReg() || MO.isUse())
1460 unsigned Reg = MO.getReg();
1463 if (Uses.count(Reg))
1467 // The condition setting instruction is not just before the conditional
1471 // Be conservative, don't insert instruction above something that may have
1472 // side-effects. And since it's potentially bad to separate flag setting
1473 // instruction from the conditional branch, just abort the optimization
1475 // Also avoid moving code above predicated instruction since it's hard to
1476 // reason about register liveness with predicated instruction.
1477 bool DontMoveAcrossStore = true;
1478 if (!PI->isSafeToMove(TII, 0, DontMoveAcrossStore) ||
1479 TII->isPredicated(PI))
1483 // Find out what registers are live. Note this routine is ignoring other live
1484 // registers which are only used by instructions in successor blocks.
1485 for (unsigned i = 0, e = PI->getNumOperands(); i != e; ++i) {
1486 const MachineOperand &MO = PI->getOperand(i);
1489 unsigned Reg = MO.getReg();
1494 for (const unsigned *AS = TRI->getAliasSet(Reg); *AS; ++AS)
1497 if (Uses.count(Reg)) {
1499 for (const unsigned *SR = TRI->getSubRegisters(Reg); *SR; ++SR)
1500 Uses.erase(*SR); // Use getSubRegisters to be conservative
1503 for (const unsigned *AS = TRI->getAliasSet(Reg); *AS; ++AS)
1511 /// HoistCommonCodeInSuccs - If the successors of MBB has common instruction
1512 /// sequence at the start of the function, move the instructions before MBB
1513 /// terminator if it's legal.
1514 bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
1515 MachineBasicBlock *TBB = 0, *FBB = 0;
1516 SmallVector<MachineOperand, 4> Cond;
1517 if (TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true) || !TBB || Cond.empty())
1520 if (!FBB) FBB = findFalseBlock(MBB, TBB);
1522 // Malformed bcc? True and false blocks are the same?
1525 // Restrict the optimization to cases where MBB is the only predecessor,
1526 // it is an obvious win.
1527 if (TBB->pred_size() > 1 || FBB->pred_size() > 1)
1530 // Find a suitable position to hoist the common instructions to. Also figure
1531 // out which registers are used or defined by instructions from the insertion
1532 // point to the end of the block.
1533 SmallSet<unsigned, 4> Uses, Defs;
1534 MachineBasicBlock::iterator Loc =
1535 findHoistingInsertPosAndDeps(MBB, TII, TRI, Uses, Defs);
1536 if (Loc == MBB->end())
1539 bool HasDups = false;
1540 SmallVector<unsigned, 4> LocalDefs;
1541 SmallSet<unsigned, 4> LocalDefsSet;
1542 MachineBasicBlock::iterator TIB = TBB->begin();
1543 MachineBasicBlock::iterator FIB = FBB->begin();
1544 MachineBasicBlock::iterator TIE = TBB->end();
1545 MachineBasicBlock::iterator FIE = FBB->end();
1546 while (TIB != TIE && FIB != FIE) {
1547 // Skip dbg_value instructions. These do not count.
1548 if (TIB->isDebugValue()) {
1549 while (TIB != TIE && TIB->isDebugValue())
1554 if (FIB->isDebugValue()) {
1555 while (FIB != FIE && FIB->isDebugValue())
1560 if (!TIB->isIdenticalTo(FIB, MachineInstr::CheckKillDead))
1563 if (TII->isPredicated(TIB))
1564 // Hard to reason about register liveness with predicated instruction.
1568 for (unsigned i = 0, e = TIB->getNumOperands(); i != e; ++i) {
1569 MachineOperand &MO = TIB->getOperand(i);
1572 unsigned Reg = MO.getReg();
1576 if (Uses.count(Reg)) {
1577 // Avoid clobbering a register that's used by the instruction at
1578 // the point of insertion.
1583 if (Defs.count(Reg) && !MO.isDead()) {
1584 // Don't hoist the instruction if the def would be clobber by the
1585 // instruction at the point insertion. FIXME: This is overly
1586 // conservative. It should be possible to hoist the instructions
1587 // in BB2 in the following example:
1589 // r1, eflag = op1 r2, r3
1598 } else if (!LocalDefsSet.count(Reg)) {
1599 if (Defs.count(Reg)) {
1600 // Use is defined by the instruction at the point of insertion.
1609 bool DontMoveAcrossStore = true;
1610 if (!TIB->isSafeToMove(TII, 0, DontMoveAcrossStore))
1613 // Track local defs so we can update liveins.
1614 for (unsigned i = 0, e = TIB->getNumOperands(); i != e; ++i) {
1615 MachineOperand &MO = TIB->getOperand(i);
1618 unsigned Reg = MO.getReg();
1623 LocalDefs.push_back(Reg);
1624 LocalDefsSet.insert(Reg);
1625 for (const unsigned *SR = TRI->getSubRegisters(Reg); *SR; ++SR)
1626 LocalDefsSet.insert(*SR);
1628 } else if (MO.isKill() && LocalDefsSet.count(Reg)) {
1629 LocalDefsSet.erase(Reg);
1630 for (const unsigned *SR = TRI->getSubRegisters(Reg); *SR; ++SR)
1631 LocalDefsSet.erase(*SR);
1643 MBB->splice(Loc, TBB, TBB->begin(), TIB);
1644 FBB->erase(FBB->begin(), FIB);
1647 for (unsigned i = 0, e = LocalDefs.size(); i != e; ++i) {
1648 unsigned Def = LocalDefs[i];
1649 if (LocalDefsSet.count(Def)) {
1650 TBB->addLiveIn(Def);
1651 FBB->addLiveIn(Def);