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(); I != E; ++I) {
804 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(I), I));
807 // See if we can do any tail merging on those.
808 if (MergePotentials.size() < TailMergeThreshold &&
809 MergePotentials.size() >= 2)
810 MadeChange |= TryTailMergeBlocks(NULL, NULL);
812 // Look at blocks (IBB) with multiple predecessors (PBB).
813 // We change each predecessor to a canonical form, by
814 // (1) temporarily removing any unconditional branch from the predecessor
816 // (2) alter conditional branches so they branch to the other block
817 // not IBB; this may require adding back an unconditional branch to IBB
818 // later, where there wasn't one coming in. E.g.
820 // fallthrough to QBB
823 // with a conceptual B to IBB after that, which never actually exists.
824 // With those changes, we see whether the predecessors' tails match,
825 // and merge them if so. We change things out of canonical form and
826 // back to the way they were later in the process. (OptimizeBranches
827 // would undo some of this, but we can't use it, because we'd get into
828 // a compile-time infinite loop repeatedly doing and undoing the same
831 for (MachineFunction::iterator I = llvm::next(MF.begin()), E = MF.end();
833 if (I->pred_size() >= 2 && I->pred_size() < TailMergeThreshold) {
834 SmallPtrSet<MachineBasicBlock *, 8> UniquePreds;
835 MachineBasicBlock *IBB = I;
836 MachineBasicBlock *PredBB = prior(I);
837 MergePotentials.clear();
838 for (MachineBasicBlock::pred_iterator P = I->pred_begin(),
841 MachineBasicBlock *PBB = *P;
842 // Skip blocks that loop to themselves, can't tail merge these.
845 // Visit each predecessor only once.
846 if (!UniquePreds.insert(PBB))
848 MachineBasicBlock *TBB = 0, *FBB = 0;
849 SmallVector<MachineOperand, 4> Cond;
850 if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond, true)) {
851 // Failing case: IBB is the target of a cbr, and
852 // we cannot reverse the branch.
853 SmallVector<MachineOperand, 4> NewCond(Cond);
854 if (!Cond.empty() && TBB == IBB) {
855 if (TII->ReverseBranchCondition(NewCond))
857 // This is the QBB case described above
859 FBB = llvm::next(MachineFunction::iterator(PBB));
861 // Failing case: the only way IBB can be reached from PBB is via
862 // exception handling. Happens for landing pads. Would be nice
863 // to have a bit in the edge so we didn't have to do all this.
864 if (IBB->isLandingPad()) {
865 MachineFunction::iterator IP = PBB; IP++;
866 MachineBasicBlock *PredNextBB = NULL;
870 if (IBB != PredNextBB) // fallthrough
873 if (TBB != IBB && FBB != IBB) // cbr then ubr
875 } else if (Cond.empty()) {
876 if (TBB != IBB) // ubr
879 if (TBB != IBB && IBB != PredNextBB) // cbr
883 // Remove the unconditional branch at the end, if any.
884 if (TBB && (Cond.empty() || FBB)) {
885 DebugLoc dl; // FIXME: this is nowhere
886 TII->RemoveBranch(*PBB);
888 // reinsert conditional branch only, for now
889 TII->InsertBranch(*PBB, (TBB == IBB) ? FBB : TBB, 0, NewCond, dl);
891 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(PBB), *P));
894 if (MergePotentials.size() >= 2)
895 MadeChange |= TryTailMergeBlocks(IBB, PredBB);
896 // Reinsert an unconditional branch if needed.
897 // The 1 below can occur as a result of removing blocks in TryTailMergeBlocks.
898 PredBB = prior(I); // this may have been changed in TryTailMergeBlocks
899 if (MergePotentials.size() == 1 &&
900 MergePotentials.begin()->getBlock() != PredBB)
901 FixTail(MergePotentials.begin()->getBlock(), IBB, TII);
907 //===----------------------------------------------------------------------===//
908 // Branch Optimization
909 //===----------------------------------------------------------------------===//
911 bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
912 bool MadeChange = false;
914 // Make sure blocks are numbered in order
917 for (MachineFunction::iterator I = llvm::next(MF.begin()), E = MF.end();
919 MachineBasicBlock *MBB = I++;
920 MadeChange |= OptimizeBlock(MBB);
922 // If it is dead, remove it.
923 if (MBB->pred_empty()) {
924 RemoveDeadBlock(MBB);
932 // Blocks should be considered empty if they contain only debug info;
933 // else the debug info would affect codegen.
934 static bool IsEmptyBlock(MachineBasicBlock *MBB) {
937 for (MachineBasicBlock::iterator MBBI = MBB->begin(), MBBE = MBB->end();
938 MBBI!=MBBE; ++MBBI) {
939 if (!MBBI->isDebugValue())
945 // Blocks with only debug info and branches should be considered the same
946 // as blocks with only branches.
947 static bool IsBranchOnlyBlock(MachineBasicBlock *MBB) {
948 MachineBasicBlock::iterator MBBI, MBBE;
949 for (MBBI = MBB->begin(), MBBE = MBB->end(); MBBI!=MBBE; ++MBBI) {
950 if (!MBBI->isDebugValue())
953 return (MBBI->getDesc().isBranch());
956 /// IsBetterFallthrough - Return true if it would be clearly better to
957 /// fall-through to MBB1 than to fall through into MBB2. This has to return
958 /// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will
959 /// result in infinite loops.
960 static bool IsBetterFallthrough(MachineBasicBlock *MBB1,
961 MachineBasicBlock *MBB2) {
962 // Right now, we use a simple heuristic. If MBB2 ends with a call, and
963 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to
964 // optimize branches that branch to either a return block or an assert block
965 // into a fallthrough to the return.
966 if (IsEmptyBlock(MBB1) || IsEmptyBlock(MBB2)) return false;
968 // If there is a clear successor ordering we make sure that one block
969 // will fall through to the next
970 if (MBB1->isSuccessor(MBB2)) return true;
971 if (MBB2->isSuccessor(MBB1)) return false;
973 // Neither block consists entirely of debug info (per IsEmptyBlock check),
974 // so we needn't test for falling off the beginning here.
975 MachineBasicBlock::iterator MBB1I = --MBB1->end();
976 while (MBB1I->isDebugValue())
978 MachineBasicBlock::iterator MBB2I = --MBB2->end();
979 while (MBB2I->isDebugValue())
981 return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall();
984 /// OptimizeBlock - Analyze and optimize control flow related to the specified
985 /// block. This is never called on the entry block.
986 bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
987 bool MadeChange = false;
988 MachineFunction &MF = *MBB->getParent();
989 DebugLoc dl; // FIXME: this is nowhere
992 MachineFunction::iterator FallThrough = MBB;
995 // If this block is empty, make everyone use its fall-through, not the block
996 // explicitly. Landing pads should not do this since the landing-pad table
997 // points to this block. Blocks with their addresses taken shouldn't be
999 if (IsEmptyBlock(MBB) && !MBB->isLandingPad() && !MBB->hasAddressTaken()) {
1000 // Dead block? Leave for cleanup later.
1001 if (MBB->pred_empty()) return MadeChange;
1003 if (FallThrough == MF.end()) {
1004 // TODO: Simplify preds to not branch here if possible!
1006 // Rewrite all predecessors of the old block to go to the fallthrough
1008 while (!MBB->pred_empty()) {
1009 MachineBasicBlock *Pred = *(MBB->pred_end()-1);
1010 Pred->ReplaceUsesOfBlockWith(MBB, FallThrough);
1012 // If MBB was the target of a jump table, update jump tables to go to the
1013 // fallthrough instead.
1014 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
1015 MJTI->ReplaceMBBInJumpTables(MBB, FallThrough);
1021 // Check to see if we can simplify the terminator of the block before this
1023 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB));
1025 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
1026 SmallVector<MachineOperand, 4> PriorCond;
1027 bool PriorUnAnalyzable =
1028 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
1029 if (!PriorUnAnalyzable) {
1030 // If the CFG for the prior block has extra edges, remove them.
1031 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB,
1032 !PriorCond.empty());
1034 // If the previous branch is conditional and both conditions go to the same
1035 // destination, remove the branch, replacing it with an unconditional one or
1037 if (PriorTBB && PriorTBB == PriorFBB) {
1038 TII->RemoveBranch(PrevBB);
1040 if (PriorTBB != MBB)
1041 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond, dl);
1044 goto ReoptimizeBlock;
1047 // If the previous block unconditionally falls through to this block and
1048 // this block has no other predecessors, move the contents of this block
1049 // into the prior block. This doesn't usually happen when SimplifyCFG
1050 // has been used, but it can happen if tail merging splits a fall-through
1051 // predecessor of a block.
1052 // This has to check PrevBB->succ_size() because EH edges are ignored by
1054 if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 &&
1055 PrevBB.succ_size() == 1 &&
1056 !MBB->hasAddressTaken() && !MBB->isLandingPad()) {
1057 DEBUG(dbgs() << "\nMerging into block: " << PrevBB
1058 << "From MBB: " << *MBB);
1059 // Remove redundant DBG_VALUEs first.
1060 if (PrevBB.begin() != PrevBB.end()) {
1061 MachineBasicBlock::iterator PrevBBIter = PrevBB.end();
1063 MachineBasicBlock::iterator MBBIter = MBB->begin();
1064 // Check if DBG_VALUE at the end of PrevBB is identical to the
1065 // DBG_VALUE at the beginning of MBB.
1066 while (PrevBBIter != PrevBB.begin() && MBBIter != MBB->end()
1067 && PrevBBIter->isDebugValue() && MBBIter->isDebugValue()) {
1068 if (!MBBIter->isIdenticalTo(PrevBBIter))
1070 MachineInstr *DuplicateDbg = MBBIter;
1071 ++MBBIter; -- PrevBBIter;
1072 DuplicateDbg->eraseFromParent();
1075 PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end());
1076 PrevBB.removeSuccessor(PrevBB.succ_begin());;
1077 assert(PrevBB.succ_empty());
1078 PrevBB.transferSuccessors(MBB);
1083 // If the previous branch *only* branches to *this* block (conditional or
1084 // not) remove the branch.
1085 if (PriorTBB == MBB && PriorFBB == 0) {
1086 TII->RemoveBranch(PrevBB);
1089 goto ReoptimizeBlock;
1092 // If the prior block branches somewhere else on the condition and here if
1093 // the condition is false, remove the uncond second branch.
1094 if (PriorFBB == MBB) {
1095 TII->RemoveBranch(PrevBB);
1096 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond, dl);
1099 goto ReoptimizeBlock;
1102 // If the prior block branches here on true and somewhere else on false, and
1103 // if the branch condition is reversible, reverse the branch to create a
1105 if (PriorTBB == MBB) {
1106 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1107 if (!TII->ReverseBranchCondition(NewPriorCond)) {
1108 TII->RemoveBranch(PrevBB);
1109 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond, dl);
1112 goto ReoptimizeBlock;
1116 // If this block has no successors (e.g. it is a return block or ends with
1117 // a call to a no-return function like abort or __cxa_throw) and if the pred
1118 // falls through into this block, and if it would otherwise fall through
1119 // into the block after this, move this block to the end of the function.
1121 // We consider it more likely that execution will stay in the function (e.g.
1122 // due to loops) than it is to exit it. This asserts in loops etc, moving
1123 // the assert condition out of the loop body.
1124 if (MBB->succ_empty() && !PriorCond.empty() && PriorFBB == 0 &&
1125 MachineFunction::iterator(PriorTBB) == FallThrough &&
1126 !MBB->canFallThrough()) {
1127 bool DoTransform = true;
1129 // We have to be careful that the succs of PredBB aren't both no-successor
1130 // blocks. If neither have successors and if PredBB is the second from
1131 // last block in the function, we'd just keep swapping the two blocks for
1132 // last. Only do the swap if one is clearly better to fall through than
1134 if (FallThrough == --MF.end() &&
1135 !IsBetterFallthrough(PriorTBB, MBB))
1136 DoTransform = false;
1139 // Reverse the branch so we will fall through on the previous true cond.
1140 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1141 if (!TII->ReverseBranchCondition(NewPriorCond)) {
1142 DEBUG(dbgs() << "\nMoving MBB: " << *MBB
1143 << "To make fallthrough to: " << *PriorTBB << "\n");
1145 TII->RemoveBranch(PrevBB);
1146 TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond, dl);
1148 // Move this block to the end of the function.
1149 MBB->moveAfter(--MF.end());
1158 // Analyze the branch in the current block.
1159 MachineBasicBlock *CurTBB = 0, *CurFBB = 0;
1160 SmallVector<MachineOperand, 4> CurCond;
1161 bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true);
1162 if (!CurUnAnalyzable) {
1163 // If the CFG for the prior block has extra edges, remove them.
1164 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty());
1166 // If this is a two-way branch, and the FBB branches to this block, reverse
1167 // the condition so the single-basic-block loop is faster. Instead of:
1168 // Loop: xxx; jcc Out; jmp Loop
1170 // Loop: xxx; jncc Loop; jmp Out
1171 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) {
1172 SmallVector<MachineOperand, 4> NewCond(CurCond);
1173 if (!TII->ReverseBranchCondition(NewCond)) {
1174 TII->RemoveBranch(*MBB);
1175 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond, dl);
1178 goto ReoptimizeBlock;
1182 // If this branch is the only thing in its block, see if we can forward
1183 // other blocks across it.
1184 if (CurTBB && CurCond.empty() && CurFBB == 0 &&
1185 IsBranchOnlyBlock(MBB) && CurTBB != MBB &&
1186 !MBB->hasAddressTaken()) {
1187 // This block may contain just an unconditional branch. Because there can
1188 // be 'non-branch terminators' in the block, try removing the branch and
1189 // then seeing if the block is empty.
1190 TII->RemoveBranch(*MBB);
1191 // If the only things remaining in the block are debug info, remove these
1192 // as well, so this will behave the same as an empty block in non-debug
1194 if (!MBB->empty()) {
1195 bool NonDebugInfoFound = false;
1196 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
1198 if (!I->isDebugValue()) {
1199 NonDebugInfoFound = true;
1203 if (!NonDebugInfoFound)
1204 // Make the block empty, losing the debug info (we could probably
1205 // improve this in some cases.)
1206 MBB->erase(MBB->begin(), MBB->end());
1208 // If this block is just an unconditional branch to CurTBB, we can
1209 // usually completely eliminate the block. The only case we cannot
1210 // completely eliminate the block is when the block before this one
1211 // falls through into MBB and we can't understand the prior block's branch
1214 bool PredHasNoFallThrough = !PrevBB.canFallThrough();
1215 if (PredHasNoFallThrough || !PriorUnAnalyzable ||
1216 !PrevBB.isSuccessor(MBB)) {
1217 // If the prior block falls through into us, turn it into an
1218 // explicit branch to us to make updates simpler.
1219 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) &&
1220 PriorTBB != MBB && PriorFBB != MBB) {
1221 if (PriorTBB == 0) {
1222 assert(PriorCond.empty() && PriorFBB == 0 &&
1223 "Bad branch analysis");
1226 assert(PriorFBB == 0 && "Machine CFG out of date!");
1229 TII->RemoveBranch(PrevBB);
1230 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, dl);
1233 // Iterate through all the predecessors, revectoring each in-turn.
1235 bool DidChange = false;
1236 bool HasBranchToSelf = false;
1237 while(PI != MBB->pred_size()) {
1238 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI);
1240 // If this block has an uncond branch to itself, leave it.
1242 HasBranchToSelf = true;
1245 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB);
1246 // If this change resulted in PMBB ending in a conditional
1247 // branch where both conditions go to the same destination,
1248 // change this to an unconditional branch (and fix the CFG).
1249 MachineBasicBlock *NewCurTBB = 0, *NewCurFBB = 0;
1250 SmallVector<MachineOperand, 4> NewCurCond;
1251 bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB,
1252 NewCurFBB, NewCurCond, true);
1253 if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) {
1254 TII->RemoveBranch(*PMBB);
1256 TII->InsertBranch(*PMBB, NewCurTBB, 0, NewCurCond, dl);
1259 PMBB->CorrectExtraCFGEdges(NewCurTBB, 0, false);
1264 // Change any jumptables to go to the new MBB.
1265 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
1266 MJTI->ReplaceMBBInJumpTables(MBB, CurTBB);
1270 if (!HasBranchToSelf) return MadeChange;
1275 // Add the branch back if the block is more than just an uncond branch.
1276 TII->InsertBranch(*MBB, CurTBB, 0, CurCond, dl);
1280 // If the prior block doesn't fall through into this block, and if this
1281 // block doesn't fall through into some other block, see if we can find a
1282 // place to move this block where a fall-through will happen.
1283 if (!PrevBB.canFallThrough()) {
1285 // Now we know that there was no fall-through into this block, check to
1286 // see if it has a fall-through into its successor.
1287 bool CurFallsThru = MBB->canFallThrough();
1289 if (!MBB->isLandingPad()) {
1290 // Check all the predecessors of this block. If one of them has no fall
1291 // throughs, move this block right after it.
1292 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
1293 E = MBB->pred_end(); PI != E; ++PI) {
1294 // Analyze the branch at the end of the pred.
1295 MachineBasicBlock *PredBB = *PI;
1296 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough;
1297 MachineBasicBlock *PredTBB = 0, *PredFBB = 0;
1298 SmallVector<MachineOperand, 4> PredCond;
1299 if (PredBB != MBB && !PredBB->canFallThrough() &&
1300 !TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true)
1301 && (!CurFallsThru || !CurTBB || !CurFBB)
1302 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
1303 // If the current block doesn't fall through, just move it.
1304 // If the current block can fall through and does not end with a
1305 // conditional branch, we need to append an unconditional jump to
1306 // the (current) next block. To avoid a possible compile-time
1307 // infinite loop, move blocks only backward in this case.
1308 // Also, if there are already 2 branches here, we cannot add a third;
1309 // this means we have the case
1314 MachineBasicBlock *NextBB = llvm::next(MachineFunction::iterator(MBB));
1316 TII->InsertBranch(*MBB, NextBB, 0, CurCond, dl);
1318 MBB->moveAfter(PredBB);
1320 goto ReoptimizeBlock;
1325 if (!CurFallsThru) {
1326 // Check all successors to see if we can move this block before it.
1327 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
1328 E = MBB->succ_end(); SI != E; ++SI) {
1329 // Analyze the branch at the end of the block before the succ.
1330 MachineBasicBlock *SuccBB = *SI;
1331 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev;
1333 // If this block doesn't already fall-through to that successor, and if
1334 // the succ doesn't already have a block that can fall through into it,
1335 // and if the successor isn't an EH destination, we can arrange for the
1336 // fallthrough to happen.
1337 if (SuccBB != MBB && &*SuccPrev != MBB &&
1338 !SuccPrev->canFallThrough() && !CurUnAnalyzable &&
1339 !SuccBB->isLandingPad()) {
1340 MBB->moveBefore(SuccBB);
1342 goto ReoptimizeBlock;
1346 // Okay, there is no really great place to put this block. If, however,
1347 // the block before this one would be a fall-through if this block were
1348 // removed, move this block to the end of the function.
1349 MachineBasicBlock *PrevTBB = 0, *PrevFBB = 0;
1350 SmallVector<MachineOperand, 4> PrevCond;
1351 if (FallThrough != MF.end() &&
1352 !TII->AnalyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) &&
1353 PrevBB.isSuccessor(FallThrough)) {
1354 MBB->moveAfter(--MF.end());
1364 //===----------------------------------------------------------------------===//
1365 // Hoist Common Code
1366 //===----------------------------------------------------------------------===//
1368 /// HoistCommonCode - Hoist common instruction sequences at the start of basic
1369 /// blocks to their common predecessor.
1370 bool BranchFolder::HoistCommonCode(MachineFunction &MF) {
1371 bool MadeChange = false;
1372 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ) {
1373 MachineBasicBlock *MBB = I++;
1374 MadeChange |= HoistCommonCodeInSuccs(MBB);
1380 /// findFalseBlock - BB has a fallthrough. Find its 'false' successor given
1381 /// its 'true' successor.
1382 static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB,
1383 MachineBasicBlock *TrueBB) {
1384 for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
1385 E = BB->succ_end(); SI != E; ++SI) {
1386 MachineBasicBlock *SuccBB = *SI;
1387 if (SuccBB != TrueBB)
1393 /// findHoistingInsertPosAndDeps - Find the location to move common instructions
1394 /// in successors to. The location is ususally just before the terminator,
1395 /// however if the terminator is a conditional branch and its previous
1396 /// instruction is the flag setting instruction, the previous instruction is
1397 /// the preferred location. This function also gathers uses and defs of the
1398 /// instructions from the insertion point to the end of the block. The data is
1399 /// used by HoistCommonCodeInSuccs to ensure safety.
1401 MachineBasicBlock::iterator findHoistingInsertPosAndDeps(MachineBasicBlock *MBB,
1402 const TargetInstrInfo *TII,
1403 const TargetRegisterInfo *TRI,
1404 SmallSet<unsigned,4> &Uses,
1405 SmallSet<unsigned,4> &Defs) {
1406 MachineBasicBlock::iterator Loc = MBB->getFirstTerminator();
1407 if (!TII->isUnpredicatedTerminator(Loc))
1410 for (unsigned i = 0, e = Loc->getNumOperands(); i != e; ++i) {
1411 const MachineOperand &MO = Loc->getOperand(i);
1414 unsigned Reg = MO.getReg();
1419 for (const unsigned *AS = TRI->getAliasSet(Reg); *AS; ++AS)
1421 } else if (!MO.isDead())
1422 // Don't try to hoist code in the rare case the terminator defines a
1423 // register that is later used.
1429 if (Loc == MBB->begin())
1432 // The terminator is probably a conditional branch, try not to separate the
1433 // branch from condition setting instruction.
1434 MachineBasicBlock::iterator PI = Loc;
1436 while (PI != MBB->begin() && Loc->isDebugValue())
1440 for (unsigned i = 0, e = PI->getNumOperands(); !IsDef && i != e; ++i) {
1441 const MachineOperand &MO = PI->getOperand(i);
1442 if (!MO.isReg() || MO.isUse())
1444 unsigned Reg = MO.getReg();
1447 if (Uses.count(Reg))
1451 // The condition setting instruction is not just before the conditional
1455 // Be conservative, don't insert instruction above something that may have
1456 // side-effects. And since it's potentially bad to separate flag setting
1457 // instruction from the conditional branch, just abort the optimization
1459 // Also avoid moving code above predicated instruction since it's hard to
1460 // reason about register liveness with predicated instruction.
1461 bool DontMoveAcrossStore = true;
1462 if (!PI->isSafeToMove(TII, 0, DontMoveAcrossStore) ||
1463 TII->isPredicated(PI))
1467 // Find out what registers are live. Note this routine is ignoring other live
1468 // registers which are only used by instructions in successor blocks.
1469 for (unsigned i = 0, e = PI->getNumOperands(); i != e; ++i) {
1470 const MachineOperand &MO = PI->getOperand(i);
1473 unsigned Reg = MO.getReg();
1478 for (const unsigned *AS = TRI->getAliasSet(Reg); *AS; ++AS)
1481 if (Uses.count(Reg)) {
1483 for (const unsigned *SR = TRI->getSubRegisters(Reg); *SR; ++SR)
1484 Uses.erase(*SR); // Use getSubRegisters to be conservative
1487 for (const unsigned *AS = TRI->getAliasSet(Reg); *AS; ++AS)
1495 /// HoistCommonCodeInSuccs - If the successors of MBB has common instruction
1496 /// sequence at the start of the function, move the instructions before MBB
1497 /// terminator if it's legal.
1498 bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
1499 MachineBasicBlock *TBB = 0, *FBB = 0;
1500 SmallVector<MachineOperand, 4> Cond;
1501 if (TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true) || !TBB || Cond.empty())
1504 if (!FBB) FBB = findFalseBlock(MBB, TBB);
1506 // Malformed bcc? True and false blocks are the same?
1509 // Restrict the optimization to cases where MBB is the only predecessor,
1510 // it is an obvious win.
1511 if (TBB->pred_size() > 1 || FBB->pred_size() > 1)
1514 // Find a suitable position to hoist the common instructions to. Also figure
1515 // out which registers are used or defined by instructions from the insertion
1516 // point to the end of the block.
1517 SmallSet<unsigned, 4> Uses, Defs;
1518 MachineBasicBlock::iterator Loc =
1519 findHoistingInsertPosAndDeps(MBB, TII, TRI, Uses, Defs);
1520 if (Loc == MBB->end())
1523 bool HasDups = false;
1524 SmallVector<unsigned, 4> LocalDefs;
1525 SmallSet<unsigned, 4> LocalDefsSet;
1526 MachineBasicBlock::iterator TIB = TBB->begin();
1527 MachineBasicBlock::iterator FIB = FBB->begin();
1528 MachineBasicBlock::iterator TIE = TBB->end();
1529 MachineBasicBlock::iterator FIE = FBB->end();
1530 while (TIB != TIE && FIB != FIE) {
1531 // Skip dbg_value instructions. These do not count.
1532 if (TIB->isDebugValue()) {
1533 while (TIB != TIE && TIB->isDebugValue())
1538 if (FIB->isDebugValue()) {
1539 while (FIB != FIE && FIB->isDebugValue())
1544 if (!TIB->isIdenticalTo(FIB, MachineInstr::CheckKillDead))
1547 if (TII->isPredicated(TIB))
1548 // Hard to reason about register liveness with predicated instruction.
1552 for (unsigned i = 0, e = TIB->getNumOperands(); i != e; ++i) {
1553 MachineOperand &MO = TIB->getOperand(i);
1556 unsigned Reg = MO.getReg();
1560 if (Uses.count(Reg)) {
1561 // Avoid clobbering a register that's used by the instruction at
1562 // the point of insertion.
1567 if (Defs.count(Reg) && !MO.isDead()) {
1568 // Don't hoist the instruction if the def would be clobber by the
1569 // instruction at the point insertion. FIXME: This is overly
1570 // conservative. It should be possible to hoist the instructions
1571 // in BB2 in the following example:
1573 // r1, eflag = op1 r2, r3
1582 } else if (!LocalDefsSet.count(Reg)) {
1583 if (Defs.count(Reg)) {
1584 // Use is defined by the instruction at the point of insertion.
1593 bool DontMoveAcrossStore = true;
1594 if (!TIB->isSafeToMove(TII, 0, DontMoveAcrossStore))
1597 // Track local defs so we can update liveins.
1598 for (unsigned i = 0, e = TIB->getNumOperands(); i != e; ++i) {
1599 MachineOperand &MO = TIB->getOperand(i);
1602 unsigned Reg = MO.getReg();
1607 LocalDefs.push_back(Reg);
1608 LocalDefsSet.insert(Reg);
1609 for (const unsigned *SR = TRI->getSubRegisters(Reg); *SR; ++SR)
1610 LocalDefsSet.insert(*SR);
1612 } else if (MO.isKill() && LocalDefsSet.count(Reg)) {
1613 LocalDefsSet.erase(Reg);
1614 for (const unsigned *SR = TRI->getSubRegisters(Reg); *SR; ++SR)
1615 LocalDefsSet.erase(*SR);
1627 MBB->splice(Loc, TBB, TBB->begin(), TIB);
1628 FBB->erase(FBB->begin(), FIB);
1631 for (unsigned i = 0, e = LocalDefs.size(); i != e; ++i) {
1632 unsigned Def = LocalDefs[i];
1633 if (LocalDefsSet.count(Def)) {
1634 TBB->addLiveIn(Def);
1635 FBB->addLiveIn(Def);