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/MachineRegisterInfo.h"
27 #include "llvm/CodeGen/RegisterScavenging.h"
28 #include "llvm/Target/TargetInstrInfo.h"
29 #include "llvm/Target/TargetMachine.h"
30 #include "llvm/Target/TargetRegisterInfo.h"
31 #include "llvm/Support/CommandLine.h"
32 #include "llvm/Support/Debug.h"
33 #include "llvm/Support/ErrorHandling.h"
34 #include "llvm/Support/raw_ostream.h"
35 #include "llvm/ADT/SmallSet.h"
36 #include "llvm/ADT/SetVector.h"
37 #include "llvm/ADT/Statistic.h"
38 #include "llvm/ADT/STLExtras.h"
42 STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
43 STATISTIC(NumBranchOpts, "Number of branches optimized");
44 STATISTIC(NumTailMerge , "Number of block tails merged");
45 STATISTIC(NumHoist , "Number of times common instructions are hoisted");
47 static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge",
48 cl::init(cl::BOU_UNSET), cl::Hidden);
50 // Throttle for huge numbers of predecessors (compile speed problems)
51 static cl::opt<unsigned>
52 TailMergeThreshold("tail-merge-threshold",
53 cl::desc("Max number of predecessors to consider tail merging"),
54 cl::init(150), cl::Hidden);
56 // Heuristic for tail merging (and, inversely, tail duplication).
57 // TODO: This should be replaced with a target query.
58 static cl::opt<unsigned>
59 TailMergeSize("tail-merge-size",
60 cl::desc("Min number of instructions to consider tail merging"),
61 cl::init(3), cl::Hidden);
64 /// BranchFolderPass - Wrap branch folder in a machine function pass.
65 class BranchFolderPass : public MachineFunctionPass {
68 explicit BranchFolderPass(): MachineFunctionPass(ID) {}
70 virtual bool runOnMachineFunction(MachineFunction &MF);
72 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
73 AU.addRequired<TargetPassConfig>();
74 MachineFunctionPass::getAnalysisUsage(AU);
79 char BranchFolderPass::ID = 0;
80 char &llvm::BranchFolderPassID = BranchFolderPass::ID;
82 INITIALIZE_PASS(BranchFolderPass, "branch-folder",
83 "Control Flow Optimizer", false, false)
85 bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) {
86 TargetPassConfig *PassConfig = &getAnalysis<TargetPassConfig>();
87 BranchFolder Folder(PassConfig->getEnableTailMerge(), /*CommonHoist=*/true);
88 return Folder.OptimizeFunction(MF,
89 MF.getTarget().getInstrInfo(),
90 MF.getTarget().getRegisterInfo(),
91 getAnalysisIfAvailable<MachineModuleInfo>());
95 BranchFolder::BranchFolder(bool defaultEnableTailMerge, bool CommonHoist) {
96 switch (FlagEnableTailMerge) {
97 case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break;
98 case cl::BOU_TRUE: EnableTailMerge = true; break;
99 case cl::BOU_FALSE: EnableTailMerge = false; break;
102 EnableHoistCommonCode = CommonHoist;
105 /// RemoveDeadBlock - Remove the specified dead machine basic block from the
106 /// function, updating the CFG.
107 void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) {
108 assert(MBB->pred_empty() && "MBB must be dead!");
109 DEBUG(dbgs() << "\nRemoving MBB: " << *MBB);
111 MachineFunction *MF = MBB->getParent();
112 // drop all successors.
113 while (!MBB->succ_empty())
114 MBB->removeSuccessor(MBB->succ_end()-1);
116 // Avoid matching if this pointer gets reused.
117 TriedMerging.erase(MBB);
123 /// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def
124 /// followed by terminators, and if the implicitly defined registers are not
125 /// used by the terminators, remove those implicit_def's. e.g.
127 /// r0 = implicit_def
128 /// r1 = implicit_def
130 /// This block can be optimized away later if the implicit instructions are
132 bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) {
133 SmallSet<unsigned, 4> ImpDefRegs;
134 MachineBasicBlock::iterator I = MBB->begin();
135 while (I != MBB->end()) {
136 if (!I->isImplicitDef())
138 unsigned Reg = I->getOperand(0).getReg();
139 ImpDefRegs.insert(Reg);
140 for (MCSubRegIterator SubRegs(Reg, TRI); SubRegs.isValid(); ++SubRegs)
141 ImpDefRegs.insert(*SubRegs);
144 if (ImpDefRegs.empty())
147 MachineBasicBlock::iterator FirstTerm = I;
148 while (I != MBB->end()) {
149 if (!TII->isUnpredicatedTerminator(I))
151 // See if it uses any of the implicitly defined registers.
152 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
153 MachineOperand &MO = I->getOperand(i);
154 if (!MO.isReg() || !MO.isUse())
156 unsigned Reg = MO.getReg();
157 if (ImpDefRegs.count(Reg))
164 while (I != FirstTerm) {
165 MachineInstr *ImpDefMI = &*I;
167 MBB->erase(ImpDefMI);
173 /// OptimizeFunction - Perhaps branch folding, tail merging and other
174 /// CFG optimizations on the given function.
175 bool BranchFolder::OptimizeFunction(MachineFunction &MF,
176 const TargetInstrInfo *tii,
177 const TargetRegisterInfo *tri,
178 MachineModuleInfo *mmi) {
179 if (!tii) return false;
181 TriedMerging.clear();
188 // Use a RegScavenger to help update liveness when required.
189 MachineRegisterInfo &MRI = MF.getRegInfo();
190 if (MRI.tracksLiveness() && TRI->trackLivenessAfterRegAlloc(MF))
191 RS = new RegScavenger();
193 MRI.invalidateLiveness();
195 // Fix CFG. The later algorithms expect it to be right.
196 bool MadeChange = false;
197 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) {
198 MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0;
199 SmallVector<MachineOperand, 4> Cond;
200 if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true))
201 MadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty());
202 MadeChange |= OptimizeImpDefsBlock(MBB);
205 bool MadeChangeThisIteration = true;
206 while (MadeChangeThisIteration) {
207 MadeChangeThisIteration = TailMergeBlocks(MF);
208 MadeChangeThisIteration |= OptimizeBranches(MF);
209 if (EnableHoistCommonCode)
210 MadeChangeThisIteration |= HoistCommonCode(MF);
211 MadeChange |= MadeChangeThisIteration;
214 // See if any jump tables have become dead as the code generator
216 MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
222 // Walk the function to find jump tables that are live.
223 BitVector JTIsLive(JTI->getJumpTables().size());
224 for (MachineFunction::iterator BB = MF.begin(), E = MF.end();
226 for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end();
228 for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) {
229 MachineOperand &Op = I->getOperand(op);
230 if (!Op.isJTI()) continue;
232 // Remember that this JT is live.
233 JTIsLive.set(Op.getIndex());
237 // Finally, remove dead jump tables. This happens when the
238 // indirect jump was unreachable (and thus deleted).
239 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i)
240 if (!JTIsLive.test(i)) {
241 JTI->RemoveJumpTable(i);
249 //===----------------------------------------------------------------------===//
250 // Tail Merging of Blocks
251 //===----------------------------------------------------------------------===//
253 /// HashMachineInstr - Compute a hash value for MI and its operands.
254 static unsigned HashMachineInstr(const MachineInstr *MI) {
255 unsigned Hash = MI->getOpcode();
256 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
257 const MachineOperand &Op = MI->getOperand(i);
259 // Merge in bits from the operand if easy.
260 unsigned OperandHash = 0;
261 switch (Op.getType()) {
262 case MachineOperand::MO_Register: OperandHash = Op.getReg(); break;
263 case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break;
264 case MachineOperand::MO_MachineBasicBlock:
265 OperandHash = Op.getMBB()->getNumber();
267 case MachineOperand::MO_FrameIndex:
268 case MachineOperand::MO_ConstantPoolIndex:
269 case MachineOperand::MO_JumpTableIndex:
270 OperandHash = Op.getIndex();
272 case MachineOperand::MO_GlobalAddress:
273 case MachineOperand::MO_ExternalSymbol:
274 // Global address / external symbol are too hard, don't bother, but do
275 // pull in the offset.
276 OperandHash = Op.getOffset();
281 Hash += ((OperandHash << 3) | Op.getType()) << (i&31);
286 /// HashEndOfMBB - Hash the last instruction in the MBB.
287 static unsigned HashEndOfMBB(const MachineBasicBlock *MBB) {
288 MachineBasicBlock::const_iterator I = MBB->end();
289 if (I == MBB->begin())
290 return 0; // Empty MBB.
293 // Skip debug info so it will not affect codegen.
294 while (I->isDebugValue()) {
296 return 0; // MBB empty except for debug info.
300 return HashMachineInstr(I);
303 /// ComputeCommonTailLength - Given two machine basic blocks, compute the number
304 /// of instructions they actually have in common together at their end. Return
305 /// iterators for the first shared instruction in each block.
306 static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1,
307 MachineBasicBlock *MBB2,
308 MachineBasicBlock::iterator &I1,
309 MachineBasicBlock::iterator &I2) {
313 unsigned TailLen = 0;
314 while (I1 != MBB1->begin() && I2 != MBB2->begin()) {
316 // Skip debugging pseudos; necessary to avoid changing the code.
317 while (I1->isDebugValue()) {
318 if (I1==MBB1->begin()) {
319 while (I2->isDebugValue()) {
320 if (I2==MBB2->begin())
321 // I1==DBG at begin; I2==DBG at begin
326 // I1==DBG at begin; I2==non-DBG, or first of DBGs not at begin
331 // I1==first (untested) non-DBG preceding known match
332 while (I2->isDebugValue()) {
333 if (I2==MBB2->begin()) {
335 // I1==non-DBG, or first of DBGs not at begin; I2==DBG at begin
340 // I1, I2==first (untested) non-DBGs preceding known match
341 if (!I1->isIdenticalTo(I2) ||
342 // FIXME: This check is dubious. It's used to get around a problem where
343 // people incorrectly expect inline asm directives to remain in the same
344 // relative order. This is untenable because normal compiler
345 // optimizations (like this one) may reorder and/or merge these
353 // Back past possible debugging pseudos at beginning of block. This matters
354 // when one block differs from the other only by whether debugging pseudos
355 // are present at the beginning. (This way, the various checks later for
356 // I1==MBB1->begin() work as expected.)
357 if (I1 == MBB1->begin() && I2 != MBB2->begin()) {
359 while (I2->isDebugValue()) {
360 if (I2 == MBB2->begin()) {
367 if (I2 == MBB2->begin() && I1 != MBB1->begin()) {
369 while (I1->isDebugValue()) {
370 if (I1 == MBB1->begin())
379 void BranchFolder::MaintainLiveIns(MachineBasicBlock *CurMBB,
380 MachineBasicBlock *NewMBB) {
382 RS->enterBasicBlock(CurMBB);
383 if (!CurMBB->empty())
384 RS->forward(prior(CurMBB->end()));
385 BitVector RegsLiveAtExit(TRI->getNumRegs());
386 RS->getRegsUsed(RegsLiveAtExit, false);
387 for (unsigned int i = 0, e = TRI->getNumRegs(); i != e; i++)
388 if (RegsLiveAtExit[i])
389 NewMBB->addLiveIn(i);
393 /// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything
394 /// after it, replacing it with an unconditional branch to NewDest.
395 void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
396 MachineBasicBlock *NewDest) {
397 MachineBasicBlock *CurMBB = OldInst->getParent();
399 TII->ReplaceTailWithBranchTo(OldInst, NewDest);
401 // For targets that use the register scavenger, we must maintain LiveIns.
402 MaintainLiveIns(CurMBB, NewDest);
407 /// SplitMBBAt - Given a machine basic block and an iterator into it, split the
408 /// MBB so that the part before the iterator falls into the part starting at the
409 /// iterator. This returns the new MBB.
410 MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB,
411 MachineBasicBlock::iterator BBI1) {
412 if (!TII->isLegalToSplitMBBAt(CurMBB, BBI1))
415 MachineFunction &MF = *CurMBB.getParent();
417 // Create the fall-through block.
418 MachineFunction::iterator MBBI = &CurMBB;
419 MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(CurMBB.getBasicBlock());
420 CurMBB.getParent()->insert(++MBBI, NewMBB);
422 // Move all the successors of this block to the specified block.
423 NewMBB->transferSuccessors(&CurMBB);
425 // Add an edge from CurMBB to NewMBB for the fall-through.
426 CurMBB.addSuccessor(NewMBB);
428 // Splice the code over.
429 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end());
431 // For targets that use the register scavenger, we must maintain LiveIns.
432 MaintainLiveIns(&CurMBB, NewMBB);
437 /// EstimateRuntime - Make a rough estimate for how long it will take to run
438 /// the specified code.
439 static unsigned EstimateRuntime(MachineBasicBlock::iterator I,
440 MachineBasicBlock::iterator E) {
442 for (; I != E; ++I) {
443 if (I->isDebugValue())
447 else if (I->mayLoad() || I->mayStore())
455 // CurMBB needs to add an unconditional branch to SuccMBB (we removed these
456 // branches temporarily for tail merging). In the case where CurMBB ends
457 // with a conditional branch to the next block, optimize by reversing the
458 // test and conditionally branching to SuccMBB instead.
459 static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB,
460 const TargetInstrInfo *TII) {
461 MachineFunction *MF = CurMBB->getParent();
462 MachineFunction::iterator I = llvm::next(MachineFunction::iterator(CurMBB));
463 MachineBasicBlock *TBB = 0, *FBB = 0;
464 SmallVector<MachineOperand, 4> Cond;
465 DebugLoc dl; // FIXME: this is nowhere
466 if (I != MF->end() &&
467 !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond, true)) {
468 MachineBasicBlock *NextBB = I;
469 if (TBB == NextBB && !Cond.empty() && !FBB) {
470 if (!TII->ReverseBranchCondition(Cond)) {
471 TII->RemoveBranch(*CurMBB);
472 TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond, dl);
477 TII->InsertBranch(*CurMBB, SuccBB, NULL,
478 SmallVector<MachineOperand, 0>(), dl);
482 BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const {
483 if (getHash() < o.getHash())
485 else if (getHash() > o.getHash())
487 else if (getBlock()->getNumber() < o.getBlock()->getNumber())
489 else if (getBlock()->getNumber() > o.getBlock()->getNumber())
492 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing
493 // an object with itself.
494 #ifndef _GLIBCXX_DEBUG
495 llvm_unreachable("Predecessor appears twice");
502 /// CountTerminators - Count the number of terminators in the given
503 /// block and set I to the position of the first non-terminator, if there
504 /// is one, or MBB->end() otherwise.
505 static unsigned CountTerminators(MachineBasicBlock *MBB,
506 MachineBasicBlock::iterator &I) {
508 unsigned NumTerms = 0;
510 if (I == MBB->begin()) {
515 if (!I->isTerminator()) break;
521 /// ProfitableToMerge - Check if two machine basic blocks have a common tail
522 /// and decide if it would be profitable to merge those tails. Return the
523 /// length of the common tail and iterators to the first common instruction
525 static bool ProfitableToMerge(MachineBasicBlock *MBB1,
526 MachineBasicBlock *MBB2,
527 unsigned minCommonTailLength,
528 unsigned &CommonTailLen,
529 MachineBasicBlock::iterator &I1,
530 MachineBasicBlock::iterator &I2,
531 MachineBasicBlock *SuccBB,
532 MachineBasicBlock *PredBB) {
533 CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2);
534 if (CommonTailLen == 0)
536 DEBUG(dbgs() << "Common tail length of BB#" << MBB1->getNumber()
537 << " and BB#" << MBB2->getNumber() << " is " << CommonTailLen
540 // It's almost always profitable to merge any number of non-terminator
541 // instructions with the block that falls through into the common successor.
542 if (MBB1 == PredBB || MBB2 == PredBB) {
543 MachineBasicBlock::iterator I;
544 unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I);
545 if (CommonTailLen > NumTerms)
549 // If one of the blocks can be completely merged and happens to be in
550 // a position where the other could fall through into it, merge any number
551 // of instructions, because it can be done without a branch.
552 // TODO: If the blocks are not adjacent, move one of them so that they are?
553 if (MBB1->isLayoutSuccessor(MBB2) && I2 == MBB2->begin())
555 if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin())
558 // If both blocks have an unconditional branch temporarily stripped out,
559 // count that as an additional common instruction for the following
561 unsigned EffectiveTailLen = CommonTailLen;
562 if (SuccBB && MBB1 != PredBB && MBB2 != PredBB &&
563 !MBB1->back().isBarrier() &&
564 !MBB2->back().isBarrier())
567 // Check if the common tail is long enough to be worthwhile.
568 if (EffectiveTailLen >= minCommonTailLength)
571 // If we are optimizing for code size, 2 instructions in common is enough if
572 // we don't have to split a block. At worst we will be introducing 1 new
573 // branch instruction, which is likely to be smaller than the 2
574 // instructions that would be deleted in the merge.
575 MachineFunction *MF = MBB1->getParent();
576 if (EffectiveTailLen >= 2 &&
577 MF->getFunction()->hasFnAttr(Attribute::OptimizeForSize) &&
578 (I1 == MBB1->begin() || I2 == MBB2->begin()))
584 /// ComputeSameTails - Look through all the blocks in MergePotentials that have
585 /// hash CurHash (guaranteed to match the last element). Build the vector
586 /// SameTails of all those that have the (same) largest number of instructions
587 /// in common of any pair of these blocks. SameTails entries contain an
588 /// iterator into MergePotentials (from which the MachineBasicBlock can be
589 /// found) and a MachineBasicBlock::iterator into that MBB indicating the
590 /// instruction where the matching code sequence begins.
591 /// Order of elements in SameTails is the reverse of the order in which
592 /// those blocks appear in MergePotentials (where they are not necessarily
594 unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
595 unsigned minCommonTailLength,
596 MachineBasicBlock *SuccBB,
597 MachineBasicBlock *PredBB) {
598 unsigned maxCommonTailLength = 0U;
600 MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
601 MPIterator HighestMPIter = prior(MergePotentials.end());
602 for (MPIterator CurMPIter = prior(MergePotentials.end()),
603 B = MergePotentials.begin();
604 CurMPIter != B && CurMPIter->getHash() == CurHash;
606 for (MPIterator I = prior(CurMPIter); I->getHash() == CurHash ; --I) {
607 unsigned CommonTailLen;
608 if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(),
610 CommonTailLen, TrialBBI1, TrialBBI2,
612 if (CommonTailLen > maxCommonTailLength) {
614 maxCommonTailLength = CommonTailLen;
615 HighestMPIter = CurMPIter;
616 SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1));
618 if (HighestMPIter == CurMPIter &&
619 CommonTailLen == maxCommonTailLength)
620 SameTails.push_back(SameTailElt(I, TrialBBI2));
626 return maxCommonTailLength;
629 /// RemoveBlocksWithHash - Remove all blocks with hash CurHash from
630 /// MergePotentials, restoring branches at ends of blocks as appropriate.
631 void BranchFolder::RemoveBlocksWithHash(unsigned CurHash,
632 MachineBasicBlock *SuccBB,
633 MachineBasicBlock *PredBB) {
634 MPIterator CurMPIter, B;
635 for (CurMPIter = prior(MergePotentials.end()), B = MergePotentials.begin();
636 CurMPIter->getHash() == CurHash;
638 // Put the unconditional branch back, if we need one.
639 MachineBasicBlock *CurMBB = CurMPIter->getBlock();
640 if (SuccBB && CurMBB != PredBB)
641 FixTail(CurMBB, SuccBB, TII);
645 if (CurMPIter->getHash() != CurHash)
647 MergePotentials.erase(CurMPIter, MergePotentials.end());
650 /// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist
651 /// only of the common tail. Create a block that does by splitting one.
652 bool BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
653 unsigned maxCommonTailLength,
654 unsigned &commonTailIndex) {
656 unsigned TimeEstimate = ~0U;
657 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
658 // Use PredBB if possible; that doesn't require a new branch.
659 if (SameTails[i].getBlock() == PredBB) {
663 // Otherwise, make a (fairly bogus) choice based on estimate of
664 // how long it will take the various blocks to execute.
665 unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(),
666 SameTails[i].getTailStartPos());
667 if (t <= TimeEstimate) {
673 MachineBasicBlock::iterator BBI =
674 SameTails[commonTailIndex].getTailStartPos();
675 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
677 // If the common tail includes any debug info we will take it pretty
678 // randomly from one of the inputs. Might be better to remove it?
679 DEBUG(dbgs() << "\nSplitting BB#" << MBB->getNumber() << ", size "
680 << maxCommonTailLength);
682 MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI);
684 DEBUG(dbgs() << "... failed!");
688 SameTails[commonTailIndex].setBlock(newMBB);
689 SameTails[commonTailIndex].setTailStartPos(newMBB->begin());
691 // If we split PredBB, newMBB is the new predecessor.
698 // See if any of the blocks in MergePotentials (which all have a common single
699 // successor, or all have no successor) can be tail-merged. If there is a
700 // successor, any blocks in MergePotentials that are not tail-merged and
701 // are not immediately before Succ must have an unconditional branch to
702 // Succ added (but the predecessor/successor lists need no adjustment).
703 // The lone predecessor of Succ that falls through into Succ,
704 // if any, is given in PredBB.
706 bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
707 MachineBasicBlock *PredBB) {
708 bool MadeChange = false;
710 // Except for the special cases below, tail-merge if there are at least
711 // this many instructions in common.
712 unsigned minCommonTailLength = TailMergeSize;
714 DEBUG(dbgs() << "\nTryTailMergeBlocks: ";
715 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
716 dbgs() << "BB#" << MergePotentials[i].getBlock()->getNumber()
717 << (i == e-1 ? "" : ", ");
720 dbgs() << " with successor BB#" << SuccBB->getNumber() << '\n';
722 dbgs() << " which has fall-through from BB#"
723 << PredBB->getNumber() << "\n";
725 dbgs() << "Looking for common tails of at least "
726 << minCommonTailLength << " instruction"
727 << (minCommonTailLength == 1 ? "" : "s") << '\n';
730 // Sort by hash value so that blocks with identical end sequences sort
732 std::stable_sort(MergePotentials.begin(), MergePotentials.end());
734 // Walk through equivalence sets looking for actual exact matches.
735 while (MergePotentials.size() > 1) {
736 unsigned CurHash = MergePotentials.back().getHash();
738 // Build SameTails, identifying the set of blocks with this hash code
739 // and with the maximum number of instructions in common.
740 unsigned maxCommonTailLength = ComputeSameTails(CurHash,
744 // If we didn't find any pair that has at least minCommonTailLength
745 // instructions in common, remove all blocks with this hash code and retry.
746 if (SameTails.empty()) {
747 RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
751 // If one of the blocks is the entire common tail (and not the entry
752 // block, which we can't jump to), we can treat all blocks with this same
753 // tail at once. Use PredBB if that is one of the possibilities, as that
754 // will not introduce any extra branches.
755 MachineBasicBlock *EntryBB = MergePotentials.begin()->getBlock()->
756 getParent()->begin();
757 unsigned commonTailIndex = SameTails.size();
758 // If there are two blocks, check to see if one can be made to fall through
760 if (SameTails.size() == 2 &&
761 SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) &&
762 SameTails[1].tailIsWholeBlock())
764 else if (SameTails.size() == 2 &&
765 SameTails[1].getBlock()->isLayoutSuccessor(
766 SameTails[0].getBlock()) &&
767 SameTails[0].tailIsWholeBlock())
770 // Otherwise just pick one, favoring the fall-through predecessor if
772 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
773 MachineBasicBlock *MBB = SameTails[i].getBlock();
774 if (MBB == EntryBB && SameTails[i].tailIsWholeBlock())
780 if (SameTails[i].tailIsWholeBlock())
785 if (commonTailIndex == SameTails.size() ||
786 (SameTails[commonTailIndex].getBlock() == PredBB &&
787 !SameTails[commonTailIndex].tailIsWholeBlock())) {
788 // None of the blocks consist entirely of the common tail.
789 // Split a block so that one does.
790 if (!CreateCommonTailOnlyBlock(PredBB,
791 maxCommonTailLength, commonTailIndex)) {
792 RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
797 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
798 // MBB is common tail. Adjust all other BB's to jump to this one.
799 // Traversal must be forwards so erases work.
800 DEBUG(dbgs() << "\nUsing common tail in BB#" << MBB->getNumber()
802 for (unsigned int i=0, e = SameTails.size(); i != e; ++i) {
803 if (commonTailIndex == i)
805 DEBUG(dbgs() << "BB#" << SameTails[i].getBlock()->getNumber()
806 << (i == e-1 ? "" : ", "));
807 // Hack the end off BB i, making it jump to BB commonTailIndex instead.
808 ReplaceTailWithBranchTo(SameTails[i].getTailStartPos(), MBB);
809 // BB i is no longer a predecessor of SuccBB; remove it from the worklist.
810 MergePotentials.erase(SameTails[i].getMPIter());
812 DEBUG(dbgs() << "\n");
813 // We leave commonTailIndex in the worklist in case there are other blocks
814 // that match it with a smaller number of instructions.
820 bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
821 bool MadeChange = false;
822 if (!EnableTailMerge) return MadeChange;
824 // First find blocks with no successors.
825 MergePotentials.clear();
826 for (MachineFunction::iterator I = MF.begin(), E = MF.end();
827 I != E && MergePotentials.size() < TailMergeThreshold; ++I) {
828 if (TriedMerging.count(I))
831 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(I), I));
834 // If this is a large problem, avoid visiting the same basic blocks
836 if (MergePotentials.size() == TailMergeThreshold)
837 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
838 TriedMerging.insert(MergePotentials[i].getBlock());
840 // See if we can do any tail merging on those.
841 if (MergePotentials.size() >= 2)
842 MadeChange |= TryTailMergeBlocks(NULL, NULL);
844 // Look at blocks (IBB) with multiple predecessors (PBB).
845 // We change each predecessor to a canonical form, by
846 // (1) temporarily removing any unconditional branch from the predecessor
848 // (2) alter conditional branches so they branch to the other block
849 // not IBB; this may require adding back an unconditional branch to IBB
850 // later, where there wasn't one coming in. E.g.
852 // fallthrough to QBB
855 // with a conceptual B to IBB after that, which never actually exists.
856 // With those changes, we see whether the predecessors' tails match,
857 // and merge them if so. We change things out of canonical form and
858 // back to the way they were later in the process. (OptimizeBranches
859 // would undo some of this, but we can't use it, because we'd get into
860 // a compile-time infinite loop repeatedly doing and undoing the same
863 for (MachineFunction::iterator I = llvm::next(MF.begin()), E = MF.end();
865 if (I->pred_size() < 2) continue;
866 SmallPtrSet<MachineBasicBlock *, 8> UniquePreds;
867 MachineBasicBlock *IBB = I;
868 MachineBasicBlock *PredBB = prior(I);
869 MergePotentials.clear();
870 for (MachineBasicBlock::pred_iterator P = I->pred_begin(),
872 P != E2 && MergePotentials.size() < TailMergeThreshold; ++P) {
873 MachineBasicBlock *PBB = *P;
874 if (TriedMerging.count(PBB))
877 // Skip blocks that loop to themselves, can't tail merge these.
881 // Visit each predecessor only once.
882 if (!UniquePreds.insert(PBB))
885 // Skip blocks which may jump to a landing pad. Can't tail merge these.
886 if (PBB->getLandingPadSuccessor())
889 MachineBasicBlock *TBB = 0, *FBB = 0;
890 SmallVector<MachineOperand, 4> Cond;
891 if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond, true)) {
892 // Failing case: IBB is the target of a cbr, and we cannot reverse the
894 SmallVector<MachineOperand, 4> NewCond(Cond);
895 if (!Cond.empty() && TBB == IBB) {
896 if (TII->ReverseBranchCondition(NewCond))
898 // This is the QBB case described above
900 FBB = llvm::next(MachineFunction::iterator(PBB));
903 // Failing case: the only way IBB can be reached from PBB is via
904 // exception handling. Happens for landing pads. Would be nice to have
905 // a bit in the edge so we didn't have to do all this.
906 if (IBB->isLandingPad()) {
907 MachineFunction::iterator IP = PBB; IP++;
908 MachineBasicBlock *PredNextBB = NULL;
912 if (IBB != PredNextBB) // fallthrough
915 if (TBB != IBB && FBB != IBB) // cbr then ubr
917 } else if (Cond.empty()) {
918 if (TBB != IBB) // ubr
921 if (TBB != IBB && IBB != PredNextBB) // cbr
926 // Remove the unconditional branch at the end, if any.
927 if (TBB && (Cond.empty() || FBB)) {
928 DebugLoc dl; // FIXME: this is nowhere
929 TII->RemoveBranch(*PBB);
931 // reinsert conditional branch only, for now
932 TII->InsertBranch(*PBB, (TBB == IBB) ? FBB : TBB, 0, NewCond, dl);
935 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(PBB), *P));
939 // If this is a large problem, avoid visiting the same basic blocks multiple
941 if (MergePotentials.size() == TailMergeThreshold)
942 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
943 TriedMerging.insert(MergePotentials[i].getBlock());
945 if (MergePotentials.size() >= 2)
946 MadeChange |= TryTailMergeBlocks(IBB, PredBB);
948 // Reinsert an unconditional branch if needed. The 1 below can occur as a
949 // result of removing blocks in TryTailMergeBlocks.
950 PredBB = prior(I); // this may have been changed in TryTailMergeBlocks
951 if (MergePotentials.size() == 1 &&
952 MergePotentials.begin()->getBlock() != PredBB)
953 FixTail(MergePotentials.begin()->getBlock(), IBB, TII);
959 //===----------------------------------------------------------------------===//
960 // Branch Optimization
961 //===----------------------------------------------------------------------===//
963 bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
964 bool MadeChange = false;
966 // Make sure blocks are numbered in order
969 for (MachineFunction::iterator I = llvm::next(MF.begin()), E = MF.end();
971 MachineBasicBlock *MBB = I++;
972 MadeChange |= OptimizeBlock(MBB);
974 // If it is dead, remove it.
975 if (MBB->pred_empty()) {
976 RemoveDeadBlock(MBB);
984 // Blocks should be considered empty if they contain only debug info;
985 // else the debug info would affect codegen.
986 static bool IsEmptyBlock(MachineBasicBlock *MBB) {
989 for (MachineBasicBlock::iterator MBBI = MBB->begin(), MBBE = MBB->end();
990 MBBI!=MBBE; ++MBBI) {
991 if (!MBBI->isDebugValue())
997 // Blocks with only debug info and branches should be considered the same
998 // as blocks with only branches.
999 static bool IsBranchOnlyBlock(MachineBasicBlock *MBB) {
1000 MachineBasicBlock::iterator MBBI, MBBE;
1001 for (MBBI = MBB->begin(), MBBE = MBB->end(); MBBI!=MBBE; ++MBBI) {
1002 if (!MBBI->isDebugValue())
1005 return (MBBI->isBranch());
1008 /// IsBetterFallthrough - Return true if it would be clearly better to
1009 /// fall-through to MBB1 than to fall through into MBB2. This has to return
1010 /// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will
1011 /// result in infinite loops.
1012 static bool IsBetterFallthrough(MachineBasicBlock *MBB1,
1013 MachineBasicBlock *MBB2) {
1014 // Right now, we use a simple heuristic. If MBB2 ends with a call, and
1015 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to
1016 // optimize branches that branch to either a return block or an assert block
1017 // into a fallthrough to the return.
1018 if (IsEmptyBlock(MBB1) || IsEmptyBlock(MBB2)) return false;
1020 // If there is a clear successor ordering we make sure that one block
1021 // will fall through to the next
1022 if (MBB1->isSuccessor(MBB2)) return true;
1023 if (MBB2->isSuccessor(MBB1)) return false;
1025 // Neither block consists entirely of debug info (per IsEmptyBlock check),
1026 // so we needn't test for falling off the beginning here.
1027 MachineBasicBlock::iterator MBB1I = --MBB1->end();
1028 while (MBB1I->isDebugValue())
1030 MachineBasicBlock::iterator MBB2I = --MBB2->end();
1031 while (MBB2I->isDebugValue())
1033 return MBB2I->isCall() && !MBB1I->isCall();
1036 /// getBranchDebugLoc - Find and return, if any, the DebugLoc of the branch
1037 /// instructions on the block. Always use the DebugLoc of the first
1038 /// branching instruction found unless its absent, in which case use the
1039 /// DebugLoc of the second if present.
1040 static DebugLoc getBranchDebugLoc(MachineBasicBlock &MBB) {
1041 MachineBasicBlock::iterator I = MBB.end();
1042 if (I == MBB.begin())
1045 while (I->isDebugValue() && I != MBB.begin())
1048 return I->getDebugLoc();
1052 /// OptimizeBlock - Analyze and optimize control flow related to the specified
1053 /// block. This is never called on the entry block.
1054 bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
1055 bool MadeChange = false;
1056 MachineFunction &MF = *MBB->getParent();
1059 MachineFunction::iterator FallThrough = MBB;
1062 // If this block is empty, make everyone use its fall-through, not the block
1063 // explicitly. Landing pads should not do this since the landing-pad table
1064 // points to this block. Blocks with their addresses taken shouldn't be
1066 if (IsEmptyBlock(MBB) && !MBB->isLandingPad() && !MBB->hasAddressTaken()) {
1067 // Dead block? Leave for cleanup later.
1068 if (MBB->pred_empty()) return MadeChange;
1070 if (FallThrough == MF.end()) {
1071 // TODO: Simplify preds to not branch here if possible!
1073 // Rewrite all predecessors of the old block to go to the fallthrough
1075 while (!MBB->pred_empty()) {
1076 MachineBasicBlock *Pred = *(MBB->pred_end()-1);
1077 Pred->ReplaceUsesOfBlockWith(MBB, FallThrough);
1079 // If MBB was the target of a jump table, update jump tables to go to the
1080 // fallthrough instead.
1081 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
1082 MJTI->ReplaceMBBInJumpTables(MBB, FallThrough);
1088 // Check to see if we can simplify the terminator of the block before this
1090 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB));
1092 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
1093 SmallVector<MachineOperand, 4> PriorCond;
1094 bool PriorUnAnalyzable =
1095 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
1096 if (!PriorUnAnalyzable) {
1097 // If the CFG for the prior block has extra edges, remove them.
1098 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB,
1099 !PriorCond.empty());
1101 // If the previous branch is conditional and both conditions go to the same
1102 // destination, remove the branch, replacing it with an unconditional one or
1104 if (PriorTBB && PriorTBB == PriorFBB) {
1105 DebugLoc dl = getBranchDebugLoc(PrevBB);
1106 TII->RemoveBranch(PrevBB);
1108 if (PriorTBB != MBB)
1109 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond, dl);
1112 goto ReoptimizeBlock;
1115 // If the previous block unconditionally falls through to this block and
1116 // this block has no other predecessors, move the contents of this block
1117 // into the prior block. This doesn't usually happen when SimplifyCFG
1118 // has been used, but it can happen if tail merging splits a fall-through
1119 // predecessor of a block.
1120 // This has to check PrevBB->succ_size() because EH edges are ignored by
1122 if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 &&
1123 PrevBB.succ_size() == 1 &&
1124 !MBB->hasAddressTaken() && !MBB->isLandingPad()) {
1125 DEBUG(dbgs() << "\nMerging into block: " << PrevBB
1126 << "From MBB: " << *MBB);
1127 // Remove redundant DBG_VALUEs first.
1128 if (PrevBB.begin() != PrevBB.end()) {
1129 MachineBasicBlock::iterator PrevBBIter = PrevBB.end();
1131 MachineBasicBlock::iterator MBBIter = MBB->begin();
1132 // Check if DBG_VALUE at the end of PrevBB is identical to the
1133 // DBG_VALUE at the beginning of MBB.
1134 while (PrevBBIter != PrevBB.begin() && MBBIter != MBB->end()
1135 && PrevBBIter->isDebugValue() && MBBIter->isDebugValue()) {
1136 if (!MBBIter->isIdenticalTo(PrevBBIter))
1138 MachineInstr *DuplicateDbg = MBBIter;
1139 ++MBBIter; -- PrevBBIter;
1140 DuplicateDbg->eraseFromParent();
1143 PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end());
1144 PrevBB.removeSuccessor(PrevBB.succ_begin());
1145 assert(PrevBB.succ_empty());
1146 PrevBB.transferSuccessors(MBB);
1151 // If the previous branch *only* branches to *this* block (conditional or
1152 // not) remove the branch.
1153 if (PriorTBB == MBB && PriorFBB == 0) {
1154 TII->RemoveBranch(PrevBB);
1157 goto ReoptimizeBlock;
1160 // If the prior block branches somewhere else on the condition and here if
1161 // the condition is false, remove the uncond second branch.
1162 if (PriorFBB == MBB) {
1163 DebugLoc dl = getBranchDebugLoc(PrevBB);
1164 TII->RemoveBranch(PrevBB);
1165 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond, dl);
1168 goto ReoptimizeBlock;
1171 // If the prior block branches here on true and somewhere else on false, and
1172 // if the branch condition is reversible, reverse the branch to create a
1174 if (PriorTBB == MBB) {
1175 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1176 if (!TII->ReverseBranchCondition(NewPriorCond)) {
1177 DebugLoc dl = getBranchDebugLoc(PrevBB);
1178 TII->RemoveBranch(PrevBB);
1179 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond, dl);
1182 goto ReoptimizeBlock;
1186 // If this block has no successors (e.g. it is a return block or ends with
1187 // a call to a no-return function like abort or __cxa_throw) and if the pred
1188 // falls through into this block, and if it would otherwise fall through
1189 // into the block after this, move this block to the end of the function.
1191 // We consider it more likely that execution will stay in the function (e.g.
1192 // due to loops) than it is to exit it. This asserts in loops etc, moving
1193 // the assert condition out of the loop body.
1194 if (MBB->succ_empty() && !PriorCond.empty() && PriorFBB == 0 &&
1195 MachineFunction::iterator(PriorTBB) == FallThrough &&
1196 !MBB->canFallThrough()) {
1197 bool DoTransform = true;
1199 // We have to be careful that the succs of PredBB aren't both no-successor
1200 // blocks. If neither have successors and if PredBB is the second from
1201 // last block in the function, we'd just keep swapping the two blocks for
1202 // last. Only do the swap if one is clearly better to fall through than
1204 if (FallThrough == --MF.end() &&
1205 !IsBetterFallthrough(PriorTBB, MBB))
1206 DoTransform = false;
1209 // Reverse the branch so we will fall through on the previous true cond.
1210 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1211 if (!TII->ReverseBranchCondition(NewPriorCond)) {
1212 DEBUG(dbgs() << "\nMoving MBB: " << *MBB
1213 << "To make fallthrough to: " << *PriorTBB << "\n");
1215 DebugLoc dl = getBranchDebugLoc(PrevBB);
1216 TII->RemoveBranch(PrevBB);
1217 TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond, dl);
1219 // Move this block to the end of the function.
1220 MBB->moveAfter(--MF.end());
1229 // Analyze the branch in the current block.
1230 MachineBasicBlock *CurTBB = 0, *CurFBB = 0;
1231 SmallVector<MachineOperand, 4> CurCond;
1232 bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true);
1233 if (!CurUnAnalyzable) {
1234 // If the CFG for the prior block has extra edges, remove them.
1235 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty());
1237 // If this is a two-way branch, and the FBB branches to this block, reverse
1238 // the condition so the single-basic-block loop is faster. Instead of:
1239 // Loop: xxx; jcc Out; jmp Loop
1241 // Loop: xxx; jncc Loop; jmp Out
1242 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) {
1243 SmallVector<MachineOperand, 4> NewCond(CurCond);
1244 if (!TII->ReverseBranchCondition(NewCond)) {
1245 DebugLoc dl = getBranchDebugLoc(*MBB);
1246 TII->RemoveBranch(*MBB);
1247 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond, dl);
1250 goto ReoptimizeBlock;
1254 // If this branch is the only thing in its block, see if we can forward
1255 // other blocks across it.
1256 if (CurTBB && CurCond.empty() && CurFBB == 0 &&
1257 IsBranchOnlyBlock(MBB) && CurTBB != MBB &&
1258 !MBB->hasAddressTaken()) {
1259 DebugLoc dl = getBranchDebugLoc(*MBB);
1260 // This block may contain just an unconditional branch. Because there can
1261 // be 'non-branch terminators' in the block, try removing the branch and
1262 // then seeing if the block is empty.
1263 TII->RemoveBranch(*MBB);
1264 // If the only things remaining in the block are debug info, remove these
1265 // as well, so this will behave the same as an empty block in non-debug
1267 if (!MBB->empty()) {
1268 bool NonDebugInfoFound = false;
1269 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
1271 if (!I->isDebugValue()) {
1272 NonDebugInfoFound = true;
1276 if (!NonDebugInfoFound)
1277 // Make the block empty, losing the debug info (we could probably
1278 // improve this in some cases.)
1279 MBB->erase(MBB->begin(), MBB->end());
1281 // If this block is just an unconditional branch to CurTBB, we can
1282 // usually completely eliminate the block. The only case we cannot
1283 // completely eliminate the block is when the block before this one
1284 // falls through into MBB and we can't understand the prior block's branch
1287 bool PredHasNoFallThrough = !PrevBB.canFallThrough();
1288 if (PredHasNoFallThrough || !PriorUnAnalyzable ||
1289 !PrevBB.isSuccessor(MBB)) {
1290 // If the prior block falls through into us, turn it into an
1291 // explicit branch to us to make updates simpler.
1292 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) &&
1293 PriorTBB != MBB && PriorFBB != MBB) {
1294 if (PriorTBB == 0) {
1295 assert(PriorCond.empty() && PriorFBB == 0 &&
1296 "Bad branch analysis");
1299 assert(PriorFBB == 0 && "Machine CFG out of date!");
1302 DebugLoc pdl = getBranchDebugLoc(PrevBB);
1303 TII->RemoveBranch(PrevBB);
1304 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, pdl);
1307 // Iterate through all the predecessors, revectoring each in-turn.
1309 bool DidChange = false;
1310 bool HasBranchToSelf = false;
1311 while(PI != MBB->pred_size()) {
1312 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI);
1314 // If this block has an uncond branch to itself, leave it.
1316 HasBranchToSelf = true;
1319 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB);
1320 // If this change resulted in PMBB ending in a conditional
1321 // branch where both conditions go to the same destination,
1322 // change this to an unconditional branch (and fix the CFG).
1323 MachineBasicBlock *NewCurTBB = 0, *NewCurFBB = 0;
1324 SmallVector<MachineOperand, 4> NewCurCond;
1325 bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB,
1326 NewCurFBB, NewCurCond, true);
1327 if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) {
1328 DebugLoc pdl = getBranchDebugLoc(*PMBB);
1329 TII->RemoveBranch(*PMBB);
1331 TII->InsertBranch(*PMBB, NewCurTBB, 0, NewCurCond, pdl);
1334 PMBB->CorrectExtraCFGEdges(NewCurTBB, 0, false);
1339 // Change any jumptables to go to the new MBB.
1340 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
1341 MJTI->ReplaceMBBInJumpTables(MBB, CurTBB);
1345 if (!HasBranchToSelf) return MadeChange;
1350 // Add the branch back if the block is more than just an uncond branch.
1351 TII->InsertBranch(*MBB, CurTBB, 0, CurCond, dl);
1355 // If the prior block doesn't fall through into this block, and if this
1356 // block doesn't fall through into some other block, see if we can find a
1357 // place to move this block where a fall-through will happen.
1358 if (!PrevBB.canFallThrough()) {
1360 // Now we know that there was no fall-through into this block, check to
1361 // see if it has a fall-through into its successor.
1362 bool CurFallsThru = MBB->canFallThrough();
1364 if (!MBB->isLandingPad()) {
1365 // Check all the predecessors of this block. If one of them has no fall
1366 // throughs, move this block right after it.
1367 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
1368 E = MBB->pred_end(); PI != E; ++PI) {
1369 // Analyze the branch at the end of the pred.
1370 MachineBasicBlock *PredBB = *PI;
1371 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough;
1372 MachineBasicBlock *PredTBB = 0, *PredFBB = 0;
1373 SmallVector<MachineOperand, 4> PredCond;
1374 if (PredBB != MBB && !PredBB->canFallThrough() &&
1375 !TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true)
1376 && (!CurFallsThru || !CurTBB || !CurFBB)
1377 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
1378 // If the current block doesn't fall through, just move it.
1379 // If the current block can fall through and does not end with a
1380 // conditional branch, we need to append an unconditional jump to
1381 // the (current) next block. To avoid a possible compile-time
1382 // infinite loop, move blocks only backward in this case.
1383 // Also, if there are already 2 branches here, we cannot add a third;
1384 // this means we have the case
1389 MachineBasicBlock *NextBB = llvm::next(MachineFunction::iterator(MBB));
1391 TII->InsertBranch(*MBB, NextBB, 0, CurCond, DebugLoc());
1393 MBB->moveAfter(PredBB);
1395 goto ReoptimizeBlock;
1400 if (!CurFallsThru) {
1401 // Check all successors to see if we can move this block before it.
1402 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
1403 E = MBB->succ_end(); SI != E; ++SI) {
1404 // Analyze the branch at the end of the block before the succ.
1405 MachineBasicBlock *SuccBB = *SI;
1406 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev;
1408 // If this block doesn't already fall-through to that successor, and if
1409 // the succ doesn't already have a block that can fall through into it,
1410 // and if the successor isn't an EH destination, we can arrange for the
1411 // fallthrough to happen.
1412 if (SuccBB != MBB && &*SuccPrev != MBB &&
1413 !SuccPrev->canFallThrough() && !CurUnAnalyzable &&
1414 !SuccBB->isLandingPad()) {
1415 MBB->moveBefore(SuccBB);
1417 goto ReoptimizeBlock;
1421 // Okay, there is no really great place to put this block. If, however,
1422 // the block before this one would be a fall-through if this block were
1423 // removed, move this block to the end of the function.
1424 MachineBasicBlock *PrevTBB = 0, *PrevFBB = 0;
1425 SmallVector<MachineOperand, 4> PrevCond;
1426 if (FallThrough != MF.end() &&
1427 !TII->AnalyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) &&
1428 PrevBB.isSuccessor(FallThrough)) {
1429 MBB->moveAfter(--MF.end());
1439 //===----------------------------------------------------------------------===//
1440 // Hoist Common Code
1441 //===----------------------------------------------------------------------===//
1443 /// HoistCommonCode - Hoist common instruction sequences at the start of basic
1444 /// blocks to their common predecessor.
1445 bool BranchFolder::HoistCommonCode(MachineFunction &MF) {
1446 bool MadeChange = false;
1447 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ) {
1448 MachineBasicBlock *MBB = I++;
1449 MadeChange |= HoistCommonCodeInSuccs(MBB);
1455 /// findFalseBlock - BB has a fallthrough. Find its 'false' successor given
1456 /// its 'true' successor.
1457 static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB,
1458 MachineBasicBlock *TrueBB) {
1459 for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(),
1460 E = BB->succ_end(); SI != E; ++SI) {
1461 MachineBasicBlock *SuccBB = *SI;
1462 if (SuccBB != TrueBB)
1468 /// findHoistingInsertPosAndDeps - Find the location to move common instructions
1469 /// in successors to. The location is usually just before the terminator,
1470 /// however if the terminator is a conditional branch and its previous
1471 /// instruction is the flag setting instruction, the previous instruction is
1472 /// the preferred location. This function also gathers uses and defs of the
1473 /// instructions from the insertion point to the end of the block. The data is
1474 /// used by HoistCommonCodeInSuccs to ensure safety.
1476 MachineBasicBlock::iterator findHoistingInsertPosAndDeps(MachineBasicBlock *MBB,
1477 const TargetInstrInfo *TII,
1478 const TargetRegisterInfo *TRI,
1479 SmallSet<unsigned,4> &Uses,
1480 SmallSet<unsigned,4> &Defs) {
1481 MachineBasicBlock::iterator Loc = MBB->getFirstTerminator();
1482 if (!TII->isUnpredicatedTerminator(Loc))
1485 for (unsigned i = 0, e = Loc->getNumOperands(); i != e; ++i) {
1486 const MachineOperand &MO = Loc->getOperand(i);
1489 unsigned Reg = MO.getReg();
1493 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
1495 } else if (!MO.isDead())
1496 // Don't try to hoist code in the rare case the terminator defines a
1497 // register that is later used.
1503 if (Loc == MBB->begin())
1506 // The terminator is probably a conditional branch, try not to separate the
1507 // branch from condition setting instruction.
1508 MachineBasicBlock::iterator PI = Loc;
1510 while (PI != MBB->begin() && Loc->isDebugValue())
1514 for (unsigned i = 0, e = PI->getNumOperands(); !IsDef && i != e; ++i) {
1515 const MachineOperand &MO = PI->getOperand(i);
1516 // If PI has a regmask operand, it is probably a call. Separate away.
1519 if (!MO.isReg() || MO.isUse())
1521 unsigned Reg = MO.getReg();
1524 if (Uses.count(Reg))
1528 // The condition setting instruction is not just before the conditional
1532 // Be conservative, don't insert instruction above something that may have
1533 // side-effects. And since it's potentially bad to separate flag setting
1534 // instruction from the conditional branch, just abort the optimization
1536 // Also avoid moving code above predicated instruction since it's hard to
1537 // reason about register liveness with predicated instruction.
1538 bool DontMoveAcrossStore = true;
1539 if (!PI->isSafeToMove(TII, 0, DontMoveAcrossStore) ||
1540 TII->isPredicated(PI))
1544 // Find out what registers are live. Note this routine is ignoring other live
1545 // registers which are only used by instructions in successor blocks.
1546 for (unsigned i = 0, e = PI->getNumOperands(); i != e; ++i) {
1547 const MachineOperand &MO = PI->getOperand(i);
1550 unsigned Reg = MO.getReg();
1554 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
1557 if (Uses.count(Reg)) {
1559 for (MCSubRegIterator SubRegs(Reg, TRI); SubRegs.isValid(); ++SubRegs)
1560 Uses.erase(*SubRegs); // Use sub-registers to be conservative
1562 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
1570 /// HoistCommonCodeInSuccs - If the successors of MBB has common instruction
1571 /// sequence at the start of the function, move the instructions before MBB
1572 /// terminator if it's legal.
1573 bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
1574 MachineBasicBlock *TBB = 0, *FBB = 0;
1575 SmallVector<MachineOperand, 4> Cond;
1576 if (TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true) || !TBB || Cond.empty())
1579 if (!FBB) FBB = findFalseBlock(MBB, TBB);
1581 // Malformed bcc? True and false blocks are the same?
1584 // Restrict the optimization to cases where MBB is the only predecessor,
1585 // it is an obvious win.
1586 if (TBB->pred_size() > 1 || FBB->pred_size() > 1)
1589 // Find a suitable position to hoist the common instructions to. Also figure
1590 // out which registers are used or defined by instructions from the insertion
1591 // point to the end of the block.
1592 SmallSet<unsigned, 4> Uses, Defs;
1593 MachineBasicBlock::iterator Loc =
1594 findHoistingInsertPosAndDeps(MBB, TII, TRI, Uses, Defs);
1595 if (Loc == MBB->end())
1598 bool HasDups = false;
1599 SmallVector<unsigned, 4> LocalDefs;
1600 SmallSet<unsigned, 4> LocalDefsSet;
1601 MachineBasicBlock::iterator TIB = TBB->begin();
1602 MachineBasicBlock::iterator FIB = FBB->begin();
1603 MachineBasicBlock::iterator TIE = TBB->end();
1604 MachineBasicBlock::iterator FIE = FBB->end();
1605 while (TIB != TIE && FIB != FIE) {
1606 // Skip dbg_value instructions. These do not count.
1607 if (TIB->isDebugValue()) {
1608 while (TIB != TIE && TIB->isDebugValue())
1613 if (FIB->isDebugValue()) {
1614 while (FIB != FIE && FIB->isDebugValue())
1619 if (!TIB->isIdenticalTo(FIB, MachineInstr::CheckKillDead))
1622 if (TII->isPredicated(TIB))
1623 // Hard to reason about register liveness with predicated instruction.
1627 for (unsigned i = 0, e = TIB->getNumOperands(); i != e; ++i) {
1628 MachineOperand &MO = TIB->getOperand(i);
1629 // Don't attempt to hoist instructions with register masks.
1630 if (MO.isRegMask()) {
1636 unsigned Reg = MO.getReg();
1640 if (Uses.count(Reg)) {
1641 // Avoid clobbering a register that's used by the instruction at
1642 // the point of insertion.
1647 if (Defs.count(Reg) && !MO.isDead()) {
1648 // Don't hoist the instruction if the def would be clobber by the
1649 // instruction at the point insertion. FIXME: This is overly
1650 // conservative. It should be possible to hoist the instructions
1651 // in BB2 in the following example:
1653 // r1, eflag = op1 r2, r3
1662 } else if (!LocalDefsSet.count(Reg)) {
1663 if (Defs.count(Reg)) {
1664 // Use is defined by the instruction at the point of insertion.
1669 if (MO.isKill() && Uses.count(Reg))
1670 // Kills a register that's read by the instruction at the point of
1671 // insertion. Remove the kill marker.
1672 MO.setIsKill(false);
1678 bool DontMoveAcrossStore = true;
1679 if (!TIB->isSafeToMove(TII, 0, DontMoveAcrossStore))
1682 // Remove kills from LocalDefsSet, these registers had short live ranges.
1683 for (unsigned i = 0, e = TIB->getNumOperands(); i != e; ++i) {
1684 MachineOperand &MO = TIB->getOperand(i);
1685 if (!MO.isReg() || !MO.isUse() || !MO.isKill())
1687 unsigned Reg = MO.getReg();
1688 if (!Reg || !LocalDefsSet.count(Reg))
1690 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
1691 LocalDefsSet.erase(*AI);
1694 // Track local defs so we can update liveins.
1695 for (unsigned i = 0, e = TIB->getNumOperands(); i != e; ++i) {
1696 MachineOperand &MO = TIB->getOperand(i);
1697 if (!MO.isReg() || !MO.isDef() || MO.isDead())
1699 unsigned Reg = MO.getReg();
1702 LocalDefs.push_back(Reg);
1703 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
1704 LocalDefsSet.insert(*AI);
1715 MBB->splice(Loc, TBB, TBB->begin(), TIB);
1716 FBB->erase(FBB->begin(), FIB);
1719 for (unsigned i = 0, e = LocalDefs.size(); i != e; ++i) {
1720 unsigned Def = LocalDefs[i];
1721 if (LocalDefsSet.count(Def)) {
1722 TBB->addLiveIn(Def);
1723 FBB->addLiveIn(Def);