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 "llvm/CodeGen/Passes.h"
21 #include "llvm/CodeGen/MachineModuleInfo.h"
22 #include "llvm/CodeGen/MachineFunctionPass.h"
23 #include "llvm/CodeGen/MachineJumpTableInfo.h"
24 #include "llvm/CodeGen/RegisterScavenging.h"
25 #include "llvm/Target/TargetInstrInfo.h"
26 #include "llvm/Target/TargetMachine.h"
27 #include "llvm/Target/TargetRegisterInfo.h"
28 #include "llvm/Support/CommandLine.h"
29 #include "llvm/Support/Debug.h"
30 #include "llvm/ADT/Statistic.h"
31 #include "llvm/ADT/STLExtras.h"
35 STATISTIC(NumDeadBlocks, "Number of dead blocks removed");
36 STATISTIC(NumBranchOpts, "Number of branches optimized");
37 STATISTIC(NumTailMerge , "Number of block tails merged");
38 static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge",
39 cl::init(cl::BOU_UNSET), cl::Hidden);
41 // Throttle for huge numbers of predecessors (compile speed problems)
43 TailMergeThreshold("tail-merge-threshold",
44 cl::desc("Max number of predecessors to consider tail merging"),
45 cl::init(100), cl::Hidden);
47 struct VISIBILITY_HIDDEN BranchFolder : public MachineFunctionPass {
49 explicit BranchFolder(bool defaultEnableTailMerge) :
50 MachineFunctionPass((intptr_t)&ID) {
51 switch (FlagEnableTailMerge) {
52 case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break;
53 case cl::BOU_TRUE: EnableTailMerge = true; break;
54 case cl::BOU_FALSE: EnableTailMerge = false; break;
58 virtual bool runOnMachineFunction(MachineFunction &MF);
59 virtual const char *getPassName() const { return "Control Flow Optimizer"; }
60 const TargetInstrInfo *TII;
61 MachineModuleInfo *MMI;
66 bool TailMergeBlocks(MachineFunction &MF);
67 bool TryMergeBlocks(MachineBasicBlock* SuccBB,
68 MachineBasicBlock* PredBB);
69 void ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
70 MachineBasicBlock *NewDest);
71 MachineBasicBlock *SplitMBBAt(MachineBasicBlock &CurMBB,
72 MachineBasicBlock::iterator BBI1);
74 std::vector<std::pair<unsigned,MachineBasicBlock*> > MergePotentials;
75 const TargetRegisterInfo *RegInfo;
78 bool OptimizeBranches(MachineFunction &MF);
79 void OptimizeBlock(MachineBasicBlock *MBB);
80 void RemoveDeadBlock(MachineBasicBlock *MBB);
82 bool CanFallThrough(MachineBasicBlock *CurBB);
83 bool CanFallThrough(MachineBasicBlock *CurBB, bool BranchUnAnalyzable,
84 MachineBasicBlock *TBB, MachineBasicBlock *FBB,
85 const std::vector<MachineOperand> &Cond);
87 char BranchFolder::ID = 0;
90 FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) {
91 return new BranchFolder(DefaultEnableTailMerge); }
93 /// RemoveDeadBlock - Remove the specified dead machine basic block from the
94 /// function, updating the CFG.
95 void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) {
96 assert(MBB->pred_empty() && "MBB must be dead!");
97 DOUT << "\nRemoving MBB: " << *MBB;
99 MachineFunction *MF = MBB->getParent();
100 // drop all successors.
101 while (!MBB->succ_empty())
102 MBB->removeSuccessor(MBB->succ_end()-1);
104 // If there is DWARF info to active, check to see if there are any LABEL
105 // records in the basic block. If so, unregister them from MachineModuleInfo.
106 if (MMI && !MBB->empty()) {
107 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end();
109 if ((unsigned)I->getOpcode() == TargetInstrInfo::LABEL) {
110 // The label ID # is always operand #0, an immediate.
111 MMI->InvalidateLabel(I->getOperand(0).getImm());
117 MF->getBasicBlockList().erase(MBB);
120 bool BranchFolder::runOnMachineFunction(MachineFunction &MF) {
121 TII = MF.getTarget().getInstrInfo();
122 if (!TII) return false;
124 // Fix CFG. The later algorithms expect it to be right.
125 bool EverMadeChange = false;
126 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) {
127 MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0;
128 std::vector<MachineOperand> Cond;
129 if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond))
130 EverMadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty());
133 RegInfo = MF.getTarget().getRegisterInfo();
134 RS = RegInfo->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL;
136 MMI = getAnalysisToUpdate<MachineModuleInfo>();
138 bool MadeChangeThisIteration = true;
139 while (MadeChangeThisIteration) {
140 MadeChangeThisIteration = false;
141 MadeChangeThisIteration |= TailMergeBlocks(MF);
142 MadeChangeThisIteration |= OptimizeBranches(MF);
143 EverMadeChange |= MadeChangeThisIteration;
146 // See if any jump tables have become mergable or dead as the code generator
148 MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
149 const std::vector<MachineJumpTableEntry> &JTs = JTI->getJumpTables();
151 // Figure out how these jump tables should be merged.
152 std::vector<unsigned> JTMapping;
153 JTMapping.reserve(JTs.size());
155 // We always keep the 0th jump table.
156 JTMapping.push_back(0);
158 // Scan the jump tables, seeing if there are any duplicates. Note that this
159 // is N^2, which should be fixed someday.
160 for (unsigned i = 1, e = JTs.size(); i != e; ++i)
161 JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs));
163 // If a jump table was merge with another one, walk the function rewriting
164 // references to jump tables to reference the new JT ID's. Keep track of
165 // whether we see a jump table idx, if not, we can delete the JT.
166 std::vector<bool> JTIsLive;
167 JTIsLive.resize(JTs.size());
168 for (MachineFunction::iterator BB = MF.begin(), E = MF.end();
170 for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end();
172 for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) {
173 MachineOperand &Op = I->getOperand(op);
174 if (!Op.isJumpTableIndex()) continue;
175 unsigned NewIdx = JTMapping[Op.getIndex()];
178 // Remember that this JT is live.
179 JTIsLive[NewIdx] = true;
183 // Finally, remove dead jump tables. This happens either because the
184 // indirect jump was unreachable (and thus deleted) or because the jump
185 // table was merged with some other one.
186 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i)
188 JTI->RemoveJumpTable(i);
189 EverMadeChange = true;
194 return EverMadeChange;
197 //===----------------------------------------------------------------------===//
198 // Tail Merging of Blocks
199 //===----------------------------------------------------------------------===//
201 /// HashMachineInstr - Compute a hash value for MI and its operands.
202 static unsigned HashMachineInstr(const MachineInstr *MI) {
203 unsigned Hash = MI->getOpcode();
204 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
205 const MachineOperand &Op = MI->getOperand(i);
207 // Merge in bits from the operand if easy.
208 unsigned OperandHash = 0;
209 switch (Op.getType()) {
210 case MachineOperand::MO_Register: OperandHash = Op.getReg(); break;
211 case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break;
212 case MachineOperand::MO_MachineBasicBlock:
213 OperandHash = Op.getMBB()->getNumber();
215 case MachineOperand::MO_FrameIndex:
216 case MachineOperand::MO_ConstantPoolIndex:
217 case MachineOperand::MO_JumpTableIndex:
218 OperandHash = Op.getIndex();
220 case MachineOperand::MO_GlobalAddress:
221 case MachineOperand::MO_ExternalSymbol:
222 // Global address / external symbol are too hard, don't bother, but do
223 // pull in the offset.
224 OperandHash = Op.getOffset();
229 Hash += ((OperandHash << 3) | Op.getType()) << (i&31);
234 /// HashEndOfMBB - Hash the last few instructions in the MBB. For blocks
235 /// with no successors, we hash two instructions, because cross-jumping
236 /// only saves code when at least two instructions are removed (since a
237 /// branch must be inserted). For blocks with a successor, one of the
238 /// two blocks to be tail-merged will end with a branch already, so
239 /// it gains to cross-jump even for one instruction.
241 static unsigned HashEndOfMBB(const MachineBasicBlock *MBB,
242 unsigned minCommonTailLength) {
243 MachineBasicBlock::const_iterator I = MBB->end();
244 if (I == MBB->begin())
245 return 0; // Empty MBB.
248 unsigned Hash = HashMachineInstr(I);
250 if (I == MBB->begin() || minCommonTailLength == 1)
251 return Hash; // Single instr MBB.
254 // Hash in the second-to-last instruction.
255 Hash ^= HashMachineInstr(I) << 2;
259 /// ComputeCommonTailLength - Given two machine basic blocks, compute the number
260 /// of instructions they actually have in common together at their end. Return
261 /// iterators for the first shared instruction in each block.
262 static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1,
263 MachineBasicBlock *MBB2,
264 MachineBasicBlock::iterator &I1,
265 MachineBasicBlock::iterator &I2) {
269 unsigned TailLen = 0;
270 while (I1 != MBB1->begin() && I2 != MBB2->begin()) {
272 if (!I1->isIdenticalTo(I2) ||
273 // FIXME: This check is dubious. It's used to get around a problem where
274 // people incorrectly expect inline asm directives to remain in the same
275 // relative order. This is untenable because normal compiler
276 // optimizations (like this one) may reorder and/or merge these
278 I1->getOpcode() == TargetInstrInfo::INLINEASM) {
287 /// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything
288 /// after it, replacing it with an unconditional branch to NewDest. This
289 /// returns true if OldInst's block is modified, false if NewDest is modified.
290 void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
291 MachineBasicBlock *NewDest) {
292 MachineBasicBlock *OldBB = OldInst->getParent();
294 // Remove all the old successors of OldBB from the CFG.
295 while (!OldBB->succ_empty())
296 OldBB->removeSuccessor(OldBB->succ_begin());
298 // Remove all the dead instructions from the end of OldBB.
299 OldBB->erase(OldInst, OldBB->end());
301 // If OldBB isn't immediately before OldBB, insert a branch to it.
302 if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest))
303 TII->InsertBranch(*OldBB, NewDest, 0, std::vector<MachineOperand>());
304 OldBB->addSuccessor(NewDest);
308 /// SplitMBBAt - Given a machine basic block and an iterator into it, split the
309 /// MBB so that the part before the iterator falls into the part starting at the
310 /// iterator. This returns the new MBB.
311 MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB,
312 MachineBasicBlock::iterator BBI1) {
313 // Create the fall-through block.
314 MachineFunction::iterator MBBI = &CurMBB;
315 MachineBasicBlock *NewMBB = new MachineBasicBlock(CurMBB.getBasicBlock());
316 CurMBB.getParent()->getBasicBlockList().insert(++MBBI, NewMBB);
318 // Move all the successors of this block to the specified block.
319 while (!CurMBB.succ_empty()) {
320 MachineBasicBlock *S = *(CurMBB.succ_end()-1);
321 NewMBB->addSuccessor(S);
322 CurMBB.removeSuccessor(S);
325 // Add an edge from CurMBB to NewMBB for the fall-through.
326 CurMBB.addSuccessor(NewMBB);
328 // Splice the code over.
329 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end());
331 // For targets that use the register scavenger, we must maintain LiveIns.
333 RS->enterBasicBlock(&CurMBB);
335 RS->forward(prior(CurMBB.end()));
336 BitVector RegsLiveAtExit(RegInfo->getNumRegs());
337 RS->getRegsUsed(RegsLiveAtExit, false);
338 for (unsigned int i=0, e=RegInfo->getNumRegs(); i!=e; i++)
339 if (RegsLiveAtExit[i])
340 NewMBB->addLiveIn(i);
346 /// EstimateRuntime - Make a rough estimate for how long it will take to run
347 /// the specified code.
348 static unsigned EstimateRuntime(MachineBasicBlock::iterator I,
349 MachineBasicBlock::iterator E) {
351 for (; I != E; ++I) {
352 const TargetInstrDesc &TID = I->getDesc();
355 else if (TID.isSimpleLoad() || TID.mayStore())
363 /// ShouldSplitFirstBlock - We need to either split MBB1 at MBB1I or MBB2 at
364 /// MBB2I and then insert an unconditional branch in the other block. Determine
365 /// which is the best to split
366 static bool ShouldSplitFirstBlock(MachineBasicBlock *MBB1,
367 MachineBasicBlock::iterator MBB1I,
368 MachineBasicBlock *MBB2,
369 MachineBasicBlock::iterator MBB2I,
370 MachineBasicBlock *PredBB) {
371 // If one block is the entry block, split the other one; we can't generate
372 // a branch to the entry block, as its label is not emitted.
373 MachineBasicBlock *Entry = MBB1->getParent()->begin();
379 // If one block falls through into the common successor, choose that
380 // one to split; it is one instruction less to do that.
384 else if (MBB2 == PredBB)
387 // TODO: if we had some notion of which block was hotter, we could split
388 // the hot block, so it is the fall-through. Since we don't have profile info
389 // make a decision based on which will hurt most to split.
390 unsigned MBB1Time = EstimateRuntime(MBB1->begin(), MBB1I);
391 unsigned MBB2Time = EstimateRuntime(MBB2->begin(), MBB2I);
393 // If the MBB1 prefix takes "less time" to run than the MBB2 prefix, split the
394 // MBB1 block so it falls through. This will penalize the MBB2 path, but will
395 // have a lower overall impact on the program execution.
396 return MBB1Time < MBB2Time;
399 // CurMBB needs to add an unconditional branch to SuccMBB (we removed these
400 // branches temporarily for tail merging). In the case where CurMBB ends
401 // with a conditional branch to the next block, optimize by reversing the
402 // test and conditionally branching to SuccMBB instead.
404 static void FixTail(MachineBasicBlock* CurMBB, MachineBasicBlock *SuccBB,
405 const TargetInstrInfo *TII) {
406 MachineFunction *MF = CurMBB->getParent();
407 MachineFunction::iterator I = next(MachineFunction::iterator(CurMBB));
408 MachineBasicBlock *TBB = 0, *FBB = 0;
409 std::vector<MachineOperand> Cond;
410 if (I != MF->end() &&
411 !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond)) {
412 MachineBasicBlock *NextBB = I;
413 if (TBB == NextBB && Cond.size() && !FBB) {
414 if (!TII->ReverseBranchCondition(Cond)) {
415 TII->RemoveBranch(*CurMBB);
416 TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond);
421 TII->InsertBranch(*CurMBB, SuccBB, NULL, std::vector<MachineOperand>());
424 static bool MergeCompare(const std::pair<unsigned,MachineBasicBlock*> &p,
425 const std::pair<unsigned,MachineBasicBlock*> &q) {
426 if (p.first < q.first)
428 else if (p.first > q.first)
430 else if (p.second->getNumber() < q.second->getNumber())
432 else if (p.second->getNumber() > q.second->getNumber())
435 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing
436 // an object with itself.
437 #ifndef _GLIBCXX_DEBUG
438 assert(0 && "Predecessor appears twice");
444 // See if any of the blocks in MergePotentials (which all have a common single
445 // successor, or all have no successor) can be tail-merged. If there is a
446 // successor, any blocks in MergePotentials that are not tail-merged and
447 // are not immediately before Succ must have an unconditional branch to
448 // Succ added (but the predecessor/successor lists need no adjustment).
449 // The lone predecessor of Succ that falls through into Succ,
450 // if any, is given in PredBB.
452 bool BranchFolder::TryMergeBlocks(MachineBasicBlock *SuccBB,
453 MachineBasicBlock* PredBB) {
454 // It doesn't make sense to save a single instruction since tail merging
456 // FIXME: Ask the target to provide the threshold?
457 unsigned minCommonTailLength = (SuccBB ? 1 : 2) + 1;
460 // Sort by hash value so that blocks with identical end sequences sort
462 std::stable_sort(MergePotentials.begin(), MergePotentials.end(), MergeCompare);
464 // Walk through equivalence sets looking for actual exact matches.
465 while (MergePotentials.size() > 1) {
466 unsigned CurHash = (MergePotentials.end()-1)->first;
467 unsigned PrevHash = (MergePotentials.end()-2)->first;
468 MachineBasicBlock *CurMBB = (MergePotentials.end()-1)->second;
470 // If there is nothing that matches the hash of the current basic block,
472 if (CurHash != PrevHash) {
473 if (SuccBB && CurMBB != PredBB)
474 FixTail(CurMBB, SuccBB, TII);
475 MergePotentials.pop_back();
479 // Look through all the pairs of blocks that have the same hash as this
480 // one, and find the pair that has the largest number of instructions in
482 // Since instructions may get combined later (e.g. single stores into
483 // store multiple) this measure is not particularly accurate.
484 MachineBasicBlock::iterator BBI1, BBI2;
486 unsigned FoundI = ~0U, FoundJ = ~0U;
487 unsigned maxCommonTailLength = 0U;
488 for (int i = MergePotentials.size()-1;
489 i != -1 && MergePotentials[i].first == CurHash; --i) {
491 j != -1 && MergePotentials[j].first == CurHash; --j) {
492 MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
493 unsigned CommonTailLen = ComputeCommonTailLength(
494 MergePotentials[i].second,
495 MergePotentials[j].second,
496 TrialBBI1, TrialBBI2);
497 if (CommonTailLen >= minCommonTailLength &&
498 CommonTailLen > maxCommonTailLength) {
501 maxCommonTailLength = CommonTailLen;
508 // If we didn't find any pair that has at least minCommonTailLength
509 // instructions in common, bail out. All entries with this
510 // hash code can go away now.
512 for (int i = MergePotentials.size()-1;
513 i != -1 && MergePotentials[i].first == CurHash; --i) {
514 // Put the unconditional branch back, if we need one.
515 CurMBB = MergePotentials[i].second;
516 if (SuccBB && CurMBB != PredBB)
517 FixTail(CurMBB, SuccBB, TII);
518 MergePotentials.pop_back();
523 // Otherwise, move the block(s) to the right position(s). So that
524 // BBI1/2 will be valid, the last must be I and the next-to-last J.
525 if (FoundI != MergePotentials.size()-1)
526 std::swap(MergePotentials[FoundI], *(MergePotentials.end()-1));
527 if (FoundJ != MergePotentials.size()-2)
528 std::swap(MergePotentials[FoundJ], *(MergePotentials.end()-2));
530 CurMBB = (MergePotentials.end()-1)->second;
531 MachineBasicBlock *MBB2 = (MergePotentials.end()-2)->second;
533 // If neither block is the entire common tail, split the tail of one block
534 // to make it redundant with the other tail. Also, we cannot jump to the
535 // entry block, so if one block is the entry block, split the other one.
536 MachineBasicBlock *Entry = CurMBB->getParent()->begin();
537 if (CurMBB->begin() == BBI1 && CurMBB != Entry)
538 ; // CurMBB is common tail
539 else if (MBB2->begin() == BBI2 && MBB2 != Entry)
540 ; // MBB2 is common tail
542 if (0) { // Enable this to disable partial tail merges.
543 MergePotentials.pop_back();
547 MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
548 unsigned CommonTailLen = ComputeCommonTailLength(CurMBB, MBB2,
549 TrialBBI1, TrialBBI2);
550 if (CommonTailLen < minCommonTailLength)
553 // Decide whether we want to split CurMBB or MBB2.
554 if (ShouldSplitFirstBlock(CurMBB, BBI1, MBB2, BBI2, PredBB)) {
555 CurMBB = SplitMBBAt(*CurMBB, BBI1);
556 BBI1 = CurMBB->begin();
557 MergePotentials.back().second = CurMBB;
559 MBB2 = SplitMBBAt(*MBB2, BBI2);
560 BBI2 = MBB2->begin();
561 (MergePotentials.end()-2)->second = MBB2;
565 if (MBB2->begin() == BBI2 && MBB2 != Entry) {
566 // Hack the end off CurMBB, making it jump to MBBI@ instead.
567 ReplaceTailWithBranchTo(BBI1, MBB2);
568 // This modifies CurMBB, so remove it from the worklist.
569 MergePotentials.pop_back();
571 assert(CurMBB->begin() == BBI1 && CurMBB != Entry &&
572 "Didn't split block correctly?");
573 // Hack the end off MBB2, making it jump to CurMBB instead.
574 ReplaceTailWithBranchTo(BBI2, CurMBB);
575 // This modifies MBB2, so remove it from the worklist.
576 MergePotentials.erase(MergePotentials.end()-2);
583 bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
585 if (!EnableTailMerge) return false;
589 // First find blocks with no successors.
590 MergePotentials.clear();
591 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
593 MergePotentials.push_back(std::make_pair(HashEndOfMBB(I, 2U), I));
595 // See if we can do any tail merging on those.
596 if (MergePotentials.size() < TailMergeThreshold)
597 MadeChange |= TryMergeBlocks(NULL, NULL);
599 // Look at blocks (IBB) with multiple predecessors (PBB).
600 // We change each predecessor to a canonical form, by
601 // (1) temporarily removing any unconditional branch from the predecessor
603 // (2) alter conditional branches so they branch to the other block
604 // not IBB; this may require adding back an unconditional branch to IBB
605 // later, where there wasn't one coming in. E.g.
607 // fallthrough to QBB
610 // with a conceptual B to IBB after that, which never actually exists.
611 // With those changes, we see whether the predecessors' tails match,
612 // and merge them if so. We change things out of canonical form and
613 // back to the way they were later in the process. (OptimizeBranches
614 // would undo some of this, but we can't use it, because we'd get into
615 // a compile-time infinite loop repeatedly doing and undoing the same
618 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
619 if (!I->succ_empty() && I->pred_size() >= 2 &&
620 I->pred_size() < TailMergeThreshold) {
621 MachineBasicBlock *IBB = I;
622 MachineBasicBlock *PredBB = prior(I);
623 MergePotentials.clear();
624 for (MachineBasicBlock::pred_iterator P = I->pred_begin(),
627 MachineBasicBlock* PBB = *P;
628 // Skip blocks that loop to themselves, can't tail merge these.
631 MachineBasicBlock *TBB = 0, *FBB = 0;
632 std::vector<MachineOperand> Cond;
633 if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond)) {
634 // Failing case: IBB is the target of a cbr, and
635 // we cannot reverse the branch.
636 std::vector<MachineOperand> NewCond(Cond);
637 if (Cond.size() && TBB==IBB) {
638 if (TII->ReverseBranchCondition(NewCond))
640 // This is the QBB case described above
642 FBB = next(MachineFunction::iterator(PBB));
644 // Failing case: the only way IBB can be reached from PBB is via
645 // exception handling. Happens for landing pads. Would be nice
646 // to have a bit in the edge so we didn't have to do all this.
647 if (IBB->isLandingPad()) {
648 MachineFunction::iterator IP = PBB; IP++;
649 MachineBasicBlock* PredNextBB = NULL;
653 if (IBB!=PredNextBB) // fallthrough
656 if (TBB!=IBB && FBB!=IBB) // cbr then ubr
658 } else if (Cond.empty()) {
662 if (TBB!=IBB && IBB!=PredNextBB) // cbr
666 // Remove the unconditional branch at the end, if any.
667 if (TBB && (Cond.size()==0 || FBB)) {
668 TII->RemoveBranch(*PBB);
670 // reinsert conditional branch only, for now
671 TII->InsertBranch(*PBB, (TBB==IBB) ? FBB : TBB, 0, NewCond);
673 MergePotentials.push_back(std::make_pair(HashEndOfMBB(PBB, 1U), *P));
676 if (MergePotentials.size() >= 2)
677 MadeChange |= TryMergeBlocks(I, PredBB);
678 // Reinsert an unconditional branch if needed.
679 // The 1 below can be either an original single predecessor, or a result
680 // of removing blocks in TryMergeBlocks.
681 PredBB = prior(I); // this may have been changed in TryMergeBlocks
682 if (MergePotentials.size()==1 &&
683 (MergePotentials.begin())->second != PredBB)
684 FixTail((MergePotentials.begin())->second, I, TII);
690 //===----------------------------------------------------------------------===//
691 // Branch Optimization
692 //===----------------------------------------------------------------------===//
694 bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
697 // Make sure blocks are numbered in order
700 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) {
701 MachineBasicBlock *MBB = I++;
704 // If it is dead, remove it.
705 if (MBB->pred_empty()) {
706 RemoveDeadBlock(MBB);
715 /// CanFallThrough - Return true if the specified block (with the specified
716 /// branch condition) can implicitly transfer control to the block after it by
717 /// falling off the end of it. This should return false if it can reach the
718 /// block after it, but it uses an explicit branch to do so (e.g. a table jump).
720 /// True is a conservative answer.
722 bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB,
723 bool BranchUnAnalyzable,
724 MachineBasicBlock *TBB, MachineBasicBlock *FBB,
725 const std::vector<MachineOperand> &Cond) {
726 MachineFunction::iterator Fallthrough = CurBB;
728 // If FallthroughBlock is off the end of the function, it can't fall through.
729 if (Fallthrough == CurBB->getParent()->end())
732 // If FallthroughBlock isn't a successor of CurBB, no fallthrough is possible.
733 if (!CurBB->isSuccessor(Fallthrough))
736 // If we couldn't analyze the branch, assume it could fall through.
737 if (BranchUnAnalyzable) return true;
739 // If there is no branch, control always falls through.
740 if (TBB == 0) return true;
742 // If there is some explicit branch to the fallthrough block, it can obviously
743 // reach, even though the branch should get folded to fall through implicitly.
744 if (MachineFunction::iterator(TBB) == Fallthrough ||
745 MachineFunction::iterator(FBB) == Fallthrough)
748 // If it's an unconditional branch to some block not the fall through, it
749 // doesn't fall through.
750 if (Cond.empty()) return false;
752 // Otherwise, if it is conditional and has no explicit false block, it falls
757 /// CanFallThrough - Return true if the specified can implicitly transfer
758 /// control to the block after it by falling off the end of it. This should
759 /// return false if it can reach the block after it, but it uses an explicit
760 /// branch to do so (e.g. a table jump).
762 /// True is a conservative answer.
764 bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) {
765 MachineBasicBlock *TBB = 0, *FBB = 0;
766 std::vector<MachineOperand> Cond;
767 bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond);
768 return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond);
771 /// IsBetterFallthrough - Return true if it would be clearly better to
772 /// fall-through to MBB1 than to fall through into MBB2. This has to return
773 /// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will
774 /// result in infinite loops.
775 static bool IsBetterFallthrough(MachineBasicBlock *MBB1,
776 MachineBasicBlock *MBB2) {
777 // Right now, we use a simple heuristic. If MBB2 ends with a call, and
778 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to
779 // optimize branches that branch to either a return block or an assert block
780 // into a fallthrough to the return.
781 if (MBB1->empty() || MBB2->empty()) return false;
783 // If there is a clear successor ordering we make sure that one block
784 // will fall through to the next
785 if (MBB1->isSuccessor(MBB2)) return true;
786 if (MBB2->isSuccessor(MBB1)) return false;
788 MachineInstr *MBB1I = --MBB1->end();
789 MachineInstr *MBB2I = --MBB2->end();
790 return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall();
793 /// OptimizeBlock - Analyze and optimize control flow related to the specified
794 /// block. This is never called on the entry block.
795 void BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
796 MachineFunction::iterator FallThrough = MBB;
799 // If this block is empty, make everyone use its fall-through, not the block
800 // explicitly. Landing pads should not do this since the landing-pad table
801 // points to this block.
802 if (MBB->empty() && !MBB->isLandingPad()) {
803 // Dead block? Leave for cleanup later.
804 if (MBB->pred_empty()) return;
806 if (FallThrough == MBB->getParent()->end()) {
807 // TODO: Simplify preds to not branch here if possible!
809 // Rewrite all predecessors of the old block to go to the fallthrough
811 while (!MBB->pred_empty()) {
812 MachineBasicBlock *Pred = *(MBB->pred_end()-1);
813 Pred->ReplaceUsesOfBlockWith(MBB, FallThrough);
816 // If MBB was the target of a jump table, update jump tables to go to the
817 // fallthrough instead.
818 MBB->getParent()->getJumpTableInfo()->
819 ReplaceMBBInJumpTables(MBB, FallThrough);
825 // Check to see if we can simplify the terminator of the block before this
827 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB));
829 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0;
830 std::vector<MachineOperand> PriorCond;
831 bool PriorUnAnalyzable =
832 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond);
833 if (!PriorUnAnalyzable) {
834 // If the CFG for the prior block has extra edges, remove them.
835 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB,
838 // If the previous branch is conditional and both conditions go to the same
839 // destination, remove the branch, replacing it with an unconditional one or
841 if (PriorTBB && PriorTBB == PriorFBB) {
842 TII->RemoveBranch(PrevBB);
845 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond);
848 return OptimizeBlock(MBB);
851 // If the previous branch *only* branches to *this* block (conditional or
852 // not) remove the branch.
853 if (PriorTBB == MBB && PriorFBB == 0) {
854 TII->RemoveBranch(PrevBB);
857 return OptimizeBlock(MBB);
860 // If the prior block branches somewhere else on the condition and here if
861 // the condition is false, remove the uncond second branch.
862 if (PriorFBB == MBB) {
863 TII->RemoveBranch(PrevBB);
864 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond);
867 return OptimizeBlock(MBB);
870 // If the prior block branches here on true and somewhere else on false, and
871 // if the branch condition is reversible, reverse the branch to create a
873 if (PriorTBB == MBB) {
874 std::vector<MachineOperand> NewPriorCond(PriorCond);
875 if (!TII->ReverseBranchCondition(NewPriorCond)) {
876 TII->RemoveBranch(PrevBB);
877 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond);
880 return OptimizeBlock(MBB);
884 // If this block doesn't fall through (e.g. it ends with an uncond branch or
885 // has no successors) and if the pred falls through into this block, and if
886 // it would otherwise fall through into the block after this, move this
887 // block to the end of the function.
889 // We consider it more likely that execution will stay in the function (e.g.
890 // due to loops) than it is to exit it. This asserts in loops etc, moving
891 // the assert condition out of the loop body.
892 if (!PriorCond.empty() && PriorFBB == 0 &&
893 MachineFunction::iterator(PriorTBB) == FallThrough &&
894 !CanFallThrough(MBB)) {
895 bool DoTransform = true;
897 // We have to be careful that the succs of PredBB aren't both no-successor
898 // blocks. If neither have successors and if PredBB is the second from
899 // last block in the function, we'd just keep swapping the two blocks for
900 // last. Only do the swap if one is clearly better to fall through than
902 if (FallThrough == --MBB->getParent()->end() &&
903 !IsBetterFallthrough(PriorTBB, MBB))
906 // We don't want to do this transformation if we have control flow like:
915 // In this case, we could actually be moving the return block *into* a
917 if (DoTransform && !MBB->succ_empty() &&
918 (!CanFallThrough(PriorTBB) || PriorTBB->empty()))
923 // Reverse the branch so we will fall through on the previous true cond.
924 std::vector<MachineOperand> NewPriorCond(PriorCond);
925 if (!TII->ReverseBranchCondition(NewPriorCond)) {
926 DOUT << "\nMoving MBB: " << *MBB;
927 DOUT << "To make fallthrough to: " << *PriorTBB << "\n";
929 TII->RemoveBranch(PrevBB);
930 TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond);
932 // Move this block to the end of the function.
933 MBB->moveAfter(--MBB->getParent()->end());
942 // Analyze the branch in the current block.
943 MachineBasicBlock *CurTBB = 0, *CurFBB = 0;
944 std::vector<MachineOperand> CurCond;
945 bool CurUnAnalyzable = TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond);
946 if (!CurUnAnalyzable) {
947 // If the CFG for the prior block has extra edges, remove them.
948 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty());
950 // If this is a two-way branch, and the FBB branches to this block, reverse
951 // the condition so the single-basic-block loop is faster. Instead of:
952 // Loop: xxx; jcc Out; jmp Loop
954 // Loop: xxx; jncc Loop; jmp Out
955 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) {
956 std::vector<MachineOperand> NewCond(CurCond);
957 if (!TII->ReverseBranchCondition(NewCond)) {
958 TII->RemoveBranch(*MBB);
959 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond);
962 return OptimizeBlock(MBB);
967 // If this branch is the only thing in its block, see if we can forward
968 // other blocks across it.
969 if (CurTBB && CurCond.empty() && CurFBB == 0 &&
970 MBB->begin()->getDesc().isBranch() && CurTBB != MBB) {
971 // This block may contain just an unconditional branch. Because there can
972 // be 'non-branch terminators' in the block, try removing the branch and
973 // then seeing if the block is empty.
974 TII->RemoveBranch(*MBB);
976 // If this block is just an unconditional branch to CurTBB, we can
977 // usually completely eliminate the block. The only case we cannot
978 // completely eliminate the block is when the block before this one
979 // falls through into MBB and we can't understand the prior block's branch
982 bool PredHasNoFallThrough = TII->BlockHasNoFallThrough(PrevBB);
983 if (PredHasNoFallThrough || !PriorUnAnalyzable ||
984 !PrevBB.isSuccessor(MBB)) {
985 // If the prior block falls through into us, turn it into an
986 // explicit branch to us to make updates simpler.
987 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) &&
988 PriorTBB != MBB && PriorFBB != MBB) {
990 assert(PriorCond.empty() && PriorFBB == 0 &&
991 "Bad branch analysis");
994 assert(PriorFBB == 0 && "Machine CFG out of date!");
997 TII->RemoveBranch(PrevBB);
998 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond);
1001 // Iterate through all the predecessors, revectoring each in-turn.
1003 bool DidChange = false;
1004 bool HasBranchToSelf = false;
1005 while(PI != MBB->pred_size()) {
1006 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI);
1008 // If this block has an uncond branch to itself, leave it.
1010 HasBranchToSelf = true;
1013 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB);
1017 // Change any jumptables to go to the new MBB.
1018 MBB->getParent()->getJumpTableInfo()->
1019 ReplaceMBBInJumpTables(MBB, CurTBB);
1023 if (!HasBranchToSelf) return;
1028 // Add the branch back if the block is more than just an uncond branch.
1029 TII->InsertBranch(*MBB, CurTBB, 0, CurCond);
1033 // If the prior block doesn't fall through into this block, and if this
1034 // block doesn't fall through into some other block, see if we can find a
1035 // place to move this block where a fall-through will happen.
1036 if (!CanFallThrough(&PrevBB, PriorUnAnalyzable,
1037 PriorTBB, PriorFBB, PriorCond)) {
1038 // Now we know that there was no fall-through into this block, check to
1039 // see if it has a fall-through into its successor.
1040 bool CurFallsThru = CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB,
1043 if (!MBB->isLandingPad()) {
1044 // Check all the predecessors of this block. If one of them has no fall
1045 // throughs, move this block right after it.
1046 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(),
1047 E = MBB->pred_end(); PI != E; ++PI) {
1048 // Analyze the branch at the end of the pred.
1049 MachineBasicBlock *PredBB = *PI;
1050 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough;
1051 if (PredBB != MBB && !CanFallThrough(PredBB)
1052 && (!CurFallsThru || !CurTBB || !CurFBB)
1053 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
1054 // If the current block doesn't fall through, just move it.
1055 // If the current block can fall through and does not end with a
1056 // conditional branch, we need to append an unconditional jump to
1057 // the (current) next block. To avoid a possible compile-time
1058 // infinite loop, move blocks only backward in this case.
1059 // Also, if there are already 2 branches here, we cannot add a third;
1060 // this means we have the case
1065 MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB));
1067 TII->InsertBranch(*MBB, NextBB, 0, CurCond);
1069 MBB->moveAfter(PredBB);
1071 return OptimizeBlock(MBB);
1076 if (!CurFallsThru) {
1077 // Check all successors to see if we can move this block before it.
1078 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
1079 E = MBB->succ_end(); SI != E; ++SI) {
1080 // Analyze the branch at the end of the block before the succ.
1081 MachineBasicBlock *SuccBB = *SI;
1082 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev;
1083 std::vector<MachineOperand> SuccPrevCond;
1085 // If this block doesn't already fall-through to that successor, and if
1086 // the succ doesn't already have a block that can fall through into it,
1087 // and if the successor isn't an EH destination, we can arrange for the
1088 // fallthrough to happen.
1089 if (SuccBB != MBB && !CanFallThrough(SuccPrev) &&
1090 !SuccBB->isLandingPad()) {
1091 MBB->moveBefore(SuccBB);
1093 return OptimizeBlock(MBB);
1097 // Okay, there is no really great place to put this block. If, however,
1098 // the block before this one would be a fall-through if this block were
1099 // removed, move this block to the end of the function.
1100 if (FallThrough != MBB->getParent()->end() &&
1101 PrevBB.isSuccessor(FallThrough)) {
1102 MBB->moveAfter(--MBB->getParent()->end());