BranchFolding.cpp revision 4a3f6c88820a204f80ca0fa3f1f2fe09ca10b02f
1//===-- BranchFolding.cpp - Fold machine code branch instructions ---------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file is distributed under the University of Illinois Open Source 6// License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 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 12// it then removes. 13// 14// Note that this pass must be run after register allocation, it cannot handle 15// SSA form. 16// 17//===----------------------------------------------------------------------===// 18 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/SmallSet.h" 31#include "llvm/ADT/Statistic.h" 32#include "llvm/ADT/STLExtras.h" 33#include <algorithm> 34using namespace llvm; 35 36STATISTIC(NumDeadBlocks, "Number of dead blocks removed"); 37STATISTIC(NumBranchOpts, "Number of branches optimized"); 38STATISTIC(NumTailMerge , "Number of block tails merged"); 39static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge", 40 cl::init(cl::BOU_UNSET), cl::Hidden); 41namespace { 42 // Throttle for huge numbers of predecessors (compile speed problems) 43 static cl::opt<unsigned> 44 TailMergeThreshold("tail-merge-threshold", 45 cl::desc("Max number of predecessors to consider tail merging"), 46 cl::init(100), cl::Hidden); 47 48 struct VISIBILITY_HIDDEN BranchFolder : public MachineFunctionPass { 49 static char ID; 50 explicit BranchFolder(bool defaultEnableTailMerge) : 51 MachineFunctionPass((intptr_t)&ID) { 52 switch (FlagEnableTailMerge) { 53 case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break; 54 case cl::BOU_TRUE: EnableTailMerge = true; break; 55 case cl::BOU_FALSE: EnableTailMerge = false; break; 56 } 57 } 58 59 virtual bool runOnMachineFunction(MachineFunction &MF); 60 virtual const char *getPassName() const { return "Control Flow Optimizer"; } 61 const TargetInstrInfo *TII; 62 MachineModuleInfo *MMI; 63 bool MadeChange; 64 private: 65 // Tail Merging. 66 bool EnableTailMerge; 67 bool TailMergeBlocks(MachineFunction &MF); 68 bool TryMergeBlocks(MachineBasicBlock* SuccBB, 69 MachineBasicBlock* PredBB); 70 void ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, 71 MachineBasicBlock *NewDest); 72 MachineBasicBlock *SplitMBBAt(MachineBasicBlock &CurMBB, 73 MachineBasicBlock::iterator BBI1); 74 75 std::vector<std::pair<unsigned,MachineBasicBlock*> > MergePotentials; 76 const TargetRegisterInfo *RegInfo; 77 RegScavenger *RS; 78 // Branch optzn. 79 bool OptimizeBranches(MachineFunction &MF); 80 void OptimizeBlock(MachineBasicBlock *MBB); 81 void RemoveDeadBlock(MachineBasicBlock *MBB); 82 bool OptimizeImpDefsBlock(MachineBasicBlock *MBB); 83 84 bool CanFallThrough(MachineBasicBlock *CurBB); 85 bool CanFallThrough(MachineBasicBlock *CurBB, bool BranchUnAnalyzable, 86 MachineBasicBlock *TBB, MachineBasicBlock *FBB, 87 const std::vector<MachineOperand> &Cond); 88 }; 89 char BranchFolder::ID = 0; 90} 91 92FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) { 93 return new BranchFolder(DefaultEnableTailMerge); } 94 95/// RemoveDeadBlock - Remove the specified dead machine basic block from the 96/// function, updating the CFG. 97void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) { 98 assert(MBB->pred_empty() && "MBB must be dead!"); 99 DOUT << "\nRemoving MBB: " << *MBB; 100 101 MachineFunction *MF = MBB->getParent(); 102 // drop all successors. 103 while (!MBB->succ_empty()) 104 MBB->removeSuccessor(MBB->succ_end()-1); 105 106 // If there is DWARF info to active, check to see if there are any LABEL 107 // records in the basic block. If so, unregister them from MachineModuleInfo. 108 if (MMI && !MBB->empty()) { 109 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); 110 I != E; ++I) { 111 if ((unsigned)I->getOpcode() == TargetInstrInfo::LABEL) { 112 // The label ID # is always operand #0, an immediate. 113 MMI->InvalidateLabel(I->getOperand(0).getImm()); 114 } 115 } 116 } 117 118 // Remove the block. 119 MF->getBasicBlockList().erase(MBB); 120} 121 122/// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def 123/// followed by terminators, and if the implicitly defined registers are not 124/// used by the terminators, remove those implicit_def's. e.g. 125/// BB1: 126/// r0 = implicit_def 127/// r1 = implicit_def 128/// br 129/// This block can be optimized away later if the implicit instructions are 130/// removed. 131bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) { 132 SmallSet<unsigned, 4> ImpDefRegs; 133 MachineBasicBlock::iterator I = MBB->begin(); 134 while (I != MBB->end()) { 135 if (I->getOpcode() != TargetInstrInfo::IMPLICIT_DEF) 136 break; 137 unsigned Reg = I->getOperand(0).getReg(); 138 ImpDefRegs.insert(Reg); 139 for (const unsigned *SubRegs = RegInfo->getSubRegisters(Reg); 140 unsigned SubReg = *SubRegs; ++SubRegs) 141 ImpDefRegs.insert(SubReg); 142 ++I; 143 } 144 if (ImpDefRegs.empty()) 145 return false; 146 147 MachineBasicBlock::iterator FirstTerm = I; 148 while (I != MBB->end()) { 149 if (!TII->isUnpredicatedTerminator(I)) 150 return false; 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()) 155 continue; 156 unsigned Reg = MO.getReg(); 157 if (ImpDefRegs.count(Reg)) 158 return false; 159 } 160 ++I; 161 } 162 163 I = MBB->begin(); 164 while (I != FirstTerm) { 165 MachineInstr *ImpDefMI = &*I; 166 ++I; 167 MBB->erase(ImpDefMI); 168 } 169 170 return true; 171} 172 173bool BranchFolder::runOnMachineFunction(MachineFunction &MF) { 174 TII = MF.getTarget().getInstrInfo(); 175 if (!TII) return false; 176 177 RegInfo = MF.getTarget().getRegisterInfo(); 178 179 // Fix CFG. The later algorithms expect it to be right. 180 bool EverMadeChange = false; 181 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) { 182 MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0; 183 std::vector<MachineOperand> Cond; 184 if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond)) 185 EverMadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty()); 186 EverMadeChange |= OptimizeImpDefsBlock(MBB); 187 } 188 189 RS = RegInfo->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL; 190 191 MMI = getAnalysisToUpdate<MachineModuleInfo>(); 192 193 bool MadeChangeThisIteration = true; 194 while (MadeChangeThisIteration) { 195 MadeChangeThisIteration = false; 196 MadeChangeThisIteration |= TailMergeBlocks(MF); 197 MadeChangeThisIteration |= OptimizeBranches(MF); 198 EverMadeChange |= MadeChangeThisIteration; 199 } 200 201 // See if any jump tables have become mergable or dead as the code generator 202 // did its thing. 203 MachineJumpTableInfo *JTI = MF.getJumpTableInfo(); 204 const std::vector<MachineJumpTableEntry> &JTs = JTI->getJumpTables(); 205 if (!JTs.empty()) { 206 // Figure out how these jump tables should be merged. 207 std::vector<unsigned> JTMapping; 208 JTMapping.reserve(JTs.size()); 209 210 // We always keep the 0th jump table. 211 JTMapping.push_back(0); 212 213 // Scan the jump tables, seeing if there are any duplicates. Note that this 214 // is N^2, which should be fixed someday. 215 for (unsigned i = 1, e = JTs.size(); i != e; ++i) 216 JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs)); 217 218 // If a jump table was merge with another one, walk the function rewriting 219 // references to jump tables to reference the new JT ID's. Keep track of 220 // whether we see a jump table idx, if not, we can delete the JT. 221 std::vector<bool> JTIsLive; 222 JTIsLive.resize(JTs.size()); 223 for (MachineFunction::iterator BB = MF.begin(), E = MF.end(); 224 BB != E; ++BB) { 225 for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end(); 226 I != E; ++I) 227 for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) { 228 MachineOperand &Op = I->getOperand(op); 229 if (!Op.isJumpTableIndex()) continue; 230 unsigned NewIdx = JTMapping[Op.getIndex()]; 231 Op.setIndex(NewIdx); 232 233 // Remember that this JT is live. 234 JTIsLive[NewIdx] = true; 235 } 236 } 237 238 // Finally, remove dead jump tables. This happens either because the 239 // indirect jump was unreachable (and thus deleted) or because the jump 240 // table was merged with some other one. 241 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i) 242 if (!JTIsLive[i]) { 243 JTI->RemoveJumpTable(i); 244 EverMadeChange = true; 245 } 246 } 247 248 delete RS; 249 return EverMadeChange; 250} 251 252//===----------------------------------------------------------------------===// 253// Tail Merging of Blocks 254//===----------------------------------------------------------------------===// 255 256/// HashMachineInstr - Compute a hash value for MI and its operands. 257static unsigned HashMachineInstr(const MachineInstr *MI) { 258 unsigned Hash = MI->getOpcode(); 259 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 260 const MachineOperand &Op = MI->getOperand(i); 261 262 // Merge in bits from the operand if easy. 263 unsigned OperandHash = 0; 264 switch (Op.getType()) { 265 case MachineOperand::MO_Register: OperandHash = Op.getReg(); break; 266 case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break; 267 case MachineOperand::MO_MachineBasicBlock: 268 OperandHash = Op.getMBB()->getNumber(); 269 break; 270 case MachineOperand::MO_FrameIndex: 271 case MachineOperand::MO_ConstantPoolIndex: 272 case MachineOperand::MO_JumpTableIndex: 273 OperandHash = Op.getIndex(); 274 break; 275 case MachineOperand::MO_GlobalAddress: 276 case MachineOperand::MO_ExternalSymbol: 277 // Global address / external symbol are too hard, don't bother, but do 278 // pull in the offset. 279 OperandHash = Op.getOffset(); 280 break; 281 default: break; 282 } 283 284 Hash += ((OperandHash << 3) | Op.getType()) << (i&31); 285 } 286 return Hash; 287} 288 289/// HashEndOfMBB - Hash the last few instructions in the MBB. For blocks 290/// with no successors, we hash two instructions, because cross-jumping 291/// only saves code when at least two instructions are removed (since a 292/// branch must be inserted). For blocks with a successor, one of the 293/// two blocks to be tail-merged will end with a branch already, so 294/// it gains to cross-jump even for one instruction. 295 296static unsigned HashEndOfMBB(const MachineBasicBlock *MBB, 297 unsigned minCommonTailLength) { 298 MachineBasicBlock::const_iterator I = MBB->end(); 299 if (I == MBB->begin()) 300 return 0; // Empty MBB. 301 302 --I; 303 unsigned Hash = HashMachineInstr(I); 304 305 if (I == MBB->begin() || minCommonTailLength == 1) 306 return Hash; // Single instr MBB. 307 308 --I; 309 // Hash in the second-to-last instruction. 310 Hash ^= HashMachineInstr(I) << 2; 311 return Hash; 312} 313 314/// ComputeCommonTailLength - Given two machine basic blocks, compute the number 315/// of instructions they actually have in common together at their end. Return 316/// iterators for the first shared instruction in each block. 317static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1, 318 MachineBasicBlock *MBB2, 319 MachineBasicBlock::iterator &I1, 320 MachineBasicBlock::iterator &I2) { 321 I1 = MBB1->end(); 322 I2 = MBB2->end(); 323 324 unsigned TailLen = 0; 325 while (I1 != MBB1->begin() && I2 != MBB2->begin()) { 326 --I1; --I2; 327 if (!I1->isIdenticalTo(I2) || 328 // FIXME: This check is dubious. It's used to get around a problem where 329 // people incorrectly expect inline asm directives to remain in the same 330 // relative order. This is untenable because normal compiler 331 // optimizations (like this one) may reorder and/or merge these 332 // directives. 333 I1->getOpcode() == TargetInstrInfo::INLINEASM) { 334 ++I1; ++I2; 335 break; 336 } 337 ++TailLen; 338 } 339 return TailLen; 340} 341 342/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything 343/// after it, replacing it with an unconditional branch to NewDest. This 344/// returns true if OldInst's block is modified, false if NewDest is modified. 345void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, 346 MachineBasicBlock *NewDest) { 347 MachineBasicBlock *OldBB = OldInst->getParent(); 348 349 // Remove all the old successors of OldBB from the CFG. 350 while (!OldBB->succ_empty()) 351 OldBB->removeSuccessor(OldBB->succ_begin()); 352 353 // Remove all the dead instructions from the end of OldBB. 354 OldBB->erase(OldInst, OldBB->end()); 355 356 // If OldBB isn't immediately before OldBB, insert a branch to it. 357 if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest)) 358 TII->InsertBranch(*OldBB, NewDest, 0, std::vector<MachineOperand>()); 359 OldBB->addSuccessor(NewDest); 360 ++NumTailMerge; 361} 362 363/// SplitMBBAt - Given a machine basic block and an iterator into it, split the 364/// MBB so that the part before the iterator falls into the part starting at the 365/// iterator. This returns the new MBB. 366MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB, 367 MachineBasicBlock::iterator BBI1) { 368 // Create the fall-through block. 369 MachineFunction::iterator MBBI = &CurMBB; 370 MachineBasicBlock *NewMBB = new MachineBasicBlock(CurMBB.getBasicBlock()); 371 CurMBB.getParent()->getBasicBlockList().insert(++MBBI, NewMBB); 372 373 // Move all the successors of this block to the specified block. 374 while (!CurMBB.succ_empty()) { 375 MachineBasicBlock *S = *(CurMBB.succ_end()-1); 376 NewMBB->addSuccessor(S); 377 CurMBB.removeSuccessor(S); 378 } 379 380 // Add an edge from CurMBB to NewMBB for the fall-through. 381 CurMBB.addSuccessor(NewMBB); 382 383 // Splice the code over. 384 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end()); 385 386 // For targets that use the register scavenger, we must maintain LiveIns. 387 if (RS) { 388 RS->enterBasicBlock(&CurMBB); 389 if (!CurMBB.empty()) 390 RS->forward(prior(CurMBB.end())); 391 BitVector RegsLiveAtExit(RegInfo->getNumRegs()); 392 RS->getRegsUsed(RegsLiveAtExit, false); 393 for (unsigned int i=0, e=RegInfo->getNumRegs(); i!=e; i++) 394 if (RegsLiveAtExit[i]) 395 NewMBB->addLiveIn(i); 396 } 397 398 return NewMBB; 399} 400 401/// EstimateRuntime - Make a rough estimate for how long it will take to run 402/// the specified code. 403static unsigned EstimateRuntime(MachineBasicBlock::iterator I, 404 MachineBasicBlock::iterator E) { 405 unsigned Time = 0; 406 for (; I != E; ++I) { 407 const TargetInstrDesc &TID = I->getDesc(); 408 if (TID.isCall()) 409 Time += 10; 410 else if (TID.isSimpleLoad() || TID.mayStore()) 411 Time += 2; 412 else 413 ++Time; 414 } 415 return Time; 416} 417 418/// ShouldSplitFirstBlock - We need to either split MBB1 at MBB1I or MBB2 at 419/// MBB2I and then insert an unconditional branch in the other block. Determine 420/// which is the best to split 421static bool ShouldSplitFirstBlock(MachineBasicBlock *MBB1, 422 MachineBasicBlock::iterator MBB1I, 423 MachineBasicBlock *MBB2, 424 MachineBasicBlock::iterator MBB2I, 425 MachineBasicBlock *PredBB) { 426 // If one block is the entry block, split the other one; we can't generate 427 // a branch to the entry block, as its label is not emitted. 428 MachineBasicBlock *Entry = MBB1->getParent()->begin(); 429 if (MBB1 == Entry) 430 return false; 431 if (MBB2 == Entry) 432 return true; 433 434 // If one block falls through into the common successor, choose that 435 // one to split; it is one instruction less to do that. 436 if (PredBB) { 437 if (MBB1 == PredBB) 438 return true; 439 else if (MBB2 == PredBB) 440 return false; 441 } 442 // TODO: if we had some notion of which block was hotter, we could split 443 // the hot block, so it is the fall-through. Since we don't have profile info 444 // make a decision based on which will hurt most to split. 445 unsigned MBB1Time = EstimateRuntime(MBB1->begin(), MBB1I); 446 unsigned MBB2Time = EstimateRuntime(MBB2->begin(), MBB2I); 447 448 // If the MBB1 prefix takes "less time" to run than the MBB2 prefix, split the 449 // MBB1 block so it falls through. This will penalize the MBB2 path, but will 450 // have a lower overall impact on the program execution. 451 return MBB1Time < MBB2Time; 452} 453 454// CurMBB needs to add an unconditional branch to SuccMBB (we removed these 455// branches temporarily for tail merging). In the case where CurMBB ends 456// with a conditional branch to the next block, optimize by reversing the 457// test and conditionally branching to SuccMBB instead. 458 459static void FixTail(MachineBasicBlock* CurMBB, MachineBasicBlock *SuccBB, 460 const TargetInstrInfo *TII) { 461 MachineFunction *MF = CurMBB->getParent(); 462 MachineFunction::iterator I = next(MachineFunction::iterator(CurMBB)); 463 MachineBasicBlock *TBB = 0, *FBB = 0; 464 std::vector<MachineOperand> Cond; 465 if (I != MF->end() && 466 !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond)) { 467 MachineBasicBlock *NextBB = I; 468 if (TBB == NextBB && Cond.size() && !FBB) { 469 if (!TII->ReverseBranchCondition(Cond)) { 470 TII->RemoveBranch(*CurMBB); 471 TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond); 472 return; 473 } 474 } 475 } 476 TII->InsertBranch(*CurMBB, SuccBB, NULL, std::vector<MachineOperand>()); 477} 478 479static bool MergeCompare(const std::pair<unsigned,MachineBasicBlock*> &p, 480 const std::pair<unsigned,MachineBasicBlock*> &q) { 481 if (p.first < q.first) 482 return true; 483 else if (p.first > q.first) 484 return false; 485 else if (p.second->getNumber() < q.second->getNumber()) 486 return true; 487 else if (p.second->getNumber() > q.second->getNumber()) 488 return false; 489 else { 490 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing 491 // an object with itself. 492#ifndef _GLIBCXX_DEBUG 493 assert(0 && "Predecessor appears twice"); 494#endif 495 return(false); 496 } 497} 498 499// See if any of the blocks in MergePotentials (which all have a common single 500// successor, or all have no successor) can be tail-merged. If there is a 501// successor, any blocks in MergePotentials that are not tail-merged and 502// are not immediately before Succ must have an unconditional branch to 503// Succ added (but the predecessor/successor lists need no adjustment). 504// The lone predecessor of Succ that falls through into Succ, 505// if any, is given in PredBB. 506 507bool BranchFolder::TryMergeBlocks(MachineBasicBlock *SuccBB, 508 MachineBasicBlock* PredBB) { 509 // It doesn't make sense to save a single instruction since tail merging 510 // will add a jump. 511 // FIXME: Ask the target to provide the threshold? 512 unsigned minCommonTailLength = (SuccBB ? 1 : 2) + 1; 513 MadeChange = false; 514 515 // Sort by hash value so that blocks with identical end sequences sort 516 // together. 517 std::stable_sort(MergePotentials.begin(), MergePotentials.end(), MergeCompare); 518 519 // Walk through equivalence sets looking for actual exact matches. 520 while (MergePotentials.size() > 1) { 521 unsigned CurHash = (MergePotentials.end()-1)->first; 522 unsigned PrevHash = (MergePotentials.end()-2)->first; 523 MachineBasicBlock *CurMBB = (MergePotentials.end()-1)->second; 524 525 // If there is nothing that matches the hash of the current basic block, 526 // give up. 527 if (CurHash != PrevHash) { 528 if (SuccBB && CurMBB != PredBB) 529 FixTail(CurMBB, SuccBB, TII); 530 MergePotentials.pop_back(); 531 continue; 532 } 533 534 // Look through all the pairs of blocks that have the same hash as this 535 // one, and find the pair that has the largest number of instructions in 536 // common. 537 // Since instructions may get combined later (e.g. single stores into 538 // store multiple) this measure is not particularly accurate. 539 MachineBasicBlock::iterator BBI1, BBI2; 540 541 unsigned FoundI = ~0U, FoundJ = ~0U; 542 unsigned maxCommonTailLength = 0U; 543 for (int i = MergePotentials.size()-1; 544 i != -1 && MergePotentials[i].first == CurHash; --i) { 545 for (int j = i-1; 546 j != -1 && MergePotentials[j].first == CurHash; --j) { 547 MachineBasicBlock::iterator TrialBBI1, TrialBBI2; 548 unsigned CommonTailLen = ComputeCommonTailLength( 549 MergePotentials[i].second, 550 MergePotentials[j].second, 551 TrialBBI1, TrialBBI2); 552 if (CommonTailLen >= minCommonTailLength && 553 CommonTailLen > maxCommonTailLength) { 554 FoundI = i; 555 FoundJ = j; 556 maxCommonTailLength = CommonTailLen; 557 BBI1 = TrialBBI1; 558 BBI2 = TrialBBI2; 559 } 560 } 561 } 562 563 // If we didn't find any pair that has at least minCommonTailLength 564 // instructions in common, bail out. All entries with this 565 // hash code can go away now. 566 if (FoundI == ~0U) { 567 for (int i = MergePotentials.size()-1; 568 i != -1 && MergePotentials[i].first == CurHash; --i) { 569 // Put the unconditional branch back, if we need one. 570 CurMBB = MergePotentials[i].second; 571 if (SuccBB && CurMBB != PredBB) 572 FixTail(CurMBB, SuccBB, TII); 573 MergePotentials.pop_back(); 574 } 575 continue; 576 } 577 578 // Otherwise, move the block(s) to the right position(s). So that 579 // BBI1/2 will be valid, the last must be I and the next-to-last J. 580 if (FoundI != MergePotentials.size()-1) 581 std::swap(MergePotentials[FoundI], *(MergePotentials.end()-1)); 582 if (FoundJ != MergePotentials.size()-2) 583 std::swap(MergePotentials[FoundJ], *(MergePotentials.end()-2)); 584 585 CurMBB = (MergePotentials.end()-1)->second; 586 MachineBasicBlock *MBB2 = (MergePotentials.end()-2)->second; 587 588 // If neither block is the entire common tail, split the tail of one block 589 // to make it redundant with the other tail. Also, we cannot jump to the 590 // entry block, so if one block is the entry block, split the other one. 591 MachineBasicBlock *Entry = CurMBB->getParent()->begin(); 592 if (CurMBB->begin() == BBI1 && CurMBB != Entry) 593 ; // CurMBB is common tail 594 else if (MBB2->begin() == BBI2 && MBB2 != Entry) 595 ; // MBB2 is common tail 596 else { 597 if (0) { // Enable this to disable partial tail merges. 598 MergePotentials.pop_back(); 599 continue; 600 } 601 602 MachineBasicBlock::iterator TrialBBI1, TrialBBI2; 603 unsigned CommonTailLen = ComputeCommonTailLength(CurMBB, MBB2, 604 TrialBBI1, TrialBBI2); 605 if (CommonTailLen < minCommonTailLength) 606 continue; 607 608 // Decide whether we want to split CurMBB or MBB2. 609 if (ShouldSplitFirstBlock(CurMBB, BBI1, MBB2, BBI2, PredBB)) { 610 CurMBB = SplitMBBAt(*CurMBB, BBI1); 611 BBI1 = CurMBB->begin(); 612 MergePotentials.back().second = CurMBB; 613 } else { 614 MBB2 = SplitMBBAt(*MBB2, BBI2); 615 BBI2 = MBB2->begin(); 616 (MergePotentials.end()-2)->second = MBB2; 617 } 618 } 619 620 if (MBB2->begin() == BBI2 && MBB2 != Entry) { 621 // Hack the end off CurMBB, making it jump to MBBI@ instead. 622 ReplaceTailWithBranchTo(BBI1, MBB2); 623 // This modifies CurMBB, so remove it from the worklist. 624 MergePotentials.pop_back(); 625 } else { 626 assert(CurMBB->begin() == BBI1 && CurMBB != Entry && 627 "Didn't split block correctly?"); 628 // Hack the end off MBB2, making it jump to CurMBB instead. 629 ReplaceTailWithBranchTo(BBI2, CurMBB); 630 // This modifies MBB2, so remove it from the worklist. 631 MergePotentials.erase(MergePotentials.end()-2); 632 } 633 MadeChange = true; 634 } 635 return MadeChange; 636} 637 638bool BranchFolder::TailMergeBlocks(MachineFunction &MF) { 639 640 if (!EnableTailMerge) return false; 641 642 MadeChange = false; 643 644 // First find blocks with no successors. 645 MergePotentials.clear(); 646 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { 647 if (I->succ_empty()) 648 MergePotentials.push_back(std::make_pair(HashEndOfMBB(I, 2U), I)); 649 } 650 // See if we can do any tail merging on those. 651 if (MergePotentials.size() < TailMergeThreshold) 652 MadeChange |= TryMergeBlocks(NULL, NULL); 653 654 // Look at blocks (IBB) with multiple predecessors (PBB). 655 // We change each predecessor to a canonical form, by 656 // (1) temporarily removing any unconditional branch from the predecessor 657 // to IBB, and 658 // (2) alter conditional branches so they branch to the other block 659 // not IBB; this may require adding back an unconditional branch to IBB 660 // later, where there wasn't one coming in. E.g. 661 // Bcc IBB 662 // fallthrough to QBB 663 // here becomes 664 // Bncc QBB 665 // with a conceptual B to IBB after that, which never actually exists. 666 // With those changes, we see whether the predecessors' tails match, 667 // and merge them if so. We change things out of canonical form and 668 // back to the way they were later in the process. (OptimizeBranches 669 // would undo some of this, but we can't use it, because we'd get into 670 // a compile-time infinite loop repeatedly doing and undoing the same 671 // transformations.) 672 673 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { 674 if (!I->succ_empty() && I->pred_size() >= 2 && 675 I->pred_size() < TailMergeThreshold) { 676 MachineBasicBlock *IBB = I; 677 MachineBasicBlock *PredBB = prior(I); 678 MergePotentials.clear(); 679 for (MachineBasicBlock::pred_iterator P = I->pred_begin(), 680 E2 = I->pred_end(); 681 P != E2; ++P) { 682 MachineBasicBlock* PBB = *P; 683 // Skip blocks that loop to themselves, can't tail merge these. 684 if (PBB==IBB) 685 continue; 686 MachineBasicBlock *TBB = 0, *FBB = 0; 687 std::vector<MachineOperand> Cond; 688 if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond)) { 689 // Failing case: IBB is the target of a cbr, and 690 // we cannot reverse the branch. 691 std::vector<MachineOperand> NewCond(Cond); 692 if (Cond.size() && TBB==IBB) { 693 if (TII->ReverseBranchCondition(NewCond)) 694 continue; 695 // This is the QBB case described above 696 if (!FBB) 697 FBB = next(MachineFunction::iterator(PBB)); 698 } 699 // Failing case: the only way IBB can be reached from PBB is via 700 // exception handling. Happens for landing pads. Would be nice 701 // to have a bit in the edge so we didn't have to do all this. 702 if (IBB->isLandingPad()) { 703 MachineFunction::iterator IP = PBB; IP++; 704 MachineBasicBlock* PredNextBB = NULL; 705 if (IP!=MF.end()) 706 PredNextBB = IP; 707 if (TBB==NULL) { 708 if (IBB!=PredNextBB) // fallthrough 709 continue; 710 } else if (FBB) { 711 if (TBB!=IBB && FBB!=IBB) // cbr then ubr 712 continue; 713 } else if (Cond.empty()) { 714 if (TBB!=IBB) // ubr 715 continue; 716 } else { 717 if (TBB!=IBB && IBB!=PredNextBB) // cbr 718 continue; 719 } 720 } 721 // Remove the unconditional branch at the end, if any. 722 if (TBB && (Cond.size()==0 || FBB)) { 723 TII->RemoveBranch(*PBB); 724 if (Cond.size()) 725 // reinsert conditional branch only, for now 726 TII->InsertBranch(*PBB, (TBB==IBB) ? FBB : TBB, 0, NewCond); 727 } 728 MergePotentials.push_back(std::make_pair(HashEndOfMBB(PBB, 1U), *P)); 729 } 730 } 731 if (MergePotentials.size() >= 2) 732 MadeChange |= TryMergeBlocks(I, PredBB); 733 // Reinsert an unconditional branch if needed. 734 // The 1 below can be either an original single predecessor, or a result 735 // of removing blocks in TryMergeBlocks. 736 PredBB = prior(I); // this may have been changed in TryMergeBlocks 737 if (MergePotentials.size()==1 && 738 (MergePotentials.begin())->second != PredBB) 739 FixTail((MergePotentials.begin())->second, I, TII); 740 } 741 } 742 return MadeChange; 743} 744 745//===----------------------------------------------------------------------===// 746// Branch Optimization 747//===----------------------------------------------------------------------===// 748 749bool BranchFolder::OptimizeBranches(MachineFunction &MF) { 750 MadeChange = false; 751 752 // Make sure blocks are numbered in order 753 MF.RenumberBlocks(); 754 755 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) { 756 MachineBasicBlock *MBB = I++; 757 OptimizeBlock(MBB); 758 759 // If it is dead, remove it. 760 if (MBB->pred_empty()) { 761 RemoveDeadBlock(MBB); 762 MadeChange = true; 763 ++NumDeadBlocks; 764 } 765 } 766 return MadeChange; 767} 768 769 770/// CanFallThrough - Return true if the specified block (with the specified 771/// branch condition) can implicitly transfer control to the block after it by 772/// falling off the end of it. This should return false if it can reach the 773/// block after it, but it uses an explicit branch to do so (e.g. a table jump). 774/// 775/// True is a conservative answer. 776/// 777bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB, 778 bool BranchUnAnalyzable, 779 MachineBasicBlock *TBB, MachineBasicBlock *FBB, 780 const std::vector<MachineOperand> &Cond) { 781 MachineFunction::iterator Fallthrough = CurBB; 782 ++Fallthrough; 783 // If FallthroughBlock is off the end of the function, it can't fall through. 784 if (Fallthrough == CurBB->getParent()->end()) 785 return false; 786 787 // If FallthroughBlock isn't a successor of CurBB, no fallthrough is possible. 788 if (!CurBB->isSuccessor(Fallthrough)) 789 return false; 790 791 // If we couldn't analyze the branch, assume it could fall through. 792 if (BranchUnAnalyzable) return true; 793 794 // If there is no branch, control always falls through. 795 if (TBB == 0) return true; 796 797 // If there is some explicit branch to the fallthrough block, it can obviously 798 // reach, even though the branch should get folded to fall through implicitly. 799 if (MachineFunction::iterator(TBB) == Fallthrough || 800 MachineFunction::iterator(FBB) == Fallthrough) 801 return true; 802 803 // If it's an unconditional branch to some block not the fall through, it 804 // doesn't fall through. 805 if (Cond.empty()) return false; 806 807 // Otherwise, if it is conditional and has no explicit false block, it falls 808 // through. 809 return FBB == 0; 810} 811 812/// CanFallThrough - Return true if the specified can implicitly transfer 813/// control to the block after it by falling off the end of it. This should 814/// return false if it can reach the block after it, but it uses an explicit 815/// branch to do so (e.g. a table jump). 816/// 817/// True is a conservative answer. 818/// 819bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) { 820 MachineBasicBlock *TBB = 0, *FBB = 0; 821 std::vector<MachineOperand> Cond; 822 bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond); 823 return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond); 824} 825 826/// IsBetterFallthrough - Return true if it would be clearly better to 827/// fall-through to MBB1 than to fall through into MBB2. This has to return 828/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will 829/// result in infinite loops. 830static bool IsBetterFallthrough(MachineBasicBlock *MBB1, 831 MachineBasicBlock *MBB2) { 832 // Right now, we use a simple heuristic. If MBB2 ends with a call, and 833 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to 834 // optimize branches that branch to either a return block or an assert block 835 // into a fallthrough to the return. 836 if (MBB1->empty() || MBB2->empty()) return false; 837 838 // If there is a clear successor ordering we make sure that one block 839 // will fall through to the next 840 if (MBB1->isSuccessor(MBB2)) return true; 841 if (MBB2->isSuccessor(MBB1)) return false; 842 843 MachineInstr *MBB1I = --MBB1->end(); 844 MachineInstr *MBB2I = --MBB2->end(); 845 return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall(); 846} 847 848/// OptimizeBlock - Analyze and optimize control flow related to the specified 849/// block. This is never called on the entry block. 850void BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) { 851 MachineFunction::iterator FallThrough = MBB; 852 ++FallThrough; 853 854 // If this block is empty, make everyone use its fall-through, not the block 855 // explicitly. Landing pads should not do this since the landing-pad table 856 // points to this block. 857 if (MBB->empty() && !MBB->isLandingPad()) { 858 // Dead block? Leave for cleanup later. 859 if (MBB->pred_empty()) return; 860 861 if (FallThrough == MBB->getParent()->end()) { 862 // TODO: Simplify preds to not branch here if possible! 863 } else { 864 // Rewrite all predecessors of the old block to go to the fallthrough 865 // instead. 866 while (!MBB->pred_empty()) { 867 MachineBasicBlock *Pred = *(MBB->pred_end()-1); 868 Pred->ReplaceUsesOfBlockWith(MBB, FallThrough); 869 } 870 871 // If MBB was the target of a jump table, update jump tables to go to the 872 // fallthrough instead. 873 MBB->getParent()->getJumpTableInfo()-> 874 ReplaceMBBInJumpTables(MBB, FallThrough); 875 MadeChange = true; 876 } 877 return; 878 } 879 880 // Check to see if we can simplify the terminator of the block before this 881 // one. 882 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB)); 883 884 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0; 885 std::vector<MachineOperand> PriorCond; 886 bool PriorUnAnalyzable = 887 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); 888 if (!PriorUnAnalyzable) { 889 // If the CFG for the prior block has extra edges, remove them. 890 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB, 891 !PriorCond.empty()); 892 893 // If the previous branch is conditional and both conditions go to the same 894 // destination, remove the branch, replacing it with an unconditional one or 895 // a fall-through. 896 if (PriorTBB && PriorTBB == PriorFBB) { 897 TII->RemoveBranch(PrevBB); 898 PriorCond.clear(); 899 if (PriorTBB != MBB) 900 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 901 MadeChange = true; 902 ++NumBranchOpts; 903 return OptimizeBlock(MBB); 904 } 905 906 // If the previous branch *only* branches to *this* block (conditional or 907 // not) remove the branch. 908 if (PriorTBB == MBB && PriorFBB == 0) { 909 TII->RemoveBranch(PrevBB); 910 MadeChange = true; 911 ++NumBranchOpts; 912 return OptimizeBlock(MBB); 913 } 914 915 // If the prior block branches somewhere else on the condition and here if 916 // the condition is false, remove the uncond second branch. 917 if (PriorFBB == MBB) { 918 TII->RemoveBranch(PrevBB); 919 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 920 MadeChange = true; 921 ++NumBranchOpts; 922 return OptimizeBlock(MBB); 923 } 924 925 // If the prior block branches here on true and somewhere else on false, and 926 // if the branch condition is reversible, reverse the branch to create a 927 // fall-through. 928 if (PriorTBB == MBB) { 929 std::vector<MachineOperand> NewPriorCond(PriorCond); 930 if (!TII->ReverseBranchCondition(NewPriorCond)) { 931 TII->RemoveBranch(PrevBB); 932 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond); 933 MadeChange = true; 934 ++NumBranchOpts; 935 return OptimizeBlock(MBB); 936 } 937 } 938 939 // If this block doesn't fall through (e.g. it ends with an uncond branch or 940 // has no successors) and if the pred falls through into this block, and if 941 // it would otherwise fall through into the block after this, move this 942 // block to the end of the function. 943 // 944 // We consider it more likely that execution will stay in the function (e.g. 945 // due to loops) than it is to exit it. This asserts in loops etc, moving 946 // the assert condition out of the loop body. 947 if (!PriorCond.empty() && PriorFBB == 0 && 948 MachineFunction::iterator(PriorTBB) == FallThrough && 949 !CanFallThrough(MBB)) { 950 bool DoTransform = true; 951 952 // We have to be careful that the succs of PredBB aren't both no-successor 953 // blocks. If neither have successors and if PredBB is the second from 954 // last block in the function, we'd just keep swapping the two blocks for 955 // last. Only do the swap if one is clearly better to fall through than 956 // the other. 957 if (FallThrough == --MBB->getParent()->end() && 958 !IsBetterFallthrough(PriorTBB, MBB)) 959 DoTransform = false; 960 961 // We don't want to do this transformation if we have control flow like: 962 // br cond BB2 963 // BB1: 964 // .. 965 // jmp BBX 966 // BB2: 967 // .. 968 // ret 969 // 970 // In this case, we could actually be moving the return block *into* a 971 // loop! 972 if (DoTransform && !MBB->succ_empty() && 973 (!CanFallThrough(PriorTBB) || PriorTBB->empty())) 974 DoTransform = false; 975 976 977 if (DoTransform) { 978 // Reverse the branch so we will fall through on the previous true cond. 979 std::vector<MachineOperand> NewPriorCond(PriorCond); 980 if (!TII->ReverseBranchCondition(NewPriorCond)) { 981 DOUT << "\nMoving MBB: " << *MBB; 982 DOUT << "To make fallthrough to: " << *PriorTBB << "\n"; 983 984 TII->RemoveBranch(PrevBB); 985 TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond); 986 987 // Move this block to the end of the function. 988 MBB->moveAfter(--MBB->getParent()->end()); 989 MadeChange = true; 990 ++NumBranchOpts; 991 return; 992 } 993 } 994 } 995 } 996 997 // Analyze the branch in the current block. 998 MachineBasicBlock *CurTBB = 0, *CurFBB = 0; 999 std::vector<MachineOperand> CurCond; 1000 bool CurUnAnalyzable = TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond); 1001 if (!CurUnAnalyzable) { 1002 // If the CFG for the prior block has extra edges, remove them. 1003 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty()); 1004 1005 // If this is a two-way branch, and the FBB branches to this block, reverse 1006 // the condition so the single-basic-block loop is faster. Instead of: 1007 // Loop: xxx; jcc Out; jmp Loop 1008 // we want: 1009 // Loop: xxx; jncc Loop; jmp Out 1010 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) { 1011 std::vector<MachineOperand> NewCond(CurCond); 1012 if (!TII->ReverseBranchCondition(NewCond)) { 1013 TII->RemoveBranch(*MBB); 1014 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond); 1015 MadeChange = true; 1016 ++NumBranchOpts; 1017 return OptimizeBlock(MBB); 1018 } 1019 } 1020 1021 1022 // If this branch is the only thing in its block, see if we can forward 1023 // other blocks across it. 1024 if (CurTBB && CurCond.empty() && CurFBB == 0 && 1025 MBB->begin()->getDesc().isBranch() && CurTBB != MBB) { 1026 // This block may contain just an unconditional branch. Because there can 1027 // be 'non-branch terminators' in the block, try removing the branch and 1028 // then seeing if the block is empty. 1029 TII->RemoveBranch(*MBB); 1030 1031 // If this block is just an unconditional branch to CurTBB, we can 1032 // usually completely eliminate the block. The only case we cannot 1033 // completely eliminate the block is when the block before this one 1034 // falls through into MBB and we can't understand the prior block's branch 1035 // condition. 1036 if (MBB->empty()) { 1037 bool PredHasNoFallThrough = TII->BlockHasNoFallThrough(PrevBB); 1038 if (PredHasNoFallThrough || !PriorUnAnalyzable || 1039 !PrevBB.isSuccessor(MBB)) { 1040 // If the prior block falls through into us, turn it into an 1041 // explicit branch to us to make updates simpler. 1042 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) && 1043 PriorTBB != MBB && PriorFBB != MBB) { 1044 if (PriorTBB == 0) { 1045 assert(PriorCond.empty() && PriorFBB == 0 && 1046 "Bad branch analysis"); 1047 PriorTBB = MBB; 1048 } else { 1049 assert(PriorFBB == 0 && "Machine CFG out of date!"); 1050 PriorFBB = MBB; 1051 } 1052 TII->RemoveBranch(PrevBB); 1053 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); 1054 } 1055 1056 // Iterate through all the predecessors, revectoring each in-turn. 1057 size_t PI = 0; 1058 bool DidChange = false; 1059 bool HasBranchToSelf = false; 1060 while(PI != MBB->pred_size()) { 1061 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI); 1062 if (PMBB == MBB) { 1063 // If this block has an uncond branch to itself, leave it. 1064 ++PI; 1065 HasBranchToSelf = true; 1066 } else { 1067 DidChange = true; 1068 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB); 1069 } 1070 } 1071 1072 // Change any jumptables to go to the new MBB. 1073 MBB->getParent()->getJumpTableInfo()-> 1074 ReplaceMBBInJumpTables(MBB, CurTBB); 1075 if (DidChange) { 1076 ++NumBranchOpts; 1077 MadeChange = true; 1078 if (!HasBranchToSelf) return; 1079 } 1080 } 1081 } 1082 1083 // Add the branch back if the block is more than just an uncond branch. 1084 TII->InsertBranch(*MBB, CurTBB, 0, CurCond); 1085 } 1086 } 1087 1088 // If the prior block doesn't fall through into this block, and if this 1089 // block doesn't fall through into some other block, see if we can find a 1090 // place to move this block where a fall-through will happen. 1091 if (!CanFallThrough(&PrevBB, PriorUnAnalyzable, 1092 PriorTBB, PriorFBB, PriorCond)) { 1093 // Now we know that there was no fall-through into this block, check to 1094 // see if it has a fall-through into its successor. 1095 bool CurFallsThru = CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB, 1096 CurCond); 1097 1098 if (!MBB->isLandingPad()) { 1099 // Check all the predecessors of this block. If one of them has no fall 1100 // throughs, move this block right after it. 1101 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(), 1102 E = MBB->pred_end(); PI != E; ++PI) { 1103 // Analyze the branch at the end of the pred. 1104 MachineBasicBlock *PredBB = *PI; 1105 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough; 1106 if (PredBB != MBB && !CanFallThrough(PredBB) 1107 && (!CurFallsThru || !CurTBB || !CurFBB) 1108 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) { 1109 // If the current block doesn't fall through, just move it. 1110 // If the current block can fall through and does not end with a 1111 // conditional branch, we need to append an unconditional jump to 1112 // the (current) next block. To avoid a possible compile-time 1113 // infinite loop, move blocks only backward in this case. 1114 // Also, if there are already 2 branches here, we cannot add a third; 1115 // this means we have the case 1116 // Bcc next 1117 // B elsewhere 1118 // next: 1119 if (CurFallsThru) { 1120 MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB)); 1121 CurCond.clear(); 1122 TII->InsertBranch(*MBB, NextBB, 0, CurCond); 1123 } 1124 MBB->moveAfter(PredBB); 1125 MadeChange = true; 1126 return OptimizeBlock(MBB); 1127 } 1128 } 1129 } 1130 1131 if (!CurFallsThru) { 1132 // Check all successors to see if we can move this block before it. 1133 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(), 1134 E = MBB->succ_end(); SI != E; ++SI) { 1135 // Analyze the branch at the end of the block before the succ. 1136 MachineBasicBlock *SuccBB = *SI; 1137 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev; 1138 std::vector<MachineOperand> SuccPrevCond; 1139 1140 // If this block doesn't already fall-through to that successor, and if 1141 // the succ doesn't already have a block that can fall through into it, 1142 // and if the successor isn't an EH destination, we can arrange for the 1143 // fallthrough to happen. 1144 if (SuccBB != MBB && !CanFallThrough(SuccPrev) && 1145 !SuccBB->isLandingPad()) { 1146 MBB->moveBefore(SuccBB); 1147 MadeChange = true; 1148 return OptimizeBlock(MBB); 1149 } 1150 } 1151 1152 // Okay, there is no really great place to put this block. If, however, 1153 // the block before this one would be a fall-through if this block were 1154 // removed, move this block to the end of the function. 1155 if (FallThrough != MBB->getParent()->end() && 1156 PrevBB.isSuccessor(FallThrough)) { 1157 MBB->moveAfter(--MBB->getParent()->end()); 1158 MadeChange = true; 1159 return; 1160 } 1161 } 1162 } 1163} 1164