BranchFolding.cpp revision b3c27428969b3cc52ab8493e91b5dd1465325fed
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/Support/ErrorHandling.h" 31#include "llvm/Support/raw_ostream.h" 32#include "llvm/ADT/SmallSet.h" 33#include "llvm/ADT/Statistic.h" 34#include "llvm/ADT/STLExtras.h" 35#include <algorithm> 36using namespace llvm; 37 38STATISTIC(NumDeadBlocks, "Number of dead blocks removed"); 39STATISTIC(NumBranchOpts, "Number of branches optimized"); 40STATISTIC(NumTailMerge , "Number of block tails merged"); 41static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge", 42 cl::init(cl::BOU_UNSET), cl::Hidden); 43// Throttle for huge numbers of predecessors (compile speed problems) 44static cl::opt<unsigned> 45TailMergeThreshold("tail-merge-threshold", 46 cl::desc("Max number of predecessors to consider tail merging"), 47 cl::init(150), cl::Hidden); 48 49namespace { 50 struct VISIBILITY_HIDDEN BranchFolder : public MachineFunctionPass { 51 static char ID; 52 explicit BranchFolder(bool defaultEnableTailMerge) : 53 MachineFunctionPass(&ID) { 54 switch (FlagEnableTailMerge) { 55 case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break; 56 case cl::BOU_TRUE: EnableTailMerge = true; break; 57 case cl::BOU_FALSE: EnableTailMerge = false; break; 58 } 59 } 60 61 virtual bool runOnMachineFunction(MachineFunction &MF); 62 virtual const char *getPassName() const { return "Control Flow Optimizer"; } 63 const TargetInstrInfo *TII; 64 MachineModuleInfo *MMI; 65 bool MadeChange; 66 private: 67 // Tail Merging. 68 bool EnableTailMerge; 69 bool TailMergeBlocks(MachineFunction &MF); 70 bool TryMergeBlocks(MachineBasicBlock* SuccBB, 71 MachineBasicBlock* PredBB); 72 void ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, 73 MachineBasicBlock *NewDest); 74 MachineBasicBlock *SplitMBBAt(MachineBasicBlock &CurMBB, 75 MachineBasicBlock::iterator BBI1); 76 unsigned ComputeSameTails(unsigned CurHash, unsigned minCommonTailLength); 77 void RemoveBlocksWithHash(unsigned CurHash, MachineBasicBlock* SuccBB, 78 MachineBasicBlock* PredBB); 79 unsigned CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB, 80 unsigned maxCommonTailLength); 81 82 typedef std::pair<unsigned,MachineBasicBlock*> MergePotentialsElt; 83 typedef std::vector<MergePotentialsElt>::iterator MPIterator; 84 std::vector<MergePotentialsElt> MergePotentials; 85 86 typedef std::pair<MPIterator, MachineBasicBlock::iterator> SameTailElt; 87 std::vector<SameTailElt> SameTails; 88 89 const TargetRegisterInfo *RegInfo; 90 RegScavenger *RS; 91 // Branch optzn. 92 bool OptimizeBranches(MachineFunction &MF); 93 void OptimizeBlock(MachineBasicBlock *MBB); 94 void RemoveDeadBlock(MachineBasicBlock *MBB); 95 bool OptimizeImpDefsBlock(MachineBasicBlock *MBB); 96 97 bool CanFallThrough(MachineBasicBlock *CurBB); 98 bool CanFallThrough(MachineBasicBlock *CurBB, bool BranchUnAnalyzable, 99 MachineBasicBlock *TBB, MachineBasicBlock *FBB, 100 const SmallVectorImpl<MachineOperand> &Cond); 101 }; 102 char BranchFolder::ID = 0; 103} 104 105FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) { 106 return new BranchFolder(DefaultEnableTailMerge); 107} 108 109/// RemoveDeadBlock - Remove the specified dead machine basic block from the 110/// function, updating the CFG. 111void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) { 112 assert(MBB->pred_empty() && "MBB must be dead!"); 113 DEBUG(errs() << "\nRemoving MBB: " << *MBB); 114 115 MachineFunction *MF = MBB->getParent(); 116 // drop all successors. 117 while (!MBB->succ_empty()) 118 MBB->removeSuccessor(MBB->succ_end()-1); 119 120 // If there are any labels in the basic block, unregister them from 121 // MachineModuleInfo. 122 if (MMI && !MBB->empty()) { 123 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); 124 I != E; ++I) { 125 if (I->isLabel()) 126 // The label ID # is always operand #0, an immediate. 127 MMI->InvalidateLabel(I->getOperand(0).getImm()); 128 } 129 } 130 131 // Remove the block. 132 MF->erase(MBB); 133} 134 135/// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def 136/// followed by terminators, and if the implicitly defined registers are not 137/// used by the terminators, remove those implicit_def's. e.g. 138/// BB1: 139/// r0 = implicit_def 140/// r1 = implicit_def 141/// br 142/// This block can be optimized away later if the implicit instructions are 143/// removed. 144bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) { 145 SmallSet<unsigned, 4> ImpDefRegs; 146 MachineBasicBlock::iterator I = MBB->begin(); 147 while (I != MBB->end()) { 148 if (I->getOpcode() != TargetInstrInfo::IMPLICIT_DEF) 149 break; 150 unsigned Reg = I->getOperand(0).getReg(); 151 ImpDefRegs.insert(Reg); 152 for (const unsigned *SubRegs = RegInfo->getSubRegisters(Reg); 153 unsigned SubReg = *SubRegs; ++SubRegs) 154 ImpDefRegs.insert(SubReg); 155 ++I; 156 } 157 if (ImpDefRegs.empty()) 158 return false; 159 160 MachineBasicBlock::iterator FirstTerm = I; 161 while (I != MBB->end()) { 162 if (!TII->isUnpredicatedTerminator(I)) 163 return false; 164 // See if it uses any of the implicitly defined registers. 165 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) { 166 MachineOperand &MO = I->getOperand(i); 167 if (!MO.isReg() || !MO.isUse()) 168 continue; 169 unsigned Reg = MO.getReg(); 170 if (ImpDefRegs.count(Reg)) 171 return false; 172 } 173 ++I; 174 } 175 176 I = MBB->begin(); 177 while (I != FirstTerm) { 178 MachineInstr *ImpDefMI = &*I; 179 ++I; 180 MBB->erase(ImpDefMI); 181 } 182 183 return true; 184} 185 186bool BranchFolder::runOnMachineFunction(MachineFunction &MF) { 187 TII = MF.getTarget().getInstrInfo(); 188 if (!TII) return false; 189 190 RegInfo = MF.getTarget().getRegisterInfo(); 191 192 // Fix CFG. The later algorithms expect it to be right. 193 bool EverMadeChange = false; 194 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) { 195 MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0; 196 SmallVector<MachineOperand, 4> Cond; 197 if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true)) 198 EverMadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty()); 199 EverMadeChange |= OptimizeImpDefsBlock(MBB); 200 } 201 202 RS = RegInfo->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL; 203 204 MMI = getAnalysisIfAvailable<MachineModuleInfo>(); 205 206 bool MadeChangeThisIteration = true; 207 while (MadeChangeThisIteration) { 208 MadeChangeThisIteration = false; 209 MadeChangeThisIteration |= TailMergeBlocks(MF); 210 MadeChangeThisIteration |= OptimizeBranches(MF); 211 EverMadeChange |= MadeChangeThisIteration; 212 } 213 214 // See if any jump tables have become mergable or dead as the code generator 215 // did its thing. 216 MachineJumpTableInfo *JTI = MF.getJumpTableInfo(); 217 const std::vector<MachineJumpTableEntry> &JTs = JTI->getJumpTables(); 218 if (!JTs.empty()) { 219 // Figure out how these jump tables should be merged. 220 std::vector<unsigned> JTMapping; 221 JTMapping.reserve(JTs.size()); 222 223 // We always keep the 0th jump table. 224 JTMapping.push_back(0); 225 226 // Scan the jump tables, seeing if there are any duplicates. Note that this 227 // is N^2, which should be fixed someday. 228 for (unsigned i = 1, e = JTs.size(); i != e; ++i) 229 JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs)); 230 231 // If a jump table was merge with another one, walk the function rewriting 232 // references to jump tables to reference the new JT ID's. Keep track of 233 // whether we see a jump table idx, if not, we can delete the JT. 234 BitVector JTIsLive(JTs.size()); 235 for (MachineFunction::iterator BB = MF.begin(), E = MF.end(); 236 BB != E; ++BB) { 237 for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end(); 238 I != E; ++I) 239 for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) { 240 MachineOperand &Op = I->getOperand(op); 241 if (!Op.isJTI()) continue; 242 unsigned NewIdx = JTMapping[Op.getIndex()]; 243 Op.setIndex(NewIdx); 244 245 // Remember that this JT is live. 246 JTIsLive.set(NewIdx); 247 } 248 } 249 250 // Finally, remove dead jump tables. This happens either because the 251 // indirect jump was unreachable (and thus deleted) or because the jump 252 // table was merged with some other one. 253 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i) 254 if (!JTIsLive.test(i)) { 255 JTI->RemoveJumpTable(i); 256 EverMadeChange = true; 257 } 258 } 259 260 delete RS; 261 return EverMadeChange; 262} 263 264//===----------------------------------------------------------------------===// 265// Tail Merging of Blocks 266//===----------------------------------------------------------------------===// 267 268/// HashMachineInstr - Compute a hash value for MI and its operands. 269static unsigned HashMachineInstr(const MachineInstr *MI) { 270 unsigned Hash = MI->getOpcode(); 271 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 272 const MachineOperand &Op = MI->getOperand(i); 273 274 // Merge in bits from the operand if easy. 275 unsigned OperandHash = 0; 276 switch (Op.getType()) { 277 case MachineOperand::MO_Register: OperandHash = Op.getReg(); break; 278 case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break; 279 case MachineOperand::MO_MachineBasicBlock: 280 OperandHash = Op.getMBB()->getNumber(); 281 break; 282 case MachineOperand::MO_FrameIndex: 283 case MachineOperand::MO_ConstantPoolIndex: 284 case MachineOperand::MO_JumpTableIndex: 285 OperandHash = Op.getIndex(); 286 break; 287 case MachineOperand::MO_GlobalAddress: 288 case MachineOperand::MO_ExternalSymbol: 289 // Global address / external symbol are too hard, don't bother, but do 290 // pull in the offset. 291 OperandHash = Op.getOffset(); 292 break; 293 default: break; 294 } 295 296 Hash += ((OperandHash << 3) | Op.getType()) << (i&31); 297 } 298 return Hash; 299} 300 301/// HashEndOfMBB - Hash the last few instructions in the MBB. For blocks 302/// with no successors, we hash two instructions, because cross-jumping 303/// only saves code when at least two instructions are removed (since a 304/// branch must be inserted). For blocks with a successor, one of the 305/// two blocks to be tail-merged will end with a branch already, so 306/// it gains to cross-jump even for one instruction. 307 308static unsigned HashEndOfMBB(const MachineBasicBlock *MBB, 309 unsigned minCommonTailLength) { 310 MachineBasicBlock::const_iterator I = MBB->end(); 311 if (I == MBB->begin()) 312 return 0; // Empty MBB. 313 314 --I; 315 unsigned Hash = HashMachineInstr(I); 316 317 if (I == MBB->begin() || minCommonTailLength == 1) 318 return Hash; // Single instr MBB. 319 320 --I; 321 // Hash in the second-to-last instruction. 322 Hash ^= HashMachineInstr(I) << 2; 323 return Hash; 324} 325 326/// ComputeCommonTailLength - Given two machine basic blocks, compute the number 327/// of instructions they actually have in common together at their end. Return 328/// iterators for the first shared instruction in each block. 329static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1, 330 MachineBasicBlock *MBB2, 331 MachineBasicBlock::iterator &I1, 332 MachineBasicBlock::iterator &I2) { 333 I1 = MBB1->end(); 334 I2 = MBB2->end(); 335 336 unsigned TailLen = 0; 337 while (I1 != MBB1->begin() && I2 != MBB2->begin()) { 338 --I1; --I2; 339 if (!I1->isIdenticalTo(I2) || 340 // FIXME: This check is dubious. It's used to get around a problem where 341 // people incorrectly expect inline asm directives to remain in the same 342 // relative order. This is untenable because normal compiler 343 // optimizations (like this one) may reorder and/or merge these 344 // directives. 345 I1->getOpcode() == TargetInstrInfo::INLINEASM) { 346 ++I1; ++I2; 347 break; 348 } 349 ++TailLen; 350 } 351 return TailLen; 352} 353 354/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything 355/// after it, replacing it with an unconditional branch to NewDest. This 356/// returns true if OldInst's block is modified, false if NewDest is modified. 357void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, 358 MachineBasicBlock *NewDest) { 359 MachineBasicBlock *OldBB = OldInst->getParent(); 360 361 // Remove all the old successors of OldBB from the CFG. 362 while (!OldBB->succ_empty()) 363 OldBB->removeSuccessor(OldBB->succ_begin()); 364 365 // Remove all the dead instructions from the end of OldBB. 366 OldBB->erase(OldInst, OldBB->end()); 367 368 // If OldBB isn't immediately before OldBB, insert a branch to it. 369 if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest)) 370 TII->InsertBranch(*OldBB, NewDest, 0, SmallVector<MachineOperand, 0>()); 371 OldBB->addSuccessor(NewDest); 372 ++NumTailMerge; 373} 374 375/// SplitMBBAt - Given a machine basic block and an iterator into it, split the 376/// MBB so that the part before the iterator falls into the part starting at the 377/// iterator. This returns the new MBB. 378MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB, 379 MachineBasicBlock::iterator BBI1) { 380 MachineFunction &MF = *CurMBB.getParent(); 381 382 // Create the fall-through block. 383 MachineFunction::iterator MBBI = &CurMBB; 384 MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(CurMBB.getBasicBlock()); 385 CurMBB.getParent()->insert(++MBBI, NewMBB); 386 387 // Move all the successors of this block to the specified block. 388 NewMBB->transferSuccessors(&CurMBB); 389 390 // Add an edge from CurMBB to NewMBB for the fall-through. 391 CurMBB.addSuccessor(NewMBB); 392 393 // Splice the code over. 394 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end()); 395 396 // For targets that use the register scavenger, we must maintain LiveIns. 397 if (RS) { 398 RS->enterBasicBlock(&CurMBB); 399 if (!CurMBB.empty()) 400 RS->forward(prior(CurMBB.end())); 401 BitVector RegsLiveAtExit(RegInfo->getNumRegs()); 402 RS->getRegsUsed(RegsLiveAtExit, false); 403 for (unsigned int i=0, e=RegInfo->getNumRegs(); i!=e; i++) 404 if (RegsLiveAtExit[i]) 405 NewMBB->addLiveIn(i); 406 } 407 408 return NewMBB; 409} 410 411/// EstimateRuntime - Make a rough estimate for how long it will take to run 412/// the specified code. 413static unsigned EstimateRuntime(MachineBasicBlock::iterator I, 414 MachineBasicBlock::iterator E) { 415 unsigned Time = 0; 416 for (; I != E; ++I) { 417 const TargetInstrDesc &TID = I->getDesc(); 418 if (TID.isCall()) 419 Time += 10; 420 else if (TID.mayLoad() || TID.mayStore()) 421 Time += 2; 422 else 423 ++Time; 424 } 425 return Time; 426} 427 428// CurMBB needs to add an unconditional branch to SuccMBB (we removed these 429// branches temporarily for tail merging). In the case where CurMBB ends 430// with a conditional branch to the next block, optimize by reversing the 431// test and conditionally branching to SuccMBB instead. 432 433static void FixTail(MachineBasicBlock* CurMBB, MachineBasicBlock *SuccBB, 434 const TargetInstrInfo *TII) { 435 MachineFunction *MF = CurMBB->getParent(); 436 MachineFunction::iterator I = next(MachineFunction::iterator(CurMBB)); 437 MachineBasicBlock *TBB = 0, *FBB = 0; 438 SmallVector<MachineOperand, 4> Cond; 439 if (I != MF->end() && 440 !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond, true)) { 441 MachineBasicBlock *NextBB = I; 442 if (TBB == NextBB && !Cond.empty() && !FBB) { 443 if (!TII->ReverseBranchCondition(Cond)) { 444 TII->RemoveBranch(*CurMBB); 445 TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond); 446 return; 447 } 448 } 449 } 450 TII->InsertBranch(*CurMBB, SuccBB, NULL, SmallVector<MachineOperand, 0>()); 451} 452 453static bool MergeCompare(const std::pair<unsigned,MachineBasicBlock*> &p, 454 const std::pair<unsigned,MachineBasicBlock*> &q) { 455 if (p.first < q.first) 456 return true; 457 else if (p.first > q.first) 458 return false; 459 else if (p.second->getNumber() < q.second->getNumber()) 460 return true; 461 else if (p.second->getNumber() > q.second->getNumber()) 462 return false; 463 else { 464 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing 465 // an object with itself. 466#ifndef _GLIBCXX_DEBUG 467 llvm_unreachable("Predecessor appears twice"); 468#endif 469 return false; 470 } 471} 472 473/// ComputeSameTails - Look through all the blocks in MergePotentials that have 474/// hash CurHash (guaranteed to match the last element). Build the vector 475/// SameTails of all those that have the (same) largest number of instructions 476/// in common of any pair of these blocks. SameTails entries contain an 477/// iterator into MergePotentials (from which the MachineBasicBlock can be 478/// found) and a MachineBasicBlock::iterator into that MBB indicating the 479/// instruction where the matching code sequence begins. 480/// Order of elements in SameTails is the reverse of the order in which 481/// those blocks appear in MergePotentials (where they are not necessarily 482/// consecutive). 483unsigned BranchFolder::ComputeSameTails(unsigned CurHash, 484 unsigned minCommonTailLength) { 485 unsigned maxCommonTailLength = 0U; 486 SameTails.clear(); 487 MachineBasicBlock::iterator TrialBBI1, TrialBBI2; 488 MPIterator HighestMPIter = prior(MergePotentials.end()); 489 for (MPIterator CurMPIter = prior(MergePotentials.end()), 490 B = MergePotentials.begin(); 491 CurMPIter!=B && CurMPIter->first==CurHash; 492 --CurMPIter) { 493 for (MPIterator I = prior(CurMPIter); I->first==CurHash ; --I) { 494 unsigned CommonTailLen = ComputeCommonTailLength( 495 CurMPIter->second, 496 I->second, 497 TrialBBI1, TrialBBI2); 498 // If we will have to split a block, there should be at least 499 // minCommonTailLength instructions in common; if not, at worst 500 // we will be replacing a fallthrough into the common tail with a 501 // branch, which at worst breaks even with falling through into 502 // the duplicated common tail, so 1 instruction in common is enough. 503 // We will always pick a block we do not have to split as the common 504 // tail if there is one. 505 // (Empty blocks will get forwarded and need not be considered.) 506 if (CommonTailLen >= minCommonTailLength || 507 (CommonTailLen > 0 && 508 (TrialBBI1==CurMPIter->second->begin() || 509 TrialBBI2==I->second->begin()))) { 510 if (CommonTailLen > maxCommonTailLength) { 511 SameTails.clear(); 512 maxCommonTailLength = CommonTailLen; 513 HighestMPIter = CurMPIter; 514 SameTails.push_back(std::make_pair(CurMPIter, TrialBBI1)); 515 } 516 if (HighestMPIter == CurMPIter && 517 CommonTailLen == maxCommonTailLength) 518 SameTails.push_back(std::make_pair(I, TrialBBI2)); 519 } 520 if (I==B) 521 break; 522 } 523 } 524 return maxCommonTailLength; 525} 526 527/// RemoveBlocksWithHash - Remove all blocks with hash CurHash from 528/// MergePotentials, restoring branches at ends of blocks as appropriate. 529void BranchFolder::RemoveBlocksWithHash(unsigned CurHash, 530 MachineBasicBlock* SuccBB, 531 MachineBasicBlock* PredBB) { 532 MPIterator CurMPIter, B; 533 for (CurMPIter = prior(MergePotentials.end()), B = MergePotentials.begin(); 534 CurMPIter->first==CurHash; 535 --CurMPIter) { 536 // Put the unconditional branch back, if we need one. 537 MachineBasicBlock *CurMBB = CurMPIter->second; 538 if (SuccBB && CurMBB != PredBB) 539 FixTail(CurMBB, SuccBB, TII); 540 if (CurMPIter==B) 541 break; 542 } 543 if (CurMPIter->first!=CurHash) 544 CurMPIter++; 545 MergePotentials.erase(CurMPIter, MergePotentials.end()); 546} 547 548/// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist 549/// only of the common tail. Create a block that does by splitting one. 550unsigned BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB, 551 unsigned maxCommonTailLength) { 552 unsigned i, commonTailIndex; 553 unsigned TimeEstimate = ~0U; 554 for (i=0, commonTailIndex=0; i<SameTails.size(); i++) { 555 // Use PredBB if possible; that doesn't require a new branch. 556 if (SameTails[i].first->second==PredBB) { 557 commonTailIndex = i; 558 break; 559 } 560 // Otherwise, make a (fairly bogus) choice based on estimate of 561 // how long it will take the various blocks to execute. 562 unsigned t = EstimateRuntime(SameTails[i].first->second->begin(), 563 SameTails[i].second); 564 if (t<=TimeEstimate) { 565 TimeEstimate = t; 566 commonTailIndex = i; 567 } 568 } 569 570 MachineBasicBlock::iterator BBI = SameTails[commonTailIndex].second; 571 MachineBasicBlock *MBB = SameTails[commonTailIndex].first->second; 572 573 DEBUG(errs() << "\nSplitting " << MBB->getNumber() << ", size " 574 << maxCommonTailLength); 575 576 MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI); 577 SameTails[commonTailIndex].first->second = newMBB; 578 SameTails[commonTailIndex].second = newMBB->begin(); 579 // If we split PredBB, newMBB is the new predecessor. 580 if (PredBB==MBB) 581 PredBB = newMBB; 582 583 return commonTailIndex; 584} 585 586// See if any of the blocks in MergePotentials (which all have a common single 587// successor, or all have no successor) can be tail-merged. If there is a 588// successor, any blocks in MergePotentials that are not tail-merged and 589// are not immediately before Succ must have an unconditional branch to 590// Succ added (but the predecessor/successor lists need no adjustment). 591// The lone predecessor of Succ that falls through into Succ, 592// if any, is given in PredBB. 593 594bool BranchFolder::TryMergeBlocks(MachineBasicBlock *SuccBB, 595 MachineBasicBlock* PredBB) { 596 // It doesn't make sense to save a single instruction since tail merging 597 // will add a jump. 598 // FIXME: Ask the target to provide the threshold? 599 unsigned minCommonTailLength = (SuccBB ? 1 : 2) + 1; 600 MadeChange = false; 601 602 DEBUG(errs() << "\nTryMergeBlocks " << MergePotentials.size() << '\n'); 603 604 // Sort by hash value so that blocks with identical end sequences sort 605 // together. 606 std::stable_sort(MergePotentials.begin(), MergePotentials.end(),MergeCompare); 607 608 // Walk through equivalence sets looking for actual exact matches. 609 while (MergePotentials.size() > 1) { 610 unsigned CurHash = prior(MergePotentials.end())->first; 611 612 // Build SameTails, identifying the set of blocks with this hash code 613 // and with the maximum number of instructions in common. 614 unsigned maxCommonTailLength = ComputeSameTails(CurHash, 615 minCommonTailLength); 616 617 // If we didn't find any pair that has at least minCommonTailLength 618 // instructions in common, remove all blocks with this hash code and retry. 619 if (SameTails.empty()) { 620 RemoveBlocksWithHash(CurHash, SuccBB, PredBB); 621 continue; 622 } 623 624 // If one of the blocks is the entire common tail (and not the entry 625 // block, which we can't jump to), we can treat all blocks with this same 626 // tail at once. Use PredBB if that is one of the possibilities, as that 627 // will not introduce any extra branches. 628 MachineBasicBlock *EntryBB = MergePotentials.begin()->second-> 629 getParent()->begin(); 630 unsigned int commonTailIndex, i; 631 for (commonTailIndex=SameTails.size(), i=0; i<SameTails.size(); i++) { 632 MachineBasicBlock *MBB = SameTails[i].first->second; 633 if (MBB->begin() == SameTails[i].second && MBB != EntryBB) { 634 commonTailIndex = i; 635 if (MBB==PredBB) 636 break; 637 } 638 } 639 640 if (commonTailIndex==SameTails.size()) { 641 // None of the blocks consist entirely of the common tail. 642 // Split a block so that one does. 643 commonTailIndex = CreateCommonTailOnlyBlock(PredBB, maxCommonTailLength); 644 } 645 646 MachineBasicBlock *MBB = SameTails[commonTailIndex].first->second; 647 // MBB is common tail. Adjust all other BB's to jump to this one. 648 // Traversal must be forwards so erases work. 649 DEBUG(errs() << "\nUsing common tail " << MBB->getNumber() << " for "); 650 for (unsigned int i=0; i<SameTails.size(); ++i) { 651 if (commonTailIndex==i) 652 continue; 653 DEBUG(errs() << SameTails[i].first->second->getNumber() << ","); 654 // Hack the end off BB i, making it jump to BB commonTailIndex instead. 655 ReplaceTailWithBranchTo(SameTails[i].second, MBB); 656 // BB i is no longer a predecessor of SuccBB; remove it from the worklist. 657 MergePotentials.erase(SameTails[i].first); 658 } 659 DEBUG(errs() << "\n"); 660 // We leave commonTailIndex in the worklist in case there are other blocks 661 // that match it with a smaller number of instructions. 662 MadeChange = true; 663 } 664 return MadeChange; 665} 666 667bool BranchFolder::TailMergeBlocks(MachineFunction &MF) { 668 669 if (!EnableTailMerge) return false; 670 671 MadeChange = false; 672 673 // First find blocks with no successors. 674 MergePotentials.clear(); 675 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { 676 if (I->succ_empty()) 677 MergePotentials.push_back(std::make_pair(HashEndOfMBB(I, 2U), I)); 678 } 679 // See if we can do any tail merging on those. 680 if (MergePotentials.size() < TailMergeThreshold && 681 MergePotentials.size() >= 2) 682 MadeChange |= TryMergeBlocks(NULL, NULL); 683 684 // Look at blocks (IBB) with multiple predecessors (PBB). 685 // We change each predecessor to a canonical form, by 686 // (1) temporarily removing any unconditional branch from the predecessor 687 // to IBB, and 688 // (2) alter conditional branches so they branch to the other block 689 // not IBB; this may require adding back an unconditional branch to IBB 690 // later, where there wasn't one coming in. E.g. 691 // Bcc IBB 692 // fallthrough to QBB 693 // here becomes 694 // Bncc QBB 695 // with a conceptual B to IBB after that, which never actually exists. 696 // With those changes, we see whether the predecessors' tails match, 697 // and merge them if so. We change things out of canonical form and 698 // back to the way they were later in the process. (OptimizeBranches 699 // would undo some of this, but we can't use it, because we'd get into 700 // a compile-time infinite loop repeatedly doing and undoing the same 701 // transformations.) 702 703 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { 704 if (I->pred_size() >= 2 && I->pred_size() < TailMergeThreshold) { 705 SmallPtrSet<MachineBasicBlock *, 8> UniquePreds; 706 MachineBasicBlock *IBB = I; 707 MachineBasicBlock *PredBB = prior(I); 708 MergePotentials.clear(); 709 for (MachineBasicBlock::pred_iterator P = I->pred_begin(), 710 E2 = I->pred_end(); 711 P != E2; ++P) { 712 MachineBasicBlock* PBB = *P; 713 // Skip blocks that loop to themselves, can't tail merge these. 714 if (PBB==IBB) 715 continue; 716 // Visit each predecessor only once. 717 if (!UniquePreds.insert(PBB)) 718 continue; 719 MachineBasicBlock *TBB = 0, *FBB = 0; 720 SmallVector<MachineOperand, 4> Cond; 721 if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond, true)) { 722 // Failing case: IBB is the target of a cbr, and 723 // we cannot reverse the branch. 724 SmallVector<MachineOperand, 4> NewCond(Cond); 725 if (!Cond.empty() && TBB==IBB) { 726 if (TII->ReverseBranchCondition(NewCond)) 727 continue; 728 // This is the QBB case described above 729 if (!FBB) 730 FBB = next(MachineFunction::iterator(PBB)); 731 } 732 // Failing case: the only way IBB can be reached from PBB is via 733 // exception handling. Happens for landing pads. Would be nice 734 // to have a bit in the edge so we didn't have to do all this. 735 if (IBB->isLandingPad()) { 736 MachineFunction::iterator IP = PBB; IP++; 737 MachineBasicBlock* PredNextBB = NULL; 738 if (IP!=MF.end()) 739 PredNextBB = IP; 740 if (TBB==NULL) { 741 if (IBB!=PredNextBB) // fallthrough 742 continue; 743 } else if (FBB) { 744 if (TBB!=IBB && FBB!=IBB) // cbr then ubr 745 continue; 746 } else if (Cond.empty()) { 747 if (TBB!=IBB) // ubr 748 continue; 749 } else { 750 if (TBB!=IBB && IBB!=PredNextBB) // cbr 751 continue; 752 } 753 } 754 // Remove the unconditional branch at the end, if any. 755 if (TBB && (Cond.empty() || FBB)) { 756 TII->RemoveBranch(*PBB); 757 if (!Cond.empty()) 758 // reinsert conditional branch only, for now 759 TII->InsertBranch(*PBB, (TBB==IBB) ? FBB : TBB, 0, NewCond); 760 } 761 MergePotentials.push_back(std::make_pair(HashEndOfMBB(PBB, 1U), *P)); 762 } 763 } 764 if (MergePotentials.size() >= 2) 765 MadeChange |= TryMergeBlocks(I, PredBB); 766 // Reinsert an unconditional branch if needed. 767 // The 1 below can occur as a result of removing blocks in TryMergeBlocks. 768 PredBB = prior(I); // this may have been changed in TryMergeBlocks 769 if (MergePotentials.size()==1 && 770 MergePotentials.begin()->second != PredBB) 771 FixTail(MergePotentials.begin()->second, I, TII); 772 } 773 } 774 return MadeChange; 775} 776 777//===----------------------------------------------------------------------===// 778// Branch Optimization 779//===----------------------------------------------------------------------===// 780 781bool BranchFolder::OptimizeBranches(MachineFunction &MF) { 782 MadeChange = false; 783 784 // Make sure blocks are numbered in order 785 MF.RenumberBlocks(); 786 787 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) { 788 MachineBasicBlock *MBB = I++; 789 OptimizeBlock(MBB); 790 791 // If it is dead, remove it. 792 if (MBB->pred_empty()) { 793 RemoveDeadBlock(MBB); 794 MadeChange = true; 795 ++NumDeadBlocks; 796 } 797 } 798 return MadeChange; 799} 800 801 802/// CanFallThrough - Return true if the specified block (with the specified 803/// branch condition) can implicitly transfer control to the block after it by 804/// falling off the end of it. This should return false if it can reach the 805/// block after it, but it uses an explicit branch to do so (e.g. a table jump). 806/// 807/// True is a conservative answer. 808/// 809bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB, 810 bool BranchUnAnalyzable, 811 MachineBasicBlock *TBB, 812 MachineBasicBlock *FBB, 813 const SmallVectorImpl<MachineOperand> &Cond) { 814 MachineFunction::iterator Fallthrough = CurBB; 815 ++Fallthrough; 816 // If FallthroughBlock is off the end of the function, it can't fall through. 817 if (Fallthrough == CurBB->getParent()->end()) 818 return false; 819 820 // If FallthroughBlock isn't a successor of CurBB, no fallthrough is possible. 821 if (!CurBB->isSuccessor(Fallthrough)) 822 return false; 823 824 // If we couldn't analyze the branch, assume it could fall through. 825 if (BranchUnAnalyzable) return true; 826 827 // If there is no branch, control always falls through. 828 if (TBB == 0) return true; 829 830 // If there is some explicit branch to the fallthrough block, it can obviously 831 // reach, even though the branch should get folded to fall through implicitly. 832 if (MachineFunction::iterator(TBB) == Fallthrough || 833 MachineFunction::iterator(FBB) == Fallthrough) 834 return true; 835 836 // If it's an unconditional branch to some block not the fall through, it 837 // doesn't fall through. 838 if (Cond.empty()) return false; 839 840 // Otherwise, if it is conditional and has no explicit false block, it falls 841 // through. 842 return FBB == 0; 843} 844 845/// CanFallThrough - Return true if the specified can implicitly transfer 846/// control to the block after it by falling off the end of it. This should 847/// return false if it can reach the block after it, but it uses an explicit 848/// branch to do so (e.g. a table jump). 849/// 850/// True is a conservative answer. 851/// 852bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) { 853 MachineBasicBlock *TBB = 0, *FBB = 0; 854 SmallVector<MachineOperand, 4> Cond; 855 bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond, true); 856 return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond); 857} 858 859/// IsBetterFallthrough - Return true if it would be clearly better to 860/// fall-through to MBB1 than to fall through into MBB2. This has to return 861/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will 862/// result in infinite loops. 863static bool IsBetterFallthrough(MachineBasicBlock *MBB1, 864 MachineBasicBlock *MBB2) { 865 // Right now, we use a simple heuristic. If MBB2 ends with a call, and 866 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to 867 // optimize branches that branch to either a return block or an assert block 868 // into a fallthrough to the return. 869 if (MBB1->empty() || MBB2->empty()) return false; 870 871 // If there is a clear successor ordering we make sure that one block 872 // will fall through to the next 873 if (MBB1->isSuccessor(MBB2)) return true; 874 if (MBB2->isSuccessor(MBB1)) return false; 875 876 MachineInstr *MBB1I = --MBB1->end(); 877 MachineInstr *MBB2I = --MBB2->end(); 878 return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall(); 879} 880 881/// OptimizeBlock - Analyze and optimize control flow related to the specified 882/// block. This is never called on the entry block. 883void BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) { 884 MachineFunction::iterator FallThrough = MBB; 885 ++FallThrough; 886 887 // If this block is empty, make everyone use its fall-through, not the block 888 // explicitly. Landing pads should not do this since the landing-pad table 889 // points to this block. 890 if (MBB->empty() && !MBB->isLandingPad()) { 891 // Dead block? Leave for cleanup later. 892 if (MBB->pred_empty()) return; 893 894 if (FallThrough == MBB->getParent()->end()) { 895 // TODO: Simplify preds to not branch here if possible! 896 } else { 897 // Rewrite all predecessors of the old block to go to the fallthrough 898 // instead. 899 while (!MBB->pred_empty()) { 900 MachineBasicBlock *Pred = *(MBB->pred_end()-1); 901 Pred->ReplaceUsesOfBlockWith(MBB, FallThrough); 902 } 903 // If MBB was the target of a jump table, update jump tables to go to the 904 // fallthrough instead. 905 MBB->getParent()->getJumpTableInfo()-> 906 ReplaceMBBInJumpTables(MBB, FallThrough); 907 MadeChange = true; 908 } 909 return; 910 } 911 912 // Check to see if we can simplify the terminator of the block before this 913 // one. 914 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB)); 915 916 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0; 917 SmallVector<MachineOperand, 4> PriorCond; 918 bool PriorUnAnalyzable = 919 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true); 920 if (!PriorUnAnalyzable) { 921 // If the CFG for the prior block has extra edges, remove them. 922 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB, 923 !PriorCond.empty()); 924 925 // If the previous branch is conditional and both conditions go to the same 926 // destination, remove the branch, replacing it with an unconditional one or 927 // a fall-through. 928 if (PriorTBB && PriorTBB == PriorFBB) { 929 TII->RemoveBranch(PrevBB); 930 PriorCond.clear(); 931 if (PriorTBB != MBB) 932 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 933 MadeChange = true; 934 ++NumBranchOpts; 935 return OptimizeBlock(MBB); 936 } 937 938 // If the previous branch *only* branches to *this* block (conditional or 939 // not) remove the branch. 940 if (PriorTBB == MBB && PriorFBB == 0) { 941 TII->RemoveBranch(PrevBB); 942 MadeChange = true; 943 ++NumBranchOpts; 944 return OptimizeBlock(MBB); 945 } 946 947 // If the prior block branches somewhere else on the condition and here if 948 // the condition is false, remove the uncond second branch. 949 if (PriorFBB == MBB) { 950 TII->RemoveBranch(PrevBB); 951 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 952 MadeChange = true; 953 ++NumBranchOpts; 954 return OptimizeBlock(MBB); 955 } 956 957 // If the prior block branches here on true and somewhere else on false, and 958 // if the branch condition is reversible, reverse the branch to create a 959 // fall-through. 960 if (PriorTBB == MBB) { 961 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond); 962 if (!TII->ReverseBranchCondition(NewPriorCond)) { 963 TII->RemoveBranch(PrevBB); 964 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond); 965 MadeChange = true; 966 ++NumBranchOpts; 967 return OptimizeBlock(MBB); 968 } 969 } 970 971 // If this block doesn't fall through (e.g. it ends with an uncond branch or 972 // has no successors) and if the pred falls through into this block, and if 973 // it would otherwise fall through into the block after this, move this 974 // block to the end of the function. 975 // 976 // We consider it more likely that execution will stay in the function (e.g. 977 // due to loops) than it is to exit it. This asserts in loops etc, moving 978 // the assert condition out of the loop body. 979 if (!PriorCond.empty() && PriorFBB == 0 && 980 MachineFunction::iterator(PriorTBB) == FallThrough && 981 !CanFallThrough(MBB)) { 982 bool DoTransform = true; 983 984 // We have to be careful that the succs of PredBB aren't both no-successor 985 // blocks. If neither have successors and if PredBB is the second from 986 // last block in the function, we'd just keep swapping the two blocks for 987 // last. Only do the swap if one is clearly better to fall through than 988 // the other. 989 if (FallThrough == --MBB->getParent()->end() && 990 !IsBetterFallthrough(PriorTBB, MBB)) 991 DoTransform = false; 992 993 // We don't want to do this transformation if we have control flow like: 994 // br cond BB2 995 // BB1: 996 // .. 997 // jmp BBX 998 // BB2: 999 // .. 1000 // ret 1001 // 1002 // In this case, we could actually be moving the return block *into* a 1003 // loop! 1004 if (DoTransform && !MBB->succ_empty() && 1005 (!CanFallThrough(PriorTBB) || PriorTBB->empty())) 1006 DoTransform = false; 1007 1008 1009 if (DoTransform) { 1010 // Reverse the branch so we will fall through on the previous true cond. 1011 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond); 1012 if (!TII->ReverseBranchCondition(NewPriorCond)) { 1013 DEBUG(errs() << "\nMoving MBB: " << *MBB 1014 << "To make fallthrough to: " << *PriorTBB << "\n"); 1015 1016 TII->RemoveBranch(PrevBB); 1017 TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond); 1018 1019 // Move this block to the end of the function. 1020 MBB->moveAfter(--MBB->getParent()->end()); 1021 MadeChange = true; 1022 ++NumBranchOpts; 1023 return; 1024 } 1025 } 1026 } 1027 } 1028 1029 // Analyze the branch in the current block. 1030 MachineBasicBlock *CurTBB = 0, *CurFBB = 0; 1031 SmallVector<MachineOperand, 4> CurCond; 1032 bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true); 1033 if (!CurUnAnalyzable) { 1034 // If the CFG for the prior block has extra edges, remove them. 1035 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty()); 1036 1037 // If this is a two-way branch, and the FBB branches to this block, reverse 1038 // the condition so the single-basic-block loop is faster. Instead of: 1039 // Loop: xxx; jcc Out; jmp Loop 1040 // we want: 1041 // Loop: xxx; jncc Loop; jmp Out 1042 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) { 1043 SmallVector<MachineOperand, 4> NewCond(CurCond); 1044 if (!TII->ReverseBranchCondition(NewCond)) { 1045 TII->RemoveBranch(*MBB); 1046 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond); 1047 MadeChange = true; 1048 ++NumBranchOpts; 1049 return OptimizeBlock(MBB); 1050 } 1051 } 1052 1053 1054 // If this branch is the only thing in its block, see if we can forward 1055 // other blocks across it. 1056 if (CurTBB && CurCond.empty() && CurFBB == 0 && 1057 MBB->begin()->getDesc().isBranch() && CurTBB != MBB) { 1058 // This block may contain just an unconditional branch. Because there can 1059 // be 'non-branch terminators' in the block, try removing the branch and 1060 // then seeing if the block is empty. 1061 TII->RemoveBranch(*MBB); 1062 1063 // If this block is just an unconditional branch to CurTBB, we can 1064 // usually completely eliminate the block. The only case we cannot 1065 // completely eliminate the block is when the block before this one 1066 // falls through into MBB and we can't understand the prior block's branch 1067 // condition. 1068 if (MBB->empty()) { 1069 bool PredHasNoFallThrough = TII->BlockHasNoFallThrough(PrevBB); 1070 if (PredHasNoFallThrough || !PriorUnAnalyzable || 1071 !PrevBB.isSuccessor(MBB)) { 1072 // If the prior block falls through into us, turn it into an 1073 // explicit branch to us to make updates simpler. 1074 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) && 1075 PriorTBB != MBB && PriorFBB != MBB) { 1076 if (PriorTBB == 0) { 1077 assert(PriorCond.empty() && PriorFBB == 0 && 1078 "Bad branch analysis"); 1079 PriorTBB = MBB; 1080 } else { 1081 assert(PriorFBB == 0 && "Machine CFG out of date!"); 1082 PriorFBB = MBB; 1083 } 1084 TII->RemoveBranch(PrevBB); 1085 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); 1086 } 1087 1088 // Iterate through all the predecessors, revectoring each in-turn. 1089 size_t PI = 0; 1090 bool DidChange = false; 1091 bool HasBranchToSelf = false; 1092 while(PI != MBB->pred_size()) { 1093 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI); 1094 if (PMBB == MBB) { 1095 // If this block has an uncond branch to itself, leave it. 1096 ++PI; 1097 HasBranchToSelf = true; 1098 } else { 1099 DidChange = true; 1100 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB); 1101 // If this change resulted in PMBB ending in a conditional 1102 // branch where both conditions go to the same destination, 1103 // change this to an unconditional branch (and fix the CFG). 1104 MachineBasicBlock *NewCurTBB = 0, *NewCurFBB = 0; 1105 SmallVector<MachineOperand, 4> NewCurCond; 1106 bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB, 1107 NewCurFBB, NewCurCond, true); 1108 if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) { 1109 TII->RemoveBranch(*PMBB); 1110 NewCurCond.clear(); 1111 TII->InsertBranch(*PMBB, NewCurTBB, 0, NewCurCond); 1112 MadeChange = true; 1113 ++NumBranchOpts; 1114 PMBB->CorrectExtraCFGEdges(NewCurTBB, NewCurFBB, false); 1115 } 1116 } 1117 } 1118 1119 // Change any jumptables to go to the new MBB. 1120 MBB->getParent()->getJumpTableInfo()-> 1121 ReplaceMBBInJumpTables(MBB, CurTBB); 1122 if (DidChange) { 1123 ++NumBranchOpts; 1124 MadeChange = true; 1125 if (!HasBranchToSelf) return; 1126 } 1127 } 1128 } 1129 1130 // Add the branch back if the block is more than just an uncond branch. 1131 TII->InsertBranch(*MBB, CurTBB, 0, CurCond); 1132 } 1133 } 1134 1135 // If the prior block doesn't fall through into this block, and if this 1136 // block doesn't fall through into some other block, see if we can find a 1137 // place to move this block where a fall-through will happen. 1138 if (!CanFallThrough(&PrevBB, PriorUnAnalyzable, 1139 PriorTBB, PriorFBB, PriorCond)) { 1140 // Now we know that there was no fall-through into this block, check to 1141 // see if it has a fall-through into its successor. 1142 bool CurFallsThru = CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB, 1143 CurCond); 1144 1145 if (!MBB->isLandingPad()) { 1146 // Check all the predecessors of this block. If one of them has no fall 1147 // throughs, move this block right after it. 1148 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(), 1149 E = MBB->pred_end(); PI != E; ++PI) { 1150 // Analyze the branch at the end of the pred. 1151 MachineBasicBlock *PredBB = *PI; 1152 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough; 1153 if (PredBB != MBB && !CanFallThrough(PredBB) 1154 && (!CurFallsThru || !CurTBB || !CurFBB) 1155 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) { 1156 // If the current block doesn't fall through, just move it. 1157 // If the current block can fall through and does not end with a 1158 // conditional branch, we need to append an unconditional jump to 1159 // the (current) next block. To avoid a possible compile-time 1160 // infinite loop, move blocks only backward in this case. 1161 // Also, if there are already 2 branches here, we cannot add a third; 1162 // this means we have the case 1163 // Bcc next 1164 // B elsewhere 1165 // next: 1166 if (CurFallsThru) { 1167 MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB)); 1168 CurCond.clear(); 1169 TII->InsertBranch(*MBB, NextBB, 0, CurCond); 1170 } 1171 MBB->moveAfter(PredBB); 1172 MadeChange = true; 1173 return OptimizeBlock(MBB); 1174 } 1175 } 1176 } 1177 1178 if (!CurFallsThru) { 1179 // Check all successors to see if we can move this block before it. 1180 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(), 1181 E = MBB->succ_end(); SI != E; ++SI) { 1182 // Analyze the branch at the end of the block before the succ. 1183 MachineBasicBlock *SuccBB = *SI; 1184 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev; 1185 std::vector<MachineOperand> SuccPrevCond; 1186 1187 // If this block doesn't already fall-through to that successor, and if 1188 // the succ doesn't already have a block that can fall through into it, 1189 // and if the successor isn't an EH destination, we can arrange for the 1190 // fallthrough to happen. 1191 if (SuccBB != MBB && !CanFallThrough(SuccPrev) && 1192 !SuccBB->isLandingPad()) { 1193 MBB->moveBefore(SuccBB); 1194 MadeChange = true; 1195 return OptimizeBlock(MBB); 1196 } 1197 } 1198 1199 // Okay, there is no really great place to put this block. If, however, 1200 // the block before this one would be a fall-through if this block were 1201 // removed, move this block to the end of the function. 1202 if (FallThrough != MBB->getParent()->end() && 1203 PrevBB.isSuccessor(FallThrough)) { 1204 MBB->moveAfter(--MBB->getParent()->end()); 1205 MadeChange = true; 1206 return; 1207 } 1208 } 1209 } 1210} 1211