BranchFolding.cpp revision cd4f04d6bcb7aefa24d92582fbadfe17519f4756
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 "BranchFolding.h" 21#include "llvm/CodeGen/Passes.h" 22#include "llvm/CodeGen/MachineModuleInfo.h" 23#include "llvm/CodeGen/MachineFunctionPass.h" 24#include "llvm/CodeGen/MachineJumpTableInfo.h" 25#include "llvm/CodeGen/RegisterScavenging.h" 26#include "llvm/Target/TargetInstrInfo.h" 27#include "llvm/Target/TargetMachine.h" 28#include "llvm/Target/TargetRegisterInfo.h" 29#include "llvm/Support/CommandLine.h" 30#include "llvm/Support/Debug.h" 31#include "llvm/Support/ErrorHandling.h" 32#include "llvm/Support/raw_ostream.h" 33#include "llvm/ADT/SmallSet.h" 34#include "llvm/ADT/Statistic.h" 35#include "llvm/ADT/STLExtras.h" 36#include <algorithm> 37using namespace llvm; 38 39STATISTIC(NumDeadBlocks, "Number of dead blocks removed"); 40STATISTIC(NumBranchOpts, "Number of branches optimized"); 41STATISTIC(NumTailMerge , "Number of block tails merged"); 42static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge", 43 cl::init(cl::BOU_UNSET), cl::Hidden); 44// Throttle for huge numbers of predecessors (compile speed problems) 45static cl::opt<unsigned> 46TailMergeThreshold("tail-merge-threshold", 47 cl::desc("Max number of predecessors to consider tail merging"), 48 cl::init(150), cl::Hidden); 49 50 51char BranchFolderPass::ID = 0; 52 53FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge, 54 CodeGenOpt::Level OptLevel) { 55 return new BranchFolderPass(DefaultEnableTailMerge, OptLevel); 56} 57 58bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) { 59 return OptimizeFunction(MF, 60 MF.getTarget().getInstrInfo(), 61 MF.getTarget().getRegisterInfo(), 62 getAnalysisIfAvailable<MachineModuleInfo>()); 63} 64 65 66 67BranchFolder::BranchFolder(bool defaultEnableTailMerge, CodeGenOpt::Level OL) { 68 OptLevel = OL; 69 switch (FlagEnableTailMerge) { 70 case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break; 71 case cl::BOU_TRUE: EnableTailMerge = true; break; 72 case cl::BOU_FALSE: EnableTailMerge = false; break; 73 } 74} 75 76/// RemoveDeadBlock - Remove the specified dead machine basic block from the 77/// function, updating the CFG. 78void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) { 79 assert(MBB->pred_empty() && "MBB must be dead!"); 80 DEBUG(errs() << "\nRemoving MBB: " << *MBB); 81 82 MachineFunction *MF = MBB->getParent(); 83 // drop all successors. 84 while (!MBB->succ_empty()) 85 MBB->removeSuccessor(MBB->succ_end()-1); 86 87 // If there are any labels in the basic block, unregister them from 88 // MachineModuleInfo. 89 if (MMI && !MBB->empty()) { 90 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); 91 I != E; ++I) { 92 if (I->isLabel()) 93 // The label ID # is always operand #0, an immediate. 94 MMI->InvalidateLabel(I->getOperand(0).getImm()); 95 } 96 } 97 98 // Remove the block. 99 MF->erase(MBB); 100} 101 102/// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def 103/// followed by terminators, and if the implicitly defined registers are not 104/// used by the terminators, remove those implicit_def's. e.g. 105/// BB1: 106/// r0 = implicit_def 107/// r1 = implicit_def 108/// br 109/// This block can be optimized away later if the implicit instructions are 110/// removed. 111bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) { 112 SmallSet<unsigned, 4> ImpDefRegs; 113 MachineBasicBlock::iterator I = MBB->begin(); 114 while (I != MBB->end()) { 115 if (I->getOpcode() != TargetInstrInfo::IMPLICIT_DEF) 116 break; 117 unsigned Reg = I->getOperand(0).getReg(); 118 ImpDefRegs.insert(Reg); 119 for (const unsigned *SubRegs = TRI->getSubRegisters(Reg); 120 unsigned SubReg = *SubRegs; ++SubRegs) 121 ImpDefRegs.insert(SubReg); 122 ++I; 123 } 124 if (ImpDefRegs.empty()) 125 return false; 126 127 MachineBasicBlock::iterator FirstTerm = I; 128 while (I != MBB->end()) { 129 if (!TII->isUnpredicatedTerminator(I)) 130 return false; 131 // See if it uses any of the implicitly defined registers. 132 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) { 133 MachineOperand &MO = I->getOperand(i); 134 if (!MO.isReg() || !MO.isUse()) 135 continue; 136 unsigned Reg = MO.getReg(); 137 if (ImpDefRegs.count(Reg)) 138 return false; 139 } 140 ++I; 141 } 142 143 I = MBB->begin(); 144 while (I != FirstTerm) { 145 MachineInstr *ImpDefMI = &*I; 146 ++I; 147 MBB->erase(ImpDefMI); 148 } 149 150 return true; 151} 152 153/// OptimizeFunction - Perhaps branch folding, tail merging and other 154/// CFG optimizations on the given function. 155bool BranchFolder::OptimizeFunction(MachineFunction &MF, 156 const TargetInstrInfo *tii, 157 const TargetRegisterInfo *tri, 158 MachineModuleInfo *mmi) { 159 if (!tii) return false; 160 161 TII = tii; 162 TRI = tri; 163 MMI = mmi; 164 165 RS = TRI->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL; 166 167 // Fix CFG. The later algorithms expect it to be right. 168 bool MadeChange = false; 169 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) { 170 MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0; 171 SmallVector<MachineOperand, 4> Cond; 172 if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true)) 173 MadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty()); 174 MadeChange |= OptimizeImpDefsBlock(MBB); 175 } 176 177 178 bool MadeChangeThisIteration = true; 179 while (MadeChangeThisIteration) { 180 MadeChangeThisIteration = false; 181 MadeChangeThisIteration |= TailMergeBlocks(MF); 182 MadeChangeThisIteration |= OptimizeBranches(MF); 183 MadeChange |= MadeChangeThisIteration; 184 } 185 186 // See if any jump tables have become mergable or dead as the code generator 187 // did its thing. 188 MachineJumpTableInfo *JTI = MF.getJumpTableInfo(); 189 const std::vector<MachineJumpTableEntry> &JTs = JTI->getJumpTables(); 190 if (!JTs.empty()) { 191 // Figure out how these jump tables should be merged. 192 std::vector<unsigned> JTMapping; 193 JTMapping.reserve(JTs.size()); 194 195 // We always keep the 0th jump table. 196 JTMapping.push_back(0); 197 198 // Scan the jump tables, seeing if there are any duplicates. Note that this 199 // is N^2, which should be fixed someday. 200 for (unsigned i = 1, e = JTs.size(); i != e; ++i) { 201 if (JTs[i].MBBs.empty()) 202 JTMapping.push_back(i); 203 else 204 JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs)); 205 } 206 207 // If a jump table was merge with another one, walk the function rewriting 208 // references to jump tables to reference the new JT ID's. Keep track of 209 // whether we see a jump table idx, if not, we can delete the JT. 210 BitVector JTIsLive(JTs.size()); 211 for (MachineFunction::iterator BB = MF.begin(), E = MF.end(); 212 BB != E; ++BB) { 213 for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end(); 214 I != E; ++I) 215 for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) { 216 MachineOperand &Op = I->getOperand(op); 217 if (!Op.isJTI()) continue; 218 unsigned NewIdx = JTMapping[Op.getIndex()]; 219 Op.setIndex(NewIdx); 220 221 // Remember that this JT is live. 222 JTIsLive.set(NewIdx); 223 } 224 } 225 226 // Finally, remove dead jump tables. This happens either because the 227 // indirect jump was unreachable (and thus deleted) or because the jump 228 // table was merged with some other one. 229 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i) 230 if (!JTIsLive.test(i)) { 231 JTI->RemoveJumpTable(i); 232 MadeChange = true; 233 } 234 } 235 236 delete RS; 237 return MadeChange; 238} 239 240//===----------------------------------------------------------------------===// 241// Tail Merging of Blocks 242//===----------------------------------------------------------------------===// 243 244/// HashMachineInstr - Compute a hash value for MI and its operands. 245static unsigned HashMachineInstr(const MachineInstr *MI) { 246 unsigned Hash = MI->getOpcode(); 247 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 248 const MachineOperand &Op = MI->getOperand(i); 249 250 // Merge in bits from the operand if easy. 251 unsigned OperandHash = 0; 252 switch (Op.getType()) { 253 case MachineOperand::MO_Register: OperandHash = Op.getReg(); break; 254 case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break; 255 case MachineOperand::MO_MachineBasicBlock: 256 OperandHash = Op.getMBB()->getNumber(); 257 break; 258 case MachineOperand::MO_FrameIndex: 259 case MachineOperand::MO_ConstantPoolIndex: 260 case MachineOperand::MO_JumpTableIndex: 261 OperandHash = Op.getIndex(); 262 break; 263 case MachineOperand::MO_GlobalAddress: 264 case MachineOperand::MO_ExternalSymbol: 265 // Global address / external symbol are too hard, don't bother, but do 266 // pull in the offset. 267 OperandHash = Op.getOffset(); 268 break; 269 default: break; 270 } 271 272 Hash += ((OperandHash << 3) | Op.getType()) << (i&31); 273 } 274 return Hash; 275} 276 277/// HashEndOfMBB - Hash the last few instructions in the MBB. For blocks 278/// with no successors, we hash two instructions, because cross-jumping 279/// only saves code when at least two instructions are removed (since a 280/// branch must be inserted). For blocks with a successor, one of the 281/// two blocks to be tail-merged will end with a branch already, so 282/// it gains to cross-jump even for one instruction. 283 284static unsigned HashEndOfMBB(const MachineBasicBlock *MBB, 285 unsigned minCommonTailLength) { 286 MachineBasicBlock::const_iterator I = MBB->end(); 287 if (I == MBB->begin()) 288 return 0; // Empty MBB. 289 290 --I; 291 unsigned Hash = HashMachineInstr(I); 292 293 if (I == MBB->begin() || minCommonTailLength == 1) 294 return Hash; // Single instr MBB. 295 296 --I; 297 // Hash in the second-to-last instruction. 298 Hash ^= HashMachineInstr(I) << 2; 299 return Hash; 300} 301 302/// ComputeCommonTailLength - Given two machine basic blocks, compute the number 303/// of instructions they actually have in common together at their end. Return 304/// iterators for the first shared instruction in each block. 305static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1, 306 MachineBasicBlock *MBB2, 307 MachineBasicBlock::iterator &I1, 308 MachineBasicBlock::iterator &I2) { 309 I1 = MBB1->end(); 310 I2 = MBB2->end(); 311 312 unsigned TailLen = 0; 313 while (I1 != MBB1->begin() && I2 != MBB2->begin()) { 314 --I1; --I2; 315 if (!I1->isIdenticalTo(I2) || 316 // FIXME: This check is dubious. It's used to get around a problem where 317 // people incorrectly expect inline asm directives to remain in the same 318 // relative order. This is untenable because normal compiler 319 // optimizations (like this one) may reorder and/or merge these 320 // directives. 321 I1->getOpcode() == TargetInstrInfo::INLINEASM) { 322 ++I1; ++I2; 323 break; 324 } 325 ++TailLen; 326 } 327 return TailLen; 328} 329 330/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything 331/// after it, replacing it with an unconditional branch to NewDest. This 332/// returns true if OldInst's block is modified, false if NewDest is modified. 333void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, 334 MachineBasicBlock *NewDest) { 335 MachineBasicBlock *OldBB = OldInst->getParent(); 336 337 // Remove all the old successors of OldBB from the CFG. 338 while (!OldBB->succ_empty()) 339 OldBB->removeSuccessor(OldBB->succ_begin()); 340 341 // Remove all the dead instructions from the end of OldBB. 342 OldBB->erase(OldInst, OldBB->end()); 343 344 // If OldBB isn't immediately before OldBB, insert a branch to it. 345 if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest)) 346 TII->InsertBranch(*OldBB, NewDest, 0, SmallVector<MachineOperand, 0>()); 347 OldBB->addSuccessor(NewDest); 348 ++NumTailMerge; 349} 350 351/// SplitMBBAt - Given a machine basic block and an iterator into it, split the 352/// MBB so that the part before the iterator falls into the part starting at the 353/// iterator. This returns the new MBB. 354MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB, 355 MachineBasicBlock::iterator BBI1) { 356 MachineFunction &MF = *CurMBB.getParent(); 357 358 // Create the fall-through block. 359 MachineFunction::iterator MBBI = &CurMBB; 360 MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(CurMBB.getBasicBlock()); 361 CurMBB.getParent()->insert(++MBBI, NewMBB); 362 363 // Move all the successors of this block to the specified block. 364 NewMBB->transferSuccessors(&CurMBB); 365 366 // Add an edge from CurMBB to NewMBB for the fall-through. 367 CurMBB.addSuccessor(NewMBB); 368 369 // Splice the code over. 370 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end()); 371 372 // For targets that use the register scavenger, we must maintain LiveIns. 373 if (RS) { 374 RS->enterBasicBlock(&CurMBB); 375 if (!CurMBB.empty()) 376 RS->forward(prior(CurMBB.end())); 377 BitVector RegsLiveAtExit(TRI->getNumRegs()); 378 RS->getRegsUsed(RegsLiveAtExit, false); 379 for (unsigned int i=0, e=TRI->getNumRegs(); i!=e; i++) 380 if (RegsLiveAtExit[i]) 381 NewMBB->addLiveIn(i); 382 } 383 384 return NewMBB; 385} 386 387/// EstimateRuntime - Make a rough estimate for how long it will take to run 388/// the specified code. 389static unsigned EstimateRuntime(MachineBasicBlock::iterator I, 390 MachineBasicBlock::iterator E) { 391 unsigned Time = 0; 392 for (; I != E; ++I) { 393 const TargetInstrDesc &TID = I->getDesc(); 394 if (TID.isCall()) 395 Time += 10; 396 else if (TID.mayLoad() || TID.mayStore()) 397 Time += 2; 398 else 399 ++Time; 400 } 401 return Time; 402} 403 404// CurMBB needs to add an unconditional branch to SuccMBB (we removed these 405// branches temporarily for tail merging). In the case where CurMBB ends 406// with a conditional branch to the next block, optimize by reversing the 407// test and conditionally branching to SuccMBB instead. 408 409static void FixTail(MachineBasicBlock* CurMBB, MachineBasicBlock *SuccBB, 410 const TargetInstrInfo *TII) { 411 MachineFunction *MF = CurMBB->getParent(); 412 MachineFunction::iterator I = next(MachineFunction::iterator(CurMBB)); 413 MachineBasicBlock *TBB = 0, *FBB = 0; 414 SmallVector<MachineOperand, 4> Cond; 415 if (I != MF->end() && 416 !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond, true)) { 417 MachineBasicBlock *NextBB = I; 418 if (TBB == NextBB && !Cond.empty() && !FBB) { 419 if (!TII->ReverseBranchCondition(Cond)) { 420 TII->RemoveBranch(*CurMBB); 421 TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond); 422 return; 423 } 424 } 425 } 426 TII->InsertBranch(*CurMBB, SuccBB, NULL, SmallVector<MachineOperand, 0>()); 427} 428 429static bool MergeCompare(const std::pair<unsigned,MachineBasicBlock*> &p, 430 const std::pair<unsigned,MachineBasicBlock*> &q) { 431 if (p.first < q.first) 432 return true; 433 else if (p.first > q.first) 434 return false; 435 else if (p.second->getNumber() < q.second->getNumber()) 436 return true; 437 else if (p.second->getNumber() > q.second->getNumber()) 438 return false; 439 else { 440 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing 441 // an object with itself. 442#ifndef _GLIBCXX_DEBUG 443 llvm_unreachable("Predecessor appears twice"); 444#endif 445 return false; 446 } 447} 448 449/// ComputeSameTails - Look through all the blocks in MergePotentials that have 450/// hash CurHash (guaranteed to match the last element). Build the vector 451/// SameTails of all those that have the (same) largest number of instructions 452/// in common of any pair of these blocks. SameTails entries contain an 453/// iterator into MergePotentials (from which the MachineBasicBlock can be 454/// found) and a MachineBasicBlock::iterator into that MBB indicating the 455/// instruction where the matching code sequence begins. 456/// Order of elements in SameTails is the reverse of the order in which 457/// those blocks appear in MergePotentials (where they are not necessarily 458/// consecutive). 459unsigned BranchFolder::ComputeSameTails(unsigned CurHash, 460 unsigned minCommonTailLength) { 461 unsigned maxCommonTailLength = 0U; 462 SameTails.clear(); 463 MachineBasicBlock::iterator TrialBBI1, TrialBBI2; 464 MPIterator HighestMPIter = prior(MergePotentials.end()); 465 for (MPIterator CurMPIter = prior(MergePotentials.end()), 466 B = MergePotentials.begin(); 467 CurMPIter!=B && CurMPIter->first==CurHash; 468 --CurMPIter) { 469 for (MPIterator I = prior(CurMPIter); I->first==CurHash ; --I) { 470 unsigned CommonTailLen = ComputeCommonTailLength( 471 CurMPIter->second, 472 I->second, 473 TrialBBI1, TrialBBI2); 474 // If we will have to split a block, there should be at least 475 // minCommonTailLength instructions in common; if not, and if we are not 476 // optimizing for performance at the expense of code size, at worst 477 // we will be replacing a fallthrough into the common tail with a 478 // branch, which at worst breaks even with falling through into 479 // the duplicated common tail, so 1 instruction in common is enough. 480 // We will always pick a block we do not have to split as the common 481 // tail if there is one. 482 // (Empty blocks will get forwarded and need not be considered.) 483 if (CommonTailLen >= minCommonTailLength || 484 (OptLevel != CodeGenOpt::Aggressive && 485 CommonTailLen > 0 && 486 (TrialBBI1==CurMPIter->second->begin() || 487 TrialBBI2==I->second->begin()))) { 488 if (CommonTailLen > maxCommonTailLength) { 489 SameTails.clear(); 490 maxCommonTailLength = CommonTailLen; 491 HighestMPIter = CurMPIter; 492 SameTails.push_back(std::make_pair(CurMPIter, TrialBBI1)); 493 } 494 if (HighestMPIter == CurMPIter && 495 CommonTailLen == maxCommonTailLength) 496 SameTails.push_back(std::make_pair(I, TrialBBI2)); 497 } 498 if (I==B) 499 break; 500 } 501 } 502 return maxCommonTailLength; 503} 504 505/// RemoveBlocksWithHash - Remove all blocks with hash CurHash from 506/// MergePotentials, restoring branches at ends of blocks as appropriate. 507void BranchFolder::RemoveBlocksWithHash(unsigned CurHash, 508 MachineBasicBlock* SuccBB, 509 MachineBasicBlock* PredBB) { 510 MPIterator CurMPIter, B; 511 for (CurMPIter = prior(MergePotentials.end()), B = MergePotentials.begin(); 512 CurMPIter->first==CurHash; 513 --CurMPIter) { 514 // Put the unconditional branch back, if we need one. 515 MachineBasicBlock *CurMBB = CurMPIter->second; 516 if (SuccBB && CurMBB != PredBB) 517 FixTail(CurMBB, SuccBB, TII); 518 if (CurMPIter==B) 519 break; 520 } 521 if (CurMPIter->first!=CurHash) 522 CurMPIter++; 523 MergePotentials.erase(CurMPIter, MergePotentials.end()); 524} 525 526/// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist 527/// only of the common tail. Create a block that does by splitting one. 528unsigned BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB, 529 unsigned maxCommonTailLength) { 530 unsigned i, commonTailIndex; 531 unsigned TimeEstimate = ~0U; 532 for (i=0, commonTailIndex=0; i<SameTails.size(); i++) { 533 // Use PredBB if possible; that doesn't require a new branch. 534 if (SameTails[i].first->second==PredBB) { 535 commonTailIndex = i; 536 break; 537 } 538 // Otherwise, make a (fairly bogus) choice based on estimate of 539 // how long it will take the various blocks to execute. 540 unsigned t = EstimateRuntime(SameTails[i].first->second->begin(), 541 SameTails[i].second); 542 if (t<=TimeEstimate) { 543 TimeEstimate = t; 544 commonTailIndex = i; 545 } 546 } 547 548 MachineBasicBlock::iterator BBI = SameTails[commonTailIndex].second; 549 MachineBasicBlock *MBB = SameTails[commonTailIndex].first->second; 550 551 DEBUG(errs() << "\nSplitting " << MBB->getNumber() << ", size " 552 << maxCommonTailLength); 553 554 MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI); 555 SameTails[commonTailIndex].first->second = newMBB; 556 SameTails[commonTailIndex].second = newMBB->begin(); 557 // If we split PredBB, newMBB is the new predecessor. 558 if (PredBB==MBB) 559 PredBB = newMBB; 560 561 return commonTailIndex; 562} 563 564// See if any of the blocks in MergePotentials (which all have a common single 565// successor, or all have no successor) can be tail-merged. If there is a 566// successor, any blocks in MergePotentials that are not tail-merged and 567// are not immediately before Succ must have an unconditional branch to 568// Succ added (but the predecessor/successor lists need no adjustment). 569// The lone predecessor of Succ that falls through into Succ, 570// if any, is given in PredBB. 571 572bool BranchFolder::TryMergeBlocks(MachineBasicBlock *SuccBB, 573 MachineBasicBlock* PredBB) { 574 bool MadeChange = false; 575 576 // It doesn't make sense to save a single instruction since tail merging 577 // will add a jump. 578 // FIXME: Ask the target to provide the threshold? 579 unsigned minCommonTailLength = (SuccBB ? 1 : 2) + 1; 580 581 DEBUG(errs() << "\nTryMergeBlocks " << MergePotentials.size() << '\n'); 582 583 // Sort by hash value so that blocks with identical end sequences sort 584 // together. 585 std::stable_sort(MergePotentials.begin(), MergePotentials.end(),MergeCompare); 586 587 // Walk through equivalence sets looking for actual exact matches. 588 while (MergePotentials.size() > 1) { 589 unsigned CurHash = prior(MergePotentials.end())->first; 590 591 // Build SameTails, identifying the set of blocks with this hash code 592 // and with the maximum number of instructions in common. 593 unsigned maxCommonTailLength = ComputeSameTails(CurHash, 594 minCommonTailLength); 595 596 // If we didn't find any pair that has at least minCommonTailLength 597 // instructions in common, remove all blocks with this hash code and retry. 598 if (SameTails.empty()) { 599 RemoveBlocksWithHash(CurHash, SuccBB, PredBB); 600 continue; 601 } 602 603 // If one of the blocks is the entire common tail (and not the entry 604 // block, which we can't jump to), we can treat all blocks with this same 605 // tail at once. Use PredBB if that is one of the possibilities, as that 606 // will not introduce any extra branches. 607 MachineBasicBlock *EntryBB = MergePotentials.begin()->second-> 608 getParent()->begin(); 609 unsigned int commonTailIndex, i; 610 for (commonTailIndex=SameTails.size(), i=0; i<SameTails.size(); i++) { 611 MachineBasicBlock *MBB = SameTails[i].first->second; 612 if (MBB->begin() == SameTails[i].second && MBB != EntryBB) { 613 commonTailIndex = i; 614 if (MBB==PredBB) 615 break; 616 } 617 } 618 619 if (commonTailIndex==SameTails.size()) { 620 // None of the blocks consist entirely of the common tail. 621 // Split a block so that one does. 622 commonTailIndex = CreateCommonTailOnlyBlock(PredBB, maxCommonTailLength); 623 } 624 625 MachineBasicBlock *MBB = SameTails[commonTailIndex].first->second; 626 // MBB is common tail. Adjust all other BB's to jump to this one. 627 // Traversal must be forwards so erases work. 628 DEBUG(errs() << "\nUsing common tail " << MBB->getNumber() << " for "); 629 for (unsigned int i=0; i<SameTails.size(); ++i) { 630 if (commonTailIndex==i) 631 continue; 632 DEBUG(errs() << SameTails[i].first->second->getNumber() << ","); 633 // Hack the end off BB i, making it jump to BB commonTailIndex instead. 634 ReplaceTailWithBranchTo(SameTails[i].second, MBB); 635 // BB i is no longer a predecessor of SuccBB; remove it from the worklist. 636 MergePotentials.erase(SameTails[i].first); 637 } 638 DEBUG(errs() << "\n"); 639 // We leave commonTailIndex in the worklist in case there are other blocks 640 // that match it with a smaller number of instructions. 641 MadeChange = true; 642 } 643 return MadeChange; 644} 645 646bool BranchFolder::TailMergeBlocks(MachineFunction &MF) { 647 648 if (!EnableTailMerge) return false; 649 650 bool MadeChange = false; 651 652 // First find blocks with no successors. 653 MergePotentials.clear(); 654 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { 655 if (I->succ_empty()) 656 MergePotentials.push_back(std::make_pair(HashEndOfMBB(I, 2U), I)); 657 } 658 // See if we can do any tail merging on those. 659 if (MergePotentials.size() < TailMergeThreshold && 660 MergePotentials.size() >= 2) 661 MadeChange |= TryMergeBlocks(NULL, NULL); 662 663 // Look at blocks (IBB) with multiple predecessors (PBB). 664 // We change each predecessor to a canonical form, by 665 // (1) temporarily removing any unconditional branch from the predecessor 666 // to IBB, and 667 // (2) alter conditional branches so they branch to the other block 668 // not IBB; this may require adding back an unconditional branch to IBB 669 // later, where there wasn't one coming in. E.g. 670 // Bcc IBB 671 // fallthrough to QBB 672 // here becomes 673 // Bncc QBB 674 // with a conceptual B to IBB after that, which never actually exists. 675 // With those changes, we see whether the predecessors' tails match, 676 // and merge them if so. We change things out of canonical form and 677 // back to the way they were later in the process. (OptimizeBranches 678 // would undo some of this, but we can't use it, because we'd get into 679 // a compile-time infinite loop repeatedly doing and undoing the same 680 // transformations.) 681 682 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { 683 if (I->pred_size() >= 2 && I->pred_size() < TailMergeThreshold) { 684 SmallPtrSet<MachineBasicBlock *, 8> UniquePreds; 685 MachineBasicBlock *IBB = I; 686 MachineBasicBlock *PredBB = prior(I); 687 MergePotentials.clear(); 688 for (MachineBasicBlock::pred_iterator P = I->pred_begin(), 689 E2 = I->pred_end(); 690 P != E2; ++P) { 691 MachineBasicBlock* PBB = *P; 692 // Skip blocks that loop to themselves, can't tail merge these. 693 if (PBB==IBB) 694 continue; 695 // Visit each predecessor only once. 696 if (!UniquePreds.insert(PBB)) 697 continue; 698 MachineBasicBlock *TBB = 0, *FBB = 0; 699 SmallVector<MachineOperand, 4> Cond; 700 if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond, true)) { 701 // Failing case: IBB is the target of a cbr, and 702 // we cannot reverse the branch. 703 SmallVector<MachineOperand, 4> NewCond(Cond); 704 if (!Cond.empty() && TBB==IBB) { 705 if (TII->ReverseBranchCondition(NewCond)) 706 continue; 707 // This is the QBB case described above 708 if (!FBB) 709 FBB = next(MachineFunction::iterator(PBB)); 710 } 711 // Failing case: the only way IBB can be reached from PBB is via 712 // exception handling. Happens for landing pads. Would be nice 713 // to have a bit in the edge so we didn't have to do all this. 714 if (IBB->isLandingPad()) { 715 MachineFunction::iterator IP = PBB; IP++; 716 MachineBasicBlock* PredNextBB = NULL; 717 if (IP!=MF.end()) 718 PredNextBB = IP; 719 if (TBB==NULL) { 720 if (IBB!=PredNextBB) // fallthrough 721 continue; 722 } else if (FBB) { 723 if (TBB!=IBB && FBB!=IBB) // cbr then ubr 724 continue; 725 } else if (Cond.empty()) { 726 if (TBB!=IBB) // ubr 727 continue; 728 } else { 729 if (TBB!=IBB && IBB!=PredNextBB) // cbr 730 continue; 731 } 732 } 733 // Remove the unconditional branch at the end, if any. 734 if (TBB && (Cond.empty() || FBB)) { 735 TII->RemoveBranch(*PBB); 736 if (!Cond.empty()) 737 // reinsert conditional branch only, for now 738 TII->InsertBranch(*PBB, (TBB==IBB) ? FBB : TBB, 0, NewCond); 739 } 740 MergePotentials.push_back(std::make_pair(HashEndOfMBB(PBB, 1U), *P)); 741 } 742 } 743 if (MergePotentials.size() >= 2) 744 MadeChange |= TryMergeBlocks(I, PredBB); 745 // Reinsert an unconditional branch if needed. 746 // The 1 below can occur as a result of removing blocks in TryMergeBlocks. 747 PredBB = prior(I); // this may have been changed in TryMergeBlocks 748 if (MergePotentials.size()==1 && 749 MergePotentials.begin()->second != PredBB) 750 FixTail(MergePotentials.begin()->second, I, TII); 751 } 752 } 753 return MadeChange; 754} 755 756//===----------------------------------------------------------------------===// 757// Branch Optimization 758//===----------------------------------------------------------------------===// 759 760bool BranchFolder::OptimizeBranches(MachineFunction &MF) { 761 bool MadeChange = false; 762 763 // Make sure blocks are numbered in order 764 MF.RenumberBlocks(); 765 766 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) { 767 MachineBasicBlock *MBB = I++; 768 MadeChange |= OptimizeBlock(MBB); 769 770 // If it is dead, remove it. 771 if (MBB->pred_empty()) { 772 RemoveDeadBlock(MBB); 773 MadeChange = true; 774 ++NumDeadBlocks; 775 } 776 } 777 return MadeChange; 778} 779 780 781/// CanFallThrough - Return true if the specified block (with the specified 782/// branch condition) can implicitly transfer control to the block after it by 783/// falling off the end of it. This should return false if it can reach the 784/// block after it, but it uses an explicit branch to do so (e.g. a table jump). 785/// 786/// True is a conservative answer. 787/// 788bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB, 789 bool BranchUnAnalyzable, 790 MachineBasicBlock *TBB, 791 MachineBasicBlock *FBB, 792 const SmallVectorImpl<MachineOperand> &Cond) { 793 MachineFunction::iterator Fallthrough = CurBB; 794 ++Fallthrough; 795 // If FallthroughBlock is off the end of the function, it can't fall through. 796 if (Fallthrough == CurBB->getParent()->end()) 797 return false; 798 799 // If FallthroughBlock isn't a successor of CurBB, no fallthrough is possible. 800 if (!CurBB->isSuccessor(Fallthrough)) 801 return false; 802 803 // If we couldn't analyze the branch, assume it could fall through. 804 if (BranchUnAnalyzable) return true; 805 806 // If there is no branch, control always falls through. 807 if (TBB == 0) return true; 808 809 // If there is some explicit branch to the fallthrough block, it can obviously 810 // reach, even though the branch should get folded to fall through implicitly. 811 if (MachineFunction::iterator(TBB) == Fallthrough || 812 MachineFunction::iterator(FBB) == Fallthrough) 813 return true; 814 815 // If it's an unconditional branch to some block not the fall through, it 816 // doesn't fall through. 817 if (Cond.empty()) return false; 818 819 // Otherwise, if it is conditional and has no explicit false block, it falls 820 // through. 821 return FBB == 0; 822} 823 824/// CanFallThrough - Return true if the specified can implicitly transfer 825/// control to the block after it by falling off the end of it. This should 826/// return false if it can reach the block after it, but it uses an explicit 827/// branch to do so (e.g. a table jump). 828/// 829/// True is a conservative answer. 830/// 831bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) { 832 MachineBasicBlock *TBB = 0, *FBB = 0; 833 SmallVector<MachineOperand, 4> Cond; 834 bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond, true); 835 return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond); 836} 837 838/// IsBetterFallthrough - Return true if it would be clearly better to 839/// fall-through to MBB1 than to fall through into MBB2. This has to return 840/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will 841/// result in infinite loops. 842static bool IsBetterFallthrough(MachineBasicBlock *MBB1, 843 MachineBasicBlock *MBB2) { 844 // Right now, we use a simple heuristic. If MBB2 ends with a call, and 845 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to 846 // optimize branches that branch to either a return block or an assert block 847 // into a fallthrough to the return. 848 if (MBB1->empty() || MBB2->empty()) return false; 849 850 // If there is a clear successor ordering we make sure that one block 851 // will fall through to the next 852 if (MBB1->isSuccessor(MBB2)) return true; 853 if (MBB2->isSuccessor(MBB1)) return false; 854 855 MachineInstr *MBB1I = --MBB1->end(); 856 MachineInstr *MBB2I = --MBB2->end(); 857 return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall(); 858} 859 860/// OptimizeBlock - Analyze and optimize control flow related to the specified 861/// block. This is never called on the entry block. 862bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) { 863 bool MadeChange = false; 864 865 MachineFunction::iterator FallThrough = MBB; 866 ++FallThrough; 867 868 // If this block is empty, make everyone use its fall-through, not the block 869 // explicitly. Landing pads should not do this since the landing-pad table 870 // points to this block. 871 if (MBB->empty() && !MBB->isLandingPad()) { 872 // Dead block? Leave for cleanup later. 873 if (MBB->pred_empty()) return MadeChange; 874 875 if (FallThrough == MBB->getParent()->end()) { 876 // TODO: Simplify preds to not branch here if possible! 877 } else { 878 // Rewrite all predecessors of the old block to go to the fallthrough 879 // instead. 880 while (!MBB->pred_empty()) { 881 MachineBasicBlock *Pred = *(MBB->pred_end()-1); 882 Pred->ReplaceUsesOfBlockWith(MBB, FallThrough); 883 } 884 // If MBB was the target of a jump table, update jump tables to go to the 885 // fallthrough instead. 886 MBB->getParent()->getJumpTableInfo()-> 887 ReplaceMBBInJumpTables(MBB, FallThrough); 888 MadeChange = true; 889 } 890 return MadeChange; 891 } 892 893 // Check to see if we can simplify the terminator of the block before this 894 // one. 895 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB)); 896 897 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0; 898 SmallVector<MachineOperand, 4> PriorCond; 899 bool PriorUnAnalyzable = 900 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true); 901 if (!PriorUnAnalyzable) { 902 // If the CFG for the prior block has extra edges, remove them. 903 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB, 904 !PriorCond.empty()); 905 906 // If the previous branch is conditional and both conditions go to the same 907 // destination, remove the branch, replacing it with an unconditional one or 908 // a fall-through. 909 if (PriorTBB && PriorTBB == PriorFBB) { 910 TII->RemoveBranch(PrevBB); 911 PriorCond.clear(); 912 if (PriorTBB != MBB) 913 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 914 MadeChange = true; 915 ++NumBranchOpts; 916 return OptimizeBlock(MBB); 917 } 918 919 // If the previous branch *only* branches to *this* block (conditional or 920 // not) remove the branch. 921 if (PriorTBB == MBB && PriorFBB == 0) { 922 TII->RemoveBranch(PrevBB); 923 MadeChange = true; 924 ++NumBranchOpts; 925 return OptimizeBlock(MBB); 926 } 927 928 // If the prior block branches somewhere else on the condition and here if 929 // the condition is false, remove the uncond second branch. 930 if (PriorFBB == MBB) { 931 TII->RemoveBranch(PrevBB); 932 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 933 MadeChange = true; 934 ++NumBranchOpts; 935 return OptimizeBlock(MBB); 936 } 937 938 // If the prior block branches here on true and somewhere else on false, and 939 // if the branch condition is reversible, reverse the branch to create a 940 // fall-through. 941 if (PriorTBB == MBB) { 942 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond); 943 if (!TII->ReverseBranchCondition(NewPriorCond)) { 944 TII->RemoveBranch(PrevBB); 945 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond); 946 MadeChange = true; 947 ++NumBranchOpts; 948 return OptimizeBlock(MBB); 949 } 950 } 951 952 // If this block has no successors (e.g. it is a return block or ends with 953 // a call to a no-return function like abort or __cxa_throw) and if the pred 954 // falls through into this block, and if it would otherwise fall through 955 // into the block after this, move this block to the end of the function. 956 // 957 // We consider it more likely that execution will stay in the function (e.g. 958 // due to loops) than it is to exit it. This asserts in loops etc, moving 959 // the assert condition out of the loop body. 960 if (MBB->succ_empty() && !PriorCond.empty() && PriorFBB == 0 && 961 MachineFunction::iterator(PriorTBB) == FallThrough && 962 !CanFallThrough(MBB)) { 963 bool DoTransform = true; 964 965 // We have to be careful that the succs of PredBB aren't both no-successor 966 // blocks. If neither have successors and if PredBB is the second from 967 // last block in the function, we'd just keep swapping the two blocks for 968 // last. Only do the swap if one is clearly better to fall through than 969 // the other. 970 if (FallThrough == --MBB->getParent()->end() && 971 !IsBetterFallthrough(PriorTBB, MBB)) 972 DoTransform = false; 973 974 // We don't want to do this transformation if we have control flow like: 975 // br cond BB2 976 // BB1: 977 // .. 978 // jmp BBX 979 // BB2: 980 // .. 981 // ret 982 // 983 // In this case, we could actually be moving the return block *into* a 984 // loop! 985 if (DoTransform && !MBB->succ_empty() && 986 (!CanFallThrough(PriorTBB) || PriorTBB->empty())) 987 DoTransform = false; 988 989 990 if (DoTransform) { 991 // Reverse the branch so we will fall through on the previous true cond. 992 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond); 993 if (!TII->ReverseBranchCondition(NewPriorCond)) { 994 DEBUG(errs() << "\nMoving MBB: " << *MBB 995 << "To make fallthrough to: " << *PriorTBB << "\n"); 996 997 TII->RemoveBranch(PrevBB); 998 TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond); 999 1000 // Move this block to the end of the function. 1001 MBB->moveAfter(--MBB->getParent()->end()); 1002 MadeChange = true; 1003 ++NumBranchOpts; 1004 return MadeChange; 1005 } 1006 } 1007 } 1008 } 1009 1010 // Analyze the branch in the current block. 1011 MachineBasicBlock *CurTBB = 0, *CurFBB = 0; 1012 SmallVector<MachineOperand, 4> CurCond; 1013 bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true); 1014 if (!CurUnAnalyzable) { 1015 // If the CFG for the prior block has extra edges, remove them. 1016 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty()); 1017 1018 // If this is a two-way branch, and the FBB branches to this block, reverse 1019 // the condition so the single-basic-block loop is faster. Instead of: 1020 // Loop: xxx; jcc Out; jmp Loop 1021 // we want: 1022 // Loop: xxx; jncc Loop; jmp Out 1023 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) { 1024 SmallVector<MachineOperand, 4> NewCond(CurCond); 1025 if (!TII->ReverseBranchCondition(NewCond)) { 1026 TII->RemoveBranch(*MBB); 1027 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond); 1028 MadeChange = true; 1029 ++NumBranchOpts; 1030 return OptimizeBlock(MBB); 1031 } 1032 } 1033 1034 1035 // If this branch is the only thing in its block, see if we can forward 1036 // other blocks across it. 1037 if (CurTBB && CurCond.empty() && CurFBB == 0 && 1038 MBB->begin()->getDesc().isBranch() && CurTBB != MBB) { 1039 // This block may contain just an unconditional branch. Because there can 1040 // be 'non-branch terminators' in the block, try removing the branch and 1041 // then seeing if the block is empty. 1042 TII->RemoveBranch(*MBB); 1043 1044 // If this block is just an unconditional branch to CurTBB, we can 1045 // usually completely eliminate the block. The only case we cannot 1046 // completely eliminate the block is when the block before this one 1047 // falls through into MBB and we can't understand the prior block's branch 1048 // condition. 1049 if (MBB->empty()) { 1050 bool PredHasNoFallThrough = TII->BlockHasNoFallThrough(PrevBB); 1051 if (PredHasNoFallThrough || !PriorUnAnalyzable || 1052 !PrevBB.isSuccessor(MBB)) { 1053 // If the prior block falls through into us, turn it into an 1054 // explicit branch to us to make updates simpler. 1055 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) && 1056 PriorTBB != MBB && PriorFBB != MBB) { 1057 if (PriorTBB == 0) { 1058 assert(PriorCond.empty() && PriorFBB == 0 && 1059 "Bad branch analysis"); 1060 PriorTBB = MBB; 1061 } else { 1062 assert(PriorFBB == 0 && "Machine CFG out of date!"); 1063 PriorFBB = MBB; 1064 } 1065 TII->RemoveBranch(PrevBB); 1066 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); 1067 } 1068 1069 // Iterate through all the predecessors, revectoring each in-turn. 1070 size_t PI = 0; 1071 bool DidChange = false; 1072 bool HasBranchToSelf = false; 1073 while(PI != MBB->pred_size()) { 1074 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI); 1075 if (PMBB == MBB) { 1076 // If this block has an uncond branch to itself, leave it. 1077 ++PI; 1078 HasBranchToSelf = true; 1079 } else { 1080 DidChange = true; 1081 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB); 1082 // If this change resulted in PMBB ending in a conditional 1083 // branch where both conditions go to the same destination, 1084 // change this to an unconditional branch (and fix the CFG). 1085 MachineBasicBlock *NewCurTBB = 0, *NewCurFBB = 0; 1086 SmallVector<MachineOperand, 4> NewCurCond; 1087 bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB, 1088 NewCurFBB, NewCurCond, true); 1089 if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) { 1090 TII->RemoveBranch(*PMBB); 1091 NewCurCond.clear(); 1092 TII->InsertBranch(*PMBB, NewCurTBB, 0, NewCurCond); 1093 MadeChange = true; 1094 ++NumBranchOpts; 1095 PMBB->CorrectExtraCFGEdges(NewCurTBB, NewCurFBB, false); 1096 } 1097 } 1098 } 1099 1100 // Change any jumptables to go to the new MBB. 1101 MBB->getParent()->getJumpTableInfo()-> 1102 ReplaceMBBInJumpTables(MBB, CurTBB); 1103 if (DidChange) { 1104 ++NumBranchOpts; 1105 MadeChange = true; 1106 if (!HasBranchToSelf) return MadeChange; 1107 } 1108 } 1109 } 1110 1111 // Add the branch back if the block is more than just an uncond branch. 1112 TII->InsertBranch(*MBB, CurTBB, 0, CurCond); 1113 } 1114 } 1115 1116 // If the prior block doesn't fall through into this block, and if this 1117 // block doesn't fall through into some other block, see if we can find a 1118 // place to move this block where a fall-through will happen. 1119 if (!CanFallThrough(&PrevBB, PriorUnAnalyzable, 1120 PriorTBB, PriorFBB, PriorCond)) { 1121 // Now we know that there was no fall-through into this block, check to 1122 // see if it has a fall-through into its successor. 1123 bool CurFallsThru = CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB, 1124 CurCond); 1125 1126 if (!MBB->isLandingPad()) { 1127 // Check all the predecessors of this block. If one of them has no fall 1128 // throughs, move this block right after it. 1129 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(), 1130 E = MBB->pred_end(); PI != E; ++PI) { 1131 // Analyze the branch at the end of the pred. 1132 MachineBasicBlock *PredBB = *PI; 1133 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough; 1134 if (PredBB != MBB && !CanFallThrough(PredBB) 1135 && (!CurFallsThru || !CurTBB || !CurFBB) 1136 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) { 1137 // If the current block doesn't fall through, just move it. 1138 // If the current block can fall through and does not end with a 1139 // conditional branch, we need to append an unconditional jump to 1140 // the (current) next block. To avoid a possible compile-time 1141 // infinite loop, move blocks only backward in this case. 1142 // Also, if there are already 2 branches here, we cannot add a third; 1143 // this means we have the case 1144 // Bcc next 1145 // B elsewhere 1146 // next: 1147 if (CurFallsThru) { 1148 MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB)); 1149 CurCond.clear(); 1150 TII->InsertBranch(*MBB, NextBB, 0, CurCond); 1151 } 1152 MBB->moveAfter(PredBB); 1153 MadeChange = true; 1154 return OptimizeBlock(MBB); 1155 } 1156 } 1157 } 1158 1159 if (!CurFallsThru) { 1160 // Check all successors to see if we can move this block before it. 1161 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(), 1162 E = MBB->succ_end(); SI != E; ++SI) { 1163 // Analyze the branch at the end of the block before the succ. 1164 MachineBasicBlock *SuccBB = *SI; 1165 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev; 1166 std::vector<MachineOperand> SuccPrevCond; 1167 1168 // If this block doesn't already fall-through to that successor, and if 1169 // the succ doesn't already have a block that can fall through into it, 1170 // and if the successor isn't an EH destination, we can arrange for the 1171 // fallthrough to happen. 1172 if (SuccBB != MBB && !CanFallThrough(SuccPrev) && 1173 !SuccBB->isLandingPad()) { 1174 MBB->moveBefore(SuccBB); 1175 MadeChange = true; 1176 return OptimizeBlock(MBB); 1177 } 1178 } 1179 1180 // Okay, there is no really great place to put this block. If, however, 1181 // the block before this one would be a fall-through if this block were 1182 // removed, move this block to the end of the function. 1183 if (FallThrough != MBB->getParent()->end() && 1184 PrevBB.isSuccessor(FallThrough)) { 1185 MBB->moveAfter(--MBB->getParent()->end()); 1186 MadeChange = true; 1187 return MadeChange; 1188 } 1189 } 1190 } 1191 1192 return MadeChange; 1193} 1194