BranchFolding.cpp revision 6d31268a7dc9854fa5a5cb9227ba9a15c5898414
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 return new BranchFolderPass(DefaultEnableTailMerge); 55} 56 57bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) { 58 return OptimizeFunction(MF, 59 MF.getTarget().getInstrInfo(), 60 MF.getTarget().getRegisterInfo(), 61 getAnalysisIfAvailable<MachineModuleInfo>()); 62} 63 64 65 66BranchFolder::BranchFolder(bool defaultEnableTailMerge) { 67 switch (FlagEnableTailMerge) { 68 case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break; 69 case cl::BOU_TRUE: EnableTailMerge = true; break; 70 case cl::BOU_FALSE: EnableTailMerge = false; break; 71 } 72} 73 74/// RemoveDeadBlock - Remove the specified dead machine basic block from the 75/// function, updating the CFG. 76void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) { 77 assert(MBB->pred_empty() && "MBB must be dead!"); 78 DEBUG(errs() << "\nRemoving MBB: " << *MBB); 79 80 MachineFunction *MF = MBB->getParent(); 81 // drop all successors. 82 while (!MBB->succ_empty()) 83 MBB->removeSuccessor(MBB->succ_end()-1); 84 85 // If there are any labels in the basic block, unregister them from 86 // MachineModuleInfo. 87 if (MMI && !MBB->empty()) { 88 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); 89 I != E; ++I) { 90 if (I->isLabel()) 91 // The label ID # is always operand #0, an immediate. 92 MMI->InvalidateLabel(I->getOperand(0).getImm()); 93 } 94 } 95 96 // Remove the block. 97 MF->erase(MBB); 98} 99 100/// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def 101/// followed by terminators, and if the implicitly defined registers are not 102/// used by the terminators, remove those implicit_def's. e.g. 103/// BB1: 104/// r0 = implicit_def 105/// r1 = implicit_def 106/// br 107/// This block can be optimized away later if the implicit instructions are 108/// removed. 109bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) { 110 SmallSet<unsigned, 4> ImpDefRegs; 111 MachineBasicBlock::iterator I = MBB->begin(); 112 while (I != MBB->end()) { 113 if (I->getOpcode() != TargetInstrInfo::IMPLICIT_DEF) 114 break; 115 unsigned Reg = I->getOperand(0).getReg(); 116 ImpDefRegs.insert(Reg); 117 for (const unsigned *SubRegs = TRI->getSubRegisters(Reg); 118 unsigned SubReg = *SubRegs; ++SubRegs) 119 ImpDefRegs.insert(SubReg); 120 ++I; 121 } 122 if (ImpDefRegs.empty()) 123 return false; 124 125 MachineBasicBlock::iterator FirstTerm = I; 126 while (I != MBB->end()) { 127 if (!TII->isUnpredicatedTerminator(I)) 128 return false; 129 // See if it uses any of the implicitly defined registers. 130 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) { 131 MachineOperand &MO = I->getOperand(i); 132 if (!MO.isReg() || !MO.isUse()) 133 continue; 134 unsigned Reg = MO.getReg(); 135 if (ImpDefRegs.count(Reg)) 136 return false; 137 } 138 ++I; 139 } 140 141 I = MBB->begin(); 142 while (I != FirstTerm) { 143 MachineInstr *ImpDefMI = &*I; 144 ++I; 145 MBB->erase(ImpDefMI); 146 } 147 148 return true; 149} 150 151/// OptimizeFunction - Perhaps branch folding, tail merging and other 152/// CFG optimizations on the given function. 153bool BranchFolder::OptimizeFunction(MachineFunction &MF, 154 const TargetInstrInfo *tii, 155 const TargetRegisterInfo *tri, 156 MachineModuleInfo *mmi) { 157 if (!tii) return false; 158 159 TII = tii; 160 TRI = tri; 161 MMI = mmi; 162 163 RS = TRI->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL; 164 165 // Fix CFG. The later algorithms expect it to be right. 166 bool MadeChange = false; 167 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) { 168 MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0; 169 SmallVector<MachineOperand, 4> Cond; 170 if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true)) 171 MadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty()); 172 MadeChange |= OptimizeImpDefsBlock(MBB); 173 } 174 175 176 bool MadeChangeThisIteration = true; 177 while (MadeChangeThisIteration) { 178 MadeChangeThisIteration = false; 179 MadeChangeThisIteration |= TailMergeBlocks(MF); 180 MadeChangeThisIteration |= OptimizeBranches(MF); 181 MadeChange |= MadeChangeThisIteration; 182 } 183 184 // See if any jump tables have become mergable or dead as the code generator 185 // did its thing. 186 MachineJumpTableInfo *JTI = MF.getJumpTableInfo(); 187 const std::vector<MachineJumpTableEntry> &JTs = JTI->getJumpTables(); 188 if (!JTs.empty()) { 189 // Figure out how these jump tables should be merged. 190 std::vector<unsigned> JTMapping; 191 JTMapping.reserve(JTs.size()); 192 193 // We always keep the 0th jump table. 194 JTMapping.push_back(0); 195 196 // Scan the jump tables, seeing if there are any duplicates. Note that this 197 // is N^2, which should be fixed someday. 198 for (unsigned i = 1, e = JTs.size(); i != e; ++i) { 199 if (JTs[i].MBBs.empty()) 200 JTMapping.push_back(i); 201 else 202 JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs)); 203 } 204 205 // If a jump table was merge with another one, walk the function rewriting 206 // references to jump tables to reference the new JT ID's. Keep track of 207 // whether we see a jump table idx, if not, we can delete the JT. 208 BitVector JTIsLive(JTs.size()); 209 for (MachineFunction::iterator BB = MF.begin(), E = MF.end(); 210 BB != E; ++BB) { 211 for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end(); 212 I != E; ++I) 213 for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) { 214 MachineOperand &Op = I->getOperand(op); 215 if (!Op.isJTI()) continue; 216 unsigned NewIdx = JTMapping[Op.getIndex()]; 217 Op.setIndex(NewIdx); 218 219 // Remember that this JT is live. 220 JTIsLive.set(NewIdx); 221 } 222 } 223 224 // Finally, remove dead jump tables. This happens either because the 225 // indirect jump was unreachable (and thus deleted) or because the jump 226 // table was merged with some other one. 227 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i) 228 if (!JTIsLive.test(i)) { 229 JTI->RemoveJumpTable(i); 230 MadeChange = true; 231 } 232 } 233 234 delete RS; 235 return MadeChange; 236} 237 238//===----------------------------------------------------------------------===// 239// Tail Merging of Blocks 240//===----------------------------------------------------------------------===// 241 242/// HashMachineInstr - Compute a hash value for MI and its operands. 243static unsigned HashMachineInstr(const MachineInstr *MI) { 244 unsigned Hash = MI->getOpcode(); 245 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 246 const MachineOperand &Op = MI->getOperand(i); 247 248 // Merge in bits from the operand if easy. 249 unsigned OperandHash = 0; 250 switch (Op.getType()) { 251 case MachineOperand::MO_Register: OperandHash = Op.getReg(); break; 252 case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break; 253 case MachineOperand::MO_MachineBasicBlock: 254 OperandHash = Op.getMBB()->getNumber(); 255 break; 256 case MachineOperand::MO_FrameIndex: 257 case MachineOperand::MO_ConstantPoolIndex: 258 case MachineOperand::MO_JumpTableIndex: 259 OperandHash = Op.getIndex(); 260 break; 261 case MachineOperand::MO_GlobalAddress: 262 case MachineOperand::MO_ExternalSymbol: 263 // Global address / external symbol are too hard, don't bother, but do 264 // pull in the offset. 265 OperandHash = Op.getOffset(); 266 break; 267 default: break; 268 } 269 270 Hash += ((OperandHash << 3) | Op.getType()) << (i&31); 271 } 272 return Hash; 273} 274 275/// HashEndOfMBB - Hash the last few instructions in the MBB. For blocks 276/// with no successors, we hash two instructions, because cross-jumping 277/// only saves code when at least two instructions are removed (since a 278/// branch must be inserted). For blocks with a successor, one of the 279/// two blocks to be tail-merged will end with a branch already, so 280/// it gains to cross-jump even for one instruction. 281 282static unsigned HashEndOfMBB(const MachineBasicBlock *MBB, 283 unsigned minCommonTailLength) { 284 MachineBasicBlock::const_iterator I = MBB->end(); 285 if (I == MBB->begin()) 286 return 0; // Empty MBB. 287 288 --I; 289 unsigned Hash = HashMachineInstr(I); 290 291 if (I == MBB->begin() || minCommonTailLength == 1) 292 return Hash; // Single instr MBB. 293 294 --I; 295 // Hash in the second-to-last instruction. 296 Hash ^= HashMachineInstr(I) << 2; 297 return Hash; 298} 299 300/// ComputeCommonTailLength - Given two machine basic blocks, compute the number 301/// of instructions they actually have in common together at their end. Return 302/// iterators for the first shared instruction in each block. 303static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1, 304 MachineBasicBlock *MBB2, 305 MachineBasicBlock::iterator &I1, 306 MachineBasicBlock::iterator &I2) { 307 I1 = MBB1->end(); 308 I2 = MBB2->end(); 309 310 unsigned TailLen = 0; 311 while (I1 != MBB1->begin() && I2 != MBB2->begin()) { 312 --I1; --I2; 313 if (!I1->isIdenticalTo(I2) || 314 // FIXME: This check is dubious. It's used to get around a problem where 315 // people incorrectly expect inline asm directives to remain in the same 316 // relative order. This is untenable because normal compiler 317 // optimizations (like this one) may reorder and/or merge these 318 // directives. 319 I1->getOpcode() == TargetInstrInfo::INLINEASM) { 320 ++I1; ++I2; 321 break; 322 } 323 ++TailLen; 324 } 325 return TailLen; 326} 327 328/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything 329/// after it, replacing it with an unconditional branch to NewDest. This 330/// returns true if OldInst's block is modified, false if NewDest is modified. 331void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, 332 MachineBasicBlock *NewDest) { 333 MachineBasicBlock *OldBB = OldInst->getParent(); 334 335 // Remove all the old successors of OldBB from the CFG. 336 while (!OldBB->succ_empty()) 337 OldBB->removeSuccessor(OldBB->succ_begin()); 338 339 // Remove all the dead instructions from the end of OldBB. 340 OldBB->erase(OldInst, OldBB->end()); 341 342 // If OldBB isn't immediately before OldBB, insert a branch to it. 343 if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest)) 344 TII->InsertBranch(*OldBB, NewDest, 0, SmallVector<MachineOperand, 0>()); 345 OldBB->addSuccessor(NewDest); 346 ++NumTailMerge; 347} 348 349/// SplitMBBAt - Given a machine basic block and an iterator into it, split the 350/// MBB so that the part before the iterator falls into the part starting at the 351/// iterator. This returns the new MBB. 352MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB, 353 MachineBasicBlock::iterator BBI1) { 354 MachineFunction &MF = *CurMBB.getParent(); 355 356 // Create the fall-through block. 357 MachineFunction::iterator MBBI = &CurMBB; 358 MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(CurMBB.getBasicBlock()); 359 CurMBB.getParent()->insert(++MBBI, NewMBB); 360 361 // Move all the successors of this block to the specified block. 362 NewMBB->transferSuccessors(&CurMBB); 363 364 // Add an edge from CurMBB to NewMBB for the fall-through. 365 CurMBB.addSuccessor(NewMBB); 366 367 // Splice the code over. 368 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end()); 369 370 // For targets that use the register scavenger, we must maintain LiveIns. 371 if (RS) { 372 RS->enterBasicBlock(&CurMBB); 373 if (!CurMBB.empty()) 374 RS->forward(prior(CurMBB.end())); 375 BitVector RegsLiveAtExit(TRI->getNumRegs()); 376 RS->getRegsUsed(RegsLiveAtExit, false); 377 for (unsigned int i=0, e=TRI->getNumRegs(); i!=e; i++) 378 if (RegsLiveAtExit[i]) 379 NewMBB->addLiveIn(i); 380 } 381 382 return NewMBB; 383} 384 385/// EstimateRuntime - Make a rough estimate for how long it will take to run 386/// the specified code. 387static unsigned EstimateRuntime(MachineBasicBlock::iterator I, 388 MachineBasicBlock::iterator E) { 389 unsigned Time = 0; 390 for (; I != E; ++I) { 391 const TargetInstrDesc &TID = I->getDesc(); 392 if (TID.isCall()) 393 Time += 10; 394 else if (TID.mayLoad() || TID.mayStore()) 395 Time += 2; 396 else 397 ++Time; 398 } 399 return Time; 400} 401 402// CurMBB needs to add an unconditional branch to SuccMBB (we removed these 403// branches temporarily for tail merging). In the case where CurMBB ends 404// with a conditional branch to the next block, optimize by reversing the 405// test and conditionally branching to SuccMBB instead. 406 407static void FixTail(MachineBasicBlock* CurMBB, MachineBasicBlock *SuccBB, 408 const TargetInstrInfo *TII) { 409 MachineFunction *MF = CurMBB->getParent(); 410 MachineFunction::iterator I = next(MachineFunction::iterator(CurMBB)); 411 MachineBasicBlock *TBB = 0, *FBB = 0; 412 SmallVector<MachineOperand, 4> Cond; 413 if (I != MF->end() && 414 !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond, true)) { 415 MachineBasicBlock *NextBB = I; 416 if (TBB == NextBB && !Cond.empty() && !FBB) { 417 if (!TII->ReverseBranchCondition(Cond)) { 418 TII->RemoveBranch(*CurMBB); 419 TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond); 420 return; 421 } 422 } 423 } 424 TII->InsertBranch(*CurMBB, SuccBB, NULL, SmallVector<MachineOperand, 0>()); 425} 426 427static bool MergeCompare(const std::pair<unsigned,MachineBasicBlock*> &p, 428 const std::pair<unsigned,MachineBasicBlock*> &q) { 429 if (p.first < q.first) 430 return true; 431 else if (p.first > q.first) 432 return false; 433 else if (p.second->getNumber() < q.second->getNumber()) 434 return true; 435 else if (p.second->getNumber() > q.second->getNumber()) 436 return false; 437 else { 438 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing 439 // an object with itself. 440#ifndef _GLIBCXX_DEBUG 441 llvm_unreachable("Predecessor appears twice"); 442#endif 443 return false; 444 } 445} 446 447/// ComputeSameTails - Look through all the blocks in MergePotentials that have 448/// hash CurHash (guaranteed to match the last element). Build the vector 449/// SameTails of all those that have the (same) largest number of instructions 450/// in common of any pair of these blocks. SameTails entries contain an 451/// iterator into MergePotentials (from which the MachineBasicBlock can be 452/// found) and a MachineBasicBlock::iterator into that MBB indicating the 453/// instruction where the matching code sequence begins. 454/// Order of elements in SameTails is the reverse of the order in which 455/// those blocks appear in MergePotentials (where they are not necessarily 456/// consecutive). 457unsigned BranchFolder::ComputeSameTails(unsigned CurHash, 458 unsigned minCommonTailLength) { 459 unsigned maxCommonTailLength = 0U; 460 SameTails.clear(); 461 MachineBasicBlock::iterator TrialBBI1, TrialBBI2; 462 MPIterator HighestMPIter = prior(MergePotentials.end()); 463 for (MPIterator CurMPIter = prior(MergePotentials.end()), 464 B = MergePotentials.begin(); 465 CurMPIter!=B && CurMPIter->first==CurHash; 466 --CurMPIter) { 467 for (MPIterator I = prior(CurMPIter); I->first==CurHash ; --I) { 468 unsigned CommonTailLen = ComputeCommonTailLength( 469 CurMPIter->second, 470 I->second, 471 TrialBBI1, TrialBBI2); 472 // If we will have to split a block, there should be at least 473 // minCommonTailLength instructions in common; if not, at worst 474 // we will be replacing a fallthrough into the common tail with a 475 // branch, which at worst breaks even with falling through into 476 // the duplicated common tail, so 1 instruction in common is enough. 477 // We will always pick a block we do not have to split as the common 478 // tail if there is one. 479 // (Empty blocks will get forwarded and need not be considered.) 480 if (CommonTailLen >= minCommonTailLength || 481 (CommonTailLen > 0 && 482 (TrialBBI1==CurMPIter->second->begin() || 483 TrialBBI2==I->second->begin()))) { 484 if (CommonTailLen > maxCommonTailLength) { 485 SameTails.clear(); 486 maxCommonTailLength = CommonTailLen; 487 HighestMPIter = CurMPIter; 488 SameTails.push_back(std::make_pair(CurMPIter, TrialBBI1)); 489 } 490 if (HighestMPIter == CurMPIter && 491 CommonTailLen == maxCommonTailLength) 492 SameTails.push_back(std::make_pair(I, TrialBBI2)); 493 } 494 if (I==B) 495 break; 496 } 497 } 498 return maxCommonTailLength; 499} 500 501/// RemoveBlocksWithHash - Remove all blocks with hash CurHash from 502/// MergePotentials, restoring branches at ends of blocks as appropriate. 503void BranchFolder::RemoveBlocksWithHash(unsigned CurHash, 504 MachineBasicBlock* SuccBB, 505 MachineBasicBlock* PredBB) { 506 MPIterator CurMPIter, B; 507 for (CurMPIter = prior(MergePotentials.end()), B = MergePotentials.begin(); 508 CurMPIter->first==CurHash; 509 --CurMPIter) { 510 // Put the unconditional branch back, if we need one. 511 MachineBasicBlock *CurMBB = CurMPIter->second; 512 if (SuccBB && CurMBB != PredBB) 513 FixTail(CurMBB, SuccBB, TII); 514 if (CurMPIter==B) 515 break; 516 } 517 if (CurMPIter->first!=CurHash) 518 CurMPIter++; 519 MergePotentials.erase(CurMPIter, MergePotentials.end()); 520} 521 522/// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist 523/// only of the common tail. Create a block that does by splitting one. 524unsigned BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB, 525 unsigned maxCommonTailLength) { 526 unsigned i, commonTailIndex; 527 unsigned TimeEstimate = ~0U; 528 for (i=0, commonTailIndex=0; i<SameTails.size(); i++) { 529 // Use PredBB if possible; that doesn't require a new branch. 530 if (SameTails[i].first->second==PredBB) { 531 commonTailIndex = i; 532 break; 533 } 534 // Otherwise, make a (fairly bogus) choice based on estimate of 535 // how long it will take the various blocks to execute. 536 unsigned t = EstimateRuntime(SameTails[i].first->second->begin(), 537 SameTails[i].second); 538 if (t<=TimeEstimate) { 539 TimeEstimate = t; 540 commonTailIndex = i; 541 } 542 } 543 544 MachineBasicBlock::iterator BBI = SameTails[commonTailIndex].second; 545 MachineBasicBlock *MBB = SameTails[commonTailIndex].first->second; 546 547 DEBUG(errs() << "\nSplitting " << MBB->getNumber() << ", size " 548 << maxCommonTailLength); 549 550 MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI); 551 SameTails[commonTailIndex].first->second = newMBB; 552 SameTails[commonTailIndex].second = newMBB->begin(); 553 // If we split PredBB, newMBB is the new predecessor. 554 if (PredBB==MBB) 555 PredBB = newMBB; 556 557 return commonTailIndex; 558} 559 560// See if any of the blocks in MergePotentials (which all have a common single 561// successor, or all have no successor) can be tail-merged. If there is a 562// successor, any blocks in MergePotentials that are not tail-merged and 563// are not immediately before Succ must have an unconditional branch to 564// Succ added (but the predecessor/successor lists need no adjustment). 565// The lone predecessor of Succ that falls through into Succ, 566// if any, is given in PredBB. 567 568bool BranchFolder::TryMergeBlocks(MachineBasicBlock *SuccBB, 569 MachineBasicBlock* PredBB) { 570 bool MadeChange = false; 571 572 // It doesn't make sense to save a single instruction since tail merging 573 // will add a jump. 574 // FIXME: Ask the target to provide the threshold? 575 unsigned minCommonTailLength = (SuccBB ? 1 : 2) + 1; 576 577 DEBUG(errs() << "\nTryMergeBlocks " << MergePotentials.size() << '\n'); 578 579 // Sort by hash value so that blocks with identical end sequences sort 580 // together. 581 std::stable_sort(MergePotentials.begin(), MergePotentials.end(),MergeCompare); 582 583 // Walk through equivalence sets looking for actual exact matches. 584 while (MergePotentials.size() > 1) { 585 unsigned CurHash = prior(MergePotentials.end())->first; 586 587 // Build SameTails, identifying the set of blocks with this hash code 588 // and with the maximum number of instructions in common. 589 unsigned maxCommonTailLength = ComputeSameTails(CurHash, 590 minCommonTailLength); 591 592 // If we didn't find any pair that has at least minCommonTailLength 593 // instructions in common, remove all blocks with this hash code and retry. 594 if (SameTails.empty()) { 595 RemoveBlocksWithHash(CurHash, SuccBB, PredBB); 596 continue; 597 } 598 599 // If one of the blocks is the entire common tail (and not the entry 600 // block, which we can't jump to), we can treat all blocks with this same 601 // tail at once. Use PredBB if that is one of the possibilities, as that 602 // will not introduce any extra branches. 603 MachineBasicBlock *EntryBB = MergePotentials.begin()->second-> 604 getParent()->begin(); 605 unsigned int commonTailIndex, i; 606 for (commonTailIndex=SameTails.size(), i=0; i<SameTails.size(); i++) { 607 MachineBasicBlock *MBB = SameTails[i].first->second; 608 if (MBB->begin() == SameTails[i].second && MBB != EntryBB) { 609 commonTailIndex = i; 610 if (MBB==PredBB) 611 break; 612 } 613 } 614 615 if (commonTailIndex==SameTails.size()) { 616 // None of the blocks consist entirely of the common tail. 617 // Split a block so that one does. 618 commonTailIndex = CreateCommonTailOnlyBlock(PredBB, maxCommonTailLength); 619 } 620 621 MachineBasicBlock *MBB = SameTails[commonTailIndex].first->second; 622 // MBB is common tail. Adjust all other BB's to jump to this one. 623 // Traversal must be forwards so erases work. 624 DEBUG(errs() << "\nUsing common tail " << MBB->getNumber() << " for "); 625 for (unsigned int i=0; i<SameTails.size(); ++i) { 626 if (commonTailIndex==i) 627 continue; 628 DEBUG(errs() << SameTails[i].first->second->getNumber() << ","); 629 // Hack the end off BB i, making it jump to BB commonTailIndex instead. 630 ReplaceTailWithBranchTo(SameTails[i].second, MBB); 631 // BB i is no longer a predecessor of SuccBB; remove it from the worklist. 632 MergePotentials.erase(SameTails[i].first); 633 } 634 DEBUG(errs() << "\n"); 635 // We leave commonTailIndex in the worklist in case there are other blocks 636 // that match it with a smaller number of instructions. 637 MadeChange = true; 638 } 639 return MadeChange; 640} 641 642bool BranchFolder::TailMergeBlocks(MachineFunction &MF) { 643 644 if (!EnableTailMerge) return false; 645 646 bool MadeChange = false; 647 648 // First find blocks with no successors. 649 MergePotentials.clear(); 650 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { 651 if (I->succ_empty()) 652 MergePotentials.push_back(std::make_pair(HashEndOfMBB(I, 2U), I)); 653 } 654 // See if we can do any tail merging on those. 655 if (MergePotentials.size() < TailMergeThreshold && 656 MergePotentials.size() >= 2) 657 MadeChange |= TryMergeBlocks(NULL, NULL); 658 659 // Look at blocks (IBB) with multiple predecessors (PBB). 660 // We change each predecessor to a canonical form, by 661 // (1) temporarily removing any unconditional branch from the predecessor 662 // to IBB, and 663 // (2) alter conditional branches so they branch to the other block 664 // not IBB; this may require adding back an unconditional branch to IBB 665 // later, where there wasn't one coming in. E.g. 666 // Bcc IBB 667 // fallthrough to QBB 668 // here becomes 669 // Bncc QBB 670 // with a conceptual B to IBB after that, which never actually exists. 671 // With those changes, we see whether the predecessors' tails match, 672 // and merge them if so. We change things out of canonical form and 673 // back to the way they were later in the process. (OptimizeBranches 674 // would undo some of this, but we can't use it, because we'd get into 675 // a compile-time infinite loop repeatedly doing and undoing the same 676 // transformations.) 677 678 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { 679 if (I->pred_size() >= 2 && I->pred_size() < TailMergeThreshold) { 680 SmallPtrSet<MachineBasicBlock *, 8> UniquePreds; 681 MachineBasicBlock *IBB = I; 682 MachineBasicBlock *PredBB = prior(I); 683 MergePotentials.clear(); 684 for (MachineBasicBlock::pred_iterator P = I->pred_begin(), 685 E2 = I->pred_end(); 686 P != E2; ++P) { 687 MachineBasicBlock* PBB = *P; 688 // Skip blocks that loop to themselves, can't tail merge these. 689 if (PBB==IBB) 690 continue; 691 // Visit each predecessor only once. 692 if (!UniquePreds.insert(PBB)) 693 continue; 694 MachineBasicBlock *TBB = 0, *FBB = 0; 695 SmallVector<MachineOperand, 4> Cond; 696 if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond, true)) { 697 // Failing case: IBB is the target of a cbr, and 698 // we cannot reverse the branch. 699 SmallVector<MachineOperand, 4> NewCond(Cond); 700 if (!Cond.empty() && TBB==IBB) { 701 if (TII->ReverseBranchCondition(NewCond)) 702 continue; 703 // This is the QBB case described above 704 if (!FBB) 705 FBB = next(MachineFunction::iterator(PBB)); 706 } 707 // Failing case: the only way IBB can be reached from PBB is via 708 // exception handling. Happens for landing pads. Would be nice 709 // to have a bit in the edge so we didn't have to do all this. 710 if (IBB->isLandingPad()) { 711 MachineFunction::iterator IP = PBB; IP++; 712 MachineBasicBlock* PredNextBB = NULL; 713 if (IP!=MF.end()) 714 PredNextBB = IP; 715 if (TBB==NULL) { 716 if (IBB!=PredNextBB) // fallthrough 717 continue; 718 } else if (FBB) { 719 if (TBB!=IBB && FBB!=IBB) // cbr then ubr 720 continue; 721 } else if (Cond.empty()) { 722 if (TBB!=IBB) // ubr 723 continue; 724 } else { 725 if (TBB!=IBB && IBB!=PredNextBB) // cbr 726 continue; 727 } 728 } 729 // Remove the unconditional branch at the end, if any. 730 if (TBB && (Cond.empty() || FBB)) { 731 TII->RemoveBranch(*PBB); 732 if (!Cond.empty()) 733 // reinsert conditional branch only, for now 734 TII->InsertBranch(*PBB, (TBB==IBB) ? FBB : TBB, 0, NewCond); 735 } 736 MergePotentials.push_back(std::make_pair(HashEndOfMBB(PBB, 1U), *P)); 737 } 738 } 739 if (MergePotentials.size() >= 2) 740 MadeChange |= TryMergeBlocks(I, PredBB); 741 // Reinsert an unconditional branch if needed. 742 // The 1 below can occur as a result of removing blocks in TryMergeBlocks. 743 PredBB = prior(I); // this may have been changed in TryMergeBlocks 744 if (MergePotentials.size()==1 && 745 MergePotentials.begin()->second != PredBB) 746 FixTail(MergePotentials.begin()->second, I, TII); 747 } 748 } 749 return MadeChange; 750} 751 752//===----------------------------------------------------------------------===// 753// Branch Optimization 754//===----------------------------------------------------------------------===// 755 756bool BranchFolder::OptimizeBranches(MachineFunction &MF) { 757 bool MadeChange = false; 758 759 // Make sure blocks are numbered in order 760 MF.RenumberBlocks(); 761 762 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) { 763 MachineBasicBlock *MBB = I++; 764 MadeChange |= OptimizeBlock(MBB); 765 766 // If it is dead, remove it. 767 if (MBB->pred_empty()) { 768 RemoveDeadBlock(MBB); 769 MadeChange = true; 770 ++NumDeadBlocks; 771 } 772 } 773 return MadeChange; 774} 775 776 777/// CanFallThrough - Return true if the specified block (with the specified 778/// branch condition) can implicitly transfer control to the block after it by 779/// falling off the end of it. This should return false if it can reach the 780/// block after it, but it uses an explicit branch to do so (e.g. a table jump). 781/// 782/// True is a conservative answer. 783/// 784bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB, 785 bool BranchUnAnalyzable, 786 MachineBasicBlock *TBB, 787 MachineBasicBlock *FBB, 788 const SmallVectorImpl<MachineOperand> &Cond) { 789 MachineFunction::iterator Fallthrough = CurBB; 790 ++Fallthrough; 791 // If FallthroughBlock is off the end of the function, it can't fall through. 792 if (Fallthrough == CurBB->getParent()->end()) 793 return false; 794 795 // If FallthroughBlock isn't a successor of CurBB, no fallthrough is possible. 796 if (!CurBB->isSuccessor(Fallthrough)) 797 return false; 798 799 // If we couldn't analyze the branch, assume it could fall through. 800 if (BranchUnAnalyzable) return true; 801 802 // If there is no branch, control always falls through. 803 if (TBB == 0) return true; 804 805 // If there is some explicit branch to the fallthrough block, it can obviously 806 // reach, even though the branch should get folded to fall through implicitly. 807 if (MachineFunction::iterator(TBB) == Fallthrough || 808 MachineFunction::iterator(FBB) == Fallthrough) 809 return true; 810 811 // If it's an unconditional branch to some block not the fall through, it 812 // doesn't fall through. 813 if (Cond.empty()) return false; 814 815 // Otherwise, if it is conditional and has no explicit false block, it falls 816 // through. 817 return FBB == 0; 818} 819 820/// CanFallThrough - Return true if the specified can implicitly transfer 821/// control to the block after it by falling off the end of it. This should 822/// return false if it can reach the block after it, but it uses an explicit 823/// branch to do so (e.g. a table jump). 824/// 825/// True is a conservative answer. 826/// 827bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) { 828 MachineBasicBlock *TBB = 0, *FBB = 0; 829 SmallVector<MachineOperand, 4> Cond; 830 bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond, true); 831 return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond); 832} 833 834/// IsBetterFallthrough - Return true if it would be clearly better to 835/// fall-through to MBB1 than to fall through into MBB2. This has to return 836/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will 837/// result in infinite loops. 838static bool IsBetterFallthrough(MachineBasicBlock *MBB1, 839 MachineBasicBlock *MBB2) { 840 // Right now, we use a simple heuristic. If MBB2 ends with a call, and 841 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to 842 // optimize branches that branch to either a return block or an assert block 843 // into a fallthrough to the return. 844 if (MBB1->empty() || MBB2->empty()) return false; 845 846 // If there is a clear successor ordering we make sure that one block 847 // will fall through to the next 848 if (MBB1->isSuccessor(MBB2)) return true; 849 if (MBB2->isSuccessor(MBB1)) return false; 850 851 MachineInstr *MBB1I = --MBB1->end(); 852 MachineInstr *MBB2I = --MBB2->end(); 853 return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall(); 854} 855 856/// OptimizeBlock - Analyze and optimize control flow related to the specified 857/// block. This is never called on the entry block. 858bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) { 859 bool MadeChange = false; 860 861 MachineFunction::iterator FallThrough = MBB; 862 ++FallThrough; 863 864 // If this block is empty, make everyone use its fall-through, not the block 865 // explicitly. Landing pads should not do this since the landing-pad table 866 // points to this block. 867 if (MBB->empty() && !MBB->isLandingPad()) { 868 // Dead block? Leave for cleanup later. 869 if (MBB->pred_empty()) return MadeChange; 870 871 if (FallThrough == MBB->getParent()->end()) { 872 // TODO: Simplify preds to not branch here if possible! 873 } else { 874 // Rewrite all predecessors of the old block to go to the fallthrough 875 // instead. 876 while (!MBB->pred_empty()) { 877 MachineBasicBlock *Pred = *(MBB->pred_end()-1); 878 Pred->ReplaceUsesOfBlockWith(MBB, FallThrough); 879 } 880 // If MBB was the target of a jump table, update jump tables to go to the 881 // fallthrough instead. 882 MBB->getParent()->getJumpTableInfo()-> 883 ReplaceMBBInJumpTables(MBB, FallThrough); 884 MadeChange = true; 885 } 886 return MadeChange; 887 } 888 889 // Check to see if we can simplify the terminator of the block before this 890 // one. 891 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB)); 892 893 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0; 894 SmallVector<MachineOperand, 4> PriorCond; 895 bool PriorUnAnalyzable = 896 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true); 897 if (!PriorUnAnalyzable) { 898 // If the CFG for the prior block has extra edges, remove them. 899 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB, 900 !PriorCond.empty()); 901 902 // If the previous branch is conditional and both conditions go to the same 903 // destination, remove the branch, replacing it with an unconditional one or 904 // a fall-through. 905 if (PriorTBB && PriorTBB == PriorFBB) { 906 TII->RemoveBranch(PrevBB); 907 PriorCond.clear(); 908 if (PriorTBB != MBB) 909 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 910 MadeChange = true; 911 ++NumBranchOpts; 912 return OptimizeBlock(MBB); 913 } 914 915 // If the previous branch *only* branches to *this* block (conditional or 916 // not) remove the branch. 917 if (PriorTBB == MBB && PriorFBB == 0) { 918 TII->RemoveBranch(PrevBB); 919 MadeChange = true; 920 ++NumBranchOpts; 921 return OptimizeBlock(MBB); 922 } 923 924 // If the prior block branches somewhere else on the condition and here if 925 // the condition is false, remove the uncond second branch. 926 if (PriorFBB == MBB) { 927 TII->RemoveBranch(PrevBB); 928 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 929 MadeChange = true; 930 ++NumBranchOpts; 931 return OptimizeBlock(MBB); 932 } 933 934 // If the prior block branches here on true and somewhere else on false, and 935 // if the branch condition is reversible, reverse the branch to create a 936 // fall-through. 937 if (PriorTBB == MBB) { 938 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond); 939 if (!TII->ReverseBranchCondition(NewPriorCond)) { 940 TII->RemoveBranch(PrevBB); 941 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond); 942 MadeChange = true; 943 ++NumBranchOpts; 944 return OptimizeBlock(MBB); 945 } 946 } 947 948 // If this block has no successors (e.g. it is a return block or ends with 949 // a call to a no-return function like abort or __cxa_throw) and if the pred 950 // falls through into this block, and if it would otherwise fall through 951 // into the block after this, move this block to the end of the function. 952 // 953 // We consider it more likely that execution will stay in the function (e.g. 954 // due to loops) than it is to exit it. This asserts in loops etc, moving 955 // the assert condition out of the loop body. 956 if (MBB->succ_empty() && !PriorCond.empty() && PriorFBB == 0 && 957 MachineFunction::iterator(PriorTBB) == FallThrough && 958 !CanFallThrough(MBB)) { 959 bool DoTransform = true; 960 961 // We have to be careful that the succs of PredBB aren't both no-successor 962 // blocks. If neither have successors and if PredBB is the second from 963 // last block in the function, we'd just keep swapping the two blocks for 964 // last. Only do the swap if one is clearly better to fall through than 965 // the other. 966 if (FallThrough == --MBB->getParent()->end() && 967 !IsBetterFallthrough(PriorTBB, MBB)) 968 DoTransform = false; 969 970 // We don't want to do this transformation if we have control flow like: 971 // br cond BB2 972 // BB1: 973 // .. 974 // jmp BBX 975 // BB2: 976 // .. 977 // ret 978 // 979 // In this case, we could actually be moving the return block *into* a 980 // loop! 981 if (DoTransform && !MBB->succ_empty() && 982 (!CanFallThrough(PriorTBB) || PriorTBB->empty())) 983 DoTransform = false; 984 985 986 if (DoTransform) { 987 // Reverse the branch so we will fall through on the previous true cond. 988 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond); 989 if (!TII->ReverseBranchCondition(NewPriorCond)) { 990 DEBUG(errs() << "\nMoving MBB: " << *MBB 991 << "To make fallthrough to: " << *PriorTBB << "\n"); 992 993 TII->RemoveBranch(PrevBB); 994 TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond); 995 996 // Move this block to the end of the function. 997 MBB->moveAfter(--MBB->getParent()->end()); 998 MadeChange = true; 999 ++NumBranchOpts; 1000 return MadeChange; 1001 } 1002 } 1003 } 1004 } 1005 1006 // Analyze the branch in the current block. 1007 MachineBasicBlock *CurTBB = 0, *CurFBB = 0; 1008 SmallVector<MachineOperand, 4> CurCond; 1009 bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true); 1010 if (!CurUnAnalyzable) { 1011 // If the CFG for the prior block has extra edges, remove them. 1012 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty()); 1013 1014 // If this is a two-way branch, and the FBB branches to this block, reverse 1015 // the condition so the single-basic-block loop is faster. Instead of: 1016 // Loop: xxx; jcc Out; jmp Loop 1017 // we want: 1018 // Loop: xxx; jncc Loop; jmp Out 1019 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) { 1020 SmallVector<MachineOperand, 4> NewCond(CurCond); 1021 if (!TII->ReverseBranchCondition(NewCond)) { 1022 TII->RemoveBranch(*MBB); 1023 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond); 1024 MadeChange = true; 1025 ++NumBranchOpts; 1026 return OptimizeBlock(MBB); 1027 } 1028 } 1029 1030 1031 // If this branch is the only thing in its block, see if we can forward 1032 // other blocks across it. 1033 if (CurTBB && CurCond.empty() && CurFBB == 0 && 1034 MBB->begin()->getDesc().isBranch() && CurTBB != MBB) { 1035 // This block may contain just an unconditional branch. Because there can 1036 // be 'non-branch terminators' in the block, try removing the branch and 1037 // then seeing if the block is empty. 1038 TII->RemoveBranch(*MBB); 1039 1040 // If this block is just an unconditional branch to CurTBB, we can 1041 // usually completely eliminate the block. The only case we cannot 1042 // completely eliminate the block is when the block before this one 1043 // falls through into MBB and we can't understand the prior block's branch 1044 // condition. 1045 if (MBB->empty()) { 1046 bool PredHasNoFallThrough = TII->BlockHasNoFallThrough(PrevBB); 1047 if (PredHasNoFallThrough || !PriorUnAnalyzable || 1048 !PrevBB.isSuccessor(MBB)) { 1049 // If the prior block falls through into us, turn it into an 1050 // explicit branch to us to make updates simpler. 1051 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) && 1052 PriorTBB != MBB && PriorFBB != MBB) { 1053 if (PriorTBB == 0) { 1054 assert(PriorCond.empty() && PriorFBB == 0 && 1055 "Bad branch analysis"); 1056 PriorTBB = MBB; 1057 } else { 1058 assert(PriorFBB == 0 && "Machine CFG out of date!"); 1059 PriorFBB = MBB; 1060 } 1061 TII->RemoveBranch(PrevBB); 1062 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); 1063 } 1064 1065 // Iterate through all the predecessors, revectoring each in-turn. 1066 size_t PI = 0; 1067 bool DidChange = false; 1068 bool HasBranchToSelf = false; 1069 while(PI != MBB->pred_size()) { 1070 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI); 1071 if (PMBB == MBB) { 1072 // If this block has an uncond branch to itself, leave it. 1073 ++PI; 1074 HasBranchToSelf = true; 1075 } else { 1076 DidChange = true; 1077 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB); 1078 // If this change resulted in PMBB ending in a conditional 1079 // branch where both conditions go to the same destination, 1080 // change this to an unconditional branch (and fix the CFG). 1081 MachineBasicBlock *NewCurTBB = 0, *NewCurFBB = 0; 1082 SmallVector<MachineOperand, 4> NewCurCond; 1083 bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB, 1084 NewCurFBB, NewCurCond, true); 1085 if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) { 1086 TII->RemoveBranch(*PMBB); 1087 NewCurCond.clear(); 1088 TII->InsertBranch(*PMBB, NewCurTBB, 0, NewCurCond); 1089 MadeChange = true; 1090 ++NumBranchOpts; 1091 PMBB->CorrectExtraCFGEdges(NewCurTBB, NewCurFBB, false); 1092 } 1093 } 1094 } 1095 1096 // Change any jumptables to go to the new MBB. 1097 MBB->getParent()->getJumpTableInfo()-> 1098 ReplaceMBBInJumpTables(MBB, CurTBB); 1099 if (DidChange) { 1100 ++NumBranchOpts; 1101 MadeChange = true; 1102 if (!HasBranchToSelf) return MadeChange; 1103 } 1104 } 1105 } 1106 1107 // Add the branch back if the block is more than just an uncond branch. 1108 TII->InsertBranch(*MBB, CurTBB, 0, CurCond); 1109 } 1110 } 1111 1112 // If the prior block doesn't fall through into this block, and if this 1113 // block doesn't fall through into some other block, see if we can find a 1114 // place to move this block where a fall-through will happen. 1115 if (!CanFallThrough(&PrevBB, PriorUnAnalyzable, 1116 PriorTBB, PriorFBB, PriorCond)) { 1117 // Now we know that there was no fall-through into this block, check to 1118 // see if it has a fall-through into its successor. 1119 bool CurFallsThru = CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB, 1120 CurCond); 1121 1122 if (!MBB->isLandingPad()) { 1123 // Check all the predecessors of this block. If one of them has no fall 1124 // throughs, move this block right after it. 1125 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(), 1126 E = MBB->pred_end(); PI != E; ++PI) { 1127 // Analyze the branch at the end of the pred. 1128 MachineBasicBlock *PredBB = *PI; 1129 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough; 1130 if (PredBB != MBB && !CanFallThrough(PredBB) 1131 && (!CurFallsThru || !CurTBB || !CurFBB) 1132 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) { 1133 // If the current block doesn't fall through, just move it. 1134 // If the current block can fall through and does not end with a 1135 // conditional branch, we need to append an unconditional jump to 1136 // the (current) next block. To avoid a possible compile-time 1137 // infinite loop, move blocks only backward in this case. 1138 // Also, if there are already 2 branches here, we cannot add a third; 1139 // this means we have the case 1140 // Bcc next 1141 // B elsewhere 1142 // next: 1143 if (CurFallsThru) { 1144 MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB)); 1145 CurCond.clear(); 1146 TII->InsertBranch(*MBB, NextBB, 0, CurCond); 1147 } 1148 MBB->moveAfter(PredBB); 1149 MadeChange = true; 1150 return OptimizeBlock(MBB); 1151 } 1152 } 1153 } 1154 1155 if (!CurFallsThru) { 1156 // Check all successors to see if we can move this block before it. 1157 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(), 1158 E = MBB->succ_end(); SI != E; ++SI) { 1159 // Analyze the branch at the end of the block before the succ. 1160 MachineBasicBlock *SuccBB = *SI; 1161 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev; 1162 std::vector<MachineOperand> SuccPrevCond; 1163 1164 // If this block doesn't already fall-through to that successor, and if 1165 // the succ doesn't already have a block that can fall through into it, 1166 // and if the successor isn't an EH destination, we can arrange for the 1167 // fallthrough to happen. 1168 if (SuccBB != MBB && !CanFallThrough(SuccPrev) && 1169 !SuccBB->isLandingPad()) { 1170 MBB->moveBefore(SuccBB); 1171 MadeChange = true; 1172 return OptimizeBlock(MBB); 1173 } 1174 } 1175 1176 // Okay, there is no really great place to put this block. If, however, 1177 // the block before this one would be a fall-through if this block were 1178 // removed, move this block to the end of the function. 1179 if (FallThrough != MBB->getParent()->end() && 1180 PrevBB.isSuccessor(FallThrough)) { 1181 MBB->moveAfter(--MBB->getParent()->end()); 1182 MadeChange = true; 1183 return MadeChange; 1184 } 1185 } 1186 } 1187 1188 return MadeChange; 1189} 1190