BranchFolding.cpp revision 1d196bc4e7da96ffa86fa5f42f411ac81f211e3e
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/Function.h" 22#include "llvm/CodeGen/Passes.h" 23#include "llvm/CodeGen/MachineModuleInfo.h" 24#include "llvm/CodeGen/MachineFunctionPass.h" 25#include "llvm/CodeGen/MachineJumpTableInfo.h" 26#include "llvm/CodeGen/RegisterScavenging.h" 27#include "llvm/Target/TargetInstrInfo.h" 28#include "llvm/Target/TargetMachine.h" 29#include "llvm/Target/TargetRegisterInfo.h" 30#include "llvm/Support/CommandLine.h" 31#include "llvm/Support/Debug.h" 32#include "llvm/Support/ErrorHandling.h" 33#include "llvm/Support/raw_ostream.h" 34#include "llvm/ADT/SmallSet.h" 35#include "llvm/ADT/SetVector.h" 36#include "llvm/ADT/Statistic.h" 37#include "llvm/ADT/STLExtras.h" 38#include <algorithm> 39using namespace llvm; 40 41STATISTIC(NumDeadBlocks, "Number of dead blocks removed"); 42STATISTIC(NumBranchOpts, "Number of branches optimized"); 43STATISTIC(NumTailMerge , "Number of block tails merged"); 44 45static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge", 46 cl::init(cl::BOU_UNSET), cl::Hidden); 47 48// Throttle for huge numbers of predecessors (compile speed problems) 49static cl::opt<unsigned> 50TailMergeThreshold("tail-merge-threshold", 51 cl::desc("Max number of predecessors to consider tail merging"), 52 cl::init(150), cl::Hidden); 53 54// Heuristic for tail merging (and, inversely, tail duplication). 55// TODO: This should be replaced with a target query. 56static cl::opt<unsigned> 57TailMergeSize("tail-merge-size", 58 cl::desc("Min number of instructions to consider tail merging"), 59 cl::init(3), cl::Hidden); 60 61namespace { 62 /// BranchFolderPass - Wrap branch folder in a machine function pass. 63 class BranchFolderPass : public MachineFunctionPass, 64 public BranchFolder { 65 public: 66 static char ID; 67 explicit BranchFolderPass(bool defaultEnableTailMerge) 68 : MachineFunctionPass(&ID), BranchFolder(defaultEnableTailMerge) {} 69 70 virtual bool runOnMachineFunction(MachineFunction &MF); 71 virtual const char *getPassName() const { return "Control Flow Optimizer"; } 72 }; 73} 74 75char BranchFolderPass::ID = 0; 76 77FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) { 78 return new BranchFolderPass(DefaultEnableTailMerge); 79} 80 81bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) { 82 return OptimizeFunction(MF, 83 MF.getTarget().getInstrInfo(), 84 MF.getTarget().getRegisterInfo(), 85 getAnalysisIfAvailable<MachineModuleInfo>()); 86} 87 88 89BranchFolder::BranchFolder(bool defaultEnableTailMerge) { 90 switch (FlagEnableTailMerge) { 91 case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break; 92 case cl::BOU_TRUE: EnableTailMerge = true; break; 93 case cl::BOU_FALSE: EnableTailMerge = false; break; 94 } 95} 96 97/// RemoveDeadBlock - Remove the specified dead machine basic block from the 98/// function, updating the CFG. 99void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) { 100 assert(MBB->pred_empty() && "MBB must be dead!"); 101 DEBUG(dbgs() << "\nRemoving MBB: " << *MBB); 102 103 MachineFunction *MF = MBB->getParent(); 104 // drop all successors. 105 while (!MBB->succ_empty()) 106 MBB->removeSuccessor(MBB->succ_end()-1); 107 108 // If there are any labels in the basic block, unregister them from 109 // MachineModuleInfo. 110 if (MMI && !MBB->empty()) { 111 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); 112 I != E; ++I) { 113 if (I->isLabel()) 114 // The label ID # is always operand #0, an immediate. 115 MMI->InvalidateLabel(I->getOperand(0).getImm()); 116 } 117 } 118 119 // Remove the block. 120 MF->erase(MBB); 121} 122 123/// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def 124/// followed by terminators, and if the implicitly defined registers are not 125/// used by the terminators, remove those implicit_def's. e.g. 126/// BB1: 127/// r0 = implicit_def 128/// r1 = implicit_def 129/// br 130/// This block can be optimized away later if the implicit instructions are 131/// removed. 132bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) { 133 SmallSet<unsigned, 4> ImpDefRegs; 134 MachineBasicBlock::iterator I = MBB->begin(); 135 while (I != MBB->end()) { 136 if (I->getOpcode() != TargetInstrInfo::IMPLICIT_DEF) 137 break; 138 unsigned Reg = I->getOperand(0).getReg(); 139 ImpDefRegs.insert(Reg); 140 for (const unsigned *SubRegs = TRI->getSubRegisters(Reg); 141 unsigned SubReg = *SubRegs; ++SubRegs) 142 ImpDefRegs.insert(SubReg); 143 ++I; 144 } 145 if (ImpDefRegs.empty()) 146 return false; 147 148 MachineBasicBlock::iterator FirstTerm = I; 149 while (I != MBB->end()) { 150 if (!TII->isUnpredicatedTerminator(I)) 151 return false; 152 // See if it uses any of the implicitly defined registers. 153 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) { 154 MachineOperand &MO = I->getOperand(i); 155 if (!MO.isReg() || !MO.isUse()) 156 continue; 157 unsigned Reg = MO.getReg(); 158 if (ImpDefRegs.count(Reg)) 159 return false; 160 } 161 ++I; 162 } 163 164 I = MBB->begin(); 165 while (I != FirstTerm) { 166 MachineInstr *ImpDefMI = &*I; 167 ++I; 168 MBB->erase(ImpDefMI); 169 } 170 171 return true; 172} 173 174/// OptimizeFunction - Perhaps branch folding, tail merging and other 175/// CFG optimizations on the given function. 176bool BranchFolder::OptimizeFunction(MachineFunction &MF, 177 const TargetInstrInfo *tii, 178 const TargetRegisterInfo *tri, 179 MachineModuleInfo *mmi) { 180 if (!tii) return false; 181 182 TII = tii; 183 TRI = tri; 184 MMI = mmi; 185 186 RS = TRI->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL; 187 188 // Fix CFG. The later algorithms expect it to be right. 189 bool MadeChange = false; 190 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) { 191 MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0; 192 SmallVector<MachineOperand, 4> Cond; 193 if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true)) 194 MadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty()); 195 MadeChange |= OptimizeImpDefsBlock(MBB); 196 } 197 198 bool MadeChangeThisIteration = true; 199 while (MadeChangeThisIteration) { 200 MadeChangeThisIteration = false; 201 MadeChangeThisIteration |= TailMergeBlocks(MF); 202 MadeChangeThisIteration |= OptimizeBranches(MF); 203 MadeChange |= MadeChangeThisIteration; 204 } 205 206 // See if any jump tables have become mergable or dead as the code generator 207 // did its thing. 208 MachineJumpTableInfo *JTI = MF.getJumpTableInfo(); 209 if (JTI == 0) { 210 delete RS; 211 return MadeChange; 212 } 213 214 const std::vector<MachineJumpTableEntry> &JTs = JTI->getJumpTables(); 215 // Figure out how these jump tables should be merged. 216 std::vector<unsigned> JTMapping; 217 JTMapping.reserve(JTs.size()); 218 219 // We always keep the 0th jump table. 220 JTMapping.push_back(0); 221 222 // Scan the jump tables, seeing if there are any duplicates. Note that this 223 // is N^2, which should be fixed someday. 224 for (unsigned i = 1, e = JTs.size(); i != e; ++i) { 225 if (JTs[i].MBBs.empty()) 226 JTMapping.push_back(i); 227 else 228 JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs)); 229 } 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 MadeChange = true; 257 } 258 259 delete RS; 260 return MadeChange; 261} 262 263//===----------------------------------------------------------------------===// 264// Tail Merging of Blocks 265//===----------------------------------------------------------------------===// 266 267/// HashMachineInstr - Compute a hash value for MI and its operands. 268static unsigned HashMachineInstr(const MachineInstr *MI) { 269 unsigned Hash = MI->getOpcode(); 270 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 271 const MachineOperand &Op = MI->getOperand(i); 272 273 // Merge in bits from the operand if easy. 274 unsigned OperandHash = 0; 275 switch (Op.getType()) { 276 case MachineOperand::MO_Register: OperandHash = Op.getReg(); break; 277 case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break; 278 case MachineOperand::MO_MachineBasicBlock: 279 OperandHash = Op.getMBB()->getNumber(); 280 break; 281 case MachineOperand::MO_FrameIndex: 282 case MachineOperand::MO_ConstantPoolIndex: 283 case MachineOperand::MO_JumpTableIndex: 284 OperandHash = Op.getIndex(); 285 break; 286 case MachineOperand::MO_GlobalAddress: 287 case MachineOperand::MO_ExternalSymbol: 288 // Global address / external symbol are too hard, don't bother, but do 289 // pull in the offset. 290 OperandHash = Op.getOffset(); 291 break; 292 default: break; 293 } 294 295 Hash += ((OperandHash << 3) | Op.getType()) << (i&31); 296 } 297 return Hash; 298} 299 300/// HashEndOfMBB - Hash the last few instructions in the MBB. For blocks 301/// with no successors, we hash two instructions, because cross-jumping 302/// only saves code when at least two instructions are removed (since a 303/// branch must be inserted). For blocks with a successor, one of the 304/// two blocks to be tail-merged will end with a branch already, so 305/// it gains to cross-jump even for one instruction. 306static unsigned HashEndOfMBB(const MachineBasicBlock *MBB, 307 unsigned minCommonTailLength) { 308 MachineBasicBlock::const_iterator I = MBB->end(); 309 if (I == MBB->begin()) 310 return 0; // Empty MBB. 311 312 --I; 313 unsigned Hash = HashMachineInstr(I); 314 315 if (I == MBB->begin() || minCommonTailLength == 1) 316 return Hash; // Single instr MBB. 317 318 --I; 319 // Hash in the second-to-last instruction. 320 Hash ^= HashMachineInstr(I) << 2; 321 return Hash; 322} 323 324/// ComputeCommonTailLength - Given two machine basic blocks, compute the number 325/// of instructions they actually have in common together at their end. Return 326/// iterators for the first shared instruction in each block. 327static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1, 328 MachineBasicBlock *MBB2, 329 MachineBasicBlock::iterator &I1, 330 MachineBasicBlock::iterator &I2) { 331 I1 = MBB1->end(); 332 I2 = MBB2->end(); 333 334 unsigned TailLen = 0; 335 while (I1 != MBB1->begin() && I2 != MBB2->begin()) { 336 --I1; --I2; 337 if (!I1->isIdenticalTo(I2) || 338 // FIXME: This check is dubious. It's used to get around a problem where 339 // people incorrectly expect inline asm directives to remain in the same 340 // relative order. This is untenable because normal compiler 341 // optimizations (like this one) may reorder and/or merge these 342 // directives. 343 I1->getOpcode() == TargetInstrInfo::INLINEASM) { 344 ++I1; ++I2; 345 break; 346 } 347 ++TailLen; 348 } 349 return TailLen; 350} 351 352/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything 353/// after it, replacing it with an unconditional branch to NewDest. This 354/// returns true if OldInst's block is modified, false if NewDest is modified. 355void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, 356 MachineBasicBlock *NewDest) { 357 MachineBasicBlock *OldBB = OldInst->getParent(); 358 359 // Remove all the old successors of OldBB from the CFG. 360 while (!OldBB->succ_empty()) 361 OldBB->removeSuccessor(OldBB->succ_begin()); 362 363 // Remove all the dead instructions from the end of OldBB. 364 OldBB->erase(OldInst, OldBB->end()); 365 366 // If OldBB isn't immediately before OldBB, insert a branch to it. 367 if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest)) 368 TII->InsertBranch(*OldBB, NewDest, 0, SmallVector<MachineOperand, 0>()); 369 OldBB->addSuccessor(NewDest); 370 ++NumTailMerge; 371} 372 373/// SplitMBBAt - Given a machine basic block and an iterator into it, split the 374/// MBB so that the part before the iterator falls into the part starting at the 375/// iterator. This returns the new MBB. 376MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB, 377 MachineBasicBlock::iterator BBI1) { 378 MachineFunction &MF = *CurMBB.getParent(); 379 380 // Create the fall-through block. 381 MachineFunction::iterator MBBI = &CurMBB; 382 MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(CurMBB.getBasicBlock()); 383 CurMBB.getParent()->insert(++MBBI, NewMBB); 384 385 // Move all the successors of this block to the specified block. 386 NewMBB->transferSuccessors(&CurMBB); 387 388 // Add an edge from CurMBB to NewMBB for the fall-through. 389 CurMBB.addSuccessor(NewMBB); 390 391 // Splice the code over. 392 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end()); 393 394 // For targets that use the register scavenger, we must maintain LiveIns. 395 if (RS) { 396 RS->enterBasicBlock(&CurMBB); 397 if (!CurMBB.empty()) 398 RS->forward(prior(CurMBB.end())); 399 BitVector RegsLiveAtExit(TRI->getNumRegs()); 400 RS->getRegsUsed(RegsLiveAtExit, false); 401 for (unsigned int i = 0, e = TRI->getNumRegs(); i != e; i++) 402 if (RegsLiveAtExit[i]) 403 NewMBB->addLiveIn(i); 404 } 405 406 return NewMBB; 407} 408 409/// EstimateRuntime - Make a rough estimate for how long it will take to run 410/// the specified code. 411static unsigned EstimateRuntime(MachineBasicBlock::iterator I, 412 MachineBasicBlock::iterator E) { 413 unsigned Time = 0; 414 for (; I != E; ++I) { 415 const TargetInstrDesc &TID = I->getDesc(); 416 if (TID.isCall()) 417 Time += 10; 418 else if (TID.mayLoad() || TID.mayStore()) 419 Time += 2; 420 else 421 ++Time; 422 } 423 return Time; 424} 425 426// CurMBB needs to add an unconditional branch to SuccMBB (we removed these 427// branches temporarily for tail merging). In the case where CurMBB ends 428// with a conditional branch to the next block, optimize by reversing the 429// test and conditionally branching to SuccMBB instead. 430static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB, 431 const TargetInstrInfo *TII) { 432 MachineFunction *MF = CurMBB->getParent(); 433 MachineFunction::iterator I = llvm::next(MachineFunction::iterator(CurMBB)); 434 MachineBasicBlock *TBB = 0, *FBB = 0; 435 SmallVector<MachineOperand, 4> Cond; 436 if (I != MF->end() && 437 !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond, true)) { 438 MachineBasicBlock *NextBB = I; 439 if (TBB == NextBB && !Cond.empty() && !FBB) { 440 if (!TII->ReverseBranchCondition(Cond)) { 441 TII->RemoveBranch(*CurMBB); 442 TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond); 443 return; 444 } 445 } 446 } 447 TII->InsertBranch(*CurMBB, SuccBB, NULL, SmallVector<MachineOperand, 0>()); 448} 449 450bool 451BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const { 452 if (getHash() < o.getHash()) 453 return true; 454 else if (getHash() > o.getHash()) 455 return false; 456 else if (getBlock()->getNumber() < o.getBlock()->getNumber()) 457 return true; 458 else if (getBlock()->getNumber() > o.getBlock()->getNumber()) 459 return false; 460 else { 461 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing 462 // an object with itself. 463#ifndef _GLIBCXX_DEBUG 464 llvm_unreachable("Predecessor appears twice"); 465#endif 466 return false; 467 } 468} 469 470/// CountTerminators - Count the number of terminators in the given 471/// block and set I to the position of the first non-terminator, if there 472/// is one, or MBB->end() otherwise. 473static unsigned CountTerminators(MachineBasicBlock *MBB, 474 MachineBasicBlock::iterator &I) { 475 I = MBB->end(); 476 unsigned NumTerms = 0; 477 for (;;) { 478 if (I == MBB->begin()) { 479 I = MBB->end(); 480 break; 481 } 482 --I; 483 if (!I->getDesc().isTerminator()) break; 484 ++NumTerms; 485 } 486 return NumTerms; 487} 488 489/// ProfitableToMerge - Check if two machine basic blocks have a common tail 490/// and decide if it would be profitable to merge those tails. Return the 491/// length of the common tail and iterators to the first common instruction 492/// in each block. 493static bool ProfitableToMerge(MachineBasicBlock *MBB1, 494 MachineBasicBlock *MBB2, 495 unsigned minCommonTailLength, 496 unsigned &CommonTailLen, 497 MachineBasicBlock::iterator &I1, 498 MachineBasicBlock::iterator &I2, 499 MachineBasicBlock *SuccBB, 500 MachineBasicBlock *PredBB) { 501 CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2); 502 MachineFunction *MF = MBB1->getParent(); 503 504 if (CommonTailLen == 0) 505 return false; 506 507 // It's almost always profitable to merge any number of non-terminator 508 // instructions with the block that falls through into the common successor. 509 if (MBB1 == PredBB || MBB2 == PredBB) { 510 MachineBasicBlock::iterator I; 511 unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I); 512 if (CommonTailLen > NumTerms) 513 return true; 514 } 515 516 // If one of the blocks can be completely merged and happens to be in 517 // a position where the other could fall through into it, merge any number 518 // of instructions, because it can be done without a branch. 519 // TODO: If the blocks are not adjacent, move one of them so that they are? 520 if (MBB1->isLayoutSuccessor(MBB2) && I2 == MBB2->begin()) 521 return true; 522 if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin()) 523 return true; 524 525 // If both blocks have an unconditional branch temporarily stripped out, 526 // count that as an additional common instruction for the following 527 // heuristics. 528 unsigned EffectiveTailLen = CommonTailLen; 529 if (SuccBB && MBB1 != PredBB && MBB2 != PredBB && 530 !MBB1->back().getDesc().isBarrier() && 531 !MBB2->back().getDesc().isBarrier()) 532 ++EffectiveTailLen; 533 534 // Check if the common tail is long enough to be worthwhile. 535 if (EffectiveTailLen >= minCommonTailLength) 536 return true; 537 538 // If we are optimizing for code size, 2 instructions in common is enough if 539 // we don't have to split a block. At worst we will be introducing 1 new 540 // branch instruction, which is likely to be smaller than the 2 541 // instructions that would be deleted in the merge. 542 if (EffectiveTailLen >= 2 && 543 MF->getFunction()->hasFnAttr(Attribute::OptimizeForSize) && 544 (I1 == MBB1->begin() || I2 == MBB2->begin())) 545 return true; 546 547 return false; 548} 549 550/// ComputeSameTails - Look through all the blocks in MergePotentials that have 551/// hash CurHash (guaranteed to match the last element). Build the vector 552/// SameTails of all those that have the (same) largest number of instructions 553/// in common of any pair of these blocks. SameTails entries contain an 554/// iterator into MergePotentials (from which the MachineBasicBlock can be 555/// found) and a MachineBasicBlock::iterator into that MBB indicating the 556/// instruction where the matching code sequence begins. 557/// Order of elements in SameTails is the reverse of the order in which 558/// those blocks appear in MergePotentials (where they are not necessarily 559/// consecutive). 560unsigned BranchFolder::ComputeSameTails(unsigned CurHash, 561 unsigned minCommonTailLength, 562 MachineBasicBlock *SuccBB, 563 MachineBasicBlock *PredBB) { 564 unsigned maxCommonTailLength = 0U; 565 SameTails.clear(); 566 MachineBasicBlock::iterator TrialBBI1, TrialBBI2; 567 MPIterator HighestMPIter = prior(MergePotentials.end()); 568 for (MPIterator CurMPIter = prior(MergePotentials.end()), 569 B = MergePotentials.begin(); 570 CurMPIter != B && CurMPIter->getHash() == CurHash; 571 --CurMPIter) { 572 for (MPIterator I = prior(CurMPIter); I->getHash() == CurHash ; --I) { 573 unsigned CommonTailLen; 574 if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(), 575 minCommonTailLength, 576 CommonTailLen, TrialBBI1, TrialBBI2, 577 SuccBB, PredBB)) { 578 if (CommonTailLen > maxCommonTailLength) { 579 SameTails.clear(); 580 maxCommonTailLength = CommonTailLen; 581 HighestMPIter = CurMPIter; 582 SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1)); 583 } 584 if (HighestMPIter == CurMPIter && 585 CommonTailLen == maxCommonTailLength) 586 SameTails.push_back(SameTailElt(I, TrialBBI2)); 587 } 588 if (I == B) 589 break; 590 } 591 } 592 return maxCommonTailLength; 593} 594 595/// RemoveBlocksWithHash - Remove all blocks with hash CurHash from 596/// MergePotentials, restoring branches at ends of blocks as appropriate. 597void BranchFolder::RemoveBlocksWithHash(unsigned CurHash, 598 MachineBasicBlock *SuccBB, 599 MachineBasicBlock *PredBB) { 600 MPIterator CurMPIter, B; 601 for (CurMPIter = prior(MergePotentials.end()), B = MergePotentials.begin(); 602 CurMPIter->getHash() == CurHash; 603 --CurMPIter) { 604 // Put the unconditional branch back, if we need one. 605 MachineBasicBlock *CurMBB = CurMPIter->getBlock(); 606 if (SuccBB && CurMBB != PredBB) 607 FixTail(CurMBB, SuccBB, TII); 608 if (CurMPIter == B) 609 break; 610 } 611 if (CurMPIter->getHash() != CurHash) 612 CurMPIter++; 613 MergePotentials.erase(CurMPIter, MergePotentials.end()); 614} 615 616/// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist 617/// only of the common tail. Create a block that does by splitting one. 618unsigned BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB, 619 unsigned maxCommonTailLength) { 620 unsigned commonTailIndex = 0; 621 unsigned TimeEstimate = ~0U; 622 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) { 623 // Use PredBB if possible; that doesn't require a new branch. 624 if (SameTails[i].getBlock() == PredBB) { 625 commonTailIndex = i; 626 break; 627 } 628 // Otherwise, make a (fairly bogus) choice based on estimate of 629 // how long it will take the various blocks to execute. 630 unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(), 631 SameTails[i].getTailStartPos()); 632 if (t <= TimeEstimate) { 633 TimeEstimate = t; 634 commonTailIndex = i; 635 } 636 } 637 638 MachineBasicBlock::iterator BBI = 639 SameTails[commonTailIndex].getTailStartPos(); 640 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock(); 641 642 DEBUG(dbgs() << "\nSplitting BB#" << MBB->getNumber() << ", size " 643 << maxCommonTailLength); 644 645 MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI); 646 SameTails[commonTailIndex].setBlock(newMBB); 647 SameTails[commonTailIndex].setTailStartPos(newMBB->begin()); 648 649 // If we split PredBB, newMBB is the new predecessor. 650 if (PredBB == MBB) 651 PredBB = newMBB; 652 653 return commonTailIndex; 654} 655 656// See if any of the blocks in MergePotentials (which all have a common single 657// successor, or all have no successor) can be tail-merged. If there is a 658// successor, any blocks in MergePotentials that are not tail-merged and 659// are not immediately before Succ must have an unconditional branch to 660// Succ added (but the predecessor/successor lists need no adjustment). 661// The lone predecessor of Succ that falls through into Succ, 662// if any, is given in PredBB. 663 664bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB, 665 MachineBasicBlock *PredBB) { 666 bool MadeChange = false; 667 668 // Except for the special cases below, tail-merge if there are at least 669 // this many instructions in common. 670 unsigned minCommonTailLength = TailMergeSize; 671 672 DEBUG(dbgs() << "\nTryTailMergeBlocks: "; 673 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i) 674 dbgs() << "BB#" << MergePotentials[i].getBlock()->getNumber() 675 << (i == e-1 ? "" : ", "); 676 dbgs() << "\n"; 677 if (SuccBB) { 678 dbgs() << " with successor BB#" << SuccBB->getNumber() << '\n'; 679 if (PredBB) 680 dbgs() << " which has fall-through from BB#" 681 << PredBB->getNumber() << "\n"; 682 } 683 dbgs() << "Looking for common tails of at least " 684 << minCommonTailLength << " instruction" 685 << (minCommonTailLength == 1 ? "" : "s") << '\n'; 686 ); 687 688 // Sort by hash value so that blocks with identical end sequences sort 689 // together. 690 std::stable_sort(MergePotentials.begin(), MergePotentials.end()); 691 692 // Walk through equivalence sets looking for actual exact matches. 693 while (MergePotentials.size() > 1) { 694 unsigned CurHash = MergePotentials.back().getHash(); 695 696 // Build SameTails, identifying the set of blocks with this hash code 697 // and with the maximum number of instructions in common. 698 unsigned maxCommonTailLength = ComputeSameTails(CurHash, 699 minCommonTailLength, 700 SuccBB, PredBB); 701 702 // If we didn't find any pair that has at least minCommonTailLength 703 // instructions in common, remove all blocks with this hash code and retry. 704 if (SameTails.empty()) { 705 RemoveBlocksWithHash(CurHash, SuccBB, PredBB); 706 continue; 707 } 708 709 // If one of the blocks is the entire common tail (and not the entry 710 // block, which we can't jump to), we can treat all blocks with this same 711 // tail at once. Use PredBB if that is one of the possibilities, as that 712 // will not introduce any extra branches. 713 MachineBasicBlock *EntryBB = MergePotentials.begin()->getBlock()-> 714 getParent()->begin(); 715 unsigned commonTailIndex = SameTails.size(); 716 // If there are two blocks, check to see if one can be made to fall through 717 // into the other. 718 if (SameTails.size() == 2 && 719 SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) && 720 SameTails[1].tailIsWholeBlock()) 721 commonTailIndex = 1; 722 else if (SameTails.size() == 2 && 723 SameTails[1].getBlock()->isLayoutSuccessor( 724 SameTails[0].getBlock()) && 725 SameTails[0].tailIsWholeBlock()) 726 commonTailIndex = 0; 727 else { 728 // Otherwise just pick one, favoring the fall-through predecessor if 729 // there is one. 730 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) { 731 MachineBasicBlock *MBB = SameTails[i].getBlock(); 732 if (MBB == EntryBB && SameTails[i].tailIsWholeBlock()) 733 continue; 734 if (MBB == PredBB) { 735 commonTailIndex = i; 736 break; 737 } 738 if (SameTails[i].tailIsWholeBlock()) 739 commonTailIndex = i; 740 } 741 } 742 743 if (commonTailIndex == SameTails.size() || 744 (SameTails[commonTailIndex].getBlock() == PredBB && 745 !SameTails[commonTailIndex].tailIsWholeBlock())) { 746 // None of the blocks consist entirely of the common tail. 747 // Split a block so that one does. 748 commonTailIndex = CreateCommonTailOnlyBlock(PredBB, maxCommonTailLength); 749 } 750 751 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock(); 752 // MBB is common tail. Adjust all other BB's to jump to this one. 753 // Traversal must be forwards so erases work. 754 DEBUG(dbgs() << "\nUsing common tail in BB#" << MBB->getNumber() 755 << " for "); 756 for (unsigned int i=0, e = SameTails.size(); i != e; ++i) { 757 if (commonTailIndex == i) 758 continue; 759 DEBUG(dbgs() << "BB#" << SameTails[i].getBlock()->getNumber() 760 << (i == e-1 ? "" : ", ")); 761 // Hack the end off BB i, making it jump to BB commonTailIndex instead. 762 ReplaceTailWithBranchTo(SameTails[i].getTailStartPos(), MBB); 763 // BB i is no longer a predecessor of SuccBB; remove it from the worklist. 764 MergePotentials.erase(SameTails[i].getMPIter()); 765 } 766 DEBUG(dbgs() << "\n"); 767 // We leave commonTailIndex in the worklist in case there are other blocks 768 // that match it with a smaller number of instructions. 769 MadeChange = true; 770 } 771 return MadeChange; 772} 773 774bool BranchFolder::TailMergeBlocks(MachineFunction &MF) { 775 776 if (!EnableTailMerge) return false; 777 778 bool MadeChange = false; 779 780 // First find blocks with no successors. 781 MergePotentials.clear(); 782 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { 783 if (I->succ_empty()) 784 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(I, 2U), I)); 785 } 786 787 // See if we can do any tail merging on those. 788 if (MergePotentials.size() < TailMergeThreshold && 789 MergePotentials.size() >= 2) 790 MadeChange |= TryTailMergeBlocks(NULL, NULL); 791 792 // Look at blocks (IBB) with multiple predecessors (PBB). 793 // We change each predecessor to a canonical form, by 794 // (1) temporarily removing any unconditional branch from the predecessor 795 // to IBB, and 796 // (2) alter conditional branches so they branch to the other block 797 // not IBB; this may require adding back an unconditional branch to IBB 798 // later, where there wasn't one coming in. E.g. 799 // Bcc IBB 800 // fallthrough to QBB 801 // here becomes 802 // Bncc QBB 803 // with a conceptual B to IBB after that, which never actually exists. 804 // With those changes, we see whether the predecessors' tails match, 805 // and merge them if so. We change things out of canonical form and 806 // back to the way they were later in the process. (OptimizeBranches 807 // would undo some of this, but we can't use it, because we'd get into 808 // a compile-time infinite loop repeatedly doing and undoing the same 809 // transformations.) 810 811 for (MachineFunction::iterator I = llvm::next(MF.begin()), E = MF.end(); 812 I != E; ++I) { 813 if (I->pred_size() >= 2 && I->pred_size() < TailMergeThreshold) { 814 SmallPtrSet<MachineBasicBlock *, 8> UniquePreds; 815 MachineBasicBlock *IBB = I; 816 MachineBasicBlock *PredBB = prior(I); 817 MergePotentials.clear(); 818 for (MachineBasicBlock::pred_iterator P = I->pred_begin(), 819 E2 = I->pred_end(); 820 P != E2; ++P) { 821 MachineBasicBlock *PBB = *P; 822 // Skip blocks that loop to themselves, can't tail merge these. 823 if (PBB == IBB) 824 continue; 825 // Visit each predecessor only once. 826 if (!UniquePreds.insert(PBB)) 827 continue; 828 MachineBasicBlock *TBB = 0, *FBB = 0; 829 SmallVector<MachineOperand, 4> Cond; 830 if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond, true)) { 831 // Failing case: IBB is the target of a cbr, and 832 // we cannot reverse the branch. 833 SmallVector<MachineOperand, 4> NewCond(Cond); 834 if (!Cond.empty() && TBB == IBB) { 835 if (TII->ReverseBranchCondition(NewCond)) 836 continue; 837 // This is the QBB case described above 838 if (!FBB) 839 FBB = llvm::next(MachineFunction::iterator(PBB)); 840 } 841 // Failing case: the only way IBB can be reached from PBB is via 842 // exception handling. Happens for landing pads. Would be nice 843 // to have a bit in the edge so we didn't have to do all this. 844 if (IBB->isLandingPad()) { 845 MachineFunction::iterator IP = PBB; IP++; 846 MachineBasicBlock *PredNextBB = NULL; 847 if (IP != MF.end()) 848 PredNextBB = IP; 849 if (TBB == NULL) { 850 if (IBB != PredNextBB) // fallthrough 851 continue; 852 } else if (FBB) { 853 if (TBB != IBB && FBB != IBB) // cbr then ubr 854 continue; 855 } else if (Cond.empty()) { 856 if (TBB != IBB) // ubr 857 continue; 858 } else { 859 if (TBB != IBB && IBB != PredNextBB) // cbr 860 continue; 861 } 862 } 863 // Remove the unconditional branch at the end, if any. 864 if (TBB && (Cond.empty() || FBB)) { 865 TII->RemoveBranch(*PBB); 866 if (!Cond.empty()) 867 // reinsert conditional branch only, for now 868 TII->InsertBranch(*PBB, (TBB == IBB) ? FBB : TBB, 0, NewCond); 869 } 870 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(PBB, 1U), 871 *P)); 872 } 873 } 874 if (MergePotentials.size() >= 2) 875 MadeChange |= TryTailMergeBlocks(IBB, PredBB); 876 // Reinsert an unconditional branch if needed. 877 // The 1 below can occur as a result of removing blocks in TryTailMergeBlocks. 878 PredBB = prior(I); // this may have been changed in TryTailMergeBlocks 879 if (MergePotentials.size() == 1 && 880 MergePotentials.begin()->getBlock() != PredBB) 881 FixTail(MergePotentials.begin()->getBlock(), IBB, TII); 882 } 883 } 884 return MadeChange; 885} 886 887//===----------------------------------------------------------------------===// 888// Branch Optimization 889//===----------------------------------------------------------------------===// 890 891bool BranchFolder::OptimizeBranches(MachineFunction &MF) { 892 bool MadeChange = false; 893 894 // Make sure blocks are numbered in order 895 MF.RenumberBlocks(); 896 897 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) { 898 MachineBasicBlock *MBB = I++; 899 MadeChange |= OptimizeBlock(MBB); 900 901 // If it is dead, remove it. 902 if (MBB->pred_empty()) { 903 RemoveDeadBlock(MBB); 904 MadeChange = true; 905 ++NumDeadBlocks; 906 } 907 } 908 return MadeChange; 909} 910 911 912/// IsBetterFallthrough - Return true if it would be clearly better to 913/// fall-through to MBB1 than to fall through into MBB2. This has to return 914/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will 915/// result in infinite loops. 916static bool IsBetterFallthrough(MachineBasicBlock *MBB1, 917 MachineBasicBlock *MBB2) { 918 // Right now, we use a simple heuristic. If MBB2 ends with a call, and 919 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to 920 // optimize branches that branch to either a return block or an assert block 921 // into a fallthrough to the return. 922 if (MBB1->empty() || MBB2->empty()) return false; 923 924 // If there is a clear successor ordering we make sure that one block 925 // will fall through to the next 926 if (MBB1->isSuccessor(MBB2)) return true; 927 if (MBB2->isSuccessor(MBB1)) return false; 928 929 MachineInstr *MBB1I = --MBB1->end(); 930 MachineInstr *MBB2I = --MBB2->end(); 931 return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall(); 932} 933 934/// OptimizeBlock - Analyze and optimize control flow related to the specified 935/// block. This is never called on the entry block. 936bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) { 937 bool MadeChange = false; 938 MachineFunction &MF = *MBB->getParent(); 939ReoptimizeBlock: 940 941 MachineFunction::iterator FallThrough = MBB; 942 ++FallThrough; 943 944 // If this block is empty, make everyone use its fall-through, not the block 945 // explicitly. Landing pads should not do this since the landing-pad table 946 // points to this block. Blocks with their addresses taken shouldn't be 947 // optimized away. 948 if (MBB->empty() && !MBB->isLandingPad() && !MBB->hasAddressTaken()) { 949 // Dead block? Leave for cleanup later. 950 if (MBB->pred_empty()) return MadeChange; 951 952 if (FallThrough == MF.end()) { 953 // TODO: Simplify preds to not branch here if possible! 954 } else { 955 // Rewrite all predecessors of the old block to go to the fallthrough 956 // instead. 957 while (!MBB->pred_empty()) { 958 MachineBasicBlock *Pred = *(MBB->pred_end()-1); 959 Pred->ReplaceUsesOfBlockWith(MBB, FallThrough); 960 } 961 // If MBB was the target of a jump table, update jump tables to go to the 962 // fallthrough instead. 963 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo()) 964 MJTI->ReplaceMBBInJumpTables(MBB, FallThrough); 965 MadeChange = true; 966 } 967 return MadeChange; 968 } 969 970 // Check to see if we can simplify the terminator of the block before this 971 // one. 972 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB)); 973 974 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0; 975 SmallVector<MachineOperand, 4> PriorCond; 976 bool PriorUnAnalyzable = 977 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true); 978 if (!PriorUnAnalyzable) { 979 // If the CFG for the prior block has extra edges, remove them. 980 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB, 981 !PriorCond.empty()); 982 983 // If the previous branch is conditional and both conditions go to the same 984 // destination, remove the branch, replacing it with an unconditional one or 985 // a fall-through. 986 if (PriorTBB && PriorTBB == PriorFBB) { 987 TII->RemoveBranch(PrevBB); 988 PriorCond.clear(); 989 if (PriorTBB != MBB) 990 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 991 MadeChange = true; 992 ++NumBranchOpts; 993 goto ReoptimizeBlock; 994 } 995 996 // If the previous block unconditionally falls through to this block and 997 // this block has no other predecessors, move the contents of this block 998 // into the prior block. This doesn't usually happen when SimplifyCFG 999 // has been used, but it can happen if tail merging splits a fall-through 1000 // predecessor of a block. 1001 // This has to check PrevBB->succ_size() because EH edges are ignored by 1002 // AnalyzeBranch. 1003 if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 && 1004 PrevBB.succ_size() == 1 && 1005 !MBB->hasAddressTaken()) { 1006 DEBUG(dbgs() << "\nMerging into block: " << PrevBB 1007 << "From MBB: " << *MBB); 1008 PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end()); 1009 PrevBB.removeSuccessor(PrevBB.succ_begin());; 1010 assert(PrevBB.succ_empty()); 1011 PrevBB.transferSuccessors(MBB); 1012 MadeChange = true; 1013 return MadeChange; 1014 } 1015 1016 // If the previous branch *only* branches to *this* block (conditional or 1017 // not) remove the branch. 1018 if (PriorTBB == MBB && PriorFBB == 0) { 1019 TII->RemoveBranch(PrevBB); 1020 MadeChange = true; 1021 ++NumBranchOpts; 1022 goto ReoptimizeBlock; 1023 } 1024 1025 // If the prior block branches somewhere else on the condition and here if 1026 // the condition is false, remove the uncond second branch. 1027 if (PriorFBB == MBB) { 1028 TII->RemoveBranch(PrevBB); 1029 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 1030 MadeChange = true; 1031 ++NumBranchOpts; 1032 goto ReoptimizeBlock; 1033 } 1034 1035 // If the prior block branches here on true and somewhere else on false, and 1036 // if the branch condition is reversible, reverse the branch to create a 1037 // fall-through. 1038 if (PriorTBB == MBB) { 1039 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond); 1040 if (!TII->ReverseBranchCondition(NewPriorCond)) { 1041 TII->RemoveBranch(PrevBB); 1042 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond); 1043 MadeChange = true; 1044 ++NumBranchOpts; 1045 goto ReoptimizeBlock; 1046 } 1047 } 1048 1049 // If this block has no successors (e.g. it is a return block or ends with 1050 // a call to a no-return function like abort or __cxa_throw) and if the pred 1051 // falls through into this block, and if it would otherwise fall through 1052 // into the block after this, move this block to the end of the function. 1053 // 1054 // We consider it more likely that execution will stay in the function (e.g. 1055 // due to loops) than it is to exit it. This asserts in loops etc, moving 1056 // the assert condition out of the loop body. 1057 if (MBB->succ_empty() && !PriorCond.empty() && PriorFBB == 0 && 1058 MachineFunction::iterator(PriorTBB) == FallThrough && 1059 !MBB->canFallThrough()) { 1060 bool DoTransform = true; 1061 1062 // We have to be careful that the succs of PredBB aren't both no-successor 1063 // blocks. If neither have successors and if PredBB is the second from 1064 // last block in the function, we'd just keep swapping the two blocks for 1065 // last. Only do the swap if one is clearly better to fall through than 1066 // the other. 1067 if (FallThrough == --MF.end() && 1068 !IsBetterFallthrough(PriorTBB, MBB)) 1069 DoTransform = false; 1070 1071 // We don't want to do this transformation if we have control flow like: 1072 // br cond BB2 1073 // BB1: 1074 // .. 1075 // jmp BBX 1076 // BB2: 1077 // .. 1078 // ret 1079 // 1080 // In this case, we could actually be moving the return block *into* a 1081 // loop! 1082 if (DoTransform && !MBB->succ_empty() && 1083 (!PriorTBB->canFallThrough() || PriorTBB->empty())) 1084 DoTransform = false; 1085 1086 1087 if (DoTransform) { 1088 // Reverse the branch so we will fall through on the previous true cond. 1089 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond); 1090 if (!TII->ReverseBranchCondition(NewPriorCond)) { 1091 DEBUG(dbgs() << "\nMoving MBB: " << *MBB 1092 << "To make fallthrough to: " << *PriorTBB << "\n"); 1093 1094 TII->RemoveBranch(PrevBB); 1095 TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond); 1096 1097 // Move this block to the end of the function. 1098 MBB->moveAfter(--MF.end()); 1099 MadeChange = true; 1100 ++NumBranchOpts; 1101 return MadeChange; 1102 } 1103 } 1104 } 1105 } 1106 1107 // Analyze the branch in the current block. 1108 MachineBasicBlock *CurTBB = 0, *CurFBB = 0; 1109 SmallVector<MachineOperand, 4> CurCond; 1110 bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true); 1111 if (!CurUnAnalyzable) { 1112 // If the CFG for the prior block has extra edges, remove them. 1113 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty()); 1114 1115 // If this is a two-way branch, and the FBB branches to this block, reverse 1116 // the condition so the single-basic-block loop is faster. Instead of: 1117 // Loop: xxx; jcc Out; jmp Loop 1118 // we want: 1119 // Loop: xxx; jncc Loop; jmp Out 1120 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) { 1121 SmallVector<MachineOperand, 4> NewCond(CurCond); 1122 if (!TII->ReverseBranchCondition(NewCond)) { 1123 TII->RemoveBranch(*MBB); 1124 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond); 1125 MadeChange = true; 1126 ++NumBranchOpts; 1127 goto ReoptimizeBlock; 1128 } 1129 } 1130 1131 // If this branch is the only thing in its block, see if we can forward 1132 // other blocks across it. 1133 if (CurTBB && CurCond.empty() && CurFBB == 0 && 1134 MBB->begin()->getDesc().isBranch() && CurTBB != MBB && 1135 !MBB->hasAddressTaken()) { 1136 // This block may contain just an unconditional branch. Because there can 1137 // be 'non-branch terminators' in the block, try removing the branch and 1138 // then seeing if the block is empty. 1139 TII->RemoveBranch(*MBB); 1140 1141 // If this block is just an unconditional branch to CurTBB, we can 1142 // usually completely eliminate the block. The only case we cannot 1143 // completely eliminate the block is when the block before this one 1144 // falls through into MBB and we can't understand the prior block's branch 1145 // condition. 1146 if (MBB->empty()) { 1147 bool PredHasNoFallThrough = !PrevBB.canFallThrough(); 1148 if (PredHasNoFallThrough || !PriorUnAnalyzable || 1149 !PrevBB.isSuccessor(MBB)) { 1150 // If the prior block falls through into us, turn it into an 1151 // explicit branch to us to make updates simpler. 1152 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) && 1153 PriorTBB != MBB && PriorFBB != MBB) { 1154 if (PriorTBB == 0) { 1155 assert(PriorCond.empty() && PriorFBB == 0 && 1156 "Bad branch analysis"); 1157 PriorTBB = MBB; 1158 } else { 1159 assert(PriorFBB == 0 && "Machine CFG out of date!"); 1160 PriorFBB = MBB; 1161 } 1162 TII->RemoveBranch(PrevBB); 1163 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); 1164 } 1165 1166 // Iterate through all the predecessors, revectoring each in-turn. 1167 size_t PI = 0; 1168 bool DidChange = false; 1169 bool HasBranchToSelf = false; 1170 while(PI != MBB->pred_size()) { 1171 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI); 1172 if (PMBB == MBB) { 1173 // If this block has an uncond branch to itself, leave it. 1174 ++PI; 1175 HasBranchToSelf = true; 1176 } else { 1177 DidChange = true; 1178 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB); 1179 // If this change resulted in PMBB ending in a conditional 1180 // branch where both conditions go to the same destination, 1181 // change this to an unconditional branch (and fix the CFG). 1182 MachineBasicBlock *NewCurTBB = 0, *NewCurFBB = 0; 1183 SmallVector<MachineOperand, 4> NewCurCond; 1184 bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB, 1185 NewCurFBB, NewCurCond, true); 1186 if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) { 1187 TII->RemoveBranch(*PMBB); 1188 NewCurCond.clear(); 1189 TII->InsertBranch(*PMBB, NewCurTBB, 0, NewCurCond); 1190 MadeChange = true; 1191 ++NumBranchOpts; 1192 PMBB->CorrectExtraCFGEdges(NewCurTBB, 0, false); 1193 } 1194 } 1195 } 1196 1197 // Change any jumptables to go to the new MBB. 1198 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo()) 1199 MJTI->ReplaceMBBInJumpTables(MBB, CurTBB); 1200 if (DidChange) { 1201 ++NumBranchOpts; 1202 MadeChange = true; 1203 if (!HasBranchToSelf) return MadeChange; 1204 } 1205 } 1206 } 1207 1208 // Add the branch back if the block is more than just an uncond branch. 1209 TII->InsertBranch(*MBB, CurTBB, 0, CurCond); 1210 } 1211 } 1212 1213 // If the prior block doesn't fall through into this block, and if this 1214 // block doesn't fall through into some other block, see if we can find a 1215 // place to move this block where a fall-through will happen. 1216 if (!PrevBB.canFallThrough()) { 1217 1218 // Now we know that there was no fall-through into this block, check to 1219 // see if it has a fall-through into its successor. 1220 bool CurFallsThru = MBB->canFallThrough(); 1221 1222 if (!MBB->isLandingPad()) { 1223 // Check all the predecessors of this block. If one of them has no fall 1224 // throughs, move this block right after it. 1225 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(), 1226 E = MBB->pred_end(); PI != E; ++PI) { 1227 // Analyze the branch at the end of the pred. 1228 MachineBasicBlock *PredBB = *PI; 1229 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough; 1230 MachineBasicBlock *PredTBB = 0, *PredFBB = 0; 1231 SmallVector<MachineOperand, 4> PredCond; 1232 if (PredBB != MBB && !PredBB->canFallThrough() && 1233 !TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true) 1234 && (!CurFallsThru || !CurTBB || !CurFBB) 1235 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) { 1236 // If the current block doesn't fall through, just move it. 1237 // If the current block can fall through and does not end with a 1238 // conditional branch, we need to append an unconditional jump to 1239 // the (current) next block. To avoid a possible compile-time 1240 // infinite loop, move blocks only backward in this case. 1241 // Also, if there are already 2 branches here, we cannot add a third; 1242 // this means we have the case 1243 // Bcc next 1244 // B elsewhere 1245 // next: 1246 if (CurFallsThru) { 1247 MachineBasicBlock *NextBB = llvm::next(MachineFunction::iterator(MBB)); 1248 CurCond.clear(); 1249 TII->InsertBranch(*MBB, NextBB, 0, CurCond); 1250 } 1251 MBB->moveAfter(PredBB); 1252 MadeChange = true; 1253 goto ReoptimizeBlock; 1254 } 1255 } 1256 } 1257 1258 if (!CurFallsThru) { 1259 // Check all successors to see if we can move this block before it. 1260 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(), 1261 E = MBB->succ_end(); SI != E; ++SI) { 1262 // Analyze the branch at the end of the block before the succ. 1263 MachineBasicBlock *SuccBB = *SI; 1264 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev; 1265 1266 // If this block doesn't already fall-through to that successor, and if 1267 // the succ doesn't already have a block that can fall through into it, 1268 // and if the successor isn't an EH destination, we can arrange for the 1269 // fallthrough to happen. 1270 if (SuccBB != MBB && &*SuccPrev != MBB && 1271 !SuccPrev->canFallThrough() && !CurUnAnalyzable && 1272 !SuccBB->isLandingPad()) { 1273 MBB->moveBefore(SuccBB); 1274 MadeChange = true; 1275 goto ReoptimizeBlock; 1276 } 1277 } 1278 1279 // Okay, there is no really great place to put this block. If, however, 1280 // the block before this one would be a fall-through if this block were 1281 // removed, move this block to the end of the function. 1282 MachineBasicBlock *PrevTBB = 0, *PrevFBB = 0; 1283 SmallVector<MachineOperand, 4> PrevCond; 1284 if (FallThrough != MF.end() && 1285 !TII->AnalyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) && 1286 PrevBB.isSuccessor(FallThrough)) { 1287 MBB->moveAfter(--MF.end()); 1288 MadeChange = true; 1289 return MadeChange; 1290 } 1291 } 1292 } 1293 1294 return MadeChange; 1295} 1296