BranchFolding.cpp revision 3ed469ccd7b028a030b550d84b7336d146f5d8fa
1//===-- BranchFolding.cpp - Fold machine code branch instructions ---------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file was developed by the LLVM research group and is distributed under 6// the University of Illinois Open Source 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#include "llvm/CodeGen/Passes.h" 20#include "llvm/CodeGen/MachineDebugInfo.h" 21#include "llvm/CodeGen/MachineFunctionPass.h" 22#include "llvm/CodeGen/MachineJumpTableInfo.h" 23#include "llvm/Target/TargetInstrInfo.h" 24#include "llvm/Target/TargetMachine.h" 25#include "llvm/Support/CommandLine.h" 26#include "llvm/ADT/Statistic.h" 27#include "llvm/ADT/STLExtras.h" 28using namespace llvm; 29 30static Statistic<> NumDeadBlocks("branchfold", "Number of dead blocks removed"); 31static Statistic<> NumBranchOpts("branchfold", "Number of branches optimized"); 32static Statistic<> NumTailMerge ("branchfold", "Number of block tails merged"); 33static cl::opt<bool> EnableTailMerge("enable-tail-merge", cl::Hidden); 34 35namespace { 36 struct BranchFolder : public MachineFunctionPass { 37 virtual bool runOnMachineFunction(MachineFunction &MF); 38 virtual const char *getPassName() const { return "Control Flow Optimizer"; } 39 const TargetInstrInfo *TII; 40 MachineDebugInfo *MDI; 41 bool MadeChange; 42 private: 43 // Tail Merging. 44 bool TailMergeBlocks(MachineFunction &MF); 45 void ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, 46 MachineBasicBlock *NewDest); 47 MachineBasicBlock *SplitMBBAt(MachineBasicBlock &CurMBB, 48 MachineBasicBlock::iterator BBI1); 49 50 // Branch optzn. 51 bool OptimizeBranches(MachineFunction &MF); 52 void OptimizeBlock(MachineBasicBlock *MBB); 53 void RemoveDeadBlock(MachineBasicBlock *MBB); 54 55 bool CanFallThrough(MachineBasicBlock *CurBB); 56 bool CanFallThrough(MachineBasicBlock *CurBB, bool BranchUnAnalyzable, 57 MachineBasicBlock *TBB, MachineBasicBlock *FBB, 58 const std::vector<MachineOperand> &Cond); 59 }; 60} 61 62FunctionPass *llvm::createBranchFoldingPass() { return new BranchFolder(); } 63 64/// RemoveDeadBlock - Remove the specified dead machine basic block from the 65/// function, updating the CFG. 66void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) { 67 assert(MBB->pred_empty() && "MBB must be dead!"); 68 69 MachineFunction *MF = MBB->getParent(); 70 // drop all successors. 71 while (!MBB->succ_empty()) 72 MBB->removeSuccessor(MBB->succ_end()-1); 73 74 // If there is DWARF info to active, check to see if there are any DWARF_LABEL 75 // records in the basic block. If so, unregister them from MachineDebugInfo. 76 if (MDI && !MBB->empty()) { 77 unsigned DWARF_LABELOpc = TII->getDWARF_LABELOpcode(); 78 assert(DWARF_LABELOpc && 79 "Target supports dwarf but didn't implement getDWARF_LABELOpcode!"); 80 81 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); 82 I != E; ++I) { 83 if ((unsigned)I->getOpcode() == DWARF_LABELOpc) { 84 // The label ID # is always operand #0, an immediate. 85 MDI->InvalidateLabel(I->getOperand(0).getImm()); 86 } 87 } 88 } 89 90 // Remove the block. 91 MF->getBasicBlockList().erase(MBB); 92} 93 94bool BranchFolder::runOnMachineFunction(MachineFunction &MF) { 95 TII = MF.getTarget().getInstrInfo(); 96 if (!TII) return false; 97 98 MDI = getAnalysisToUpdate<MachineDebugInfo>(); 99 100 bool EverMadeChange = false; 101 bool MadeChangeThisIteration = true; 102 while (MadeChangeThisIteration) { 103 MadeChangeThisIteration = false; 104 MadeChangeThisIteration |= TailMergeBlocks(MF); 105 MadeChangeThisIteration |= OptimizeBranches(MF); 106 EverMadeChange |= MadeChangeThisIteration; 107 } 108 109 // See if any jump tables have become mergable or dead as the code generator 110 // did its thing. 111 MachineJumpTableInfo *JTI = MF.getJumpTableInfo(); 112 const std::vector<MachineJumpTableEntry> &JTs = JTI->getJumpTables(); 113 if (!JTs.empty()) { 114 // Figure out how these jump tables should be merged. 115 std::vector<unsigned> JTMapping; 116 JTMapping.reserve(JTs.size()); 117 118 // We always keep the 0th jump table. 119 JTMapping.push_back(0); 120 121 // Scan the jump tables, seeing if there are any duplicates. Note that this 122 // is N^2, which should be fixed someday. 123 for (unsigned i = 1, e = JTs.size(); i != e; ++i) 124 JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs)); 125 126 // If a jump table was merge with another one, walk the function rewriting 127 // references to jump tables to reference the new JT ID's. Keep track of 128 // whether we see a jump table idx, if not, we can delete the JT. 129 std::vector<bool> JTIsLive; 130 JTIsLive.resize(JTs.size()); 131 for (MachineFunction::iterator BB = MF.begin(), E = MF.end(); 132 BB != E; ++BB) { 133 for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end(); 134 I != E; ++I) 135 for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) { 136 MachineOperand &Op = I->getOperand(op); 137 if (!Op.isJumpTableIndex()) continue; 138 unsigned NewIdx = JTMapping[Op.getJumpTableIndex()]; 139 Op.setJumpTableIndex(NewIdx); 140 141 // Remember that this JT is live. 142 JTIsLive[NewIdx] = true; 143 } 144 } 145 146 // Finally, remove dead jump tables. This happens either because the 147 // indirect jump was unreachable (and thus deleted) or because the jump 148 // table was merged with some other one. 149 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i) 150 if (!JTIsLive[i]) { 151 JTI->RemoveJumpTable(i); 152 EverMadeChange = true; 153 } 154 } 155 156 return EverMadeChange; 157} 158 159//===----------------------------------------------------------------------===// 160// Tail Merging of Blocks 161//===----------------------------------------------------------------------===// 162 163/// HashMachineInstr - Compute a hash value for MI and its operands. 164static unsigned HashMachineInstr(const MachineInstr *MI) { 165 unsigned Hash = MI->getOpcode(); 166 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 167 const MachineOperand &Op = MI->getOperand(i); 168 169 // Merge in bits from the operand if easy. 170 unsigned OperandHash = 0; 171 switch (Op.getType()) { 172 case MachineOperand::MO_Register: OperandHash = Op.getReg(); break; 173 case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break; 174 case MachineOperand::MO_MachineBasicBlock: 175 OperandHash = Op.getMachineBasicBlock()->getNumber(); 176 break; 177 case MachineOperand::MO_FrameIndex: OperandHash = Op.getFrameIndex(); break; 178 case MachineOperand::MO_ConstantPoolIndex: 179 OperandHash = Op.getConstantPoolIndex(); 180 break; 181 case MachineOperand::MO_JumpTableIndex: 182 OperandHash = Op.getJumpTableIndex(); 183 break; 184 case MachineOperand::MO_GlobalAddress: 185 case MachineOperand::MO_ExternalSymbol: 186 // Global address / external symbol are too hard, don't bother, but do 187 // pull in the offset. 188 OperandHash = Op.getOffset(); 189 break; 190 default: break; 191 } 192 193 Hash += ((OperandHash << 3) | Op.getType()) << (i&31); 194 } 195 return Hash; 196} 197 198/// HashEndOfMBB - Hash the last two instructions in the MBB. We hash two 199/// instructions, because cross-jumping only saves code when at least two 200/// instructions are removed (since a branch must be inserted). 201static unsigned HashEndOfMBB(const MachineBasicBlock *MBB) { 202 MachineBasicBlock::const_iterator I = MBB->end(); 203 if (I == MBB->begin()) 204 return 0; // Empty MBB. 205 206 --I; 207 unsigned Hash = HashMachineInstr(I); 208 209 if (I == MBB->begin()) 210 return Hash; // Single instr MBB. 211 212 --I; 213 // Hash in the second-to-last instruction. 214 Hash ^= HashMachineInstr(I) << 2; 215 return Hash; 216} 217 218/// ComputeCommonTailLength - Given two machine basic blocks, compute the number 219/// of instructions they actually have in common together at their end. Return 220/// iterators for the first shared instruction in each block. 221static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1, 222 MachineBasicBlock *MBB2, 223 MachineBasicBlock::iterator &I1, 224 MachineBasicBlock::iterator &I2) { 225 I1 = MBB1->end(); 226 I2 = MBB2->end(); 227 228 unsigned TailLen = 0; 229 while (I1 != MBB1->begin() && I2 != MBB2->begin()) { 230 --I1; --I2; 231 if (!I1->isIdenticalTo(I2)) { 232 ++I1; ++I2; 233 break; 234 } 235 ++TailLen; 236 } 237 return TailLen; 238} 239 240/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything 241/// after it, replacing it with an unconditional branch to NewDest. This 242/// returns true if OldInst's block is modified, false if NewDest is modified. 243void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, 244 MachineBasicBlock *NewDest) { 245 MachineBasicBlock *OldBB = OldInst->getParent(); 246 247 // Remove all the old successors of OldBB from the CFG. 248 while (!OldBB->succ_empty()) 249 OldBB->removeSuccessor(OldBB->succ_begin()); 250 251 // Remove all the dead instructions from the end of OldBB. 252 OldBB->erase(OldInst, OldBB->end()); 253 254 // If OldBB isn't immediately before OldBB, insert a branch to it. 255 if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest)) 256 TII->InsertBranch(*OldBB, NewDest, 0, std::vector<MachineOperand>()); 257 OldBB->addSuccessor(NewDest); 258 ++NumTailMerge; 259} 260 261/// SplitMBBAt - Given a machine basic block and an iterator into it, split the 262/// MBB so that the part before the iterator falls into the part starting at the 263/// iterator. This returns the new MBB. 264MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB, 265 MachineBasicBlock::iterator BBI1) { 266 // Create the fall-through block. 267 MachineFunction::iterator MBBI = &CurMBB; 268 MachineBasicBlock *NewMBB = new MachineBasicBlock(CurMBB.getBasicBlock()); 269 CurMBB.getParent()->getBasicBlockList().insert(++MBBI, NewMBB); 270 271 // Move all the successors of this block to the specified block. 272 while (!CurMBB.succ_empty()) { 273 MachineBasicBlock *S = *(CurMBB.succ_end()-1); 274 NewMBB->addSuccessor(S); 275 CurMBB.removeSuccessor(S); 276 } 277 278 // Add an edge from CurMBB to NewMBB for the fall-through. 279 CurMBB.addSuccessor(NewMBB); 280 281 // Splice the code over. 282 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end()); 283 return NewMBB; 284} 285 286/// EstimateRuntime - Make a rough estimate for how long it will take to run 287/// the specified code. 288static unsigned EstimateRuntime(MachineBasicBlock::iterator I, 289 MachineBasicBlock::iterator E, 290 const TargetInstrInfo *TII) { 291 unsigned Time = 0; 292 for (; I != E; ++I) { 293 const TargetInstrDescriptor &TID = TII->get(I->getOpcode()); 294 if (TID.Flags & M_CALL_FLAG) 295 Time += 10; 296 else if (TID.Flags & (M_LOAD_FLAG|M_STORE_FLAG)) 297 Time += 2; 298 else 299 ++Time; 300 } 301 return Time; 302} 303 304/// ShouldSplitFirstBlock - We need to either split MBB1 at MBB1I or MBB2 at 305/// MBB2I and then insert an unconditional branch in the other block. Determine 306/// which is the best to split 307static bool ShouldSplitFirstBlock(MachineBasicBlock *MBB1, 308 MachineBasicBlock::iterator MBB1I, 309 MachineBasicBlock *MBB2, 310 MachineBasicBlock::iterator MBB2I, 311 const TargetInstrInfo *TII) { 312 // TODO: if we had some notion of which block was hotter, we could split 313 // the hot block, so it is the fall-through. Since we don't have profile info 314 // make a decision based on which will hurt most to split. 315 unsigned MBB1Time = EstimateRuntime(MBB1->begin(), MBB1I, TII); 316 unsigned MBB2Time = EstimateRuntime(MBB2->begin(), MBB2I, TII); 317 318 // If the MBB1 prefix takes "less time" to run than the MBB2 prefix, split the 319 // MBB1 block so it falls through. This will penalize the MBB2 path, but will 320 // have a lower overall impact on the program execution. 321 return MBB1Time < MBB2Time; 322} 323 324bool BranchFolder::TailMergeBlocks(MachineFunction &MF) { 325 MadeChange = false; 326 327 if (!EnableTailMerge) return false; 328 329 // Find blocks with no successors. 330 std::vector<std::pair<unsigned,MachineBasicBlock*> > MergePotentials; 331 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { 332 if (I->succ_empty()) 333 MergePotentials.push_back(std::make_pair(HashEndOfMBB(I), I)); 334 } 335 336 // Sort by hash value so that blocks with identical end sequences sort 337 // together. 338 std::stable_sort(MergePotentials.begin(), MergePotentials.end()); 339 340 // Walk through equivalence sets looking for actual exact matches. 341 while (MergePotentials.size() > 1) { 342 unsigned CurHash = (MergePotentials.end()-1)->first; 343 unsigned PrevHash = (MergePotentials.end()-2)->first; 344 MachineBasicBlock *CurMBB = (MergePotentials.end()-1)->second; 345 346 // If there is nothing that matches the hash of the current basic block, 347 // give up. 348 if (CurHash != PrevHash) { 349 MergePotentials.pop_back(); 350 continue; 351 } 352 353 // Determine the actual length of the shared tail between these two basic 354 // blocks. Because the hash can have collisions, it's possible that this is 355 // less than 2. 356 MachineBasicBlock::iterator BBI1, BBI2; 357 unsigned CommonTailLen = 358 ComputeCommonTailLength(CurMBB, (MergePotentials.end()-2)->second, 359 BBI1, BBI2); 360 361 // If the tails don't have at least two instructions in common, see if there 362 // is anything else in the equivalence class that does match. 363 if (CommonTailLen < 2) { 364 unsigned FoundMatch = ~0U; 365 for (int i = MergePotentials.size()-2; 366 i != -1 && MergePotentials[i].first == CurHash; --i) { 367 CommonTailLen = ComputeCommonTailLength(CurMBB, 368 MergePotentials[i].second, 369 BBI1, BBI2); 370 if (CommonTailLen >= 2) { 371 FoundMatch = i; 372 break; 373 } 374 } 375 376 // If we didn't find anything that has at least two instructions matching 377 // this one, bail out. 378 if (FoundMatch == ~0U) { 379 MergePotentials.pop_back(); 380 continue; 381 } 382 383 // Otherwise, move the matching block to the right position. 384 std::swap(MergePotentials[FoundMatch], *(MergePotentials.end()-2)); 385 } 386 387 MachineBasicBlock *MBB2 = (MergePotentials.end()-2)->second; 388 389 // If neither block is the entire common tail, split the tail of one block 390 // to make it redundant with the other tail. 391 if (CurMBB->begin() != BBI1 && MBB2->begin() != BBI2) { 392 if (0) { // Enable this to disable partial tail merges. 393 MergePotentials.pop_back(); 394 continue; 395 } 396 397 // Decide whether we want to split CurMBB or MBB2. 398 if (ShouldSplitFirstBlock(CurMBB, BBI1, MBB2, BBI2, TII)) { 399 CurMBB = SplitMBBAt(*CurMBB, BBI1); 400 BBI1 = CurMBB->begin(); 401 MergePotentials.back().second = CurMBB; 402 } else { 403 MBB2 = SplitMBBAt(*MBB2, BBI2); 404 BBI2 = MBB2->begin(); 405 (MergePotentials.end()-2)->second = MBB2; 406 } 407 } 408 409 if (MBB2->begin() == BBI2) { 410 // Hack the end off CurMBB, making it jump to MBBI@ instead. 411 ReplaceTailWithBranchTo(BBI1, MBB2); 412 // This modifies CurMBB, so remove it from the worklist. 413 MergePotentials.pop_back(); 414 } else { 415 assert(CurMBB->begin() == BBI1 && "Didn't split block correctly?"); 416 // Hack the end off MBB2, making it jump to CurMBB instead. 417 ReplaceTailWithBranchTo(BBI2, CurMBB); 418 // This modifies MBB2, so remove it from the worklist. 419 MergePotentials.erase(MergePotentials.end()-2); 420 } 421 MadeChange = true; 422 } 423 424 return MadeChange; 425} 426 427 428//===----------------------------------------------------------------------===// 429// Branch Optimization 430//===----------------------------------------------------------------------===// 431 432bool BranchFolder::OptimizeBranches(MachineFunction &MF) { 433 MadeChange = false; 434 435 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) { 436 MachineBasicBlock *MBB = I++; 437 OptimizeBlock(MBB); 438 439 // If it is dead, remove it. 440 if (MBB->pred_empty()) { 441 RemoveDeadBlock(MBB); 442 MadeChange = true; 443 ++NumDeadBlocks; 444 } 445 } 446 return MadeChange; 447} 448 449 450/// CorrectExtraCFGEdges - Various pieces of code can cause excess edges in the 451/// CFG to be inserted. If we have proven that MBB can only branch to DestA and 452/// DestB, remove any other MBB successors from the CFG. DestA and DestB can 453/// be null. 454static bool CorrectExtraCFGEdges(MachineBasicBlock &MBB, 455 MachineBasicBlock *DestA, 456 MachineBasicBlock *DestB, 457 bool isCond, 458 MachineFunction::iterator FallThru) { 459 bool MadeChange = false; 460 bool AddedFallThrough = false; 461 462 // If this block ends with a conditional branch that falls through to its 463 // successor, set DestB as the successor. 464 if (isCond) { 465 if (DestB == 0 && FallThru != MBB.getParent()->end()) { 466 DestB = FallThru; 467 AddedFallThrough = true; 468 } 469 } else { 470 // If this is an unconditional branch with no explicit dest, it must just be 471 // a fallthrough into DestB. 472 if (DestA == 0 && FallThru != MBB.getParent()->end()) { 473 DestA = FallThru; 474 AddedFallThrough = true; 475 } 476 } 477 478 MachineBasicBlock::pred_iterator SI = MBB.succ_begin(); 479 while (SI != MBB.succ_end()) { 480 if (*SI == DestA) { 481 DestA = 0; 482 ++SI; 483 } else if (*SI == DestB) { 484 DestB = 0; 485 ++SI; 486 } else { 487 // Otherwise, this is a superfluous edge, remove it. 488 MBB.removeSuccessor(SI); 489 MadeChange = true; 490 } 491 } 492 if (!AddedFallThrough) { 493 assert(DestA == 0 && DestB == 0 && 494 "MachineCFG is missing edges!"); 495 } else if (isCond) { 496 assert(DestA == 0 && "MachineCFG is missing edges!"); 497 } 498 return MadeChange; 499} 500 501 502/// ReplaceUsesOfBlockWith - Given a machine basic block 'BB' that branched to 503/// 'Old', change the code and CFG so that it branches to 'New' instead. 504static void ReplaceUsesOfBlockWith(MachineBasicBlock *BB, 505 MachineBasicBlock *Old, 506 MachineBasicBlock *New, 507 const TargetInstrInfo *TII) { 508 assert(Old != New && "Cannot replace self with self!"); 509 510 MachineBasicBlock::iterator I = BB->end(); 511 while (I != BB->begin()) { 512 --I; 513 if (!TII->isTerminatorInstr(I->getOpcode())) break; 514 515 // Scan the operands of this machine instruction, replacing any uses of Old 516 // with New. 517 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) 518 if (I->getOperand(i).isMachineBasicBlock() && 519 I->getOperand(i).getMachineBasicBlock() == Old) 520 I->getOperand(i).setMachineBasicBlock(New); 521 } 522 523 // Update the successor information. 524 std::vector<MachineBasicBlock*> Succs(BB->succ_begin(), BB->succ_end()); 525 for (int i = Succs.size()-1; i >= 0; --i) 526 if (Succs[i] == Old) { 527 BB->removeSuccessor(Old); 528 BB->addSuccessor(New); 529 } 530} 531 532/// CanFallThrough - Return true if the specified block (with the specified 533/// branch condition) can implicitly transfer control to the block after it by 534/// falling off the end of it. This should return false if it can reach the 535/// block after it, but it uses an explicit branch to do so (e.g. a table jump). 536/// 537/// True is a conservative answer. 538/// 539bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB, 540 bool BranchUnAnalyzable, 541 MachineBasicBlock *TBB, MachineBasicBlock *FBB, 542 const std::vector<MachineOperand> &Cond) { 543 MachineFunction::iterator Fallthrough = CurBB; 544 ++Fallthrough; 545 // If FallthroughBlock is off the end of the function, it can't fall through. 546 if (Fallthrough == CurBB->getParent()->end()) 547 return false; 548 549 // If FallthroughBlock isn't a successor of CurBB, no fallthrough is possible. 550 if (!CurBB->isSuccessor(Fallthrough)) 551 return false; 552 553 // If we couldn't analyze the branch, assume it could fall through. 554 if (BranchUnAnalyzable) return true; 555 556 // If there is no branch, control always falls through. 557 if (TBB == 0) return true; 558 559 // If there is some explicit branch to the fallthrough block, it can obviously 560 // reach, even though the branch should get folded to fall through implicitly. 561 if (MachineFunction::iterator(TBB) == Fallthrough || 562 MachineFunction::iterator(FBB) == Fallthrough) 563 return true; 564 565 // If it's an unconditional branch to some block not the fall through, it 566 // doesn't fall through. 567 if (Cond.empty()) return false; 568 569 // Otherwise, if it is conditional and has no explicit false block, it falls 570 // through. 571 return FBB == 0; 572} 573 574/// CanFallThrough - Return true if the specified can implicitly transfer 575/// control to the block after it by falling off the end of it. This should 576/// return false if it can reach the block after it, but it uses an explicit 577/// branch to do so (e.g. a table jump). 578/// 579/// True is a conservative answer. 580/// 581bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) { 582 MachineBasicBlock *TBB = 0, *FBB = 0; 583 std::vector<MachineOperand> Cond; 584 bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond); 585 return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond); 586} 587 588/// OptimizeBlock - Analyze and optimize control flow related to the specified 589/// block. This is never called on the entry block. 590void BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) { 591 MachineFunction::iterator FallThrough = MBB; 592 ++FallThrough; 593 594 // If this block is empty, make everyone use its fall-through, not the block 595 // explicitly. 596 if (MBB->empty()) { 597 // Dead block? Leave for cleanup later. 598 if (MBB->pred_empty()) return; 599 600 if (FallThrough == MBB->getParent()->end()) { 601 // TODO: Simplify preds to not branch here if possible! 602 } else { 603 // Rewrite all predecessors of the old block to go to the fallthrough 604 // instead. 605 while (!MBB->pred_empty()) { 606 MachineBasicBlock *Pred = *(MBB->pred_end()-1); 607 ReplaceUsesOfBlockWith(Pred, MBB, FallThrough, TII); 608 } 609 610 // If MBB was the target of a jump table, update jump tables to go to the 611 // fallthrough instead. 612 MBB->getParent()->getJumpTableInfo()-> 613 ReplaceMBBInJumpTables(MBB, FallThrough); 614 MadeChange = true; 615 } 616 return; 617 } 618 619 // Check to see if we can simplify the terminator of the block before this 620 // one. 621 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB)); 622 623 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0; 624 std::vector<MachineOperand> PriorCond; 625 bool PriorUnAnalyzable = 626 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); 627 if (!PriorUnAnalyzable) { 628 // If the CFG for the prior block has extra edges, remove them. 629 MadeChange |= CorrectExtraCFGEdges(PrevBB, PriorTBB, PriorFBB, 630 !PriorCond.empty(), MBB); 631 632 // If the previous branch is conditional and both conditions go to the same 633 // destination, remove the branch, replacing it with an unconditional one or 634 // a fall-through. 635 if (PriorTBB && PriorTBB == PriorFBB) { 636 TII->RemoveBranch(PrevBB); 637 PriorCond.clear(); 638 if (PriorTBB != MBB) 639 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 640 MadeChange = true; 641 ++NumBranchOpts; 642 return OptimizeBlock(MBB); 643 } 644 645 // If the previous branch *only* branches to *this* block (conditional or 646 // not) remove the branch. 647 if (PriorTBB == MBB && PriorFBB == 0) { 648 TII->RemoveBranch(PrevBB); 649 MadeChange = true; 650 ++NumBranchOpts; 651 return OptimizeBlock(MBB); 652 } 653 654 // If the prior block branches somewhere else on the condition and here if 655 // the condition is false, remove the uncond second branch. 656 if (PriorFBB == MBB) { 657 TII->RemoveBranch(PrevBB); 658 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 659 MadeChange = true; 660 ++NumBranchOpts; 661 return OptimizeBlock(MBB); 662 } 663 664 // If the prior block branches here on true and somewhere else on false, and 665 // if the branch condition is reversible, reverse the branch to create a 666 // fall-through. 667 if (PriorTBB == MBB) { 668 std::vector<MachineOperand> NewPriorCond(PriorCond); 669 if (!TII->ReverseBranchCondition(NewPriorCond)) { 670 TII->RemoveBranch(PrevBB); 671 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond); 672 MadeChange = true; 673 ++NumBranchOpts; 674 return OptimizeBlock(MBB); 675 } 676 } 677 } 678 679 // Analyze the branch in the current block. 680 MachineBasicBlock *CurTBB = 0, *CurFBB = 0; 681 std::vector<MachineOperand> CurCond; 682 bool CurUnAnalyzable = TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond); 683 if (!CurUnAnalyzable) { 684 // If the CFG for the prior block has extra edges, remove them. 685 MadeChange |= CorrectExtraCFGEdges(*MBB, CurTBB, CurFBB, 686 !CurCond.empty(), 687 ++MachineFunction::iterator(MBB)); 688 689 // If this branch is the only thing in its block, see if we can forward 690 // other blocks across it. 691 if (CurTBB && CurCond.empty() && CurFBB == 0 && 692 TII->isBranch(MBB->begin()->getOpcode()) && CurTBB != MBB) { 693 // This block may contain just an unconditional branch. Because there can 694 // be 'non-branch terminators' in the block, try removing the branch and 695 // then seeing if the block is empty. 696 TII->RemoveBranch(*MBB); 697 698 // If this block is just an unconditional branch to CurTBB, we can 699 // usually completely eliminate the block. The only case we cannot 700 // completely eliminate the block is when the block before this one 701 // falls through into MBB and we can't understand the prior block's branch 702 // condition. 703 if (MBB->empty()) { 704 bool PredHasNoFallThrough = TII->BlockHasNoFallThrough(PrevBB); 705 if (PredHasNoFallThrough || !PriorUnAnalyzable || 706 !PrevBB.isSuccessor(MBB)) { 707 // If the prior block falls through into us, turn it into an 708 // explicit branch to us to make updates simpler. 709 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) && 710 PriorTBB != MBB && PriorFBB != MBB) { 711 if (PriorTBB == 0) { 712 assert(PriorCond.empty() && PriorFBB == 0 && 713 "Bad branch analysis"); 714 PriorTBB = MBB; 715 } else { 716 assert(PriorFBB == 0 && "Machine CFG out of date!"); 717 PriorFBB = MBB; 718 } 719 TII->RemoveBranch(PrevBB); 720 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); 721 } 722 723 // Iterate through all the predecessors, revectoring each in-turn. 724 MachineBasicBlock::pred_iterator PI = MBB->pred_begin(); 725 bool DidChange = false; 726 bool HasBranchToSelf = false; 727 while (PI != MBB->pred_end()) { 728 if (*PI == MBB) { 729 // If this block has an uncond branch to itself, leave it. 730 ++PI; 731 HasBranchToSelf = true; 732 } else { 733 DidChange = true; 734 ReplaceUsesOfBlockWith(*PI, MBB, CurTBB, TII); 735 } 736 } 737 738 // Change any jumptables to go to the new MBB. 739 MBB->getParent()->getJumpTableInfo()-> 740 ReplaceMBBInJumpTables(MBB, CurTBB); 741 if (DidChange) { 742 ++NumBranchOpts; 743 MadeChange = true; 744 if (!HasBranchToSelf) return; 745 } 746 } 747 } 748 749 // Add the branch back if the block is more than just an uncond branch. 750 TII->InsertBranch(*MBB, CurTBB, 0, CurCond); 751 } 752 } 753 754 // If the prior block doesn't fall through into this block, and if this 755 // block doesn't fall through into some other block, see if we can find a 756 // place to move this block where a fall-through will happen. 757 if (!CanFallThrough(&PrevBB, PriorUnAnalyzable, 758 PriorTBB, PriorFBB, PriorCond)) { 759 // Now we know that there was no fall-through into this block, check to 760 // see if it has a fall-through into its successor. 761 if (!CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB, CurCond)) { 762 // Check all the predecessors of this block. If one of them has no fall 763 // throughs, move this block right after it. 764 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(), 765 E = MBB->pred_end(); PI != E; ++PI) { 766 // Analyze the branch at the end of the pred. 767 MachineBasicBlock *PredBB = *PI; 768 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough; 769 std::vector<MachineOperand> PredCond; 770 if (PredBB != MBB && !CanFallThrough(PredBB)) { 771 MBB->moveAfter(PredBB); 772 MadeChange = true; 773 return OptimizeBlock(MBB); 774 } 775 } 776 777 // Check all successors to see if we can move this block before it. 778 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(), 779 E = MBB->succ_end(); SI != E; ++SI) { 780 // Analyze the branch at the end of the block before the succ. 781 MachineBasicBlock *SuccBB = *SI; 782 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev; 783 std::vector<MachineOperand> SuccPrevCond; 784 if (SuccBB != MBB && !CanFallThrough(SuccPrev)) { 785 MBB->moveBefore(SuccBB); 786 MadeChange = true; 787 return OptimizeBlock(MBB); 788 } 789 } 790 791 // Okay, there is no really great place to put this block. If, however, 792 // the block before this one would be a fall-through if this block were 793 // removed, move this block to the end of the function. 794 if (FallThrough != MBB->getParent()->end() && 795 PrevBB.isSuccessor(FallThrough)) { 796 MBB->moveAfter(--MBB->getParent()->end()); 797 MadeChange = true; 798 return; 799 } 800 } 801 } 802} 803