BranchFolding.cpp revision 1ee29257428960fede862fcfdbe80d5d007927e9
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#define DEBUG_TYPE "branchfolding" 20#include "llvm/CodeGen/Passes.h" 21#include "llvm/CodeGen/MachineDebugInfo.h" 22#include "llvm/CodeGen/MachineFunctionPass.h" 23#include "llvm/CodeGen/MachineJumpTableInfo.h" 24#include "llvm/Target/TargetInstrInfo.h" 25#include "llvm/Target/TargetMachine.h" 26#include "llvm/Support/CommandLine.h" 27#include "llvm/Support/Debug.h" 28#include "llvm/ADT/Statistic.h" 29#include "llvm/ADT/STLExtras.h" 30#include <algorithm> 31using namespace llvm; 32 33STATISTIC(NumDeadBlocks, "Number of dead blocks removed"); 34STATISTIC(NumBranchOpts, "Number of branches optimized"); 35STATISTIC(NumTailMerge , "Number of block tails merged"); 36static cl::opt<bool> EnableTailMerge("enable-tail-merge", cl::Hidden); 37 38namespace { 39 struct BranchFolder : public MachineFunctionPass { 40 virtual bool runOnMachineFunction(MachineFunction &MF); 41 virtual const char *getPassName() const { return "Control Flow Optimizer"; } 42 const TargetInstrInfo *TII; 43 MachineDebugInfo *MDI; 44 bool MadeChange; 45 private: 46 // Tail Merging. 47 bool TailMergeBlocks(MachineFunction &MF); 48 void ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, 49 MachineBasicBlock *NewDest); 50 MachineBasicBlock *SplitMBBAt(MachineBasicBlock &CurMBB, 51 MachineBasicBlock::iterator BBI1); 52 53 // Branch optzn. 54 bool OptimizeBranches(MachineFunction &MF); 55 void OptimizeBlock(MachineBasicBlock *MBB); 56 void RemoveDeadBlock(MachineBasicBlock *MBB); 57 58 bool CanFallThrough(MachineBasicBlock *CurBB); 59 bool CanFallThrough(MachineBasicBlock *CurBB, bool BranchUnAnalyzable, 60 MachineBasicBlock *TBB, MachineBasicBlock *FBB, 61 const std::vector<MachineOperand> &Cond); 62 }; 63} 64 65FunctionPass *llvm::createBranchFoldingPass() { return new BranchFolder(); } 66 67/// RemoveDeadBlock - Remove the specified dead machine basic block from the 68/// function, updating the CFG. 69void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) { 70 assert(MBB->pred_empty() && "MBB must be dead!"); 71 72 MachineFunction *MF = MBB->getParent(); 73 // drop all successors. 74 while (!MBB->succ_empty()) 75 MBB->removeSuccessor(MBB->succ_end()-1); 76 77 // If there is DWARF info to active, check to see if there are any LABEL 78 // records in the basic block. If so, unregister them from MachineDebugInfo. 79 if (MDI && !MBB->empty()) { 80 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); 81 I != E; ++I) { 82 if ((unsigned)I->getOpcode() == TargetInstrInfo::LABEL) { 83 // The label ID # is always operand #0, an immediate. 84 MDI->InvalidateLabel(I->getOperand(0).getImm()); 85 } 86 } 87 } 88 89 // Remove the block. 90 MF->getBasicBlockList().erase(MBB); 91} 92 93bool BranchFolder::runOnMachineFunction(MachineFunction &MF) { 94 TII = MF.getTarget().getInstrInfo(); 95 if (!TII) return false; 96 97 MDI = getAnalysisToUpdate<MachineDebugInfo>(); 98 99 bool EverMadeChange = false; 100 bool MadeChangeThisIteration = true; 101 while (MadeChangeThisIteration) { 102 MadeChangeThisIteration = false; 103 MadeChangeThisIteration |= TailMergeBlocks(MF); 104 MadeChangeThisIteration |= OptimizeBranches(MF); 105 EverMadeChange |= MadeChangeThisIteration; 106 } 107 108 // See if any jump tables have become mergable or dead as the code generator 109 // did its thing. 110 MachineJumpTableInfo *JTI = MF.getJumpTableInfo(); 111 const std::vector<MachineJumpTableEntry> &JTs = JTI->getJumpTables(); 112 if (!JTs.empty()) { 113 // Figure out how these jump tables should be merged. 114 std::vector<unsigned> JTMapping; 115 JTMapping.reserve(JTs.size()); 116 117 // We always keep the 0th jump table. 118 JTMapping.push_back(0); 119 120 // Scan the jump tables, seeing if there are any duplicates. Note that this 121 // is N^2, which should be fixed someday. 122 for (unsigned i = 1, e = JTs.size(); i != e; ++i) 123 JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs)); 124 125 // If a jump table was merge with another one, walk the function rewriting 126 // references to jump tables to reference the new JT ID's. Keep track of 127 // whether we see a jump table idx, if not, we can delete the JT. 128 std::vector<bool> JTIsLive; 129 JTIsLive.resize(JTs.size()); 130 for (MachineFunction::iterator BB = MF.begin(), E = MF.end(); 131 BB != E; ++BB) { 132 for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end(); 133 I != E; ++I) 134 for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) { 135 MachineOperand &Op = I->getOperand(op); 136 if (!Op.isJumpTableIndex()) continue; 137 unsigned NewIdx = JTMapping[Op.getJumpTableIndex()]; 138 Op.setJumpTableIndex(NewIdx); 139 140 // Remember that this JT is live. 141 JTIsLive[NewIdx] = true; 142 } 143 } 144 145 // Finally, remove dead jump tables. This happens either because the 146 // indirect jump was unreachable (and thus deleted) or because the jump 147 // table was merged with some other one. 148 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i) 149 if (!JTIsLive[i]) { 150 JTI->RemoveJumpTable(i); 151 EverMadeChange = true; 152 } 153 } 154 155 return EverMadeChange; 156} 157 158//===----------------------------------------------------------------------===// 159// Tail Merging of Blocks 160//===----------------------------------------------------------------------===// 161 162/// HashMachineInstr - Compute a hash value for MI and its operands. 163static unsigned HashMachineInstr(const MachineInstr *MI) { 164 unsigned Hash = MI->getOpcode(); 165 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 166 const MachineOperand &Op = MI->getOperand(i); 167 168 // Merge in bits from the operand if easy. 169 unsigned OperandHash = 0; 170 switch (Op.getType()) { 171 case MachineOperand::MO_Register: OperandHash = Op.getReg(); break; 172 case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break; 173 case MachineOperand::MO_MachineBasicBlock: 174 OperandHash = Op.getMachineBasicBlock()->getNumber(); 175 break; 176 case MachineOperand::MO_FrameIndex: OperandHash = Op.getFrameIndex(); break; 177 case MachineOperand::MO_ConstantPoolIndex: 178 OperandHash = Op.getConstantPoolIndex(); 179 break; 180 case MachineOperand::MO_JumpTableIndex: 181 OperandHash = Op.getJumpTableIndex(); 182 break; 183 case MachineOperand::MO_GlobalAddress: 184 case MachineOperand::MO_ExternalSymbol: 185 // Global address / external symbol are too hard, don't bother, but do 186 // pull in the offset. 187 OperandHash = Op.getOffset(); 188 break; 189 default: break; 190 } 191 192 Hash += ((OperandHash << 3) | Op.getType()) << (i&31); 193 } 194 return Hash; 195} 196 197/// HashEndOfMBB - Hash the last two instructions in the MBB. We hash two 198/// instructions, because cross-jumping only saves code when at least two 199/// instructions are removed (since a branch must be inserted). 200static unsigned HashEndOfMBB(const MachineBasicBlock *MBB) { 201 MachineBasicBlock::const_iterator I = MBB->end(); 202 if (I == MBB->begin()) 203 return 0; // Empty MBB. 204 205 --I; 206 unsigned Hash = HashMachineInstr(I); 207 208 if (I == MBB->begin()) 209 return Hash; // Single instr MBB. 210 211 --I; 212 // Hash in the second-to-last instruction. 213 Hash ^= HashMachineInstr(I) << 2; 214 return Hash; 215} 216 217/// ComputeCommonTailLength - Given two machine basic blocks, compute the number 218/// of instructions they actually have in common together at their end. Return 219/// iterators for the first shared instruction in each block. 220static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1, 221 MachineBasicBlock *MBB2, 222 MachineBasicBlock::iterator &I1, 223 MachineBasicBlock::iterator &I2) { 224 I1 = MBB1->end(); 225 I2 = MBB2->end(); 226 227 unsigned TailLen = 0; 228 while (I1 != MBB1->begin() && I2 != MBB2->begin()) { 229 --I1; --I2; 230 if (!I1->isIdenticalTo(I2)) { 231 ++I1; ++I2; 232 break; 233 } 234 ++TailLen; 235 } 236 return TailLen; 237} 238 239/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything 240/// after it, replacing it with an unconditional branch to NewDest. This 241/// returns true if OldInst's block is modified, false if NewDest is modified. 242void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, 243 MachineBasicBlock *NewDest) { 244 MachineBasicBlock *OldBB = OldInst->getParent(); 245 246 // Remove all the old successors of OldBB from the CFG. 247 while (!OldBB->succ_empty()) 248 OldBB->removeSuccessor(OldBB->succ_begin()); 249 250 // Remove all the dead instructions from the end of OldBB. 251 OldBB->erase(OldInst, OldBB->end()); 252 253 // If OldBB isn't immediately before OldBB, insert a branch to it. 254 if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest)) 255 TII->InsertBranch(*OldBB, NewDest, 0, std::vector<MachineOperand>()); 256 OldBB->addSuccessor(NewDest); 257 ++NumTailMerge; 258} 259 260/// SplitMBBAt - Given a machine basic block and an iterator into it, split the 261/// MBB so that the part before the iterator falls into the part starting at the 262/// iterator. This returns the new MBB. 263MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB, 264 MachineBasicBlock::iterator BBI1) { 265 // Create the fall-through block. 266 MachineFunction::iterator MBBI = &CurMBB; 267 MachineBasicBlock *NewMBB = new MachineBasicBlock(CurMBB.getBasicBlock()); 268 CurMBB.getParent()->getBasicBlockList().insert(++MBBI, NewMBB); 269 270 // Move all the successors of this block to the specified block. 271 while (!CurMBB.succ_empty()) { 272 MachineBasicBlock *S = *(CurMBB.succ_end()-1); 273 NewMBB->addSuccessor(S); 274 CurMBB.removeSuccessor(S); 275 } 276 277 // Add an edge from CurMBB to NewMBB for the fall-through. 278 CurMBB.addSuccessor(NewMBB); 279 280 // Splice the code over. 281 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end()); 282 return NewMBB; 283} 284 285/// EstimateRuntime - Make a rough estimate for how long it will take to run 286/// the specified code. 287static unsigned EstimateRuntime(MachineBasicBlock::iterator I, 288 MachineBasicBlock::iterator E, 289 const TargetInstrInfo *TII) { 290 unsigned Time = 0; 291 for (; I != E; ++I) { 292 const TargetInstrDescriptor &TID = TII->get(I->getOpcode()); 293 if (TID.Flags & M_CALL_FLAG) 294 Time += 10; 295 else if (TID.Flags & (M_LOAD_FLAG|M_STORE_FLAG)) 296 Time += 2; 297 else 298 ++Time; 299 } 300 return Time; 301} 302 303/// ShouldSplitFirstBlock - We need to either split MBB1 at MBB1I or MBB2 at 304/// MBB2I and then insert an unconditional branch in the other block. Determine 305/// which is the best to split 306static bool ShouldSplitFirstBlock(MachineBasicBlock *MBB1, 307 MachineBasicBlock::iterator MBB1I, 308 MachineBasicBlock *MBB2, 309 MachineBasicBlock::iterator MBB2I, 310 const TargetInstrInfo *TII) { 311 // TODO: if we had some notion of which block was hotter, we could split 312 // the hot block, so it is the fall-through. Since we don't have profile info 313 // make a decision based on which will hurt most to split. 314 unsigned MBB1Time = EstimateRuntime(MBB1->begin(), MBB1I, TII); 315 unsigned MBB2Time = EstimateRuntime(MBB2->begin(), MBB2I, TII); 316 317 // If the MBB1 prefix takes "less time" to run than the MBB2 prefix, split the 318 // MBB1 block so it falls through. This will penalize the MBB2 path, but will 319 // have a lower overall impact on the program execution. 320 return MBB1Time < MBB2Time; 321} 322 323bool BranchFolder::TailMergeBlocks(MachineFunction &MF) { 324 MadeChange = false; 325 326 if (!EnableTailMerge) return false; 327 328 // Find blocks with no successors. 329 std::vector<std::pair<unsigned,MachineBasicBlock*> > MergePotentials; 330 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { 331 if (I->succ_empty()) 332 MergePotentials.push_back(std::make_pair(HashEndOfMBB(I), I)); 333 } 334 335 // Sort by hash value so that blocks with identical end sequences sort 336 // together. 337 std::stable_sort(MergePotentials.begin(), MergePotentials.end()); 338 339 // Walk through equivalence sets looking for actual exact matches. 340 while (MergePotentials.size() > 1) { 341 unsigned CurHash = (MergePotentials.end()-1)->first; 342 unsigned PrevHash = (MergePotentials.end()-2)->first; 343 MachineBasicBlock *CurMBB = (MergePotentials.end()-1)->second; 344 345 // If there is nothing that matches the hash of the current basic block, 346 // give up. 347 if (CurHash != PrevHash) { 348 MergePotentials.pop_back(); 349 continue; 350 } 351 352 // Determine the actual length of the shared tail between these two basic 353 // blocks. Because the hash can have collisions, it's possible that this is 354 // less than 2. 355 MachineBasicBlock::iterator BBI1, BBI2; 356 unsigned CommonTailLen = 357 ComputeCommonTailLength(CurMBB, (MergePotentials.end()-2)->second, 358 BBI1, BBI2); 359 360 // If the tails don't have at least two instructions in common, see if there 361 // is anything else in the equivalence class that does match. 362 if (CommonTailLen < 2) { 363 unsigned FoundMatch = ~0U; 364 for (int i = MergePotentials.size()-2; 365 i != -1 && MergePotentials[i].first == CurHash; --i) { 366 CommonTailLen = ComputeCommonTailLength(CurMBB, 367 MergePotentials[i].second, 368 BBI1, BBI2); 369 if (CommonTailLen >= 2) { 370 FoundMatch = i; 371 break; 372 } 373 } 374 375 // If we didn't find anything that has at least two instructions matching 376 // this one, bail out. 377 if (FoundMatch == ~0U) { 378 MergePotentials.pop_back(); 379 continue; 380 } 381 382 // Otherwise, move the matching block to the right position. 383 std::swap(MergePotentials[FoundMatch], *(MergePotentials.end()-2)); 384 } 385 386 MachineBasicBlock *MBB2 = (MergePotentials.end()-2)->second; 387 388 // If neither block is the entire common tail, split the tail of one block 389 // to make it redundant with the other tail. 390 if (CurMBB->begin() != BBI1 && MBB2->begin() != BBI2) { 391 if (0) { // Enable this to disable partial tail merges. 392 MergePotentials.pop_back(); 393 continue; 394 } 395 396 // Decide whether we want to split CurMBB or MBB2. 397 if (ShouldSplitFirstBlock(CurMBB, BBI1, MBB2, BBI2, TII)) { 398 CurMBB = SplitMBBAt(*CurMBB, BBI1); 399 BBI1 = CurMBB->begin(); 400 MergePotentials.back().second = CurMBB; 401 } else { 402 MBB2 = SplitMBBAt(*MBB2, BBI2); 403 BBI2 = MBB2->begin(); 404 (MergePotentials.end()-2)->second = MBB2; 405 } 406 } 407 408 if (MBB2->begin() == BBI2) { 409 // Hack the end off CurMBB, making it jump to MBBI@ instead. 410 ReplaceTailWithBranchTo(BBI1, MBB2); 411 // This modifies CurMBB, so remove it from the worklist. 412 MergePotentials.pop_back(); 413 } else { 414 assert(CurMBB->begin() == BBI1 && "Didn't split block correctly?"); 415 // Hack the end off MBB2, making it jump to CurMBB instead. 416 ReplaceTailWithBranchTo(BBI2, CurMBB); 417 // This modifies MBB2, so remove it from the worklist. 418 MergePotentials.erase(MergePotentials.end()-2); 419 } 420 MadeChange = true; 421 } 422 423 return MadeChange; 424} 425 426 427//===----------------------------------------------------------------------===// 428// Branch Optimization 429//===----------------------------------------------------------------------===// 430 431bool BranchFolder::OptimizeBranches(MachineFunction &MF) { 432 MadeChange = false; 433 434 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) { 435 MachineBasicBlock *MBB = I++; 436 OptimizeBlock(MBB); 437 438 // If it is dead, remove it. 439 if (MBB->pred_empty()) { 440 RemoveDeadBlock(MBB); 441 MadeChange = true; 442 ++NumDeadBlocks; 443 } 444 } 445 return MadeChange; 446} 447 448 449/// CorrectExtraCFGEdges - Various pieces of code can cause excess edges in the 450/// CFG to be inserted. If we have proven that MBB can only branch to DestA and 451/// DestB, remove any other MBB successors from the CFG. DestA and DestB can 452/// be null. 453static bool CorrectExtraCFGEdges(MachineBasicBlock &MBB, 454 MachineBasicBlock *DestA, 455 MachineBasicBlock *DestB, 456 bool isCond, 457 MachineFunction::iterator FallThru) { 458 bool MadeChange = false; 459 bool AddedFallThrough = false; 460 461 // If this block ends with a conditional branch that falls through to its 462 // successor, set DestB as the successor. 463 if (isCond) { 464 if (DestB == 0 && FallThru != MBB.getParent()->end()) { 465 DestB = FallThru; 466 AddedFallThrough = true; 467 } 468 } else { 469 // If this is an unconditional branch with no explicit dest, it must just be 470 // a fallthrough into DestB. 471 if (DestA == 0 && FallThru != MBB.getParent()->end()) { 472 DestA = FallThru; 473 AddedFallThrough = true; 474 } 475 } 476 477 MachineBasicBlock::pred_iterator SI = MBB.succ_begin(); 478 while (SI != MBB.succ_end()) { 479 if (*SI == DestA) { 480 DestA = 0; 481 ++SI; 482 } else if (*SI == DestB) { 483 DestB = 0; 484 ++SI; 485 } else { 486 // Otherwise, this is a superfluous edge, remove it. 487 MBB.removeSuccessor(SI); 488 MadeChange = true; 489 } 490 } 491 if (!AddedFallThrough) { 492 assert(DestA == 0 && DestB == 0 && 493 "MachineCFG is missing edges!"); 494 } else if (isCond) { 495 assert(DestA == 0 && "MachineCFG is missing edges!"); 496 } 497 return MadeChange; 498} 499 500 501/// ReplaceUsesOfBlockWith - Given a machine basic block 'BB' that branched to 502/// 'Old', change the code and CFG so that it branches to 'New' instead. 503static void ReplaceUsesOfBlockWith(MachineBasicBlock *BB, 504 MachineBasicBlock *Old, 505 MachineBasicBlock *New, 506 const TargetInstrInfo *TII) { 507 assert(Old != New && "Cannot replace self with self!"); 508 509 MachineBasicBlock::iterator I = BB->end(); 510 while (I != BB->begin()) { 511 --I; 512 if (!TII->isTerminatorInstr(I->getOpcode())) break; 513 514 // Scan the operands of this machine instruction, replacing any uses of Old 515 // with New. 516 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) 517 if (I->getOperand(i).isMachineBasicBlock() && 518 I->getOperand(i).getMachineBasicBlock() == Old) 519 I->getOperand(i).setMachineBasicBlock(New); 520 } 521 522 // Update the successor information. 523 std::vector<MachineBasicBlock*> Succs(BB->succ_begin(), BB->succ_end()); 524 for (int i = Succs.size()-1; i >= 0; --i) 525 if (Succs[i] == Old) { 526 BB->removeSuccessor(Old); 527 BB->addSuccessor(New); 528 } 529} 530 531/// CanFallThrough - Return true if the specified block (with the specified 532/// branch condition) can implicitly transfer control to the block after it by 533/// falling off the end of it. This should return false if it can reach the 534/// block after it, but it uses an explicit branch to do so (e.g. a table jump). 535/// 536/// True is a conservative answer. 537/// 538bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB, 539 bool BranchUnAnalyzable, 540 MachineBasicBlock *TBB, MachineBasicBlock *FBB, 541 const std::vector<MachineOperand> &Cond) { 542 MachineFunction::iterator Fallthrough = CurBB; 543 ++Fallthrough; 544 // If FallthroughBlock is off the end of the function, it can't fall through. 545 if (Fallthrough == CurBB->getParent()->end()) 546 return false; 547 548 // If FallthroughBlock isn't a successor of CurBB, no fallthrough is possible. 549 if (!CurBB->isSuccessor(Fallthrough)) 550 return false; 551 552 // If we couldn't analyze the branch, assume it could fall through. 553 if (BranchUnAnalyzable) return true; 554 555 // If there is no branch, control always falls through. 556 if (TBB == 0) return true; 557 558 // If there is some explicit branch to the fallthrough block, it can obviously 559 // reach, even though the branch should get folded to fall through implicitly. 560 if (MachineFunction::iterator(TBB) == Fallthrough || 561 MachineFunction::iterator(FBB) == Fallthrough) 562 return true; 563 564 // If it's an unconditional branch to some block not the fall through, it 565 // doesn't fall through. 566 if (Cond.empty()) return false; 567 568 // Otherwise, if it is conditional and has no explicit false block, it falls 569 // through. 570 return FBB == 0; 571} 572 573/// CanFallThrough - Return true if the specified can implicitly transfer 574/// control to the block after it by falling off the end of it. This should 575/// return false if it can reach the block after it, but it uses an explicit 576/// branch to do so (e.g. a table jump). 577/// 578/// True is a conservative answer. 579/// 580bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) { 581 MachineBasicBlock *TBB = 0, *FBB = 0; 582 std::vector<MachineOperand> Cond; 583 bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond); 584 return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond); 585} 586 587/// IsBetterFallthrough - Return true if it would be clearly better to 588/// fall-through to MBB1 than to fall through into MBB2. This has to return 589/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will 590/// result in infinite loops. 591static bool IsBetterFallthrough(MachineBasicBlock *MBB1, 592 MachineBasicBlock *MBB2, 593 const TargetInstrInfo &TII) { 594 // Right now, we use a simple heuristic. If MBB2 ends with a call, and 595 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to 596 // optimize branches that branch to either a return block or an assert block 597 // into a fallthrough to the return. 598 if (MBB1->empty() || MBB2->empty()) return false; 599 600 MachineInstr *MBB1I = --MBB1->end(); 601 MachineInstr *MBB2I = --MBB2->end(); 602 return TII.isCall(MBB2I->getOpcode()) && !TII.isCall(MBB1I->getOpcode()); 603} 604 605/// OptimizeBlock - Analyze and optimize control flow related to the specified 606/// block. This is never called on the entry block. 607void BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) { 608 MachineFunction::iterator FallThrough = MBB; 609 ++FallThrough; 610 611 // If this block is empty, make everyone use its fall-through, not the block 612 // explicitly. 613 if (MBB->empty()) { 614 // Dead block? Leave for cleanup later. 615 if (MBB->pred_empty()) return; 616 617 if (FallThrough == MBB->getParent()->end()) { 618 // TODO: Simplify preds to not branch here if possible! 619 } else { 620 // Rewrite all predecessors of the old block to go to the fallthrough 621 // instead. 622 while (!MBB->pred_empty()) { 623 MachineBasicBlock *Pred = *(MBB->pred_end()-1); 624 ReplaceUsesOfBlockWith(Pred, MBB, FallThrough, TII); 625 } 626 627 // If MBB was the target of a jump table, update jump tables to go to the 628 // fallthrough instead. 629 MBB->getParent()->getJumpTableInfo()-> 630 ReplaceMBBInJumpTables(MBB, FallThrough); 631 MadeChange = true; 632 } 633 return; 634 } 635 636 // Check to see if we can simplify the terminator of the block before this 637 // one. 638 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB)); 639 640 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0; 641 std::vector<MachineOperand> PriorCond; 642 bool PriorUnAnalyzable = 643 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); 644 if (!PriorUnAnalyzable) { 645 // If the CFG for the prior block has extra edges, remove them. 646 MadeChange |= CorrectExtraCFGEdges(PrevBB, PriorTBB, PriorFBB, 647 !PriorCond.empty(), MBB); 648 649 // If the previous branch is conditional and both conditions go to the same 650 // destination, remove the branch, replacing it with an unconditional one or 651 // a fall-through. 652 if (PriorTBB && PriorTBB == PriorFBB) { 653 TII->RemoveBranch(PrevBB); 654 PriorCond.clear(); 655 if (PriorTBB != MBB) 656 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 657 MadeChange = true; 658 ++NumBranchOpts; 659 return OptimizeBlock(MBB); 660 } 661 662 // If the previous branch *only* branches to *this* block (conditional or 663 // not) remove the branch. 664 if (PriorTBB == MBB && PriorFBB == 0) { 665 TII->RemoveBranch(PrevBB); 666 MadeChange = true; 667 ++NumBranchOpts; 668 return OptimizeBlock(MBB); 669 } 670 671 // If the prior block branches somewhere else on the condition and here if 672 // the condition is false, remove the uncond second branch. 673 if (PriorFBB == MBB) { 674 TII->RemoveBranch(PrevBB); 675 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 676 MadeChange = true; 677 ++NumBranchOpts; 678 return OptimizeBlock(MBB); 679 } 680 681 // If the prior block branches here on true and somewhere else on false, and 682 // if the branch condition is reversible, reverse the branch to create a 683 // fall-through. 684 if (PriorTBB == MBB) { 685 std::vector<MachineOperand> NewPriorCond(PriorCond); 686 if (!TII->ReverseBranchCondition(NewPriorCond)) { 687 TII->RemoveBranch(PrevBB); 688 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond); 689 MadeChange = true; 690 ++NumBranchOpts; 691 return OptimizeBlock(MBB); 692 } 693 } 694 695 // If this block doesn't fall through (e.g. it ends with an uncond branch or 696 // has no successors) and if the pred falls through into this block, and if 697 // it would otherwise fall through into the block after this, move this 698 // block to the end of the function. 699 // 700 // We consider it more likely that execution will stay in the function (e.g. 701 // due to loops) than it is to exit it. This asserts in loops etc, moving 702 // the assert condition out of the loop body. 703 if (!PriorCond.empty() && PriorFBB == 0 && 704 MachineFunction::iterator(PriorTBB) == FallThrough && 705 !CanFallThrough(MBB)) { 706 bool DoTransform = true; 707 708 // We have to be careful that the succs of PredBB aren't both no-successor 709 // blocks. If neither have successors and if PredBB is the second from 710 // last block in the function, we'd just keep swapping the two blocks for 711 // last. Only do the swap if one is clearly better to fall through than 712 // the other. 713 if (FallThrough == --MBB->getParent()->end() && 714 !IsBetterFallthrough(PriorTBB, MBB, *TII)) 715 DoTransform = false; 716 717 // We don't want to do this transformation if we have control flow like: 718 // br cond BB2 719 // BB1: 720 // .. 721 // jmp BBX 722 // BB2: 723 // .. 724 // ret 725 // 726 // In this case, we could actually be moving the return block *into* a 727 // loop! 728 if (DoTransform && !MBB->succ_empty() && 729 (!CanFallThrough(PriorTBB) || PriorTBB->empty())) 730 DoTransform = false; 731 732 733 if (DoTransform) { 734 // Reverse the branch so we will fall through on the previous true cond. 735 std::vector<MachineOperand> NewPriorCond(PriorCond); 736 if (!TII->ReverseBranchCondition(NewPriorCond)) { 737 DOUT << "\nMoving MBB: " << *MBB; 738 DOUT << "To make fallthrough to: " << *PriorTBB << "\n"; 739 740 TII->RemoveBranch(PrevBB); 741 TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond); 742 743 // Move this block to the end of the function. 744 MBB->moveAfter(--MBB->getParent()->end()); 745 MadeChange = true; 746 ++NumBranchOpts; 747 return; 748 } 749 } 750 } 751 } 752 753 // Analyze the branch in the current block. 754 MachineBasicBlock *CurTBB = 0, *CurFBB = 0; 755 std::vector<MachineOperand> CurCond; 756 bool CurUnAnalyzable = TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond); 757 if (!CurUnAnalyzable) { 758 // If the CFG for the prior block has extra edges, remove them. 759 MadeChange |= CorrectExtraCFGEdges(*MBB, CurTBB, CurFBB, 760 !CurCond.empty(), 761 ++MachineFunction::iterator(MBB)); 762 763 // If this is a two-way branch, and the FBB branches to this block, reverse 764 // the condition so the single-basic-block loop is faster. Instead of: 765 // Loop: xxx; jcc Out; jmp Loop 766 // we want: 767 // Loop: xxx; jncc Loop; jmp Out 768 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) { 769 std::vector<MachineOperand> NewCond(CurCond); 770 if (!TII->ReverseBranchCondition(NewCond)) { 771 TII->RemoveBranch(*MBB); 772 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond); 773 MadeChange = true; 774 ++NumBranchOpts; 775 return OptimizeBlock(MBB); 776 } 777 } 778 779 780 // If this branch is the only thing in its block, see if we can forward 781 // other blocks across it. 782 if (CurTBB && CurCond.empty() && CurFBB == 0 && 783 TII->isBranch(MBB->begin()->getOpcode()) && CurTBB != MBB) { 784 // This block may contain just an unconditional branch. Because there can 785 // be 'non-branch terminators' in the block, try removing the branch and 786 // then seeing if the block is empty. 787 TII->RemoveBranch(*MBB); 788 789 // If this block is just an unconditional branch to CurTBB, we can 790 // usually completely eliminate the block. The only case we cannot 791 // completely eliminate the block is when the block before this one 792 // falls through into MBB and we can't understand the prior block's branch 793 // condition. 794 if (MBB->empty()) { 795 bool PredHasNoFallThrough = TII->BlockHasNoFallThrough(PrevBB); 796 if (PredHasNoFallThrough || !PriorUnAnalyzable || 797 !PrevBB.isSuccessor(MBB)) { 798 // If the prior block falls through into us, turn it into an 799 // explicit branch to us to make updates simpler. 800 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) && 801 PriorTBB != MBB && PriorFBB != MBB) { 802 if (PriorTBB == 0) { 803 assert(PriorCond.empty() && PriorFBB == 0 && 804 "Bad branch analysis"); 805 PriorTBB = MBB; 806 } else { 807 assert(PriorFBB == 0 && "Machine CFG out of date!"); 808 PriorFBB = MBB; 809 } 810 TII->RemoveBranch(PrevBB); 811 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); 812 } 813 814 // Iterate through all the predecessors, revectoring each in-turn. 815 MachineBasicBlock::pred_iterator PI = MBB->pred_begin(); 816 bool DidChange = false; 817 bool HasBranchToSelf = false; 818 while (PI != MBB->pred_end()) { 819 if (*PI == MBB) { 820 // If this block has an uncond branch to itself, leave it. 821 ++PI; 822 HasBranchToSelf = true; 823 } else { 824 DidChange = true; 825 ReplaceUsesOfBlockWith(*PI, MBB, CurTBB, TII); 826 } 827 } 828 829 // Change any jumptables to go to the new MBB. 830 MBB->getParent()->getJumpTableInfo()-> 831 ReplaceMBBInJumpTables(MBB, CurTBB); 832 if (DidChange) { 833 ++NumBranchOpts; 834 MadeChange = true; 835 if (!HasBranchToSelf) return; 836 } 837 } 838 } 839 840 // Add the branch back if the block is more than just an uncond branch. 841 TII->InsertBranch(*MBB, CurTBB, 0, CurCond); 842 } 843 } 844 845 // If the prior block doesn't fall through into this block, and if this 846 // block doesn't fall through into some other block, see if we can find a 847 // place to move this block where a fall-through will happen. 848 if (!CanFallThrough(&PrevBB, PriorUnAnalyzable, 849 PriorTBB, PriorFBB, PriorCond)) { 850 // Now we know that there was no fall-through into this block, check to 851 // see if it has a fall-through into its successor. 852 if (!CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB, CurCond)) { 853 // Check all the predecessors of this block. If one of them has no fall 854 // throughs, move this block right after it. 855 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(), 856 E = MBB->pred_end(); PI != E; ++PI) { 857 // Analyze the branch at the end of the pred. 858 MachineBasicBlock *PredBB = *PI; 859 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough; 860 std::vector<MachineOperand> PredCond; 861 if (PredBB != MBB && !CanFallThrough(PredBB)) { 862 MBB->moveAfter(PredBB); 863 MadeChange = true; 864 return OptimizeBlock(MBB); 865 } 866 } 867 868 // Check all successors to see if we can move this block before it. 869 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(), 870 E = MBB->succ_end(); SI != E; ++SI) { 871 // Analyze the branch at the end of the block before the succ. 872 MachineBasicBlock *SuccBB = *SI; 873 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev; 874 std::vector<MachineOperand> SuccPrevCond; 875 if (SuccBB != MBB && !CanFallThrough(SuccPrev)) { 876 MBB->moveBefore(SuccBB); 877 MadeChange = true; 878 return OptimizeBlock(MBB); 879 } 880 } 881 882 // Okay, there is no really great place to put this block. If, however, 883 // the block before this one would be a fall-through if this block were 884 // removed, move this block to the end of the function. 885 if (FallThrough != MBB->getParent()->end() && 886 PrevBB.isSuccessor(FallThrough)) { 887 MBB->moveAfter(--MBB->getParent()->end()); 888 MadeChange = true; 889 return; 890 } 891 } 892 } 893} 894