BranchFolding.cpp revision 3e15bf33e024b9df9e89351a165acfdb1dde51ed
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/MachineModuleInfo.h" 22#include "llvm/CodeGen/MachineFunctionPass.h" 23#include "llvm/CodeGen/MachineJumpTableInfo.h" 24#include "llvm/CodeGen/RegisterScavenging.h" 25#include "llvm/Target/TargetInstrInfo.h" 26#include "llvm/Target/TargetMachine.h" 27#include "llvm/Target/MRegisterInfo.h" 28#include "llvm/Support/CommandLine.h" 29#include "llvm/Support/Debug.h" 30#include "llvm/ADT/Statistic.h" 31#include "llvm/ADT/STLExtras.h" 32#include <algorithm> 33using namespace llvm; 34 35STATISTIC(NumDeadBlocks, "Number of dead blocks removed"); 36STATISTIC(NumBranchOpts, "Number of branches optimized"); 37STATISTIC(NumTailMerge , "Number of block tails merged"); 38static cl::opt<bool> EnableTailMerge("enable-tail-merge", cl::Hidden); 39 40namespace { 41 struct BranchFolder : public MachineFunctionPass { 42 static const char ID; 43 BranchFolder() : MachineFunctionPass((intptr_t)&ID) {} 44 45 virtual bool runOnMachineFunction(MachineFunction &MF); 46 virtual const char *getPassName() const { return "Control Flow Optimizer"; } 47 const TargetInstrInfo *TII; 48 MachineModuleInfo *MMI; 49 bool MadeChange; 50 private: 51 // Tail Merging. 52 bool TailMergeBlocks(MachineFunction &MF); 53 void ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, 54 MachineBasicBlock *NewDest); 55 MachineBasicBlock *SplitMBBAt(MachineBasicBlock &CurMBB, 56 MachineBasicBlock::iterator BBI1); 57 58 const MRegisterInfo *RegInfo; 59 RegScavenger *RS; 60 // Branch optzn. 61 bool OptimizeBranches(MachineFunction &MF); 62 void OptimizeBlock(MachineBasicBlock *MBB); 63 void RemoveDeadBlock(MachineBasicBlock *MBB); 64 65 bool CanFallThrough(MachineBasicBlock *CurBB); 66 bool CanFallThrough(MachineBasicBlock *CurBB, bool BranchUnAnalyzable, 67 MachineBasicBlock *TBB, MachineBasicBlock *FBB, 68 const std::vector<MachineOperand> &Cond); 69 }; 70 const char BranchFolder::ID = 0; 71} 72 73FunctionPass *llvm::createBranchFoldingPass() { return new BranchFolder(); } 74 75/// RemoveDeadBlock - Remove the specified dead machine basic block from the 76/// function, updating the CFG. 77void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) { 78 assert(MBB->pred_empty() && "MBB must be dead!"); 79 DOUT << "\nRemoving MBB: " << *MBB; 80 81 MachineFunction *MF = MBB->getParent(); 82 // drop all successors. 83 while (!MBB->succ_empty()) 84 MBB->removeSuccessor(MBB->succ_end()-1); 85 86 // If there is DWARF info to active, check to see if there are any LABEL 87 // records in the basic block. If so, unregister them from MachineModuleInfo. 88 if (MMI && !MBB->empty()) { 89 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); 90 I != E; ++I) { 91 if ((unsigned)I->getOpcode() == TargetInstrInfo::LABEL) { 92 // The label ID # is always operand #0, an immediate. 93 MMI->InvalidateLabel(I->getOperand(0).getImm()); 94 } 95 } 96 } 97 98 // Remove the block. 99 MF->getBasicBlockList().erase(MBB); 100} 101 102bool BranchFolder::runOnMachineFunction(MachineFunction &MF) { 103 TII = MF.getTarget().getInstrInfo(); 104 if (!TII) return false; 105 106 RegInfo = MF.getTarget().getRegisterInfo(); 107 RS = RegInfo->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL; 108 109 MMI = getAnalysisToUpdate<MachineModuleInfo>(); 110 111 bool EverMadeChange = false; 112 bool MadeChangeThisIteration = true; 113 while (MadeChangeThisIteration) { 114 MadeChangeThisIteration = false; 115 MadeChangeThisIteration |= TailMergeBlocks(MF); 116 MadeChangeThisIteration |= OptimizeBranches(MF); 117 EverMadeChange |= MadeChangeThisIteration; 118 } 119 120 // See if any jump tables have become mergable or dead as the code generator 121 // did its thing. 122 MachineJumpTableInfo *JTI = MF.getJumpTableInfo(); 123 const std::vector<MachineJumpTableEntry> &JTs = JTI->getJumpTables(); 124 if (!JTs.empty()) { 125 // Figure out how these jump tables should be merged. 126 std::vector<unsigned> JTMapping; 127 JTMapping.reserve(JTs.size()); 128 129 // We always keep the 0th jump table. 130 JTMapping.push_back(0); 131 132 // Scan the jump tables, seeing if there are any duplicates. Note that this 133 // is N^2, which should be fixed someday. 134 for (unsigned i = 1, e = JTs.size(); i != e; ++i) 135 JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs)); 136 137 // If a jump table was merge with another one, walk the function rewriting 138 // references to jump tables to reference the new JT ID's. Keep track of 139 // whether we see a jump table idx, if not, we can delete the JT. 140 std::vector<bool> JTIsLive; 141 JTIsLive.resize(JTs.size()); 142 for (MachineFunction::iterator BB = MF.begin(), E = MF.end(); 143 BB != E; ++BB) { 144 for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end(); 145 I != E; ++I) 146 for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) { 147 MachineOperand &Op = I->getOperand(op); 148 if (!Op.isJumpTableIndex()) continue; 149 unsigned NewIdx = JTMapping[Op.getJumpTableIndex()]; 150 Op.setJumpTableIndex(NewIdx); 151 152 // Remember that this JT is live. 153 JTIsLive[NewIdx] = true; 154 } 155 } 156 157 // Finally, remove dead jump tables. This happens either because the 158 // indirect jump was unreachable (and thus deleted) or because the jump 159 // table was merged with some other one. 160 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i) 161 if (!JTIsLive[i]) { 162 JTI->RemoveJumpTable(i); 163 EverMadeChange = true; 164 } 165 } 166 167 delete RS; 168 return EverMadeChange; 169} 170 171//===----------------------------------------------------------------------===// 172// Tail Merging of Blocks 173//===----------------------------------------------------------------------===// 174 175/// HashMachineInstr - Compute a hash value for MI and its operands. 176static unsigned HashMachineInstr(const MachineInstr *MI) { 177 unsigned Hash = MI->getOpcode(); 178 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 179 const MachineOperand &Op = MI->getOperand(i); 180 181 // Merge in bits from the operand if easy. 182 unsigned OperandHash = 0; 183 switch (Op.getType()) { 184 case MachineOperand::MO_Register: OperandHash = Op.getReg(); break; 185 case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break; 186 case MachineOperand::MO_MachineBasicBlock: 187 OperandHash = Op.getMachineBasicBlock()->getNumber(); 188 break; 189 case MachineOperand::MO_FrameIndex: OperandHash = Op.getFrameIndex(); break; 190 case MachineOperand::MO_ConstantPoolIndex: 191 OperandHash = Op.getConstantPoolIndex(); 192 break; 193 case MachineOperand::MO_JumpTableIndex: 194 OperandHash = Op.getJumpTableIndex(); 195 break; 196 case MachineOperand::MO_GlobalAddress: 197 case MachineOperand::MO_ExternalSymbol: 198 // Global address / external symbol are too hard, don't bother, but do 199 // pull in the offset. 200 OperandHash = Op.getOffset(); 201 break; 202 default: break; 203 } 204 205 Hash += ((OperandHash << 3) | Op.getType()) << (i&31); 206 } 207 return Hash; 208} 209 210/// HashEndOfMBB - Hash the last two instructions in the MBB. We hash two 211/// instructions, because cross-jumping only saves code when at least two 212/// instructions are removed (since a branch must be inserted). 213static unsigned HashEndOfMBB(const MachineBasicBlock *MBB) { 214 MachineBasicBlock::const_iterator I = MBB->end(); 215 if (I == MBB->begin()) 216 return 0; // Empty MBB. 217 218 --I; 219 unsigned Hash = HashMachineInstr(I); 220 221 if (I == MBB->begin()) 222 return Hash; // Single instr MBB. 223 224 --I; 225 // Hash in the second-to-last instruction. 226 Hash ^= HashMachineInstr(I) << 2; 227 return Hash; 228} 229 230/// ComputeCommonTailLength - Given two machine basic blocks, compute the number 231/// of instructions they actually have in common together at their end. Return 232/// iterators for the first shared instruction in each block. 233static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1, 234 MachineBasicBlock *MBB2, 235 MachineBasicBlock::iterator &I1, 236 MachineBasicBlock::iterator &I2) { 237 I1 = MBB1->end(); 238 I2 = MBB2->end(); 239 240 unsigned TailLen = 0; 241 while (I1 != MBB1->begin() && I2 != MBB2->begin()) { 242 --I1; --I2; 243 if (!I1->isIdenticalTo(I2)) { 244 ++I1; ++I2; 245 break; 246 } 247 ++TailLen; 248 } 249 return TailLen; 250} 251 252/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything 253/// after it, replacing it with an unconditional branch to NewDest. This 254/// returns true if OldInst's block is modified, false if NewDest is modified. 255void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, 256 MachineBasicBlock *NewDest) { 257 MachineBasicBlock *OldBB = OldInst->getParent(); 258 259 // Remove all the old successors of OldBB from the CFG. 260 while (!OldBB->succ_empty()) 261 OldBB->removeSuccessor(OldBB->succ_begin()); 262 263 // Remove all the dead instructions from the end of OldBB. 264 OldBB->erase(OldInst, OldBB->end()); 265 266 // If OldBB isn't immediately before OldBB, insert a branch to it. 267 if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest)) 268 TII->InsertBranch(*OldBB, NewDest, 0, std::vector<MachineOperand>()); 269 OldBB->addSuccessor(NewDest); 270 ++NumTailMerge; 271} 272 273/// SplitMBBAt - Given a machine basic block and an iterator into it, split the 274/// MBB so that the part before the iterator falls into the part starting at the 275/// iterator. This returns the new MBB. 276MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB, 277 MachineBasicBlock::iterator BBI1) { 278 // Create the fall-through block. 279 MachineFunction::iterator MBBI = &CurMBB; 280 MachineBasicBlock *NewMBB = new MachineBasicBlock(CurMBB.getBasicBlock()); 281 CurMBB.getParent()->getBasicBlockList().insert(++MBBI, NewMBB); 282 283 // Move all the successors of this block to the specified block. 284 while (!CurMBB.succ_empty()) { 285 MachineBasicBlock *S = *(CurMBB.succ_end()-1); 286 NewMBB->addSuccessor(S); 287 CurMBB.removeSuccessor(S); 288 } 289 290 // Add an edge from CurMBB to NewMBB for the fall-through. 291 CurMBB.addSuccessor(NewMBB); 292 293 // Splice the code over. 294 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end()); 295 296 // For targets that use the register scavenger, we must maintain LiveIns. 297 if (RS) { 298 RS->enterBasicBlock(&CurMBB); 299 if (!CurMBB.empty()) 300 RS->forward(prior(CurMBB.end())); 301 BitVector RegsLiveAtExit(RegInfo->getNumRegs()); 302 RS->getRegsUsed(RegsLiveAtExit, false); 303 for (unsigned int i=0, e=RegInfo->getNumRegs(); i!=e; i++) 304 if (RegsLiveAtExit[i]) 305 NewMBB->addLiveIn(i); 306 } 307 308 return NewMBB; 309} 310 311/// EstimateRuntime - Make a rough estimate for how long it will take to run 312/// the specified code. 313static unsigned EstimateRuntime(MachineBasicBlock::iterator I, 314 MachineBasicBlock::iterator E, 315 const TargetInstrInfo *TII) { 316 unsigned Time = 0; 317 for (; I != E; ++I) { 318 const TargetInstrDescriptor &TID = TII->get(I->getOpcode()); 319 if (TID.Flags & M_CALL_FLAG) 320 Time += 10; 321 else if (TID.Flags & (M_LOAD_FLAG|M_STORE_FLAG)) 322 Time += 2; 323 else 324 ++Time; 325 } 326 return Time; 327} 328 329/// ShouldSplitFirstBlock - We need to either split MBB1 at MBB1I or MBB2 at 330/// MBB2I and then insert an unconditional branch in the other block. Determine 331/// which is the best to split 332static bool ShouldSplitFirstBlock(MachineBasicBlock *MBB1, 333 MachineBasicBlock::iterator MBB1I, 334 MachineBasicBlock *MBB2, 335 MachineBasicBlock::iterator MBB2I, 336 const TargetInstrInfo *TII) { 337 // TODO: if we had some notion of which block was hotter, we could split 338 // the hot block, so it is the fall-through. Since we don't have profile info 339 // make a decision based on which will hurt most to split. 340 unsigned MBB1Time = EstimateRuntime(MBB1->begin(), MBB1I, TII); 341 unsigned MBB2Time = EstimateRuntime(MBB2->begin(), MBB2I, TII); 342 343 // If the MBB1 prefix takes "less time" to run than the MBB2 prefix, split the 344 // MBB1 block so it falls through. This will penalize the MBB2 path, but will 345 // have a lower overall impact on the program execution. 346 return MBB1Time < MBB2Time; 347} 348 349bool BranchFolder::TailMergeBlocks(MachineFunction &MF) { 350 MadeChange = false; 351 352 if (!EnableTailMerge) return false; 353 354 // Find blocks with no successors. 355 std::vector<std::pair<unsigned,MachineBasicBlock*> > MergePotentials; 356 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { 357 if (I->succ_empty()) 358 MergePotentials.push_back(std::make_pair(HashEndOfMBB(I), I)); 359 } 360 361 // Sort by hash value so that blocks with identical end sequences sort 362 // together. 363 std::stable_sort(MergePotentials.begin(), MergePotentials.end()); 364 365 // Walk through equivalence sets looking for actual exact matches. 366 while (MergePotentials.size() > 1) { 367 unsigned CurHash = (MergePotentials.end()-1)->first; 368 unsigned PrevHash = (MergePotentials.end()-2)->first; 369 MachineBasicBlock *CurMBB = (MergePotentials.end()-1)->second; 370 371 // If there is nothing that matches the hash of the current basic block, 372 // give up. 373 if (CurHash != PrevHash) { 374 MergePotentials.pop_back(); 375 continue; 376 } 377 378 // Determine the actual length of the shared tail between these two basic 379 // blocks. Because the hash can have collisions, it's possible that this is 380 // less than 2. 381 MachineBasicBlock::iterator BBI1, BBI2; 382 unsigned CommonTailLen = 383 ComputeCommonTailLength(CurMBB, (MergePotentials.end()-2)->second, 384 BBI1, BBI2); 385 386 // If the tails don't have at least two instructions in common, see if there 387 // is anything else in the equivalence class that does match. 388 if (CommonTailLen < 2) { 389 unsigned FoundMatch = ~0U; 390 for (int i = MergePotentials.size()-2; 391 i != -1 && MergePotentials[i].first == CurHash; --i) { 392 CommonTailLen = ComputeCommonTailLength(CurMBB, 393 MergePotentials[i].second, 394 BBI1, BBI2); 395 if (CommonTailLen >= 2) { 396 FoundMatch = i; 397 break; 398 } 399 } 400 401 // If we didn't find anything that has at least two instructions matching 402 // this one, bail out. 403 if (FoundMatch == ~0U) { 404 MergePotentials.pop_back(); 405 continue; 406 } 407 408 // Otherwise, move the matching block to the right position. 409 std::swap(MergePotentials[FoundMatch], *(MergePotentials.end()-2)); 410 } 411 412 MachineBasicBlock *MBB2 = (MergePotentials.end()-2)->second; 413 414 // If neither block is the entire common tail, split the tail of one block 415 // to make it redundant with the other tail. 416 if (CurMBB->begin() != BBI1 && MBB2->begin() != BBI2) { 417 if (0) { // Enable this to disable partial tail merges. 418 MergePotentials.pop_back(); 419 continue; 420 } 421 422 // Decide whether we want to split CurMBB or MBB2. 423 if (ShouldSplitFirstBlock(CurMBB, BBI1, MBB2, BBI2, TII)) { 424 CurMBB = SplitMBBAt(*CurMBB, BBI1); 425 BBI1 = CurMBB->begin(); 426 MergePotentials.back().second = CurMBB; 427 } else { 428 MBB2 = SplitMBBAt(*MBB2, BBI2); 429 BBI2 = MBB2->begin(); 430 (MergePotentials.end()-2)->second = MBB2; 431 } 432 } 433 434 if (MBB2->begin() == BBI2) { 435 // Hack the end off CurMBB, making it jump to MBBI@ instead. 436 ReplaceTailWithBranchTo(BBI1, MBB2); 437 // This modifies CurMBB, so remove it from the worklist. 438 MergePotentials.pop_back(); 439 } else { 440 assert(CurMBB->begin() == BBI1 && "Didn't split block correctly?"); 441 // Hack the end off MBB2, making it jump to CurMBB instead. 442 ReplaceTailWithBranchTo(BBI2, CurMBB); 443 // This modifies MBB2, so remove it from the worklist. 444 MergePotentials.erase(MergePotentials.end()-2); 445 } 446 MadeChange = true; 447 } 448 449 return MadeChange; 450} 451 452 453//===----------------------------------------------------------------------===// 454// Branch Optimization 455//===----------------------------------------------------------------------===// 456 457bool BranchFolder::OptimizeBranches(MachineFunction &MF) { 458 MadeChange = false; 459 460 // Make sure blocks are numbered in order 461 MF.RenumberBlocks(); 462 463 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) { 464 MachineBasicBlock *MBB = I++; 465 OptimizeBlock(MBB); 466 467 // If it is dead, remove it. 468 if (MBB->pred_empty()) { 469 RemoveDeadBlock(MBB); 470 MadeChange = true; 471 ++NumDeadBlocks; 472 } 473 } 474 return MadeChange; 475} 476 477 478/// CorrectExtraCFGEdges - Various pieces of code can cause excess edges in the 479/// CFG to be inserted. If we have proven that MBB can only branch to DestA and 480/// DestB, remove any other MBB successors from the CFG. DestA and DestB can 481/// be null. 482static bool CorrectExtraCFGEdges(MachineBasicBlock &MBB, 483 MachineBasicBlock *DestA, 484 MachineBasicBlock *DestB, 485 bool isCond, 486 MachineFunction::iterator FallThru) { 487 bool MadeChange = false; 488 bool AddedFallThrough = false; 489 490 // If this block ends with a conditional branch that falls through to its 491 // successor, set DestB as the successor. 492 if (isCond) { 493 if (DestB == 0 && FallThru != MBB.getParent()->end()) { 494 DestB = FallThru; 495 AddedFallThrough = true; 496 } 497 } else { 498 // If this is an unconditional branch with no explicit dest, it must just be 499 // a fallthrough into DestB. 500 if (DestA == 0 && FallThru != MBB.getParent()->end()) { 501 DestA = FallThru; 502 AddedFallThrough = true; 503 } 504 } 505 506 MachineBasicBlock::pred_iterator SI = MBB.succ_begin(); 507 while (SI != MBB.succ_end()) { 508 if (*SI == DestA) { 509 DestA = 0; 510 ++SI; 511 } else if (*SI == DestB) { 512 DestB = 0; 513 ++SI; 514 } else if ((*SI)->isLandingPad()) { 515 ++SI; 516 } else { 517 // Otherwise, this is a superfluous edge, remove it. 518 MBB.removeSuccessor(SI); 519 MadeChange = true; 520 } 521 } 522 if (!AddedFallThrough) { 523 assert(DestA == 0 && DestB == 0 && 524 "MachineCFG is missing edges!"); 525 } else if (isCond) { 526 assert(DestA == 0 && "MachineCFG is missing edges!"); 527 } 528 return MadeChange; 529} 530 531 532/// ReplaceUsesOfBlockWith - Given a machine basic block 'BB' that branched to 533/// 'Old', change the code and CFG so that it branches to 'New' instead. 534static void ReplaceUsesOfBlockWith(MachineBasicBlock *BB, 535 MachineBasicBlock *Old, 536 MachineBasicBlock *New, 537 const TargetInstrInfo *TII) { 538 assert(Old != New && "Cannot replace self with self!"); 539 540 MachineBasicBlock::iterator I = BB->end(); 541 while (I != BB->begin()) { 542 --I; 543 if (!TII->isTerminatorInstr(I->getOpcode())) break; 544 545 // Scan the operands of this machine instruction, replacing any uses of Old 546 // with New. 547 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) 548 if (I->getOperand(i).isMachineBasicBlock() && 549 I->getOperand(i).getMachineBasicBlock() == Old) 550 I->getOperand(i).setMachineBasicBlock(New); 551 } 552 553 // Update the successor information. 554 std::vector<MachineBasicBlock*> Succs(BB->succ_begin(), BB->succ_end()); 555 for (int i = Succs.size()-1; i >= 0; --i) 556 if (Succs[i] == Old) { 557 BB->removeSuccessor(Old); 558 BB->addSuccessor(New); 559 } 560} 561 562/// CanFallThrough - Return true if the specified block (with the specified 563/// branch condition) can implicitly transfer control to the block after it by 564/// falling off the end of it. This should return false if it can reach the 565/// block after it, but it uses an explicit branch to do so (e.g. a table jump). 566/// 567/// True is a conservative answer. 568/// 569bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB, 570 bool BranchUnAnalyzable, 571 MachineBasicBlock *TBB, MachineBasicBlock *FBB, 572 const std::vector<MachineOperand> &Cond) { 573 MachineFunction::iterator Fallthrough = CurBB; 574 ++Fallthrough; 575 // If FallthroughBlock is off the end of the function, it can't fall through. 576 if (Fallthrough == CurBB->getParent()->end()) 577 return false; 578 579 // If FallthroughBlock isn't a successor of CurBB, no fallthrough is possible. 580 if (!CurBB->isSuccessor(Fallthrough)) 581 return false; 582 583 // If we couldn't analyze the branch, assume it could fall through. 584 if (BranchUnAnalyzable) return true; 585 586 // If there is no branch, control always falls through. 587 if (TBB == 0) return true; 588 589 // If there is some explicit branch to the fallthrough block, it can obviously 590 // reach, even though the branch should get folded to fall through implicitly. 591 if (MachineFunction::iterator(TBB) == Fallthrough || 592 MachineFunction::iterator(FBB) == Fallthrough) 593 return true; 594 595 // If it's an unconditional branch to some block not the fall through, it 596 // doesn't fall through. 597 if (Cond.empty()) return false; 598 599 // Otherwise, if it is conditional and has no explicit false block, it falls 600 // through. 601 return FBB == 0; 602} 603 604/// CanFallThrough - Return true if the specified can implicitly transfer 605/// control to the block after it by falling off the end of it. This should 606/// return false if it can reach the block after it, but it uses an explicit 607/// branch to do so (e.g. a table jump). 608/// 609/// True is a conservative answer. 610/// 611bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) { 612 MachineBasicBlock *TBB = 0, *FBB = 0; 613 std::vector<MachineOperand> Cond; 614 bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond); 615 return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond); 616} 617 618/// IsBetterFallthrough - Return true if it would be clearly better to 619/// fall-through to MBB1 than to fall through into MBB2. This has to return 620/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will 621/// result in infinite loops. 622static bool IsBetterFallthrough(MachineBasicBlock *MBB1, 623 MachineBasicBlock *MBB2, 624 const TargetInstrInfo &TII) { 625 // Right now, we use a simple heuristic. If MBB2 ends with a call, and 626 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to 627 // optimize branches that branch to either a return block or an assert block 628 // into a fallthrough to the return. 629 if (MBB1->empty() || MBB2->empty()) return false; 630 631 MachineInstr *MBB1I = --MBB1->end(); 632 MachineInstr *MBB2I = --MBB2->end(); 633 return TII.isCall(MBB2I->getOpcode()) && !TII.isCall(MBB1I->getOpcode()); 634} 635 636/// OptimizeBlock - Analyze and optimize control flow related to the specified 637/// block. This is never called on the entry block. 638void BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) { 639 MachineFunction::iterator FallThrough = MBB; 640 ++FallThrough; 641 642 // If this block is empty, make everyone use its fall-through, not the block 643 // explicitly. 644 if (MBB->empty()) { 645 // Dead block? Leave for cleanup later. 646 if (MBB->pred_empty()) return; 647 648 if (FallThrough == MBB->getParent()->end()) { 649 // TODO: Simplify preds to not branch here if possible! 650 } else { 651 // Rewrite all predecessors of the old block to go to the fallthrough 652 // instead. 653 while (!MBB->pred_empty()) { 654 MachineBasicBlock *Pred = *(MBB->pred_end()-1); 655 ReplaceUsesOfBlockWith(Pred, MBB, FallThrough, TII); 656 } 657 658 // If MBB was the target of a jump table, update jump tables to go to the 659 // fallthrough instead. 660 MBB->getParent()->getJumpTableInfo()-> 661 ReplaceMBBInJumpTables(MBB, FallThrough); 662 MadeChange = true; 663 } 664 return; 665 } 666 667 // Check to see if we can simplify the terminator of the block before this 668 // one. 669 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB)); 670 671 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0; 672 std::vector<MachineOperand> PriorCond; 673 bool PriorUnAnalyzable = 674 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); 675 if (!PriorUnAnalyzable) { 676 // If the CFG for the prior block has extra edges, remove them. 677 MadeChange |= CorrectExtraCFGEdges(PrevBB, PriorTBB, PriorFBB, 678 !PriorCond.empty(), MBB); 679 680 // If the previous branch is conditional and both conditions go to the same 681 // destination, remove the branch, replacing it with an unconditional one or 682 // a fall-through. 683 if (PriorTBB && PriorTBB == PriorFBB) { 684 TII->RemoveBranch(PrevBB); 685 PriorCond.clear(); 686 if (PriorTBB != MBB) 687 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 688 MadeChange = true; 689 ++NumBranchOpts; 690 return OptimizeBlock(MBB); 691 } 692 693 // If the previous branch *only* branches to *this* block (conditional or 694 // not) remove the branch. 695 if (PriorTBB == MBB && PriorFBB == 0) { 696 TII->RemoveBranch(PrevBB); 697 MadeChange = true; 698 ++NumBranchOpts; 699 return OptimizeBlock(MBB); 700 } 701 702 // If the prior block branches somewhere else on the condition and here if 703 // the condition is false, remove the uncond second branch. 704 if (PriorFBB == MBB) { 705 TII->RemoveBranch(PrevBB); 706 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 707 MadeChange = true; 708 ++NumBranchOpts; 709 return OptimizeBlock(MBB); 710 } 711 712 // If the prior block branches here on true and somewhere else on false, and 713 // if the branch condition is reversible, reverse the branch to create a 714 // fall-through. 715 if (PriorTBB == MBB) { 716 std::vector<MachineOperand> NewPriorCond(PriorCond); 717 if (!TII->ReverseBranchCondition(NewPriorCond)) { 718 TII->RemoveBranch(PrevBB); 719 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond); 720 MadeChange = true; 721 ++NumBranchOpts; 722 return OptimizeBlock(MBB); 723 } 724 } 725 726 // If this block doesn't fall through (e.g. it ends with an uncond branch or 727 // has no successors) and if the pred falls through into this block, and if 728 // it would otherwise fall through into the block after this, move this 729 // block to the end of the function. 730 // 731 // We consider it more likely that execution will stay in the function (e.g. 732 // due to loops) than it is to exit it. This asserts in loops etc, moving 733 // the assert condition out of the loop body. 734 if (!PriorCond.empty() && PriorFBB == 0 && 735 MachineFunction::iterator(PriorTBB) == FallThrough && 736 !CanFallThrough(MBB)) { 737 bool DoTransform = true; 738 739 // We have to be careful that the succs of PredBB aren't both no-successor 740 // blocks. If neither have successors and if PredBB is the second from 741 // last block in the function, we'd just keep swapping the two blocks for 742 // last. Only do the swap if one is clearly better to fall through than 743 // the other. 744 if (FallThrough == --MBB->getParent()->end() && 745 !IsBetterFallthrough(PriorTBB, MBB, *TII)) 746 DoTransform = false; 747 748 // We don't want to do this transformation if we have control flow like: 749 // br cond BB2 750 // BB1: 751 // .. 752 // jmp BBX 753 // BB2: 754 // .. 755 // ret 756 // 757 // In this case, we could actually be moving the return block *into* a 758 // loop! 759 if (DoTransform && !MBB->succ_empty() && 760 (!CanFallThrough(PriorTBB) || PriorTBB->empty())) 761 DoTransform = false; 762 763 764 if (DoTransform) { 765 // Reverse the branch so we will fall through on the previous true cond. 766 std::vector<MachineOperand> NewPriorCond(PriorCond); 767 if (!TII->ReverseBranchCondition(NewPriorCond)) { 768 DOUT << "\nMoving MBB: " << *MBB; 769 DOUT << "To make fallthrough to: " << *PriorTBB << "\n"; 770 771 TII->RemoveBranch(PrevBB); 772 TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond); 773 774 // Move this block to the end of the function. 775 MBB->moveAfter(--MBB->getParent()->end()); 776 MadeChange = true; 777 ++NumBranchOpts; 778 return; 779 } 780 } 781 } 782 } 783 784 // Analyze the branch in the current block. 785 MachineBasicBlock *CurTBB = 0, *CurFBB = 0; 786 std::vector<MachineOperand> CurCond; 787 bool CurUnAnalyzable = TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond); 788 if (!CurUnAnalyzable) { 789 // If the CFG for the prior block has extra edges, remove them. 790 MadeChange |= CorrectExtraCFGEdges(*MBB, CurTBB, CurFBB, 791 !CurCond.empty(), 792 ++MachineFunction::iterator(MBB)); 793 794 // If this is a two-way branch, and the FBB branches to this block, reverse 795 // the condition so the single-basic-block loop is faster. Instead of: 796 // Loop: xxx; jcc Out; jmp Loop 797 // we want: 798 // Loop: xxx; jncc Loop; jmp Out 799 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) { 800 std::vector<MachineOperand> NewCond(CurCond); 801 if (!TII->ReverseBranchCondition(NewCond)) { 802 TII->RemoveBranch(*MBB); 803 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond); 804 MadeChange = true; 805 ++NumBranchOpts; 806 return OptimizeBlock(MBB); 807 } 808 } 809 810 811 // If this branch is the only thing in its block, see if we can forward 812 // other blocks across it. 813 if (CurTBB && CurCond.empty() && CurFBB == 0 && 814 TII->isBranch(MBB->begin()->getOpcode()) && CurTBB != MBB) { 815 // This block may contain just an unconditional branch. Because there can 816 // be 'non-branch terminators' in the block, try removing the branch and 817 // then seeing if the block is empty. 818 TII->RemoveBranch(*MBB); 819 820 // If this block is just an unconditional branch to CurTBB, we can 821 // usually completely eliminate the block. The only case we cannot 822 // completely eliminate the block is when the block before this one 823 // falls through into MBB and we can't understand the prior block's branch 824 // condition. 825 if (MBB->empty()) { 826 bool PredHasNoFallThrough = TII->BlockHasNoFallThrough(PrevBB); 827 if (PredHasNoFallThrough || !PriorUnAnalyzable || 828 !PrevBB.isSuccessor(MBB)) { 829 // If the prior block falls through into us, turn it into an 830 // explicit branch to us to make updates simpler. 831 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) && 832 PriorTBB != MBB && PriorFBB != MBB) { 833 if (PriorTBB == 0) { 834 assert(PriorCond.empty() && PriorFBB == 0 && 835 "Bad branch analysis"); 836 PriorTBB = MBB; 837 } else { 838 assert(PriorFBB == 0 && "Machine CFG out of date!"); 839 PriorFBB = MBB; 840 } 841 TII->RemoveBranch(PrevBB); 842 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); 843 } 844 845 // Iterate through all the predecessors, revectoring each in-turn. 846 MachineBasicBlock::pred_iterator PI = MBB->pred_begin(); 847 bool DidChange = false; 848 bool HasBranchToSelf = false; 849 while (PI != MBB->pred_end()) { 850 if (*PI == MBB) { 851 // If this block has an uncond branch to itself, leave it. 852 ++PI; 853 HasBranchToSelf = true; 854 } else { 855 DidChange = true; 856 ReplaceUsesOfBlockWith(*PI, MBB, CurTBB, TII); 857 } 858 } 859 860 // Change any jumptables to go to the new MBB. 861 MBB->getParent()->getJumpTableInfo()-> 862 ReplaceMBBInJumpTables(MBB, CurTBB); 863 if (DidChange) { 864 ++NumBranchOpts; 865 MadeChange = true; 866 if (!HasBranchToSelf) return; 867 } 868 } 869 } 870 871 // Add the branch back if the block is more than just an uncond branch. 872 TII->InsertBranch(*MBB, CurTBB, 0, CurCond); 873 } 874 } 875 876 // If the prior block doesn't fall through into this block, and if this 877 // block doesn't fall through into some other block, see if we can find a 878 // place to move this block where a fall-through will happen. 879 if (!CanFallThrough(&PrevBB, PriorUnAnalyzable, 880 PriorTBB, PriorFBB, PriorCond)) { 881 // Now we know that there was no fall-through into this block, check to 882 // see if it has a fall-through into its successor. 883 bool CurFallsThru = CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB, 884 CurCond); 885 886 if (!MBB->isLandingPad()) { 887 // Check all the predecessors of this block. If one of them has no fall 888 // throughs, move this block right after it. 889 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(), 890 E = MBB->pred_end(); PI != E; ++PI) { 891 // Analyze the branch at the end of the pred. 892 MachineBasicBlock *PredBB = *PI; 893 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough; 894 if (PredBB != MBB && !CanFallThrough(PredBB) 895 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) { 896 // If the current block doesn't fall through, just move it. 897 // If the current block can fall through and does not end with a 898 // conditional branch, we need to append an unconditional jump to 899 // the (current) next block. To avoid a possible compile-time 900 // infinite loop, move blocks only backward in this case. 901 if (CurFallsThru) { 902 MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB)); 903 CurCond.clear(); 904 TII->InsertBranch(*MBB, NextBB, 0, CurCond); 905 } 906 MBB->moveAfter(PredBB); 907 MadeChange = true; 908 return OptimizeBlock(MBB); 909 } 910 } 911 } 912 913 if (!CurFallsThru) { 914 // Check all successors to see if we can move this block before it. 915 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(), 916 E = MBB->succ_end(); SI != E; ++SI) { 917 // Analyze the branch at the end of the block before the succ. 918 MachineBasicBlock *SuccBB = *SI; 919 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev; 920 std::vector<MachineOperand> SuccPrevCond; 921 922 // If this block doesn't already fall-through to that successor, and if 923 // the succ doesn't already have a block that can fall through into it, 924 // and if the successor isn't an EH destination, we can arrange for the 925 // fallthrough to happen. 926 if (SuccBB != MBB && !CanFallThrough(SuccPrev) && 927 !SuccBB->isLandingPad()) { 928 MBB->moveBefore(SuccBB); 929 MadeChange = true; 930 return OptimizeBlock(MBB); 931 } 932 } 933 934 // Okay, there is no really great place to put this block. If, however, 935 // the block before this one would be a fall-through if this block were 936 // removed, move this block to the end of the function. 937 if (FallThrough != MBB->getParent()->end() && 938 PrevBB.isSuccessor(FallThrough)) { 939 MBB->moveAfter(--MBB->getParent()->end()); 940 MadeChange = true; 941 return; 942 } 943 } 944 } 945} 946