BranchFolding.cpp revision 8a46d342d8cbca7c9c7be6c66007d41329babad0
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<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge", 39 cl::init(cl::BOU_UNSET), cl::Hidden); 40namespace { 41 // Throttle for huge numbers of predecessors (compile speed problems) 42 cl::opt<unsigned> 43 TailMergeThreshold("tail-merge-threshold", 44 cl::desc("Max number of predecessors to consider tail merging"), 45 cl::init(100), cl::Hidden); 46 47 struct BranchFolder : public MachineFunctionPass { 48 static char ID; 49 BranchFolder(bool defaultEnableTailMerge) : 50 MachineFunctionPass((intptr_t)&ID) { 51 switch (FlagEnableTailMerge) { 52 case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break; 53 case cl::BOU_TRUE: EnableTailMerge = true; break; 54 case cl::BOU_FALSE: EnableTailMerge = false; break; 55 } 56 } 57 58 virtual bool runOnMachineFunction(MachineFunction &MF); 59 virtual const char *getPassName() const { return "Control Flow Optimizer"; } 60 const TargetInstrInfo *TII; 61 MachineModuleInfo *MMI; 62 bool MadeChange; 63 private: 64 // Tail Merging. 65 bool EnableTailMerge; 66 bool TailMergeBlocks(MachineFunction &MF); 67 bool TryMergeBlocks(MachineBasicBlock* SuccBB, 68 MachineBasicBlock* PredBB); 69 void ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, 70 MachineBasicBlock *NewDest); 71 MachineBasicBlock *SplitMBBAt(MachineBasicBlock &CurMBB, 72 MachineBasicBlock::iterator BBI1); 73 74 std::vector<std::pair<unsigned,MachineBasicBlock*> > MergePotentials; 75 const MRegisterInfo *RegInfo; 76 RegScavenger *RS; 77 // Branch optzn. 78 bool OptimizeBranches(MachineFunction &MF); 79 void OptimizeBlock(MachineBasicBlock *MBB); 80 void RemoveDeadBlock(MachineBasicBlock *MBB); 81 82 bool CanFallThrough(MachineBasicBlock *CurBB); 83 bool CanFallThrough(MachineBasicBlock *CurBB, bool BranchUnAnalyzable, 84 MachineBasicBlock *TBB, MachineBasicBlock *FBB, 85 const std::vector<MachineOperand> &Cond); 86 }; 87 char BranchFolder::ID = 0; 88} 89 90FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) { 91 return new BranchFolder(DefaultEnableTailMerge); } 92 93/// RemoveDeadBlock - Remove the specified dead machine basic block from the 94/// function, updating the CFG. 95void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) { 96 assert(MBB->pred_empty() && "MBB must be dead!"); 97 DOUT << "\nRemoving MBB: " << *MBB; 98 99 MachineFunction *MF = MBB->getParent(); 100 // drop all successors. 101 while (!MBB->succ_empty()) 102 MBB->removeSuccessor(MBB->succ_end()-1); 103 104 // If there is DWARF info to active, check to see if there are any LABEL 105 // records in the basic block. If so, unregister them from MachineModuleInfo. 106 if (MMI && !MBB->empty()) { 107 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); 108 I != E; ++I) { 109 if ((unsigned)I->getOpcode() == TargetInstrInfo::LABEL) { 110 // The label ID # is always operand #0, an immediate. 111 MMI->InvalidateLabel(I->getOperand(0).getImm()); 112 } 113 } 114 } 115 116 // Remove the block. 117 MF->getBasicBlockList().erase(MBB); 118} 119 120bool BranchFolder::runOnMachineFunction(MachineFunction &MF) { 121 TII = MF.getTarget().getInstrInfo(); 122 if (!TII) return false; 123 124 // Fix CFG. The later algorithms expect it to be right. 125 bool EverMadeChange = false; 126 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) { 127 MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0; 128 std::vector<MachineOperand> Cond; 129 if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond)) 130 EverMadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty()); 131 } 132 133 RegInfo = MF.getTarget().getRegisterInfo(); 134 RS = RegInfo->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL; 135 136 MMI = getAnalysisToUpdate<MachineModuleInfo>(); 137 138 bool MadeChangeThisIteration = true; 139 while (MadeChangeThisIteration) { 140 MadeChangeThisIteration = false; 141 MadeChangeThisIteration |= TailMergeBlocks(MF); 142 MadeChangeThisIteration |= OptimizeBranches(MF); 143 EverMadeChange |= MadeChangeThisIteration; 144 } 145 146 // See if any jump tables have become mergable or dead as the code generator 147 // did its thing. 148 MachineJumpTableInfo *JTI = MF.getJumpTableInfo(); 149 const std::vector<MachineJumpTableEntry> &JTs = JTI->getJumpTables(); 150 if (!JTs.empty()) { 151 // Figure out how these jump tables should be merged. 152 std::vector<unsigned> JTMapping; 153 JTMapping.reserve(JTs.size()); 154 155 // We always keep the 0th jump table. 156 JTMapping.push_back(0); 157 158 // Scan the jump tables, seeing if there are any duplicates. Note that this 159 // is N^2, which should be fixed someday. 160 for (unsigned i = 1, e = JTs.size(); i != e; ++i) 161 JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs)); 162 163 // If a jump table was merge with another one, walk the function rewriting 164 // references to jump tables to reference the new JT ID's. Keep track of 165 // whether we see a jump table idx, if not, we can delete the JT. 166 std::vector<bool> JTIsLive; 167 JTIsLive.resize(JTs.size()); 168 for (MachineFunction::iterator BB = MF.begin(), E = MF.end(); 169 BB != E; ++BB) { 170 for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end(); 171 I != E; ++I) 172 for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) { 173 MachineOperand &Op = I->getOperand(op); 174 if (!Op.isJumpTableIndex()) continue; 175 unsigned NewIdx = JTMapping[Op.getJumpTableIndex()]; 176 Op.setJumpTableIndex(NewIdx); 177 178 // Remember that this JT is live. 179 JTIsLive[NewIdx] = true; 180 } 181 } 182 183 // Finally, remove dead jump tables. This happens either because the 184 // indirect jump was unreachable (and thus deleted) or because the jump 185 // table was merged with some other one. 186 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i) 187 if (!JTIsLive[i]) { 188 JTI->RemoveJumpTable(i); 189 EverMadeChange = true; 190 } 191 } 192 193 delete RS; 194 return EverMadeChange; 195} 196 197//===----------------------------------------------------------------------===// 198// Tail Merging of Blocks 199//===----------------------------------------------------------------------===// 200 201/// HashMachineInstr - Compute a hash value for MI and its operands. 202static unsigned HashMachineInstr(const MachineInstr *MI) { 203 unsigned Hash = MI->getOpcode(); 204 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 205 const MachineOperand &Op = MI->getOperand(i); 206 207 // Merge in bits from the operand if easy. 208 unsigned OperandHash = 0; 209 switch (Op.getType()) { 210 case MachineOperand::MO_Register: OperandHash = Op.getReg(); break; 211 case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break; 212 case MachineOperand::MO_MachineBasicBlock: 213 OperandHash = Op.getMachineBasicBlock()->getNumber(); 214 break; 215 case MachineOperand::MO_FrameIndex: OperandHash = Op.getFrameIndex(); break; 216 case MachineOperand::MO_ConstantPoolIndex: 217 OperandHash = Op.getConstantPoolIndex(); 218 break; 219 case MachineOperand::MO_JumpTableIndex: 220 OperandHash = Op.getJumpTableIndex(); 221 break; 222 case MachineOperand::MO_GlobalAddress: 223 case MachineOperand::MO_ExternalSymbol: 224 // Global address / external symbol are too hard, don't bother, but do 225 // pull in the offset. 226 OperandHash = Op.getOffset(); 227 break; 228 default: break; 229 } 230 231 Hash += ((OperandHash << 3) | Op.getType()) << (i&31); 232 } 233 return Hash; 234} 235 236/// HashEndOfMBB - Hash the last few instructions in the MBB. For blocks 237/// with no successors, we hash two instructions, because cross-jumping 238/// only saves code when at least two instructions are removed (since a 239/// branch must be inserted). For blocks with a successor, one of the 240/// two blocks to be tail-merged will end with a branch already, so 241/// it gains to cross-jump even for one instruction. 242 243static unsigned HashEndOfMBB(const MachineBasicBlock *MBB, 244 unsigned minCommonTailLength) { 245 MachineBasicBlock::const_iterator I = MBB->end(); 246 if (I == MBB->begin()) 247 return 0; // Empty MBB. 248 249 --I; 250 unsigned Hash = HashMachineInstr(I); 251 252 if (I == MBB->begin() || minCommonTailLength == 1) 253 return Hash; // Single instr MBB. 254 255 --I; 256 // Hash in the second-to-last instruction. 257 Hash ^= HashMachineInstr(I) << 2; 258 return Hash; 259} 260 261/// ComputeCommonTailLength - Given two machine basic blocks, compute the number 262/// of instructions they actually have in common together at their end. Return 263/// iterators for the first shared instruction in each block. 264static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1, 265 MachineBasicBlock *MBB2, 266 MachineBasicBlock::iterator &I1, 267 MachineBasicBlock::iterator &I2) { 268 I1 = MBB1->end(); 269 I2 = MBB2->end(); 270 271 unsigned TailLen = 0; 272 while (I1 != MBB1->begin() && I2 != MBB2->begin()) { 273 --I1; --I2; 274 if (!I1->isIdenticalTo(I2)) { 275 ++I1; ++I2; 276 break; 277 } 278 ++TailLen; 279 } 280 return TailLen; 281} 282 283/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything 284/// after it, replacing it with an unconditional branch to NewDest. This 285/// returns true if OldInst's block is modified, false if NewDest is modified. 286void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, 287 MachineBasicBlock *NewDest) { 288 MachineBasicBlock *OldBB = OldInst->getParent(); 289 290 // Remove all the old successors of OldBB from the CFG. 291 while (!OldBB->succ_empty()) 292 OldBB->removeSuccessor(OldBB->succ_begin()); 293 294 // Remove all the dead instructions from the end of OldBB. 295 OldBB->erase(OldInst, OldBB->end()); 296 297 // If OldBB isn't immediately before OldBB, insert a branch to it. 298 if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest)) 299 TII->InsertBranch(*OldBB, NewDest, 0, std::vector<MachineOperand>()); 300 OldBB->addSuccessor(NewDest); 301 ++NumTailMerge; 302} 303 304/// SplitMBBAt - Given a machine basic block and an iterator into it, split the 305/// MBB so that the part before the iterator falls into the part starting at the 306/// iterator. This returns the new MBB. 307MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB, 308 MachineBasicBlock::iterator BBI1) { 309 // Create the fall-through block. 310 MachineFunction::iterator MBBI = &CurMBB; 311 MachineBasicBlock *NewMBB = new MachineBasicBlock(CurMBB.getBasicBlock()); 312 CurMBB.getParent()->getBasicBlockList().insert(++MBBI, NewMBB); 313 314 // Move all the successors of this block to the specified block. 315 while (!CurMBB.succ_empty()) { 316 MachineBasicBlock *S = *(CurMBB.succ_end()-1); 317 NewMBB->addSuccessor(S); 318 CurMBB.removeSuccessor(S); 319 } 320 321 // Add an edge from CurMBB to NewMBB for the fall-through. 322 CurMBB.addSuccessor(NewMBB); 323 324 // Splice the code over. 325 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end()); 326 327 // For targets that use the register scavenger, we must maintain LiveIns. 328 if (RS) { 329 RS->enterBasicBlock(&CurMBB); 330 if (!CurMBB.empty()) 331 RS->forward(prior(CurMBB.end())); 332 BitVector RegsLiveAtExit(RegInfo->getNumRegs()); 333 RS->getRegsUsed(RegsLiveAtExit, false); 334 for (unsigned int i=0, e=RegInfo->getNumRegs(); i!=e; i++) 335 if (RegsLiveAtExit[i]) 336 NewMBB->addLiveIn(i); 337 } 338 339 return NewMBB; 340} 341 342/// EstimateRuntime - Make a rough estimate for how long it will take to run 343/// the specified code. 344static unsigned EstimateRuntime(MachineBasicBlock::iterator I, 345 MachineBasicBlock::iterator E, 346 const TargetInstrInfo *TII) { 347 unsigned Time = 0; 348 for (; I != E; ++I) { 349 const TargetInstrDescriptor &TID = TII->get(I->getOpcode()); 350 if (TID.Flags & M_CALL_FLAG) 351 Time += 10; 352 else if (TID.Flags & (M_LOAD_FLAG|M_STORE_FLAG)) 353 Time += 2; 354 else 355 ++Time; 356 } 357 return Time; 358} 359 360/// ShouldSplitFirstBlock - We need to either split MBB1 at MBB1I or MBB2 at 361/// MBB2I and then insert an unconditional branch in the other block. Determine 362/// which is the best to split 363static bool ShouldSplitFirstBlock(MachineBasicBlock *MBB1, 364 MachineBasicBlock::iterator MBB1I, 365 MachineBasicBlock *MBB2, 366 MachineBasicBlock::iterator MBB2I, 367 const TargetInstrInfo *TII, 368 MachineBasicBlock *PredBB) { 369 // If one block is the entry block, split the other one; we can't generate 370 // a branch to the entry block, as its label is not emitted. 371 MachineBasicBlock *Entry = MBB1->getParent()->begin(); 372 if (MBB1 == Entry) 373 return false; 374 if (MBB2 == Entry) 375 return true; 376 377 // If one block falls through into the common successor, choose that 378 // one to split; it is one instruction less to do that. 379 if (PredBB) { 380 if (MBB1 == PredBB) 381 return true; 382 else if (MBB2 == PredBB) 383 return false; 384 } 385 // TODO: if we had some notion of which block was hotter, we could split 386 // the hot block, so it is the fall-through. Since we don't have profile info 387 // make a decision based on which will hurt most to split. 388 unsigned MBB1Time = EstimateRuntime(MBB1->begin(), MBB1I, TII); 389 unsigned MBB2Time = EstimateRuntime(MBB2->begin(), MBB2I, TII); 390 391 // If the MBB1 prefix takes "less time" to run than the MBB2 prefix, split the 392 // MBB1 block so it falls through. This will penalize the MBB2 path, but will 393 // have a lower overall impact on the program execution. 394 return MBB1Time < MBB2Time; 395} 396 397// CurMBB needs to add an unconditional branch to SuccMBB (we removed these 398// branches temporarily for tail merging). In the case where CurMBB ends 399// with a conditional branch to the next block, optimize by reversing the 400// test and conditionally branching to SuccMBB instead. 401 402static void FixTail(MachineBasicBlock* CurMBB, MachineBasicBlock *SuccBB, 403 const TargetInstrInfo *TII) { 404 MachineFunction *MF = CurMBB->getParent(); 405 MachineFunction::iterator I = next(MachineFunction::iterator(CurMBB)); 406 MachineBasicBlock *TBB = 0, *FBB = 0; 407 std::vector<MachineOperand> Cond; 408 if (I != MF->end() && 409 !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond)) { 410 MachineBasicBlock *NextBB = I; 411 if (TBB == NextBB && Cond.size() && !FBB) { 412 if (!TII->ReverseBranchCondition(Cond)) { 413 TII->RemoveBranch(*CurMBB); 414 TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond); 415 return; 416 } 417 } 418 } 419 TII->InsertBranch(*CurMBB, SuccBB, NULL, std::vector<MachineOperand>()); 420} 421 422static bool MergeCompare(const std::pair<unsigned,MachineBasicBlock*> &p, 423 const std::pair<unsigned,MachineBasicBlock*> &q) { 424 if (p.first < q.first) 425 return true; 426 else if (p.first > q.first) 427 return false; 428 else if (p.second->getNumber() < q.second->getNumber()) 429 return true; 430 else if (p.second->getNumber() > q.second->getNumber()) 431 return false; 432 else 433 assert(0 && "Predecessor appears twice"); 434} 435 436// See if any of the blocks in MergePotentials (which all have a common single 437// successor, or all have no successor) can be tail-merged. If there is a 438// successor, any blocks in MergePotentials that are not tail-merged and 439// are not immediately before Succ must have an unconditional branch to 440// Succ added (but the predecessor/successor lists need no adjustment). 441// The lone predecessor of Succ that falls through into Succ, 442// if any, is given in PredBB. 443 444bool BranchFolder::TryMergeBlocks(MachineBasicBlock *SuccBB, 445 MachineBasicBlock* PredBB) { 446 unsigned minCommonTailLength = (SuccBB ? 1 : 2); 447 MadeChange = false; 448 449 // Sort by hash value so that blocks with identical end sequences sort 450 // together. 451 std::stable_sort(MergePotentials.begin(), MergePotentials.end(), MergeCompare); 452 453 // Walk through equivalence sets looking for actual exact matches. 454 while (MergePotentials.size() > 1) { 455 unsigned CurHash = (MergePotentials.end()-1)->first; 456 unsigned PrevHash = (MergePotentials.end()-2)->first; 457 MachineBasicBlock *CurMBB = (MergePotentials.end()-1)->second; 458 459 // If there is nothing that matches the hash of the current basic block, 460 // give up. 461 if (CurHash != PrevHash) { 462 if (SuccBB && CurMBB != PredBB) 463 FixTail(CurMBB, SuccBB, TII); 464 MergePotentials.pop_back(); 465 continue; 466 } 467 468 // Look through all the pairs of blocks that have the same hash as this 469 // one, and find the pair that has the largest number of instructions in 470 // common. 471 // Since instructions may get combined later (e.g. single stores into 472 // store multiple) this measure is not particularly accurate. 473 MachineBasicBlock::iterator BBI1, BBI2; 474 475 unsigned FoundI = ~0U, FoundJ = ~0U; 476 unsigned maxCommonTailLength = 0U; 477 for (int i = MergePotentials.size()-1; 478 i != -1 && MergePotentials[i].first == CurHash; --i) { 479 for (int j = i-1; 480 j != -1 && MergePotentials[j].first == CurHash; --j) { 481 MachineBasicBlock::iterator TrialBBI1, TrialBBI2; 482 unsigned CommonTailLen = ComputeCommonTailLength( 483 MergePotentials[i].second, 484 MergePotentials[j].second, 485 TrialBBI1, TrialBBI2); 486 if (CommonTailLen >= minCommonTailLength && 487 CommonTailLen > maxCommonTailLength) { 488 FoundI = i; 489 FoundJ = j; 490 maxCommonTailLength = CommonTailLen; 491 BBI1 = TrialBBI1; 492 BBI2 = TrialBBI2; 493 } 494 } 495 } 496 497 // If we didn't find any pair that has at least minCommonTailLength 498 // instructions in common, bail out. All entries with this 499 // hash code can go away now. 500 if (FoundI == ~0U) { 501 for (int i = MergePotentials.size()-1; 502 i != -1 && MergePotentials[i].first == CurHash; --i) { 503 // Put the unconditional branch back, if we need one. 504 CurMBB = MergePotentials[i].second; 505 if (SuccBB && CurMBB != PredBB) 506 FixTail(CurMBB, SuccBB, TII); 507 MergePotentials.pop_back(); 508 } 509 continue; 510 } 511 512 // Otherwise, move the block(s) to the right position(s). So that 513 // BBI1/2 will be valid, the last must be I and the next-to-last J. 514 if (FoundI != MergePotentials.size()-1) 515 std::swap(MergePotentials[FoundI], *(MergePotentials.end()-1)); 516 if (FoundJ != MergePotentials.size()-2) 517 std::swap(MergePotentials[FoundJ], *(MergePotentials.end()-2)); 518 519 CurMBB = (MergePotentials.end()-1)->second; 520 MachineBasicBlock *MBB2 = (MergePotentials.end()-2)->second; 521 522 // If neither block is the entire common tail, split the tail of one block 523 // to make it redundant with the other tail. Also, we cannot jump to the 524 // entry block, so if one block is the entry block, split the other one. 525 MachineBasicBlock *Entry = CurMBB->getParent()->begin(); 526 if (CurMBB->begin() == BBI1 && CurMBB != Entry) 527 ; // CurMBB is common tail 528 else if (MBB2->begin() == BBI2 && MBB2 != Entry) 529 ; // MBB2 is common tail 530 else { 531 if (0) { // Enable this to disable partial tail merges. 532 MergePotentials.pop_back(); 533 continue; 534 } 535 536 // Decide whether we want to split CurMBB or MBB2. 537 if (ShouldSplitFirstBlock(CurMBB, BBI1, MBB2, BBI2, TII, PredBB)) { 538 CurMBB = SplitMBBAt(*CurMBB, BBI1); 539 BBI1 = CurMBB->begin(); 540 MergePotentials.back().second = CurMBB; 541 } else { 542 MBB2 = SplitMBBAt(*MBB2, BBI2); 543 BBI2 = MBB2->begin(); 544 (MergePotentials.end()-2)->second = MBB2; 545 } 546 } 547 548 if (MBB2->begin() == BBI2 && MBB2 != Entry) { 549 // Hack the end off CurMBB, making it jump to MBBI@ instead. 550 ReplaceTailWithBranchTo(BBI1, MBB2); 551 // This modifies CurMBB, so remove it from the worklist. 552 MergePotentials.pop_back(); 553 } else { 554 assert(CurMBB->begin() == BBI1 && CurMBB != Entry && 555 "Didn't split block correctly?"); 556 // Hack the end off MBB2, making it jump to CurMBB instead. 557 ReplaceTailWithBranchTo(BBI2, CurMBB); 558 // This modifies MBB2, so remove it from the worklist. 559 MergePotentials.erase(MergePotentials.end()-2); 560 } 561 MadeChange = true; 562 } 563 return MadeChange; 564} 565 566bool BranchFolder::TailMergeBlocks(MachineFunction &MF) { 567 568 if (!EnableTailMerge) return false; 569 570 MadeChange = false; 571 572 // First find blocks with no successors. 573 MergePotentials.clear(); 574 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { 575 if (I->succ_empty()) 576 MergePotentials.push_back(std::make_pair(HashEndOfMBB(I, 2U), I)); 577 } 578 // See if we can do any tail merging on those. 579 if (MergePotentials.size() < TailMergeThreshold) 580 MadeChange |= TryMergeBlocks(NULL, NULL); 581 582 // Look at blocks (IBB) with multiple predecessors (PBB). 583 // We change each predecessor to a canonical form, by 584 // (1) temporarily removing any unconditional branch from the predecessor 585 // to IBB, and 586 // (2) alter conditional branches so they branch to the other block 587 // not IBB; this may require adding back an unconditional branch to IBB 588 // later, where there wasn't one coming in. E.g. 589 // Bcc IBB 590 // fallthrough to QBB 591 // here becomes 592 // Bncc QBB 593 // with a conceptual B to IBB after that, which never actually exists. 594 // With those changes, we see whether the predecessors' tails match, 595 // and merge them if so. We change things out of canonical form and 596 // back to the way they were later in the process. (OptimizeBranches 597 // would undo some of this, but we can't use it, because we'd get into 598 // a compile-time infinite loop repeatedly doing and undoing the same 599 // transformations.) 600 601 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { 602 if (!I->succ_empty() && I->pred_size() >= 2 && 603 I->pred_size() < TailMergeThreshold) { 604 MachineBasicBlock *IBB = I; 605 MachineBasicBlock *PredBB = prior(I); 606 MergePotentials.clear(); 607 for (MachineBasicBlock::pred_iterator P = I->pred_begin(), 608 E2 = I->pred_end(); 609 P != E2; ++P) { 610 MachineBasicBlock* PBB = *P; 611 // Skip blocks that loop to themselves, can't tail merge these. 612 if (PBB==IBB) 613 continue; 614 MachineBasicBlock *TBB = 0, *FBB = 0; 615 std::vector<MachineOperand> Cond; 616 if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond)) { 617 // Failing case: IBB is the target of a cbr, and 618 // we cannot reverse the branch. 619 std::vector<MachineOperand> NewCond(Cond); 620 if (Cond.size() && TBB==IBB) { 621 if (TII->ReverseBranchCondition(NewCond)) 622 continue; 623 // This is the QBB case described above 624 if (!FBB) 625 FBB = next(MachineFunction::iterator(PBB)); 626 } 627 // Failing case: the only way IBB can be reached from PBB is via 628 // exception handling. Happens for landing pads. Would be nice 629 // to have a bit in the edge so we didn't have to do all this. 630 if (IBB->isLandingPad()) { 631 MachineFunction::iterator IP = PBB; IP++; 632 MachineBasicBlock* PredNextBB = NULL; 633 if (IP!=MF.end()) 634 PredNextBB = IP; 635 if (TBB==NULL) { 636 if (IBB!=PredNextBB) // fallthrough 637 continue; 638 } else if (FBB) { 639 if (TBB!=IBB && FBB!=IBB) // cbr then ubr 640 continue; 641 } else if (Cond.size() == 0) { 642 if (TBB!=IBB) // ubr 643 continue; 644 } else { 645 if (TBB!=IBB && IBB!=PredNextBB) // cbr 646 continue; 647 } 648 } 649 // Remove the unconditional branch at the end, if any. 650 if (TBB && (Cond.size()==0 || FBB)) { 651 TII->RemoveBranch(*PBB); 652 if (Cond.size()) 653 // reinsert conditional branch only, for now 654 TII->InsertBranch(*PBB, (TBB==IBB) ? FBB : TBB, 0, NewCond); 655 } 656 MergePotentials.push_back(std::make_pair(HashEndOfMBB(PBB, 1U), *P)); 657 } 658 } 659 if (MergePotentials.size() >= 2) 660 MadeChange |= TryMergeBlocks(I, PredBB); 661 // Reinsert an unconditional branch if needed. 662 // The 1 below can be either an original single predecessor, or a result 663 // of removing blocks in TryMergeBlocks. 664 PredBB = prior(I); // this may have been changed in TryMergeBlocks 665 if (MergePotentials.size()==1 && 666 (MergePotentials.begin())->second != PredBB) 667 FixTail((MergePotentials.begin())->second, I, TII); 668 } 669 } 670 return MadeChange; 671} 672 673//===----------------------------------------------------------------------===// 674// Branch Optimization 675//===----------------------------------------------------------------------===// 676 677bool BranchFolder::OptimizeBranches(MachineFunction &MF) { 678 MadeChange = false; 679 680 // Make sure blocks are numbered in order 681 MF.RenumberBlocks(); 682 683 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) { 684 MachineBasicBlock *MBB = I++; 685 OptimizeBlock(MBB); 686 687 // If it is dead, remove it. 688 if (MBB->pred_empty()) { 689 RemoveDeadBlock(MBB); 690 MadeChange = true; 691 ++NumDeadBlocks; 692 } 693 } 694 return MadeChange; 695} 696 697 698/// CanFallThrough - Return true if the specified block (with the specified 699/// branch condition) can implicitly transfer control to the block after it by 700/// falling off the end of it. This should return false if it can reach the 701/// block after it, but it uses an explicit branch to do so (e.g. a table jump). 702/// 703/// True is a conservative answer. 704/// 705bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB, 706 bool BranchUnAnalyzable, 707 MachineBasicBlock *TBB, MachineBasicBlock *FBB, 708 const std::vector<MachineOperand> &Cond) { 709 MachineFunction::iterator Fallthrough = CurBB; 710 ++Fallthrough; 711 // If FallthroughBlock is off the end of the function, it can't fall through. 712 if (Fallthrough == CurBB->getParent()->end()) 713 return false; 714 715 // If FallthroughBlock isn't a successor of CurBB, no fallthrough is possible. 716 if (!CurBB->isSuccessor(Fallthrough)) 717 return false; 718 719 // If we couldn't analyze the branch, assume it could fall through. 720 if (BranchUnAnalyzable) return true; 721 722 // If there is no branch, control always falls through. 723 if (TBB == 0) return true; 724 725 // If there is some explicit branch to the fallthrough block, it can obviously 726 // reach, even though the branch should get folded to fall through implicitly. 727 if (MachineFunction::iterator(TBB) == Fallthrough || 728 MachineFunction::iterator(FBB) == Fallthrough) 729 return true; 730 731 // If it's an unconditional branch to some block not the fall through, it 732 // doesn't fall through. 733 if (Cond.empty()) return false; 734 735 // Otherwise, if it is conditional and has no explicit false block, it falls 736 // through. 737 return FBB == 0; 738} 739 740/// CanFallThrough - Return true if the specified can implicitly transfer 741/// control to the block after it by falling off the end of it. This should 742/// return false if it can reach the block after it, but it uses an explicit 743/// branch to do so (e.g. a table jump). 744/// 745/// True is a conservative answer. 746/// 747bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) { 748 MachineBasicBlock *TBB = 0, *FBB = 0; 749 std::vector<MachineOperand> Cond; 750 bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond); 751 return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond); 752} 753 754/// IsBetterFallthrough - Return true if it would be clearly better to 755/// fall-through to MBB1 than to fall through into MBB2. This has to return 756/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will 757/// result in infinite loops. 758static bool IsBetterFallthrough(MachineBasicBlock *MBB1, 759 MachineBasicBlock *MBB2, 760 const TargetInstrInfo &TII) { 761 // Right now, we use a simple heuristic. If MBB2 ends with a call, and 762 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to 763 // optimize branches that branch to either a return block or an assert block 764 // into a fallthrough to the return. 765 if (MBB1->empty() || MBB2->empty()) return false; 766 767 MachineInstr *MBB1I = --MBB1->end(); 768 MachineInstr *MBB2I = --MBB2->end(); 769 return TII.isCall(MBB2I->getOpcode()) && !TII.isCall(MBB1I->getOpcode()); 770} 771 772/// OptimizeBlock - Analyze and optimize control flow related to the specified 773/// block. This is never called on the entry block. 774void BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) { 775 MachineFunction::iterator FallThrough = MBB; 776 ++FallThrough; 777 778 // If this block is empty, make everyone use its fall-through, not the block 779 // explicitly. Landing pads should not do this since the landing-pad table 780 // points to this block. 781 if (MBB->empty() && !MBB->isLandingPad()) { 782 // Dead block? Leave for cleanup later. 783 if (MBB->pred_empty()) return; 784 785 if (FallThrough == MBB->getParent()->end()) { 786 // TODO: Simplify preds to not branch here if possible! 787 } else { 788 // Rewrite all predecessors of the old block to go to the fallthrough 789 // instead. 790 while (!MBB->pred_empty()) { 791 MachineBasicBlock *Pred = *(MBB->pred_end()-1); 792 Pred->ReplaceUsesOfBlockWith(MBB, FallThrough); 793 } 794 795 // If MBB was the target of a jump table, update jump tables to go to the 796 // fallthrough instead. 797 MBB->getParent()->getJumpTableInfo()-> 798 ReplaceMBBInJumpTables(MBB, FallThrough); 799 MadeChange = true; 800 } 801 return; 802 } 803 804 // Check to see if we can simplify the terminator of the block before this 805 // one. 806 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB)); 807 808 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0; 809 std::vector<MachineOperand> PriorCond; 810 bool PriorUnAnalyzable = 811 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); 812 if (!PriorUnAnalyzable) { 813 // If the CFG for the prior block has extra edges, remove them. 814 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB, 815 !PriorCond.empty()); 816 817 // If the previous branch is conditional and both conditions go to the same 818 // destination, remove the branch, replacing it with an unconditional one or 819 // a fall-through. 820 if (PriorTBB && PriorTBB == PriorFBB) { 821 TII->RemoveBranch(PrevBB); 822 PriorCond.clear(); 823 if (PriorTBB != MBB) 824 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 825 MadeChange = true; 826 ++NumBranchOpts; 827 return OptimizeBlock(MBB); 828 } 829 830 // If the previous branch *only* branches to *this* block (conditional or 831 // not) remove the branch. 832 if (PriorTBB == MBB && PriorFBB == 0) { 833 TII->RemoveBranch(PrevBB); 834 MadeChange = true; 835 ++NumBranchOpts; 836 return OptimizeBlock(MBB); 837 } 838 839 // If the prior block branches somewhere else on the condition and here if 840 // the condition is false, remove the uncond second branch. 841 if (PriorFBB == MBB) { 842 TII->RemoveBranch(PrevBB); 843 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 844 MadeChange = true; 845 ++NumBranchOpts; 846 return OptimizeBlock(MBB); 847 } 848 849 // If the prior block branches here on true and somewhere else on false, and 850 // if the branch condition is reversible, reverse the branch to create a 851 // fall-through. 852 if (PriorTBB == MBB) { 853 std::vector<MachineOperand> NewPriorCond(PriorCond); 854 if (!TII->ReverseBranchCondition(NewPriorCond)) { 855 TII->RemoveBranch(PrevBB); 856 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond); 857 MadeChange = true; 858 ++NumBranchOpts; 859 return OptimizeBlock(MBB); 860 } 861 } 862 863 // If this block doesn't fall through (e.g. it ends with an uncond branch or 864 // has no successors) and if the pred falls through into this block, and if 865 // it would otherwise fall through into the block after this, move this 866 // block to the end of the function. 867 // 868 // We consider it more likely that execution will stay in the function (e.g. 869 // due to loops) than it is to exit it. This asserts in loops etc, moving 870 // the assert condition out of the loop body. 871 if (!PriorCond.empty() && PriorFBB == 0 && 872 MachineFunction::iterator(PriorTBB) == FallThrough && 873 !CanFallThrough(MBB)) { 874 bool DoTransform = true; 875 876 // We have to be careful that the succs of PredBB aren't both no-successor 877 // blocks. If neither have successors and if PredBB is the second from 878 // last block in the function, we'd just keep swapping the two blocks for 879 // last. Only do the swap if one is clearly better to fall through than 880 // the other. 881 if (FallThrough == --MBB->getParent()->end() && 882 !IsBetterFallthrough(PriorTBB, MBB, *TII)) 883 DoTransform = false; 884 885 // We don't want to do this transformation if we have control flow like: 886 // br cond BB2 887 // BB1: 888 // .. 889 // jmp BBX 890 // BB2: 891 // .. 892 // ret 893 // 894 // In this case, we could actually be moving the return block *into* a 895 // loop! 896 if (DoTransform && !MBB->succ_empty() && 897 (!CanFallThrough(PriorTBB) || PriorTBB->empty())) 898 DoTransform = false; 899 900 901 if (DoTransform) { 902 // Reverse the branch so we will fall through on the previous true cond. 903 std::vector<MachineOperand> NewPriorCond(PriorCond); 904 if (!TII->ReverseBranchCondition(NewPriorCond)) { 905 DOUT << "\nMoving MBB: " << *MBB; 906 DOUT << "To make fallthrough to: " << *PriorTBB << "\n"; 907 908 TII->RemoveBranch(PrevBB); 909 TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond); 910 911 // Move this block to the end of the function. 912 MBB->moveAfter(--MBB->getParent()->end()); 913 MadeChange = true; 914 ++NumBranchOpts; 915 return; 916 } 917 } 918 } 919 } 920 921 // Analyze the branch in the current block. 922 MachineBasicBlock *CurTBB = 0, *CurFBB = 0; 923 std::vector<MachineOperand> CurCond; 924 bool CurUnAnalyzable = TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond); 925 if (!CurUnAnalyzable) { 926 // If the CFG for the prior block has extra edges, remove them. 927 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty()); 928 929 // If this is a two-way branch, and the FBB branches to this block, reverse 930 // the condition so the single-basic-block loop is faster. Instead of: 931 // Loop: xxx; jcc Out; jmp Loop 932 // we want: 933 // Loop: xxx; jncc Loop; jmp Out 934 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) { 935 std::vector<MachineOperand> NewCond(CurCond); 936 if (!TII->ReverseBranchCondition(NewCond)) { 937 TII->RemoveBranch(*MBB); 938 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond); 939 MadeChange = true; 940 ++NumBranchOpts; 941 return OptimizeBlock(MBB); 942 } 943 } 944 945 946 // If this branch is the only thing in its block, see if we can forward 947 // other blocks across it. 948 if (CurTBB && CurCond.empty() && CurFBB == 0 && 949 TII->isBranch(MBB->begin()->getOpcode()) && CurTBB != MBB) { 950 // This block may contain just an unconditional branch. Because there can 951 // be 'non-branch terminators' in the block, try removing the branch and 952 // then seeing if the block is empty. 953 TII->RemoveBranch(*MBB); 954 955 // If this block is just an unconditional branch to CurTBB, we can 956 // usually completely eliminate the block. The only case we cannot 957 // completely eliminate the block is when the block before this one 958 // falls through into MBB and we can't understand the prior block's branch 959 // condition. 960 if (MBB->empty()) { 961 bool PredHasNoFallThrough = TII->BlockHasNoFallThrough(PrevBB); 962 if (PredHasNoFallThrough || !PriorUnAnalyzable || 963 !PrevBB.isSuccessor(MBB)) { 964 // If the prior block falls through into us, turn it into an 965 // explicit branch to us to make updates simpler. 966 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) && 967 PriorTBB != MBB && PriorFBB != MBB) { 968 if (PriorTBB == 0) { 969 assert(PriorCond.empty() && PriorFBB == 0 && 970 "Bad branch analysis"); 971 PriorTBB = MBB; 972 } else { 973 assert(PriorFBB == 0 && "Machine CFG out of date!"); 974 PriorFBB = MBB; 975 } 976 TII->RemoveBranch(PrevBB); 977 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); 978 } 979 980 // Iterate through all the predecessors, revectoring each in-turn. 981 size_t PI = 0; 982 bool DidChange = false; 983 bool HasBranchToSelf = false; 984 while(PI != MBB->pred_size()) { 985 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI); 986 if (PMBB == MBB) { 987 // If this block has an uncond branch to itself, leave it. 988 ++PI; 989 HasBranchToSelf = true; 990 } else { 991 DidChange = true; 992 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB); 993 } 994 } 995 996 // Change any jumptables to go to the new MBB. 997 MBB->getParent()->getJumpTableInfo()-> 998 ReplaceMBBInJumpTables(MBB, CurTBB); 999 if (DidChange) { 1000 ++NumBranchOpts; 1001 MadeChange = true; 1002 if (!HasBranchToSelf) return; 1003 } 1004 } 1005 } 1006 1007 // Add the branch back if the block is more than just an uncond branch. 1008 TII->InsertBranch(*MBB, CurTBB, 0, CurCond); 1009 } 1010 } 1011 1012 // If the prior block doesn't fall through into this block, and if this 1013 // block doesn't fall through into some other block, see if we can find a 1014 // place to move this block where a fall-through will happen. 1015 if (!CanFallThrough(&PrevBB, PriorUnAnalyzable, 1016 PriorTBB, PriorFBB, PriorCond)) { 1017 // Now we know that there was no fall-through into this block, check to 1018 // see if it has a fall-through into its successor. 1019 bool CurFallsThru = CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB, 1020 CurCond); 1021 1022 if (!MBB->isLandingPad()) { 1023 // Check all the predecessors of this block. If one of them has no fall 1024 // throughs, move this block right after it. 1025 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(), 1026 E = MBB->pred_end(); PI != E; ++PI) { 1027 // Analyze the branch at the end of the pred. 1028 MachineBasicBlock *PredBB = *PI; 1029 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough; 1030 if (PredBB != MBB && !CanFallThrough(PredBB) 1031 && (!CurFallsThru || !CurTBB || !CurFBB) 1032 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) { 1033 // If the current block doesn't fall through, just move it. 1034 // If the current block can fall through and does not end with a 1035 // conditional branch, we need to append an unconditional jump to 1036 // the (current) next block. To avoid a possible compile-time 1037 // infinite loop, move blocks only backward in this case. 1038 // Also, if there are already 2 branches here, we cannot add a third; 1039 // this means we have the case 1040 // Bcc next 1041 // B elsewhere 1042 // next: 1043 if (CurFallsThru) { 1044 MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB)); 1045 CurCond.clear(); 1046 TII->InsertBranch(*MBB, NextBB, 0, CurCond); 1047 } 1048 MBB->moveAfter(PredBB); 1049 MadeChange = true; 1050 return OptimizeBlock(MBB); 1051 } 1052 } 1053 } 1054 1055 if (!CurFallsThru) { 1056 // Check all successors to see if we can move this block before it. 1057 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(), 1058 E = MBB->succ_end(); SI != E; ++SI) { 1059 // Analyze the branch at the end of the block before the succ. 1060 MachineBasicBlock *SuccBB = *SI; 1061 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev; 1062 std::vector<MachineOperand> SuccPrevCond; 1063 1064 // If this block doesn't already fall-through to that successor, and if 1065 // the succ doesn't already have a block that can fall through into it, 1066 // and if the successor isn't an EH destination, we can arrange for the 1067 // fallthrough to happen. 1068 if (SuccBB != MBB && !CanFallThrough(SuccPrev) && 1069 !SuccBB->isLandingPad()) { 1070 MBB->moveBefore(SuccBB); 1071 MadeChange = true; 1072 return OptimizeBlock(MBB); 1073 } 1074 } 1075 1076 // Okay, there is no really great place to put this block. If, however, 1077 // the block before this one would be a fall-through if this block were 1078 // removed, move this block to the end of the function. 1079 if (FallThrough != MBB->getParent()->end() && 1080 PrevBB.isSuccessor(FallThrough)) { 1081 MBB->moveAfter(--MBB->getParent()->end()); 1082 MadeChange = true; 1083 return; 1084 } 1085 } 1086 } 1087} 1088