BranchFolding.cpp revision 1e57df3b4fd69b2a316d8931dee1b9e033ae3a9a
1//===-- BranchFolding.cpp - Fold machine code branch instructions ---------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file is distributed under the University of Illinois Open Source 6// License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// This pass forwards branches to unconditional branches to make them branch 11// directly to the target block. This pass often results in dead MBB's, which 12// it then removes. 13// 14// Note that this pass must be run after register allocation, it cannot handle 15// SSA form. 16// 17//===----------------------------------------------------------------------===// 18 19#define DEBUG_TYPE "branchfolding" 20#include "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/TargetRegisterInfo.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 explicit 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 TargetRegisterInfo *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.getIndex()]; 176 Op.setIndex(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.getMBB()->getNumber(); 214 break; 215 case MachineOperand::MO_FrameIndex: 216 case MachineOperand::MO_ConstantPoolIndex: 217 case MachineOperand::MO_JumpTableIndex: 218 OperandHash = Op.getIndex(); 219 break; 220 case MachineOperand::MO_GlobalAddress: 221 case MachineOperand::MO_ExternalSymbol: 222 // Global address / external symbol are too hard, don't bother, but do 223 // pull in the offset. 224 OperandHash = Op.getOffset(); 225 break; 226 default: break; 227 } 228 229 Hash += ((OperandHash << 3) | Op.getType()) << (i&31); 230 } 231 return Hash; 232} 233 234/// HashEndOfMBB - Hash the last few instructions in the MBB. For blocks 235/// with no successors, we hash two instructions, because cross-jumping 236/// only saves code when at least two instructions are removed (since a 237/// branch must be inserted). For blocks with a successor, one of the 238/// two blocks to be tail-merged will end with a branch already, so 239/// it gains to cross-jump even for one instruction. 240 241static unsigned HashEndOfMBB(const MachineBasicBlock *MBB, 242 unsigned minCommonTailLength) { 243 MachineBasicBlock::const_iterator I = MBB->end(); 244 if (I == MBB->begin()) 245 return 0; // Empty MBB. 246 247 --I; 248 unsigned Hash = HashMachineInstr(I); 249 250 if (I == MBB->begin() || minCommonTailLength == 1) 251 return Hash; // Single instr MBB. 252 253 --I; 254 // Hash in the second-to-last instruction. 255 Hash ^= HashMachineInstr(I) << 2; 256 return Hash; 257} 258 259/// ComputeCommonTailLength - Given two machine basic blocks, compute the number 260/// of instructions they actually have in common together at their end. Return 261/// iterators for the first shared instruction in each block. 262static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1, 263 MachineBasicBlock *MBB2, 264 MachineBasicBlock::iterator &I1, 265 MachineBasicBlock::iterator &I2) { 266 I1 = MBB1->end(); 267 I2 = MBB2->end(); 268 269 unsigned TailLen = 0; 270 while (I1 != MBB1->begin() && I2 != MBB2->begin()) { 271 --I1; --I2; 272 if (!I1->isIdenticalTo(I2) || 273 // FIXME: This check is dubious. It's used to get around a problem where 274 // people incorrectly expect inline asm directives to remain in the same 275 // relative order. This is untenable because normal compiler 276 // optimizations (like this one) may reorder and/or merge these 277 // directives. 278 I1->getOpcode() == TargetInstrInfo::INLINEASM) { 279 ++I1; ++I2; 280 break; 281 } 282 ++TailLen; 283 } 284 return TailLen; 285} 286 287/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything 288/// after it, replacing it with an unconditional branch to NewDest. This 289/// returns true if OldInst's block is modified, false if NewDest is modified. 290void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, 291 MachineBasicBlock *NewDest) { 292 MachineBasicBlock *OldBB = OldInst->getParent(); 293 294 // Remove all the old successors of OldBB from the CFG. 295 while (!OldBB->succ_empty()) 296 OldBB->removeSuccessor(OldBB->succ_begin()); 297 298 // Remove all the dead instructions from the end of OldBB. 299 OldBB->erase(OldInst, OldBB->end()); 300 301 // If OldBB isn't immediately before OldBB, insert a branch to it. 302 if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest)) 303 TII->InsertBranch(*OldBB, NewDest, 0, std::vector<MachineOperand>()); 304 OldBB->addSuccessor(NewDest); 305 ++NumTailMerge; 306} 307 308/// SplitMBBAt - Given a machine basic block and an iterator into it, split the 309/// MBB so that the part before the iterator falls into the part starting at the 310/// iterator. This returns the new MBB. 311MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB, 312 MachineBasicBlock::iterator BBI1) { 313 // Create the fall-through block. 314 MachineFunction::iterator MBBI = &CurMBB; 315 MachineBasicBlock *NewMBB = new MachineBasicBlock(CurMBB.getBasicBlock()); 316 CurMBB.getParent()->getBasicBlockList().insert(++MBBI, NewMBB); 317 318 // Move all the successors of this block to the specified block. 319 while (!CurMBB.succ_empty()) { 320 MachineBasicBlock *S = *(CurMBB.succ_end()-1); 321 NewMBB->addSuccessor(S); 322 CurMBB.removeSuccessor(S); 323 } 324 325 // Add an edge from CurMBB to NewMBB for the fall-through. 326 CurMBB.addSuccessor(NewMBB); 327 328 // Splice the code over. 329 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end()); 330 331 // For targets that use the register scavenger, we must maintain LiveIns. 332 if (RS) { 333 RS->enterBasicBlock(&CurMBB); 334 if (!CurMBB.empty()) 335 RS->forward(prior(CurMBB.end())); 336 BitVector RegsLiveAtExit(RegInfo->getNumRegs()); 337 RS->getRegsUsed(RegsLiveAtExit, false); 338 for (unsigned int i=0, e=RegInfo->getNumRegs(); i!=e; i++) 339 if (RegsLiveAtExit[i]) 340 NewMBB->addLiveIn(i); 341 } 342 343 return NewMBB; 344} 345 346/// EstimateRuntime - Make a rough estimate for how long it will take to run 347/// the specified code. 348static unsigned EstimateRuntime(MachineBasicBlock::iterator I, 349 MachineBasicBlock::iterator E) { 350 unsigned Time = 0; 351 for (; I != E; ++I) { 352 const TargetInstrDesc &TID = I->getDesc(); 353 if (TID.isCall()) 354 Time += 10; 355 else if (TID.isSimpleLoad() || TID.mayStore()) 356 Time += 2; 357 else 358 ++Time; 359 } 360 return Time; 361} 362 363/// ShouldSplitFirstBlock - We need to either split MBB1 at MBB1I or MBB2 at 364/// MBB2I and then insert an unconditional branch in the other block. Determine 365/// which is the best to split 366static bool ShouldSplitFirstBlock(MachineBasicBlock *MBB1, 367 MachineBasicBlock::iterator MBB1I, 368 MachineBasicBlock *MBB2, 369 MachineBasicBlock::iterator MBB2I, 370 MachineBasicBlock *PredBB) { 371 // If one block is the entry block, split the other one; we can't generate 372 // a branch to the entry block, as its label is not emitted. 373 MachineBasicBlock *Entry = MBB1->getParent()->begin(); 374 if (MBB1 == Entry) 375 return false; 376 if (MBB2 == Entry) 377 return true; 378 379 // If one block falls through into the common successor, choose that 380 // one to split; it is one instruction less to do that. 381 if (PredBB) { 382 if (MBB1 == PredBB) 383 return true; 384 else if (MBB2 == PredBB) 385 return false; 386 } 387 // TODO: if we had some notion of which block was hotter, we could split 388 // the hot block, so it is the fall-through. Since we don't have profile info 389 // make a decision based on which will hurt most to split. 390 unsigned MBB1Time = EstimateRuntime(MBB1->begin(), MBB1I); 391 unsigned MBB2Time = EstimateRuntime(MBB2->begin(), MBB2I); 392 393 // If the MBB1 prefix takes "less time" to run than the MBB2 prefix, split the 394 // MBB1 block so it falls through. This will penalize the MBB2 path, but will 395 // have a lower overall impact on the program execution. 396 return MBB1Time < MBB2Time; 397} 398 399// CurMBB needs to add an unconditional branch to SuccMBB (we removed these 400// branches temporarily for tail merging). In the case where CurMBB ends 401// with a conditional branch to the next block, optimize by reversing the 402// test and conditionally branching to SuccMBB instead. 403 404static void FixTail(MachineBasicBlock* CurMBB, MachineBasicBlock *SuccBB, 405 const TargetInstrInfo *TII) { 406 MachineFunction *MF = CurMBB->getParent(); 407 MachineFunction::iterator I = next(MachineFunction::iterator(CurMBB)); 408 MachineBasicBlock *TBB = 0, *FBB = 0; 409 std::vector<MachineOperand> Cond; 410 if (I != MF->end() && 411 !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond)) { 412 MachineBasicBlock *NextBB = I; 413 if (TBB == NextBB && Cond.size() && !FBB) { 414 if (!TII->ReverseBranchCondition(Cond)) { 415 TII->RemoveBranch(*CurMBB); 416 TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond); 417 return; 418 } 419 } 420 } 421 TII->InsertBranch(*CurMBB, SuccBB, NULL, std::vector<MachineOperand>()); 422} 423 424static bool MergeCompare(const std::pair<unsigned,MachineBasicBlock*> &p, 425 const std::pair<unsigned,MachineBasicBlock*> &q) { 426 if (p.first < q.first) 427 return true; 428 else if (p.first > q.first) 429 return false; 430 else if (p.second->getNumber() < q.second->getNumber()) 431 return true; 432 else if (p.second->getNumber() > q.second->getNumber()) 433 return false; 434 else { 435 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing 436 // an object with itself. 437#ifndef _GLIBCXX_DEBUG 438 assert(0 && "Predecessor appears twice"); 439#endif 440 return(false); 441 } 442} 443 444// See if any of the blocks in MergePotentials (which all have a common single 445// successor, or all have no successor) can be tail-merged. If there is a 446// successor, any blocks in MergePotentials that are not tail-merged and 447// are not immediately before Succ must have an unconditional branch to 448// Succ added (but the predecessor/successor lists need no adjustment). 449// The lone predecessor of Succ that falls through into Succ, 450// if any, is given in PredBB. 451 452bool BranchFolder::TryMergeBlocks(MachineBasicBlock *SuccBB, 453 MachineBasicBlock* PredBB) { 454 unsigned minCommonTailLength = (SuccBB ? 1 : 2); 455 MadeChange = false; 456 457 // Sort by hash value so that blocks with identical end sequences sort 458 // together. 459 std::stable_sort(MergePotentials.begin(), MergePotentials.end(), MergeCompare); 460 461 // Walk through equivalence sets looking for actual exact matches. 462 while (MergePotentials.size() > 1) { 463 unsigned CurHash = (MergePotentials.end()-1)->first; 464 unsigned PrevHash = (MergePotentials.end()-2)->first; 465 MachineBasicBlock *CurMBB = (MergePotentials.end()-1)->second; 466 467 // If there is nothing that matches the hash of the current basic block, 468 // give up. 469 if (CurHash != PrevHash) { 470 if (SuccBB && CurMBB != PredBB) 471 FixTail(CurMBB, SuccBB, TII); 472 MergePotentials.pop_back(); 473 continue; 474 } 475 476 // Look through all the pairs of blocks that have the same hash as this 477 // one, and find the pair that has the largest number of instructions in 478 // common. 479 // Since instructions may get combined later (e.g. single stores into 480 // store multiple) this measure is not particularly accurate. 481 MachineBasicBlock::iterator BBI1, BBI2; 482 483 unsigned FoundI = ~0U, FoundJ = ~0U; 484 unsigned maxCommonTailLength = 0U; 485 for (int i = MergePotentials.size()-1; 486 i != -1 && MergePotentials[i].first == CurHash; --i) { 487 for (int j = i-1; 488 j != -1 && MergePotentials[j].first == CurHash; --j) { 489 MachineBasicBlock::iterator TrialBBI1, TrialBBI2; 490 unsigned CommonTailLen = ComputeCommonTailLength( 491 MergePotentials[i].second, 492 MergePotentials[j].second, 493 TrialBBI1, TrialBBI2); 494 if (CommonTailLen >= minCommonTailLength && 495 CommonTailLen > maxCommonTailLength) { 496 FoundI = i; 497 FoundJ = j; 498 maxCommonTailLength = CommonTailLen; 499 BBI1 = TrialBBI1; 500 BBI2 = TrialBBI2; 501 } 502 } 503 } 504 505 // If we didn't find any pair that has at least minCommonTailLength 506 // instructions in common, bail out. All entries with this 507 // hash code can go away now. 508 if (FoundI == ~0U) { 509 for (int i = MergePotentials.size()-1; 510 i != -1 && MergePotentials[i].first == CurHash; --i) { 511 // Put the unconditional branch back, if we need one. 512 CurMBB = MergePotentials[i].second; 513 if (SuccBB && CurMBB != PredBB) 514 FixTail(CurMBB, SuccBB, TII); 515 MergePotentials.pop_back(); 516 } 517 continue; 518 } 519 520 // Otherwise, move the block(s) to the right position(s). So that 521 // BBI1/2 will be valid, the last must be I and the next-to-last J. 522 if (FoundI != MergePotentials.size()-1) 523 std::swap(MergePotentials[FoundI], *(MergePotentials.end()-1)); 524 if (FoundJ != MergePotentials.size()-2) 525 std::swap(MergePotentials[FoundJ], *(MergePotentials.end()-2)); 526 527 CurMBB = (MergePotentials.end()-1)->second; 528 MachineBasicBlock *MBB2 = (MergePotentials.end()-2)->second; 529 530 // If neither block is the entire common tail, split the tail of one block 531 // to make it redundant with the other tail. Also, we cannot jump to the 532 // entry block, so if one block is the entry block, split the other one. 533 MachineBasicBlock *Entry = CurMBB->getParent()->begin(); 534 if (CurMBB->begin() == BBI1 && CurMBB != Entry) 535 ; // CurMBB is common tail 536 else if (MBB2->begin() == BBI2 && MBB2 != Entry) 537 ; // MBB2 is common tail 538 else { 539 if (0) { // Enable this to disable partial tail merges. 540 MergePotentials.pop_back(); 541 continue; 542 } 543 544 // Decide whether we want to split CurMBB or MBB2. 545 if (ShouldSplitFirstBlock(CurMBB, BBI1, MBB2, BBI2, PredBB)) { 546 CurMBB = SplitMBBAt(*CurMBB, BBI1); 547 BBI1 = CurMBB->begin(); 548 MergePotentials.back().second = CurMBB; 549 } else { 550 MBB2 = SplitMBBAt(*MBB2, BBI2); 551 BBI2 = MBB2->begin(); 552 (MergePotentials.end()-2)->second = MBB2; 553 } 554 } 555 556 if (MBB2->begin() == BBI2 && MBB2 != Entry) { 557 // Hack the end off CurMBB, making it jump to MBBI@ instead. 558 ReplaceTailWithBranchTo(BBI1, MBB2); 559 // This modifies CurMBB, so remove it from the worklist. 560 MergePotentials.pop_back(); 561 } else { 562 assert(CurMBB->begin() == BBI1 && CurMBB != Entry && 563 "Didn't split block correctly?"); 564 // Hack the end off MBB2, making it jump to CurMBB instead. 565 ReplaceTailWithBranchTo(BBI2, CurMBB); 566 // This modifies MBB2, so remove it from the worklist. 567 MergePotentials.erase(MergePotentials.end()-2); 568 } 569 MadeChange = true; 570 } 571 return MadeChange; 572} 573 574bool BranchFolder::TailMergeBlocks(MachineFunction &MF) { 575 576 if (!EnableTailMerge) return false; 577 578 MadeChange = false; 579 580 // First find blocks with no successors. 581 MergePotentials.clear(); 582 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { 583 if (I->succ_empty()) 584 MergePotentials.push_back(std::make_pair(HashEndOfMBB(I, 2U), I)); 585 } 586 // See if we can do any tail merging on those. 587 if (MergePotentials.size() < TailMergeThreshold) 588 MadeChange |= TryMergeBlocks(NULL, NULL); 589 590 // Look at blocks (IBB) with multiple predecessors (PBB). 591 // We change each predecessor to a canonical form, by 592 // (1) temporarily removing any unconditional branch from the predecessor 593 // to IBB, and 594 // (2) alter conditional branches so they branch to the other block 595 // not IBB; this may require adding back an unconditional branch to IBB 596 // later, where there wasn't one coming in. E.g. 597 // Bcc IBB 598 // fallthrough to QBB 599 // here becomes 600 // Bncc QBB 601 // with a conceptual B to IBB after that, which never actually exists. 602 // With those changes, we see whether the predecessors' tails match, 603 // and merge them if so. We change things out of canonical form and 604 // back to the way they were later in the process. (OptimizeBranches 605 // would undo some of this, but we can't use it, because we'd get into 606 // a compile-time infinite loop repeatedly doing and undoing the same 607 // transformations.) 608 609 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { 610 if (!I->succ_empty() && I->pred_size() >= 2 && 611 I->pred_size() < TailMergeThreshold) { 612 MachineBasicBlock *IBB = I; 613 MachineBasicBlock *PredBB = prior(I); 614 MergePotentials.clear(); 615 for (MachineBasicBlock::pred_iterator P = I->pred_begin(), 616 E2 = I->pred_end(); 617 P != E2; ++P) { 618 MachineBasicBlock* PBB = *P; 619 // Skip blocks that loop to themselves, can't tail merge these. 620 if (PBB==IBB) 621 continue; 622 MachineBasicBlock *TBB = 0, *FBB = 0; 623 std::vector<MachineOperand> Cond; 624 if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond)) { 625 // Failing case: IBB is the target of a cbr, and 626 // we cannot reverse the branch. 627 std::vector<MachineOperand> NewCond(Cond); 628 if (Cond.size() && TBB==IBB) { 629 if (TII->ReverseBranchCondition(NewCond)) 630 continue; 631 // This is the QBB case described above 632 if (!FBB) 633 FBB = next(MachineFunction::iterator(PBB)); 634 } 635 // Failing case: the only way IBB can be reached from PBB is via 636 // exception handling. Happens for landing pads. Would be nice 637 // to have a bit in the edge so we didn't have to do all this. 638 if (IBB->isLandingPad()) { 639 MachineFunction::iterator IP = PBB; IP++; 640 MachineBasicBlock* PredNextBB = NULL; 641 if (IP!=MF.end()) 642 PredNextBB = IP; 643 if (TBB==NULL) { 644 if (IBB!=PredNextBB) // fallthrough 645 continue; 646 } else if (FBB) { 647 if (TBB!=IBB && FBB!=IBB) // cbr then ubr 648 continue; 649 } else if (Cond.empty()) { 650 if (TBB!=IBB) // ubr 651 continue; 652 } else { 653 if (TBB!=IBB && IBB!=PredNextBB) // cbr 654 continue; 655 } 656 } 657 // Remove the unconditional branch at the end, if any. 658 if (TBB && (Cond.size()==0 || FBB)) { 659 TII->RemoveBranch(*PBB); 660 if (Cond.size()) 661 // reinsert conditional branch only, for now 662 TII->InsertBranch(*PBB, (TBB==IBB) ? FBB : TBB, 0, NewCond); 663 } 664 MergePotentials.push_back(std::make_pair(HashEndOfMBB(PBB, 1U), *P)); 665 } 666 } 667 if (MergePotentials.size() >= 2) 668 MadeChange |= TryMergeBlocks(I, PredBB); 669 // Reinsert an unconditional branch if needed. 670 // The 1 below can be either an original single predecessor, or a result 671 // of removing blocks in TryMergeBlocks. 672 PredBB = prior(I); // this may have been changed in TryMergeBlocks 673 if (MergePotentials.size()==1 && 674 (MergePotentials.begin())->second != PredBB) 675 FixTail((MergePotentials.begin())->second, I, TII); 676 } 677 } 678 return MadeChange; 679} 680 681//===----------------------------------------------------------------------===// 682// Branch Optimization 683//===----------------------------------------------------------------------===// 684 685bool BranchFolder::OptimizeBranches(MachineFunction &MF) { 686 MadeChange = false; 687 688 // Make sure blocks are numbered in order 689 MF.RenumberBlocks(); 690 691 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) { 692 MachineBasicBlock *MBB = I++; 693 OptimizeBlock(MBB); 694 695 // If it is dead, remove it. 696 if (MBB->pred_empty()) { 697 RemoveDeadBlock(MBB); 698 MadeChange = true; 699 ++NumDeadBlocks; 700 } 701 } 702 return MadeChange; 703} 704 705 706/// CanFallThrough - Return true if the specified block (with the specified 707/// branch condition) can implicitly transfer control to the block after it by 708/// falling off the end of it. This should return false if it can reach the 709/// block after it, but it uses an explicit branch to do so (e.g. a table jump). 710/// 711/// True is a conservative answer. 712/// 713bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB, 714 bool BranchUnAnalyzable, 715 MachineBasicBlock *TBB, MachineBasicBlock *FBB, 716 const std::vector<MachineOperand> &Cond) { 717 MachineFunction::iterator Fallthrough = CurBB; 718 ++Fallthrough; 719 // If FallthroughBlock is off the end of the function, it can't fall through. 720 if (Fallthrough == CurBB->getParent()->end()) 721 return false; 722 723 // If FallthroughBlock isn't a successor of CurBB, no fallthrough is possible. 724 if (!CurBB->isSuccessor(Fallthrough)) 725 return false; 726 727 // If we couldn't analyze the branch, assume it could fall through. 728 if (BranchUnAnalyzable) return true; 729 730 // If there is no branch, control always falls through. 731 if (TBB == 0) return true; 732 733 // If there is some explicit branch to the fallthrough block, it can obviously 734 // reach, even though the branch should get folded to fall through implicitly. 735 if (MachineFunction::iterator(TBB) == Fallthrough || 736 MachineFunction::iterator(FBB) == Fallthrough) 737 return true; 738 739 // If it's an unconditional branch to some block not the fall through, it 740 // doesn't fall through. 741 if (Cond.empty()) return false; 742 743 // Otherwise, if it is conditional and has no explicit false block, it falls 744 // through. 745 return FBB == 0; 746} 747 748/// CanFallThrough - Return true if the specified can implicitly transfer 749/// control to the block after it by falling off the end of it. This should 750/// return false if it can reach the block after it, but it uses an explicit 751/// branch to do so (e.g. a table jump). 752/// 753/// True is a conservative answer. 754/// 755bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) { 756 MachineBasicBlock *TBB = 0, *FBB = 0; 757 std::vector<MachineOperand> Cond; 758 bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond); 759 return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond); 760} 761 762/// IsBetterFallthrough - Return true if it would be clearly better to 763/// fall-through to MBB1 than to fall through into MBB2. This has to return 764/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will 765/// result in infinite loops. 766static bool IsBetterFallthrough(MachineBasicBlock *MBB1, 767 MachineBasicBlock *MBB2) { 768 // Right now, we use a simple heuristic. If MBB2 ends with a call, and 769 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to 770 // optimize branches that branch to either a return block or an assert block 771 // into a fallthrough to the return. 772 if (MBB1->empty() || MBB2->empty()) return false; 773 774 // If there is a clear successor ordering we make sure that one block 775 // will fall through to the next 776 if (MBB1->isSuccessor(MBB2)) return true; 777 if (MBB2->isSuccessor(MBB1)) return false; 778 779 MachineInstr *MBB1I = --MBB1->end(); 780 MachineInstr *MBB2I = --MBB2->end(); 781 return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall(); 782} 783 784/// OptimizeBlock - Analyze and optimize control flow related to the specified 785/// block. This is never called on the entry block. 786void BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) { 787 MachineFunction::iterator FallThrough = MBB; 788 ++FallThrough; 789 790 // If this block is empty, make everyone use its fall-through, not the block 791 // explicitly. Landing pads should not do this since the landing-pad table 792 // points to this block. 793 if (MBB->empty() && !MBB->isLandingPad()) { 794 // Dead block? Leave for cleanup later. 795 if (MBB->pred_empty()) return; 796 797 if (FallThrough == MBB->getParent()->end()) { 798 // TODO: Simplify preds to not branch here if possible! 799 } else { 800 // Rewrite all predecessors of the old block to go to the fallthrough 801 // instead. 802 while (!MBB->pred_empty()) { 803 MachineBasicBlock *Pred = *(MBB->pred_end()-1); 804 Pred->ReplaceUsesOfBlockWith(MBB, FallThrough); 805 } 806 807 // If MBB was the target of a jump table, update jump tables to go to the 808 // fallthrough instead. 809 MBB->getParent()->getJumpTableInfo()-> 810 ReplaceMBBInJumpTables(MBB, FallThrough); 811 MadeChange = true; 812 } 813 return; 814 } 815 816 // Check to see if we can simplify the terminator of the block before this 817 // one. 818 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB)); 819 820 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0; 821 std::vector<MachineOperand> PriorCond; 822 bool PriorUnAnalyzable = 823 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); 824 if (!PriorUnAnalyzable) { 825 // If the CFG for the prior block has extra edges, remove them. 826 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB, 827 !PriorCond.empty()); 828 829 // If the previous branch is conditional and both conditions go to the same 830 // destination, remove the branch, replacing it with an unconditional one or 831 // a fall-through. 832 if (PriorTBB && PriorTBB == PriorFBB) { 833 TII->RemoveBranch(PrevBB); 834 PriorCond.clear(); 835 if (PriorTBB != MBB) 836 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 837 MadeChange = true; 838 ++NumBranchOpts; 839 return OptimizeBlock(MBB); 840 } 841 842 // If the previous branch *only* branches to *this* block (conditional or 843 // not) remove the branch. 844 if (PriorTBB == MBB && PriorFBB == 0) { 845 TII->RemoveBranch(PrevBB); 846 MadeChange = true; 847 ++NumBranchOpts; 848 return OptimizeBlock(MBB); 849 } 850 851 // If the prior block branches somewhere else on the condition and here if 852 // the condition is false, remove the uncond second branch. 853 if (PriorFBB == MBB) { 854 TII->RemoveBranch(PrevBB); 855 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 856 MadeChange = true; 857 ++NumBranchOpts; 858 return OptimizeBlock(MBB); 859 } 860 861 // If the prior block branches here on true and somewhere else on false, and 862 // if the branch condition is reversible, reverse the branch to create a 863 // fall-through. 864 if (PriorTBB == MBB) { 865 std::vector<MachineOperand> NewPriorCond(PriorCond); 866 if (!TII->ReverseBranchCondition(NewPriorCond)) { 867 TII->RemoveBranch(PrevBB); 868 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond); 869 MadeChange = true; 870 ++NumBranchOpts; 871 return OptimizeBlock(MBB); 872 } 873 } 874 875 // If this block doesn't fall through (e.g. it ends with an uncond branch or 876 // has no successors) and if the pred falls through into this block, and if 877 // it would otherwise fall through into the block after this, move this 878 // block to the end of the function. 879 // 880 // We consider it more likely that execution will stay in the function (e.g. 881 // due to loops) than it is to exit it. This asserts in loops etc, moving 882 // the assert condition out of the loop body. 883 if (!PriorCond.empty() && PriorFBB == 0 && 884 MachineFunction::iterator(PriorTBB) == FallThrough && 885 !CanFallThrough(MBB)) { 886 bool DoTransform = true; 887 888 // We have to be careful that the succs of PredBB aren't both no-successor 889 // blocks. If neither have successors and if PredBB is the second from 890 // last block in the function, we'd just keep swapping the two blocks for 891 // last. Only do the swap if one is clearly better to fall through than 892 // the other. 893 if (FallThrough == --MBB->getParent()->end() && 894 !IsBetterFallthrough(PriorTBB, MBB)) 895 DoTransform = false; 896 897 // We don't want to do this transformation if we have control flow like: 898 // br cond BB2 899 // BB1: 900 // .. 901 // jmp BBX 902 // BB2: 903 // .. 904 // ret 905 // 906 // In this case, we could actually be moving the return block *into* a 907 // loop! 908 if (DoTransform && !MBB->succ_empty() && 909 (!CanFallThrough(PriorTBB) || PriorTBB->empty())) 910 DoTransform = false; 911 912 913 if (DoTransform) { 914 // Reverse the branch so we will fall through on the previous true cond. 915 std::vector<MachineOperand> NewPriorCond(PriorCond); 916 if (!TII->ReverseBranchCondition(NewPriorCond)) { 917 DOUT << "\nMoving MBB: " << *MBB; 918 DOUT << "To make fallthrough to: " << *PriorTBB << "\n"; 919 920 TII->RemoveBranch(PrevBB); 921 TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond); 922 923 // Move this block to the end of the function. 924 MBB->moveAfter(--MBB->getParent()->end()); 925 MadeChange = true; 926 ++NumBranchOpts; 927 return; 928 } 929 } 930 } 931 } 932 933 // Analyze the branch in the current block. 934 MachineBasicBlock *CurTBB = 0, *CurFBB = 0; 935 std::vector<MachineOperand> CurCond; 936 bool CurUnAnalyzable = TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond); 937 if (!CurUnAnalyzable) { 938 // If the CFG for the prior block has extra edges, remove them. 939 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty()); 940 941 // If this is a two-way branch, and the FBB branches to this block, reverse 942 // the condition so the single-basic-block loop is faster. Instead of: 943 // Loop: xxx; jcc Out; jmp Loop 944 // we want: 945 // Loop: xxx; jncc Loop; jmp Out 946 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) { 947 std::vector<MachineOperand> NewCond(CurCond); 948 if (!TII->ReverseBranchCondition(NewCond)) { 949 TII->RemoveBranch(*MBB); 950 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond); 951 MadeChange = true; 952 ++NumBranchOpts; 953 return OptimizeBlock(MBB); 954 } 955 } 956 957 958 // If this branch is the only thing in its block, see if we can forward 959 // other blocks across it. 960 if (CurTBB && CurCond.empty() && CurFBB == 0 && 961 MBB->begin()->getDesc().isBranch() && CurTBB != MBB) { 962 // This block may contain just an unconditional branch. Because there can 963 // be 'non-branch terminators' in the block, try removing the branch and 964 // then seeing if the block is empty. 965 TII->RemoveBranch(*MBB); 966 967 // If this block is just an unconditional branch to CurTBB, we can 968 // usually completely eliminate the block. The only case we cannot 969 // completely eliminate the block is when the block before this one 970 // falls through into MBB and we can't understand the prior block's branch 971 // condition. 972 if (MBB->empty()) { 973 bool PredHasNoFallThrough = TII->BlockHasNoFallThrough(PrevBB); 974 if (PredHasNoFallThrough || !PriorUnAnalyzable || 975 !PrevBB.isSuccessor(MBB)) { 976 // If the prior block falls through into us, turn it into an 977 // explicit branch to us to make updates simpler. 978 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) && 979 PriorTBB != MBB && PriorFBB != MBB) { 980 if (PriorTBB == 0) { 981 assert(PriorCond.empty() && PriorFBB == 0 && 982 "Bad branch analysis"); 983 PriorTBB = MBB; 984 } else { 985 assert(PriorFBB == 0 && "Machine CFG out of date!"); 986 PriorFBB = MBB; 987 } 988 TII->RemoveBranch(PrevBB); 989 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); 990 } 991 992 // Iterate through all the predecessors, revectoring each in-turn. 993 size_t PI = 0; 994 bool DidChange = false; 995 bool HasBranchToSelf = false; 996 while(PI != MBB->pred_size()) { 997 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI); 998 if (PMBB == MBB) { 999 // If this block has an uncond branch to itself, leave it. 1000 ++PI; 1001 HasBranchToSelf = true; 1002 } else { 1003 DidChange = true; 1004 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB); 1005 } 1006 } 1007 1008 // Change any jumptables to go to the new MBB. 1009 MBB->getParent()->getJumpTableInfo()-> 1010 ReplaceMBBInJumpTables(MBB, CurTBB); 1011 if (DidChange) { 1012 ++NumBranchOpts; 1013 MadeChange = true; 1014 if (!HasBranchToSelf) return; 1015 } 1016 } 1017 } 1018 1019 // Add the branch back if the block is more than just an uncond branch. 1020 TII->InsertBranch(*MBB, CurTBB, 0, CurCond); 1021 } 1022 } 1023 1024 // If the prior block doesn't fall through into this block, and if this 1025 // block doesn't fall through into some other block, see if we can find a 1026 // place to move this block where a fall-through will happen. 1027 if (!CanFallThrough(&PrevBB, PriorUnAnalyzable, 1028 PriorTBB, PriorFBB, PriorCond)) { 1029 // Now we know that there was no fall-through into this block, check to 1030 // see if it has a fall-through into its successor. 1031 bool CurFallsThru = CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB, 1032 CurCond); 1033 1034 if (!MBB->isLandingPad()) { 1035 // Check all the predecessors of this block. If one of them has no fall 1036 // throughs, move this block right after it. 1037 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(), 1038 E = MBB->pred_end(); PI != E; ++PI) { 1039 // Analyze the branch at the end of the pred. 1040 MachineBasicBlock *PredBB = *PI; 1041 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough; 1042 if (PredBB != MBB && !CanFallThrough(PredBB) 1043 && (!CurFallsThru || !CurTBB || !CurFBB) 1044 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) { 1045 // If the current block doesn't fall through, just move it. 1046 // If the current block can fall through and does not end with a 1047 // conditional branch, we need to append an unconditional jump to 1048 // the (current) next block. To avoid a possible compile-time 1049 // infinite loop, move blocks only backward in this case. 1050 // Also, if there are already 2 branches here, we cannot add a third; 1051 // this means we have the case 1052 // Bcc next 1053 // B elsewhere 1054 // next: 1055 if (CurFallsThru) { 1056 MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB)); 1057 CurCond.clear(); 1058 TII->InsertBranch(*MBB, NextBB, 0, CurCond); 1059 } 1060 MBB->moveAfter(PredBB); 1061 MadeChange = true; 1062 return OptimizeBlock(MBB); 1063 } 1064 } 1065 } 1066 1067 if (!CurFallsThru) { 1068 // Check all successors to see if we can move this block before it. 1069 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(), 1070 E = MBB->succ_end(); SI != E; ++SI) { 1071 // Analyze the branch at the end of the block before the succ. 1072 MachineBasicBlock *SuccBB = *SI; 1073 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev; 1074 std::vector<MachineOperand> SuccPrevCond; 1075 1076 // If this block doesn't already fall-through to that successor, and if 1077 // the succ doesn't already have a block that can fall through into it, 1078 // and if the successor isn't an EH destination, we can arrange for the 1079 // fallthrough to happen. 1080 if (SuccBB != MBB && !CanFallThrough(SuccPrev) && 1081 !SuccBB->isLandingPad()) { 1082 MBB->moveBefore(SuccBB); 1083 MadeChange = true; 1084 return OptimizeBlock(MBB); 1085 } 1086 } 1087 1088 // Okay, there is no really great place to put this block. If, however, 1089 // the block before this one would be a fall-through if this block were 1090 // removed, move this block to the end of the function. 1091 if (FallThrough != MBB->getParent()->end() && 1092 PrevBB.isSuccessor(FallThrough)) { 1093 MBB->moveAfter(--MBB->getParent()->end()); 1094 MadeChange = true; 1095 return; 1096 } 1097 } 1098 } 1099} 1100