BranchFolding.cpp revision 2e48a70b35635165703838fc8d3796b664207aa1
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/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 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 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.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 const TargetInstrInfo *TII) { 351 unsigned Time = 0; 352 for (; I != E; ++I) { 353 const TargetInstrDescriptor &TID = TII->get(I->getOpcode()); 354 if (TID.Flags & M_CALL_FLAG) 355 Time += 10; 356 else if (TID.Flags & (M_LOAD_FLAG|M_MAY_STORE_FLAG)) 357 Time += 2; 358 else 359 ++Time; 360 } 361 return Time; 362} 363 364/// ShouldSplitFirstBlock - We need to either split MBB1 at MBB1I or MBB2 at 365/// MBB2I and then insert an unconditional branch in the other block. Determine 366/// which is the best to split 367static bool ShouldSplitFirstBlock(MachineBasicBlock *MBB1, 368 MachineBasicBlock::iterator MBB1I, 369 MachineBasicBlock *MBB2, 370 MachineBasicBlock::iterator MBB2I, 371 const TargetInstrInfo *TII, 372 MachineBasicBlock *PredBB) { 373 // If one block is the entry block, split the other one; we can't generate 374 // a branch to the entry block, as its label is not emitted. 375 MachineBasicBlock *Entry = MBB1->getParent()->begin(); 376 if (MBB1 == Entry) 377 return false; 378 if (MBB2 == Entry) 379 return true; 380 381 // If one block falls through into the common successor, choose that 382 // one to split; it is one instruction less to do that. 383 if (PredBB) { 384 if (MBB1 == PredBB) 385 return true; 386 else if (MBB2 == PredBB) 387 return false; 388 } 389 // TODO: if we had some notion of which block was hotter, we could split 390 // the hot block, so it is the fall-through. Since we don't have profile info 391 // make a decision based on which will hurt most to split. 392 unsigned MBB1Time = EstimateRuntime(MBB1->begin(), MBB1I, TII); 393 unsigned MBB2Time = EstimateRuntime(MBB2->begin(), MBB2I, TII); 394 395 // If the MBB1 prefix takes "less time" to run than the MBB2 prefix, split the 396 // MBB1 block so it falls through. This will penalize the MBB2 path, but will 397 // have a lower overall impact on the program execution. 398 return MBB1Time < MBB2Time; 399} 400 401// CurMBB needs to add an unconditional branch to SuccMBB (we removed these 402// branches temporarily for tail merging). In the case where CurMBB ends 403// with a conditional branch to the next block, optimize by reversing the 404// test and conditionally branching to SuccMBB instead. 405 406static void FixTail(MachineBasicBlock* CurMBB, MachineBasicBlock *SuccBB, 407 const TargetInstrInfo *TII) { 408 MachineFunction *MF = CurMBB->getParent(); 409 MachineFunction::iterator I = next(MachineFunction::iterator(CurMBB)); 410 MachineBasicBlock *TBB = 0, *FBB = 0; 411 std::vector<MachineOperand> Cond; 412 if (I != MF->end() && 413 !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond)) { 414 MachineBasicBlock *NextBB = I; 415 if (TBB == NextBB && Cond.size() && !FBB) { 416 if (!TII->ReverseBranchCondition(Cond)) { 417 TII->RemoveBranch(*CurMBB); 418 TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond); 419 return; 420 } 421 } 422 } 423 TII->InsertBranch(*CurMBB, SuccBB, NULL, std::vector<MachineOperand>()); 424} 425 426static bool MergeCompare(const std::pair<unsigned,MachineBasicBlock*> &p, 427 const std::pair<unsigned,MachineBasicBlock*> &q) { 428 if (p.first < q.first) 429 return true; 430 else if (p.first > q.first) 431 return false; 432 else if (p.second->getNumber() < q.second->getNumber()) 433 return true; 434 else if (p.second->getNumber() > q.second->getNumber()) 435 return false; 436 else { 437 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing 438 // an object with itself. 439#ifndef _GLIBCXX_DEBUG 440 assert(0 && "Predecessor appears twice"); 441#endif 442 return(false); 443 } 444} 445 446// See if any of the blocks in MergePotentials (which all have a common single 447// successor, or all have no successor) can be tail-merged. If there is a 448// successor, any blocks in MergePotentials that are not tail-merged and 449// are not immediately before Succ must have an unconditional branch to 450// Succ added (but the predecessor/successor lists need no adjustment). 451// The lone predecessor of Succ that falls through into Succ, 452// if any, is given in PredBB. 453 454bool BranchFolder::TryMergeBlocks(MachineBasicBlock *SuccBB, 455 MachineBasicBlock* PredBB) { 456 unsigned minCommonTailLength = (SuccBB ? 1 : 2); 457 MadeChange = false; 458 459 // Sort by hash value so that blocks with identical end sequences sort 460 // together. 461 std::stable_sort(MergePotentials.begin(), MergePotentials.end(), MergeCompare); 462 463 // Walk through equivalence sets looking for actual exact matches. 464 while (MergePotentials.size() > 1) { 465 unsigned CurHash = (MergePotentials.end()-1)->first; 466 unsigned PrevHash = (MergePotentials.end()-2)->first; 467 MachineBasicBlock *CurMBB = (MergePotentials.end()-1)->second; 468 469 // If there is nothing that matches the hash of the current basic block, 470 // give up. 471 if (CurHash != PrevHash) { 472 if (SuccBB && CurMBB != PredBB) 473 FixTail(CurMBB, SuccBB, TII); 474 MergePotentials.pop_back(); 475 continue; 476 } 477 478 // Look through all the pairs of blocks that have the same hash as this 479 // one, and find the pair that has the largest number of instructions in 480 // common. 481 // Since instructions may get combined later (e.g. single stores into 482 // store multiple) this measure is not particularly accurate. 483 MachineBasicBlock::iterator BBI1, BBI2; 484 485 unsigned FoundI = ~0U, FoundJ = ~0U; 486 unsigned maxCommonTailLength = 0U; 487 for (int i = MergePotentials.size()-1; 488 i != -1 && MergePotentials[i].first == CurHash; --i) { 489 for (int j = i-1; 490 j != -1 && MergePotentials[j].first == CurHash; --j) { 491 MachineBasicBlock::iterator TrialBBI1, TrialBBI2; 492 unsigned CommonTailLen = ComputeCommonTailLength( 493 MergePotentials[i].second, 494 MergePotentials[j].second, 495 TrialBBI1, TrialBBI2); 496 if (CommonTailLen >= minCommonTailLength && 497 CommonTailLen > maxCommonTailLength) { 498 FoundI = i; 499 FoundJ = j; 500 maxCommonTailLength = CommonTailLen; 501 BBI1 = TrialBBI1; 502 BBI2 = TrialBBI2; 503 } 504 } 505 } 506 507 // If we didn't find any pair that has at least minCommonTailLength 508 // instructions in common, bail out. All entries with this 509 // hash code can go away now. 510 if (FoundI == ~0U) { 511 for (int i = MergePotentials.size()-1; 512 i != -1 && MergePotentials[i].first == CurHash; --i) { 513 // Put the unconditional branch back, if we need one. 514 CurMBB = MergePotentials[i].second; 515 if (SuccBB && CurMBB != PredBB) 516 FixTail(CurMBB, SuccBB, TII); 517 MergePotentials.pop_back(); 518 } 519 continue; 520 } 521 522 // Otherwise, move the block(s) to the right position(s). So that 523 // BBI1/2 will be valid, the last must be I and the next-to-last J. 524 if (FoundI != MergePotentials.size()-1) 525 std::swap(MergePotentials[FoundI], *(MergePotentials.end()-1)); 526 if (FoundJ != MergePotentials.size()-2) 527 std::swap(MergePotentials[FoundJ], *(MergePotentials.end()-2)); 528 529 CurMBB = (MergePotentials.end()-1)->second; 530 MachineBasicBlock *MBB2 = (MergePotentials.end()-2)->second; 531 532 // If neither block is the entire common tail, split the tail of one block 533 // to make it redundant with the other tail. Also, we cannot jump to the 534 // entry block, so if one block is the entry block, split the other one. 535 MachineBasicBlock *Entry = CurMBB->getParent()->begin(); 536 if (CurMBB->begin() == BBI1 && CurMBB != Entry) 537 ; // CurMBB is common tail 538 else if (MBB2->begin() == BBI2 && MBB2 != Entry) 539 ; // MBB2 is common tail 540 else { 541 if (0) { // Enable this to disable partial tail merges. 542 MergePotentials.pop_back(); 543 continue; 544 } 545 546 // Decide whether we want to split CurMBB or MBB2. 547 if (ShouldSplitFirstBlock(CurMBB, BBI1, MBB2, BBI2, TII, PredBB)) { 548 CurMBB = SplitMBBAt(*CurMBB, BBI1); 549 BBI1 = CurMBB->begin(); 550 MergePotentials.back().second = CurMBB; 551 } else { 552 MBB2 = SplitMBBAt(*MBB2, BBI2); 553 BBI2 = MBB2->begin(); 554 (MergePotentials.end()-2)->second = MBB2; 555 } 556 } 557 558 if (MBB2->begin() == BBI2 && MBB2 != Entry) { 559 // Hack the end off CurMBB, making it jump to MBBI@ instead. 560 ReplaceTailWithBranchTo(BBI1, MBB2); 561 // This modifies CurMBB, so remove it from the worklist. 562 MergePotentials.pop_back(); 563 } else { 564 assert(CurMBB->begin() == BBI1 && CurMBB != Entry && 565 "Didn't split block correctly?"); 566 // Hack the end off MBB2, making it jump to CurMBB instead. 567 ReplaceTailWithBranchTo(BBI2, CurMBB); 568 // This modifies MBB2, so remove it from the worklist. 569 MergePotentials.erase(MergePotentials.end()-2); 570 } 571 MadeChange = true; 572 } 573 return MadeChange; 574} 575 576bool BranchFolder::TailMergeBlocks(MachineFunction &MF) { 577 578 if (!EnableTailMerge) return false; 579 580 MadeChange = false; 581 582 // First find blocks with no successors. 583 MergePotentials.clear(); 584 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { 585 if (I->succ_empty()) 586 MergePotentials.push_back(std::make_pair(HashEndOfMBB(I, 2U), I)); 587 } 588 // See if we can do any tail merging on those. 589 if (MergePotentials.size() < TailMergeThreshold) 590 MadeChange |= TryMergeBlocks(NULL, NULL); 591 592 // Look at blocks (IBB) with multiple predecessors (PBB). 593 // We change each predecessor to a canonical form, by 594 // (1) temporarily removing any unconditional branch from the predecessor 595 // to IBB, and 596 // (2) alter conditional branches so they branch to the other block 597 // not IBB; this may require adding back an unconditional branch to IBB 598 // later, where there wasn't one coming in. E.g. 599 // Bcc IBB 600 // fallthrough to QBB 601 // here becomes 602 // Bncc QBB 603 // with a conceptual B to IBB after that, which never actually exists. 604 // With those changes, we see whether the predecessors' tails match, 605 // and merge them if so. We change things out of canonical form and 606 // back to the way they were later in the process. (OptimizeBranches 607 // would undo some of this, but we can't use it, because we'd get into 608 // a compile-time infinite loop repeatedly doing and undoing the same 609 // transformations.) 610 611 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { 612 if (!I->succ_empty() && I->pred_size() >= 2 && 613 I->pred_size() < TailMergeThreshold) { 614 MachineBasicBlock *IBB = I; 615 MachineBasicBlock *PredBB = prior(I); 616 MergePotentials.clear(); 617 for (MachineBasicBlock::pred_iterator P = I->pred_begin(), 618 E2 = I->pred_end(); 619 P != E2; ++P) { 620 MachineBasicBlock* PBB = *P; 621 // Skip blocks that loop to themselves, can't tail merge these. 622 if (PBB==IBB) 623 continue; 624 MachineBasicBlock *TBB = 0, *FBB = 0; 625 std::vector<MachineOperand> Cond; 626 if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond)) { 627 // Failing case: IBB is the target of a cbr, and 628 // we cannot reverse the branch. 629 std::vector<MachineOperand> NewCond(Cond); 630 if (Cond.size() && TBB==IBB) { 631 if (TII->ReverseBranchCondition(NewCond)) 632 continue; 633 // This is the QBB case described above 634 if (!FBB) 635 FBB = next(MachineFunction::iterator(PBB)); 636 } 637 // Failing case: the only way IBB can be reached from PBB is via 638 // exception handling. Happens for landing pads. Would be nice 639 // to have a bit in the edge so we didn't have to do all this. 640 if (IBB->isLandingPad()) { 641 MachineFunction::iterator IP = PBB; IP++; 642 MachineBasicBlock* PredNextBB = NULL; 643 if (IP!=MF.end()) 644 PredNextBB = IP; 645 if (TBB==NULL) { 646 if (IBB!=PredNextBB) // fallthrough 647 continue; 648 } else if (FBB) { 649 if (TBB!=IBB && FBB!=IBB) // cbr then ubr 650 continue; 651 } else if (Cond.size() == 0) { 652 if (TBB!=IBB) // ubr 653 continue; 654 } else { 655 if (TBB!=IBB && IBB!=PredNextBB) // cbr 656 continue; 657 } 658 } 659 // Remove the unconditional branch at the end, if any. 660 if (TBB && (Cond.size()==0 || FBB)) { 661 TII->RemoveBranch(*PBB); 662 if (Cond.size()) 663 // reinsert conditional branch only, for now 664 TII->InsertBranch(*PBB, (TBB==IBB) ? FBB : TBB, 0, NewCond); 665 } 666 MergePotentials.push_back(std::make_pair(HashEndOfMBB(PBB, 1U), *P)); 667 } 668 } 669 if (MergePotentials.size() >= 2) 670 MadeChange |= TryMergeBlocks(I, PredBB); 671 // Reinsert an unconditional branch if needed. 672 // The 1 below can be either an original single predecessor, or a result 673 // of removing blocks in TryMergeBlocks. 674 PredBB = prior(I); // this may have been changed in TryMergeBlocks 675 if (MergePotentials.size()==1 && 676 (MergePotentials.begin())->second != PredBB) 677 FixTail((MergePotentials.begin())->second, I, TII); 678 } 679 } 680 return MadeChange; 681} 682 683//===----------------------------------------------------------------------===// 684// Branch Optimization 685//===----------------------------------------------------------------------===// 686 687bool BranchFolder::OptimizeBranches(MachineFunction &MF) { 688 MadeChange = false; 689 690 // Make sure blocks are numbered in order 691 MF.RenumberBlocks(); 692 693 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) { 694 MachineBasicBlock *MBB = I++; 695 OptimizeBlock(MBB); 696 697 // If it is dead, remove it. 698 if (MBB->pred_empty()) { 699 RemoveDeadBlock(MBB); 700 MadeChange = true; 701 ++NumDeadBlocks; 702 } 703 } 704 return MadeChange; 705} 706 707 708/// CanFallThrough - Return true if the specified block (with the specified 709/// branch condition) can implicitly transfer control to the block after it by 710/// falling off the end of it. This should return false if it can reach the 711/// block after it, but it uses an explicit branch to do so (e.g. a table jump). 712/// 713/// True is a conservative answer. 714/// 715bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB, 716 bool BranchUnAnalyzable, 717 MachineBasicBlock *TBB, MachineBasicBlock *FBB, 718 const std::vector<MachineOperand> &Cond) { 719 MachineFunction::iterator Fallthrough = CurBB; 720 ++Fallthrough; 721 // If FallthroughBlock is off the end of the function, it can't fall through. 722 if (Fallthrough == CurBB->getParent()->end()) 723 return false; 724 725 // If FallthroughBlock isn't a successor of CurBB, no fallthrough is possible. 726 if (!CurBB->isSuccessor(Fallthrough)) 727 return false; 728 729 // If we couldn't analyze the branch, assume it could fall through. 730 if (BranchUnAnalyzable) return true; 731 732 // If there is no branch, control always falls through. 733 if (TBB == 0) return true; 734 735 // If there is some explicit branch to the fallthrough block, it can obviously 736 // reach, even though the branch should get folded to fall through implicitly. 737 if (MachineFunction::iterator(TBB) == Fallthrough || 738 MachineFunction::iterator(FBB) == Fallthrough) 739 return true; 740 741 // If it's an unconditional branch to some block not the fall through, it 742 // doesn't fall through. 743 if (Cond.empty()) return false; 744 745 // Otherwise, if it is conditional and has no explicit false block, it falls 746 // through. 747 return FBB == 0; 748} 749 750/// CanFallThrough - Return true if the specified can implicitly transfer 751/// control to the block after it by falling off the end of it. This should 752/// return false if it can reach the block after it, but it uses an explicit 753/// branch to do so (e.g. a table jump). 754/// 755/// True is a conservative answer. 756/// 757bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) { 758 MachineBasicBlock *TBB = 0, *FBB = 0; 759 std::vector<MachineOperand> Cond; 760 bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond); 761 return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond); 762} 763 764/// IsBetterFallthrough - Return true if it would be clearly better to 765/// fall-through to MBB1 than to fall through into MBB2. This has to return 766/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will 767/// result in infinite loops. 768static bool IsBetterFallthrough(MachineBasicBlock *MBB1, 769 MachineBasicBlock *MBB2, 770 const TargetInstrInfo &TII) { 771 // Right now, we use a simple heuristic. If MBB2 ends with a call, and 772 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to 773 // optimize branches that branch to either a return block or an assert block 774 // into a fallthrough to the return. 775 if (MBB1->empty() || MBB2->empty()) return false; 776 777 // If there is a clear successor ordering we make sure that one block 778 // will fall through to the next 779 if (MBB1->isSuccessor(MBB2)) return true; 780 if (MBB2->isSuccessor(MBB1)) return false; 781 782 MachineInstr *MBB1I = --MBB1->end(); 783 MachineInstr *MBB2I = --MBB2->end(); 784 return TII.isCall(MBB2I->getOpcode()) && !TII.isCall(MBB1I->getOpcode()); 785} 786 787/// OptimizeBlock - Analyze and optimize control flow related to the specified 788/// block. This is never called on the entry block. 789void BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) { 790 MachineFunction::iterator FallThrough = MBB; 791 ++FallThrough; 792 793 // If this block is empty, make everyone use its fall-through, not the block 794 // explicitly. Landing pads should not do this since the landing-pad table 795 // points to this block. 796 if (MBB->empty() && !MBB->isLandingPad()) { 797 // Dead block? Leave for cleanup later. 798 if (MBB->pred_empty()) return; 799 800 if (FallThrough == MBB->getParent()->end()) { 801 // TODO: Simplify preds to not branch here if possible! 802 } else { 803 // Rewrite all predecessors of the old block to go to the fallthrough 804 // instead. 805 while (!MBB->pred_empty()) { 806 MachineBasicBlock *Pred = *(MBB->pred_end()-1); 807 Pred->ReplaceUsesOfBlockWith(MBB, FallThrough); 808 } 809 810 // If MBB was the target of a jump table, update jump tables to go to the 811 // fallthrough instead. 812 MBB->getParent()->getJumpTableInfo()-> 813 ReplaceMBBInJumpTables(MBB, FallThrough); 814 MadeChange = true; 815 } 816 return; 817 } 818 819 // Check to see if we can simplify the terminator of the block before this 820 // one. 821 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB)); 822 823 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0; 824 std::vector<MachineOperand> PriorCond; 825 bool PriorUnAnalyzable = 826 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); 827 if (!PriorUnAnalyzable) { 828 // If the CFG for the prior block has extra edges, remove them. 829 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB, 830 !PriorCond.empty()); 831 832 // If the previous branch is conditional and both conditions go to the same 833 // destination, remove the branch, replacing it with an unconditional one or 834 // a fall-through. 835 if (PriorTBB && PriorTBB == PriorFBB) { 836 TII->RemoveBranch(PrevBB); 837 PriorCond.clear(); 838 if (PriorTBB != MBB) 839 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 840 MadeChange = true; 841 ++NumBranchOpts; 842 return OptimizeBlock(MBB); 843 } 844 845 // If the previous branch *only* branches to *this* block (conditional or 846 // not) remove the branch. 847 if (PriorTBB == MBB && PriorFBB == 0) { 848 TII->RemoveBranch(PrevBB); 849 MadeChange = true; 850 ++NumBranchOpts; 851 return OptimizeBlock(MBB); 852 } 853 854 // If the prior block branches somewhere else on the condition and here if 855 // the condition is false, remove the uncond second branch. 856 if (PriorFBB == MBB) { 857 TII->RemoveBranch(PrevBB); 858 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 859 MadeChange = true; 860 ++NumBranchOpts; 861 return OptimizeBlock(MBB); 862 } 863 864 // If the prior block branches here on true and somewhere else on false, and 865 // if the branch condition is reversible, reverse the branch to create a 866 // fall-through. 867 if (PriorTBB == MBB) { 868 std::vector<MachineOperand> NewPriorCond(PriorCond); 869 if (!TII->ReverseBranchCondition(NewPriorCond)) { 870 TII->RemoveBranch(PrevBB); 871 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond); 872 MadeChange = true; 873 ++NumBranchOpts; 874 return OptimizeBlock(MBB); 875 } 876 } 877 878 // If this block doesn't fall through (e.g. it ends with an uncond branch or 879 // has no successors) and if the pred falls through into this block, and if 880 // it would otherwise fall through into the block after this, move this 881 // block to the end of the function. 882 // 883 // We consider it more likely that execution will stay in the function (e.g. 884 // due to loops) than it is to exit it. This asserts in loops etc, moving 885 // the assert condition out of the loop body. 886 if (!PriorCond.empty() && PriorFBB == 0 && 887 MachineFunction::iterator(PriorTBB) == FallThrough && 888 !CanFallThrough(MBB)) { 889 bool DoTransform = true; 890 891 // We have to be careful that the succs of PredBB aren't both no-successor 892 // blocks. If neither have successors and if PredBB is the second from 893 // last block in the function, we'd just keep swapping the two blocks for 894 // last. Only do the swap if one is clearly better to fall through than 895 // the other. 896 if (FallThrough == --MBB->getParent()->end() && 897 !IsBetterFallthrough(PriorTBB, MBB, *TII)) 898 DoTransform = false; 899 900 // We don't want to do this transformation if we have control flow like: 901 // br cond BB2 902 // BB1: 903 // .. 904 // jmp BBX 905 // BB2: 906 // .. 907 // ret 908 // 909 // In this case, we could actually be moving the return block *into* a 910 // loop! 911 if (DoTransform && !MBB->succ_empty() && 912 (!CanFallThrough(PriorTBB) || PriorTBB->empty())) 913 DoTransform = false; 914 915 916 if (DoTransform) { 917 // Reverse the branch so we will fall through on the previous true cond. 918 std::vector<MachineOperand> NewPriorCond(PriorCond); 919 if (!TII->ReverseBranchCondition(NewPriorCond)) { 920 DOUT << "\nMoving MBB: " << *MBB; 921 DOUT << "To make fallthrough to: " << *PriorTBB << "\n"; 922 923 TII->RemoveBranch(PrevBB); 924 TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond); 925 926 // Move this block to the end of the function. 927 MBB->moveAfter(--MBB->getParent()->end()); 928 MadeChange = true; 929 ++NumBranchOpts; 930 return; 931 } 932 } 933 } 934 } 935 936 // Analyze the branch in the current block. 937 MachineBasicBlock *CurTBB = 0, *CurFBB = 0; 938 std::vector<MachineOperand> CurCond; 939 bool CurUnAnalyzable = TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond); 940 if (!CurUnAnalyzable) { 941 // If the CFG for the prior block has extra edges, remove them. 942 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty()); 943 944 // If this is a two-way branch, and the FBB branches to this block, reverse 945 // the condition so the single-basic-block loop is faster. Instead of: 946 // Loop: xxx; jcc Out; jmp Loop 947 // we want: 948 // Loop: xxx; jncc Loop; jmp Out 949 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) { 950 std::vector<MachineOperand> NewCond(CurCond); 951 if (!TII->ReverseBranchCondition(NewCond)) { 952 TII->RemoveBranch(*MBB); 953 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond); 954 MadeChange = true; 955 ++NumBranchOpts; 956 return OptimizeBlock(MBB); 957 } 958 } 959 960 961 // If this branch is the only thing in its block, see if we can forward 962 // other blocks across it. 963 if (CurTBB && CurCond.empty() && CurFBB == 0 && 964 TII->isBranch(MBB->begin()->getOpcode()) && CurTBB != MBB) { 965 // This block may contain just an unconditional branch. Because there can 966 // be 'non-branch terminators' in the block, try removing the branch and 967 // then seeing if the block is empty. 968 TII->RemoveBranch(*MBB); 969 970 // If this block is just an unconditional branch to CurTBB, we can 971 // usually completely eliminate the block. The only case we cannot 972 // completely eliminate the block is when the block before this one 973 // falls through into MBB and we can't understand the prior block's branch 974 // condition. 975 if (MBB->empty()) { 976 bool PredHasNoFallThrough = TII->BlockHasNoFallThrough(PrevBB); 977 if (PredHasNoFallThrough || !PriorUnAnalyzable || 978 !PrevBB.isSuccessor(MBB)) { 979 // If the prior block falls through into us, turn it into an 980 // explicit branch to us to make updates simpler. 981 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) && 982 PriorTBB != MBB && PriorFBB != MBB) { 983 if (PriorTBB == 0) { 984 assert(PriorCond.empty() && PriorFBB == 0 && 985 "Bad branch analysis"); 986 PriorTBB = MBB; 987 } else { 988 assert(PriorFBB == 0 && "Machine CFG out of date!"); 989 PriorFBB = MBB; 990 } 991 TII->RemoveBranch(PrevBB); 992 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); 993 } 994 995 // Iterate through all the predecessors, revectoring each in-turn. 996 size_t PI = 0; 997 bool DidChange = false; 998 bool HasBranchToSelf = false; 999 while(PI != MBB->pred_size()) { 1000 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI); 1001 if (PMBB == MBB) { 1002 // If this block has an uncond branch to itself, leave it. 1003 ++PI; 1004 HasBranchToSelf = true; 1005 } else { 1006 DidChange = true; 1007 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB); 1008 } 1009 } 1010 1011 // Change any jumptables to go to the new MBB. 1012 MBB->getParent()->getJumpTableInfo()-> 1013 ReplaceMBBInJumpTables(MBB, CurTBB); 1014 if (DidChange) { 1015 ++NumBranchOpts; 1016 MadeChange = true; 1017 if (!HasBranchToSelf) return; 1018 } 1019 } 1020 } 1021 1022 // Add the branch back if the block is more than just an uncond branch. 1023 TII->InsertBranch(*MBB, CurTBB, 0, CurCond); 1024 } 1025 } 1026 1027 // If the prior block doesn't fall through into this block, and if this 1028 // block doesn't fall through into some other block, see if we can find a 1029 // place to move this block where a fall-through will happen. 1030 if (!CanFallThrough(&PrevBB, PriorUnAnalyzable, 1031 PriorTBB, PriorFBB, PriorCond)) { 1032 // Now we know that there was no fall-through into this block, check to 1033 // see if it has a fall-through into its successor. 1034 bool CurFallsThru = CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB, 1035 CurCond); 1036 1037 if (!MBB->isLandingPad()) { 1038 // Check all the predecessors of this block. If one of them has no fall 1039 // throughs, move this block right after it. 1040 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(), 1041 E = MBB->pred_end(); PI != E; ++PI) { 1042 // Analyze the branch at the end of the pred. 1043 MachineBasicBlock *PredBB = *PI; 1044 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough; 1045 if (PredBB != MBB && !CanFallThrough(PredBB) 1046 && (!CurFallsThru || !CurTBB || !CurFBB) 1047 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) { 1048 // If the current block doesn't fall through, just move it. 1049 // If the current block can fall through and does not end with a 1050 // conditional branch, we need to append an unconditional jump to 1051 // the (current) next block. To avoid a possible compile-time 1052 // infinite loop, move blocks only backward in this case. 1053 // Also, if there are already 2 branches here, we cannot add a third; 1054 // this means we have the case 1055 // Bcc next 1056 // B elsewhere 1057 // next: 1058 if (CurFallsThru) { 1059 MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB)); 1060 CurCond.clear(); 1061 TII->InsertBranch(*MBB, NextBB, 0, CurCond); 1062 } 1063 MBB->moveAfter(PredBB); 1064 MadeChange = true; 1065 return OptimizeBlock(MBB); 1066 } 1067 } 1068 } 1069 1070 if (!CurFallsThru) { 1071 // Check all successors to see if we can move this block before it. 1072 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(), 1073 E = MBB->succ_end(); SI != E; ++SI) { 1074 // Analyze the branch at the end of the block before the succ. 1075 MachineBasicBlock *SuccBB = *SI; 1076 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev; 1077 std::vector<MachineOperand> SuccPrevCond; 1078 1079 // If this block doesn't already fall-through to that successor, and if 1080 // the succ doesn't already have a block that can fall through into it, 1081 // and if the successor isn't an EH destination, we can arrange for the 1082 // fallthrough to happen. 1083 if (SuccBB != MBB && !CanFallThrough(SuccPrev) && 1084 !SuccBB->isLandingPad()) { 1085 MBB->moveBefore(SuccBB); 1086 MadeChange = true; 1087 return OptimizeBlock(MBB); 1088 } 1089 } 1090 1091 // Okay, there is no really great place to put this block. If, however, 1092 // the block before this one would be a fall-through if this block were 1093 // removed, move this block to the end of the function. 1094 if (FallThrough != MBB->getParent()->end() && 1095 PrevBB.isSuccessor(FallThrough)) { 1096 MBB->moveAfter(--MBB->getParent()->end()); 1097 MadeChange = true; 1098 return; 1099 } 1100 } 1101 } 1102} 1103