BranchFolding.cpp revision 80629c85f1041df41b5158ebb03a4725af6ecd90
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 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.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->getOpcode() == TargetInstrInfo::INLINEASM) { 276 ++I1; ++I2; 277 break; 278 } 279 ++TailLen; 280 } 281 return TailLen; 282} 283 284/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything 285/// after it, replacing it with an unconditional branch to NewDest. This 286/// returns true if OldInst's block is modified, false if NewDest is modified. 287void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, 288 MachineBasicBlock *NewDest) { 289 MachineBasicBlock *OldBB = OldInst->getParent(); 290 291 // Remove all the old successors of OldBB from the CFG. 292 while (!OldBB->succ_empty()) 293 OldBB->removeSuccessor(OldBB->succ_begin()); 294 295 // Remove all the dead instructions from the end of OldBB. 296 OldBB->erase(OldInst, OldBB->end()); 297 298 // If OldBB isn't immediately before OldBB, insert a branch to it. 299 if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest)) 300 TII->InsertBranch(*OldBB, NewDest, 0, std::vector<MachineOperand>()); 301 OldBB->addSuccessor(NewDest); 302 ++NumTailMerge; 303} 304 305/// SplitMBBAt - Given a machine basic block and an iterator into it, split the 306/// MBB so that the part before the iterator falls into the part starting at the 307/// iterator. This returns the new MBB. 308MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB, 309 MachineBasicBlock::iterator BBI1) { 310 // Create the fall-through block. 311 MachineFunction::iterator MBBI = &CurMBB; 312 MachineBasicBlock *NewMBB = new MachineBasicBlock(CurMBB.getBasicBlock()); 313 CurMBB.getParent()->getBasicBlockList().insert(++MBBI, NewMBB); 314 315 // Move all the successors of this block to the specified block. 316 while (!CurMBB.succ_empty()) { 317 MachineBasicBlock *S = *(CurMBB.succ_end()-1); 318 NewMBB->addSuccessor(S); 319 CurMBB.removeSuccessor(S); 320 } 321 322 // Add an edge from CurMBB to NewMBB for the fall-through. 323 CurMBB.addSuccessor(NewMBB); 324 325 // Splice the code over. 326 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end()); 327 328 // For targets that use the register scavenger, we must maintain LiveIns. 329 if (RS) { 330 RS->enterBasicBlock(&CurMBB); 331 if (!CurMBB.empty()) 332 RS->forward(prior(CurMBB.end())); 333 BitVector RegsLiveAtExit(RegInfo->getNumRegs()); 334 RS->getRegsUsed(RegsLiveAtExit, false); 335 for (unsigned int i=0, e=RegInfo->getNumRegs(); i!=e; i++) 336 if (RegsLiveAtExit[i]) 337 NewMBB->addLiveIn(i); 338 } 339 340 return NewMBB; 341} 342 343/// EstimateRuntime - Make a rough estimate for how long it will take to run 344/// the specified code. 345static unsigned EstimateRuntime(MachineBasicBlock::iterator I, 346 MachineBasicBlock::iterator E, 347 const TargetInstrInfo *TII) { 348 unsigned Time = 0; 349 for (; I != E; ++I) { 350 const TargetInstrDescriptor &TID = TII->get(I->getOpcode()); 351 if (TID.Flags & M_CALL_FLAG) 352 Time += 10; 353 else if (TID.Flags & (M_LOAD_FLAG|M_STORE_FLAG)) 354 Time += 2; 355 else 356 ++Time; 357 } 358 return Time; 359} 360 361/// ShouldSplitFirstBlock - We need to either split MBB1 at MBB1I or MBB2 at 362/// MBB2I and then insert an unconditional branch in the other block. Determine 363/// which is the best to split 364static bool ShouldSplitFirstBlock(MachineBasicBlock *MBB1, 365 MachineBasicBlock::iterator MBB1I, 366 MachineBasicBlock *MBB2, 367 MachineBasicBlock::iterator MBB2I, 368 const TargetInstrInfo *TII, 369 MachineBasicBlock *PredBB) { 370 // If one block is the entry block, split the other one; we can't generate 371 // a branch to the entry block, as its label is not emitted. 372 MachineBasicBlock *Entry = MBB1->getParent()->begin(); 373 if (MBB1 == Entry) 374 return false; 375 if (MBB2 == Entry) 376 return true; 377 378 // If one block falls through into the common successor, choose that 379 // one to split; it is one instruction less to do that. 380 if (PredBB) { 381 if (MBB1 == PredBB) 382 return true; 383 else if (MBB2 == PredBB) 384 return false; 385 } 386 // TODO: if we had some notion of which block was hotter, we could split 387 // the hot block, so it is the fall-through. Since we don't have profile info 388 // make a decision based on which will hurt most to split. 389 unsigned MBB1Time = EstimateRuntime(MBB1->begin(), MBB1I, TII); 390 unsigned MBB2Time = EstimateRuntime(MBB2->begin(), MBB2I, TII); 391 392 // If the MBB1 prefix takes "less time" to run than the MBB2 prefix, split the 393 // MBB1 block so it falls through. This will penalize the MBB2 path, but will 394 // have a lower overall impact on the program execution. 395 return MBB1Time < MBB2Time; 396} 397 398// CurMBB needs to add an unconditional branch to SuccMBB (we removed these 399// branches temporarily for tail merging). In the case where CurMBB ends 400// with a conditional branch to the next block, optimize by reversing the 401// test and conditionally branching to SuccMBB instead. 402 403static void FixTail(MachineBasicBlock* CurMBB, MachineBasicBlock *SuccBB, 404 const TargetInstrInfo *TII) { 405 MachineFunction *MF = CurMBB->getParent(); 406 MachineFunction::iterator I = next(MachineFunction::iterator(CurMBB)); 407 MachineBasicBlock *TBB = 0, *FBB = 0; 408 std::vector<MachineOperand> Cond; 409 if (I != MF->end() && 410 !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond)) { 411 MachineBasicBlock *NextBB = I; 412 if (TBB == NextBB && Cond.size() && !FBB) { 413 if (!TII->ReverseBranchCondition(Cond)) { 414 TII->RemoveBranch(*CurMBB); 415 TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond); 416 return; 417 } 418 } 419 } 420 TII->InsertBranch(*CurMBB, SuccBB, NULL, std::vector<MachineOperand>()); 421} 422 423static bool MergeCompare(const std::pair<unsigned,MachineBasicBlock*> &p, 424 const std::pair<unsigned,MachineBasicBlock*> &q) { 425 if (p.first < q.first) 426 return true; 427 else if (p.first > q.first) 428 return false; 429 else if (p.second->getNumber() < q.second->getNumber()) 430 return true; 431 else if (p.second->getNumber() > q.second->getNumber()) 432 return false; 433 else { 434 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing 435 // an object with itself. 436#ifndef _GLIBCXX_DEBUG 437 assert(0 && "Predecessor appears twice"); 438#endif 439 return(false); 440 } 441} 442 443// See if any of the blocks in MergePotentials (which all have a common single 444// successor, or all have no successor) can be tail-merged. If there is a 445// successor, any blocks in MergePotentials that are not tail-merged and 446// are not immediately before Succ must have an unconditional branch to 447// Succ added (but the predecessor/successor lists need no adjustment). 448// The lone predecessor of Succ that falls through into Succ, 449// if any, is given in PredBB. 450 451bool BranchFolder::TryMergeBlocks(MachineBasicBlock *SuccBB, 452 MachineBasicBlock* PredBB) { 453 unsigned minCommonTailLength = (SuccBB ? 1 : 2); 454 MadeChange = false; 455 456 // Sort by hash value so that blocks with identical end sequences sort 457 // together. 458 std::stable_sort(MergePotentials.begin(), MergePotentials.end(), MergeCompare); 459 460 // Walk through equivalence sets looking for actual exact matches. 461 while (MergePotentials.size() > 1) { 462 unsigned CurHash = (MergePotentials.end()-1)->first; 463 unsigned PrevHash = (MergePotentials.end()-2)->first; 464 MachineBasicBlock *CurMBB = (MergePotentials.end()-1)->second; 465 466 // If there is nothing that matches the hash of the current basic block, 467 // give up. 468 if (CurHash != PrevHash) { 469 if (SuccBB && CurMBB != PredBB) 470 FixTail(CurMBB, SuccBB, TII); 471 MergePotentials.pop_back(); 472 continue; 473 } 474 475 // Look through all the pairs of blocks that have the same hash as this 476 // one, and find the pair that has the largest number of instructions in 477 // common. 478 // Since instructions may get combined later (e.g. single stores into 479 // store multiple) this measure is not particularly accurate. 480 MachineBasicBlock::iterator BBI1, BBI2; 481 482 unsigned FoundI = ~0U, FoundJ = ~0U; 483 unsigned maxCommonTailLength = 0U; 484 for (int i = MergePotentials.size()-1; 485 i != -1 && MergePotentials[i].first == CurHash; --i) { 486 for (int j = i-1; 487 j != -1 && MergePotentials[j].first == CurHash; --j) { 488 MachineBasicBlock::iterator TrialBBI1, TrialBBI2; 489 unsigned CommonTailLen = ComputeCommonTailLength( 490 MergePotentials[i].second, 491 MergePotentials[j].second, 492 TrialBBI1, TrialBBI2); 493 if (CommonTailLen >= minCommonTailLength && 494 CommonTailLen > maxCommonTailLength) { 495 FoundI = i; 496 FoundJ = j; 497 maxCommonTailLength = CommonTailLen; 498 BBI1 = TrialBBI1; 499 BBI2 = TrialBBI2; 500 } 501 } 502 } 503 504 // If we didn't find any pair that has at least minCommonTailLength 505 // instructions in common, bail out. All entries with this 506 // hash code can go away now. 507 if (FoundI == ~0U) { 508 for (int i = MergePotentials.size()-1; 509 i != -1 && MergePotentials[i].first == CurHash; --i) { 510 // Put the unconditional branch back, if we need one. 511 CurMBB = MergePotentials[i].second; 512 if (SuccBB && CurMBB != PredBB) 513 FixTail(CurMBB, SuccBB, TII); 514 MergePotentials.pop_back(); 515 } 516 continue; 517 } 518 519 // Otherwise, move the block(s) to the right position(s). So that 520 // BBI1/2 will be valid, the last must be I and the next-to-last J. 521 if (FoundI != MergePotentials.size()-1) 522 std::swap(MergePotentials[FoundI], *(MergePotentials.end()-1)); 523 if (FoundJ != MergePotentials.size()-2) 524 std::swap(MergePotentials[FoundJ], *(MergePotentials.end()-2)); 525 526 CurMBB = (MergePotentials.end()-1)->second; 527 MachineBasicBlock *MBB2 = (MergePotentials.end()-2)->second; 528 529 // If neither block is the entire common tail, split the tail of one block 530 // to make it redundant with the other tail. Also, we cannot jump to the 531 // entry block, so if one block is the entry block, split the other one. 532 MachineBasicBlock *Entry = CurMBB->getParent()->begin(); 533 if (CurMBB->begin() == BBI1 && CurMBB != Entry) 534 ; // CurMBB is common tail 535 else if (MBB2->begin() == BBI2 && MBB2 != Entry) 536 ; // MBB2 is common tail 537 else { 538 if (0) { // Enable this to disable partial tail merges. 539 MergePotentials.pop_back(); 540 continue; 541 } 542 543 // Decide whether we want to split CurMBB or MBB2. 544 if (ShouldSplitFirstBlock(CurMBB, BBI1, MBB2, BBI2, TII, PredBB)) { 545 CurMBB = SplitMBBAt(*CurMBB, BBI1); 546 BBI1 = CurMBB->begin(); 547 MergePotentials.back().second = CurMBB; 548 } else { 549 MBB2 = SplitMBBAt(*MBB2, BBI2); 550 BBI2 = MBB2->begin(); 551 (MergePotentials.end()-2)->second = MBB2; 552 } 553 } 554 555 if (MBB2->begin() == BBI2 && MBB2 != Entry) { 556 // Hack the end off CurMBB, making it jump to MBBI@ instead. 557 ReplaceTailWithBranchTo(BBI1, MBB2); 558 // This modifies CurMBB, so remove it from the worklist. 559 MergePotentials.pop_back(); 560 } else { 561 assert(CurMBB->begin() == BBI1 && CurMBB != Entry && 562 "Didn't split block correctly?"); 563 // Hack the end off MBB2, making it jump to CurMBB instead. 564 ReplaceTailWithBranchTo(BBI2, CurMBB); 565 // This modifies MBB2, so remove it from the worklist. 566 MergePotentials.erase(MergePotentials.end()-2); 567 } 568 MadeChange = true; 569 } 570 return MadeChange; 571} 572 573bool BranchFolder::TailMergeBlocks(MachineFunction &MF) { 574 575 if (!EnableTailMerge) return false; 576 577 MadeChange = false; 578 579 // First find blocks with no successors. 580 MergePotentials.clear(); 581 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { 582 if (I->succ_empty()) 583 MergePotentials.push_back(std::make_pair(HashEndOfMBB(I, 2U), I)); 584 } 585 // See if we can do any tail merging on those. 586 if (MergePotentials.size() < TailMergeThreshold) 587 MadeChange |= TryMergeBlocks(NULL, NULL); 588 589 // Look at blocks (IBB) with multiple predecessors (PBB). 590 // We change each predecessor to a canonical form, by 591 // (1) temporarily removing any unconditional branch from the predecessor 592 // to IBB, and 593 // (2) alter conditional branches so they branch to the other block 594 // not IBB; this may require adding back an unconditional branch to IBB 595 // later, where there wasn't one coming in. E.g. 596 // Bcc IBB 597 // fallthrough to QBB 598 // here becomes 599 // Bncc QBB 600 // with a conceptual B to IBB after that, which never actually exists. 601 // With those changes, we see whether the predecessors' tails match, 602 // and merge them if so. We change things out of canonical form and 603 // back to the way they were later in the process. (OptimizeBranches 604 // would undo some of this, but we can't use it, because we'd get into 605 // a compile-time infinite loop repeatedly doing and undoing the same 606 // transformations.) 607 608 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { 609 if (!I->succ_empty() && I->pred_size() >= 2 && 610 I->pred_size() < TailMergeThreshold) { 611 MachineBasicBlock *IBB = I; 612 MachineBasicBlock *PredBB = prior(I); 613 MergePotentials.clear(); 614 for (MachineBasicBlock::pred_iterator P = I->pred_begin(), 615 E2 = I->pred_end(); 616 P != E2; ++P) { 617 MachineBasicBlock* PBB = *P; 618 // Skip blocks that loop to themselves, can't tail merge these. 619 if (PBB==IBB) 620 continue; 621 MachineBasicBlock *TBB = 0, *FBB = 0; 622 std::vector<MachineOperand> Cond; 623 if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond)) { 624 // Failing case: IBB is the target of a cbr, and 625 // we cannot reverse the branch. 626 std::vector<MachineOperand> NewCond(Cond); 627 if (Cond.size() && TBB==IBB) { 628 if (TII->ReverseBranchCondition(NewCond)) 629 continue; 630 // This is the QBB case described above 631 if (!FBB) 632 FBB = next(MachineFunction::iterator(PBB)); 633 } 634 // Failing case: the only way IBB can be reached from PBB is via 635 // exception handling. Happens for landing pads. Would be nice 636 // to have a bit in the edge so we didn't have to do all this. 637 if (IBB->isLandingPad()) { 638 MachineFunction::iterator IP = PBB; IP++; 639 MachineBasicBlock* PredNextBB = NULL; 640 if (IP!=MF.end()) 641 PredNextBB = IP; 642 if (TBB==NULL) { 643 if (IBB!=PredNextBB) // fallthrough 644 continue; 645 } else if (FBB) { 646 if (TBB!=IBB && FBB!=IBB) // cbr then ubr 647 continue; 648 } else if (Cond.size() == 0) { 649 if (TBB!=IBB) // ubr 650 continue; 651 } else { 652 if (TBB!=IBB && IBB!=PredNextBB) // cbr 653 continue; 654 } 655 } 656 // Remove the unconditional branch at the end, if any. 657 if (TBB && (Cond.size()==0 || FBB)) { 658 TII->RemoveBranch(*PBB); 659 if (Cond.size()) 660 // reinsert conditional branch only, for now 661 TII->InsertBranch(*PBB, (TBB==IBB) ? FBB : TBB, 0, NewCond); 662 } 663 MergePotentials.push_back(std::make_pair(HashEndOfMBB(PBB, 1U), *P)); 664 } 665 } 666 if (MergePotentials.size() >= 2) 667 MadeChange |= TryMergeBlocks(I, PredBB); 668 // Reinsert an unconditional branch if needed. 669 // The 1 below can be either an original single predecessor, or a result 670 // of removing blocks in TryMergeBlocks. 671 PredBB = prior(I); // this may have been changed in TryMergeBlocks 672 if (MergePotentials.size()==1 && 673 (MergePotentials.begin())->second != PredBB) 674 FixTail((MergePotentials.begin())->second, I, TII); 675 } 676 } 677 return MadeChange; 678} 679 680//===----------------------------------------------------------------------===// 681// Branch Optimization 682//===----------------------------------------------------------------------===// 683 684bool BranchFolder::OptimizeBranches(MachineFunction &MF) { 685 MadeChange = false; 686 687 // Make sure blocks are numbered in order 688 MF.RenumberBlocks(); 689 690 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) { 691 MachineBasicBlock *MBB = I++; 692 OptimizeBlock(MBB); 693 694 // If it is dead, remove it. 695 if (MBB->pred_empty()) { 696 RemoveDeadBlock(MBB); 697 MadeChange = true; 698 ++NumDeadBlocks; 699 } 700 } 701 return MadeChange; 702} 703 704 705/// CanFallThrough - Return true if the specified block (with the specified 706/// branch condition) can implicitly transfer control to the block after it by 707/// falling off the end of it. This should return false if it can reach the 708/// block after it, but it uses an explicit branch to do so (e.g. a table jump). 709/// 710/// True is a conservative answer. 711/// 712bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB, 713 bool BranchUnAnalyzable, 714 MachineBasicBlock *TBB, MachineBasicBlock *FBB, 715 const std::vector<MachineOperand> &Cond) { 716 MachineFunction::iterator Fallthrough = CurBB; 717 ++Fallthrough; 718 // If FallthroughBlock is off the end of the function, it can't fall through. 719 if (Fallthrough == CurBB->getParent()->end()) 720 return false; 721 722 // If FallthroughBlock isn't a successor of CurBB, no fallthrough is possible. 723 if (!CurBB->isSuccessor(Fallthrough)) 724 return false; 725 726 // If we couldn't analyze the branch, assume it could fall through. 727 if (BranchUnAnalyzable) return true; 728 729 // If there is no branch, control always falls through. 730 if (TBB == 0) return true; 731 732 // If there is some explicit branch to the fallthrough block, it can obviously 733 // reach, even though the branch should get folded to fall through implicitly. 734 if (MachineFunction::iterator(TBB) == Fallthrough || 735 MachineFunction::iterator(FBB) == Fallthrough) 736 return true; 737 738 // If it's an unconditional branch to some block not the fall through, it 739 // doesn't fall through. 740 if (Cond.empty()) return false; 741 742 // Otherwise, if it is conditional and has no explicit false block, it falls 743 // through. 744 return FBB == 0; 745} 746 747/// CanFallThrough - Return true if the specified can implicitly transfer 748/// control to the block after it by falling off the end of it. This should 749/// return false if it can reach the block after it, but it uses an explicit 750/// branch to do so (e.g. a table jump). 751/// 752/// True is a conservative answer. 753/// 754bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) { 755 MachineBasicBlock *TBB = 0, *FBB = 0; 756 std::vector<MachineOperand> Cond; 757 bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond); 758 return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond); 759} 760 761/// IsBetterFallthrough - Return true if it would be clearly better to 762/// fall-through to MBB1 than to fall through into MBB2. This has to return 763/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will 764/// result in infinite loops. 765static bool IsBetterFallthrough(MachineBasicBlock *MBB1, 766 MachineBasicBlock *MBB2, 767 const TargetInstrInfo &TII) { 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 MachineInstr *MBB1I = --MBB1->end(); 775 MachineInstr *MBB2I = --MBB2->end(); 776 return TII.isCall(MBB2I->getOpcode()) && !TII.isCall(MBB1I->getOpcode()); 777} 778 779/// OptimizeBlock - Analyze and optimize control flow related to the specified 780/// block. This is never called on the entry block. 781void BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) { 782 MachineFunction::iterator FallThrough = MBB; 783 ++FallThrough; 784 785 // If this block is empty, make everyone use its fall-through, not the block 786 // explicitly. Landing pads should not do this since the landing-pad table 787 // points to this block. 788 if (MBB->empty() && !MBB->isLandingPad()) { 789 // Dead block? Leave for cleanup later. 790 if (MBB->pred_empty()) return; 791 792 if (FallThrough == MBB->getParent()->end()) { 793 // TODO: Simplify preds to not branch here if possible! 794 } else { 795 // Rewrite all predecessors of the old block to go to the fallthrough 796 // instead. 797 while (!MBB->pred_empty()) { 798 MachineBasicBlock *Pred = *(MBB->pred_end()-1); 799 Pred->ReplaceUsesOfBlockWith(MBB, FallThrough); 800 } 801 802 // If MBB was the target of a jump table, update jump tables to go to the 803 // fallthrough instead. 804 MBB->getParent()->getJumpTableInfo()-> 805 ReplaceMBBInJumpTables(MBB, FallThrough); 806 MadeChange = true; 807 } 808 return; 809 } 810 811 // Check to see if we can simplify the terminator of the block before this 812 // one. 813 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB)); 814 815 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0; 816 std::vector<MachineOperand> PriorCond; 817 bool PriorUnAnalyzable = 818 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); 819 if (!PriorUnAnalyzable) { 820 // If the CFG for the prior block has extra edges, remove them. 821 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB, 822 !PriorCond.empty()); 823 824 // If the previous branch is conditional and both conditions go to the same 825 // destination, remove the branch, replacing it with an unconditional one or 826 // a fall-through. 827 if (PriorTBB && PriorTBB == PriorFBB) { 828 TII->RemoveBranch(PrevBB); 829 PriorCond.clear(); 830 if (PriorTBB != MBB) 831 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 832 MadeChange = true; 833 ++NumBranchOpts; 834 return OptimizeBlock(MBB); 835 } 836 837 // If the previous branch *only* branches to *this* block (conditional or 838 // not) remove the branch. 839 if (PriorTBB == MBB && PriorFBB == 0) { 840 TII->RemoveBranch(PrevBB); 841 MadeChange = true; 842 ++NumBranchOpts; 843 return OptimizeBlock(MBB); 844 } 845 846 // If the prior block branches somewhere else on the condition and here if 847 // the condition is false, remove the uncond second branch. 848 if (PriorFBB == MBB) { 849 TII->RemoveBranch(PrevBB); 850 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 851 MadeChange = true; 852 ++NumBranchOpts; 853 return OptimizeBlock(MBB); 854 } 855 856 // If the prior block branches here on true and somewhere else on false, and 857 // if the branch condition is reversible, reverse the branch to create a 858 // fall-through. 859 if (PriorTBB == MBB) { 860 std::vector<MachineOperand> NewPriorCond(PriorCond); 861 if (!TII->ReverseBranchCondition(NewPriorCond)) { 862 TII->RemoveBranch(PrevBB); 863 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond); 864 MadeChange = true; 865 ++NumBranchOpts; 866 return OptimizeBlock(MBB); 867 } 868 } 869 870 // If this block doesn't fall through (e.g. it ends with an uncond branch or 871 // has no successors) and if the pred falls through into this block, and if 872 // it would otherwise fall through into the block after this, move this 873 // block to the end of the function. 874 // 875 // We consider it more likely that execution will stay in the function (e.g. 876 // due to loops) than it is to exit it. This asserts in loops etc, moving 877 // the assert condition out of the loop body. 878 if (!PriorCond.empty() && PriorFBB == 0 && 879 MachineFunction::iterator(PriorTBB) == FallThrough && 880 !CanFallThrough(MBB)) { 881 bool DoTransform = true; 882 883 // We have to be careful that the succs of PredBB aren't both no-successor 884 // blocks. If neither have successors and if PredBB is the second from 885 // last block in the function, we'd just keep swapping the two blocks for 886 // last. Only do the swap if one is clearly better to fall through than 887 // the other. 888 if (FallThrough == --MBB->getParent()->end() && 889 !IsBetterFallthrough(PriorTBB, MBB, *TII)) 890 DoTransform = false; 891 892 // We don't want to do this transformation if we have control flow like: 893 // br cond BB2 894 // BB1: 895 // .. 896 // jmp BBX 897 // BB2: 898 // .. 899 // ret 900 // 901 // In this case, we could actually be moving the return block *into* a 902 // loop! 903 if (DoTransform && !MBB->succ_empty() && 904 (!CanFallThrough(PriorTBB) || PriorTBB->empty())) 905 DoTransform = false; 906 907 908 if (DoTransform) { 909 // Reverse the branch so we will fall through on the previous true cond. 910 std::vector<MachineOperand> NewPriorCond(PriorCond); 911 if (!TII->ReverseBranchCondition(NewPriorCond)) { 912 DOUT << "\nMoving MBB: " << *MBB; 913 DOUT << "To make fallthrough to: " << *PriorTBB << "\n"; 914 915 TII->RemoveBranch(PrevBB); 916 TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond); 917 918 // Move this block to the end of the function. 919 MBB->moveAfter(--MBB->getParent()->end()); 920 MadeChange = true; 921 ++NumBranchOpts; 922 return; 923 } 924 } 925 } 926 } 927 928 // Analyze the branch in the current block. 929 MachineBasicBlock *CurTBB = 0, *CurFBB = 0; 930 std::vector<MachineOperand> CurCond; 931 bool CurUnAnalyzable = TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond); 932 if (!CurUnAnalyzable) { 933 // If the CFG for the prior block has extra edges, remove them. 934 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty()); 935 936 // If this is a two-way branch, and the FBB branches to this block, reverse 937 // the condition so the single-basic-block loop is faster. Instead of: 938 // Loop: xxx; jcc Out; jmp Loop 939 // we want: 940 // Loop: xxx; jncc Loop; jmp Out 941 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) { 942 std::vector<MachineOperand> NewCond(CurCond); 943 if (!TII->ReverseBranchCondition(NewCond)) { 944 TII->RemoveBranch(*MBB); 945 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond); 946 MadeChange = true; 947 ++NumBranchOpts; 948 return OptimizeBlock(MBB); 949 } 950 } 951 952 953 // If this branch is the only thing in its block, see if we can forward 954 // other blocks across it. 955 if (CurTBB && CurCond.empty() && CurFBB == 0 && 956 TII->isBranch(MBB->begin()->getOpcode()) && CurTBB != MBB) { 957 // This block may contain just an unconditional branch. Because there can 958 // be 'non-branch terminators' in the block, try removing the branch and 959 // then seeing if the block is empty. 960 TII->RemoveBranch(*MBB); 961 962 // If this block is just an unconditional branch to CurTBB, we can 963 // usually completely eliminate the block. The only case we cannot 964 // completely eliminate the block is when the block before this one 965 // falls through into MBB and we can't understand the prior block's branch 966 // condition. 967 if (MBB->empty()) { 968 bool PredHasNoFallThrough = TII->BlockHasNoFallThrough(PrevBB); 969 if (PredHasNoFallThrough || !PriorUnAnalyzable || 970 !PrevBB.isSuccessor(MBB)) { 971 // If the prior block falls through into us, turn it into an 972 // explicit branch to us to make updates simpler. 973 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) && 974 PriorTBB != MBB && PriorFBB != MBB) { 975 if (PriorTBB == 0) { 976 assert(PriorCond.empty() && PriorFBB == 0 && 977 "Bad branch analysis"); 978 PriorTBB = MBB; 979 } else { 980 assert(PriorFBB == 0 && "Machine CFG out of date!"); 981 PriorFBB = MBB; 982 } 983 TII->RemoveBranch(PrevBB); 984 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); 985 } 986 987 // Iterate through all the predecessors, revectoring each in-turn. 988 size_t PI = 0; 989 bool DidChange = false; 990 bool HasBranchToSelf = false; 991 while(PI != MBB->pred_size()) { 992 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI); 993 if (PMBB == MBB) { 994 // If this block has an uncond branch to itself, leave it. 995 ++PI; 996 HasBranchToSelf = true; 997 } else { 998 DidChange = true; 999 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB); 1000 } 1001 } 1002 1003 // Change any jumptables to go to the new MBB. 1004 MBB->getParent()->getJumpTableInfo()-> 1005 ReplaceMBBInJumpTables(MBB, CurTBB); 1006 if (DidChange) { 1007 ++NumBranchOpts; 1008 MadeChange = true; 1009 if (!HasBranchToSelf) return; 1010 } 1011 } 1012 } 1013 1014 // Add the branch back if the block is more than just an uncond branch. 1015 TII->InsertBranch(*MBB, CurTBB, 0, CurCond); 1016 } 1017 } 1018 1019 // If the prior block doesn't fall through into this block, and if this 1020 // block doesn't fall through into some other block, see if we can find a 1021 // place to move this block where a fall-through will happen. 1022 if (!CanFallThrough(&PrevBB, PriorUnAnalyzable, 1023 PriorTBB, PriorFBB, PriorCond)) { 1024 // Now we know that there was no fall-through into this block, check to 1025 // see if it has a fall-through into its successor. 1026 bool CurFallsThru = CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB, 1027 CurCond); 1028 1029 if (!MBB->isLandingPad()) { 1030 // Check all the predecessors of this block. If one of them has no fall 1031 // throughs, move this block right after it. 1032 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(), 1033 E = MBB->pred_end(); PI != E; ++PI) { 1034 // Analyze the branch at the end of the pred. 1035 MachineBasicBlock *PredBB = *PI; 1036 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough; 1037 if (PredBB != MBB && !CanFallThrough(PredBB) 1038 && (!CurFallsThru || !CurTBB || !CurFBB) 1039 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) { 1040 // If the current block doesn't fall through, just move it. 1041 // If the current block can fall through and does not end with a 1042 // conditional branch, we need to append an unconditional jump to 1043 // the (current) next block. To avoid a possible compile-time 1044 // infinite loop, move blocks only backward in this case. 1045 // Also, if there are already 2 branches here, we cannot add a third; 1046 // this means we have the case 1047 // Bcc next 1048 // B elsewhere 1049 // next: 1050 if (CurFallsThru) { 1051 MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB)); 1052 CurCond.clear(); 1053 TII->InsertBranch(*MBB, NextBB, 0, CurCond); 1054 } 1055 MBB->moveAfter(PredBB); 1056 MadeChange = true; 1057 return OptimizeBlock(MBB); 1058 } 1059 } 1060 } 1061 1062 if (!CurFallsThru) { 1063 // Check all successors to see if we can move this block before it. 1064 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(), 1065 E = MBB->succ_end(); SI != E; ++SI) { 1066 // Analyze the branch at the end of the block before the succ. 1067 MachineBasicBlock *SuccBB = *SI; 1068 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev; 1069 std::vector<MachineOperand> SuccPrevCond; 1070 1071 // If this block doesn't already fall-through to that successor, and if 1072 // the succ doesn't already have a block that can fall through into it, 1073 // and if the successor isn't an EH destination, we can arrange for the 1074 // fallthrough to happen. 1075 if (SuccBB != MBB && !CanFallThrough(SuccPrev) && 1076 !SuccBB->isLandingPad()) { 1077 MBB->moveBefore(SuccBB); 1078 MadeChange = true; 1079 return OptimizeBlock(MBB); 1080 } 1081 } 1082 1083 // Okay, there is no really great place to put this block. If, however, 1084 // the block before this one would be a fall-through if this block were 1085 // removed, move this block to the end of the function. 1086 if (FallThrough != MBB->getParent()->end() && 1087 PrevBB.isSuccessor(FallThrough)) { 1088 MBB->moveAfter(--MBB->getParent()->end()); 1089 MadeChange = true; 1090 return; 1091 } 1092 } 1093 } 1094} 1095