BranchFolding.cpp revision 7fda037ed8e65b25550b1b439af1153b40df0739
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 "BranchFolding.h" 21#include "llvm/Function.h" 22#include "llvm/CodeGen/Passes.h" 23#include "llvm/CodeGen/MachineModuleInfo.h" 24#include "llvm/CodeGen/MachineFunctionPass.h" 25#include "llvm/CodeGen/MachineJumpTableInfo.h" 26#include "llvm/CodeGen/RegisterScavenging.h" 27#include "llvm/Target/TargetInstrInfo.h" 28#include "llvm/Target/TargetMachine.h" 29#include "llvm/Target/TargetRegisterInfo.h" 30#include "llvm/Support/CommandLine.h" 31#include "llvm/Support/Debug.h" 32#include "llvm/Support/ErrorHandling.h" 33#include "llvm/Support/raw_ostream.h" 34#include "llvm/ADT/SmallSet.h" 35#include "llvm/ADT/SetVector.h" 36#include "llvm/ADT/Statistic.h" 37#include "llvm/ADT/STLExtras.h" 38#include <algorithm> 39using namespace llvm; 40 41STATISTIC(NumDeadBlocks, "Number of dead blocks removed"); 42STATISTIC(NumBranchOpts, "Number of branches optimized"); 43STATISTIC(NumTailMerge , "Number of block tails merged"); 44STATISTIC(NumTailDups , "Number of tail duplicated blocks"); 45STATISTIC(NumInstrDups , "Additional instructions due to tail duplication"); 46 47static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge", 48 cl::init(cl::BOU_UNSET), cl::Hidden); 49 50// Throttle for huge numbers of predecessors (compile speed problems) 51static cl::opt<unsigned> 52TailMergeThreshold("tail-merge-threshold", 53 cl::desc("Max number of predecessors to consider tail merging"), 54 cl::init(150), cl::Hidden); 55 56// Heuristic for tail merging (and, inversely, tail duplication). 57// TODO: This should be replaced with a target query. 58static cl::opt<unsigned> 59TailMergeSize("tail-merge-size", 60 cl::desc("Min number of instructions to consider tail merging"), 61 cl::init(3), cl::Hidden); 62 63namespace { 64 /// BranchFolderPass - Wrap branch folder in a machine function pass. 65 class BranchFolderPass : public MachineFunctionPass, 66 public BranchFolder { 67 public: 68 static char ID; 69 explicit BranchFolderPass(bool defaultEnableTailMerge) 70 : MachineFunctionPass(&ID), BranchFolder(defaultEnableTailMerge) {} 71 72 virtual bool runOnMachineFunction(MachineFunction &MF); 73 virtual const char *getPassName() const { return "Control Flow Optimizer"; } 74 }; 75} 76 77char BranchFolderPass::ID = 0; 78 79FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) { 80 return new BranchFolderPass(DefaultEnableTailMerge); 81} 82 83bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) { 84 return OptimizeFunction(MF, 85 MF.getTarget().getInstrInfo(), 86 MF.getTarget().getRegisterInfo(), 87 getAnalysisIfAvailable<MachineModuleInfo>()); 88} 89 90 91BranchFolder::BranchFolder(bool defaultEnableTailMerge) { 92 switch (FlagEnableTailMerge) { 93 case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break; 94 case cl::BOU_TRUE: EnableTailMerge = true; break; 95 case cl::BOU_FALSE: EnableTailMerge = false; break; 96 } 97} 98 99/// RemoveDeadBlock - Remove the specified dead machine basic block from the 100/// function, updating the CFG. 101void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) { 102 assert(MBB->pred_empty() && "MBB must be dead!"); 103 DEBUG(errs() << "\nRemoving MBB: " << *MBB); 104 105 MachineFunction *MF = MBB->getParent(); 106 // drop all successors. 107 while (!MBB->succ_empty()) 108 MBB->removeSuccessor(MBB->succ_end()-1); 109 110 // If there are any labels in the basic block, unregister them from 111 // MachineModuleInfo. 112 if (MMI && !MBB->empty()) { 113 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); 114 I != E; ++I) { 115 if (I->isLabel()) 116 // The label ID # is always operand #0, an immediate. 117 MMI->InvalidateLabel(I->getOperand(0).getImm()); 118 } 119 } 120 121 // Remove the block. 122 MF->erase(MBB); 123} 124 125/// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def 126/// followed by terminators, and if the implicitly defined registers are not 127/// used by the terminators, remove those implicit_def's. e.g. 128/// BB1: 129/// r0 = implicit_def 130/// r1 = implicit_def 131/// br 132/// This block can be optimized away later if the implicit instructions are 133/// removed. 134bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) { 135 SmallSet<unsigned, 4> ImpDefRegs; 136 MachineBasicBlock::iterator I = MBB->begin(); 137 while (I != MBB->end()) { 138 if (I->getOpcode() != TargetInstrInfo::IMPLICIT_DEF) 139 break; 140 unsigned Reg = I->getOperand(0).getReg(); 141 ImpDefRegs.insert(Reg); 142 for (const unsigned *SubRegs = TRI->getSubRegisters(Reg); 143 unsigned SubReg = *SubRegs; ++SubRegs) 144 ImpDefRegs.insert(SubReg); 145 ++I; 146 } 147 if (ImpDefRegs.empty()) 148 return false; 149 150 MachineBasicBlock::iterator FirstTerm = I; 151 while (I != MBB->end()) { 152 if (!TII->isUnpredicatedTerminator(I)) 153 return false; 154 // See if it uses any of the implicitly defined registers. 155 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) { 156 MachineOperand &MO = I->getOperand(i); 157 if (!MO.isReg() || !MO.isUse()) 158 continue; 159 unsigned Reg = MO.getReg(); 160 if (ImpDefRegs.count(Reg)) 161 return false; 162 } 163 ++I; 164 } 165 166 I = MBB->begin(); 167 while (I != FirstTerm) { 168 MachineInstr *ImpDefMI = &*I; 169 ++I; 170 MBB->erase(ImpDefMI); 171 } 172 173 return true; 174} 175 176/// OptimizeFunction - Perhaps branch folding, tail merging and other 177/// CFG optimizations on the given function. 178bool BranchFolder::OptimizeFunction(MachineFunction &MF, 179 const TargetInstrInfo *tii, 180 const TargetRegisterInfo *tri, 181 MachineModuleInfo *mmi) { 182 if (!tii) return false; 183 184 TII = tii; 185 TRI = tri; 186 MMI = mmi; 187 188 RS = TRI->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL; 189 190 // Fix CFG. The later algorithms expect it to be right. 191 bool MadeChange = false; 192 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) { 193 MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0; 194 SmallVector<MachineOperand, 4> Cond; 195 if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true)) 196 MadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty()); 197 MadeChange |= OptimizeImpDefsBlock(MBB); 198 } 199 200 201 bool MadeChangeThisIteration = true; 202 while (MadeChangeThisIteration) { 203 MadeChangeThisIteration = false; 204 MadeChangeThisIteration |= TailMergeBlocks(MF); 205 MadeChangeThisIteration |= OptimizeBranches(MF); 206 MadeChange |= MadeChangeThisIteration; 207 } 208 209 // Do tail duplication once after tail merging is done. Otherwise it is 210 // tough to avoid situations where tail duplication and tail merging undo 211 // each other's transformations ad infinitum. 212 MadeChange |= TailDuplicateBlocks(MF); 213 214 // See if any jump tables have become mergable or dead as the code generator 215 // did its thing. 216 MachineJumpTableInfo *JTI = MF.getJumpTableInfo(); 217 const std::vector<MachineJumpTableEntry> &JTs = JTI->getJumpTables(); 218 if (!JTs.empty()) { 219 // Figure out how these jump tables should be merged. 220 std::vector<unsigned> JTMapping; 221 JTMapping.reserve(JTs.size()); 222 223 // We always keep the 0th jump table. 224 JTMapping.push_back(0); 225 226 // Scan the jump tables, seeing if there are any duplicates. Note that this 227 // is N^2, which should be fixed someday. 228 for (unsigned i = 1, e = JTs.size(); i != e; ++i) { 229 if (JTs[i].MBBs.empty()) 230 JTMapping.push_back(i); 231 else 232 JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs)); 233 } 234 235 // If a jump table was merge with another one, walk the function rewriting 236 // references to jump tables to reference the new JT ID's. Keep track of 237 // whether we see a jump table idx, if not, we can delete the JT. 238 BitVector JTIsLive(JTs.size()); 239 for (MachineFunction::iterator BB = MF.begin(), E = MF.end(); 240 BB != E; ++BB) { 241 for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end(); 242 I != E; ++I) 243 for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) { 244 MachineOperand &Op = I->getOperand(op); 245 if (!Op.isJTI()) continue; 246 unsigned NewIdx = JTMapping[Op.getIndex()]; 247 Op.setIndex(NewIdx); 248 249 // Remember that this JT is live. 250 JTIsLive.set(NewIdx); 251 } 252 } 253 254 // Finally, remove dead jump tables. This happens either because the 255 // indirect jump was unreachable (and thus deleted) or because the jump 256 // table was merged with some other one. 257 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i) 258 if (!JTIsLive.test(i)) { 259 JTI->RemoveJumpTable(i); 260 MadeChange = true; 261 } 262 } 263 264 delete RS; 265 return MadeChange; 266} 267 268//===----------------------------------------------------------------------===// 269// Tail Merging of Blocks 270//===----------------------------------------------------------------------===// 271 272/// HashMachineInstr - Compute a hash value for MI and its operands. 273static unsigned HashMachineInstr(const MachineInstr *MI) { 274 unsigned Hash = MI->getOpcode(); 275 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 276 const MachineOperand &Op = MI->getOperand(i); 277 278 // Merge in bits from the operand if easy. 279 unsigned OperandHash = 0; 280 switch (Op.getType()) { 281 case MachineOperand::MO_Register: OperandHash = Op.getReg(); break; 282 case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break; 283 case MachineOperand::MO_MachineBasicBlock: 284 OperandHash = Op.getMBB()->getNumber(); 285 break; 286 case MachineOperand::MO_FrameIndex: 287 case MachineOperand::MO_ConstantPoolIndex: 288 case MachineOperand::MO_JumpTableIndex: 289 OperandHash = Op.getIndex(); 290 break; 291 case MachineOperand::MO_GlobalAddress: 292 case MachineOperand::MO_ExternalSymbol: 293 // Global address / external symbol are too hard, don't bother, but do 294 // pull in the offset. 295 OperandHash = Op.getOffset(); 296 break; 297 default: break; 298 } 299 300 Hash += ((OperandHash << 3) | Op.getType()) << (i&31); 301 } 302 return Hash; 303} 304 305/// HashEndOfMBB - Hash the last few instructions in the MBB. For blocks 306/// with no successors, we hash two instructions, because cross-jumping 307/// only saves code when at least two instructions are removed (since a 308/// branch must be inserted). For blocks with a successor, one of the 309/// two blocks to be tail-merged will end with a branch already, so 310/// it gains to cross-jump even for one instruction. 311static unsigned HashEndOfMBB(const MachineBasicBlock *MBB, 312 unsigned minCommonTailLength) { 313 MachineBasicBlock::const_iterator I = MBB->end(); 314 if (I == MBB->begin()) 315 return 0; // Empty MBB. 316 317 --I; 318 unsigned Hash = HashMachineInstr(I); 319 320 if (I == MBB->begin() || minCommonTailLength == 1) 321 return Hash; // Single instr MBB. 322 323 --I; 324 // Hash in the second-to-last instruction. 325 Hash ^= HashMachineInstr(I) << 2; 326 return Hash; 327} 328 329/// ComputeCommonTailLength - Given two machine basic blocks, compute the number 330/// of instructions they actually have in common together at their end. Return 331/// iterators for the first shared instruction in each block. 332static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1, 333 MachineBasicBlock *MBB2, 334 MachineBasicBlock::iterator &I1, 335 MachineBasicBlock::iterator &I2) { 336 I1 = MBB1->end(); 337 I2 = MBB2->end(); 338 339 unsigned TailLen = 0; 340 while (I1 != MBB1->begin() && I2 != MBB2->begin()) { 341 --I1; --I2; 342 if (!I1->isIdenticalTo(I2) || 343 // FIXME: This check is dubious. It's used to get around a problem where 344 // people incorrectly expect inline asm directives to remain in the same 345 // relative order. This is untenable because normal compiler 346 // optimizations (like this one) may reorder and/or merge these 347 // directives. 348 I1->getOpcode() == TargetInstrInfo::INLINEASM) { 349 ++I1; ++I2; 350 break; 351 } 352 ++TailLen; 353 } 354 return TailLen; 355} 356 357/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything 358/// after it, replacing it with an unconditional branch to NewDest. This 359/// returns true if OldInst's block is modified, false if NewDest is modified. 360void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, 361 MachineBasicBlock *NewDest) { 362 MachineBasicBlock *OldBB = OldInst->getParent(); 363 364 // Remove all the old successors of OldBB from the CFG. 365 while (!OldBB->succ_empty()) 366 OldBB->removeSuccessor(OldBB->succ_begin()); 367 368 // Remove all the dead instructions from the end of OldBB. 369 OldBB->erase(OldInst, OldBB->end()); 370 371 // If OldBB isn't immediately before OldBB, insert a branch to it. 372 if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest)) 373 TII->InsertBranch(*OldBB, NewDest, 0, SmallVector<MachineOperand, 0>()); 374 OldBB->addSuccessor(NewDest); 375 ++NumTailMerge; 376} 377 378/// SplitMBBAt - Given a machine basic block and an iterator into it, split the 379/// MBB so that the part before the iterator falls into the part starting at the 380/// iterator. This returns the new MBB. 381MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB, 382 MachineBasicBlock::iterator BBI1) { 383 MachineFunction &MF = *CurMBB.getParent(); 384 385 // Create the fall-through block. 386 MachineFunction::iterator MBBI = &CurMBB; 387 MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(CurMBB.getBasicBlock()); 388 CurMBB.getParent()->insert(++MBBI, NewMBB); 389 390 // Move all the successors of this block to the specified block. 391 NewMBB->transferSuccessors(&CurMBB); 392 393 // Add an edge from CurMBB to NewMBB for the fall-through. 394 CurMBB.addSuccessor(NewMBB); 395 396 // Splice the code over. 397 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end()); 398 399 // For targets that use the register scavenger, we must maintain LiveIns. 400 if (RS) { 401 RS->enterBasicBlock(&CurMBB); 402 if (!CurMBB.empty()) 403 RS->forward(prior(CurMBB.end())); 404 BitVector RegsLiveAtExit(TRI->getNumRegs()); 405 RS->getRegsUsed(RegsLiveAtExit, false); 406 for (unsigned int i = 0, e = TRI->getNumRegs(); i != e; i++) 407 if (RegsLiveAtExit[i]) 408 NewMBB->addLiveIn(i); 409 } 410 411 return NewMBB; 412} 413 414/// EstimateRuntime - Make a rough estimate for how long it will take to run 415/// the specified code. 416static unsigned EstimateRuntime(MachineBasicBlock::iterator I, 417 MachineBasicBlock::iterator E) { 418 unsigned Time = 0; 419 for (; I != E; ++I) { 420 const TargetInstrDesc &TID = I->getDesc(); 421 if (TID.isCall()) 422 Time += 10; 423 else if (TID.mayLoad() || TID.mayStore()) 424 Time += 2; 425 else 426 ++Time; 427 } 428 return Time; 429} 430 431// CurMBB needs to add an unconditional branch to SuccMBB (we removed these 432// branches temporarily for tail merging). In the case where CurMBB ends 433// with a conditional branch to the next block, optimize by reversing the 434// test and conditionally branching to SuccMBB instead. 435static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB, 436 const TargetInstrInfo *TII) { 437 MachineFunction *MF = CurMBB->getParent(); 438 MachineFunction::iterator I = next(MachineFunction::iterator(CurMBB)); 439 MachineBasicBlock *TBB = 0, *FBB = 0; 440 SmallVector<MachineOperand, 4> Cond; 441 if (I != MF->end() && 442 !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond, true)) { 443 MachineBasicBlock *NextBB = I; 444 if (TBB == NextBB && !Cond.empty() && !FBB) { 445 if (!TII->ReverseBranchCondition(Cond)) { 446 TII->RemoveBranch(*CurMBB); 447 TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond); 448 return; 449 } 450 } 451 } 452 TII->InsertBranch(*CurMBB, SuccBB, NULL, SmallVector<MachineOperand, 0>()); 453} 454 455bool 456BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const { 457 if (getHash() < o.getHash()) 458 return true; 459 else if (getHash() > o.getHash()) 460 return false; 461 else if (getBlock()->getNumber() < o.getBlock()->getNumber()) 462 return true; 463 else if (getBlock()->getNumber() > o.getBlock()->getNumber()) 464 return false; 465 else { 466 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing 467 // an object with itself. 468#ifndef _GLIBCXX_DEBUG 469 llvm_unreachable("Predecessor appears twice"); 470#endif 471 return false; 472 } 473} 474 475/// CountTerminators - Count the number of terminators in the given 476/// block and set I to the position of the first non-terminator, if there 477/// is one, or MBB->end() otherwise. 478static unsigned CountTerminators(MachineBasicBlock *MBB, 479 MachineBasicBlock::iterator &I) { 480 I = MBB->end(); 481 unsigned NumTerms = 0; 482 for (;;) { 483 if (I == MBB->begin()) { 484 I = MBB->end(); 485 break; 486 } 487 --I; 488 if (!I->getDesc().isTerminator()) break; 489 ++NumTerms; 490 } 491 return NumTerms; 492} 493 494/// ProfitableToMerge - Check if two machine basic blocks have a common tail 495/// and decide if it would be profitable to merge those tails. Return the 496/// length of the common tail and iterators to the first common instruction 497/// in each block. 498static bool ProfitableToMerge(MachineBasicBlock *MBB1, 499 MachineBasicBlock *MBB2, 500 unsigned minCommonTailLength, 501 unsigned &CommonTailLen, 502 MachineBasicBlock::iterator &I1, 503 MachineBasicBlock::iterator &I2, 504 MachineBasicBlock *SuccBB, 505 MachineBasicBlock *PredBB) { 506 CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2); 507 MachineFunction *MF = MBB1->getParent(); 508 509 if (CommonTailLen == 0) 510 return false; 511 512 // It's almost always profitable to merge any number of non-terminator 513 // instructions with the block that falls through into the common successor. 514 if (MBB1 == PredBB || MBB2 == PredBB) { 515 MachineBasicBlock::iterator I; 516 unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I); 517 if (CommonTailLen > NumTerms) 518 return true; 519 } 520 521 // If one of the blocks can be completely merged and happens to be in 522 // a position where the other could fall through into it, merge any number 523 // of instructions, because it can be done without a branch. 524 // TODO: If the blocks are not adjacent, move one of them so that they are? 525 if (MBB1->isLayoutSuccessor(MBB2) && I2 == MBB2->begin()) 526 return true; 527 if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin()) 528 return true; 529 530 // If both blocks have an unconditional branch temporarily stripped out, 531 // count that as an additional common instruction for the following 532 // heuristics. 533 unsigned EffectiveTailLen = CommonTailLen; 534 if (SuccBB && MBB1 != PredBB && MBB2 != PredBB && 535 !MBB1->back().getDesc().isBarrier() && 536 !MBB2->back().getDesc().isBarrier()) 537 ++EffectiveTailLen; 538 539 // Check if the common tail is long enough to be worthwhile. 540 if (EffectiveTailLen >= minCommonTailLength) 541 return true; 542 543 // If we are optimizing for code size, 2 instructions in common is enough if 544 // we don't have to split a block. At worst we will be introducing 1 new 545 // branch instruction, which is likely to be smaller than the 2 546 // instructions that would be deleted in the merge. 547 if (EffectiveTailLen >= 2 && 548 MF->getFunction()->hasFnAttr(Attribute::OptimizeForSize) && 549 (I1 == MBB1->begin() || I2 == MBB2->begin())) 550 return true; 551 552 return false; 553} 554 555/// ComputeSameTails - Look through all the blocks in MergePotentials that have 556/// hash CurHash (guaranteed to match the last element). Build the vector 557/// SameTails of all those that have the (same) largest number of instructions 558/// in common of any pair of these blocks. SameTails entries contain an 559/// iterator into MergePotentials (from which the MachineBasicBlock can be 560/// found) and a MachineBasicBlock::iterator into that MBB indicating the 561/// instruction where the matching code sequence begins. 562/// Order of elements in SameTails is the reverse of the order in which 563/// those blocks appear in MergePotentials (where they are not necessarily 564/// consecutive). 565unsigned BranchFolder::ComputeSameTails(unsigned CurHash, 566 unsigned minCommonTailLength, 567 MachineBasicBlock *SuccBB, 568 MachineBasicBlock *PredBB) { 569 unsigned maxCommonTailLength = 0U; 570 SameTails.clear(); 571 MachineBasicBlock::iterator TrialBBI1, TrialBBI2; 572 MPIterator HighestMPIter = prior(MergePotentials.end()); 573 for (MPIterator CurMPIter = prior(MergePotentials.end()), 574 B = MergePotentials.begin(); 575 CurMPIter != B && CurMPIter->getHash() == CurHash; 576 --CurMPIter) { 577 for (MPIterator I = prior(CurMPIter); I->getHash() == CurHash ; --I) { 578 unsigned CommonTailLen; 579 if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(), 580 minCommonTailLength, 581 CommonTailLen, TrialBBI1, TrialBBI2, 582 SuccBB, PredBB)) { 583 if (CommonTailLen > maxCommonTailLength) { 584 SameTails.clear(); 585 maxCommonTailLength = CommonTailLen; 586 HighestMPIter = CurMPIter; 587 SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1)); 588 } 589 if (HighestMPIter == CurMPIter && 590 CommonTailLen == maxCommonTailLength) 591 SameTails.push_back(SameTailElt(I, TrialBBI2)); 592 } 593 if (I == B) 594 break; 595 } 596 } 597 return maxCommonTailLength; 598} 599 600/// RemoveBlocksWithHash - Remove all blocks with hash CurHash from 601/// MergePotentials, restoring branches at ends of blocks as appropriate. 602void BranchFolder::RemoveBlocksWithHash(unsigned CurHash, 603 MachineBasicBlock *SuccBB, 604 MachineBasicBlock *PredBB) { 605 MPIterator CurMPIter, B; 606 for (CurMPIter = prior(MergePotentials.end()), B = MergePotentials.begin(); 607 CurMPIter->getHash() == CurHash; 608 --CurMPIter) { 609 // Put the unconditional branch back, if we need one. 610 MachineBasicBlock *CurMBB = CurMPIter->getBlock(); 611 if (SuccBB && CurMBB != PredBB) 612 FixTail(CurMBB, SuccBB, TII); 613 if (CurMPIter == B) 614 break; 615 } 616 if (CurMPIter->getHash() != CurHash) 617 CurMPIter++; 618 MergePotentials.erase(CurMPIter, MergePotentials.end()); 619} 620 621/// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist 622/// only of the common tail. Create a block that does by splitting one. 623unsigned BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB, 624 unsigned maxCommonTailLength) { 625 unsigned commonTailIndex = 0; 626 unsigned TimeEstimate = ~0U; 627 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) { 628 // Use PredBB if possible; that doesn't require a new branch. 629 if (SameTails[i].getBlock() == PredBB) { 630 commonTailIndex = i; 631 break; 632 } 633 // Otherwise, make a (fairly bogus) choice based on estimate of 634 // how long it will take the various blocks to execute. 635 unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(), 636 SameTails[i].getTailStartPos()); 637 if (t <= TimeEstimate) { 638 TimeEstimate = t; 639 commonTailIndex = i; 640 } 641 } 642 643 MachineBasicBlock::iterator BBI = 644 SameTails[commonTailIndex].getTailStartPos(); 645 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock(); 646 647 DEBUG(errs() << "\nSplitting BB#" << MBB->getNumber() << ", size " 648 << maxCommonTailLength); 649 650 MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI); 651 SameTails[commonTailIndex].setBlock(newMBB); 652 SameTails[commonTailIndex].setTailStartPos(newMBB->begin()); 653 654 // If we split PredBB, newMBB is the new predecessor. 655 if (PredBB == MBB) 656 PredBB = newMBB; 657 658 return commonTailIndex; 659} 660 661// See if any of the blocks in MergePotentials (which all have a common single 662// successor, or all have no successor) can be tail-merged. If there is a 663// successor, any blocks in MergePotentials that are not tail-merged and 664// are not immediately before Succ must have an unconditional branch to 665// Succ added (but the predecessor/successor lists need no adjustment). 666// The lone predecessor of Succ that falls through into Succ, 667// if any, is given in PredBB. 668 669bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB, 670 MachineBasicBlock *PredBB) { 671 bool MadeChange = false; 672 673 // Except for the special cases below, tail-merge if there are at least 674 // this many instructions in common. 675 unsigned minCommonTailLength = TailMergeSize; 676 677 DEBUG(errs() << "\nTryTailMergeBlocks: "; 678 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i) 679 errs() << "BB#" << MergePotentials[i].getBlock()->getNumber() 680 << (i == e-1 ? "" : ", "); 681 errs() << "\n"; 682 if (SuccBB) { 683 errs() << " with successor BB#" << SuccBB->getNumber() << '\n'; 684 if (PredBB) 685 errs() << " which has fall-through from BB#" 686 << PredBB->getNumber() << "\n"; 687 } 688 errs() << "Looking for common tails of at least " 689 << minCommonTailLength << " instruction" 690 << (minCommonTailLength == 1 ? "" : "s") << '\n'; 691 ); 692 693 // Sort by hash value so that blocks with identical end sequences sort 694 // together. 695 std::stable_sort(MergePotentials.begin(), MergePotentials.end()); 696 697 // Walk through equivalence sets looking for actual exact matches. 698 while (MergePotentials.size() > 1) { 699 unsigned CurHash = MergePotentials.back().getHash(); 700 701 // Build SameTails, identifying the set of blocks with this hash code 702 // and with the maximum number of instructions in common. 703 unsigned maxCommonTailLength = ComputeSameTails(CurHash, 704 minCommonTailLength, 705 SuccBB, PredBB); 706 707 // If we didn't find any pair that has at least minCommonTailLength 708 // instructions in common, remove all blocks with this hash code and retry. 709 if (SameTails.empty()) { 710 RemoveBlocksWithHash(CurHash, SuccBB, PredBB); 711 continue; 712 } 713 714 // If one of the blocks is the entire common tail (and not the entry 715 // block, which we can't jump to), we can treat all blocks with this same 716 // tail at once. Use PredBB if that is one of the possibilities, as that 717 // will not introduce any extra branches. 718 MachineBasicBlock *EntryBB = MergePotentials.begin()->getBlock()-> 719 getParent()->begin(); 720 unsigned commonTailIndex = SameTails.size(); 721 // If there are two blocks, check to see if one can be made to fall through 722 // into the other. 723 if (SameTails.size() == 2 && 724 SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) && 725 SameTails[1].tailIsWholeBlock()) 726 commonTailIndex = 1; 727 else if (SameTails.size() == 2 && 728 SameTails[1].getBlock()->isLayoutSuccessor( 729 SameTails[0].getBlock()) && 730 SameTails[0].tailIsWholeBlock()) 731 commonTailIndex = 0; 732 else { 733 // Otherwise just pick one, favoring the fall-through predecessor if 734 // there is one. 735 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) { 736 MachineBasicBlock *MBB = SameTails[i].getBlock(); 737 if (MBB == EntryBB && SameTails[i].tailIsWholeBlock()) 738 continue; 739 if (MBB == PredBB) { 740 commonTailIndex = i; 741 break; 742 } 743 if (SameTails[i].tailIsWholeBlock()) 744 commonTailIndex = i; 745 } 746 } 747 748 if (commonTailIndex == SameTails.size() || 749 (SameTails[commonTailIndex].getBlock() == PredBB && 750 !SameTails[commonTailIndex].tailIsWholeBlock())) { 751 // None of the blocks consist entirely of the common tail. 752 // Split a block so that one does. 753 commonTailIndex = CreateCommonTailOnlyBlock(PredBB, maxCommonTailLength); 754 } 755 756 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock(); 757 // MBB is common tail. Adjust all other BB's to jump to this one. 758 // Traversal must be forwards so erases work. 759 DEBUG(errs() << "\nUsing common tail in BB#" << MBB->getNumber() 760 << " for "); 761 for (unsigned int i=0, e = SameTails.size(); i != e; ++i) { 762 if (commonTailIndex == i) 763 continue; 764 DEBUG(errs() << "BB#" << SameTails[i].getBlock()->getNumber() 765 << (i == e-1 ? "" : ", ")); 766 // Hack the end off BB i, making it jump to BB commonTailIndex instead. 767 ReplaceTailWithBranchTo(SameTails[i].getTailStartPos(), MBB); 768 // BB i is no longer a predecessor of SuccBB; remove it from the worklist. 769 MergePotentials.erase(SameTails[i].getMPIter()); 770 } 771 DEBUG(errs() << "\n"); 772 // We leave commonTailIndex in the worklist in case there are other blocks 773 // that match it with a smaller number of instructions. 774 MadeChange = true; 775 } 776 return MadeChange; 777} 778 779bool BranchFolder::TailMergeBlocks(MachineFunction &MF) { 780 781 if (!EnableTailMerge) return false; 782 783 bool MadeChange = false; 784 785 // First find blocks with no successors. 786 MergePotentials.clear(); 787 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { 788 if (I->succ_empty()) 789 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(I, 2U), I)); 790 } 791 792 // See if we can do any tail merging on those. 793 if (MergePotentials.size() < TailMergeThreshold && 794 MergePotentials.size() >= 2) 795 MadeChange |= TryTailMergeBlocks(NULL, NULL); 796 797 // Look at blocks (IBB) with multiple predecessors (PBB). 798 // We change each predecessor to a canonical form, by 799 // (1) temporarily removing any unconditional branch from the predecessor 800 // to IBB, and 801 // (2) alter conditional branches so they branch to the other block 802 // not IBB; this may require adding back an unconditional branch to IBB 803 // later, where there wasn't one coming in. E.g. 804 // Bcc IBB 805 // fallthrough to QBB 806 // here becomes 807 // Bncc QBB 808 // with a conceptual B to IBB after that, which never actually exists. 809 // With those changes, we see whether the predecessors' tails match, 810 // and merge them if so. We change things out of canonical form and 811 // back to the way they were later in the process. (OptimizeBranches 812 // would undo some of this, but we can't use it, because we'd get into 813 // a compile-time infinite loop repeatedly doing and undoing the same 814 // transformations.) 815 816 for (MachineFunction::iterator I = next(MF.begin()), E = MF.end(); 817 I != E; ++I) { 818 if (I->pred_size() >= 2 && I->pred_size() < TailMergeThreshold) { 819 SmallPtrSet<MachineBasicBlock *, 8> UniquePreds; 820 MachineBasicBlock *IBB = I; 821 MachineBasicBlock *PredBB = prior(I); 822 MergePotentials.clear(); 823 for (MachineBasicBlock::pred_iterator P = I->pred_begin(), 824 E2 = I->pred_end(); 825 P != E2; ++P) { 826 MachineBasicBlock *PBB = *P; 827 // Skip blocks that loop to themselves, can't tail merge these. 828 if (PBB == IBB) 829 continue; 830 // Visit each predecessor only once. 831 if (!UniquePreds.insert(PBB)) 832 continue; 833 MachineBasicBlock *TBB = 0, *FBB = 0; 834 SmallVector<MachineOperand, 4> Cond; 835 if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond, true)) { 836 // Failing case: IBB is the target of a cbr, and 837 // we cannot reverse the branch. 838 SmallVector<MachineOperand, 4> NewCond(Cond); 839 if (!Cond.empty() && TBB == IBB) { 840 if (TII->ReverseBranchCondition(NewCond)) 841 continue; 842 // This is the QBB case described above 843 if (!FBB) 844 FBB = next(MachineFunction::iterator(PBB)); 845 } 846 // Failing case: the only way IBB can be reached from PBB is via 847 // exception handling. Happens for landing pads. Would be nice 848 // to have a bit in the edge so we didn't have to do all this. 849 if (IBB->isLandingPad()) { 850 MachineFunction::iterator IP = PBB; IP++; 851 MachineBasicBlock *PredNextBB = NULL; 852 if (IP != MF.end()) 853 PredNextBB = IP; 854 if (TBB == NULL) { 855 if (IBB != PredNextBB) // fallthrough 856 continue; 857 } else if (FBB) { 858 if (TBB != IBB && FBB != IBB) // cbr then ubr 859 continue; 860 } else if (Cond.empty()) { 861 if (TBB != IBB) // ubr 862 continue; 863 } else { 864 if (TBB != IBB && IBB != PredNextBB) // cbr 865 continue; 866 } 867 } 868 // Remove the unconditional branch at the end, if any. 869 if (TBB && (Cond.empty() || FBB)) { 870 TII->RemoveBranch(*PBB); 871 if (!Cond.empty()) 872 // reinsert conditional branch only, for now 873 TII->InsertBranch(*PBB, (TBB == IBB) ? FBB : TBB, 0, NewCond); 874 } 875 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(PBB, 1U), 876 *P)); 877 } 878 } 879 if (MergePotentials.size() >= 2) 880 MadeChange |= TryTailMergeBlocks(IBB, PredBB); 881 // Reinsert an unconditional branch if needed. 882 // The 1 below can occur as a result of removing blocks in TryTailMergeBlocks. 883 PredBB = prior(I); // this may have been changed in TryTailMergeBlocks 884 if (MergePotentials.size() == 1 && 885 MergePotentials.begin()->getBlock() != PredBB) 886 FixTail(MergePotentials.begin()->getBlock(), IBB, TII); 887 } 888 } 889 return MadeChange; 890} 891 892//===----------------------------------------------------------------------===// 893// Branch Optimization 894//===----------------------------------------------------------------------===// 895 896bool BranchFolder::OptimizeBranches(MachineFunction &MF) { 897 bool MadeChange = false; 898 899 // Make sure blocks are numbered in order 900 MF.RenumberBlocks(); 901 902 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) { 903 MachineBasicBlock *MBB = I++; 904 MadeChange |= OptimizeBlock(MBB); 905 906 // If it is dead, remove it. 907 if (MBB->pred_empty()) { 908 RemoveDeadBlock(MBB); 909 MadeChange = true; 910 ++NumDeadBlocks; 911 } 912 } 913 return MadeChange; 914} 915 916 917/// CanFallThrough - Return true if the specified block (with the specified 918/// branch condition) can implicitly transfer control to the block after it by 919/// falling off the end of it. This should return false if it can reach the 920/// block after it, but it uses an explicit branch to do so (e.g. a table jump). 921/// 922/// True is a conservative answer. 923/// 924bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB, 925 bool BranchUnAnalyzable, 926 MachineBasicBlock *TBB, 927 MachineBasicBlock *FBB, 928 const SmallVectorImpl<MachineOperand> &Cond) { 929 MachineFunction::iterator Fallthrough = CurBB; 930 ++Fallthrough; 931 // If FallthroughBlock is off the end of the function, it can't fall through. 932 if (Fallthrough == CurBB->getParent()->end()) 933 return false; 934 935 // If FallthroughBlock isn't a successor of CurBB, no fallthrough is possible. 936 if (!CurBB->isSuccessor(Fallthrough)) 937 return false; 938 939 // If we couldn't analyze the branch, examine the last instruction. 940 // If the block doesn't end in a known control barrier, assume fallthrough 941 // is possible. The isPredicable check is needed because this code can be 942 // called during IfConversion, where an instruction which is normally a 943 // Barrier is predicated and thus no longer an actual control barrier. This 944 // is over-conservative though, because if an instruction isn't actually 945 // predicated we could still treat it like a barrier. 946 if (BranchUnAnalyzable) 947 return CurBB->empty() || !CurBB->back().getDesc().isBarrier() || 948 CurBB->back().getDesc().isPredicable(); 949 950 // If there is no branch, control always falls through. 951 if (TBB == 0) return true; 952 953 // If there is some explicit branch to the fallthrough block, it can obviously 954 // reach, even though the branch should get folded to fall through implicitly. 955 if (MachineFunction::iterator(TBB) == Fallthrough || 956 MachineFunction::iterator(FBB) == Fallthrough) 957 return true; 958 959 // If it's an unconditional branch to some block not the fall through, it 960 // doesn't fall through. 961 if (Cond.empty()) return false; 962 963 // Otherwise, if it is conditional and has no explicit false block, it falls 964 // through. 965 return FBB == 0; 966} 967 968/// CanFallThrough - Return true if the specified can implicitly transfer 969/// control to the block after it by falling off the end of it. This should 970/// return false if it can reach the block after it, but it uses an explicit 971/// branch to do so (e.g. a table jump). 972/// 973/// True is a conservative answer. 974/// 975bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) { 976 MachineBasicBlock *TBB = 0, *FBB = 0; 977 SmallVector<MachineOperand, 4> Cond; 978 bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond, true); 979 return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond); 980} 981 982/// IsBetterFallthrough - Return true if it would be clearly better to 983/// fall-through to MBB1 than to fall through into MBB2. This has to return 984/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will 985/// result in infinite loops. 986static bool IsBetterFallthrough(MachineBasicBlock *MBB1, 987 MachineBasicBlock *MBB2) { 988 // Right now, we use a simple heuristic. If MBB2 ends with a call, and 989 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to 990 // optimize branches that branch to either a return block or an assert block 991 // into a fallthrough to the return. 992 if (MBB1->empty() || MBB2->empty()) return false; 993 994 // If there is a clear successor ordering we make sure that one block 995 // will fall through to the next 996 if (MBB1->isSuccessor(MBB2)) return true; 997 if (MBB2->isSuccessor(MBB1)) return false; 998 999 MachineInstr *MBB1I = --MBB1->end(); 1000 MachineInstr *MBB2I = --MBB2->end(); 1001 return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall(); 1002} 1003 1004/// TailDuplicateBlocks - Look for small blocks that are unconditionally 1005/// branched to and do not fall through. Tail-duplicate their instructions 1006/// into their predecessors to eliminate (dynamic) branches. 1007bool BranchFolder::TailDuplicateBlocks(MachineFunction &MF) { 1008 bool MadeChange = false; 1009 1010 // Make sure blocks are numbered in order 1011 MF.RenumberBlocks(); 1012 1013 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) { 1014 MachineBasicBlock *MBB = I++; 1015 1016 // Only duplicate blocks that end with unconditional branches. 1017 if (CanFallThrough(MBB)) 1018 continue; 1019 1020 MadeChange |= TailDuplicate(MBB, MF); 1021 1022 // If it is dead, remove it. 1023 if (MBB->pred_empty()) { 1024 NumInstrDups -= MBB->size(); 1025 RemoveDeadBlock(MBB); 1026 MadeChange = true; 1027 ++NumDeadBlocks; 1028 } 1029 } 1030 return MadeChange; 1031} 1032 1033/// TailDuplicate - If it is profitable, duplicate TailBB's contents in each 1034/// of its predecessors. 1035bool BranchFolder::TailDuplicate(MachineBasicBlock *TailBB, 1036 MachineFunction &MF) { 1037 // Don't try to tail-duplicate single-block loops. 1038 if (TailBB->isSuccessor(TailBB)) 1039 return false; 1040 1041 // Set the limit on the number of instructions to duplicate, with a default 1042 // of one less than the tail-merge threshold. When optimizing for size, 1043 // duplicate only one, because one branch instruction can be eliminated to 1044 // compensate for the duplication. 1045 unsigned MaxDuplicateCount = 1046 MF.getFunction()->hasFnAttr(Attribute::OptimizeForSize) ? 1047 1 : TII->TailDuplicationLimit(*TailBB, TailMergeSize - 1); 1048 1049 // Check the instructions in the block to determine whether tail-duplication 1050 // is invalid or unlikely to be profitable. 1051 unsigned i = 0; 1052 bool HasCall = false; 1053 for (MachineBasicBlock::iterator I = TailBB->begin(); 1054 I != TailBB->end(); ++I, ++i) { 1055 // Non-duplicable things shouldn't be tail-duplicated. 1056 if (I->getDesc().isNotDuplicable()) return false; 1057 // Don't duplicate more than the threshold. 1058 if (i == MaxDuplicateCount) return false; 1059 // Remember if we saw a call. 1060 if (I->getDesc().isCall()) HasCall = true; 1061 } 1062 // Heuristically, don't tail-duplicate calls if it would expand code size, 1063 // as it's less likely to be worth the extra cost. 1064 if (i > 1 && HasCall) 1065 return false; 1066 1067 // Iterate through all the unique predecessors and tail-duplicate this 1068 // block into them, if possible. Copying the list ahead of time also 1069 // avoids trouble with the predecessor list reallocating. 1070 bool Changed = false; 1071 SmallSetVector<MachineBasicBlock *, 8> Preds(TailBB->pred_begin(), 1072 TailBB->pred_end()); 1073 for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(), 1074 PE = Preds.end(); PI != PE; ++PI) { 1075 MachineBasicBlock *PredBB = *PI; 1076 1077 assert(TailBB != PredBB && 1078 "Single-block loop should have been rejected earlier!"); 1079 if (PredBB->succ_size() > 1) continue; 1080 1081 MachineBasicBlock *PredTBB, *PredFBB; 1082 SmallVector<MachineOperand, 4> PredCond; 1083 if (TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true)) 1084 continue; 1085 if (!PredCond.empty()) 1086 continue; 1087 // EH edges are ignored by AnalyzeBranch. 1088 if (PredBB->succ_size() != 1) 1089 continue; 1090 // Don't duplicate into a fall-through predecessor (at least for now). 1091 if (PredBB->isLayoutSuccessor(TailBB) && CanFallThrough(PredBB)) 1092 continue; 1093 1094 DEBUG(errs() << "\nTail-duplicating into PredBB: " << *PredBB 1095 << "From Succ: " << *TailBB); 1096 1097 // Remove PredBB's unconditional branch. 1098 TII->RemoveBranch(*PredBB); 1099 // Clone the contents of TailBB into PredBB. 1100 for (MachineBasicBlock::iterator I = TailBB->begin(), E = TailBB->end(); 1101 I != E; ++I) { 1102 MachineInstr *NewMI = MF.CloneMachineInstr(I); 1103 PredBB->insert(PredBB->end(), NewMI); 1104 } 1105 NumInstrDups += TailBB->size() - 1; // subtract one for removed branch 1106 1107 // Update the CFG. 1108 PredBB->removeSuccessor(PredBB->succ_begin()); 1109 assert(PredBB->succ_empty() && 1110 "TailDuplicate called on block with multiple successors!"); 1111 for (MachineBasicBlock::succ_iterator I = TailBB->succ_begin(), 1112 E = TailBB->succ_end(); I != E; ++I) 1113 PredBB->addSuccessor(*I); 1114 1115 Changed = true; 1116 ++NumTailDups; 1117 } 1118 1119 // If TailBB was duplicated into all its predecessors except for the prior 1120 // block, which falls through unconditionally, move the contents of this 1121 // block into the prior block. 1122 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(TailBB)); 1123 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0; 1124 SmallVector<MachineOperand, 4> PriorCond; 1125 bool PriorUnAnalyzable = 1126 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true); 1127 // This has to check PrevBB->succ_size() because EH edges are ignored by 1128 // AnalyzeBranch. 1129 if (!PriorUnAnalyzable && PriorCond.empty() && !PriorTBB && 1130 TailBB->pred_size() == 1 && PrevBB.succ_size() == 1 && 1131 !TailBB->hasAddressTaken()) { 1132 DEBUG(errs() << "\nMerging into block: " << PrevBB 1133 << "From MBB: " << *TailBB); 1134 PrevBB.splice(PrevBB.end(), TailBB, TailBB->begin(), TailBB->end()); 1135 PrevBB.removeSuccessor(PrevBB.succ_begin());; 1136 assert(PrevBB.succ_empty()); 1137 PrevBB.transferSuccessors(TailBB); 1138 Changed = true; 1139 } 1140 1141 return Changed; 1142} 1143 1144/// OptimizeBlock - Analyze and optimize control flow related to the specified 1145/// block. This is never called on the entry block. 1146bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) { 1147 bool MadeChange = false; 1148 MachineFunction &MF = *MBB->getParent(); 1149ReoptimizeBlock: 1150 1151 MachineFunction::iterator FallThrough = MBB; 1152 ++FallThrough; 1153 1154 // If this block is empty, make everyone use its fall-through, not the block 1155 // explicitly. Landing pads should not do this since the landing-pad table 1156 // points to this block. Blocks with their addresses taken shouldn't be 1157 // optimized away. 1158 if (MBB->empty() && !MBB->isLandingPad() && !MBB->hasAddressTaken()) { 1159 // Dead block? Leave for cleanup later. 1160 if (MBB->pred_empty()) return MadeChange; 1161 1162 if (FallThrough == MF.end()) { 1163 // TODO: Simplify preds to not branch here if possible! 1164 } else { 1165 // Rewrite all predecessors of the old block to go to the fallthrough 1166 // instead. 1167 while (!MBB->pred_empty()) { 1168 MachineBasicBlock *Pred = *(MBB->pred_end()-1); 1169 Pred->ReplaceUsesOfBlockWith(MBB, FallThrough); 1170 } 1171 // If MBB was the target of a jump table, update jump tables to go to the 1172 // fallthrough instead. 1173 MF.getJumpTableInfo()->ReplaceMBBInJumpTables(MBB, FallThrough); 1174 MadeChange = true; 1175 } 1176 return MadeChange; 1177 } 1178 1179 // Check to see if we can simplify the terminator of the block before this 1180 // one. 1181 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB)); 1182 1183 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0; 1184 SmallVector<MachineOperand, 4> PriorCond; 1185 bool PriorUnAnalyzable = 1186 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true); 1187 if (!PriorUnAnalyzable) { 1188 // If the CFG for the prior block has extra edges, remove them. 1189 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB, 1190 !PriorCond.empty()); 1191 1192 // If the previous branch is conditional and both conditions go to the same 1193 // destination, remove the branch, replacing it with an unconditional one or 1194 // a fall-through. 1195 if (PriorTBB && PriorTBB == PriorFBB) { 1196 TII->RemoveBranch(PrevBB); 1197 PriorCond.clear(); 1198 if (PriorTBB != MBB) 1199 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 1200 MadeChange = true; 1201 ++NumBranchOpts; 1202 goto ReoptimizeBlock; 1203 } 1204 1205 // If the previous block unconditionally falls through to this block and 1206 // this block has no other predecessors, move the contents of this block 1207 // into the prior block. This doesn't usually happen when SimplifyCFG 1208 // has been used, but it can happen if tail merging splits a fall-through 1209 // predecessor of a block. 1210 // This has to check PrevBB->succ_size() because EH edges are ignored by 1211 // AnalyzeBranch. 1212 if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 && 1213 PrevBB.succ_size() == 1 && 1214 !MBB->hasAddressTaken()) { 1215 DEBUG(errs() << "\nMerging into block: " << PrevBB 1216 << "From MBB: " << *MBB); 1217 PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end()); 1218 PrevBB.removeSuccessor(PrevBB.succ_begin());; 1219 assert(PrevBB.succ_empty()); 1220 PrevBB.transferSuccessors(MBB); 1221 MadeChange = true; 1222 return MadeChange; 1223 } 1224 1225 // If the previous branch *only* branches to *this* block (conditional or 1226 // not) remove the branch. 1227 if (PriorTBB == MBB && PriorFBB == 0) { 1228 TII->RemoveBranch(PrevBB); 1229 MadeChange = true; 1230 ++NumBranchOpts; 1231 goto ReoptimizeBlock; 1232 } 1233 1234 // If the prior block branches somewhere else on the condition and here if 1235 // the condition is false, remove the uncond second branch. 1236 if (PriorFBB == MBB) { 1237 TII->RemoveBranch(PrevBB); 1238 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 1239 MadeChange = true; 1240 ++NumBranchOpts; 1241 goto ReoptimizeBlock; 1242 } 1243 1244 // If the prior block branches here on true and somewhere else on false, and 1245 // if the branch condition is reversible, reverse the branch to create a 1246 // fall-through. 1247 if (PriorTBB == MBB) { 1248 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond); 1249 if (!TII->ReverseBranchCondition(NewPriorCond)) { 1250 TII->RemoveBranch(PrevBB); 1251 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond); 1252 MadeChange = true; 1253 ++NumBranchOpts; 1254 goto ReoptimizeBlock; 1255 } 1256 } 1257 1258 // If this block has no successors (e.g. it is a return block or ends with 1259 // a call to a no-return function like abort or __cxa_throw) and if the pred 1260 // falls through into this block, and if it would otherwise fall through 1261 // into the block after this, move this block to the end of the function. 1262 // 1263 // We consider it more likely that execution will stay in the function (e.g. 1264 // due to loops) than it is to exit it. This asserts in loops etc, moving 1265 // the assert condition out of the loop body. 1266 if (MBB->succ_empty() && !PriorCond.empty() && PriorFBB == 0 && 1267 MachineFunction::iterator(PriorTBB) == FallThrough && 1268 !CanFallThrough(MBB)) { 1269 bool DoTransform = true; 1270 1271 // We have to be careful that the succs of PredBB aren't both no-successor 1272 // blocks. If neither have successors and if PredBB is the second from 1273 // last block in the function, we'd just keep swapping the two blocks for 1274 // last. Only do the swap if one is clearly better to fall through than 1275 // the other. 1276 if (FallThrough == --MF.end() && 1277 !IsBetterFallthrough(PriorTBB, MBB)) 1278 DoTransform = false; 1279 1280 // We don't want to do this transformation if we have control flow like: 1281 // br cond BB2 1282 // BB1: 1283 // .. 1284 // jmp BBX 1285 // BB2: 1286 // .. 1287 // ret 1288 // 1289 // In this case, we could actually be moving the return block *into* a 1290 // loop! 1291 if (DoTransform && !MBB->succ_empty() && 1292 (!CanFallThrough(PriorTBB) || PriorTBB->empty())) 1293 DoTransform = false; 1294 1295 1296 if (DoTransform) { 1297 // Reverse the branch so we will fall through on the previous true cond. 1298 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond); 1299 if (!TII->ReverseBranchCondition(NewPriorCond)) { 1300 DEBUG(errs() << "\nMoving MBB: " << *MBB 1301 << "To make fallthrough to: " << *PriorTBB << "\n"); 1302 1303 TII->RemoveBranch(PrevBB); 1304 TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond); 1305 1306 // Move this block to the end of the function. 1307 MBB->moveAfter(--MF.end()); 1308 MadeChange = true; 1309 ++NumBranchOpts; 1310 return MadeChange; 1311 } 1312 } 1313 } 1314 } 1315 1316 // Analyze the branch in the current block. 1317 MachineBasicBlock *CurTBB = 0, *CurFBB = 0; 1318 SmallVector<MachineOperand, 4> CurCond; 1319 bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true); 1320 if (!CurUnAnalyzable) { 1321 // If the CFG for the prior block has extra edges, remove them. 1322 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty()); 1323 1324 // If this is a two-way branch, and the FBB branches to this block, reverse 1325 // the condition so the single-basic-block loop is faster. Instead of: 1326 // Loop: xxx; jcc Out; jmp Loop 1327 // we want: 1328 // Loop: xxx; jncc Loop; jmp Out 1329 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) { 1330 SmallVector<MachineOperand, 4> NewCond(CurCond); 1331 if (!TII->ReverseBranchCondition(NewCond)) { 1332 TII->RemoveBranch(*MBB); 1333 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond); 1334 MadeChange = true; 1335 ++NumBranchOpts; 1336 goto ReoptimizeBlock; 1337 } 1338 } 1339 1340 // If this branch is the only thing in its block, see if we can forward 1341 // other blocks across it. 1342 if (CurTBB && CurCond.empty() && CurFBB == 0 && 1343 MBB->begin()->getDesc().isBranch() && CurTBB != MBB && 1344 !MBB->hasAddressTaken()) { 1345 // This block may contain just an unconditional branch. Because there can 1346 // be 'non-branch terminators' in the block, try removing the branch and 1347 // then seeing if the block is empty. 1348 TII->RemoveBranch(*MBB); 1349 1350 // If this block is just an unconditional branch to CurTBB, we can 1351 // usually completely eliminate the block. The only case we cannot 1352 // completely eliminate the block is when the block before this one 1353 // falls through into MBB and we can't understand the prior block's branch 1354 // condition. 1355 if (MBB->empty()) { 1356 bool PredHasNoFallThrough = TII->BlockHasNoFallThrough(PrevBB); 1357 if (PredHasNoFallThrough || !PriorUnAnalyzable || 1358 !PrevBB.isSuccessor(MBB)) { 1359 // If the prior block falls through into us, turn it into an 1360 // explicit branch to us to make updates simpler. 1361 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) && 1362 PriorTBB != MBB && PriorFBB != MBB) { 1363 if (PriorTBB == 0) { 1364 assert(PriorCond.empty() && PriorFBB == 0 && 1365 "Bad branch analysis"); 1366 PriorTBB = MBB; 1367 } else { 1368 assert(PriorFBB == 0 && "Machine CFG out of date!"); 1369 PriorFBB = MBB; 1370 } 1371 TII->RemoveBranch(PrevBB); 1372 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); 1373 } 1374 1375 // Iterate through all the predecessors, revectoring each in-turn. 1376 size_t PI = 0; 1377 bool DidChange = false; 1378 bool HasBranchToSelf = false; 1379 while(PI != MBB->pred_size()) { 1380 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI); 1381 if (PMBB == MBB) { 1382 // If this block has an uncond branch to itself, leave it. 1383 ++PI; 1384 HasBranchToSelf = true; 1385 } else { 1386 DidChange = true; 1387 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB); 1388 // If this change resulted in PMBB ending in a conditional 1389 // branch where both conditions go to the same destination, 1390 // change this to an unconditional branch (and fix the CFG). 1391 MachineBasicBlock *NewCurTBB = 0, *NewCurFBB = 0; 1392 SmallVector<MachineOperand, 4> NewCurCond; 1393 bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB, 1394 NewCurFBB, NewCurCond, true); 1395 if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) { 1396 TII->RemoveBranch(*PMBB); 1397 NewCurCond.clear(); 1398 TII->InsertBranch(*PMBB, NewCurTBB, 0, NewCurCond); 1399 MadeChange = true; 1400 ++NumBranchOpts; 1401 PMBB->CorrectExtraCFGEdges(NewCurTBB, 0, false); 1402 } 1403 } 1404 } 1405 1406 // Change any jumptables to go to the new MBB. 1407 MF.getJumpTableInfo()->ReplaceMBBInJumpTables(MBB, CurTBB); 1408 if (DidChange) { 1409 ++NumBranchOpts; 1410 MadeChange = true; 1411 if (!HasBranchToSelf) return MadeChange; 1412 } 1413 } 1414 } 1415 1416 // Add the branch back if the block is more than just an uncond branch. 1417 TII->InsertBranch(*MBB, CurTBB, 0, CurCond); 1418 } 1419 } 1420 1421 // If the prior block doesn't fall through into this block, and if this 1422 // block doesn't fall through into some other block, see if we can find a 1423 // place to move this block where a fall-through will happen. 1424 if (!CanFallThrough(&PrevBB, PriorUnAnalyzable, 1425 PriorTBB, PriorFBB, PriorCond)) { 1426 1427 // Now we know that there was no fall-through into this block, check to 1428 // see if it has a fall-through into its successor. 1429 bool CurFallsThru = CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB, 1430 CurCond); 1431 1432 if (!MBB->isLandingPad()) { 1433 // Check all the predecessors of this block. If one of them has no fall 1434 // throughs, move this block right after it. 1435 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(), 1436 E = MBB->pred_end(); PI != E; ++PI) { 1437 // Analyze the branch at the end of the pred. 1438 MachineBasicBlock *PredBB = *PI; 1439 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough; 1440 MachineBasicBlock *PredTBB, *PredFBB; 1441 SmallVector<MachineOperand, 4> PredCond; 1442 if (PredBB != MBB && !CanFallThrough(PredBB) && 1443 !TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true) 1444 && (!CurFallsThru || !CurTBB || !CurFBB) 1445 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) { 1446 // If the current block doesn't fall through, just move it. 1447 // If the current block can fall through and does not end with a 1448 // conditional branch, we need to append an unconditional jump to 1449 // the (current) next block. To avoid a possible compile-time 1450 // infinite loop, move blocks only backward in this case. 1451 // Also, if there are already 2 branches here, we cannot add a third; 1452 // this means we have the case 1453 // Bcc next 1454 // B elsewhere 1455 // next: 1456 if (CurFallsThru) { 1457 MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB)); 1458 CurCond.clear(); 1459 TII->InsertBranch(*MBB, NextBB, 0, CurCond); 1460 } 1461 MBB->moveAfter(PredBB); 1462 MadeChange = true; 1463 goto ReoptimizeBlock; 1464 } 1465 } 1466 } 1467 1468 if (!CurFallsThru) { 1469 // Check all successors to see if we can move this block before it. 1470 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(), 1471 E = MBB->succ_end(); SI != E; ++SI) { 1472 // Analyze the branch at the end of the block before the succ. 1473 MachineBasicBlock *SuccBB = *SI; 1474 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev; 1475 1476 // If this block doesn't already fall-through to that successor, and if 1477 // the succ doesn't already have a block that can fall through into it, 1478 // and if the successor isn't an EH destination, we can arrange for the 1479 // fallthrough to happen. 1480 if (SuccBB != MBB && &*SuccPrev != MBB && 1481 !CanFallThrough(SuccPrev) && !CurUnAnalyzable && 1482 !SuccBB->isLandingPad()) { 1483 MBB->moveBefore(SuccBB); 1484 MadeChange = true; 1485 goto ReoptimizeBlock; 1486 } 1487 } 1488 1489 // Okay, there is no really great place to put this block. If, however, 1490 // the block before this one would be a fall-through if this block were 1491 // removed, move this block to the end of the function. 1492 MachineBasicBlock *PrevTBB, *PrevFBB; 1493 SmallVector<MachineOperand, 4> PrevCond; 1494 if (FallThrough != MF.end() && 1495 !TII->AnalyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) && 1496 PrevBB.isSuccessor(FallThrough)) { 1497 MBB->moveAfter(--MF.end()); 1498 MadeChange = true; 1499 return MadeChange; 1500 } 1501 } 1502 } 1503 1504 return MadeChange; 1505} 1506