BranchFolding.cpp revision e7bfb8796a10f92b4749d8c2510437693a8722e7
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"); 44static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge", 45 cl::init(cl::BOU_UNSET), cl::Hidden); 46// Throttle for huge numbers of predecessors (compile speed problems) 47static cl::opt<unsigned> 48TailMergeThreshold("tail-merge-threshold", 49 cl::desc("Max number of predecessors to consider tail merging"), 50 cl::init(150), cl::Hidden); 51 52// Heuristic for tail merging (and, inversely, tail duplication). 53// TODO: This should be replaced with a target query. 54static cl::opt<unsigned> 55TailMergeSize("tail-merge-size", 56 cl::desc("Min number of instructions to consider tail merging"), 57 cl::init(3), cl::Hidden); 58 59char BranchFolderPass::ID = 0; 60 61FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) { 62 return new BranchFolderPass(DefaultEnableTailMerge); 63} 64 65bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) { 66 return OptimizeFunction(MF, 67 MF.getTarget().getInstrInfo(), 68 MF.getTarget().getRegisterInfo(), 69 getAnalysisIfAvailable<MachineModuleInfo>()); 70} 71 72 73BranchFolder::BranchFolder(bool defaultEnableTailMerge) { 74 switch (FlagEnableTailMerge) { 75 case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break; 76 case cl::BOU_TRUE: EnableTailMerge = true; break; 77 case cl::BOU_FALSE: EnableTailMerge = false; break; 78 } 79} 80 81/// RemoveDeadBlock - Remove the specified dead machine basic block from the 82/// function, updating the CFG. 83void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) { 84 assert(MBB->pred_empty() && "MBB must be dead!"); 85 DEBUG(errs() << "\nRemoving MBB: " << *MBB); 86 87 MachineFunction *MF = MBB->getParent(); 88 // drop all successors. 89 while (!MBB->succ_empty()) 90 MBB->removeSuccessor(MBB->succ_end()-1); 91 92 // If there are any labels in the basic block, unregister them from 93 // MachineModuleInfo. 94 if (MMI && !MBB->empty()) { 95 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); 96 I != E; ++I) { 97 if (I->isLabel()) 98 // The label ID # is always operand #0, an immediate. 99 MMI->InvalidateLabel(I->getOperand(0).getImm()); 100 } 101 } 102 103 // Remove the block. 104 MF->erase(MBB); 105} 106 107/// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def 108/// followed by terminators, and if the implicitly defined registers are not 109/// used by the terminators, remove those implicit_def's. e.g. 110/// BB1: 111/// r0 = implicit_def 112/// r1 = implicit_def 113/// br 114/// This block can be optimized away later if the implicit instructions are 115/// removed. 116bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) { 117 SmallSet<unsigned, 4> ImpDefRegs; 118 MachineBasicBlock::iterator I = MBB->begin(); 119 while (I != MBB->end()) { 120 if (I->getOpcode() != TargetInstrInfo::IMPLICIT_DEF) 121 break; 122 unsigned Reg = I->getOperand(0).getReg(); 123 ImpDefRegs.insert(Reg); 124 for (const unsigned *SubRegs = TRI->getSubRegisters(Reg); 125 unsigned SubReg = *SubRegs; ++SubRegs) 126 ImpDefRegs.insert(SubReg); 127 ++I; 128 } 129 if (ImpDefRegs.empty()) 130 return false; 131 132 MachineBasicBlock::iterator FirstTerm = I; 133 while (I != MBB->end()) { 134 if (!TII->isUnpredicatedTerminator(I)) 135 return false; 136 // See if it uses any of the implicitly defined registers. 137 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) { 138 MachineOperand &MO = I->getOperand(i); 139 if (!MO.isReg() || !MO.isUse()) 140 continue; 141 unsigned Reg = MO.getReg(); 142 if (ImpDefRegs.count(Reg)) 143 return false; 144 } 145 ++I; 146 } 147 148 I = MBB->begin(); 149 while (I != FirstTerm) { 150 MachineInstr *ImpDefMI = &*I; 151 ++I; 152 MBB->erase(ImpDefMI); 153 } 154 155 return true; 156} 157 158/// OptimizeFunction - Perhaps branch folding, tail merging and other 159/// CFG optimizations on the given function. 160bool BranchFolder::OptimizeFunction(MachineFunction &MF, 161 const TargetInstrInfo *tii, 162 const TargetRegisterInfo *tri, 163 MachineModuleInfo *mmi) { 164 if (!tii) return false; 165 166 TII = tii; 167 TRI = tri; 168 MMI = mmi; 169 170 RS = TRI->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL; 171 172 // Fix CFG. The later algorithms expect it to be right. 173 bool MadeChange = false; 174 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) { 175 MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0; 176 SmallVector<MachineOperand, 4> Cond; 177 if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true)) 178 MadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty()); 179 MadeChange |= OptimizeImpDefsBlock(MBB); 180 } 181 182 183 bool MadeChangeThisIteration = true; 184 while (MadeChangeThisIteration) { 185 MadeChangeThisIteration = false; 186 MadeChangeThisIteration |= TailMergeBlocks(MF); 187 MadeChangeThisIteration |= OptimizeBranches(MF); 188 MadeChange |= MadeChangeThisIteration; 189 } 190 191 // See if any jump tables have become mergable or dead as the code generator 192 // did its thing. 193 MachineJumpTableInfo *JTI = MF.getJumpTableInfo(); 194 const std::vector<MachineJumpTableEntry> &JTs = JTI->getJumpTables(); 195 if (!JTs.empty()) { 196 // Figure out how these jump tables should be merged. 197 std::vector<unsigned> JTMapping; 198 JTMapping.reserve(JTs.size()); 199 200 // We always keep the 0th jump table. 201 JTMapping.push_back(0); 202 203 // Scan the jump tables, seeing if there are any duplicates. Note that this 204 // is N^2, which should be fixed someday. 205 for (unsigned i = 1, e = JTs.size(); i != e; ++i) { 206 if (JTs[i].MBBs.empty()) 207 JTMapping.push_back(i); 208 else 209 JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs)); 210 } 211 212 // If a jump table was merge with another one, walk the function rewriting 213 // references to jump tables to reference the new JT ID's. Keep track of 214 // whether we see a jump table idx, if not, we can delete the JT. 215 BitVector JTIsLive(JTs.size()); 216 for (MachineFunction::iterator BB = MF.begin(), E = MF.end(); 217 BB != E; ++BB) { 218 for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end(); 219 I != E; ++I) 220 for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) { 221 MachineOperand &Op = I->getOperand(op); 222 if (!Op.isJTI()) continue; 223 unsigned NewIdx = JTMapping[Op.getIndex()]; 224 Op.setIndex(NewIdx); 225 226 // Remember that this JT is live. 227 JTIsLive.set(NewIdx); 228 } 229 } 230 231 // Finally, remove dead jump tables. This happens either because the 232 // indirect jump was unreachable (and thus deleted) or because the jump 233 // table was merged with some other one. 234 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i) 235 if (!JTIsLive.test(i)) { 236 JTI->RemoveJumpTable(i); 237 MadeChange = true; 238 } 239 } 240 241 delete RS; 242 return MadeChange; 243} 244 245//===----------------------------------------------------------------------===// 246// Tail Merging of Blocks 247//===----------------------------------------------------------------------===// 248 249/// HashMachineInstr - Compute a hash value for MI and its operands. 250static unsigned HashMachineInstr(const MachineInstr *MI) { 251 unsigned Hash = MI->getOpcode(); 252 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 253 const MachineOperand &Op = MI->getOperand(i); 254 255 // Merge in bits from the operand if easy. 256 unsigned OperandHash = 0; 257 switch (Op.getType()) { 258 case MachineOperand::MO_Register: OperandHash = Op.getReg(); break; 259 case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break; 260 case MachineOperand::MO_MachineBasicBlock: 261 OperandHash = Op.getMBB()->getNumber(); 262 break; 263 case MachineOperand::MO_FrameIndex: 264 case MachineOperand::MO_ConstantPoolIndex: 265 case MachineOperand::MO_JumpTableIndex: 266 OperandHash = Op.getIndex(); 267 break; 268 case MachineOperand::MO_GlobalAddress: 269 case MachineOperand::MO_ExternalSymbol: 270 // Global address / external symbol are too hard, don't bother, but do 271 // pull in the offset. 272 OperandHash = Op.getOffset(); 273 break; 274 default: break; 275 } 276 277 Hash += ((OperandHash << 3) | Op.getType()) << (i&31); 278 } 279 return Hash; 280} 281 282/// HashEndOfMBB - Hash the last few instructions in the MBB. For blocks 283/// with no successors, we hash two instructions, because cross-jumping 284/// only saves code when at least two instructions are removed (since a 285/// branch must be inserted). For blocks with a successor, one of the 286/// two blocks to be tail-merged will end with a branch already, so 287/// it gains to cross-jump even for one instruction. 288static unsigned HashEndOfMBB(const MachineBasicBlock *MBB, 289 unsigned minCommonTailLength) { 290 MachineBasicBlock::const_iterator I = MBB->end(); 291 if (I == MBB->begin()) 292 return 0; // Empty MBB. 293 294 --I; 295 unsigned Hash = HashMachineInstr(I); 296 297 if (I == MBB->begin() || minCommonTailLength == 1) 298 return Hash; // Single instr MBB. 299 300 --I; 301 // Hash in the second-to-last instruction. 302 Hash ^= HashMachineInstr(I) << 2; 303 return Hash; 304} 305 306/// ComputeCommonTailLength - Given two machine basic blocks, compute the number 307/// of instructions they actually have in common together at their end. Return 308/// iterators for the first shared instruction in each block. 309static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1, 310 MachineBasicBlock *MBB2, 311 MachineBasicBlock::iterator &I1, 312 MachineBasicBlock::iterator &I2) { 313 I1 = MBB1->end(); 314 I2 = MBB2->end(); 315 316 unsigned TailLen = 0; 317 while (I1 != MBB1->begin() && I2 != MBB2->begin()) { 318 --I1; --I2; 319 if (!I1->isIdenticalTo(I2) || 320 // FIXME: This check is dubious. It's used to get around a problem where 321 // people incorrectly expect inline asm directives to remain in the same 322 // relative order. This is untenable because normal compiler 323 // optimizations (like this one) may reorder and/or merge these 324 // directives. 325 I1->getOpcode() == TargetInstrInfo::INLINEASM) { 326 ++I1; ++I2; 327 break; 328 } 329 ++TailLen; 330 } 331 return TailLen; 332} 333 334/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything 335/// after it, replacing it with an unconditional branch to NewDest. This 336/// returns true if OldInst's block is modified, false if NewDest is modified. 337void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, 338 MachineBasicBlock *NewDest) { 339 MachineBasicBlock *OldBB = OldInst->getParent(); 340 341 // Remove all the old successors of OldBB from the CFG. 342 while (!OldBB->succ_empty()) 343 OldBB->removeSuccessor(OldBB->succ_begin()); 344 345 // Remove all the dead instructions from the end of OldBB. 346 OldBB->erase(OldInst, OldBB->end()); 347 348 // If OldBB isn't immediately before OldBB, insert a branch to it. 349 if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest)) 350 TII->InsertBranch(*OldBB, NewDest, 0, SmallVector<MachineOperand, 0>()); 351 OldBB->addSuccessor(NewDest); 352 ++NumTailMerge; 353} 354 355/// SplitMBBAt - Given a machine basic block and an iterator into it, split the 356/// MBB so that the part before the iterator falls into the part starting at the 357/// iterator. This returns the new MBB. 358MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB, 359 MachineBasicBlock::iterator BBI1) { 360 MachineFunction &MF = *CurMBB.getParent(); 361 362 // Create the fall-through block. 363 MachineFunction::iterator MBBI = &CurMBB; 364 MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(CurMBB.getBasicBlock()); 365 CurMBB.getParent()->insert(++MBBI, NewMBB); 366 367 // Move all the successors of this block to the specified block. 368 NewMBB->transferSuccessors(&CurMBB); 369 370 // Add an edge from CurMBB to NewMBB for the fall-through. 371 CurMBB.addSuccessor(NewMBB); 372 373 // Splice the code over. 374 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end()); 375 376 // For targets that use the register scavenger, we must maintain LiveIns. 377 if (RS) { 378 RS->enterBasicBlock(&CurMBB); 379 if (!CurMBB.empty()) 380 RS->forward(prior(CurMBB.end())); 381 BitVector RegsLiveAtExit(TRI->getNumRegs()); 382 RS->getRegsUsed(RegsLiveAtExit, false); 383 for (unsigned int i=0, e=TRI->getNumRegs(); i!=e; i++) 384 if (RegsLiveAtExit[i]) 385 NewMBB->addLiveIn(i); 386 } 387 388 return NewMBB; 389} 390 391/// EstimateRuntime - Make a rough estimate for how long it will take to run 392/// the specified code. 393static unsigned EstimateRuntime(MachineBasicBlock::iterator I, 394 MachineBasicBlock::iterator E) { 395 unsigned Time = 0; 396 for (; I != E; ++I) { 397 const TargetInstrDesc &TID = I->getDesc(); 398 if (TID.isCall()) 399 Time += 10; 400 else if (TID.mayLoad() || TID.mayStore()) 401 Time += 2; 402 else 403 ++Time; 404 } 405 return Time; 406} 407 408// CurMBB needs to add an unconditional branch to SuccMBB (we removed these 409// branches temporarily for tail merging). In the case where CurMBB ends 410// with a conditional branch to the next block, optimize by reversing the 411// test and conditionally branching to SuccMBB instead. 412static void FixTail(MachineBasicBlock* CurMBB, MachineBasicBlock *SuccBB, 413 const TargetInstrInfo *TII) { 414 MachineFunction *MF = CurMBB->getParent(); 415 MachineFunction::iterator I = next(MachineFunction::iterator(CurMBB)); 416 MachineBasicBlock *TBB = 0, *FBB = 0; 417 SmallVector<MachineOperand, 4> Cond; 418 if (I != MF->end() && 419 !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond, true)) { 420 MachineBasicBlock *NextBB = I; 421 if (TBB == NextBB && !Cond.empty() && !FBB) { 422 if (!TII->ReverseBranchCondition(Cond)) { 423 TII->RemoveBranch(*CurMBB); 424 TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond); 425 return; 426 } 427 } 428 } 429 TII->InsertBranch(*CurMBB, SuccBB, NULL, SmallVector<MachineOperand, 0>()); 430} 431 432bool 433BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const { 434 if (getHash() < o.getHash()) 435 return true; 436 else if (getHash() > o.getHash()) 437 return false; 438 else if (getBlock()->getNumber() < o.getBlock()->getNumber()) 439 return true; 440 else if (getBlock()->getNumber() > o.getBlock()->getNumber()) 441 return false; 442 else { 443 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing 444 // an object with itself. 445#ifndef _GLIBCXX_DEBUG 446 llvm_unreachable("Predecessor appears twice"); 447#endif 448 return false; 449 } 450} 451 452/// CountTerminators - Count the number of terminators in the given 453/// block and set I to the position of the first non-terminator, if there 454/// is one, or MBB->end() otherwise. 455static unsigned CountTerminators(MachineBasicBlock *MBB, 456 MachineBasicBlock::iterator &I) { 457 I = MBB->end(); 458 unsigned NumTerms = 0; 459 for (;;) { 460 if (I == MBB->begin()) { 461 I = MBB->end(); 462 break; 463 } 464 --I; 465 if (!I->getDesc().isTerminator()) break; 466 ++NumTerms; 467 } 468 return NumTerms; 469} 470 471/// ProfitableToMerge - Check if two machine basic blocks have a common tail 472/// and decide if it would be profitable to merge those tails. Return the 473/// length of the common tail and iterators to the first common instruction 474/// in each block. 475static bool ProfitableToMerge(MachineBasicBlock *MBB1, 476 MachineBasicBlock *MBB2, 477 unsigned minCommonTailLength, 478 unsigned &CommonTailLen, 479 MachineBasicBlock::iterator &I1, 480 MachineBasicBlock::iterator &I2, 481 MachineBasicBlock *SuccBB, 482 MachineBasicBlock *PredBB) { 483 CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2); 484 MachineFunction *MF = MBB1->getParent(); 485 486 if (CommonTailLen == 0) 487 return false; 488 489 // It's almost always profitable to merge any number of non-terminator 490 // instructions with the block that falls through into the common successor. 491 if (MBB1 == PredBB || MBB2 == PredBB) { 492 MachineBasicBlock::iterator I; 493 unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I); 494 if (CommonTailLen > NumTerms) 495 return true; 496 } 497 498 // If both blocks have an unconditional branch temporarily stripped out, 499 // treat that as an additional common instruction. 500 if (MBB1 != PredBB && MBB2 != PredBB && 501 !MBB1->back().getDesc().isBarrier() && 502 !MBB2->back().getDesc().isBarrier()) 503 --minCommonTailLength; 504 505 // Check if the common tail is long enough to be worthwhile. 506 if (CommonTailLen >= minCommonTailLength) 507 return true; 508 509 // If we are optimizing for code size, 1 instruction in common is enough if 510 // we don't have to split a block. At worst we will be replacing a 511 // fallthrough into the common tail with a branch, which at worst breaks 512 // even with falling through into the duplicated common tail. 513 if (MF->getFunction()->hasFnAttr(Attribute::OptimizeForSize) && 514 (I1 == MBB1->begin() || I2 == MBB2->begin())) 515 return true; 516 517 return false; 518} 519 520/// ComputeSameTails - Look through all the blocks in MergePotentials that have 521/// hash CurHash (guaranteed to match the last element). Build the vector 522/// SameTails of all those that have the (same) largest number of instructions 523/// in common of any pair of these blocks. SameTails entries contain an 524/// iterator into MergePotentials (from which the MachineBasicBlock can be 525/// found) and a MachineBasicBlock::iterator into that MBB indicating the 526/// instruction where the matching code sequence begins. 527/// Order of elements in SameTails is the reverse of the order in which 528/// those blocks appear in MergePotentials (where they are not necessarily 529/// consecutive). 530unsigned BranchFolder::ComputeSameTails(unsigned CurHash, 531 unsigned minCommonTailLength, 532 MachineBasicBlock *SuccBB, 533 MachineBasicBlock *PredBB) { 534 unsigned maxCommonTailLength = 0U; 535 SameTails.clear(); 536 MachineBasicBlock::iterator TrialBBI1, TrialBBI2; 537 MPIterator HighestMPIter = prior(MergePotentials.end()); 538 for (MPIterator CurMPIter = prior(MergePotentials.end()), 539 B = MergePotentials.begin(); 540 CurMPIter!=B && CurMPIter->getHash() == CurHash; 541 --CurMPIter) { 542 for (MPIterator I = prior(CurMPIter); I->getHash() == CurHash ; --I) { 543 unsigned CommonTailLen; 544 if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(), 545 minCommonTailLength, 546 CommonTailLen, TrialBBI1, TrialBBI2, 547 SuccBB, PredBB)) { 548 if (CommonTailLen > maxCommonTailLength) { 549 SameTails.clear(); 550 maxCommonTailLength = CommonTailLen; 551 HighestMPIter = CurMPIter; 552 SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1)); 553 } 554 if (HighestMPIter == CurMPIter && 555 CommonTailLen == maxCommonTailLength) 556 SameTails.push_back(SameTailElt(I, TrialBBI2)); 557 } 558 if (I == B) 559 break; 560 } 561 } 562 return maxCommonTailLength; 563} 564 565/// RemoveBlocksWithHash - Remove all blocks with hash CurHash from 566/// MergePotentials, restoring branches at ends of blocks as appropriate. 567void BranchFolder::RemoveBlocksWithHash(unsigned CurHash, 568 MachineBasicBlock* SuccBB, 569 MachineBasicBlock* PredBB) { 570 MPIterator CurMPIter, B; 571 for (CurMPIter = prior(MergePotentials.end()), B = MergePotentials.begin(); 572 CurMPIter->getHash() == CurHash; 573 --CurMPIter) { 574 // Put the unconditional branch back, if we need one. 575 MachineBasicBlock *CurMBB = CurMPIter->getBlock(); 576 if (SuccBB && CurMBB != PredBB) 577 FixTail(CurMBB, SuccBB, TII); 578 if (CurMPIter == B) 579 break; 580 } 581 if (CurMPIter->getHash() != CurHash) 582 CurMPIter++; 583 MergePotentials.erase(CurMPIter, MergePotentials.end()); 584} 585 586/// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist 587/// only of the common tail. Create a block that does by splitting one. 588unsigned BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB, 589 unsigned maxCommonTailLength) { 590 unsigned i, commonTailIndex; 591 unsigned TimeEstimate = ~0U; 592 for (i=0, commonTailIndex=0; i<SameTails.size(); i++) { 593 // Use PredBB if possible; that doesn't require a new branch. 594 if (SameTails[i].getBlock() == PredBB) { 595 commonTailIndex = i; 596 break; 597 } 598 // Otherwise, make a (fairly bogus) choice based on estimate of 599 // how long it will take the various blocks to execute. 600 unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(), 601 SameTails[i].getTailStartPos()); 602 if (t <= TimeEstimate) { 603 TimeEstimate = t; 604 commonTailIndex = i; 605 } 606 } 607 608 MachineBasicBlock::iterator BBI = 609 SameTails[commonTailIndex].getTailStartPos(); 610 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock(); 611 612 DEBUG(errs() << "\nSplitting BB#" << MBB->getNumber() << ", size " 613 << maxCommonTailLength); 614 615 MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI); 616 SameTails[commonTailIndex].setBlock(newMBB); 617 SameTails[commonTailIndex].setTailStartPos(newMBB->begin()); 618 619 // If we split PredBB, newMBB is the new predecessor. 620 if (PredBB == MBB) 621 PredBB = newMBB; 622 623 return commonTailIndex; 624} 625 626// See if any of the blocks in MergePotentials (which all have a common single 627// successor, or all have no successor) can be tail-merged. If there is a 628// successor, any blocks in MergePotentials that are not tail-merged and 629// are not immediately before Succ must have an unconditional branch to 630// Succ added (but the predecessor/successor lists need no adjustment). 631// The lone predecessor of Succ that falls through into Succ, 632// if any, is given in PredBB. 633 634bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB, 635 MachineBasicBlock* PredBB) { 636 bool MadeChange = false; 637 638 // Except for the special cases below, tail-merge if there are at least 639 // this many instructions in common. 640 unsigned minCommonTailLength = TailMergeSize; 641 642 // If there's a successor block, there are some cases which don't require 643 // new branching and as such are very likely to be profitable. 644 if (SuccBB) { 645 if (SuccBB->pred_size() == MergePotentials.size() && 646 !MergePotentials[0].getBlock()->empty()) { 647 // If all the predecessors have at least one tail instruction in common, 648 // merging is very likely to be a win since it won't require an increase 649 // in static branches, and it will decrease the static instruction count. 650 bool AllPredsMatch = true; 651 MachineBasicBlock::iterator FirstNonTerm; 652 unsigned MinNumTerms = CountTerminators(MergePotentials[0].getBlock(), 653 FirstNonTerm); 654 if (FirstNonTerm != MergePotentials[0].getBlock()->end()) { 655 for (unsigned i = 1, e = MergePotentials.size(); i != e; ++i) { 656 MachineBasicBlock::iterator OtherFirstNonTerm; 657 unsigned NumTerms = CountTerminators(MergePotentials[0].getBlock(), 658 OtherFirstNonTerm); 659 if (NumTerms < MinNumTerms) 660 MinNumTerms = NumTerms; 661 if (OtherFirstNonTerm == MergePotentials[i].getBlock()->end() || 662 OtherFirstNonTerm->isIdenticalTo(FirstNonTerm)) { 663 AllPredsMatch = false; 664 break; 665 } 666 } 667 668 // If they all have an instruction in common, do any amount of merging. 669 if (AllPredsMatch) 670 minCommonTailLength = MinNumTerms + 1; 671 } 672 } 673 } 674 675 DEBUG(errs() << "\nTryTailMergeBlocks: "; 676 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i) 677 errs() << "BB#" << MergePotentials[i].getBlock()->getNumber() 678 << (i == e-1 ? "" : ", "); 679 errs() << "\n"; 680 if (SuccBB) { 681 errs() << " with successor BB#" << SuccBB->getNumber() << '\n'; 682 if (PredBB) 683 errs() << " which has fall-through from BB#" 684 << PredBB->getNumber() << "\n"; 685 } 686 errs() << "Looking for common tails of at least " 687 << minCommonTailLength << " instruction" 688 << (minCommonTailLength == 1 ? "" : "s") << '\n'; 689 ); 690 691 // Sort by hash value so that blocks with identical end sequences sort 692 // together. 693 std::stable_sort(MergePotentials.begin(), MergePotentials.end()); 694 695 // Walk through equivalence sets looking for actual exact matches. 696 while (MergePotentials.size() > 1) { 697 unsigned CurHash = MergePotentials.back().getHash(); 698 699 // Build SameTails, identifying the set of blocks with this hash code 700 // and with the maximum number of instructions in common. 701 unsigned maxCommonTailLength = ComputeSameTails(CurHash, 702 minCommonTailLength, 703 SuccBB, PredBB); 704 705 // If we didn't find any pair that has at least minCommonTailLength 706 // instructions in common, remove all blocks with this hash code and retry. 707 if (SameTails.empty()) { 708 RemoveBlocksWithHash(CurHash, SuccBB, PredBB); 709 continue; 710 } 711 712 // If one of the blocks is the entire common tail (and not the entry 713 // block, which we can't jump to), we can treat all blocks with this same 714 // tail at once. Use PredBB if that is one of the possibilities, as that 715 // will not introduce any extra branches. 716 MachineBasicBlock *EntryBB = MergePotentials.begin()->getBlock()-> 717 getParent()->begin(); 718 unsigned commonTailIndex = SameTails.size(); 719 for (unsigned i=0; i<SameTails.size(); i++) { 720 MachineBasicBlock *MBB = SameTails[i].getBlock(); 721 if (MBB == EntryBB) 722 continue; 723 if (MBB == PredBB) { 724 commonTailIndex = i; 725 break; 726 } 727 if (MBB->begin() == SameTails[i].getTailStartPos()) 728 commonTailIndex = i; 729 } 730 731 if (commonTailIndex == SameTails.size() || 732 (SameTails[commonTailIndex].getBlock() == PredBB && 733 !SameTails[commonTailIndex].tailIsWholeBlock())) { 734 // None of the blocks consist entirely of the common tail. 735 // Split a block so that one does. 736 commonTailIndex = CreateCommonTailOnlyBlock(PredBB, maxCommonTailLength); 737 } 738 739 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock(); 740 // MBB is common tail. Adjust all other BB's to jump to this one. 741 // Traversal must be forwards so erases work. 742 DEBUG(errs() << "\nUsing common tail in BB#" << MBB->getNumber() 743 << " for "); 744 for (unsigned int i=0, e = SameTails.size(); i != e; ++i) { 745 if (commonTailIndex == i) 746 continue; 747 DEBUG(errs() << "BB#" << SameTails[i].getBlock()->getNumber() 748 << (i == e-1 ? "" : ", ")); 749 // Hack the end off BB i, making it jump to BB commonTailIndex instead. 750 ReplaceTailWithBranchTo(SameTails[i].getTailStartPos(), MBB); 751 // BB i is no longer a predecessor of SuccBB; remove it from the worklist. 752 MergePotentials.erase(SameTails[i].getMPIter()); 753 } 754 DEBUG(errs() << "\n"); 755 // We leave commonTailIndex in the worklist in case there are other blocks 756 // that match it with a smaller number of instructions. 757 MadeChange = true; 758 } 759 return MadeChange; 760} 761 762bool BranchFolder::TailMergeBlocks(MachineFunction &MF) { 763 764 if (!EnableTailMerge) return false; 765 766 bool MadeChange = false; 767 768 // First find blocks with no successors. 769 MergePotentials.clear(); 770 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { 771 if (I->succ_empty()) 772 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(I, 2U), I)); 773 } 774 775 // See if we can do any tail merging on those. 776 if (MergePotentials.size() < TailMergeThreshold && 777 MergePotentials.size() >= 2) 778 MadeChange |= TryTailMergeBlocks(NULL, NULL); 779 780 // Look at blocks (IBB) with multiple predecessors (PBB). 781 // We change each predecessor to a canonical form, by 782 // (1) temporarily removing any unconditional branch from the predecessor 783 // to IBB, and 784 // (2) alter conditional branches so they branch to the other block 785 // not IBB; this may require adding back an unconditional branch to IBB 786 // later, where there wasn't one coming in. E.g. 787 // Bcc IBB 788 // fallthrough to QBB 789 // here becomes 790 // Bncc QBB 791 // with a conceptual B to IBB after that, which never actually exists. 792 // With those changes, we see whether the predecessors' tails match, 793 // and merge them if so. We change things out of canonical form and 794 // back to the way they were later in the process. (OptimizeBranches 795 // would undo some of this, but we can't use it, because we'd get into 796 // a compile-time infinite loop repeatedly doing and undoing the same 797 // transformations.) 798 799 for (MachineFunction::iterator I = next(MF.begin()), E = MF.end(); 800 I != E; ++I) { 801 if (I->pred_size() >= 2 && I->pred_size() < TailMergeThreshold) { 802 SmallPtrSet<MachineBasicBlock *, 8> UniquePreds; 803 MachineBasicBlock *IBB = I; 804 MachineBasicBlock *PredBB = prior(I); 805 MergePotentials.clear(); 806 for (MachineBasicBlock::pred_iterator P = I->pred_begin(), 807 E2 = I->pred_end(); 808 P != E2; ++P) { 809 MachineBasicBlock* PBB = *P; 810 // Skip blocks that loop to themselves, can't tail merge these. 811 if (PBB == IBB) 812 continue; 813 // Visit each predecessor only once. 814 if (!UniquePreds.insert(PBB)) 815 continue; 816 MachineBasicBlock *TBB = 0, *FBB = 0; 817 SmallVector<MachineOperand, 4> Cond; 818 if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond, true)) { 819 // Failing case: IBB is the target of a cbr, and 820 // we cannot reverse the branch. 821 SmallVector<MachineOperand, 4> NewCond(Cond); 822 if (!Cond.empty() && TBB == IBB) { 823 if (TII->ReverseBranchCondition(NewCond)) 824 continue; 825 // This is the QBB case described above 826 if (!FBB) 827 FBB = next(MachineFunction::iterator(PBB)); 828 } 829 // Failing case: the only way IBB can be reached from PBB is via 830 // exception handling. Happens for landing pads. Would be nice 831 // to have a bit in the edge so we didn't have to do all this. 832 if (IBB->isLandingPad()) { 833 MachineFunction::iterator IP = PBB; IP++; 834 MachineBasicBlock* PredNextBB = NULL; 835 if (IP!=MF.end()) 836 PredNextBB = IP; 837 if (TBB == NULL) { 838 if (IBB!=PredNextBB) // fallthrough 839 continue; 840 } else if (FBB) { 841 if (TBB!=IBB && FBB!=IBB) // cbr then ubr 842 continue; 843 } else if (Cond.empty()) { 844 if (TBB!=IBB) // ubr 845 continue; 846 } else { 847 if (TBB!=IBB && IBB!=PredNextBB) // cbr 848 continue; 849 } 850 } 851 // Remove the unconditional branch at the end, if any. 852 if (TBB && (Cond.empty() || FBB)) { 853 TII->RemoveBranch(*PBB); 854 if (!Cond.empty()) 855 // reinsert conditional branch only, for now 856 TII->InsertBranch(*PBB, (TBB == IBB) ? FBB : TBB, 0, NewCond); 857 } 858 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(PBB, 1U), 859 *P)); 860 } 861 } 862 if (MergePotentials.size() >= 2) 863 MadeChange |= TryTailMergeBlocks(IBB, PredBB); 864 // Reinsert an unconditional branch if needed. 865 // The 1 below can occur as a result of removing blocks in TryTailMergeBlocks. 866 PredBB = prior(I); // this may have been changed in TryTailMergeBlocks 867 if (MergePotentials.size() == 1 && 868 MergePotentials.begin()->getBlock() != PredBB) 869 FixTail(MergePotentials.begin()->getBlock(), IBB, TII); 870 } 871 } 872 return MadeChange; 873} 874 875//===----------------------------------------------------------------------===// 876// Branch Optimization 877//===----------------------------------------------------------------------===// 878 879bool BranchFolder::OptimizeBranches(MachineFunction &MF) { 880 bool MadeChange = false; 881 882 // Make sure blocks are numbered in order 883 MF.RenumberBlocks(); 884 885 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) { 886 MachineBasicBlock *MBB = I++; 887 MadeChange |= OptimizeBlock(MBB); 888 889 // If it is dead, remove it. 890 if (MBB->pred_empty()) { 891 RemoveDeadBlock(MBB); 892 MadeChange = true; 893 ++NumDeadBlocks; 894 } 895 } 896 return MadeChange; 897} 898 899 900/// CanFallThrough - Return true if the specified block (with the specified 901/// branch condition) can implicitly transfer control to the block after it by 902/// falling off the end of it. This should return false if it can reach the 903/// block after it, but it uses an explicit branch to do so (e.g. a table jump). 904/// 905/// True is a conservative answer. 906/// 907bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB, 908 bool BranchUnAnalyzable, 909 MachineBasicBlock *TBB, 910 MachineBasicBlock *FBB, 911 const SmallVectorImpl<MachineOperand> &Cond) { 912 MachineFunction::iterator Fallthrough = CurBB; 913 ++Fallthrough; 914 // If FallthroughBlock is off the end of the function, it can't fall through. 915 if (Fallthrough == CurBB->getParent()->end()) 916 return false; 917 918 // If FallthroughBlock isn't a successor of CurBB, no fallthrough is possible. 919 if (!CurBB->isSuccessor(Fallthrough)) 920 return false; 921 922 // If we couldn't analyze the branch, examine the last instruction. 923 // If the block doesn't end in a known control barrier, assume fallthrough 924 // is possible. The isPredicable check is needed because this code can be 925 // called during IfConversion, where an instruction which is normally a 926 // Barrier is predicated and thus no longer an actual control barrier. This 927 // is over-conservative though, because if an instruction isn't actually 928 // predicated we could still treat it like a barrier. 929 if (BranchUnAnalyzable) 930 return CurBB->empty() || !CurBB->back().getDesc().isBarrier() || 931 CurBB->back().getDesc().isPredicable(); 932 933 // If there is no branch, control always falls through. 934 if (TBB == 0) return true; 935 936 // If there is some explicit branch to the fallthrough block, it can obviously 937 // reach, even though the branch should get folded to fall through implicitly. 938 if (MachineFunction::iterator(TBB) == Fallthrough || 939 MachineFunction::iterator(FBB) == Fallthrough) 940 return true; 941 942 // If it's an unconditional branch to some block not the fall through, it 943 // doesn't fall through. 944 if (Cond.empty()) return false; 945 946 // Otherwise, if it is conditional and has no explicit false block, it falls 947 // through. 948 return FBB == 0; 949} 950 951/// CanFallThrough - Return true if the specified can implicitly transfer 952/// control to the block after it by falling off the end of it. This should 953/// return false if it can reach the block after it, but it uses an explicit 954/// branch to do so (e.g. a table jump). 955/// 956/// True is a conservative answer. 957/// 958bool BranchFolder::CanFallThrough(MachineBasicBlock *CurBB) { 959 MachineBasicBlock *TBB = 0, *FBB = 0; 960 SmallVector<MachineOperand, 4> Cond; 961 bool CurUnAnalyzable = TII->AnalyzeBranch(*CurBB, TBB, FBB, Cond, true); 962 return CanFallThrough(CurBB, CurUnAnalyzable, TBB, FBB, Cond); 963} 964 965/// IsBetterFallthrough - Return true if it would be clearly better to 966/// fall-through to MBB1 than to fall through into MBB2. This has to return 967/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will 968/// result in infinite loops. 969static bool IsBetterFallthrough(MachineBasicBlock *MBB1, 970 MachineBasicBlock *MBB2) { 971 // Right now, we use a simple heuristic. If MBB2 ends with a call, and 972 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to 973 // optimize branches that branch to either a return block or an assert block 974 // into a fallthrough to the return. 975 if (MBB1->empty() || MBB2->empty()) return false; 976 977 // If there is a clear successor ordering we make sure that one block 978 // will fall through to the next 979 if (MBB1->isSuccessor(MBB2)) return true; 980 if (MBB2->isSuccessor(MBB1)) return false; 981 982 MachineInstr *MBB1I = --MBB1->end(); 983 MachineInstr *MBB2I = --MBB2->end(); 984 return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall(); 985} 986 987/// TailDuplicate - MBB unconditionally branches to SuccBB. If it is profitable, 988/// duplicate SuccBB's contents in MBB to eliminate the branch. 989bool BranchFolder::TailDuplicate(MachineBasicBlock *TailBB, 990 bool PrevFallsThrough, 991 MachineFunction &MF) { 992 // Don't try to tail-duplicate single-block loops. 993 if (TailBB->isSuccessor(TailBB)) 994 return false; 995 996 // Don't tail-duplicate a block which will soon be folded into its successor. 997 if (TailBB->succ_size() == 1 && 998 TailBB->succ_begin()[0]->pred_size() == 1) 999 return false; 1000 1001 // Duplicate up to one less that the tail-merge threshold, so that we don't 1002 // get into an infinite loop between duplicating and merging. When optimizing 1003 // for size, duplicate only one, because one branch instruction can be 1004 // eliminated to compensate for the duplication. 1005 unsigned MaxDuplicateCount = 1006 MF.getFunction()->hasFnAttr(Attribute::OptimizeForSize) ? 1007 1 : (TailMergeSize - 1); 1008 1009 // Check the instructions in the block to determine whether tail-duplication 1010 // is invalid or unlikely to be unprofitable. 1011 unsigned i = 0; 1012 bool HasCall = false; 1013 for (MachineBasicBlock::iterator I = TailBB->begin(); 1014 I != TailBB->end(); ++I, ++i) { 1015 // Non-duplicable things shouldn't be tail-duplicated. 1016 if (I->getDesc().isNotDuplicable()) return false; 1017 // Don't duplicate more than the threshold. 1018 if (i == MaxDuplicateCount) return false; 1019 // Remember if we saw a call. 1020 if (I->getDesc().isCall()) HasCall = true; 1021 } 1022 // Heuristically, don't tail-duplicate calls if it would expand code size, 1023 // as it's less likely to be worth the extra cost. 1024 if (i > 1 && HasCall) 1025 return false; 1026 1027 // Iterate through all the unique predecessors and tail-duplicate this 1028 // block into them, if possible. Copying the list ahead of time also 1029 // avoids trouble with the predecessor list reallocating. 1030 bool Changed = false; 1031 SmallSetVector<MachineBasicBlock *, 8> Preds(TailBB->pred_begin(), 1032 TailBB->pred_end()); 1033 for (SmallSetVector<MachineBasicBlock *, 8>::iterator PI = Preds.begin(), 1034 PE = Preds.end(); PI != PE; ++PI) { 1035 MachineBasicBlock *PredBB = *PI; 1036 1037 assert(TailBB != PredBB && 1038 "Single-block loop should have been rejected earlier!"); 1039 if (PredBB->succ_size() > 1) continue; 1040 1041 MachineBasicBlock *PredTBB, *PredFBB; 1042 SmallVector<MachineOperand, 4> PredCond; 1043 if (TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true)) 1044 continue; 1045 if (!PredCond.empty()) 1046 continue; 1047 // EH edges are ignored by AnalyzeBranch. 1048 if (PredBB->succ_size() != 1) 1049 continue; 1050 // Don't duplicate into a fall-through predecessor unless its the 1051 // only predecessor. 1052 if (&*next(MachineFunction::iterator(PredBB)) == TailBB && 1053 PrevFallsThrough && 1054 TailBB->pred_size() != 1) 1055 continue; 1056 1057 DEBUG(errs() << "\nTail-duplicating into PredBB: " << *PredBB 1058 << "From Succ: " << *TailBB); 1059 1060 // Remove PredBB's unconditional branch. 1061 TII->RemoveBranch(*PredBB); 1062 // Clone the contents of TailBB into PredBB. 1063 for (MachineBasicBlock::iterator I = TailBB->begin(), E = TailBB->end(); 1064 I != E; ++I) { 1065 MachineInstr *NewMI = MF.CloneMachineInstr(I); 1066 PredBB->insert(PredBB->end(), NewMI); 1067 } 1068 1069 // Update the CFG. 1070 PredBB->removeSuccessor(PredBB->succ_begin()); 1071 assert(PredBB->succ_empty() && 1072 "TailDuplicate called on block with multiple successors!"); 1073 for (MachineBasicBlock::succ_iterator I = TailBB->succ_begin(), 1074 E = TailBB->succ_end(); I != E; ++I) 1075 PredBB->addSuccessor(*I); 1076 1077 Changed = true; 1078 } 1079 1080 return Changed; 1081} 1082 1083/// OptimizeBlock - Analyze and optimize control flow related to the specified 1084/// block. This is never called on the entry block. 1085bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) { 1086 bool MadeChange = false; 1087 MachineFunction &MF = *MBB->getParent(); 1088ReoptimizeBlock: 1089 1090 MachineFunction::iterator FallThrough = MBB; 1091 ++FallThrough; 1092 1093 // If this block is empty, make everyone use its fall-through, not the block 1094 // explicitly. Landing pads should not do this since the landing-pad table 1095 // points to this block. Blocks with their addresses taken shouldn't be 1096 // optimized away. 1097 if (MBB->empty() && !MBB->isLandingPad() && !MBB->hasAddressTaken()) { 1098 // Dead block? Leave for cleanup later. 1099 if (MBB->pred_empty()) return MadeChange; 1100 1101 if (FallThrough == MF.end()) { 1102 // TODO: Simplify preds to not branch here if possible! 1103 } else { 1104 // Rewrite all predecessors of the old block to go to the fallthrough 1105 // instead. 1106 while (!MBB->pred_empty()) { 1107 MachineBasicBlock *Pred = *(MBB->pred_end()-1); 1108 Pred->ReplaceUsesOfBlockWith(MBB, FallThrough); 1109 } 1110 // If MBB was the target of a jump table, update jump tables to go to the 1111 // fallthrough instead. 1112 MF.getJumpTableInfo()->ReplaceMBBInJumpTables(MBB, FallThrough); 1113 MadeChange = true; 1114 } 1115 return MadeChange; 1116 } 1117 1118 // Check to see if we can simplify the terminator of the block before this 1119 // one. 1120 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB)); 1121 1122 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0; 1123 SmallVector<MachineOperand, 4> PriorCond; 1124 bool PriorUnAnalyzable = 1125 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true); 1126 if (!PriorUnAnalyzable) { 1127 // If the CFG for the prior block has extra edges, remove them. 1128 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB, 1129 !PriorCond.empty()); 1130 1131 // If the previous branch is conditional and both conditions go to the same 1132 // destination, remove the branch, replacing it with an unconditional one or 1133 // a fall-through. 1134 if (PriorTBB && PriorTBB == PriorFBB) { 1135 TII->RemoveBranch(PrevBB); 1136 PriorCond.clear(); 1137 if (PriorTBB != MBB) 1138 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 1139 MadeChange = true; 1140 ++NumBranchOpts; 1141 goto ReoptimizeBlock; 1142 } 1143 1144 // If the previous block unconditionally falls through to this block and 1145 // this block has no other predecessors, move the contents of this block 1146 // into the prior block. This doesn't usually happen when SimplifyCFG 1147 // has been used, but it can happen tail duplication eliminates all the 1148 // non-branch predecessors of a block leaving only the fall-through edge. 1149 // This has to check PrevBB->succ_size() because EH edges are ignored by 1150 // AnalyzeBranch. 1151 if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 && 1152 PrevBB.succ_size() == 1 && 1153 !MBB->hasAddressTaken()) { 1154 DEBUG(errs() << "\nMerging into block: " << PrevBB 1155 << "From MBB: " << *MBB); 1156 PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end()); 1157 PrevBB.removeSuccessor(PrevBB.succ_begin());; 1158 assert(PrevBB.succ_empty()); 1159 PrevBB.transferSuccessors(MBB); 1160 MadeChange = true; 1161 return MadeChange; 1162 } 1163 1164 // If the previous branch *only* branches to *this* block (conditional or 1165 // not) remove the branch. 1166 if (PriorTBB == MBB && PriorFBB == 0) { 1167 TII->RemoveBranch(PrevBB); 1168 MadeChange = true; 1169 ++NumBranchOpts; 1170 goto ReoptimizeBlock; 1171 } 1172 1173 // If the prior block branches somewhere else on the condition and here if 1174 // the condition is false, remove the uncond second branch. 1175 if (PriorFBB == MBB) { 1176 TII->RemoveBranch(PrevBB); 1177 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 1178 MadeChange = true; 1179 ++NumBranchOpts; 1180 goto ReoptimizeBlock; 1181 } 1182 1183 // If the prior block branches here on true and somewhere else on false, and 1184 // if the branch condition is reversible, reverse the branch to create a 1185 // fall-through. 1186 if (PriorTBB == MBB) { 1187 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond); 1188 if (!TII->ReverseBranchCondition(NewPriorCond)) { 1189 TII->RemoveBranch(PrevBB); 1190 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond); 1191 MadeChange = true; 1192 ++NumBranchOpts; 1193 goto ReoptimizeBlock; 1194 } 1195 } 1196 1197 // If this block has no successors (e.g. it is a return block or ends with 1198 // a call to a no-return function like abort or __cxa_throw) and if the pred 1199 // falls through into this block, and if it would otherwise fall through 1200 // into the block after this, move this block to the end of the function. 1201 // 1202 // We consider it more likely that execution will stay in the function (e.g. 1203 // due to loops) than it is to exit it. This asserts in loops etc, moving 1204 // the assert condition out of the loop body. 1205 if (MBB->succ_empty() && !PriorCond.empty() && PriorFBB == 0 && 1206 MachineFunction::iterator(PriorTBB) == FallThrough && 1207 !CanFallThrough(MBB)) { 1208 bool DoTransform = true; 1209 1210 // We have to be careful that the succs of PredBB aren't both no-successor 1211 // blocks. If neither have successors and if PredBB is the second from 1212 // last block in the function, we'd just keep swapping the two blocks for 1213 // last. Only do the swap if one is clearly better to fall through than 1214 // the other. 1215 if (FallThrough == --MF.end() && 1216 !IsBetterFallthrough(PriorTBB, MBB)) 1217 DoTransform = false; 1218 1219 // We don't want to do this transformation if we have control flow like: 1220 // br cond BB2 1221 // BB1: 1222 // .. 1223 // jmp BBX 1224 // BB2: 1225 // .. 1226 // ret 1227 // 1228 // In this case, we could actually be moving the return block *into* a 1229 // loop! 1230 if (DoTransform && !MBB->succ_empty() && 1231 (!CanFallThrough(PriorTBB) || PriorTBB->empty())) 1232 DoTransform = false; 1233 1234 1235 if (DoTransform) { 1236 // Reverse the branch so we will fall through on the previous true cond. 1237 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond); 1238 if (!TII->ReverseBranchCondition(NewPriorCond)) { 1239 DEBUG(errs() << "\nMoving MBB: " << *MBB 1240 << "To make fallthrough to: " << *PriorTBB << "\n"); 1241 1242 TII->RemoveBranch(PrevBB); 1243 TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond); 1244 1245 // Move this block to the end of the function. 1246 MBB->moveAfter(--MF.end()); 1247 MadeChange = true; 1248 ++NumBranchOpts; 1249 return MadeChange; 1250 } 1251 } 1252 } 1253 } 1254 1255 // Analyze the branch in the current block. 1256 MachineBasicBlock *CurTBB = 0, *CurFBB = 0; 1257 SmallVector<MachineOperand, 4> CurCond; 1258 bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true); 1259 if (!CurUnAnalyzable) { 1260 // If the CFG for the prior block has extra edges, remove them. 1261 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty()); 1262 1263 // If this is a two-way branch, and the FBB branches to this block, reverse 1264 // the condition so the single-basic-block loop is faster. Instead of: 1265 // Loop: xxx; jcc Out; jmp Loop 1266 // we want: 1267 // Loop: xxx; jncc Loop; jmp Out 1268 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) { 1269 SmallVector<MachineOperand, 4> NewCond(CurCond); 1270 if (!TII->ReverseBranchCondition(NewCond)) { 1271 TII->RemoveBranch(*MBB); 1272 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond); 1273 MadeChange = true; 1274 ++NumBranchOpts; 1275 goto ReoptimizeBlock; 1276 } 1277 } 1278 1279 1280 // If this branch is the only thing in its block, see if we can forward 1281 // other blocks across it. 1282 if (CurTBB && CurCond.empty() && CurFBB == 0 && 1283 MBB->begin()->getDesc().isBranch() && CurTBB != MBB && 1284 !MBB->hasAddressTaken()) { 1285 // This block may contain just an unconditional branch. Because there can 1286 // be 'non-branch terminators' in the block, try removing the branch and 1287 // then seeing if the block is empty. 1288 TII->RemoveBranch(*MBB); 1289 1290 // If this block is just an unconditional branch to CurTBB, we can 1291 // usually completely eliminate the block. The only case we cannot 1292 // completely eliminate the block is when the block before this one 1293 // falls through into MBB and we can't understand the prior block's branch 1294 // condition. 1295 if (MBB->empty()) { 1296 bool PredHasNoFallThrough = TII->BlockHasNoFallThrough(PrevBB); 1297 if (PredHasNoFallThrough || !PriorUnAnalyzable || 1298 !PrevBB.isSuccessor(MBB)) { 1299 // If the prior block falls through into us, turn it into an 1300 // explicit branch to us to make updates simpler. 1301 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) && 1302 PriorTBB != MBB && PriorFBB != MBB) { 1303 if (PriorTBB == 0) { 1304 assert(PriorCond.empty() && PriorFBB == 0 && 1305 "Bad branch analysis"); 1306 PriorTBB = MBB; 1307 } else { 1308 assert(PriorFBB == 0 && "Machine CFG out of date!"); 1309 PriorFBB = MBB; 1310 } 1311 TII->RemoveBranch(PrevBB); 1312 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); 1313 } 1314 1315 // Iterate through all the predecessors, revectoring each in-turn. 1316 size_t PI = 0; 1317 bool DidChange = false; 1318 bool HasBranchToSelf = false; 1319 while(PI != MBB->pred_size()) { 1320 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI); 1321 if (PMBB == MBB) { 1322 // If this block has an uncond branch to itself, leave it. 1323 ++PI; 1324 HasBranchToSelf = true; 1325 } else { 1326 DidChange = true; 1327 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB); 1328 // If this change resulted in PMBB ending in a conditional 1329 // branch where both conditions go to the same destination, 1330 // change this to an unconditional branch (and fix the CFG). 1331 MachineBasicBlock *NewCurTBB = 0, *NewCurFBB = 0; 1332 SmallVector<MachineOperand, 4> NewCurCond; 1333 bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB, 1334 NewCurFBB, NewCurCond, true); 1335 if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) { 1336 TII->RemoveBranch(*PMBB); 1337 NewCurCond.clear(); 1338 TII->InsertBranch(*PMBB, NewCurTBB, 0, NewCurCond); 1339 MadeChange = true; 1340 ++NumBranchOpts; 1341 PMBB->CorrectExtraCFGEdges(NewCurTBB, 0, false); 1342 } 1343 } 1344 } 1345 1346 // Change any jumptables to go to the new MBB. 1347 MF.getJumpTableInfo()->ReplaceMBBInJumpTables(MBB, CurTBB); 1348 if (DidChange) { 1349 ++NumBranchOpts; 1350 MadeChange = true; 1351 if (!HasBranchToSelf) return MadeChange; 1352 } 1353 } 1354 } 1355 1356 // Add the branch back if the block is more than just an uncond branch. 1357 TII->InsertBranch(*MBB, CurTBB, 0, CurCond); 1358 } 1359 } 1360 1361 // Now we know that there was no fall-through into this block, check to 1362 // see if it has a fall-through into its successor. 1363 bool CurFallsThru = CanFallThrough(MBB, CurUnAnalyzable, CurTBB, CurFBB, 1364 CurCond); 1365 bool PrevFallsThru = CanFallThrough(&PrevBB, PriorUnAnalyzable, 1366 PriorTBB, PriorFBB, PriorCond); 1367 1368 // If this block is small, unconditionally branched to, and does not 1369 // fall through, tail-duplicate its instructions into its predecessors 1370 // to eliminate a (dynamic) branch. 1371 if (!CurFallsThru) 1372 if (TailDuplicate(MBB, PrevFallsThru, MF)) { 1373 MadeChange = true; 1374 return MadeChange; 1375 } 1376 1377 // If the prior block doesn't fall through into this block, and if this 1378 // block doesn't fall through into some other block, see if we can find a 1379 // place to move this block where a fall-through will happen. 1380 if (!PrevFallsThru) { 1381 if (!MBB->isLandingPad()) { 1382 // Check all the predecessors of this block. If one of them has no fall 1383 // throughs, move this block right after it. 1384 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(), 1385 E = MBB->pred_end(); PI != E; ++PI) { 1386 // Analyze the branch at the end of the pred. 1387 MachineBasicBlock *PredBB = *PI; 1388 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough; 1389 MachineBasicBlock *PredTBB, *PredFBB; 1390 SmallVector<MachineOperand, 4> PredCond; 1391 if (PredBB != MBB && !CanFallThrough(PredBB) && 1392 !TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true) 1393 && (!CurFallsThru || !CurTBB || !CurFBB) 1394 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) { 1395 // If the current block doesn't fall through, just move it. 1396 // If the current block can fall through and does not end with a 1397 // conditional branch, we need to append an unconditional jump to 1398 // the (current) next block. To avoid a possible compile-time 1399 // infinite loop, move blocks only backward in this case. 1400 // Also, if there are already 2 branches here, we cannot add a third; 1401 // this means we have the case 1402 // Bcc next 1403 // B elsewhere 1404 // next: 1405 if (CurFallsThru) { 1406 MachineBasicBlock *NextBB = next(MachineFunction::iterator(MBB)); 1407 CurCond.clear(); 1408 TII->InsertBranch(*MBB, NextBB, 0, CurCond); 1409 } 1410 MBB->moveAfter(PredBB); 1411 MadeChange = true; 1412 goto ReoptimizeBlock; 1413 } 1414 } 1415 } 1416 1417 if (!CurFallsThru) { 1418 // Check all successors to see if we can move this block before it. 1419 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(), 1420 E = MBB->succ_end(); SI != E; ++SI) { 1421 // Analyze the branch at the end of the block before the succ. 1422 MachineBasicBlock *SuccBB = *SI; 1423 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev; 1424 1425 // If this block doesn't already fall-through to that successor, and if 1426 // the succ doesn't already have a block that can fall through into it, 1427 // and if the successor isn't an EH destination, we can arrange for the 1428 // fallthrough to happen. 1429 if (SuccBB != MBB && &*SuccPrev != MBB && 1430 !CanFallThrough(SuccPrev) && !CurUnAnalyzable && 1431 !SuccBB->isLandingPad()) { 1432 MBB->moveBefore(SuccBB); 1433 MadeChange = true; 1434 goto ReoptimizeBlock; 1435 } 1436 } 1437 1438 // Okay, there is no really great place to put this block. If, however, 1439 // the block before this one would be a fall-through if this block were 1440 // removed, move this block to the end of the function. 1441 MachineBasicBlock *PrevTBB, *PrevFBB; 1442 SmallVector<MachineOperand, 4> PrevCond; 1443 if (FallThrough != MF.end() && 1444 !TII->AnalyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) && 1445 PrevBB.isSuccessor(FallThrough)) { 1446 MBB->moveAfter(--MF.end()); 1447 MadeChange = true; 1448 return MadeChange; 1449 } 1450 } 1451 } 1452 1453 return MadeChange; 1454} 1455