BranchFolding.cpp revision 61f1e3db43e556f495b6b9360d2f550291f78471
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(NumHoist , "Number of times common instructions are hoisted"); 45 46static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge", 47 cl::init(cl::BOU_UNSET), cl::Hidden); 48 49// Throttle for huge numbers of predecessors (compile speed problems) 50static cl::opt<unsigned> 51TailMergeThreshold("tail-merge-threshold", 52 cl::desc("Max number of predecessors to consider tail merging"), 53 cl::init(150), cl::Hidden); 54 55// Heuristic for tail merging (and, inversely, tail duplication). 56// TODO: This should be replaced with a target query. 57static cl::opt<unsigned> 58TailMergeSize("tail-merge-size", 59 cl::desc("Min number of instructions to consider tail merging"), 60 cl::init(3), cl::Hidden); 61 62namespace { 63 /// BranchFolderPass - Wrap branch folder in a machine function pass. 64 class BranchFolderPass : public MachineFunctionPass { 65 public: 66 static char ID; 67 explicit BranchFolderPass(): MachineFunctionPass(ID) {} 68 69 virtual bool runOnMachineFunction(MachineFunction &MF); 70 71 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 72 AU.addRequired<TargetPassConfig>(); 73 MachineFunctionPass::getAnalysisUsage(AU); 74 } 75 }; 76} 77 78char BranchFolderPass::ID = 0; 79char &llvm::BranchFolderPassID = BranchFolderPass::ID; 80 81INITIALIZE_PASS(BranchFolderPass, "branch-folder", 82 "Control Flow Optimizer", false, false) 83 84bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) { 85 TargetPassConfig *PassConfig = &getAnalysis<TargetPassConfig>(); 86 BranchFolder Folder(PassConfig->getEnableTailMerge(), /*CommonHoist=*/true); 87 return Folder.OptimizeFunction(MF, 88 MF.getTarget().getInstrInfo(), 89 MF.getTarget().getRegisterInfo(), 90 getAnalysisIfAvailable<MachineModuleInfo>()); 91} 92 93 94BranchFolder::BranchFolder(bool defaultEnableTailMerge, bool CommonHoist) { 95 switch (FlagEnableTailMerge) { 96 case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break; 97 case cl::BOU_TRUE: EnableTailMerge = true; break; 98 case cl::BOU_FALSE: EnableTailMerge = false; break; 99 } 100 101 EnableHoistCommonCode = CommonHoist; 102} 103 104/// RemoveDeadBlock - Remove the specified dead machine basic block from the 105/// function, updating the CFG. 106void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) { 107 assert(MBB->pred_empty() && "MBB must be dead!"); 108 DEBUG(dbgs() << "\nRemoving MBB: " << *MBB); 109 110 MachineFunction *MF = MBB->getParent(); 111 // drop all successors. 112 while (!MBB->succ_empty()) 113 MBB->removeSuccessor(MBB->succ_end()-1); 114 115 // Avoid matching if this pointer gets reused. 116 TriedMerging.erase(MBB); 117 118 // Remove the block. 119 MF->erase(MBB); 120} 121 122/// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def 123/// followed by terminators, and if the implicitly defined registers are not 124/// used by the terminators, remove those implicit_def's. e.g. 125/// BB1: 126/// r0 = implicit_def 127/// r1 = implicit_def 128/// br 129/// This block can be optimized away later if the implicit instructions are 130/// removed. 131bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) { 132 SmallSet<unsigned, 4> ImpDefRegs; 133 MachineBasicBlock::iterator I = MBB->begin(); 134 while (I != MBB->end()) { 135 if (!I->isImplicitDef()) 136 break; 137 unsigned Reg = I->getOperand(0).getReg(); 138 ImpDefRegs.insert(Reg); 139 for (const unsigned *SubRegs = TRI->getSubRegisters(Reg); 140 unsigned SubReg = *SubRegs; ++SubRegs) 141 ImpDefRegs.insert(SubReg); 142 ++I; 143 } 144 if (ImpDefRegs.empty()) 145 return false; 146 147 MachineBasicBlock::iterator FirstTerm = I; 148 while (I != MBB->end()) { 149 if (!TII->isUnpredicatedTerminator(I)) 150 return false; 151 // See if it uses any of the implicitly defined registers. 152 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) { 153 MachineOperand &MO = I->getOperand(i); 154 if (!MO.isReg() || !MO.isUse()) 155 continue; 156 unsigned Reg = MO.getReg(); 157 if (ImpDefRegs.count(Reg)) 158 return false; 159 } 160 ++I; 161 } 162 163 I = MBB->begin(); 164 while (I != FirstTerm) { 165 MachineInstr *ImpDefMI = &*I; 166 ++I; 167 MBB->erase(ImpDefMI); 168 } 169 170 return true; 171} 172 173/// OptimizeFunction - Perhaps branch folding, tail merging and other 174/// CFG optimizations on the given function. 175bool BranchFolder::OptimizeFunction(MachineFunction &MF, 176 const TargetInstrInfo *tii, 177 const TargetRegisterInfo *tri, 178 MachineModuleInfo *mmi) { 179 if (!tii) return false; 180 181 TriedMerging.clear(); 182 183 TII = tii; 184 TRI = tri; 185 MMI = mmi; 186 187 RS = TRI->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL; 188 189 // Fix CFG. The later algorithms expect it to be right. 190 bool MadeChange = false; 191 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) { 192 MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0; 193 SmallVector<MachineOperand, 4> Cond; 194 if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true)) 195 MadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty()); 196 MadeChange |= OptimizeImpDefsBlock(MBB); 197 } 198 199 bool MadeChangeThisIteration = true; 200 while (MadeChangeThisIteration) { 201 MadeChangeThisIteration = TailMergeBlocks(MF); 202 MadeChangeThisIteration |= OptimizeBranches(MF); 203 if (EnableHoistCommonCode) 204 MadeChangeThisIteration |= HoistCommonCode(MF); 205 MadeChange |= MadeChangeThisIteration; 206 } 207 208 // See if any jump tables have become dead as the code generator 209 // did its thing. 210 MachineJumpTableInfo *JTI = MF.getJumpTableInfo(); 211 if (JTI == 0) { 212 delete RS; 213 return MadeChange; 214 } 215 216 // Walk the function to find jump tables that are live. 217 BitVector JTIsLive(JTI->getJumpTables().size()); 218 for (MachineFunction::iterator BB = MF.begin(), E = MF.end(); 219 BB != E; ++BB) { 220 for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end(); 221 I != E; ++I) 222 for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) { 223 MachineOperand &Op = I->getOperand(op); 224 if (!Op.isJTI()) continue; 225 226 // Remember that this JT is live. 227 JTIsLive.set(Op.getIndex()); 228 } 229 } 230 231 // Finally, remove dead jump tables. This happens when the 232 // indirect jump was unreachable (and thus deleted). 233 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i) 234 if (!JTIsLive.test(i)) { 235 JTI->RemoveJumpTable(i); 236 MadeChange = true; 237 } 238 239 delete RS; 240 return MadeChange; 241} 242 243//===----------------------------------------------------------------------===// 244// Tail Merging of Blocks 245//===----------------------------------------------------------------------===// 246 247/// HashMachineInstr - Compute a hash value for MI and its operands. 248static unsigned HashMachineInstr(const MachineInstr *MI) { 249 unsigned Hash = MI->getOpcode(); 250 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 251 const MachineOperand &Op = MI->getOperand(i); 252 253 // Merge in bits from the operand if easy. 254 unsigned OperandHash = 0; 255 switch (Op.getType()) { 256 case MachineOperand::MO_Register: OperandHash = Op.getReg(); break; 257 case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break; 258 case MachineOperand::MO_MachineBasicBlock: 259 OperandHash = Op.getMBB()->getNumber(); 260 break; 261 case MachineOperand::MO_FrameIndex: 262 case MachineOperand::MO_ConstantPoolIndex: 263 case MachineOperand::MO_JumpTableIndex: 264 OperandHash = Op.getIndex(); 265 break; 266 case MachineOperand::MO_GlobalAddress: 267 case MachineOperand::MO_ExternalSymbol: 268 // Global address / external symbol are too hard, don't bother, but do 269 // pull in the offset. 270 OperandHash = Op.getOffset(); 271 break; 272 default: break; 273 } 274 275 Hash += ((OperandHash << 3) | Op.getType()) << (i&31); 276 } 277 return Hash; 278} 279 280/// HashEndOfMBB - Hash the last instruction in the MBB. 281static unsigned HashEndOfMBB(const MachineBasicBlock *MBB) { 282 MachineBasicBlock::const_iterator I = MBB->end(); 283 if (I == MBB->begin()) 284 return 0; // Empty MBB. 285 286 --I; 287 // Skip debug info so it will not affect codegen. 288 while (I->isDebugValue()) { 289 if (I==MBB->begin()) 290 return 0; // MBB empty except for debug info. 291 --I; 292 } 293 294 return HashMachineInstr(I); 295} 296 297/// ComputeCommonTailLength - Given two machine basic blocks, compute the number 298/// of instructions they actually have in common together at their end. Return 299/// iterators for the first shared instruction in each block. 300static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1, 301 MachineBasicBlock *MBB2, 302 MachineBasicBlock::iterator &I1, 303 MachineBasicBlock::iterator &I2) { 304 I1 = MBB1->end(); 305 I2 = MBB2->end(); 306 307 unsigned TailLen = 0; 308 while (I1 != MBB1->begin() && I2 != MBB2->begin()) { 309 --I1; --I2; 310 // Skip debugging pseudos; necessary to avoid changing the code. 311 while (I1->isDebugValue()) { 312 if (I1==MBB1->begin()) { 313 while (I2->isDebugValue()) { 314 if (I2==MBB2->begin()) 315 // I1==DBG at begin; I2==DBG at begin 316 return TailLen; 317 --I2; 318 } 319 ++I2; 320 // I1==DBG at begin; I2==non-DBG, or first of DBGs not at begin 321 return TailLen; 322 } 323 --I1; 324 } 325 // I1==first (untested) non-DBG preceding known match 326 while (I2->isDebugValue()) { 327 if (I2==MBB2->begin()) { 328 ++I1; 329 // I1==non-DBG, or first of DBGs not at begin; I2==DBG at begin 330 return TailLen; 331 } 332 --I2; 333 } 334 // I1, I2==first (untested) non-DBGs preceding known match 335 if (!I1->isIdenticalTo(I2) || 336 // FIXME: This check is dubious. It's used to get around a problem where 337 // people incorrectly expect inline asm directives to remain in the same 338 // relative order. This is untenable because normal compiler 339 // optimizations (like this one) may reorder and/or merge these 340 // directives. 341 I1->isInlineAsm()) { 342 ++I1; ++I2; 343 break; 344 } 345 ++TailLen; 346 } 347 // Back past possible debugging pseudos at beginning of block. This matters 348 // when one block differs from the other only by whether debugging pseudos 349 // are present at the beginning. (This way, the various checks later for 350 // I1==MBB1->begin() work as expected.) 351 if (I1 == MBB1->begin() && I2 != MBB2->begin()) { 352 --I2; 353 while (I2->isDebugValue()) { 354 if (I2 == MBB2->begin()) { 355 return TailLen; 356 } 357 --I2; 358 } 359 ++I2; 360 } 361 if (I2 == MBB2->begin() && I1 != MBB1->begin()) { 362 --I1; 363 while (I1->isDebugValue()) { 364 if (I1 == MBB1->begin()) 365 return TailLen; 366 --I1; 367 } 368 ++I1; 369 } 370 return TailLen; 371} 372 373void BranchFolder::MaintainLiveIns(MachineBasicBlock *CurMBB, 374 MachineBasicBlock *NewMBB) { 375 if (RS) { 376 RS->enterBasicBlock(CurMBB); 377 if (!CurMBB->empty()) 378 RS->forward(prior(CurMBB->end())); 379 BitVector RegsLiveAtExit(TRI->getNumRegs()); 380 RS->getRegsUsed(RegsLiveAtExit, false); 381 for (unsigned int i = 0, e = TRI->getNumRegs(); i != e; i++) 382 if (RegsLiveAtExit[i]) 383 NewMBB->addLiveIn(i); 384 } 385} 386 387/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything 388/// after it, replacing it with an unconditional branch to NewDest. 389void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, 390 MachineBasicBlock *NewDest) { 391 MachineBasicBlock *CurMBB = OldInst->getParent(); 392 393 TII->ReplaceTailWithBranchTo(OldInst, NewDest); 394 395 // For targets that use the register scavenger, we must maintain LiveIns. 396 MaintainLiveIns(CurMBB, NewDest); 397 398 ++NumTailMerge; 399} 400 401/// SplitMBBAt - Given a machine basic block and an iterator into it, split the 402/// MBB so that the part before the iterator falls into the part starting at the 403/// iterator. This returns the new MBB. 404MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB, 405 MachineBasicBlock::iterator BBI1) { 406 if (!TII->isLegalToSplitMBBAt(CurMBB, BBI1)) 407 return 0; 408 409 MachineFunction &MF = *CurMBB.getParent(); 410 411 // Create the fall-through block. 412 MachineFunction::iterator MBBI = &CurMBB; 413 MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(CurMBB.getBasicBlock()); 414 CurMBB.getParent()->insert(++MBBI, NewMBB); 415 416 // Move all the successors of this block to the specified block. 417 NewMBB->transferSuccessors(&CurMBB); 418 419 // Add an edge from CurMBB to NewMBB for the fall-through. 420 CurMBB.addSuccessor(NewMBB); 421 422 // Splice the code over. 423 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end()); 424 425 // For targets that use the register scavenger, we must maintain LiveIns. 426 MaintainLiveIns(&CurMBB, NewMBB); 427 428 return NewMBB; 429} 430 431/// EstimateRuntime - Make a rough estimate for how long it will take to run 432/// the specified code. 433static unsigned EstimateRuntime(MachineBasicBlock::iterator I, 434 MachineBasicBlock::iterator E) { 435 unsigned Time = 0; 436 for (; I != E; ++I) { 437 if (I->isDebugValue()) 438 continue; 439 if (I->isCall()) 440 Time += 10; 441 else if (I->mayLoad() || I->mayStore()) 442 Time += 2; 443 else 444 ++Time; 445 } 446 return Time; 447} 448 449// CurMBB needs to add an unconditional branch to SuccMBB (we removed these 450// branches temporarily for tail merging). In the case where CurMBB ends 451// with a conditional branch to the next block, optimize by reversing the 452// test and conditionally branching to SuccMBB instead. 453static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB, 454 const TargetInstrInfo *TII) { 455 MachineFunction *MF = CurMBB->getParent(); 456 MachineFunction::iterator I = llvm::next(MachineFunction::iterator(CurMBB)); 457 MachineBasicBlock *TBB = 0, *FBB = 0; 458 SmallVector<MachineOperand, 4> Cond; 459 DebugLoc dl; // FIXME: this is nowhere 460 if (I != MF->end() && 461 !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond, true)) { 462 MachineBasicBlock *NextBB = I; 463 if (TBB == NextBB && !Cond.empty() && !FBB) { 464 if (!TII->ReverseBranchCondition(Cond)) { 465 TII->RemoveBranch(*CurMBB); 466 TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond, dl); 467 return; 468 } 469 } 470 } 471 TII->InsertBranch(*CurMBB, SuccBB, NULL, 472 SmallVector<MachineOperand, 0>(), dl); 473} 474 475bool 476BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const { 477 if (getHash() < o.getHash()) 478 return true; 479 else if (getHash() > o.getHash()) 480 return false; 481 else if (getBlock()->getNumber() < o.getBlock()->getNumber()) 482 return true; 483 else if (getBlock()->getNumber() > o.getBlock()->getNumber()) 484 return false; 485 else { 486 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing 487 // an object with itself. 488#ifndef _GLIBCXX_DEBUG 489 llvm_unreachable("Predecessor appears twice"); 490#else 491 return false; 492#endif 493 } 494} 495 496/// CountTerminators - Count the number of terminators in the given 497/// block and set I to the position of the first non-terminator, if there 498/// is one, or MBB->end() otherwise. 499static unsigned CountTerminators(MachineBasicBlock *MBB, 500 MachineBasicBlock::iterator &I) { 501 I = MBB->end(); 502 unsigned NumTerms = 0; 503 for (;;) { 504 if (I == MBB->begin()) { 505 I = MBB->end(); 506 break; 507 } 508 --I; 509 if (!I->isTerminator()) break; 510 ++NumTerms; 511 } 512 return NumTerms; 513} 514 515/// ProfitableToMerge - Check if two machine basic blocks have a common tail 516/// and decide if it would be profitable to merge those tails. Return the 517/// length of the common tail and iterators to the first common instruction 518/// in each block. 519static bool ProfitableToMerge(MachineBasicBlock *MBB1, 520 MachineBasicBlock *MBB2, 521 unsigned minCommonTailLength, 522 unsigned &CommonTailLen, 523 MachineBasicBlock::iterator &I1, 524 MachineBasicBlock::iterator &I2, 525 MachineBasicBlock *SuccBB, 526 MachineBasicBlock *PredBB) { 527 CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2); 528 if (CommonTailLen == 0) 529 return false; 530 DEBUG(dbgs() << "Common tail length of BB#" << MBB1->getNumber() 531 << " and BB#" << MBB2->getNumber() << " is " << CommonTailLen 532 << '\n'); 533 534 // It's almost always profitable to merge any number of non-terminator 535 // instructions with the block that falls through into the common successor. 536 if (MBB1 == PredBB || MBB2 == PredBB) { 537 MachineBasicBlock::iterator I; 538 unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I); 539 if (CommonTailLen > NumTerms) 540 return true; 541 } 542 543 // If one of the blocks can be completely merged and happens to be in 544 // a position where the other could fall through into it, merge any number 545 // of instructions, because it can be done without a branch. 546 // TODO: If the blocks are not adjacent, move one of them so that they are? 547 if (MBB1->isLayoutSuccessor(MBB2) && I2 == MBB2->begin()) 548 return true; 549 if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin()) 550 return true; 551 552 // If both blocks have an unconditional branch temporarily stripped out, 553 // count that as an additional common instruction for the following 554 // heuristics. 555 unsigned EffectiveTailLen = CommonTailLen; 556 if (SuccBB && MBB1 != PredBB && MBB2 != PredBB && 557 !MBB1->back().isBarrier() && 558 !MBB2->back().isBarrier()) 559 ++EffectiveTailLen; 560 561 // Check if the common tail is long enough to be worthwhile. 562 if (EffectiveTailLen >= minCommonTailLength) 563 return true; 564 565 // If we are optimizing for code size, 2 instructions in common is enough if 566 // we don't have to split a block. At worst we will be introducing 1 new 567 // branch instruction, which is likely to be smaller than the 2 568 // instructions that would be deleted in the merge. 569 MachineFunction *MF = MBB1->getParent(); 570 if (EffectiveTailLen >= 2 && 571 MF->getFunction()->hasFnAttr(Attribute::OptimizeForSize) && 572 (I1 == MBB1->begin() || I2 == MBB2->begin())) 573 return true; 574 575 return false; 576} 577 578/// ComputeSameTails - Look through all the blocks in MergePotentials that have 579/// hash CurHash (guaranteed to match the last element). Build the vector 580/// SameTails of all those that have the (same) largest number of instructions 581/// in common of any pair of these blocks. SameTails entries contain an 582/// iterator into MergePotentials (from which the MachineBasicBlock can be 583/// found) and a MachineBasicBlock::iterator into that MBB indicating the 584/// instruction where the matching code sequence begins. 585/// Order of elements in SameTails is the reverse of the order in which 586/// those blocks appear in MergePotentials (where they are not necessarily 587/// consecutive). 588unsigned BranchFolder::ComputeSameTails(unsigned CurHash, 589 unsigned minCommonTailLength, 590 MachineBasicBlock *SuccBB, 591 MachineBasicBlock *PredBB) { 592 unsigned maxCommonTailLength = 0U; 593 SameTails.clear(); 594 MachineBasicBlock::iterator TrialBBI1, TrialBBI2; 595 MPIterator HighestMPIter = prior(MergePotentials.end()); 596 for (MPIterator CurMPIter = prior(MergePotentials.end()), 597 B = MergePotentials.begin(); 598 CurMPIter != B && CurMPIter->getHash() == CurHash; 599 --CurMPIter) { 600 for (MPIterator I = prior(CurMPIter); I->getHash() == CurHash ; --I) { 601 unsigned CommonTailLen; 602 if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(), 603 minCommonTailLength, 604 CommonTailLen, TrialBBI1, TrialBBI2, 605 SuccBB, PredBB)) { 606 if (CommonTailLen > maxCommonTailLength) { 607 SameTails.clear(); 608 maxCommonTailLength = CommonTailLen; 609 HighestMPIter = CurMPIter; 610 SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1)); 611 } 612 if (HighestMPIter == CurMPIter && 613 CommonTailLen == maxCommonTailLength) 614 SameTails.push_back(SameTailElt(I, TrialBBI2)); 615 } 616 if (I == B) 617 break; 618 } 619 } 620 return maxCommonTailLength; 621} 622 623/// RemoveBlocksWithHash - Remove all blocks with hash CurHash from 624/// MergePotentials, restoring branches at ends of blocks as appropriate. 625void BranchFolder::RemoveBlocksWithHash(unsigned CurHash, 626 MachineBasicBlock *SuccBB, 627 MachineBasicBlock *PredBB) { 628 MPIterator CurMPIter, B; 629 for (CurMPIter = prior(MergePotentials.end()), B = MergePotentials.begin(); 630 CurMPIter->getHash() == CurHash; 631 --CurMPIter) { 632 // Put the unconditional branch back, if we need one. 633 MachineBasicBlock *CurMBB = CurMPIter->getBlock(); 634 if (SuccBB && CurMBB != PredBB) 635 FixTail(CurMBB, SuccBB, TII); 636 if (CurMPIter == B) 637 break; 638 } 639 if (CurMPIter->getHash() != CurHash) 640 CurMPIter++; 641 MergePotentials.erase(CurMPIter, MergePotentials.end()); 642} 643 644/// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist 645/// only of the common tail. Create a block that does by splitting one. 646bool BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB, 647 unsigned maxCommonTailLength, 648 unsigned &commonTailIndex) { 649 commonTailIndex = 0; 650 unsigned TimeEstimate = ~0U; 651 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) { 652 // Use PredBB if possible; that doesn't require a new branch. 653 if (SameTails[i].getBlock() == PredBB) { 654 commonTailIndex = i; 655 break; 656 } 657 // Otherwise, make a (fairly bogus) choice based on estimate of 658 // how long it will take the various blocks to execute. 659 unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(), 660 SameTails[i].getTailStartPos()); 661 if (t <= TimeEstimate) { 662 TimeEstimate = t; 663 commonTailIndex = i; 664 } 665 } 666 667 MachineBasicBlock::iterator BBI = 668 SameTails[commonTailIndex].getTailStartPos(); 669 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock(); 670 671 // If the common tail includes any debug info we will take it pretty 672 // randomly from one of the inputs. Might be better to remove it? 673 DEBUG(dbgs() << "\nSplitting BB#" << MBB->getNumber() << ", size " 674 << maxCommonTailLength); 675 676 MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI); 677 if (!newMBB) { 678 DEBUG(dbgs() << "... failed!"); 679 return false; 680 } 681 682 SameTails[commonTailIndex].setBlock(newMBB); 683 SameTails[commonTailIndex].setTailStartPos(newMBB->begin()); 684 685 // If we split PredBB, newMBB is the new predecessor. 686 if (PredBB == MBB) 687 PredBB = newMBB; 688 689 return true; 690} 691 692// See if any of the blocks in MergePotentials (which all have a common single 693// successor, or all have no successor) can be tail-merged. If there is a 694// successor, any blocks in MergePotentials that are not tail-merged and 695// are not immediately before Succ must have an unconditional branch to 696// Succ added (but the predecessor/successor lists need no adjustment). 697// The lone predecessor of Succ that falls through into Succ, 698// if any, is given in PredBB. 699 700bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB, 701 MachineBasicBlock *PredBB) { 702 bool MadeChange = false; 703 704 // Except for the special cases below, tail-merge if there are at least 705 // this many instructions in common. 706 unsigned minCommonTailLength = TailMergeSize; 707 708 DEBUG(dbgs() << "\nTryTailMergeBlocks: "; 709 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i) 710 dbgs() << "BB#" << MergePotentials[i].getBlock()->getNumber() 711 << (i == e-1 ? "" : ", "); 712 dbgs() << "\n"; 713 if (SuccBB) { 714 dbgs() << " with successor BB#" << SuccBB->getNumber() << '\n'; 715 if (PredBB) 716 dbgs() << " which has fall-through from BB#" 717 << PredBB->getNumber() << "\n"; 718 } 719 dbgs() << "Looking for common tails of at least " 720 << minCommonTailLength << " instruction" 721 << (minCommonTailLength == 1 ? "" : "s") << '\n'; 722 ); 723 724 // Sort by hash value so that blocks with identical end sequences sort 725 // together. 726 std::stable_sort(MergePotentials.begin(), MergePotentials.end()); 727 728 // Walk through equivalence sets looking for actual exact matches. 729 while (MergePotentials.size() > 1) { 730 unsigned CurHash = MergePotentials.back().getHash(); 731 732 // Build SameTails, identifying the set of blocks with this hash code 733 // and with the maximum number of instructions in common. 734 unsigned maxCommonTailLength = ComputeSameTails(CurHash, 735 minCommonTailLength, 736 SuccBB, PredBB); 737 738 // If we didn't find any pair that has at least minCommonTailLength 739 // instructions in common, remove all blocks with this hash code and retry. 740 if (SameTails.empty()) { 741 RemoveBlocksWithHash(CurHash, SuccBB, PredBB); 742 continue; 743 } 744 745 // If one of the blocks is the entire common tail (and not the entry 746 // block, which we can't jump to), we can treat all blocks with this same 747 // tail at once. Use PredBB if that is one of the possibilities, as that 748 // will not introduce any extra branches. 749 MachineBasicBlock *EntryBB = MergePotentials.begin()->getBlock()-> 750 getParent()->begin(); 751 unsigned commonTailIndex = SameTails.size(); 752 // If there are two blocks, check to see if one can be made to fall through 753 // into the other. 754 if (SameTails.size() == 2 && 755 SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) && 756 SameTails[1].tailIsWholeBlock()) 757 commonTailIndex = 1; 758 else if (SameTails.size() == 2 && 759 SameTails[1].getBlock()->isLayoutSuccessor( 760 SameTails[0].getBlock()) && 761 SameTails[0].tailIsWholeBlock()) 762 commonTailIndex = 0; 763 else { 764 // Otherwise just pick one, favoring the fall-through predecessor if 765 // there is one. 766 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) { 767 MachineBasicBlock *MBB = SameTails[i].getBlock(); 768 if (MBB == EntryBB && SameTails[i].tailIsWholeBlock()) 769 continue; 770 if (MBB == PredBB) { 771 commonTailIndex = i; 772 break; 773 } 774 if (SameTails[i].tailIsWholeBlock()) 775 commonTailIndex = i; 776 } 777 } 778 779 if (commonTailIndex == SameTails.size() || 780 (SameTails[commonTailIndex].getBlock() == PredBB && 781 !SameTails[commonTailIndex].tailIsWholeBlock())) { 782 // None of the blocks consist entirely of the common tail. 783 // Split a block so that one does. 784 if (!CreateCommonTailOnlyBlock(PredBB, 785 maxCommonTailLength, commonTailIndex)) { 786 RemoveBlocksWithHash(CurHash, SuccBB, PredBB); 787 continue; 788 } 789 } 790 791 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock(); 792 // MBB is common tail. Adjust all other BB's to jump to this one. 793 // Traversal must be forwards so erases work. 794 DEBUG(dbgs() << "\nUsing common tail in BB#" << MBB->getNumber() 795 << " for "); 796 for (unsigned int i=0, e = SameTails.size(); i != e; ++i) { 797 if (commonTailIndex == i) 798 continue; 799 DEBUG(dbgs() << "BB#" << SameTails[i].getBlock()->getNumber() 800 << (i == e-1 ? "" : ", ")); 801 // Hack the end off BB i, making it jump to BB commonTailIndex instead. 802 ReplaceTailWithBranchTo(SameTails[i].getTailStartPos(), MBB); 803 // BB i is no longer a predecessor of SuccBB; remove it from the worklist. 804 MergePotentials.erase(SameTails[i].getMPIter()); 805 } 806 DEBUG(dbgs() << "\n"); 807 // We leave commonTailIndex in the worklist in case there are other blocks 808 // that match it with a smaller number of instructions. 809 MadeChange = true; 810 } 811 return MadeChange; 812} 813 814bool BranchFolder::TailMergeBlocks(MachineFunction &MF) { 815 816 if (!EnableTailMerge) return false; 817 818 bool MadeChange = false; 819 820 // First find blocks with no successors. 821 MergePotentials.clear(); 822 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); 823 I != E && MergePotentials.size() < TailMergeThreshold; ++I) { 824 if (TriedMerging.count(I)) 825 continue; 826 if (I->succ_empty()) 827 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(I), I)); 828 } 829 830 // If this is a large problem, avoid visiting the same basic blocks 831 // multiple times. 832 if (MergePotentials.size() == TailMergeThreshold) 833 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i) 834 TriedMerging.insert(MergePotentials[i].getBlock()); 835 // See if we can do any tail merging on those. 836 if (MergePotentials.size() >= 2) 837 MadeChange |= TryTailMergeBlocks(NULL, NULL); 838 839 // Look at blocks (IBB) with multiple predecessors (PBB). 840 // We change each predecessor to a canonical form, by 841 // (1) temporarily removing any unconditional branch from the predecessor 842 // to IBB, and 843 // (2) alter conditional branches so they branch to the other block 844 // not IBB; this may require adding back an unconditional branch to IBB 845 // later, where there wasn't one coming in. E.g. 846 // Bcc IBB 847 // fallthrough to QBB 848 // here becomes 849 // Bncc QBB 850 // with a conceptual B to IBB after that, which never actually exists. 851 // With those changes, we see whether the predecessors' tails match, 852 // and merge them if so. We change things out of canonical form and 853 // back to the way they were later in the process. (OptimizeBranches 854 // would undo some of this, but we can't use it, because we'd get into 855 // a compile-time infinite loop repeatedly doing and undoing the same 856 // transformations.) 857 858 for (MachineFunction::iterator I = llvm::next(MF.begin()), E = MF.end(); 859 I != E; ++I) { 860 if (I->pred_size() >= 2) { 861 SmallPtrSet<MachineBasicBlock *, 8> UniquePreds; 862 MachineBasicBlock *IBB = I; 863 MachineBasicBlock *PredBB = prior(I); 864 MergePotentials.clear(); 865 for (MachineBasicBlock::pred_iterator P = I->pred_begin(), 866 E2 = I->pred_end(); 867 P != E2 && MergePotentials.size() < TailMergeThreshold; ++P) { 868 MachineBasicBlock *PBB = *P; 869 if (TriedMerging.count(PBB)) 870 continue; 871 // Skip blocks that loop to themselves, can't tail merge these. 872 if (PBB == IBB) 873 continue; 874 // Visit each predecessor only once. 875 if (!UniquePreds.insert(PBB)) 876 continue; 877 // Skip blocks which may jump to a landing pad. Can't tail merge these. 878 if (PBB->getLandingPadSuccessor()) 879 continue; 880 MachineBasicBlock *TBB = 0, *FBB = 0; 881 SmallVector<MachineOperand, 4> Cond; 882 if (!TII->AnalyzeBranch(*PBB, TBB, FBB, Cond, true)) { 883 // Failing case: IBB is the target of a cbr, and 884 // we cannot reverse the branch. 885 SmallVector<MachineOperand, 4> NewCond(Cond); 886 if (!Cond.empty() && TBB == IBB) { 887 if (TII->ReverseBranchCondition(NewCond)) 888 continue; 889 // This is the QBB case described above 890 if (!FBB) 891 FBB = llvm::next(MachineFunction::iterator(PBB)); 892 } 893 // Failing case: the only way IBB can be reached from PBB is via 894 // exception handling. Happens for landing pads. Would be nice 895 // to have a bit in the edge so we didn't have to do all this. 896 if (IBB->isLandingPad()) { 897 MachineFunction::iterator IP = PBB; IP++; 898 MachineBasicBlock *PredNextBB = NULL; 899 if (IP != MF.end()) 900 PredNextBB = IP; 901 if (TBB == NULL) { 902 if (IBB != PredNextBB) // fallthrough 903 continue; 904 } else if (FBB) { 905 if (TBB != IBB && FBB != IBB) // cbr then ubr 906 continue; 907 } else if (Cond.empty()) { 908 if (TBB != IBB) // ubr 909 continue; 910 } else { 911 if (TBB != IBB && IBB != PredNextBB) // cbr 912 continue; 913 } 914 } 915 // Remove the unconditional branch at the end, if any. 916 if (TBB && (Cond.empty() || FBB)) { 917 DebugLoc dl; // FIXME: this is nowhere 918 TII->RemoveBranch(*PBB); 919 if (!Cond.empty()) 920 // reinsert conditional branch only, for now 921 TII->InsertBranch(*PBB, (TBB == IBB) ? FBB : TBB, 0, NewCond, dl); 922 } 923 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(PBB), *P)); 924 } 925 } 926 // If this is a large problem, avoid visiting the same basic blocks 927 // multiple times. 928 if (MergePotentials.size() == TailMergeThreshold) 929 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i) 930 TriedMerging.insert(MergePotentials[i].getBlock()); 931 if (MergePotentials.size() >= 2) 932 MadeChange |= TryTailMergeBlocks(IBB, PredBB); 933 // Reinsert an unconditional branch if needed. 934 // The 1 below can occur as a result of removing blocks in 935 // TryTailMergeBlocks. 936 PredBB = prior(I); // this may have been changed in TryTailMergeBlocks 937 if (MergePotentials.size() == 1 && 938 MergePotentials.begin()->getBlock() != PredBB) 939 FixTail(MergePotentials.begin()->getBlock(), IBB, TII); 940 } 941 } 942 return MadeChange; 943} 944 945//===----------------------------------------------------------------------===// 946// Branch Optimization 947//===----------------------------------------------------------------------===// 948 949bool BranchFolder::OptimizeBranches(MachineFunction &MF) { 950 bool MadeChange = false; 951 952 // Make sure blocks are numbered in order 953 MF.RenumberBlocks(); 954 955 for (MachineFunction::iterator I = llvm::next(MF.begin()), E = MF.end(); 956 I != E; ) { 957 MachineBasicBlock *MBB = I++; 958 MadeChange |= OptimizeBlock(MBB); 959 960 // If it is dead, remove it. 961 if (MBB->pred_empty()) { 962 RemoveDeadBlock(MBB); 963 MadeChange = true; 964 ++NumDeadBlocks; 965 } 966 } 967 return MadeChange; 968} 969 970// Blocks should be considered empty if they contain only debug info; 971// else the debug info would affect codegen. 972static bool IsEmptyBlock(MachineBasicBlock *MBB) { 973 if (MBB->empty()) 974 return true; 975 for (MachineBasicBlock::iterator MBBI = MBB->begin(), MBBE = MBB->end(); 976 MBBI!=MBBE; ++MBBI) { 977 if (!MBBI->isDebugValue()) 978 return false; 979 } 980 return true; 981} 982 983// Blocks with only debug info and branches should be considered the same 984// as blocks with only branches. 985static bool IsBranchOnlyBlock(MachineBasicBlock *MBB) { 986 MachineBasicBlock::iterator MBBI, MBBE; 987 for (MBBI = MBB->begin(), MBBE = MBB->end(); MBBI!=MBBE; ++MBBI) { 988 if (!MBBI->isDebugValue()) 989 break; 990 } 991 return (MBBI->isBranch()); 992} 993 994/// IsBetterFallthrough - Return true if it would be clearly better to 995/// fall-through to MBB1 than to fall through into MBB2. This has to return 996/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will 997/// result in infinite loops. 998static bool IsBetterFallthrough(MachineBasicBlock *MBB1, 999 MachineBasicBlock *MBB2) { 1000 // Right now, we use a simple heuristic. If MBB2 ends with a call, and 1001 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to 1002 // optimize branches that branch to either a return block or an assert block 1003 // into a fallthrough to the return. 1004 if (IsEmptyBlock(MBB1) || IsEmptyBlock(MBB2)) return false; 1005 1006 // If there is a clear successor ordering we make sure that one block 1007 // will fall through to the next 1008 if (MBB1->isSuccessor(MBB2)) return true; 1009 if (MBB2->isSuccessor(MBB1)) return false; 1010 1011 // Neither block consists entirely of debug info (per IsEmptyBlock check), 1012 // so we needn't test for falling off the beginning here. 1013 MachineBasicBlock::iterator MBB1I = --MBB1->end(); 1014 while (MBB1I->isDebugValue()) 1015 --MBB1I; 1016 MachineBasicBlock::iterator MBB2I = --MBB2->end(); 1017 while (MBB2I->isDebugValue()) 1018 --MBB2I; 1019 return MBB2I->isCall() && !MBB1I->isCall(); 1020} 1021 1022/// OptimizeBlock - Analyze and optimize control flow related to the specified 1023/// block. This is never called on the entry block. 1024bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) { 1025 bool MadeChange = false; 1026 MachineFunction &MF = *MBB->getParent(); 1027 DebugLoc dl; // FIXME: this is nowhere 1028ReoptimizeBlock: 1029 1030 MachineFunction::iterator FallThrough = MBB; 1031 ++FallThrough; 1032 1033 // If this block is empty, make everyone use its fall-through, not the block 1034 // explicitly. Landing pads should not do this since the landing-pad table 1035 // points to this block. Blocks with their addresses taken shouldn't be 1036 // optimized away. 1037 if (IsEmptyBlock(MBB) && !MBB->isLandingPad() && !MBB->hasAddressTaken()) { 1038 // Dead block? Leave for cleanup later. 1039 if (MBB->pred_empty()) return MadeChange; 1040 1041 if (FallThrough == MF.end()) { 1042 // TODO: Simplify preds to not branch here if possible! 1043 } else { 1044 // Rewrite all predecessors of the old block to go to the fallthrough 1045 // instead. 1046 while (!MBB->pred_empty()) { 1047 MachineBasicBlock *Pred = *(MBB->pred_end()-1); 1048 Pred->ReplaceUsesOfBlockWith(MBB, FallThrough); 1049 } 1050 // If MBB was the target of a jump table, update jump tables to go to the 1051 // fallthrough instead. 1052 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo()) 1053 MJTI->ReplaceMBBInJumpTables(MBB, FallThrough); 1054 MadeChange = true; 1055 } 1056 return MadeChange; 1057 } 1058 1059 // Check to see if we can simplify the terminator of the block before this 1060 // one. 1061 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB)); 1062 1063 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0; 1064 SmallVector<MachineOperand, 4> PriorCond; 1065 bool PriorUnAnalyzable = 1066 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true); 1067 if (!PriorUnAnalyzable) { 1068 // If the CFG for the prior block has extra edges, remove them. 1069 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB, 1070 !PriorCond.empty()); 1071 1072 // If the previous branch is conditional and both conditions go to the same 1073 // destination, remove the branch, replacing it with an unconditional one or 1074 // a fall-through. 1075 if (PriorTBB && PriorTBB == PriorFBB) { 1076 TII->RemoveBranch(PrevBB); 1077 PriorCond.clear(); 1078 if (PriorTBB != MBB) 1079 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond, dl); 1080 MadeChange = true; 1081 ++NumBranchOpts; 1082 goto ReoptimizeBlock; 1083 } 1084 1085 // If the previous block unconditionally falls through to this block and 1086 // this block has no other predecessors, move the contents of this block 1087 // into the prior block. This doesn't usually happen when SimplifyCFG 1088 // has been used, but it can happen if tail merging splits a fall-through 1089 // predecessor of a block. 1090 // This has to check PrevBB->succ_size() because EH edges are ignored by 1091 // AnalyzeBranch. 1092 if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 && 1093 PrevBB.succ_size() == 1 && 1094 !MBB->hasAddressTaken() && !MBB->isLandingPad()) { 1095 DEBUG(dbgs() << "\nMerging into block: " << PrevBB 1096 << "From MBB: " << *MBB); 1097 // Remove redundant DBG_VALUEs first. 1098 if (PrevBB.begin() != PrevBB.end()) { 1099 MachineBasicBlock::iterator PrevBBIter = PrevBB.end(); 1100 --PrevBBIter; 1101 MachineBasicBlock::iterator MBBIter = MBB->begin(); 1102 // Check if DBG_VALUE at the end of PrevBB is identical to the 1103 // DBG_VALUE at the beginning of MBB. 1104 while (PrevBBIter != PrevBB.begin() && MBBIter != MBB->end() 1105 && PrevBBIter->isDebugValue() && MBBIter->isDebugValue()) { 1106 if (!MBBIter->isIdenticalTo(PrevBBIter)) 1107 break; 1108 MachineInstr *DuplicateDbg = MBBIter; 1109 ++MBBIter; -- PrevBBIter; 1110 DuplicateDbg->eraseFromParent(); 1111 } 1112 } 1113 PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end()); 1114 PrevBB.removeSuccessor(PrevBB.succ_begin());; 1115 assert(PrevBB.succ_empty()); 1116 PrevBB.transferSuccessors(MBB); 1117 MadeChange = true; 1118 return MadeChange; 1119 } 1120 1121 // If the previous branch *only* branches to *this* block (conditional or 1122 // not) remove the branch. 1123 if (PriorTBB == MBB && PriorFBB == 0) { 1124 TII->RemoveBranch(PrevBB); 1125 MadeChange = true; 1126 ++NumBranchOpts; 1127 goto ReoptimizeBlock; 1128 } 1129 1130 // If the prior block branches somewhere else on the condition and here if 1131 // the condition is false, remove the uncond second branch. 1132 if (PriorFBB == MBB) { 1133 TII->RemoveBranch(PrevBB); 1134 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond, dl); 1135 MadeChange = true; 1136 ++NumBranchOpts; 1137 goto ReoptimizeBlock; 1138 } 1139 1140 // If the prior block branches here on true and somewhere else on false, and 1141 // if the branch condition is reversible, reverse the branch to create a 1142 // fall-through. 1143 if (PriorTBB == MBB) { 1144 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond); 1145 if (!TII->ReverseBranchCondition(NewPriorCond)) { 1146 TII->RemoveBranch(PrevBB); 1147 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond, dl); 1148 MadeChange = true; 1149 ++NumBranchOpts; 1150 goto ReoptimizeBlock; 1151 } 1152 } 1153 1154 // If this block has no successors (e.g. it is a return block or ends with 1155 // a call to a no-return function like abort or __cxa_throw) and if the pred 1156 // falls through into this block, and if it would otherwise fall through 1157 // into the block after this, move this block to the end of the function. 1158 // 1159 // We consider it more likely that execution will stay in the function (e.g. 1160 // due to loops) than it is to exit it. This asserts in loops etc, moving 1161 // the assert condition out of the loop body. 1162 if (MBB->succ_empty() && !PriorCond.empty() && PriorFBB == 0 && 1163 MachineFunction::iterator(PriorTBB) == FallThrough && 1164 !MBB->canFallThrough()) { 1165 bool DoTransform = true; 1166 1167 // We have to be careful that the succs of PredBB aren't both no-successor 1168 // blocks. If neither have successors and if PredBB is the second from 1169 // last block in the function, we'd just keep swapping the two blocks for 1170 // last. Only do the swap if one is clearly better to fall through than 1171 // the other. 1172 if (FallThrough == --MF.end() && 1173 !IsBetterFallthrough(PriorTBB, MBB)) 1174 DoTransform = false; 1175 1176 if (DoTransform) { 1177 // Reverse the branch so we will fall through on the previous true cond. 1178 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond); 1179 if (!TII->ReverseBranchCondition(NewPriorCond)) { 1180 DEBUG(dbgs() << "\nMoving MBB: " << *MBB 1181 << "To make fallthrough to: " << *PriorTBB << "\n"); 1182 1183 TII->RemoveBranch(PrevBB); 1184 TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond, dl); 1185 1186 // Move this block to the end of the function. 1187 MBB->moveAfter(--MF.end()); 1188 MadeChange = true; 1189 ++NumBranchOpts; 1190 return MadeChange; 1191 } 1192 } 1193 } 1194 } 1195 1196 // Analyze the branch in the current block. 1197 MachineBasicBlock *CurTBB = 0, *CurFBB = 0; 1198 SmallVector<MachineOperand, 4> CurCond; 1199 bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true); 1200 if (!CurUnAnalyzable) { 1201 // If the CFG for the prior block has extra edges, remove them. 1202 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty()); 1203 1204 // If this is a two-way branch, and the FBB branches to this block, reverse 1205 // the condition so the single-basic-block loop is faster. Instead of: 1206 // Loop: xxx; jcc Out; jmp Loop 1207 // we want: 1208 // Loop: xxx; jncc Loop; jmp Out 1209 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) { 1210 SmallVector<MachineOperand, 4> NewCond(CurCond); 1211 if (!TII->ReverseBranchCondition(NewCond)) { 1212 TII->RemoveBranch(*MBB); 1213 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond, dl); 1214 MadeChange = true; 1215 ++NumBranchOpts; 1216 goto ReoptimizeBlock; 1217 } 1218 } 1219 1220 // If this branch is the only thing in its block, see if we can forward 1221 // other blocks across it. 1222 if (CurTBB && CurCond.empty() && CurFBB == 0 && 1223 IsBranchOnlyBlock(MBB) && CurTBB != MBB && 1224 !MBB->hasAddressTaken()) { 1225 // This block may contain just an unconditional branch. Because there can 1226 // be 'non-branch terminators' in the block, try removing the branch and 1227 // then seeing if the block is empty. 1228 TII->RemoveBranch(*MBB); 1229 // If the only things remaining in the block are debug info, remove these 1230 // as well, so this will behave the same as an empty block in non-debug 1231 // mode. 1232 if (!MBB->empty()) { 1233 bool NonDebugInfoFound = false; 1234 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); 1235 I != E; ++I) { 1236 if (!I->isDebugValue()) { 1237 NonDebugInfoFound = true; 1238 break; 1239 } 1240 } 1241 if (!NonDebugInfoFound) 1242 // Make the block empty, losing the debug info (we could probably 1243 // improve this in some cases.) 1244 MBB->erase(MBB->begin(), MBB->end()); 1245 } 1246 // If this block is just an unconditional branch to CurTBB, we can 1247 // usually completely eliminate the block. The only case we cannot 1248 // completely eliminate the block is when the block before this one 1249 // falls through into MBB and we can't understand the prior block's branch 1250 // condition. 1251 if (MBB->empty()) { 1252 bool PredHasNoFallThrough = !PrevBB.canFallThrough(); 1253 if (PredHasNoFallThrough || !PriorUnAnalyzable || 1254 !PrevBB.isSuccessor(MBB)) { 1255 // If the prior block falls through into us, turn it into an 1256 // explicit branch to us to make updates simpler. 1257 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) && 1258 PriorTBB != MBB && PriorFBB != MBB) { 1259 if (PriorTBB == 0) { 1260 assert(PriorCond.empty() && PriorFBB == 0 && 1261 "Bad branch analysis"); 1262 PriorTBB = MBB; 1263 } else { 1264 assert(PriorFBB == 0 && "Machine CFG out of date!"); 1265 PriorFBB = MBB; 1266 } 1267 TII->RemoveBranch(PrevBB); 1268 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, dl); 1269 } 1270 1271 // Iterate through all the predecessors, revectoring each in-turn. 1272 size_t PI = 0; 1273 bool DidChange = false; 1274 bool HasBranchToSelf = false; 1275 while(PI != MBB->pred_size()) { 1276 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI); 1277 if (PMBB == MBB) { 1278 // If this block has an uncond branch to itself, leave it. 1279 ++PI; 1280 HasBranchToSelf = true; 1281 } else { 1282 DidChange = true; 1283 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB); 1284 // If this change resulted in PMBB ending in a conditional 1285 // branch where both conditions go to the same destination, 1286 // change this to an unconditional branch (and fix the CFG). 1287 MachineBasicBlock *NewCurTBB = 0, *NewCurFBB = 0; 1288 SmallVector<MachineOperand, 4> NewCurCond; 1289 bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB, 1290 NewCurFBB, NewCurCond, true); 1291 if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) { 1292 TII->RemoveBranch(*PMBB); 1293 NewCurCond.clear(); 1294 TII->InsertBranch(*PMBB, NewCurTBB, 0, NewCurCond, dl); 1295 MadeChange = true; 1296 ++NumBranchOpts; 1297 PMBB->CorrectExtraCFGEdges(NewCurTBB, 0, false); 1298 } 1299 } 1300 } 1301 1302 // Change any jumptables to go to the new MBB. 1303 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo()) 1304 MJTI->ReplaceMBBInJumpTables(MBB, CurTBB); 1305 if (DidChange) { 1306 ++NumBranchOpts; 1307 MadeChange = true; 1308 if (!HasBranchToSelf) return MadeChange; 1309 } 1310 } 1311 } 1312 1313 // Add the branch back if the block is more than just an uncond branch. 1314 TII->InsertBranch(*MBB, CurTBB, 0, CurCond, dl); 1315 } 1316 } 1317 1318 // If the prior block doesn't fall through into this block, and if this 1319 // block doesn't fall through into some other block, see if we can find a 1320 // place to move this block where a fall-through will happen. 1321 if (!PrevBB.canFallThrough()) { 1322 1323 // Now we know that there was no fall-through into this block, check to 1324 // see if it has a fall-through into its successor. 1325 bool CurFallsThru = MBB->canFallThrough(); 1326 1327 if (!MBB->isLandingPad()) { 1328 // Check all the predecessors of this block. If one of them has no fall 1329 // throughs, move this block right after it. 1330 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(), 1331 E = MBB->pred_end(); PI != E; ++PI) { 1332 // Analyze the branch at the end of the pred. 1333 MachineBasicBlock *PredBB = *PI; 1334 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough; 1335 MachineBasicBlock *PredTBB = 0, *PredFBB = 0; 1336 SmallVector<MachineOperand, 4> PredCond; 1337 if (PredBB != MBB && !PredBB->canFallThrough() && 1338 !TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true) 1339 && (!CurFallsThru || !CurTBB || !CurFBB) 1340 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) { 1341 // If the current block doesn't fall through, just move it. 1342 // If the current block can fall through and does not end with a 1343 // conditional branch, we need to append an unconditional jump to 1344 // the (current) next block. To avoid a possible compile-time 1345 // infinite loop, move blocks only backward in this case. 1346 // Also, if there are already 2 branches here, we cannot add a third; 1347 // this means we have the case 1348 // Bcc next 1349 // B elsewhere 1350 // next: 1351 if (CurFallsThru) { 1352 MachineBasicBlock *NextBB = llvm::next(MachineFunction::iterator(MBB)); 1353 CurCond.clear(); 1354 TII->InsertBranch(*MBB, NextBB, 0, CurCond, dl); 1355 } 1356 MBB->moveAfter(PredBB); 1357 MadeChange = true; 1358 goto ReoptimizeBlock; 1359 } 1360 } 1361 } 1362 1363 if (!CurFallsThru) { 1364 // Check all successors to see if we can move this block before it. 1365 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(), 1366 E = MBB->succ_end(); SI != E; ++SI) { 1367 // Analyze the branch at the end of the block before the succ. 1368 MachineBasicBlock *SuccBB = *SI; 1369 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev; 1370 1371 // If this block doesn't already fall-through to that successor, and if 1372 // the succ doesn't already have a block that can fall through into it, 1373 // and if the successor isn't an EH destination, we can arrange for the 1374 // fallthrough to happen. 1375 if (SuccBB != MBB && &*SuccPrev != MBB && 1376 !SuccPrev->canFallThrough() && !CurUnAnalyzable && 1377 !SuccBB->isLandingPad()) { 1378 MBB->moveBefore(SuccBB); 1379 MadeChange = true; 1380 goto ReoptimizeBlock; 1381 } 1382 } 1383 1384 // Okay, there is no really great place to put this block. If, however, 1385 // the block before this one would be a fall-through if this block were 1386 // removed, move this block to the end of the function. 1387 MachineBasicBlock *PrevTBB = 0, *PrevFBB = 0; 1388 SmallVector<MachineOperand, 4> PrevCond; 1389 if (FallThrough != MF.end() && 1390 !TII->AnalyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) && 1391 PrevBB.isSuccessor(FallThrough)) { 1392 MBB->moveAfter(--MF.end()); 1393 MadeChange = true; 1394 return MadeChange; 1395 } 1396 } 1397 } 1398 1399 return MadeChange; 1400} 1401 1402//===----------------------------------------------------------------------===// 1403// Hoist Common Code 1404//===----------------------------------------------------------------------===// 1405 1406/// HoistCommonCode - Hoist common instruction sequences at the start of basic 1407/// blocks to their common predecessor. 1408bool BranchFolder::HoistCommonCode(MachineFunction &MF) { 1409 bool MadeChange = false; 1410 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ) { 1411 MachineBasicBlock *MBB = I++; 1412 MadeChange |= HoistCommonCodeInSuccs(MBB); 1413 } 1414 1415 return MadeChange; 1416} 1417 1418/// findFalseBlock - BB has a fallthrough. Find its 'false' successor given 1419/// its 'true' successor. 1420static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB, 1421 MachineBasicBlock *TrueBB) { 1422 for (MachineBasicBlock::succ_iterator SI = BB->succ_begin(), 1423 E = BB->succ_end(); SI != E; ++SI) { 1424 MachineBasicBlock *SuccBB = *SI; 1425 if (SuccBB != TrueBB) 1426 return SuccBB; 1427 } 1428 return NULL; 1429} 1430 1431/// findHoistingInsertPosAndDeps - Find the location to move common instructions 1432/// in successors to. The location is ususally just before the terminator, 1433/// however if the terminator is a conditional branch and its previous 1434/// instruction is the flag setting instruction, the previous instruction is 1435/// the preferred location. This function also gathers uses and defs of the 1436/// instructions from the insertion point to the end of the block. The data is 1437/// used by HoistCommonCodeInSuccs to ensure safety. 1438static 1439MachineBasicBlock::iterator findHoistingInsertPosAndDeps(MachineBasicBlock *MBB, 1440 const TargetInstrInfo *TII, 1441 const TargetRegisterInfo *TRI, 1442 SmallSet<unsigned,4> &Uses, 1443 SmallSet<unsigned,4> &Defs) { 1444 MachineBasicBlock::iterator Loc = MBB->getFirstTerminator(); 1445 if (!TII->isUnpredicatedTerminator(Loc)) 1446 return MBB->end(); 1447 1448 for (unsigned i = 0, e = Loc->getNumOperands(); i != e; ++i) { 1449 const MachineOperand &MO = Loc->getOperand(i); 1450 if (!MO.isReg()) 1451 continue; 1452 unsigned Reg = MO.getReg(); 1453 if (!Reg) 1454 continue; 1455 if (MO.isUse()) { 1456 Uses.insert(Reg); 1457 for (const unsigned *AS = TRI->getAliasSet(Reg); *AS; ++AS) 1458 Uses.insert(*AS); 1459 } else if (!MO.isDead()) 1460 // Don't try to hoist code in the rare case the terminator defines a 1461 // register that is later used. 1462 return MBB->end(); 1463 } 1464 1465 if (Uses.empty()) 1466 return Loc; 1467 if (Loc == MBB->begin()) 1468 return MBB->end(); 1469 1470 // The terminator is probably a conditional branch, try not to separate the 1471 // branch from condition setting instruction. 1472 MachineBasicBlock::iterator PI = Loc; 1473 --PI; 1474 while (PI != MBB->begin() && Loc->isDebugValue()) 1475 --PI; 1476 1477 bool IsDef = false; 1478 for (unsigned i = 0, e = PI->getNumOperands(); !IsDef && i != e; ++i) { 1479 const MachineOperand &MO = PI->getOperand(i); 1480 if (!MO.isReg() || MO.isUse()) 1481 continue; 1482 unsigned Reg = MO.getReg(); 1483 if (!Reg) 1484 continue; 1485 if (Uses.count(Reg)) 1486 IsDef = true; 1487 } 1488 if (!IsDef) 1489 // The condition setting instruction is not just before the conditional 1490 // branch. 1491 return Loc; 1492 1493 // Be conservative, don't insert instruction above something that may have 1494 // side-effects. And since it's potentially bad to separate flag setting 1495 // instruction from the conditional branch, just abort the optimization 1496 // completely. 1497 // Also avoid moving code above predicated instruction since it's hard to 1498 // reason about register liveness with predicated instruction. 1499 bool DontMoveAcrossStore = true; 1500 if (!PI->isSafeToMove(TII, 0, DontMoveAcrossStore) || 1501 TII->isPredicated(PI)) 1502 return MBB->end(); 1503 1504 1505 // Find out what registers are live. Note this routine is ignoring other live 1506 // registers which are only used by instructions in successor blocks. 1507 for (unsigned i = 0, e = PI->getNumOperands(); i != e; ++i) { 1508 const MachineOperand &MO = PI->getOperand(i); 1509 if (!MO.isReg()) 1510 continue; 1511 unsigned Reg = MO.getReg(); 1512 if (!Reg) 1513 continue; 1514 if (MO.isUse()) { 1515 Uses.insert(Reg); 1516 for (const unsigned *AS = TRI->getAliasSet(Reg); *AS; ++AS) 1517 Uses.insert(*AS); 1518 } else { 1519 if (Uses.count(Reg)) { 1520 Uses.erase(Reg); 1521 for (const unsigned *SR = TRI->getSubRegisters(Reg); *SR; ++SR) 1522 Uses.erase(*SR); // Use getSubRegisters to be conservative 1523 } 1524 Defs.insert(Reg); 1525 for (const unsigned *AS = TRI->getAliasSet(Reg); *AS; ++AS) 1526 Defs.insert(*AS); 1527 } 1528 } 1529 1530 return PI; 1531} 1532 1533/// HoistCommonCodeInSuccs - If the successors of MBB has common instruction 1534/// sequence at the start of the function, move the instructions before MBB 1535/// terminator if it's legal. 1536bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) { 1537 MachineBasicBlock *TBB = 0, *FBB = 0; 1538 SmallVector<MachineOperand, 4> Cond; 1539 if (TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true) || !TBB || Cond.empty()) 1540 return false; 1541 1542 if (!FBB) FBB = findFalseBlock(MBB, TBB); 1543 if (!FBB) 1544 // Malformed bcc? True and false blocks are the same? 1545 return false; 1546 1547 // Restrict the optimization to cases where MBB is the only predecessor, 1548 // it is an obvious win. 1549 if (TBB->pred_size() > 1 || FBB->pred_size() > 1) 1550 return false; 1551 1552 // Find a suitable position to hoist the common instructions to. Also figure 1553 // out which registers are used or defined by instructions from the insertion 1554 // point to the end of the block. 1555 SmallSet<unsigned, 4> Uses, Defs; 1556 MachineBasicBlock::iterator Loc = 1557 findHoistingInsertPosAndDeps(MBB, TII, TRI, Uses, Defs); 1558 if (Loc == MBB->end()) 1559 return false; 1560 1561 bool HasDups = false; 1562 SmallVector<unsigned, 4> LocalDefs; 1563 SmallSet<unsigned, 4> LocalDefsSet; 1564 MachineBasicBlock::iterator TIB = TBB->begin(); 1565 MachineBasicBlock::iterator FIB = FBB->begin(); 1566 MachineBasicBlock::iterator TIE = TBB->end(); 1567 MachineBasicBlock::iterator FIE = FBB->end(); 1568 while (TIB != TIE && FIB != FIE) { 1569 // Skip dbg_value instructions. These do not count. 1570 if (TIB->isDebugValue()) { 1571 while (TIB != TIE && TIB->isDebugValue()) 1572 ++TIB; 1573 if (TIB == TIE) 1574 break; 1575 } 1576 if (FIB->isDebugValue()) { 1577 while (FIB != FIE && FIB->isDebugValue()) 1578 ++FIB; 1579 if (FIB == FIE) 1580 break; 1581 } 1582 if (!TIB->isIdenticalTo(FIB, MachineInstr::CheckKillDead)) 1583 break; 1584 1585 if (TII->isPredicated(TIB)) 1586 // Hard to reason about register liveness with predicated instruction. 1587 break; 1588 1589 bool IsSafe = true; 1590 for (unsigned i = 0, e = TIB->getNumOperands(); i != e; ++i) { 1591 MachineOperand &MO = TIB->getOperand(i); 1592 if (!MO.isReg()) 1593 continue; 1594 unsigned Reg = MO.getReg(); 1595 if (!Reg) 1596 continue; 1597 if (MO.isDef()) { 1598 if (Uses.count(Reg)) { 1599 // Avoid clobbering a register that's used by the instruction at 1600 // the point of insertion. 1601 IsSafe = false; 1602 break; 1603 } 1604 1605 if (Defs.count(Reg) && !MO.isDead()) { 1606 // Don't hoist the instruction if the def would be clobber by the 1607 // instruction at the point insertion. FIXME: This is overly 1608 // conservative. It should be possible to hoist the instructions 1609 // in BB2 in the following example: 1610 // BB1: 1611 // r1, eflag = op1 r2, r3 1612 // brcc eflag 1613 // 1614 // BB2: 1615 // r1 = op2, ... 1616 // = op3, r1<kill> 1617 IsSafe = false; 1618 break; 1619 } 1620 } else if (!LocalDefsSet.count(Reg)) { 1621 if (Defs.count(Reg)) { 1622 // Use is defined by the instruction at the point of insertion. 1623 IsSafe = false; 1624 break; 1625 } 1626 1627 if (MO.isKill() && Uses.count(Reg)) 1628 // Kills a register that's read by the instruction at the point of 1629 // insertion. Remove the kill marker. 1630 MO.setIsKill(false); 1631 } 1632 } 1633 if (!IsSafe) 1634 break; 1635 1636 bool DontMoveAcrossStore = true; 1637 if (!TIB->isSafeToMove(TII, 0, DontMoveAcrossStore)) 1638 break; 1639 1640 // Remove kills from LocalDefsSet, these registers had short live ranges. 1641 for (unsigned i = 0, e = TIB->getNumOperands(); i != e; ++i) { 1642 MachineOperand &MO = TIB->getOperand(i); 1643 if (!MO.isReg() || !MO.isUse() || !MO.isKill()) 1644 continue; 1645 unsigned Reg = MO.getReg(); 1646 if (!Reg || !LocalDefsSet.count(Reg)) 1647 continue; 1648 for (const unsigned *OR = TRI->getOverlaps(Reg); *OR; ++OR) 1649 LocalDefsSet.erase(*OR); 1650 } 1651 1652 // Track local defs so we can update liveins. 1653 for (unsigned i = 0, e = TIB->getNumOperands(); i != e; ++i) { 1654 MachineOperand &MO = TIB->getOperand(i); 1655 if (!MO.isReg() || !MO.isDef() || MO.isDead()) 1656 continue; 1657 unsigned Reg = MO.getReg(); 1658 if (!Reg) 1659 continue; 1660 LocalDefs.push_back(Reg); 1661 for (const unsigned *OR = TRI->getOverlaps(Reg); *OR; ++OR) 1662 LocalDefsSet.insert(*OR); 1663 } 1664 1665 HasDups = true;; 1666 ++TIB; 1667 ++FIB; 1668 } 1669 1670 if (!HasDups) 1671 return false; 1672 1673 MBB->splice(Loc, TBB, TBB->begin(), TIB); 1674 FBB->erase(FBB->begin(), FIB); 1675 1676 // Update livein's. 1677 for (unsigned i = 0, e = LocalDefs.size(); i != e; ++i) { 1678 unsigned Def = LocalDefs[i]; 1679 if (LocalDefsSet.count(Def)) { 1680 TBB->addLiveIn(Def); 1681 FBB->addLiveIn(Def); 1682 } 1683 } 1684 1685 ++NumHoist; 1686 return true; 1687} 1688