BranchFolding.cpp revision 8ef7e2159fc1cccf6308d20f5ef7872c344e2253
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"); 44 45static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge", 46 cl::init(cl::BOU_UNSET), cl::Hidden); 47 48// Throttle for huge numbers of predecessors (compile speed problems) 49static cl::opt<unsigned> 50TailMergeThreshold("tail-merge-threshold", 51 cl::desc("Max number of predecessors to consider tail merging"), 52 cl::init(150), cl::Hidden); 53 54// Heuristic for tail merging (and, inversely, tail duplication). 55// TODO: This should be replaced with a target query. 56static cl::opt<unsigned> 57TailMergeSize("tail-merge-size", 58 cl::desc("Min number of instructions to consider tail merging"), 59 cl::init(3), cl::Hidden); 60 61namespace { 62 /// BranchFolderPass - Wrap branch folder in a machine function pass. 63 class BranchFolderPass : public MachineFunctionPass, 64 public BranchFolder { 65 public: 66 static char ID; 67 explicit BranchFolderPass(bool defaultEnableTailMerge) 68 : MachineFunctionPass(&ID), BranchFolder(defaultEnableTailMerge) {} 69 70 virtual bool runOnMachineFunction(MachineFunction &MF); 71 virtual const char *getPassName() const { return "Control Flow Optimizer"; } 72 }; 73} 74 75char BranchFolderPass::ID = 0; 76 77FunctionPass *llvm::createBranchFoldingPass(bool DefaultEnableTailMerge) { 78 return new BranchFolderPass(DefaultEnableTailMerge); 79} 80 81bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) { 82 return OptimizeFunction(MF, 83 MF.getTarget().getInstrInfo(), 84 MF.getTarget().getRegisterInfo(), 85 getAnalysisIfAvailable<MachineModuleInfo>()); 86} 87 88 89BranchFolder::BranchFolder(bool defaultEnableTailMerge) { 90 switch (FlagEnableTailMerge) { 91 case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break; 92 case cl::BOU_TRUE: EnableTailMerge = true; break; 93 case cl::BOU_FALSE: EnableTailMerge = false; break; 94 } 95} 96 97/// RemoveDeadBlock - Remove the specified dead machine basic block from the 98/// function, updating the CFG. 99void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) { 100 assert(MBB->pred_empty() && "MBB must be dead!"); 101 DEBUG(dbgs() << "\nRemoving MBB: " << *MBB); 102 103 MachineFunction *MF = MBB->getParent(); 104 // drop all successors. 105 while (!MBB->succ_empty()) 106 MBB->removeSuccessor(MBB->succ_end()-1); 107 108 // Remove the block. 109 MF->erase(MBB); 110} 111 112/// OptimizeImpDefsBlock - If a basic block is just a bunch of implicit_def 113/// followed by terminators, and if the implicitly defined registers are not 114/// used by the terminators, remove those implicit_def's. e.g. 115/// BB1: 116/// r0 = implicit_def 117/// r1 = implicit_def 118/// br 119/// This block can be optimized away later if the implicit instructions are 120/// removed. 121bool BranchFolder::OptimizeImpDefsBlock(MachineBasicBlock *MBB) { 122 SmallSet<unsigned, 4> ImpDefRegs; 123 MachineBasicBlock::iterator I = MBB->begin(); 124 while (I != MBB->end()) { 125 if (!I->isImplicitDef()) 126 break; 127 unsigned Reg = I->getOperand(0).getReg(); 128 ImpDefRegs.insert(Reg); 129 for (const unsigned *SubRegs = TRI->getSubRegisters(Reg); 130 unsigned SubReg = *SubRegs; ++SubRegs) 131 ImpDefRegs.insert(SubReg); 132 ++I; 133 } 134 if (ImpDefRegs.empty()) 135 return false; 136 137 MachineBasicBlock::iterator FirstTerm = I; 138 while (I != MBB->end()) { 139 if (!TII->isUnpredicatedTerminator(I)) 140 return false; 141 // See if it uses any of the implicitly defined registers. 142 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) { 143 MachineOperand &MO = I->getOperand(i); 144 if (!MO.isReg() || !MO.isUse()) 145 continue; 146 unsigned Reg = MO.getReg(); 147 if (ImpDefRegs.count(Reg)) 148 return false; 149 } 150 ++I; 151 } 152 153 I = MBB->begin(); 154 while (I != FirstTerm) { 155 MachineInstr *ImpDefMI = &*I; 156 ++I; 157 MBB->erase(ImpDefMI); 158 } 159 160 return true; 161} 162 163/// OptimizeFunction - Perhaps branch folding, tail merging and other 164/// CFG optimizations on the given function. 165bool BranchFolder::OptimizeFunction(MachineFunction &MF, 166 const TargetInstrInfo *tii, 167 const TargetRegisterInfo *tri, 168 MachineModuleInfo *mmi) { 169 if (!tii) return false; 170 171 TII = tii; 172 TRI = tri; 173 MMI = mmi; 174 175 RS = TRI->requiresRegisterScavenging(MF) ? new RegScavenger() : NULL; 176 177 // Fix CFG. The later algorithms expect it to be right. 178 bool MadeChange = false; 179 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; I++) { 180 MachineBasicBlock *MBB = I, *TBB = 0, *FBB = 0; 181 SmallVector<MachineOperand, 4> Cond; 182 if (!TII->AnalyzeBranch(*MBB, TBB, FBB, Cond, true)) 183 MadeChange |= MBB->CorrectExtraCFGEdges(TBB, FBB, !Cond.empty()); 184 MadeChange |= OptimizeImpDefsBlock(MBB); 185 } 186 187 bool MadeChangeThisIteration = true; 188 while (MadeChangeThisIteration) { 189 MadeChangeThisIteration = false; 190 MadeChangeThisIteration |= TailMergeBlocks(MF); 191 MadeChangeThisIteration |= OptimizeBranches(MF); 192 MadeChange |= MadeChangeThisIteration; 193 } 194 195 // See if any jump tables have become mergable or dead as the code generator 196 // did its thing. 197 MachineJumpTableInfo *JTI = MF.getJumpTableInfo(); 198 if (JTI == 0) { 199 delete RS; 200 return MadeChange; 201 } 202 203 const std::vector<MachineJumpTableEntry> &JTs = JTI->getJumpTables(); 204 // Figure out how these jump tables should be merged. 205 std::vector<unsigned> JTMapping; 206 JTMapping.reserve(JTs.size()); 207 208 // We always keep the 0th jump table. 209 JTMapping.push_back(0); 210 211 // Scan the jump tables, seeing if there are any duplicates. Note that this 212 // is N^2, which should be fixed someday. 213 for (unsigned i = 1, e = JTs.size(); i != e; ++i) { 214 if (JTs[i].MBBs.empty()) 215 JTMapping.push_back(i); 216 else 217 JTMapping.push_back(JTI->getJumpTableIndex(JTs[i].MBBs)); 218 } 219 220 // If a jump table was merge with another one, walk the function rewriting 221 // references to jump tables to reference the new JT ID's. Keep track of 222 // whether we see a jump table idx, if not, we can delete the JT. 223 BitVector JTIsLive(JTs.size()); 224 for (MachineFunction::iterator BB = MF.begin(), E = MF.end(); 225 BB != E; ++BB) { 226 for (MachineBasicBlock::iterator I = BB->begin(), E = BB->end(); 227 I != E; ++I) 228 for (unsigned op = 0, e = I->getNumOperands(); op != e; ++op) { 229 MachineOperand &Op = I->getOperand(op); 230 if (!Op.isJTI()) continue; 231 unsigned NewIdx = JTMapping[Op.getIndex()]; 232 Op.setIndex(NewIdx); 233 234 // Remember that this JT is live. 235 JTIsLive.set(NewIdx); 236 } 237 } 238 239 // Finally, remove dead jump tables. This happens either because the 240 // indirect jump was unreachable (and thus deleted) or because the jump 241 // table was merged with some other one. 242 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i) 243 if (!JTIsLive.test(i)) { 244 JTI->RemoveJumpTable(i); 245 MadeChange = true; 246 } 247 248 delete RS; 249 return MadeChange; 250} 251 252//===----------------------------------------------------------------------===// 253// Tail Merging of Blocks 254//===----------------------------------------------------------------------===// 255 256/// HashMachineInstr - Compute a hash value for MI and its operands. 257static unsigned HashMachineInstr(const MachineInstr *MI) { 258 unsigned Hash = MI->getOpcode(); 259 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { 260 const MachineOperand &Op = MI->getOperand(i); 261 262 // Merge in bits from the operand if easy. 263 unsigned OperandHash = 0; 264 switch (Op.getType()) { 265 case MachineOperand::MO_Register: OperandHash = Op.getReg(); break; 266 case MachineOperand::MO_Immediate: OperandHash = Op.getImm(); break; 267 case MachineOperand::MO_MachineBasicBlock: 268 OperandHash = Op.getMBB()->getNumber(); 269 break; 270 case MachineOperand::MO_FrameIndex: 271 case MachineOperand::MO_ConstantPoolIndex: 272 case MachineOperand::MO_JumpTableIndex: 273 OperandHash = Op.getIndex(); 274 break; 275 case MachineOperand::MO_GlobalAddress: 276 case MachineOperand::MO_ExternalSymbol: 277 // Global address / external symbol are too hard, don't bother, but do 278 // pull in the offset. 279 OperandHash = Op.getOffset(); 280 break; 281 default: break; 282 } 283 284 Hash += ((OperandHash << 3) | Op.getType()) << (i&31); 285 } 286 return Hash; 287} 288 289/// HashEndOfMBB - Hash the last few instructions in the MBB. For blocks 290/// with no successors, we hash two instructions, because cross-jumping 291/// only saves code when at least two instructions are removed (since a 292/// branch must be inserted). For blocks with a successor, one of the 293/// two blocks to be tail-merged will end with a branch already, so 294/// it gains to cross-jump even for one instruction. 295static unsigned HashEndOfMBB(const MachineBasicBlock *MBB, 296 unsigned minCommonTailLength) { 297 MachineBasicBlock::const_iterator I = MBB->end(); 298 if (I == MBB->begin()) 299 return 0; // Empty MBB. 300 301 --I; 302 // Skip debug info so it will not affect codegen. 303 while (I->isDebugValue()) { 304 if (I==MBB->begin()) 305 return 0; // MBB empty except for debug info. 306 --I; 307 } 308 unsigned Hash = HashMachineInstr(I); 309 310 if (I == MBB->begin() || minCommonTailLength == 1) 311 return Hash; // Single instr MBB. 312 313 --I; 314 while (I->isDebugValue()) { 315 if (I==MBB->begin()) 316 return Hash; // MBB with single non-debug instr. 317 --I; 318 } 319 // Hash in the second-to-last instruction. 320 Hash ^= HashMachineInstr(I) << 2; 321 return Hash; 322} 323 324/// ComputeCommonTailLength - Given two machine basic blocks, compute the number 325/// of instructions they actually have in common together at their end. Return 326/// iterators for the first shared instruction in each block. 327static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1, 328 MachineBasicBlock *MBB2, 329 MachineBasicBlock::iterator &I1, 330 MachineBasicBlock::iterator &I2) { 331 I1 = MBB1->end(); 332 I2 = MBB2->end(); 333 334 unsigned TailLen = 0; 335 while (I1 != MBB1->begin() && I2 != MBB2->begin()) { 336 --I1; --I2; 337 // Skip debugging pseudos; necessary to avoid changing the code. 338 while (I1->isDebugValue()) { 339 if (I1==MBB1->begin()) { 340 while (I2->isDebugValue()) { 341 if (I2==MBB2->begin()) 342 // I1==DBG at begin; I2==DBG at begin 343 return TailLen; 344 --I2; 345 } 346 ++I2; 347 // I1==DBG at begin; I2==non-DBG, or first of DBGs not at begin 348 return TailLen; 349 } 350 --I1; 351 } 352 // I1==first (untested) non-DBG preceding known match 353 while (I2->isDebugValue()) { 354 if (I2==MBB2->begin()) { 355 ++I1; 356 // I1==non-DBG, or first of DBGs not at begin; I2==DBG at begin 357 return TailLen; 358 } 359 --I2; 360 } 361 // I1, I2==first (untested) non-DBGs preceding known match 362 if (!I1->isIdenticalTo(I2) || 363 // FIXME: This check is dubious. It's used to get around a problem where 364 // people incorrectly expect inline asm directives to remain in the same 365 // relative order. This is untenable because normal compiler 366 // optimizations (like this one) may reorder and/or merge these 367 // directives. 368 I1->isInlineAsm()) { 369 ++I1; ++I2; 370 break; 371 } 372 ++TailLen; 373 } 374 // Back past possible debugging pseudos at beginning of block. This matters 375 // when one block differs from the other only by whether debugging pseudos 376 // are present at the beginning. (This way, the various checks later for 377 // I1==MBB1->begin() work as expected.) 378 if (I1 == MBB1->begin() && I2 != MBB2->begin()) { 379 --I2; 380 while (I2->isDebugValue()) { 381 if (I2 == MBB2->begin()) { 382 return TailLen; 383 } 384 --I2; 385 } 386 ++I2; 387 } 388 if (I2 == MBB2->begin() && I1 != MBB1->begin()) { 389 --I1; 390 while (I1->isDebugValue()) { 391 if (I1 == MBB1->begin()) 392 return TailLen; 393 --I1; 394 } 395 ++I1; 396 } 397 return TailLen; 398} 399 400/// ReplaceTailWithBranchTo - Delete the instruction OldInst and everything 401/// after it, replacing it with an unconditional branch to NewDest. This 402/// returns true if OldInst's block is modified, false if NewDest is modified. 403void BranchFolder::ReplaceTailWithBranchTo(MachineBasicBlock::iterator OldInst, 404 MachineBasicBlock *NewDest) { 405 MachineBasicBlock *OldBB = OldInst->getParent(); 406 407 // Remove all the old successors of OldBB from the CFG. 408 while (!OldBB->succ_empty()) 409 OldBB->removeSuccessor(OldBB->succ_begin()); 410 411 // Remove all the dead instructions from the end of OldBB. 412 OldBB->erase(OldInst, OldBB->end()); 413 414 // If OldBB isn't immediately before OldBB, insert a branch to it. 415 if (++MachineFunction::iterator(OldBB) != MachineFunction::iterator(NewDest)) 416 TII->InsertBranch(*OldBB, NewDest, 0, SmallVector<MachineOperand, 0>()); 417 OldBB->addSuccessor(NewDest); 418 ++NumTailMerge; 419} 420 421/// SplitMBBAt - Given a machine basic block and an iterator into it, split the 422/// MBB so that the part before the iterator falls into the part starting at the 423/// iterator. This returns the new MBB. 424MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB, 425 MachineBasicBlock::iterator BBI1) { 426 MachineFunction &MF = *CurMBB.getParent(); 427 428 // Create the fall-through block. 429 MachineFunction::iterator MBBI = &CurMBB; 430 MachineBasicBlock *NewMBB =MF.CreateMachineBasicBlock(CurMBB.getBasicBlock()); 431 CurMBB.getParent()->insert(++MBBI, NewMBB); 432 433 // Move all the successors of this block to the specified block. 434 NewMBB->transferSuccessors(&CurMBB); 435 436 // Add an edge from CurMBB to NewMBB for the fall-through. 437 CurMBB.addSuccessor(NewMBB); 438 439 // Splice the code over. 440 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end()); 441 442 // For targets that use the register scavenger, we must maintain LiveIns. 443 if (RS) { 444 RS->enterBasicBlock(&CurMBB); 445 if (!CurMBB.empty()) 446 RS->forward(prior(CurMBB.end())); 447 BitVector RegsLiveAtExit(TRI->getNumRegs()); 448 RS->getRegsUsed(RegsLiveAtExit, false); 449 for (unsigned int i = 0, e = TRI->getNumRegs(); i != e; i++) 450 if (RegsLiveAtExit[i]) 451 NewMBB->addLiveIn(i); 452 } 453 454 return NewMBB; 455} 456 457/// EstimateRuntime - Make a rough estimate for how long it will take to run 458/// the specified code. 459static unsigned EstimateRuntime(MachineBasicBlock::iterator I, 460 MachineBasicBlock::iterator E) { 461 unsigned Time = 0; 462 for (; I != E; ++I) { 463 if (I->isDebugValue()) 464 continue; 465 const TargetInstrDesc &TID = I->getDesc(); 466 if (TID.isCall()) 467 Time += 10; 468 else if (TID.mayLoad() || TID.mayStore()) 469 Time += 2; 470 else 471 ++Time; 472 } 473 return Time; 474} 475 476// CurMBB needs to add an unconditional branch to SuccMBB (we removed these 477// branches temporarily for tail merging). In the case where CurMBB ends 478// with a conditional branch to the next block, optimize by reversing the 479// test and conditionally branching to SuccMBB instead. 480static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB, 481 const TargetInstrInfo *TII) { 482 MachineFunction *MF = CurMBB->getParent(); 483 MachineFunction::iterator I = llvm::next(MachineFunction::iterator(CurMBB)); 484 MachineBasicBlock *TBB = 0, *FBB = 0; 485 SmallVector<MachineOperand, 4> Cond; 486 if (I != MF->end() && 487 !TII->AnalyzeBranch(*CurMBB, TBB, FBB, Cond, true)) { 488 MachineBasicBlock *NextBB = I; 489 if (TBB == NextBB && !Cond.empty() && !FBB) { 490 if (!TII->ReverseBranchCondition(Cond)) { 491 TII->RemoveBranch(*CurMBB); 492 TII->InsertBranch(*CurMBB, SuccBB, NULL, Cond); 493 return; 494 } 495 } 496 } 497 TII->InsertBranch(*CurMBB, SuccBB, NULL, SmallVector<MachineOperand, 0>()); 498} 499 500bool 501BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const { 502 if (getHash() < o.getHash()) 503 return true; 504 else if (getHash() > o.getHash()) 505 return false; 506 else if (getBlock()->getNumber() < o.getBlock()->getNumber()) 507 return true; 508 else if (getBlock()->getNumber() > o.getBlock()->getNumber()) 509 return false; 510 else { 511 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing 512 // an object with itself. 513#ifndef _GLIBCXX_DEBUG 514 llvm_unreachable("Predecessor appears twice"); 515#endif 516 return false; 517 } 518} 519 520/// CountTerminators - Count the number of terminators in the given 521/// block and set I to the position of the first non-terminator, if there 522/// is one, or MBB->end() otherwise. 523static unsigned CountTerminators(MachineBasicBlock *MBB, 524 MachineBasicBlock::iterator &I) { 525 I = MBB->end(); 526 unsigned NumTerms = 0; 527 for (;;) { 528 if (I == MBB->begin()) { 529 I = MBB->end(); 530 break; 531 } 532 --I; 533 if (!I->getDesc().isTerminator()) break; 534 ++NumTerms; 535 } 536 return NumTerms; 537} 538 539/// ProfitableToMerge - Check if two machine basic blocks have a common tail 540/// and decide if it would be profitable to merge those tails. Return the 541/// length of the common tail and iterators to the first common instruction 542/// in each block. 543static bool ProfitableToMerge(MachineBasicBlock *MBB1, 544 MachineBasicBlock *MBB2, 545 unsigned minCommonTailLength, 546 unsigned &CommonTailLen, 547 MachineBasicBlock::iterator &I1, 548 MachineBasicBlock::iterator &I2, 549 MachineBasicBlock *SuccBB, 550 MachineBasicBlock *PredBB) { 551 CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2); 552 MachineFunction *MF = MBB1->getParent(); 553 554 if (CommonTailLen == 0) 555 return false; 556 557 // It's almost always profitable to merge any number of non-terminator 558 // instructions with the block that falls through into the common successor. 559 if (MBB1 == PredBB || MBB2 == PredBB) { 560 MachineBasicBlock::iterator I; 561 unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I); 562 if (CommonTailLen > NumTerms) 563 return true; 564 } 565 566 // If one of the blocks can be completely merged and happens to be in 567 // a position where the other could fall through into it, merge any number 568 // of instructions, because it can be done without a branch. 569 // TODO: If the blocks are not adjacent, move one of them so that they are? 570 if (MBB1->isLayoutSuccessor(MBB2) && I2 == MBB2->begin()) 571 return true; 572 if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin()) 573 return true; 574 575 // If both blocks have an unconditional branch temporarily stripped out, 576 // count that as an additional common instruction for the following 577 // heuristics. 578 unsigned EffectiveTailLen = CommonTailLen; 579 if (SuccBB && MBB1 != PredBB && MBB2 != PredBB && 580 !MBB1->back().getDesc().isBarrier() && 581 !MBB2->back().getDesc().isBarrier()) 582 ++EffectiveTailLen; 583 584 // Check if the common tail is long enough to be worthwhile. 585 if (EffectiveTailLen >= minCommonTailLength) 586 return true; 587 588 // If we are optimizing for code size, 2 instructions in common is enough if 589 // we don't have to split a block. At worst we will be introducing 1 new 590 // branch instruction, which is likely to be smaller than the 2 591 // instructions that would be deleted in the merge. 592 if (EffectiveTailLen >= 2 && 593 MF->getFunction()->hasFnAttr(Attribute::OptimizeForSize) && 594 (I1 == MBB1->begin() || I2 == MBB2->begin())) 595 return true; 596 597 return false; 598} 599 600/// ComputeSameTails - Look through all the blocks in MergePotentials that have 601/// hash CurHash (guaranteed to match the last element). Build the vector 602/// SameTails of all those that have the (same) largest number of instructions 603/// in common of any pair of these blocks. SameTails entries contain an 604/// iterator into MergePotentials (from which the MachineBasicBlock can be 605/// found) and a MachineBasicBlock::iterator into that MBB indicating the 606/// instruction where the matching code sequence begins. 607/// Order of elements in SameTails is the reverse of the order in which 608/// those blocks appear in MergePotentials (where they are not necessarily 609/// consecutive). 610unsigned BranchFolder::ComputeSameTails(unsigned CurHash, 611 unsigned minCommonTailLength, 612 MachineBasicBlock *SuccBB, 613 MachineBasicBlock *PredBB) { 614 unsigned maxCommonTailLength = 0U; 615 SameTails.clear(); 616 MachineBasicBlock::iterator TrialBBI1, TrialBBI2; 617 MPIterator HighestMPIter = prior(MergePotentials.end()); 618 for (MPIterator CurMPIter = prior(MergePotentials.end()), 619 B = MergePotentials.begin(); 620 CurMPIter != B && CurMPIter->getHash() == CurHash; 621 --CurMPIter) { 622 for (MPIterator I = prior(CurMPIter); I->getHash() == CurHash ; --I) { 623 unsigned CommonTailLen; 624 if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(), 625 minCommonTailLength, 626 CommonTailLen, TrialBBI1, TrialBBI2, 627 SuccBB, PredBB)) { 628 if (CommonTailLen > maxCommonTailLength) { 629 SameTails.clear(); 630 maxCommonTailLength = CommonTailLen; 631 HighestMPIter = CurMPIter; 632 SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1)); 633 } 634 if (HighestMPIter == CurMPIter && 635 CommonTailLen == maxCommonTailLength) 636 SameTails.push_back(SameTailElt(I, TrialBBI2)); 637 } 638 if (I == B) 639 break; 640 } 641 } 642 return maxCommonTailLength; 643} 644 645/// RemoveBlocksWithHash - Remove all blocks with hash CurHash from 646/// MergePotentials, restoring branches at ends of blocks as appropriate. 647void BranchFolder::RemoveBlocksWithHash(unsigned CurHash, 648 MachineBasicBlock *SuccBB, 649 MachineBasicBlock *PredBB) { 650 MPIterator CurMPIter, B; 651 for (CurMPIter = prior(MergePotentials.end()), B = MergePotentials.begin(); 652 CurMPIter->getHash() == CurHash; 653 --CurMPIter) { 654 // Put the unconditional branch back, if we need one. 655 MachineBasicBlock *CurMBB = CurMPIter->getBlock(); 656 if (SuccBB && CurMBB != PredBB) 657 FixTail(CurMBB, SuccBB, TII); 658 if (CurMPIter == B) 659 break; 660 } 661 if (CurMPIter->getHash() != CurHash) 662 CurMPIter++; 663 MergePotentials.erase(CurMPIter, MergePotentials.end()); 664} 665 666/// CreateCommonTailOnlyBlock - None of the blocks to be tail-merged consist 667/// only of the common tail. Create a block that does by splitting one. 668unsigned BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB, 669 unsigned maxCommonTailLength) { 670 unsigned commonTailIndex = 0; 671 unsigned TimeEstimate = ~0U; 672 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) { 673 // Use PredBB if possible; that doesn't require a new branch. 674 if (SameTails[i].getBlock() == PredBB) { 675 commonTailIndex = i; 676 break; 677 } 678 // Otherwise, make a (fairly bogus) choice based on estimate of 679 // how long it will take the various blocks to execute. 680 unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(), 681 SameTails[i].getTailStartPos()); 682 if (t <= TimeEstimate) { 683 TimeEstimate = t; 684 commonTailIndex = i; 685 } 686 } 687 688 MachineBasicBlock::iterator BBI = 689 SameTails[commonTailIndex].getTailStartPos(); 690 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock(); 691 692 // If the common tail includes any debug info we will take it pretty 693 // randomly from one of the inputs. Might be better to remove it? 694 DEBUG(dbgs() << "\nSplitting BB#" << MBB->getNumber() << ", size " 695 << maxCommonTailLength); 696 697 MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI); 698 SameTails[commonTailIndex].setBlock(newMBB); 699 SameTails[commonTailIndex].setTailStartPos(newMBB->begin()); 700 701 // If we split PredBB, newMBB is the new predecessor. 702 if (PredBB == MBB) 703 PredBB = newMBB; 704 705 return commonTailIndex; 706} 707 708// See if any of the blocks in MergePotentials (which all have a common single 709// successor, or all have no successor) can be tail-merged. If there is a 710// successor, any blocks in MergePotentials that are not tail-merged and 711// are not immediately before Succ must have an unconditional branch to 712// Succ added (but the predecessor/successor lists need no adjustment). 713// The lone predecessor of Succ that falls through into Succ, 714// if any, is given in PredBB. 715 716bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB, 717 MachineBasicBlock *PredBB) { 718 bool MadeChange = false; 719 720 // Except for the special cases below, tail-merge if there are at least 721 // this many instructions in common. 722 unsigned minCommonTailLength = TailMergeSize; 723 724 DEBUG(dbgs() << "\nTryTailMergeBlocks: "; 725 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i) 726 dbgs() << "BB#" << MergePotentials[i].getBlock()->getNumber() 727 << (i == e-1 ? "" : ", "); 728 dbgs() << "\n"; 729 if (SuccBB) { 730 dbgs() << " with successor BB#" << SuccBB->getNumber() << '\n'; 731 if (PredBB) 732 dbgs() << " which has fall-through from BB#" 733 << PredBB->getNumber() << "\n"; 734 } 735 dbgs() << "Looking for common tails of at least " 736 << minCommonTailLength << " instruction" 737 << (minCommonTailLength == 1 ? "" : "s") << '\n'; 738 ); 739 740 // Sort by hash value so that blocks with identical end sequences sort 741 // together. 742 std::stable_sort(MergePotentials.begin(), MergePotentials.end()); 743 744 // Walk through equivalence sets looking for actual exact matches. 745 while (MergePotentials.size() > 1) { 746 unsigned CurHash = MergePotentials.back().getHash(); 747 748 // Build SameTails, identifying the set of blocks with this hash code 749 // and with the maximum number of instructions in common. 750 unsigned maxCommonTailLength = ComputeSameTails(CurHash, 751 minCommonTailLength, 752 SuccBB, PredBB); 753 754 // If we didn't find any pair that has at least minCommonTailLength 755 // instructions in common, remove all blocks with this hash code and retry. 756 if (SameTails.empty()) { 757 RemoveBlocksWithHash(CurHash, SuccBB, PredBB); 758 continue; 759 } 760 761 // If one of the blocks is the entire common tail (and not the entry 762 // block, which we can't jump to), we can treat all blocks with this same 763 // tail at once. Use PredBB if that is one of the possibilities, as that 764 // will not introduce any extra branches. 765 MachineBasicBlock *EntryBB = MergePotentials.begin()->getBlock()-> 766 getParent()->begin(); 767 unsigned commonTailIndex = SameTails.size(); 768 // If there are two blocks, check to see if one can be made to fall through 769 // into the other. 770 if (SameTails.size() == 2 && 771 SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) && 772 SameTails[1].tailIsWholeBlock()) 773 commonTailIndex = 1; 774 else if (SameTails.size() == 2 && 775 SameTails[1].getBlock()->isLayoutSuccessor( 776 SameTails[0].getBlock()) && 777 SameTails[0].tailIsWholeBlock()) 778 commonTailIndex = 0; 779 else { 780 // Otherwise just pick one, favoring the fall-through predecessor if 781 // there is one. 782 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) { 783 MachineBasicBlock *MBB = SameTails[i].getBlock(); 784 if (MBB == EntryBB && SameTails[i].tailIsWholeBlock()) 785 continue; 786 if (MBB == PredBB) { 787 commonTailIndex = i; 788 break; 789 } 790 if (SameTails[i].tailIsWholeBlock()) 791 commonTailIndex = i; 792 } 793 } 794 795 if (commonTailIndex == SameTails.size() || 796 (SameTails[commonTailIndex].getBlock() == PredBB && 797 !SameTails[commonTailIndex].tailIsWholeBlock())) { 798 // None of the blocks consist entirely of the common tail. 799 // Split a block so that one does. 800 commonTailIndex = CreateCommonTailOnlyBlock(PredBB, maxCommonTailLength); 801 } 802 803 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock(); 804 // MBB is common tail. Adjust all other BB's to jump to this one. 805 // Traversal must be forwards so erases work. 806 DEBUG(dbgs() << "\nUsing common tail in BB#" << MBB->getNumber() 807 << " for "); 808 for (unsigned int i=0, e = SameTails.size(); i != e; ++i) { 809 if (commonTailIndex == i) 810 continue; 811 DEBUG(dbgs() << "BB#" << SameTails[i].getBlock()->getNumber() 812 << (i == e-1 ? "" : ", ")); 813 // Hack the end off BB i, making it jump to BB commonTailIndex instead. 814 ReplaceTailWithBranchTo(SameTails[i].getTailStartPos(), MBB); 815 // BB i is no longer a predecessor of SuccBB; remove it from the worklist. 816 MergePotentials.erase(SameTails[i].getMPIter()); 817 } 818 DEBUG(dbgs() << "\n"); 819 // We leave commonTailIndex in the worklist in case there are other blocks 820 // that match it with a smaller number of instructions. 821 MadeChange = true; 822 } 823 return MadeChange; 824} 825 826bool BranchFolder::TailMergeBlocks(MachineFunction &MF) { 827 828 if (!EnableTailMerge) return false; 829 830 bool MadeChange = false; 831 832 // First find blocks with no successors. 833 MergePotentials.clear(); 834 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) { 835 if (I->succ_empty()) 836 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(I, 2U), I)); 837 } 838 839 // See if we can do any tail merging on those. 840 if (MergePotentials.size() < TailMergeThreshold && 841 MergePotentials.size() >= 2) 842 MadeChange |= TryTailMergeBlocks(NULL, NULL); 843 844 // Look at blocks (IBB) with multiple predecessors (PBB). 845 // We change each predecessor to a canonical form, by 846 // (1) temporarily removing any unconditional branch from the predecessor 847 // to IBB, and 848 // (2) alter conditional branches so they branch to the other block 849 // not IBB; this may require adding back an unconditional branch to IBB 850 // later, where there wasn't one coming in. E.g. 851 // Bcc IBB 852 // fallthrough to QBB 853 // here becomes 854 // Bncc QBB 855 // with a conceptual B to IBB after that, which never actually exists. 856 // With those changes, we see whether the predecessors' tails match, 857 // and merge them if so. We change things out of canonical form and 858 // back to the way they were later in the process. (OptimizeBranches 859 // would undo some of this, but we can't use it, because we'd get into 860 // a compile-time infinite loop repeatedly doing and undoing the same 861 // transformations.) 862 863 for (MachineFunction::iterator I = llvm::next(MF.begin()), E = MF.end(); 864 I != E; ++I) { 865 if (I->pred_size() >= 2 && I->pred_size() < TailMergeThreshold) { 866 SmallPtrSet<MachineBasicBlock *, 8> UniquePreds; 867 MachineBasicBlock *IBB = I; 868 MachineBasicBlock *PredBB = prior(I); 869 MergePotentials.clear(); 870 for (MachineBasicBlock::pred_iterator P = I->pred_begin(), 871 E2 = I->pred_end(); 872 P != E2; ++P) { 873 MachineBasicBlock *PBB = *P; 874 // Skip blocks that loop to themselves, can't tail merge these. 875 if (PBB == IBB) 876 continue; 877 // Visit each predecessor only once. 878 if (!UniquePreds.insert(PBB)) 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 TII->RemoveBranch(*PBB); 918 if (!Cond.empty()) 919 // reinsert conditional branch only, for now 920 TII->InsertBranch(*PBB, (TBB == IBB) ? FBB : TBB, 0, NewCond); 921 } 922 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(PBB, 1U), 923 *P)); 924 } 925 } 926 if (MergePotentials.size() >= 2) 927 MadeChange |= TryTailMergeBlocks(IBB, PredBB); 928 // Reinsert an unconditional branch if needed. 929 // The 1 below can occur as a result of removing blocks in TryTailMergeBlocks. 930 PredBB = prior(I); // this may have been changed in TryTailMergeBlocks 931 if (MergePotentials.size() == 1 && 932 MergePotentials.begin()->getBlock() != PredBB) 933 FixTail(MergePotentials.begin()->getBlock(), IBB, TII); 934 } 935 } 936 return MadeChange; 937} 938 939//===----------------------------------------------------------------------===// 940// Branch Optimization 941//===----------------------------------------------------------------------===// 942 943bool BranchFolder::OptimizeBranches(MachineFunction &MF) { 944 bool MadeChange = false; 945 946 // Make sure blocks are numbered in order 947 MF.RenumberBlocks(); 948 949 for (MachineFunction::iterator I = ++MF.begin(), E = MF.end(); I != E; ) { 950 MachineBasicBlock *MBB = I++; 951 MadeChange |= OptimizeBlock(MBB); 952 953 // If it is dead, remove it. 954 if (MBB->pred_empty()) { 955 RemoveDeadBlock(MBB); 956 MadeChange = true; 957 ++NumDeadBlocks; 958 } 959 } 960 return MadeChange; 961} 962 963// Blocks should be considered empty if they contain only debug info; 964// else the debug info would affect codegen. 965static bool IsEmptyBlock(MachineBasicBlock *MBB) { 966 if (MBB->empty()) 967 return true; 968 for (MachineBasicBlock::iterator MBBI = MBB->begin(), MBBE = MBB->end(); 969 MBBI!=MBBE; ++MBBI) { 970 if (!MBBI->isDebugValue()) 971 return false; 972 } 973 return true; 974} 975 976// Blocks with only debug info and branches should be considered the same 977// as blocks with only branches. 978static bool IsBranchOnlyBlock(MachineBasicBlock *MBB) { 979 MachineBasicBlock::iterator MBBI, MBBE; 980 for (MBBI = MBB->begin(), MBBE = MBB->end(); MBBI!=MBBE; ++MBBI) { 981 if (!MBBI->isDebugValue()) 982 break; 983 } 984 return (MBBI->getDesc().isBranch()); 985} 986 987/// IsBetterFallthrough - Return true if it would be clearly better to 988/// fall-through to MBB1 than to fall through into MBB2. This has to return 989/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will 990/// result in infinite loops. 991static bool IsBetterFallthrough(MachineBasicBlock *MBB1, 992 MachineBasicBlock *MBB2) { 993 // Right now, we use a simple heuristic. If MBB2 ends with a call, and 994 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to 995 // optimize branches that branch to either a return block or an assert block 996 // into a fallthrough to the return. 997 if (MBB1->empty() || MBB2->empty()) return false; 998 999 // If there is a clear successor ordering we make sure that one block 1000 // will fall through to the next 1001 if (MBB1->isSuccessor(MBB2)) return true; 1002 if (MBB2->isSuccessor(MBB1)) return false; 1003 1004 MachineInstr *MBB1I = --MBB1->end(); 1005 MachineInstr *MBB2I = --MBB2->end(); 1006 return MBB2I->getDesc().isCall() && !MBB1I->getDesc().isCall(); 1007} 1008 1009/// OptimizeBlock - Analyze and optimize control flow related to the specified 1010/// block. This is never called on the entry block. 1011bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) { 1012 bool MadeChange = false; 1013 MachineFunction &MF = *MBB->getParent(); 1014ReoptimizeBlock: 1015 1016 MachineFunction::iterator FallThrough = MBB; 1017 ++FallThrough; 1018 1019 // If this block is empty, make everyone use its fall-through, not the block 1020 // explicitly. Landing pads should not do this since the landing-pad table 1021 // points to this block. Blocks with their addresses taken shouldn't be 1022 // optimized away. 1023 if (IsEmptyBlock(MBB) && !MBB->isLandingPad() && !MBB->hasAddressTaken()) { 1024 // Dead block? Leave for cleanup later. 1025 if (MBB->pred_empty()) return MadeChange; 1026 1027 if (FallThrough == MF.end()) { 1028 // TODO: Simplify preds to not branch here if possible! 1029 } else { 1030 // Rewrite all predecessors of the old block to go to the fallthrough 1031 // instead. 1032 while (!MBB->pred_empty()) { 1033 MachineBasicBlock *Pred = *(MBB->pred_end()-1); 1034 Pred->ReplaceUsesOfBlockWith(MBB, FallThrough); 1035 } 1036 // If MBB was the target of a jump table, update jump tables to go to the 1037 // fallthrough instead. 1038 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo()) 1039 MJTI->ReplaceMBBInJumpTables(MBB, FallThrough); 1040 MadeChange = true; 1041 } 1042 return MadeChange; 1043 } 1044 1045 // Check to see if we can simplify the terminator of the block before this 1046 // one. 1047 MachineBasicBlock &PrevBB = *prior(MachineFunction::iterator(MBB)); 1048 1049 MachineBasicBlock *PriorTBB = 0, *PriorFBB = 0; 1050 SmallVector<MachineOperand, 4> PriorCond; 1051 bool PriorUnAnalyzable = 1052 TII->AnalyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true); 1053 if (!PriorUnAnalyzable) { 1054 // If the CFG for the prior block has extra edges, remove them. 1055 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB, 1056 !PriorCond.empty()); 1057 1058 // If the previous branch is conditional and both conditions go to the same 1059 // destination, remove the branch, replacing it with an unconditional one or 1060 // a fall-through. 1061 if (PriorTBB && PriorTBB == PriorFBB) { 1062 TII->RemoveBranch(PrevBB); 1063 PriorCond.clear(); 1064 if (PriorTBB != MBB) 1065 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 1066 MadeChange = true; 1067 ++NumBranchOpts; 1068 goto ReoptimizeBlock; 1069 } 1070 1071 // If the previous block unconditionally falls through to this block and 1072 // this block has no other predecessors, move the contents of this block 1073 // into the prior block. This doesn't usually happen when SimplifyCFG 1074 // has been used, but it can happen if tail merging splits a fall-through 1075 // predecessor of a block. 1076 // This has to check PrevBB->succ_size() because EH edges are ignored by 1077 // AnalyzeBranch. 1078 if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 && 1079 PrevBB.succ_size() == 1 && 1080 !MBB->hasAddressTaken()) { 1081 DEBUG(dbgs() << "\nMerging into block: " << PrevBB 1082 << "From MBB: " << *MBB); 1083 PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end()); 1084 PrevBB.removeSuccessor(PrevBB.succ_begin());; 1085 assert(PrevBB.succ_empty()); 1086 PrevBB.transferSuccessors(MBB); 1087 MadeChange = true; 1088 return MadeChange; 1089 } 1090 1091 // If the previous branch *only* branches to *this* block (conditional or 1092 // not) remove the branch. 1093 if (PriorTBB == MBB && PriorFBB == 0) { 1094 TII->RemoveBranch(PrevBB); 1095 MadeChange = true; 1096 ++NumBranchOpts; 1097 goto ReoptimizeBlock; 1098 } 1099 1100 // If the prior block branches somewhere else on the condition and here if 1101 // the condition is false, remove the uncond second branch. 1102 if (PriorFBB == MBB) { 1103 TII->RemoveBranch(PrevBB); 1104 TII->InsertBranch(PrevBB, PriorTBB, 0, PriorCond); 1105 MadeChange = true; 1106 ++NumBranchOpts; 1107 goto ReoptimizeBlock; 1108 } 1109 1110 // If the prior block branches here on true and somewhere else on false, and 1111 // if the branch condition is reversible, reverse the branch to create a 1112 // fall-through. 1113 if (PriorTBB == MBB) { 1114 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond); 1115 if (!TII->ReverseBranchCondition(NewPriorCond)) { 1116 TII->RemoveBranch(PrevBB); 1117 TII->InsertBranch(PrevBB, PriorFBB, 0, NewPriorCond); 1118 MadeChange = true; 1119 ++NumBranchOpts; 1120 goto ReoptimizeBlock; 1121 } 1122 } 1123 1124 // If this block has no successors (e.g. it is a return block or ends with 1125 // a call to a no-return function like abort or __cxa_throw) and if the pred 1126 // falls through into this block, and if it would otherwise fall through 1127 // into the block after this, move this block to the end of the function. 1128 // 1129 // We consider it more likely that execution will stay in the function (e.g. 1130 // due to loops) than it is to exit it. This asserts in loops etc, moving 1131 // the assert condition out of the loop body. 1132 if (MBB->succ_empty() && !PriorCond.empty() && PriorFBB == 0 && 1133 MachineFunction::iterator(PriorTBB) == FallThrough && 1134 !MBB->canFallThrough()) { 1135 bool DoTransform = true; 1136 1137 // We have to be careful that the succs of PredBB aren't both no-successor 1138 // blocks. If neither have successors and if PredBB is the second from 1139 // last block in the function, we'd just keep swapping the two blocks for 1140 // last. Only do the swap if one is clearly better to fall through than 1141 // the other. 1142 if (FallThrough == --MF.end() && 1143 !IsBetterFallthrough(PriorTBB, MBB)) 1144 DoTransform = false; 1145 1146 if (DoTransform) { 1147 // Reverse the branch so we will fall through on the previous true cond. 1148 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond); 1149 if (!TII->ReverseBranchCondition(NewPriorCond)) { 1150 DEBUG(dbgs() << "\nMoving MBB: " << *MBB 1151 << "To make fallthrough to: " << *PriorTBB << "\n"); 1152 1153 TII->RemoveBranch(PrevBB); 1154 TII->InsertBranch(PrevBB, MBB, 0, NewPriorCond); 1155 1156 // Move this block to the end of the function. 1157 MBB->moveAfter(--MF.end()); 1158 MadeChange = true; 1159 ++NumBranchOpts; 1160 return MadeChange; 1161 } 1162 } 1163 } 1164 } 1165 1166 // Analyze the branch in the current block. 1167 MachineBasicBlock *CurTBB = 0, *CurFBB = 0; 1168 SmallVector<MachineOperand, 4> CurCond; 1169 bool CurUnAnalyzable= TII->AnalyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true); 1170 if (!CurUnAnalyzable) { 1171 // If the CFG for the prior block has extra edges, remove them. 1172 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty()); 1173 1174 // If this is a two-way branch, and the FBB branches to this block, reverse 1175 // the condition so the single-basic-block loop is faster. Instead of: 1176 // Loop: xxx; jcc Out; jmp Loop 1177 // we want: 1178 // Loop: xxx; jncc Loop; jmp Out 1179 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) { 1180 SmallVector<MachineOperand, 4> NewCond(CurCond); 1181 if (!TII->ReverseBranchCondition(NewCond)) { 1182 TII->RemoveBranch(*MBB); 1183 TII->InsertBranch(*MBB, CurFBB, CurTBB, NewCond); 1184 MadeChange = true; 1185 ++NumBranchOpts; 1186 goto ReoptimizeBlock; 1187 } 1188 } 1189 1190 // If this branch is the only thing in its block, see if we can forward 1191 // other blocks across it. 1192 if (CurTBB && CurCond.empty() && CurFBB == 0 && 1193 IsBranchOnlyBlock(MBB) && CurTBB != MBB && 1194 !MBB->hasAddressTaken()) { 1195 // This block may contain just an unconditional branch. Because there can 1196 // be 'non-branch terminators' in the block, try removing the branch and 1197 // then seeing if the block is empty. 1198 TII->RemoveBranch(*MBB); 1199 // If the only things remaining in the block are debug info, remove these 1200 // as well, so this will behave the same as an empty block in non-debug 1201 // mode. 1202 if (!MBB->empty()) { 1203 bool NonDebugInfoFound = false; 1204 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); 1205 I != E; ++I) { 1206 if (!I->isDebugValue()) { 1207 NonDebugInfoFound = true; 1208 break; 1209 } 1210 } 1211 if (!NonDebugInfoFound) 1212 // Make the block empty, losing the debug info (we could probably 1213 // improve this in some cases.) 1214 MBB->erase(MBB->begin(), MBB->end()); 1215 } 1216 // If this block is just an unconditional branch to CurTBB, we can 1217 // usually completely eliminate the block. The only case we cannot 1218 // completely eliminate the block is when the block before this one 1219 // falls through into MBB and we can't understand the prior block's branch 1220 // condition. 1221 if (MBB->empty()) { 1222 bool PredHasNoFallThrough = !PrevBB.canFallThrough(); 1223 if (PredHasNoFallThrough || !PriorUnAnalyzable || 1224 !PrevBB.isSuccessor(MBB)) { 1225 // If the prior block falls through into us, turn it into an 1226 // explicit branch to us to make updates simpler. 1227 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) && 1228 PriorTBB != MBB && PriorFBB != MBB) { 1229 if (PriorTBB == 0) { 1230 assert(PriorCond.empty() && PriorFBB == 0 && 1231 "Bad branch analysis"); 1232 PriorTBB = MBB; 1233 } else { 1234 assert(PriorFBB == 0 && "Machine CFG out of date!"); 1235 PriorFBB = MBB; 1236 } 1237 TII->RemoveBranch(PrevBB); 1238 TII->InsertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond); 1239 } 1240 1241 // Iterate through all the predecessors, revectoring each in-turn. 1242 size_t PI = 0; 1243 bool DidChange = false; 1244 bool HasBranchToSelf = false; 1245 while(PI != MBB->pred_size()) { 1246 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI); 1247 if (PMBB == MBB) { 1248 // If this block has an uncond branch to itself, leave it. 1249 ++PI; 1250 HasBranchToSelf = true; 1251 } else { 1252 DidChange = true; 1253 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB); 1254 // If this change resulted in PMBB ending in a conditional 1255 // branch where both conditions go to the same destination, 1256 // change this to an unconditional branch (and fix the CFG). 1257 MachineBasicBlock *NewCurTBB = 0, *NewCurFBB = 0; 1258 SmallVector<MachineOperand, 4> NewCurCond; 1259 bool NewCurUnAnalyzable = TII->AnalyzeBranch(*PMBB, NewCurTBB, 1260 NewCurFBB, NewCurCond, true); 1261 if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) { 1262 TII->RemoveBranch(*PMBB); 1263 NewCurCond.clear(); 1264 TII->InsertBranch(*PMBB, NewCurTBB, 0, NewCurCond); 1265 MadeChange = true; 1266 ++NumBranchOpts; 1267 PMBB->CorrectExtraCFGEdges(NewCurTBB, 0, false); 1268 } 1269 } 1270 } 1271 1272 // Change any jumptables to go to the new MBB. 1273 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo()) 1274 MJTI->ReplaceMBBInJumpTables(MBB, CurTBB); 1275 if (DidChange) { 1276 ++NumBranchOpts; 1277 MadeChange = true; 1278 if (!HasBranchToSelf) return MadeChange; 1279 } 1280 } 1281 } 1282 1283 // Add the branch back if the block is more than just an uncond branch. 1284 TII->InsertBranch(*MBB, CurTBB, 0, CurCond); 1285 } 1286 } 1287 1288 // If the prior block doesn't fall through into this block, and if this 1289 // block doesn't fall through into some other block, see if we can find a 1290 // place to move this block where a fall-through will happen. 1291 if (!PrevBB.canFallThrough()) { 1292 1293 // Now we know that there was no fall-through into this block, check to 1294 // see if it has a fall-through into its successor. 1295 bool CurFallsThru = MBB->canFallThrough(); 1296 1297 if (!MBB->isLandingPad()) { 1298 // Check all the predecessors of this block. If one of them has no fall 1299 // throughs, move this block right after it. 1300 for (MachineBasicBlock::pred_iterator PI = MBB->pred_begin(), 1301 E = MBB->pred_end(); PI != E; ++PI) { 1302 // Analyze the branch at the end of the pred. 1303 MachineBasicBlock *PredBB = *PI; 1304 MachineFunction::iterator PredFallthrough = PredBB; ++PredFallthrough; 1305 MachineBasicBlock *PredTBB = 0, *PredFBB = 0; 1306 SmallVector<MachineOperand, 4> PredCond; 1307 if (PredBB != MBB && !PredBB->canFallThrough() && 1308 !TII->AnalyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true) 1309 && (!CurFallsThru || !CurTBB || !CurFBB) 1310 && (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) { 1311 // If the current block doesn't fall through, just move it. 1312 // If the current block can fall through and does not end with a 1313 // conditional branch, we need to append an unconditional jump to 1314 // the (current) next block. To avoid a possible compile-time 1315 // infinite loop, move blocks only backward in this case. 1316 // Also, if there are already 2 branches here, we cannot add a third; 1317 // this means we have the case 1318 // Bcc next 1319 // B elsewhere 1320 // next: 1321 if (CurFallsThru) { 1322 MachineBasicBlock *NextBB = llvm::next(MachineFunction::iterator(MBB)); 1323 CurCond.clear(); 1324 TII->InsertBranch(*MBB, NextBB, 0, CurCond); 1325 } 1326 MBB->moveAfter(PredBB); 1327 MadeChange = true; 1328 goto ReoptimizeBlock; 1329 } 1330 } 1331 } 1332 1333 if (!CurFallsThru) { 1334 // Check all successors to see if we can move this block before it. 1335 for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(), 1336 E = MBB->succ_end(); SI != E; ++SI) { 1337 // Analyze the branch at the end of the block before the succ. 1338 MachineBasicBlock *SuccBB = *SI; 1339 MachineFunction::iterator SuccPrev = SuccBB; --SuccPrev; 1340 1341 // If this block doesn't already fall-through to that successor, and if 1342 // the succ doesn't already have a block that can fall through into it, 1343 // and if the successor isn't an EH destination, we can arrange for the 1344 // fallthrough to happen. 1345 if (SuccBB != MBB && &*SuccPrev != MBB && 1346 !SuccPrev->canFallThrough() && !CurUnAnalyzable && 1347 !SuccBB->isLandingPad()) { 1348 MBB->moveBefore(SuccBB); 1349 MadeChange = true; 1350 goto ReoptimizeBlock; 1351 } 1352 } 1353 1354 // Okay, there is no really great place to put this block. If, however, 1355 // the block before this one would be a fall-through if this block were 1356 // removed, move this block to the end of the function. 1357 MachineBasicBlock *PrevTBB = 0, *PrevFBB = 0; 1358 SmallVector<MachineOperand, 4> PrevCond; 1359 if (FallThrough != MF.end() && 1360 !TII->AnalyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) && 1361 PrevBB.isSuccessor(FallThrough)) { 1362 MBB->moveAfter(--MF.end()); 1363 MadeChange = true; 1364 return MadeChange; 1365 } 1366 } 1367 } 1368 1369 return MadeChange; 1370} 1371