SimplifyCFG.cpp revision 0289929c6212229980c9057853860ccfd3f81678
1//===- SimplifyCFG.cpp - Code to perform CFG simplification ---------------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file was developed by the LLVM research group and is distributed under 6// the University of Illinois Open Source License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// Peephole optimize the CFG. 11// 12//===----------------------------------------------------------------------===// 13 14#define DEBUG_TYPE "simplifycfg" 15#include "llvm/Transforms/Utils/Local.h" 16#include "llvm/Constants.h" 17#include "llvm/Instructions.h" 18#include "llvm/Type.h" 19#include "llvm/Support/CFG.h" 20#include "llvm/Support/Debug.h" 21#include <algorithm> 22#include <functional> 23#include <set> 24#include <map> 25using namespace llvm; 26 27// PropagatePredecessorsForPHIs - This gets "Succ" ready to have the 28// predecessors from "BB". This is a little tricky because "Succ" has PHI 29// nodes, which need to have extra slots added to them to hold the merge edges 30// from BB's predecessors, and BB itself might have had PHI nodes in it. This 31// function returns true (failure) if the Succ BB already has a predecessor that 32// is a predecessor of BB and incoming PHI arguments would not be discernible. 33// 34// Assumption: Succ is the single successor for BB. 35// 36static bool PropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) { 37 assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!"); 38 39 if (!isa<PHINode>(Succ->front())) 40 return false; // We can make the transformation, no problem. 41 42 // If there is more than one predecessor, and there are PHI nodes in 43 // the successor, then we need to add incoming edges for the PHI nodes 44 // 45 const std::vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB)); 46 47 // Check to see if one of the predecessors of BB is already a predecessor of 48 // Succ. If so, we cannot do the transformation if there are any PHI nodes 49 // with incompatible values coming in from the two edges! 50 // 51 for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ); PI != PE; ++PI) 52 if (std::find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end()) { 53 // Loop over all of the PHI nodes checking to see if there are 54 // incompatible values coming in. 55 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) { 56 PHINode *PN = cast<PHINode>(I); 57 // Loop up the entries in the PHI node for BB and for *PI if the values 58 // coming in are non-equal, we cannot merge these two blocks (instead we 59 // should insert a conditional move or something, then merge the 60 // blocks). 61 int Idx1 = PN->getBasicBlockIndex(BB); 62 int Idx2 = PN->getBasicBlockIndex(*PI); 63 assert(Idx1 != -1 && Idx2 != -1 && 64 "Didn't have entries for my predecessors??"); 65 if (PN->getIncomingValue(Idx1) != PN->getIncomingValue(Idx2)) 66 return true; // Values are not equal... 67 } 68 } 69 70 // Loop over all of the PHI nodes in the successor BB. 71 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) { 72 PHINode *PN = cast<PHINode>(I); 73 Value *OldVal = PN->removeIncomingValue(BB, false); 74 assert(OldVal && "No entry in PHI for Pred BB!"); 75 76 // If this incoming value is one of the PHI nodes in BB, the new entries in 77 // the PHI node are the entries from the old PHI. 78 if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) { 79 PHINode *OldValPN = cast<PHINode>(OldVal); 80 for (unsigned i = 0, e = OldValPN->getNumIncomingValues(); i != e; ++i) 81 PN->addIncoming(OldValPN->getIncomingValue(i), 82 OldValPN->getIncomingBlock(i)); 83 } else { 84 for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(), 85 End = BBPreds.end(); PredI != End; ++PredI) { 86 // Add an incoming value for each of the new incoming values... 87 PN->addIncoming(OldVal, *PredI); 88 } 89 } 90 } 91 return false; 92} 93 94/// GetIfCondition - Given a basic block (BB) with two predecessors (and 95/// presumably PHI nodes in it), check to see if the merge at this block is due 96/// to an "if condition". If so, return the boolean condition that determines 97/// which entry into BB will be taken. Also, return by references the block 98/// that will be entered from if the condition is true, and the block that will 99/// be entered if the condition is false. 100/// 101/// 102static Value *GetIfCondition(BasicBlock *BB, 103 BasicBlock *&IfTrue, BasicBlock *&IfFalse) { 104 assert(std::distance(pred_begin(BB), pred_end(BB)) == 2 && 105 "Function can only handle blocks with 2 predecessors!"); 106 BasicBlock *Pred1 = *pred_begin(BB); 107 BasicBlock *Pred2 = *++pred_begin(BB); 108 109 // We can only handle branches. Other control flow will be lowered to 110 // branches if possible anyway. 111 if (!isa<BranchInst>(Pred1->getTerminator()) || 112 !isa<BranchInst>(Pred2->getTerminator())) 113 return 0; 114 BranchInst *Pred1Br = cast<BranchInst>(Pred1->getTerminator()); 115 BranchInst *Pred2Br = cast<BranchInst>(Pred2->getTerminator()); 116 117 // Eliminate code duplication by ensuring that Pred1Br is conditional if 118 // either are. 119 if (Pred2Br->isConditional()) { 120 // If both branches are conditional, we don't have an "if statement". In 121 // reality, we could transform this case, but since the condition will be 122 // required anyway, we stand no chance of eliminating it, so the xform is 123 // probably not profitable. 124 if (Pred1Br->isConditional()) 125 return 0; 126 127 std::swap(Pred1, Pred2); 128 std::swap(Pred1Br, Pred2Br); 129 } 130 131 if (Pred1Br->isConditional()) { 132 // If we found a conditional branch predecessor, make sure that it branches 133 // to BB and Pred2Br. If it doesn't, this isn't an "if statement". 134 if (Pred1Br->getSuccessor(0) == BB && 135 Pred1Br->getSuccessor(1) == Pred2) { 136 IfTrue = Pred1; 137 IfFalse = Pred2; 138 } else if (Pred1Br->getSuccessor(0) == Pred2 && 139 Pred1Br->getSuccessor(1) == BB) { 140 IfTrue = Pred2; 141 IfFalse = Pred1; 142 } else { 143 // We know that one arm of the conditional goes to BB, so the other must 144 // go somewhere unrelated, and this must not be an "if statement". 145 return 0; 146 } 147 148 // The only thing we have to watch out for here is to make sure that Pred2 149 // doesn't have incoming edges from other blocks. If it does, the condition 150 // doesn't dominate BB. 151 if (++pred_begin(Pred2) != pred_end(Pred2)) 152 return 0; 153 154 return Pred1Br->getCondition(); 155 } 156 157 // Ok, if we got here, both predecessors end with an unconditional branch to 158 // BB. Don't panic! If both blocks only have a single (identical) 159 // predecessor, and THAT is a conditional branch, then we're all ok! 160 if (pred_begin(Pred1) == pred_end(Pred1) || 161 ++pred_begin(Pred1) != pred_end(Pred1) || 162 pred_begin(Pred2) == pred_end(Pred2) || 163 ++pred_begin(Pred2) != pred_end(Pred2) || 164 *pred_begin(Pred1) != *pred_begin(Pred2)) 165 return 0; 166 167 // Otherwise, if this is a conditional branch, then we can use it! 168 BasicBlock *CommonPred = *pred_begin(Pred1); 169 if (BranchInst *BI = dyn_cast<BranchInst>(CommonPred->getTerminator())) { 170 assert(BI->isConditional() && "Two successors but not conditional?"); 171 if (BI->getSuccessor(0) == Pred1) { 172 IfTrue = Pred1; 173 IfFalse = Pred2; 174 } else { 175 IfTrue = Pred2; 176 IfFalse = Pred1; 177 } 178 return BI->getCondition(); 179 } 180 return 0; 181} 182 183 184// If we have a merge point of an "if condition" as accepted above, return true 185// if the specified value dominates the block. We don't handle the true 186// generality of domination here, just a special case which works well enough 187// for us. 188// 189// If AggressiveInsts is non-null, and if V does not dominate BB, we check to 190// see if V (which must be an instruction) is cheap to compute and is 191// non-trapping. If both are true, the instruction is inserted into the set and 192// true is returned. 193static bool DominatesMergePoint(Value *V, BasicBlock *BB, 194 std::set<Instruction*> *AggressiveInsts) { 195 Instruction *I = dyn_cast<Instruction>(V); 196 if (!I) return true; // Non-instructions all dominate instructions. 197 BasicBlock *PBB = I->getParent(); 198 199 // We don't want to allow weird loops that might have the "if condition" in 200 // the bottom of this block. 201 if (PBB == BB) return false; 202 203 // If this instruction is defined in a block that contains an unconditional 204 // branch to BB, then it must be in the 'conditional' part of the "if 205 // statement". 206 if (BranchInst *BI = dyn_cast<BranchInst>(PBB->getTerminator())) 207 if (BI->isUnconditional() && BI->getSuccessor(0) == BB) { 208 if (!AggressiveInsts) return false; 209 // Okay, it looks like the instruction IS in the "condition". Check to 210 // see if its a cheap instruction to unconditionally compute, and if it 211 // only uses stuff defined outside of the condition. If so, hoist it out. 212 switch (I->getOpcode()) { 213 default: return false; // Cannot hoist this out safely. 214 case Instruction::Load: 215 // We can hoist loads that are non-volatile and obviously cannot trap. 216 if (cast<LoadInst>(I)->isVolatile()) 217 return false; 218 if (!isa<AllocaInst>(I->getOperand(0)) && 219 !isa<Constant>(I->getOperand(0))) 220 return false; 221 222 // Finally, we have to check to make sure there are no instructions 223 // before the load in its basic block, as we are going to hoist the loop 224 // out to its predecessor. 225 if (PBB->begin() != BasicBlock::iterator(I)) 226 return false; 227 break; 228 case Instruction::Add: 229 case Instruction::Sub: 230 case Instruction::And: 231 case Instruction::Or: 232 case Instruction::Xor: 233 case Instruction::Shl: 234 case Instruction::Shr: 235 case Instruction::SetEQ: 236 case Instruction::SetNE: 237 case Instruction::SetLT: 238 case Instruction::SetGT: 239 case Instruction::SetLE: 240 case Instruction::SetGE: 241 break; // These are all cheap and non-trapping instructions. 242 } 243 244 // Okay, we can only really hoist these out if their operands are not 245 // defined in the conditional region. 246 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) 247 if (!DominatesMergePoint(I->getOperand(i), BB, 0)) 248 return false; 249 // Okay, it's safe to do this! Remember this instruction. 250 AggressiveInsts->insert(I); 251 } 252 253 return true; 254} 255 256// GatherConstantSetEQs - Given a potentially 'or'd together collection of seteq 257// instructions that compare a value against a constant, return the value being 258// compared, and stick the constant into the Values vector. 259static Value *GatherConstantSetEQs(Value *V, std::vector<ConstantInt*> &Values){ 260 if (Instruction *Inst = dyn_cast<Instruction>(V)) 261 if (Inst->getOpcode() == Instruction::SetEQ) { 262 if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(1))) { 263 Values.push_back(C); 264 return Inst->getOperand(0); 265 } else if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(0))) { 266 Values.push_back(C); 267 return Inst->getOperand(1); 268 } 269 } else if (Inst->getOpcode() == Instruction::Or) { 270 if (Value *LHS = GatherConstantSetEQs(Inst->getOperand(0), Values)) 271 if (Value *RHS = GatherConstantSetEQs(Inst->getOperand(1), Values)) 272 if (LHS == RHS) 273 return LHS; 274 } 275 return 0; 276} 277 278// GatherConstantSetNEs - Given a potentially 'and'd together collection of 279// setne instructions that compare a value against a constant, return the value 280// being compared, and stick the constant into the Values vector. 281static Value *GatherConstantSetNEs(Value *V, std::vector<ConstantInt*> &Values){ 282 if (Instruction *Inst = dyn_cast<Instruction>(V)) 283 if (Inst->getOpcode() == Instruction::SetNE) { 284 if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(1))) { 285 Values.push_back(C); 286 return Inst->getOperand(0); 287 } else if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(0))) { 288 Values.push_back(C); 289 return Inst->getOperand(1); 290 } 291 } else if (Inst->getOpcode() == Instruction::Cast) { 292 // Cast of X to bool is really a comparison against zero. 293 assert(Inst->getType() == Type::BoolTy && "Can only handle bool values!"); 294 Values.push_back(ConstantInt::get(Inst->getOperand(0)->getType(), 0)); 295 return Inst->getOperand(0); 296 } else if (Inst->getOpcode() == Instruction::And) { 297 if (Value *LHS = GatherConstantSetNEs(Inst->getOperand(0), Values)) 298 if (Value *RHS = GatherConstantSetNEs(Inst->getOperand(1), Values)) 299 if (LHS == RHS) 300 return LHS; 301 } 302 return 0; 303} 304 305 306 307/// GatherValueComparisons - If the specified Cond is an 'and' or 'or' of a 308/// bunch of comparisons of one value against constants, return the value and 309/// the constants being compared. 310static bool GatherValueComparisons(Instruction *Cond, Value *&CompVal, 311 std::vector<ConstantInt*> &Values) { 312 if (Cond->getOpcode() == Instruction::Or) { 313 CompVal = GatherConstantSetEQs(Cond, Values); 314 315 // Return true to indicate that the condition is true if the CompVal is 316 // equal to one of the constants. 317 return true; 318 } else if (Cond->getOpcode() == Instruction::And) { 319 CompVal = GatherConstantSetNEs(Cond, Values); 320 321 // Return false to indicate that the condition is false if the CompVal is 322 // equal to one of the constants. 323 return false; 324 } 325 return false; 326} 327 328/// ErasePossiblyDeadInstructionTree - If the specified instruction is dead and 329/// has no side effects, nuke it. If it uses any instructions that become dead 330/// because the instruction is now gone, nuke them too. 331static void ErasePossiblyDeadInstructionTree(Instruction *I) { 332 if (isInstructionTriviallyDead(I)) { 333 std::vector<Value*> Operands(I->op_begin(), I->op_end()); 334 I->getParent()->getInstList().erase(I); 335 for (unsigned i = 0, e = Operands.size(); i != e; ++i) 336 if (Instruction *OpI = dyn_cast<Instruction>(Operands[i])) 337 ErasePossiblyDeadInstructionTree(OpI); 338 } 339} 340 341/// SafeToMergeTerminators - Return true if it is safe to merge these two 342/// terminator instructions together. 343/// 344static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) { 345 if (SI1 == SI2) return false; // Can't merge with self! 346 347 // It is not safe to merge these two switch instructions if they have a common 348 // successor, and if that successor has a PHI node, and if *that* PHI node has 349 // conflicting incoming values from the two switch blocks. 350 BasicBlock *SI1BB = SI1->getParent(); 351 BasicBlock *SI2BB = SI2->getParent(); 352 std::set<BasicBlock*> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB)); 353 354 for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I) 355 if (SI1Succs.count(*I)) 356 for (BasicBlock::iterator BBI = (*I)->begin(); 357 isa<PHINode>(BBI); ++BBI) { 358 PHINode *PN = cast<PHINode>(BBI); 359 if (PN->getIncomingValueForBlock(SI1BB) != 360 PN->getIncomingValueForBlock(SI2BB)) 361 return false; 362 } 363 364 return true; 365} 366 367/// AddPredecessorToBlock - Update PHI nodes in Succ to indicate that there will 368/// now be entries in it from the 'NewPred' block. The values that will be 369/// flowing into the PHI nodes will be the same as those coming in from 370/// ExistPred, an existing predecessor of Succ. 371static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred, 372 BasicBlock *ExistPred) { 373 assert(std::find(succ_begin(ExistPred), succ_end(ExistPred), Succ) != 374 succ_end(ExistPred) && "ExistPred is not a predecessor of Succ!"); 375 if (!isa<PHINode>(Succ->begin())) return; // Quick exit if nothing to do 376 377 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) { 378 PHINode *PN = cast<PHINode>(I); 379 Value *V = PN->getIncomingValueForBlock(ExistPred); 380 PN->addIncoming(V, NewPred); 381 } 382} 383 384// isValueEqualityComparison - Return true if the specified terminator checks to 385// see if a value is equal to constant integer value. 386static Value *isValueEqualityComparison(TerminatorInst *TI) { 387 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) { 388 // Do not permit merging of large switch instructions into their 389 // predecessors unless there is only one predecessor. 390 if (SI->getNumSuccessors() * std::distance(pred_begin(SI->getParent()), 391 pred_end(SI->getParent())) > 128) 392 return 0; 393 394 return SI->getCondition(); 395 } 396 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) 397 if (BI->isConditional() && BI->getCondition()->hasOneUse()) 398 if (SetCondInst *SCI = dyn_cast<SetCondInst>(BI->getCondition())) 399 if ((SCI->getOpcode() == Instruction::SetEQ || 400 SCI->getOpcode() == Instruction::SetNE) && 401 isa<ConstantInt>(SCI->getOperand(1))) 402 return SCI->getOperand(0); 403 return 0; 404} 405 406// Given a value comparison instruction, decode all of the 'cases' that it 407// represents and return the 'default' block. 408static BasicBlock * 409GetValueEqualityComparisonCases(TerminatorInst *TI, 410 std::vector<std::pair<ConstantInt*, 411 BasicBlock*> > &Cases) { 412 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) { 413 Cases.reserve(SI->getNumCases()); 414 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) 415 Cases.push_back(std::make_pair(SI->getCaseValue(i), SI->getSuccessor(i))); 416 return SI->getDefaultDest(); 417 } 418 419 BranchInst *BI = cast<BranchInst>(TI); 420 SetCondInst *SCI = cast<SetCondInst>(BI->getCondition()); 421 Cases.push_back(std::make_pair(cast<ConstantInt>(SCI->getOperand(1)), 422 BI->getSuccessor(SCI->getOpcode() == 423 Instruction::SetNE))); 424 return BI->getSuccessor(SCI->getOpcode() == Instruction::SetEQ); 425} 426 427 428// EliminateBlockCases - Given an vector of bb/value pairs, remove any entries 429// in the list that match the specified block. 430static void EliminateBlockCases(BasicBlock *BB, 431 std::vector<std::pair<ConstantInt*, BasicBlock*> > &Cases) { 432 for (unsigned i = 0, e = Cases.size(); i != e; ++i) 433 if (Cases[i].second == BB) { 434 Cases.erase(Cases.begin()+i); 435 --i; --e; 436 } 437} 438 439// ValuesOverlap - Return true if there are any keys in C1 that exist in C2 as 440// well. 441static bool 442ValuesOverlap(std::vector<std::pair<ConstantInt*, BasicBlock*> > &C1, 443 std::vector<std::pair<ConstantInt*, BasicBlock*> > &C2) { 444 std::vector<std::pair<ConstantInt*, BasicBlock*> > *V1 = &C1, *V2 = &C2; 445 446 // Make V1 be smaller than V2. 447 if (V1->size() > V2->size()) 448 std::swap(V1, V2); 449 450 if (V1->size() == 0) return false; 451 if (V1->size() == 1) { 452 // Just scan V2. 453 ConstantInt *TheVal = (*V1)[0].first; 454 for (unsigned i = 0, e = V2->size(); i != e; ++i) 455 if (TheVal == (*V2)[i].first) 456 return true; 457 } 458 459 // Otherwise, just sort both lists and compare element by element. 460 std::sort(V1->begin(), V1->end()); 461 std::sort(V2->begin(), V2->end()); 462 unsigned i1 = 0, i2 = 0, e1 = V1->size(), e2 = V2->size(); 463 while (i1 != e1 && i2 != e2) { 464 if ((*V1)[i1].first == (*V2)[i2].first) 465 return true; 466 if ((*V1)[i1].first < (*V2)[i2].first) 467 ++i1; 468 else 469 ++i2; 470 } 471 return false; 472} 473 474// SimplifyEqualityComparisonWithOnlyPredecessor - If TI is known to be a 475// terminator instruction and its block is known to only have a single 476// predecessor block, check to see if that predecessor is also a value 477// comparison with the same value, and if that comparison determines the outcome 478// of this comparison. If so, simplify TI. This does a very limited form of 479// jump threading. 480static bool SimplifyEqualityComparisonWithOnlyPredecessor(TerminatorInst *TI, 481 BasicBlock *Pred) { 482 Value *PredVal = isValueEqualityComparison(Pred->getTerminator()); 483 if (!PredVal) return false; // Not a value comparison in predecessor. 484 485 Value *ThisVal = isValueEqualityComparison(TI); 486 assert(ThisVal && "This isn't a value comparison!!"); 487 if (ThisVal != PredVal) return false; // Different predicates. 488 489 // Find out information about when control will move from Pred to TI's block. 490 std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases; 491 BasicBlock *PredDef = GetValueEqualityComparisonCases(Pred->getTerminator(), 492 PredCases); 493 EliminateBlockCases(PredDef, PredCases); // Remove default from cases. 494 495 // Find information about how control leaves this block. 496 std::vector<std::pair<ConstantInt*, BasicBlock*> > ThisCases; 497 BasicBlock *ThisDef = GetValueEqualityComparisonCases(TI, ThisCases); 498 EliminateBlockCases(ThisDef, ThisCases); // Remove default from cases. 499 500 // If TI's block is the default block from Pred's comparison, potentially 501 // simplify TI based on this knowledge. 502 if (PredDef == TI->getParent()) { 503 // If we are here, we know that the value is none of those cases listed in 504 // PredCases. If there are any cases in ThisCases that are in PredCases, we 505 // can simplify TI. 506 if (ValuesOverlap(PredCases, ThisCases)) { 507 if (BranchInst *BTI = dyn_cast<BranchInst>(TI)) { 508 // Okay, one of the successors of this condbr is dead. Convert it to a 509 // uncond br. 510 assert(ThisCases.size() == 1 && "Branch can only have one case!"); 511 Value *Cond = BTI->getCondition(); 512 // Insert the new branch. 513 Instruction *NI = new BranchInst(ThisDef, TI); 514 515 // Remove PHI node entries for the dead edge. 516 ThisCases[0].second->removePredecessor(TI->getParent()); 517 518 DEBUG(std::cerr << "Threading pred instr: " << *Pred->getTerminator() 519 << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n"); 520 521 TI->eraseFromParent(); // Nuke the old one. 522 // If condition is now dead, nuke it. 523 if (Instruction *CondI = dyn_cast<Instruction>(Cond)) 524 ErasePossiblyDeadInstructionTree(CondI); 525 return true; 526 527 } else { 528 SwitchInst *SI = cast<SwitchInst>(TI); 529 // Okay, TI has cases that are statically dead, prune them away. 530 std::set<Constant*> DeadCases; 531 for (unsigned i = 0, e = PredCases.size(); i != e; ++i) 532 DeadCases.insert(PredCases[i].first); 533 534 DEBUG(std::cerr << "Threading pred instr: " << *Pred->getTerminator() 535 << "Through successor TI: " << *TI); 536 537 for (unsigned i = SI->getNumCases()-1; i != 0; --i) 538 if (DeadCases.count(SI->getCaseValue(i))) { 539 SI->getSuccessor(i)->removePredecessor(TI->getParent()); 540 SI->removeCase(i); 541 } 542 543 DEBUG(std::cerr << "Leaving: " << *TI << "\n"); 544 return true; 545 } 546 } 547 548 } else { 549 // Otherwise, TI's block must correspond to some matched value. Find out 550 // which value (or set of values) this is. 551 ConstantInt *TIV = 0; 552 BasicBlock *TIBB = TI->getParent(); 553 for (unsigned i = 0, e = PredCases.size(); i != e; ++i) 554 if (PredCases[i].second == TIBB) 555 if (TIV == 0) 556 TIV = PredCases[i].first; 557 else 558 return false; // Cannot handle multiple values coming to this block. 559 assert(TIV && "No edge from pred to succ?"); 560 561 // Okay, we found the one constant that our value can be if we get into TI's 562 // BB. Find out which successor will unconditionally be branched to. 563 BasicBlock *TheRealDest = 0; 564 for (unsigned i = 0, e = ThisCases.size(); i != e; ++i) 565 if (ThisCases[i].first == TIV) { 566 TheRealDest = ThisCases[i].second; 567 break; 568 } 569 570 // If not handled by any explicit cases, it is handled by the default case. 571 if (TheRealDest == 0) TheRealDest = ThisDef; 572 573 // Remove PHI node entries for dead edges. 574 BasicBlock *CheckEdge = TheRealDest; 575 for (succ_iterator SI = succ_begin(TIBB), e = succ_end(TIBB); SI != e; ++SI) 576 if (*SI != CheckEdge) 577 (*SI)->removePredecessor(TIBB); 578 else 579 CheckEdge = 0; 580 581 // Insert the new branch. 582 Instruction *NI = new BranchInst(TheRealDest, TI); 583 584 DEBUG(std::cerr << "Threading pred instr: " << *Pred->getTerminator() 585 << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n"); 586 Instruction *Cond = 0; 587 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) 588 Cond = dyn_cast<Instruction>(BI->getCondition()); 589 TI->eraseFromParent(); // Nuke the old one. 590 591 if (Cond) ErasePossiblyDeadInstructionTree(Cond); 592 return true; 593 } 594 return false; 595} 596 597// FoldValueComparisonIntoPredecessors - The specified terminator is a value 598// equality comparison instruction (either a switch or a branch on "X == c"). 599// See if any of the predecessors of the terminator block are value comparisons 600// on the same value. If so, and if safe to do so, fold them together. 601static bool FoldValueComparisonIntoPredecessors(TerminatorInst *TI) { 602 BasicBlock *BB = TI->getParent(); 603 Value *CV = isValueEqualityComparison(TI); // CondVal 604 assert(CV && "Not a comparison?"); 605 bool Changed = false; 606 607 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB)); 608 while (!Preds.empty()) { 609 BasicBlock *Pred = Preds.back(); 610 Preds.pop_back(); 611 612 // See if the predecessor is a comparison with the same value. 613 TerminatorInst *PTI = Pred->getTerminator(); 614 Value *PCV = isValueEqualityComparison(PTI); // PredCondVal 615 616 if (PCV == CV && SafeToMergeTerminators(TI, PTI)) { 617 // Figure out which 'cases' to copy from SI to PSI. 618 std::vector<std::pair<ConstantInt*, BasicBlock*> > BBCases; 619 BasicBlock *BBDefault = GetValueEqualityComparisonCases(TI, BBCases); 620 621 std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases; 622 BasicBlock *PredDefault = GetValueEqualityComparisonCases(PTI, PredCases); 623 624 // Based on whether the default edge from PTI goes to BB or not, fill in 625 // PredCases and PredDefault with the new switch cases we would like to 626 // build. 627 std::vector<BasicBlock*> NewSuccessors; 628 629 if (PredDefault == BB) { 630 // If this is the default destination from PTI, only the edges in TI 631 // that don't occur in PTI, or that branch to BB will be activated. 632 std::set<ConstantInt*> PTIHandled; 633 for (unsigned i = 0, e = PredCases.size(); i != e; ++i) 634 if (PredCases[i].second != BB) 635 PTIHandled.insert(PredCases[i].first); 636 else { 637 // The default destination is BB, we don't need explicit targets. 638 std::swap(PredCases[i], PredCases.back()); 639 PredCases.pop_back(); 640 --i; --e; 641 } 642 643 // Reconstruct the new switch statement we will be building. 644 if (PredDefault != BBDefault) { 645 PredDefault->removePredecessor(Pred); 646 PredDefault = BBDefault; 647 NewSuccessors.push_back(BBDefault); 648 } 649 for (unsigned i = 0, e = BBCases.size(); i != e; ++i) 650 if (!PTIHandled.count(BBCases[i].first) && 651 BBCases[i].second != BBDefault) { 652 PredCases.push_back(BBCases[i]); 653 NewSuccessors.push_back(BBCases[i].second); 654 } 655 656 } else { 657 // If this is not the default destination from PSI, only the edges 658 // in SI that occur in PSI with a destination of BB will be 659 // activated. 660 std::set<ConstantInt*> PTIHandled; 661 for (unsigned i = 0, e = PredCases.size(); i != e; ++i) 662 if (PredCases[i].second == BB) { 663 PTIHandled.insert(PredCases[i].first); 664 std::swap(PredCases[i], PredCases.back()); 665 PredCases.pop_back(); 666 --i; --e; 667 } 668 669 // Okay, now we know which constants were sent to BB from the 670 // predecessor. Figure out where they will all go now. 671 for (unsigned i = 0, e = BBCases.size(); i != e; ++i) 672 if (PTIHandled.count(BBCases[i].first)) { 673 // If this is one we are capable of getting... 674 PredCases.push_back(BBCases[i]); 675 NewSuccessors.push_back(BBCases[i].second); 676 PTIHandled.erase(BBCases[i].first);// This constant is taken care of 677 } 678 679 // If there are any constants vectored to BB that TI doesn't handle, 680 // they must go to the default destination of TI. 681 for (std::set<ConstantInt*>::iterator I = PTIHandled.begin(), 682 E = PTIHandled.end(); I != E; ++I) { 683 PredCases.push_back(std::make_pair(*I, BBDefault)); 684 NewSuccessors.push_back(BBDefault); 685 } 686 } 687 688 // Okay, at this point, we know which new successor Pred will get. Make 689 // sure we update the number of entries in the PHI nodes for these 690 // successors. 691 for (unsigned i = 0, e = NewSuccessors.size(); i != e; ++i) 692 AddPredecessorToBlock(NewSuccessors[i], Pred, BB); 693 694 // Now that the successors are updated, create the new Switch instruction. 695 SwitchInst *NewSI = new SwitchInst(CV, PredDefault, PredCases.size(),PTI); 696 for (unsigned i = 0, e = PredCases.size(); i != e; ++i) 697 NewSI->addCase(PredCases[i].first, PredCases[i].second); 698 699 Instruction *DeadCond = 0; 700 if (BranchInst *BI = dyn_cast<BranchInst>(PTI)) 701 // If PTI is a branch, remember the condition. 702 DeadCond = dyn_cast<Instruction>(BI->getCondition()); 703 Pred->getInstList().erase(PTI); 704 705 // If the condition is dead now, remove the instruction tree. 706 if (DeadCond) ErasePossiblyDeadInstructionTree(DeadCond); 707 708 // Okay, last check. If BB is still a successor of PSI, then we must 709 // have an infinite loop case. If so, add an infinitely looping block 710 // to handle the case to preserve the behavior of the code. 711 BasicBlock *InfLoopBlock = 0; 712 for (unsigned i = 0, e = NewSI->getNumSuccessors(); i != e; ++i) 713 if (NewSI->getSuccessor(i) == BB) { 714 if (InfLoopBlock == 0) { 715 // Insert it at the end of the loop, because it's either code, 716 // or it won't matter if it's hot. :) 717 InfLoopBlock = new BasicBlock("infloop", BB->getParent()); 718 new BranchInst(InfLoopBlock, InfLoopBlock); 719 } 720 NewSI->setSuccessor(i, InfLoopBlock); 721 } 722 723 Changed = true; 724 } 725 } 726 return Changed; 727} 728 729/// HoistThenElseCodeToIf - Given a conditional branch that codes to BB1 and 730/// BB2, hoist any common code in the two blocks up into the branch block. The 731/// caller of this function guarantees that BI's block dominates BB1 and BB2. 732static bool HoistThenElseCodeToIf(BranchInst *BI) { 733 // This does very trivial matching, with limited scanning, to find identical 734 // instructions in the two blocks. In particular, we don't want to get into 735 // O(M*N) situations here where M and N are the sizes of BB1 and BB2. As 736 // such, we currently just scan for obviously identical instructions in an 737 // identical order. 738 BasicBlock *BB1 = BI->getSuccessor(0); // The true destination. 739 BasicBlock *BB2 = BI->getSuccessor(1); // The false destination 740 741 Instruction *I1 = BB1->begin(), *I2 = BB2->begin(); 742 if (I1->getOpcode() != I2->getOpcode() || !I1->isIdenticalTo(I2)) 743 return false; 744 745 // If we get here, we can hoist at least one instruction. 746 BasicBlock *BIParent = BI->getParent(); 747 748 do { 749 // If we are hoisting the terminator instruction, don't move one (making a 750 // broken BB), instead clone it, and remove BI. 751 if (isa<TerminatorInst>(I1)) 752 goto HoistTerminator; 753 754 // For a normal instruction, we just move one to right before the branch, 755 // then replace all uses of the other with the first. Finally, we remove 756 // the now redundant second instruction. 757 BIParent->getInstList().splice(BI, BB1->getInstList(), I1); 758 if (!I2->use_empty()) 759 I2->replaceAllUsesWith(I1); 760 BB2->getInstList().erase(I2); 761 762 I1 = BB1->begin(); 763 I2 = BB2->begin(); 764 } while (I1->getOpcode() == I2->getOpcode() && I1->isIdenticalTo(I2)); 765 766 return true; 767 768HoistTerminator: 769 // Okay, it is safe to hoist the terminator. 770 Instruction *NT = I1->clone(); 771 BIParent->getInstList().insert(BI, NT); 772 if (NT->getType() != Type::VoidTy) { 773 I1->replaceAllUsesWith(NT); 774 I2->replaceAllUsesWith(NT); 775 NT->setName(I1->getName()); 776 } 777 778 // Hoisting one of the terminators from our successor is a great thing. 779 // Unfortunately, the successors of the if/else blocks may have PHI nodes in 780 // them. If they do, all PHI entries for BB1/BB2 must agree for all PHI 781 // nodes, so we insert select instruction to compute the final result. 782 std::map<std::pair<Value*,Value*>, SelectInst*> InsertedSelects; 783 for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) { 784 PHINode *PN; 785 for (BasicBlock::iterator BBI = SI->begin(); 786 (PN = dyn_cast<PHINode>(BBI)); ++BBI) { 787 Value *BB1V = PN->getIncomingValueForBlock(BB1); 788 Value *BB2V = PN->getIncomingValueForBlock(BB2); 789 if (BB1V != BB2V) { 790 // These values do not agree. Insert a select instruction before NT 791 // that determines the right value. 792 SelectInst *&SI = InsertedSelects[std::make_pair(BB1V, BB2V)]; 793 if (SI == 0) 794 SI = new SelectInst(BI->getCondition(), BB1V, BB2V, 795 BB1V->getName()+"."+BB2V->getName(), NT); 796 // Make the PHI node use the select for all incoming values for BB1/BB2 797 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 798 if (PN->getIncomingBlock(i) == BB1 || PN->getIncomingBlock(i) == BB2) 799 PN->setIncomingValue(i, SI); 800 } 801 } 802 } 803 804 // Update any PHI nodes in our new successors. 805 for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) 806 AddPredecessorToBlock(*SI, BIParent, BB1); 807 808 BI->eraseFromParent(); 809 return true; 810} 811 812namespace { 813 /// ConstantIntOrdering - This class implements a stable ordering of constant 814 /// integers that does not depend on their address. This is important for 815 /// applications that sort ConstantInt's to ensure uniqueness. 816 struct ConstantIntOrdering { 817 bool operator()(const ConstantInt *LHS, const ConstantInt *RHS) const { 818 return LHS->getRawValue() < RHS->getRawValue(); 819 } 820 }; 821} 822 823 824// SimplifyCFG - This function is used to do simplification of a CFG. For 825// example, it adjusts branches to branches to eliminate the extra hop, it 826// eliminates unreachable basic blocks, and does other "peephole" optimization 827// of the CFG. It returns true if a modification was made. 828// 829// WARNING: The entry node of a function may not be simplified. 830// 831bool llvm::SimplifyCFG(BasicBlock *BB) { 832 bool Changed = false; 833 Function *M = BB->getParent(); 834 835 assert(BB && BB->getParent() && "Block not embedded in function!"); 836 assert(BB->getTerminator() && "Degenerate basic block encountered!"); 837 assert(&BB->getParent()->front() != BB && "Can't Simplify entry block!"); 838 839 // Remove basic blocks that have no predecessors... which are unreachable. 840 if (pred_begin(BB) == pred_end(BB) || 841 *pred_begin(BB) == BB && ++pred_begin(BB) == pred_end(BB)) { 842 DEBUG(std::cerr << "Removing BB: \n" << *BB); 843 844 // Loop through all of our successors and make sure they know that one 845 // of their predecessors is going away. 846 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) 847 SI->removePredecessor(BB); 848 849 while (!BB->empty()) { 850 Instruction &I = BB->back(); 851 // If this instruction is used, replace uses with an arbitrary 852 // constant value. Because control flow can't get here, we don't care 853 // what we replace the value with. Note that since this block is 854 // unreachable, and all values contained within it must dominate their 855 // uses, that all uses will eventually be removed. 856 if (!I.use_empty()) 857 // Make all users of this instruction reference the constant instead 858 I.replaceAllUsesWith(Constant::getNullValue(I.getType())); 859 860 // Remove the instruction from the basic block 861 BB->getInstList().pop_back(); 862 } 863 M->getBasicBlockList().erase(BB); 864 return true; 865 } 866 867 // Check to see if we can constant propagate this terminator instruction 868 // away... 869 Changed |= ConstantFoldTerminator(BB); 870 871 // Check to see if this block has no non-phi instructions and only a single 872 // successor. If so, replace references to this basic block with references 873 // to the successor. 874 succ_iterator SI(succ_begin(BB)); 875 if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ? 876 BasicBlock::iterator BBI = BB->begin(); // Skip over phi nodes... 877 while (isa<PHINode>(*BBI)) ++BBI; 878 879 BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor. 880 if (BBI->isTerminator() && // Terminator is the only non-phi instruction! 881 Succ != BB) { // Don't hurt infinite loops! 882 // If our successor has PHI nodes, then we need to update them to include 883 // entries for BB's predecessors, not for BB itself. Be careful though, 884 // if this transformation fails (returns true) then we cannot do this 885 // transformation! 886 // 887 if (!PropagatePredecessorsForPHIs(BB, Succ)) { 888 DEBUG(std::cerr << "Killing Trivial BB: \n" << *BB); 889 890 if (isa<PHINode>(&BB->front())) { 891 std::vector<BasicBlock*> 892 OldSuccPreds(pred_begin(Succ), pred_end(Succ)); 893 894 // Move all PHI nodes in BB to Succ if they are alive, otherwise 895 // delete them. 896 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) 897 if (PN->use_empty()) 898 BB->getInstList().erase(BB->begin()); // Nuke instruction. 899 else { 900 // The instruction is alive, so this means that Succ must have 901 // *ONLY* had BB as a predecessor, and the PHI node is still valid 902 // now. Simply move it into Succ, because we know that BB 903 // strictly dominated Succ. 904 BB->getInstList().remove(BB->begin()); 905 Succ->getInstList().push_front(PN); 906 907 // We need to add new entries for the PHI node to account for 908 // predecessors of Succ that the PHI node does not take into 909 // account. At this point, since we know that BB dominated succ, 910 // this means that we should any newly added incoming edges should 911 // use the PHI node as the value for these edges, because they are 912 // loop back edges. 913 for (unsigned i = 0, e = OldSuccPreds.size(); i != e; ++i) 914 if (OldSuccPreds[i] != BB) 915 PN->addIncoming(PN, OldSuccPreds[i]); 916 } 917 } 918 919 // Everything that jumped to BB now goes to Succ. 920 std::string OldName = BB->getName(); 921 BB->replaceAllUsesWith(Succ); 922 BB->eraseFromParent(); // Delete the old basic block. 923 924 if (!OldName.empty() && !Succ->hasName()) // Transfer name if we can 925 Succ->setName(OldName); 926 return true; 927 } 928 } 929 } 930 931 // If this is a returning block with only PHI nodes in it, fold the return 932 // instruction into any unconditional branch predecessors. 933 // 934 // If any predecessor is a conditional branch that just selects among 935 // different return values, fold the replace the branch/return with a select 936 // and return. 937 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) { 938 BasicBlock::iterator BBI = BB->getTerminator(); 939 if (BBI == BB->begin() || isa<PHINode>(--BBI)) { 940 // Find predecessors that end with branches. 941 std::vector<BasicBlock*> UncondBranchPreds; 942 std::vector<BranchInst*> CondBranchPreds; 943 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { 944 TerminatorInst *PTI = (*PI)->getTerminator(); 945 if (BranchInst *BI = dyn_cast<BranchInst>(PTI)) 946 if (BI->isUnconditional()) 947 UncondBranchPreds.push_back(*PI); 948 else 949 CondBranchPreds.push_back(BI); 950 } 951 952 // If we found some, do the transformation! 953 if (!UncondBranchPreds.empty()) { 954 while (!UncondBranchPreds.empty()) { 955 BasicBlock *Pred = UncondBranchPreds.back(); 956 UncondBranchPreds.pop_back(); 957 Instruction *UncondBranch = Pred->getTerminator(); 958 // Clone the return and add it to the end of the predecessor. 959 Instruction *NewRet = RI->clone(); 960 Pred->getInstList().push_back(NewRet); 961 962 // If the return instruction returns a value, and if the value was a 963 // PHI node in "BB", propagate the right value into the return. 964 if (NewRet->getNumOperands() == 1) 965 if (PHINode *PN = dyn_cast<PHINode>(NewRet->getOperand(0))) 966 if (PN->getParent() == BB) 967 NewRet->setOperand(0, PN->getIncomingValueForBlock(Pred)); 968 // Update any PHI nodes in the returning block to realize that we no 969 // longer branch to them. 970 BB->removePredecessor(Pred); 971 Pred->getInstList().erase(UncondBranch); 972 } 973 974 // If we eliminated all predecessors of the block, delete the block now. 975 if (pred_begin(BB) == pred_end(BB)) 976 // We know there are no successors, so just nuke the block. 977 M->getBasicBlockList().erase(BB); 978 979 return true; 980 } 981 982 // Check out all of the conditional branches going to this return 983 // instruction. If any of them just select between returns, change the 984 // branch itself into a select/return pair. 985 while (!CondBranchPreds.empty()) { 986 BranchInst *BI = CondBranchPreds.back(); 987 CondBranchPreds.pop_back(); 988 BasicBlock *TrueSucc = BI->getSuccessor(0); 989 BasicBlock *FalseSucc = BI->getSuccessor(1); 990 BasicBlock *OtherSucc = TrueSucc == BB ? FalseSucc : TrueSucc; 991 992 // Check to see if the non-BB successor is also a return block. 993 if (isa<ReturnInst>(OtherSucc->getTerminator())) { 994 // Check to see if there are only PHI instructions in this block. 995 BasicBlock::iterator OSI = OtherSucc->getTerminator(); 996 if (OSI == OtherSucc->begin() || isa<PHINode>(--OSI)) { 997 // Okay, we found a branch that is going to two return nodes. If 998 // there is no return value for this function, just change the 999 // branch into a return. 1000 if (RI->getNumOperands() == 0) { 1001 TrueSucc->removePredecessor(BI->getParent()); 1002 FalseSucc->removePredecessor(BI->getParent()); 1003 new ReturnInst(0, BI); 1004 BI->getParent()->getInstList().erase(BI); 1005 return true; 1006 } 1007 1008 // Otherwise, figure out what the true and false return values are 1009 // so we can insert a new select instruction. 1010 Value *TrueValue = TrueSucc->getTerminator()->getOperand(0); 1011 Value *FalseValue = FalseSucc->getTerminator()->getOperand(0); 1012 1013 // Unwrap any PHI nodes in the return blocks. 1014 if (PHINode *TVPN = dyn_cast<PHINode>(TrueValue)) 1015 if (TVPN->getParent() == TrueSucc) 1016 TrueValue = TVPN->getIncomingValueForBlock(BI->getParent()); 1017 if (PHINode *FVPN = dyn_cast<PHINode>(FalseValue)) 1018 if (FVPN->getParent() == FalseSucc) 1019 FalseValue = FVPN->getIncomingValueForBlock(BI->getParent()); 1020 1021 TrueSucc->removePredecessor(BI->getParent()); 1022 FalseSucc->removePredecessor(BI->getParent()); 1023 1024 // Insert a new select instruction. 1025 Value *NewRetVal; 1026 Value *BrCond = BI->getCondition(); 1027 if (TrueValue != FalseValue) 1028 NewRetVal = new SelectInst(BrCond, TrueValue, 1029 FalseValue, "retval", BI); 1030 else 1031 NewRetVal = TrueValue; 1032 1033 new ReturnInst(NewRetVal, BI); 1034 BI->getParent()->getInstList().erase(BI); 1035 if (BrCond->use_empty()) 1036 if (Instruction *BrCondI = dyn_cast<Instruction>(BrCond)) 1037 BrCondI->getParent()->getInstList().erase(BrCondI); 1038 return true; 1039 } 1040 } 1041 } 1042 } 1043 } else if (UnwindInst *UI = dyn_cast<UnwindInst>(BB->begin())) { 1044 // Check to see if the first instruction in this block is just an unwind. 1045 // If so, replace any invoke instructions which use this as an exception 1046 // destination with call instructions, and any unconditional branch 1047 // predecessor with an unwind. 1048 // 1049 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB)); 1050 while (!Preds.empty()) { 1051 BasicBlock *Pred = Preds.back(); 1052 if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator())) { 1053 if (BI->isUnconditional()) { 1054 Pred->getInstList().pop_back(); // nuke uncond branch 1055 new UnwindInst(Pred); // Use unwind. 1056 Changed = true; 1057 } 1058 } else if (InvokeInst *II = dyn_cast<InvokeInst>(Pred->getTerminator())) 1059 if (II->getUnwindDest() == BB) { 1060 // Insert a new branch instruction before the invoke, because this 1061 // is now a fall through... 1062 BranchInst *BI = new BranchInst(II->getNormalDest(), II); 1063 Pred->getInstList().remove(II); // Take out of symbol table 1064 1065 // Insert the call now... 1066 std::vector<Value*> Args(II->op_begin()+3, II->op_end()); 1067 CallInst *CI = new CallInst(II->getCalledValue(), Args, 1068 II->getName(), BI); 1069 CI->setCallingConv(II->getCallingConv()); 1070 // If the invoke produced a value, the Call now does instead 1071 II->replaceAllUsesWith(CI); 1072 delete II; 1073 Changed = true; 1074 } 1075 1076 Preds.pop_back(); 1077 } 1078 1079 // If this block is now dead, remove it. 1080 if (pred_begin(BB) == pred_end(BB)) { 1081 // We know there are no successors, so just nuke the block. 1082 M->getBasicBlockList().erase(BB); 1083 return true; 1084 } 1085 1086 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) { 1087 if (isValueEqualityComparison(SI)) { 1088 // If we only have one predecessor, and if it is a branch on this value, 1089 // see if that predecessor totally determines the outcome of this switch. 1090 if (BasicBlock *OnlyPred = BB->getSinglePredecessor()) 1091 if (SimplifyEqualityComparisonWithOnlyPredecessor(SI, OnlyPred)) 1092 return SimplifyCFG(BB) || 1; 1093 1094 // If the block only contains the switch, see if we can fold the block 1095 // away into any preds. 1096 if (SI == &BB->front()) 1097 if (FoldValueComparisonIntoPredecessors(SI)) 1098 return SimplifyCFG(BB) || 1; 1099 } 1100 } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) { 1101 if (BI->isConditional()) { 1102 if (Value *CompVal = isValueEqualityComparison(BI)) { 1103 // If we only have one predecessor, and if it is a branch on this value, 1104 // see if that predecessor totally determines the outcome of this 1105 // switch. 1106 if (BasicBlock *OnlyPred = BB->getSinglePredecessor()) 1107 if (SimplifyEqualityComparisonWithOnlyPredecessor(BI, OnlyPred)) 1108 return SimplifyCFG(BB) || 1; 1109 1110 // This block must be empty, except for the setcond inst, if it exists. 1111 BasicBlock::iterator I = BB->begin(); 1112 if (&*I == BI || 1113 (&*I == cast<Instruction>(BI->getCondition()) && 1114 &*++I == BI)) 1115 if (FoldValueComparisonIntoPredecessors(BI)) 1116 return SimplifyCFG(BB) | true; 1117 } 1118 1119 // If this basic block is ONLY a setcc and a branch, and if a predecessor 1120 // branches to us and one of our successors, fold the setcc into the 1121 // predecessor and use logical operations to pick the right destination. 1122 BasicBlock *TrueDest = BI->getSuccessor(0); 1123 BasicBlock *FalseDest = BI->getSuccessor(1); 1124 if (BinaryOperator *Cond = dyn_cast<BinaryOperator>(BI->getCondition())) 1125 if (Cond->getParent() == BB && &BB->front() == Cond && 1126 Cond->getNext() == BI && Cond->hasOneUse() && 1127 TrueDest != BB && FalseDest != BB) 1128 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI!=E; ++PI) 1129 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator())) 1130 if (PBI->isConditional() && SafeToMergeTerminators(BI, PBI)) { 1131 BasicBlock *PredBlock = *PI; 1132 if (PBI->getSuccessor(0) == FalseDest || 1133 PBI->getSuccessor(1) == TrueDest) { 1134 // Invert the predecessors condition test (xor it with true), 1135 // which allows us to write this code once. 1136 Value *NewCond = 1137 BinaryOperator::createNot(PBI->getCondition(), 1138 PBI->getCondition()->getName()+".not", PBI); 1139 PBI->setCondition(NewCond); 1140 BasicBlock *OldTrue = PBI->getSuccessor(0); 1141 BasicBlock *OldFalse = PBI->getSuccessor(1); 1142 PBI->setSuccessor(0, OldFalse); 1143 PBI->setSuccessor(1, OldTrue); 1144 } 1145 1146 if (PBI->getSuccessor(0) == TrueDest || 1147 PBI->getSuccessor(1) == FalseDest) { 1148 // Clone Cond into the predecessor basic block, and or/and the 1149 // two conditions together. 1150 Instruction *New = Cond->clone(); 1151 New->setName(Cond->getName()); 1152 Cond->setName(Cond->getName()+".old"); 1153 PredBlock->getInstList().insert(PBI, New); 1154 Instruction::BinaryOps Opcode = 1155 PBI->getSuccessor(0) == TrueDest ? 1156 Instruction::Or : Instruction::And; 1157 Value *NewCond = 1158 BinaryOperator::create(Opcode, PBI->getCondition(), 1159 New, "bothcond", PBI); 1160 PBI->setCondition(NewCond); 1161 if (PBI->getSuccessor(0) == BB) { 1162 AddPredecessorToBlock(TrueDest, PredBlock, BB); 1163 PBI->setSuccessor(0, TrueDest); 1164 } 1165 if (PBI->getSuccessor(1) == BB) { 1166 AddPredecessorToBlock(FalseDest, PredBlock, BB); 1167 PBI->setSuccessor(1, FalseDest); 1168 } 1169 return SimplifyCFG(BB) | 1; 1170 } 1171 } 1172 1173 // If this block ends with a branch instruction, and if there is one 1174 // predecessor, see if the previous block ended with a branch on the same 1175 // condition, which makes this conditional branch redundant. 1176 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB)); 1177 BasicBlock *OnlyPred = *PI++; 1178 for (; PI != PE; ++PI)// Search all predecessors, see if they are all same 1179 if (*PI != OnlyPred) { 1180 OnlyPred = 0; // There are multiple different predecessors... 1181 break; 1182 } 1183 1184 if (OnlyPred) 1185 if (BranchInst *PBI = dyn_cast<BranchInst>(OnlyPred->getTerminator())) 1186 if (PBI->isConditional() && 1187 PBI->getCondition() == BI->getCondition() && 1188 (PBI->getSuccessor(0) != BB || PBI->getSuccessor(1) != BB)) { 1189 // Okay, the outcome of this conditional branch is statically 1190 // knowable. Delete the outgoing CFG edge that is impossible to 1191 // execute. 1192 bool CondIsTrue = PBI->getSuccessor(0) == BB; 1193 BI->getSuccessor(CondIsTrue)->removePredecessor(BB); 1194 new BranchInst(BI->getSuccessor(!CondIsTrue), BB); 1195 BB->getInstList().erase(BI); 1196 return SimplifyCFG(BB) | true; 1197 } 1198 } 1199 } else if (isa<UnreachableInst>(BB->getTerminator())) { 1200 // If there are any instructions immediately before the unreachable that can 1201 // be removed, do so. 1202 Instruction *Unreachable = BB->getTerminator(); 1203 while (Unreachable != BB->begin()) { 1204 BasicBlock::iterator BBI = Unreachable; 1205 --BBI; 1206 if (isa<CallInst>(BBI)) break; 1207 // Delete this instruction 1208 BB->getInstList().erase(BBI); 1209 Changed = true; 1210 } 1211 1212 // If the unreachable instruction is the first in the block, take a gander 1213 // at all of the predecessors of this instruction, and simplify them. 1214 if (&BB->front() == Unreachable) { 1215 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB)); 1216 for (unsigned i = 0, e = Preds.size(); i != e; ++i) { 1217 TerminatorInst *TI = Preds[i]->getTerminator(); 1218 1219 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) { 1220 if (BI->isUnconditional()) { 1221 if (BI->getSuccessor(0) == BB) { 1222 new UnreachableInst(TI); 1223 TI->eraseFromParent(); 1224 Changed = true; 1225 } 1226 } else { 1227 if (BI->getSuccessor(0) == BB) { 1228 new BranchInst(BI->getSuccessor(1), BI); 1229 BI->eraseFromParent(); 1230 } else if (BI->getSuccessor(1) == BB) { 1231 new BranchInst(BI->getSuccessor(0), BI); 1232 BI->eraseFromParent(); 1233 Changed = true; 1234 } 1235 } 1236 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) { 1237 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) 1238 if (SI->getSuccessor(i) == BB) { 1239 BB->removePredecessor(SI->getParent()); 1240 SI->removeCase(i); 1241 --i; --e; 1242 Changed = true; 1243 } 1244 // If the default value is unreachable, figure out the most popular 1245 // destination and make it the default. 1246 if (SI->getSuccessor(0) == BB) { 1247 std::map<BasicBlock*, unsigned> Popularity; 1248 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) 1249 Popularity[SI->getSuccessor(i)]++; 1250 1251 // Find the most popular block. 1252 unsigned MaxPop = 0; 1253 BasicBlock *MaxBlock = 0; 1254 for (std::map<BasicBlock*, unsigned>::iterator 1255 I = Popularity.begin(), E = Popularity.end(); I != E; ++I) { 1256 if (I->second > MaxPop) { 1257 MaxPop = I->second; 1258 MaxBlock = I->first; 1259 } 1260 } 1261 if (MaxBlock) { 1262 // Make this the new default, allowing us to delete any explicit 1263 // edges to it. 1264 SI->setSuccessor(0, MaxBlock); 1265 Changed = true; 1266 1267 // If MaxBlock has phinodes in it, remove MaxPop-1 entries from 1268 // it. 1269 if (isa<PHINode>(MaxBlock->begin())) 1270 for (unsigned i = 0; i != MaxPop-1; ++i) 1271 MaxBlock->removePredecessor(SI->getParent()); 1272 1273 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) 1274 if (SI->getSuccessor(i) == MaxBlock) { 1275 SI->removeCase(i); 1276 --i; --e; 1277 } 1278 } 1279 } 1280 } else if (InvokeInst *II = dyn_cast<InvokeInst>(TI)) { 1281 if (II->getUnwindDest() == BB) { 1282 // Convert the invoke to a call instruction. This would be a good 1283 // place to note that the call does not throw though. 1284 BranchInst *BI = new BranchInst(II->getNormalDest(), II); 1285 II->removeFromParent(); // Take out of symbol table 1286 1287 // Insert the call now... 1288 std::vector<Value*> Args(II->op_begin()+3, II->op_end()); 1289 CallInst *CI = new CallInst(II->getCalledValue(), Args, 1290 II->getName(), BI); 1291 CI->setCallingConv(II->getCallingConv()); 1292 // If the invoke produced a value, the Call does now instead. 1293 II->replaceAllUsesWith(CI); 1294 delete II; 1295 Changed = true; 1296 } 1297 } 1298 } 1299 1300 // If this block is now dead, remove it. 1301 if (pred_begin(BB) == pred_end(BB)) { 1302 // We know there are no successors, so just nuke the block. 1303 M->getBasicBlockList().erase(BB); 1304 return true; 1305 } 1306 } 1307 } 1308 1309 // Merge basic blocks into their predecessor if there is only one distinct 1310 // pred, and if there is only one distinct successor of the predecessor, and 1311 // if there are no PHI nodes. 1312 // 1313 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB)); 1314 BasicBlock *OnlyPred = *PI++; 1315 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same 1316 if (*PI != OnlyPred) { 1317 OnlyPred = 0; // There are multiple different predecessors... 1318 break; 1319 } 1320 1321 BasicBlock *OnlySucc = 0; 1322 if (OnlyPred && OnlyPred != BB && // Don't break self loops 1323 OnlyPred->getTerminator()->getOpcode() != Instruction::Invoke) { 1324 // Check to see if there is only one distinct successor... 1325 succ_iterator SI(succ_begin(OnlyPred)), SE(succ_end(OnlyPred)); 1326 OnlySucc = BB; 1327 for (; SI != SE; ++SI) 1328 if (*SI != OnlySucc) { 1329 OnlySucc = 0; // There are multiple distinct successors! 1330 break; 1331 } 1332 } 1333 1334 if (OnlySucc) { 1335 DEBUG(std::cerr << "Merging: " << *BB << "into: " << *OnlyPred); 1336 TerminatorInst *Term = OnlyPred->getTerminator(); 1337 1338 // Resolve any PHI nodes at the start of the block. They are all 1339 // guaranteed to have exactly one entry if they exist, unless there are 1340 // multiple duplicate (but guaranteed to be equal) entries for the 1341 // incoming edges. This occurs when there are multiple edges from 1342 // OnlyPred to OnlySucc. 1343 // 1344 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) { 1345 PN->replaceAllUsesWith(PN->getIncomingValue(0)); 1346 BB->getInstList().pop_front(); // Delete the phi node... 1347 } 1348 1349 // Delete the unconditional branch from the predecessor... 1350 OnlyPred->getInstList().pop_back(); 1351 1352 // Move all definitions in the successor to the predecessor... 1353 OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList()); 1354 1355 // Make all PHI nodes that referred to BB now refer to Pred as their 1356 // source... 1357 BB->replaceAllUsesWith(OnlyPred); 1358 1359 std::string OldName = BB->getName(); 1360 1361 // Erase basic block from the function... 1362 M->getBasicBlockList().erase(BB); 1363 1364 // Inherit predecessors name if it exists... 1365 if (!OldName.empty() && !OnlyPred->hasName()) 1366 OnlyPred->setName(OldName); 1367 1368 return true; 1369 } 1370 1371 // Otherwise, if this block only has a single predecessor, and if that block 1372 // is a conditional branch, see if we can hoist any code from this block up 1373 // into our predecessor. 1374 if (OnlyPred) 1375 if (BranchInst *BI = dyn_cast<BranchInst>(OnlyPred->getTerminator())) 1376 if (BI->isConditional()) { 1377 // Get the other block. 1378 BasicBlock *OtherBB = BI->getSuccessor(BI->getSuccessor(0) == BB); 1379 PI = pred_begin(OtherBB); 1380 ++PI; 1381 if (PI == pred_end(OtherBB)) { 1382 // We have a conditional branch to two blocks that are only reachable 1383 // from the condbr. We know that the condbr dominates the two blocks, 1384 // so see if there is any identical code in the "then" and "else" 1385 // blocks. If so, we can hoist it up to the branching block. 1386 Changed |= HoistThenElseCodeToIf(BI); 1387 } 1388 } 1389 1390 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) 1391 if (BranchInst *BI = dyn_cast<BranchInst>((*PI)->getTerminator())) 1392 // Change br (X == 0 | X == 1), T, F into a switch instruction. 1393 if (BI->isConditional() && isa<Instruction>(BI->getCondition())) { 1394 Instruction *Cond = cast<Instruction>(BI->getCondition()); 1395 // If this is a bunch of seteq's or'd together, or if it's a bunch of 1396 // 'setne's and'ed together, collect them. 1397 Value *CompVal = 0; 1398 std::vector<ConstantInt*> Values; 1399 bool TrueWhenEqual = GatherValueComparisons(Cond, CompVal, Values); 1400 if (CompVal && CompVal->getType()->isInteger()) { 1401 // There might be duplicate constants in the list, which the switch 1402 // instruction can't handle, remove them now. 1403 std::sort(Values.begin(), Values.end(), ConstantIntOrdering()); 1404 Values.erase(std::unique(Values.begin(), Values.end()), Values.end()); 1405 1406 // Figure out which block is which destination. 1407 BasicBlock *DefaultBB = BI->getSuccessor(1); 1408 BasicBlock *EdgeBB = BI->getSuccessor(0); 1409 if (!TrueWhenEqual) std::swap(DefaultBB, EdgeBB); 1410 1411 // Create the new switch instruction now. 1412 SwitchInst *New = new SwitchInst(CompVal, DefaultBB,Values.size(),BI); 1413 1414 // Add all of the 'cases' to the switch instruction. 1415 for (unsigned i = 0, e = Values.size(); i != e; ++i) 1416 New->addCase(Values[i], EdgeBB); 1417 1418 // We added edges from PI to the EdgeBB. As such, if there were any 1419 // PHI nodes in EdgeBB, they need entries to be added corresponding to 1420 // the number of edges added. 1421 for (BasicBlock::iterator BBI = EdgeBB->begin(); 1422 isa<PHINode>(BBI); ++BBI) { 1423 PHINode *PN = cast<PHINode>(BBI); 1424 Value *InVal = PN->getIncomingValueForBlock(*PI); 1425 for (unsigned i = 0, e = Values.size()-1; i != e; ++i) 1426 PN->addIncoming(InVal, *PI); 1427 } 1428 1429 // Erase the old branch instruction. 1430 (*PI)->getInstList().erase(BI); 1431 1432 // Erase the potentially condition tree that was used to computed the 1433 // branch condition. 1434 ErasePossiblyDeadInstructionTree(Cond); 1435 return true; 1436 } 1437 } 1438 1439 // If there is a trivial two-entry PHI node in this basic block, and we can 1440 // eliminate it, do so now. 1441 if (PHINode *PN = dyn_cast<PHINode>(BB->begin())) 1442 if (PN->getNumIncomingValues() == 2) { 1443 // Ok, this is a two entry PHI node. Check to see if this is a simple "if 1444 // statement", which has a very simple dominance structure. Basically, we 1445 // are trying to find the condition that is being branched on, which 1446 // subsequently causes this merge to happen. We really want control 1447 // dependence information for this check, but simplifycfg can't keep it up 1448 // to date, and this catches most of the cases we care about anyway. 1449 // 1450 BasicBlock *IfTrue, *IfFalse; 1451 if (Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse)) { 1452 DEBUG(std::cerr << "FOUND IF CONDITION! " << *IfCond << " T: " 1453 << IfTrue->getName() << " F: " << IfFalse->getName() << "\n"); 1454 1455 // Loop over the PHI's seeing if we can promote them all to select 1456 // instructions. While we are at it, keep track of the instructions 1457 // that need to be moved to the dominating block. 1458 std::set<Instruction*> AggressiveInsts; 1459 bool CanPromote = true; 1460 1461 BasicBlock::iterator AfterPHIIt = BB->begin(); 1462 while (isa<PHINode>(AfterPHIIt)) { 1463 PHINode *PN = cast<PHINode>(AfterPHIIt++); 1464 if (PN->getIncomingValue(0) == PN->getIncomingValue(1)) { 1465 if (PN->getIncomingValue(0) != PN) 1466 PN->replaceAllUsesWith(PN->getIncomingValue(0)); 1467 else 1468 PN->replaceAllUsesWith(UndefValue::get(PN->getType())); 1469 } else if (!DominatesMergePoint(PN->getIncomingValue(0), BB, 1470 &AggressiveInsts) || 1471 !DominatesMergePoint(PN->getIncomingValue(1), BB, 1472 &AggressiveInsts)) { 1473 CanPromote = false; 1474 break; 1475 } 1476 } 1477 1478 // Did we eliminate all PHI's? 1479 CanPromote |= AfterPHIIt == BB->begin(); 1480 1481 // If we all PHI nodes are promotable, check to make sure that all 1482 // instructions in the predecessor blocks can be promoted as well. If 1483 // not, we won't be able to get rid of the control flow, so it's not 1484 // worth promoting to select instructions. 1485 BasicBlock *DomBlock = 0, *IfBlock1 = 0, *IfBlock2 = 0; 1486 if (CanPromote) { 1487 PN = cast<PHINode>(BB->begin()); 1488 BasicBlock *Pred = PN->getIncomingBlock(0); 1489 if (cast<BranchInst>(Pred->getTerminator())->isUnconditional()) { 1490 IfBlock1 = Pred; 1491 DomBlock = *pred_begin(Pred); 1492 for (BasicBlock::iterator I = Pred->begin(); 1493 !isa<TerminatorInst>(I); ++I) 1494 if (!AggressiveInsts.count(I)) { 1495 // This is not an aggressive instruction that we can promote. 1496 // Because of this, we won't be able to get rid of the control 1497 // flow, so the xform is not worth it. 1498 CanPromote = false; 1499 break; 1500 } 1501 } 1502 1503 Pred = PN->getIncomingBlock(1); 1504 if (CanPromote && 1505 cast<BranchInst>(Pred->getTerminator())->isUnconditional()) { 1506 IfBlock2 = Pred; 1507 DomBlock = *pred_begin(Pred); 1508 for (BasicBlock::iterator I = Pred->begin(); 1509 !isa<TerminatorInst>(I); ++I) 1510 if (!AggressiveInsts.count(I)) { 1511 // This is not an aggressive instruction that we can promote. 1512 // Because of this, we won't be able to get rid of the control 1513 // flow, so the xform is not worth it. 1514 CanPromote = false; 1515 break; 1516 } 1517 } 1518 } 1519 1520 // If we can still promote the PHI nodes after this gauntlet of tests, 1521 // do all of the PHI's now. 1522 if (CanPromote) { 1523 // Move all 'aggressive' instructions, which are defined in the 1524 // conditional parts of the if's up to the dominating block. 1525 if (IfBlock1) { 1526 DomBlock->getInstList().splice(DomBlock->getTerminator(), 1527 IfBlock1->getInstList(), 1528 IfBlock1->begin(), 1529 IfBlock1->getTerminator()); 1530 } 1531 if (IfBlock2) { 1532 DomBlock->getInstList().splice(DomBlock->getTerminator(), 1533 IfBlock2->getInstList(), 1534 IfBlock2->begin(), 1535 IfBlock2->getTerminator()); 1536 } 1537 1538 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) { 1539 // Change the PHI node into a select instruction. 1540 Value *TrueVal = 1541 PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse); 1542 Value *FalseVal = 1543 PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue); 1544 1545 std::string Name = PN->getName(); PN->setName(""); 1546 PN->replaceAllUsesWith(new SelectInst(IfCond, TrueVal, FalseVal, 1547 Name, AfterPHIIt)); 1548 BB->getInstList().erase(PN); 1549 } 1550 Changed = true; 1551 } 1552 } 1553 } 1554 1555 return Changed; 1556} 1557