SimplifyCFG.cpp revision 203954
1//===- SimplifyCFG.cpp - Code to perform CFG simplification ---------------===// 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// 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/IntrinsicInst.h" 19#include "llvm/Type.h" 20#include "llvm/DerivedTypes.h" 21#include "llvm/GlobalVariable.h" 22#include "llvm/Support/CFG.h" 23#include "llvm/Support/Debug.h" 24#include "llvm/Support/raw_ostream.h" 25#include "llvm/Analysis/ConstantFolding.h" 26#include "llvm/Target/TargetData.h" 27#include "llvm/Transforms/Utils/BasicBlockUtils.h" 28#include "llvm/ADT/DenseMap.h" 29#include "llvm/ADT/SmallVector.h" 30#include "llvm/ADT/SmallPtrSet.h" 31#include "llvm/ADT/Statistic.h" 32#include <algorithm> 33#include <functional> 34#include <set> 35#include <map> 36using namespace llvm; 37 38STATISTIC(NumSpeculations, "Number of speculative executed instructions"); 39 40namespace { 41class SimplifyCFGOpt { 42 const TargetData *const TD; 43 44 ConstantInt *GetConstantInt(Value *V); 45 Value *GatherConstantSetEQs(Value *V, std::vector<ConstantInt*> &Values); 46 Value *GatherConstantSetNEs(Value *V, std::vector<ConstantInt*> &Values); 47 bool GatherValueComparisons(Instruction *Cond, Value *&CompVal, 48 std::vector<ConstantInt*> &Values); 49 Value *isValueEqualityComparison(TerminatorInst *TI); 50 BasicBlock *GetValueEqualityComparisonCases(TerminatorInst *TI, 51 std::vector<std::pair<ConstantInt*, BasicBlock*> > &Cases); 52 bool SimplifyEqualityComparisonWithOnlyPredecessor(TerminatorInst *TI, 53 BasicBlock *Pred); 54 bool FoldValueComparisonIntoPredecessors(TerminatorInst *TI); 55 56public: 57 explicit SimplifyCFGOpt(const TargetData *td) : TD(td) {} 58 bool run(BasicBlock *BB); 59}; 60} 61 62/// SafeToMergeTerminators - Return true if it is safe to merge these two 63/// terminator instructions together. 64/// 65static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) { 66 if (SI1 == SI2) return false; // Can't merge with self! 67 68 // It is not safe to merge these two switch instructions if they have a common 69 // successor, and if that successor has a PHI node, and if *that* PHI node has 70 // conflicting incoming values from the two switch blocks. 71 BasicBlock *SI1BB = SI1->getParent(); 72 BasicBlock *SI2BB = SI2->getParent(); 73 SmallPtrSet<BasicBlock*, 16> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB)); 74 75 for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I) 76 if (SI1Succs.count(*I)) 77 for (BasicBlock::iterator BBI = (*I)->begin(); 78 isa<PHINode>(BBI); ++BBI) { 79 PHINode *PN = cast<PHINode>(BBI); 80 if (PN->getIncomingValueForBlock(SI1BB) != 81 PN->getIncomingValueForBlock(SI2BB)) 82 return false; 83 } 84 85 return true; 86} 87 88/// AddPredecessorToBlock - Update PHI nodes in Succ to indicate that there will 89/// now be entries in it from the 'NewPred' block. The values that will be 90/// flowing into the PHI nodes will be the same as those coming in from 91/// ExistPred, an existing predecessor of Succ. 92static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred, 93 BasicBlock *ExistPred) { 94 assert(std::find(succ_begin(ExistPred), succ_end(ExistPred), Succ) != 95 succ_end(ExistPred) && "ExistPred is not a predecessor of Succ!"); 96 if (!isa<PHINode>(Succ->begin())) return; // Quick exit if nothing to do 97 98 PHINode *PN; 99 for (BasicBlock::iterator I = Succ->begin(); 100 (PN = dyn_cast<PHINode>(I)); ++I) 101 PN->addIncoming(PN->getIncomingValueForBlock(ExistPred), NewPred); 102} 103 104 105/// GetIfCondition - Given a basic block (BB) with two predecessors (and 106/// presumably PHI nodes in it), check to see if the merge at this block is due 107/// to an "if condition". If so, return the boolean condition that determines 108/// which entry into BB will be taken. Also, return by references the block 109/// that will be entered from if the condition is true, and the block that will 110/// be entered if the condition is false. 111/// 112/// 113static Value *GetIfCondition(BasicBlock *BB, 114 BasicBlock *&IfTrue, BasicBlock *&IfFalse) { 115 assert(std::distance(pred_begin(BB), pred_end(BB)) == 2 && 116 "Function can only handle blocks with 2 predecessors!"); 117 BasicBlock *Pred1 = *pred_begin(BB); 118 BasicBlock *Pred2 = *++pred_begin(BB); 119 120 // We can only handle branches. Other control flow will be lowered to 121 // branches if possible anyway. 122 if (!isa<BranchInst>(Pred1->getTerminator()) || 123 !isa<BranchInst>(Pred2->getTerminator())) 124 return 0; 125 BranchInst *Pred1Br = cast<BranchInst>(Pred1->getTerminator()); 126 BranchInst *Pred2Br = cast<BranchInst>(Pred2->getTerminator()); 127 128 // Eliminate code duplication by ensuring that Pred1Br is conditional if 129 // either are. 130 if (Pred2Br->isConditional()) { 131 // If both branches are conditional, we don't have an "if statement". In 132 // reality, we could transform this case, but since the condition will be 133 // required anyway, we stand no chance of eliminating it, so the xform is 134 // probably not profitable. 135 if (Pred1Br->isConditional()) 136 return 0; 137 138 std::swap(Pred1, Pred2); 139 std::swap(Pred1Br, Pred2Br); 140 } 141 142 if (Pred1Br->isConditional()) { 143 // If we found a conditional branch predecessor, make sure that it branches 144 // to BB and Pred2Br. If it doesn't, this isn't an "if statement". 145 if (Pred1Br->getSuccessor(0) == BB && 146 Pred1Br->getSuccessor(1) == Pred2) { 147 IfTrue = Pred1; 148 IfFalse = Pred2; 149 } else if (Pred1Br->getSuccessor(0) == Pred2 && 150 Pred1Br->getSuccessor(1) == BB) { 151 IfTrue = Pred2; 152 IfFalse = Pred1; 153 } else { 154 // We know that one arm of the conditional goes to BB, so the other must 155 // go somewhere unrelated, and this must not be an "if statement". 156 return 0; 157 } 158 159 // The only thing we have to watch out for here is to make sure that Pred2 160 // doesn't have incoming edges from other blocks. If it does, the condition 161 // doesn't dominate BB. 162 if (++pred_begin(Pred2) != pred_end(Pred2)) 163 return 0; 164 165 return Pred1Br->getCondition(); 166 } 167 168 // Ok, if we got here, both predecessors end with an unconditional branch to 169 // BB. Don't panic! If both blocks only have a single (identical) 170 // predecessor, and THAT is a conditional branch, then we're all ok! 171 if (pred_begin(Pred1) == pred_end(Pred1) || 172 ++pred_begin(Pred1) != pred_end(Pred1) || 173 pred_begin(Pred2) == pred_end(Pred2) || 174 ++pred_begin(Pred2) != pred_end(Pred2) || 175 *pred_begin(Pred1) != *pred_begin(Pred2)) 176 return 0; 177 178 // Otherwise, if this is a conditional branch, then we can use it! 179 BasicBlock *CommonPred = *pred_begin(Pred1); 180 if (BranchInst *BI = dyn_cast<BranchInst>(CommonPred->getTerminator())) { 181 assert(BI->isConditional() && "Two successors but not conditional?"); 182 if (BI->getSuccessor(0) == Pred1) { 183 IfTrue = Pred1; 184 IfFalse = Pred2; 185 } else { 186 IfTrue = Pred2; 187 IfFalse = Pred1; 188 } 189 return BI->getCondition(); 190 } 191 return 0; 192} 193 194/// DominatesMergePoint - If we have a merge point of an "if condition" as 195/// accepted above, return true if the specified value dominates the block. We 196/// don't handle the true generality of domination here, just a special case 197/// which works well enough for us. 198/// 199/// If AggressiveInsts is non-null, and if V does not dominate BB, we check to 200/// see if V (which must be an instruction) is cheap to compute and is 201/// non-trapping. If both are true, the instruction is inserted into the set 202/// and true is returned. 203static bool DominatesMergePoint(Value *V, BasicBlock *BB, 204 std::set<Instruction*> *AggressiveInsts) { 205 Instruction *I = dyn_cast<Instruction>(V); 206 if (!I) { 207 // Non-instructions all dominate instructions, but not all constantexprs 208 // can be executed unconditionally. 209 if (ConstantExpr *C = dyn_cast<ConstantExpr>(V)) 210 if (C->canTrap()) 211 return false; 212 return true; 213 } 214 BasicBlock *PBB = I->getParent(); 215 216 // We don't want to allow weird loops that might have the "if condition" in 217 // the bottom of this block. 218 if (PBB == BB) return false; 219 220 // If this instruction is defined in a block that contains an unconditional 221 // branch to BB, then it must be in the 'conditional' part of the "if 222 // statement". 223 if (BranchInst *BI = dyn_cast<BranchInst>(PBB->getTerminator())) 224 if (BI->isUnconditional() && BI->getSuccessor(0) == BB) { 225 if (!AggressiveInsts) return false; 226 // Okay, it looks like the instruction IS in the "condition". Check to 227 // see if its a cheap instruction to unconditionally compute, and if it 228 // only uses stuff defined outside of the condition. If so, hoist it out. 229 if (!I->isSafeToSpeculativelyExecute()) 230 return false; 231 232 switch (I->getOpcode()) { 233 default: return false; // Cannot hoist this out safely. 234 case Instruction::Load: { 235 // We have to check to make sure there are no instructions before the 236 // load in its basic block, as we are going to hoist the loop out to 237 // its predecessor. 238 BasicBlock::iterator IP = PBB->begin(); 239 while (isa<DbgInfoIntrinsic>(IP)) 240 IP++; 241 if (IP != BasicBlock::iterator(I)) 242 return false; 243 break; 244 } 245 case Instruction::Add: 246 case Instruction::Sub: 247 case Instruction::And: 248 case Instruction::Or: 249 case Instruction::Xor: 250 case Instruction::Shl: 251 case Instruction::LShr: 252 case Instruction::AShr: 253 case Instruction::ICmp: 254 break; // These are all cheap and non-trapping instructions. 255 } 256 257 // Okay, we can only really hoist these out if their operands are not 258 // defined in the conditional region. 259 for (User::op_iterator i = I->op_begin(), e = I->op_end(); i != e; ++i) 260 if (!DominatesMergePoint(*i, BB, 0)) 261 return false; 262 // Okay, it's safe to do this! Remember this instruction. 263 AggressiveInsts->insert(I); 264 } 265 266 return true; 267} 268 269/// GetConstantInt - Extract ConstantInt from value, looking through IntToPtr 270/// and PointerNullValue. Return NULL if value is not a constant int. 271ConstantInt *SimplifyCFGOpt::GetConstantInt(Value *V) { 272 // Normal constant int. 273 ConstantInt *CI = dyn_cast<ConstantInt>(V); 274 if (CI || !TD || !isa<Constant>(V) || !isa<PointerType>(V->getType())) 275 return CI; 276 277 // This is some kind of pointer constant. Turn it into a pointer-sized 278 // ConstantInt if possible. 279 const IntegerType *PtrTy = TD->getIntPtrType(V->getContext()); 280 281 // Null pointer means 0, see SelectionDAGBuilder::getValue(const Value*). 282 if (isa<ConstantPointerNull>(V)) 283 return ConstantInt::get(PtrTy, 0); 284 285 // IntToPtr const int. 286 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) 287 if (CE->getOpcode() == Instruction::IntToPtr) 288 if (ConstantInt *CI = dyn_cast<ConstantInt>(CE->getOperand(0))) { 289 // The constant is very likely to have the right type already. 290 if (CI->getType() == PtrTy) 291 return CI; 292 else 293 return cast<ConstantInt> 294 (ConstantExpr::getIntegerCast(CI, PtrTy, /*isSigned=*/false)); 295 } 296 return 0; 297} 298 299/// GatherConstantSetEQs - Given a potentially 'or'd together collection of 300/// icmp_eq instructions that compare a value against a constant, return the 301/// value being compared, and stick the constant into the Values vector. 302Value *SimplifyCFGOpt:: 303GatherConstantSetEQs(Value *V, std::vector<ConstantInt*> &Values) { 304 if (Instruction *Inst = dyn_cast<Instruction>(V)) { 305 if (Inst->getOpcode() == Instruction::ICmp && 306 cast<ICmpInst>(Inst)->getPredicate() == ICmpInst::ICMP_EQ) { 307 if (ConstantInt *C = GetConstantInt(Inst->getOperand(1))) { 308 Values.push_back(C); 309 return Inst->getOperand(0); 310 } else if (ConstantInt *C = GetConstantInt(Inst->getOperand(0))) { 311 Values.push_back(C); 312 return Inst->getOperand(1); 313 } 314 } else if (Inst->getOpcode() == Instruction::Or) { 315 if (Value *LHS = GatherConstantSetEQs(Inst->getOperand(0), Values)) 316 if (Value *RHS = GatherConstantSetEQs(Inst->getOperand(1), Values)) 317 if (LHS == RHS) 318 return LHS; 319 } 320 } 321 return 0; 322} 323 324/// GatherConstantSetNEs - Given a potentially 'and'd together collection of 325/// setne instructions that compare a value against a constant, return the value 326/// being compared, and stick the constant into the Values vector. 327Value *SimplifyCFGOpt:: 328GatherConstantSetNEs(Value *V, std::vector<ConstantInt*> &Values) { 329 if (Instruction *Inst = dyn_cast<Instruction>(V)) { 330 if (Inst->getOpcode() == Instruction::ICmp && 331 cast<ICmpInst>(Inst)->getPredicate() == ICmpInst::ICMP_NE) { 332 if (ConstantInt *C = GetConstantInt(Inst->getOperand(1))) { 333 Values.push_back(C); 334 return Inst->getOperand(0); 335 } else if (ConstantInt *C = GetConstantInt(Inst->getOperand(0))) { 336 Values.push_back(C); 337 return Inst->getOperand(1); 338 } 339 } else if (Inst->getOpcode() == Instruction::And) { 340 if (Value *LHS = GatherConstantSetNEs(Inst->getOperand(0), Values)) 341 if (Value *RHS = GatherConstantSetNEs(Inst->getOperand(1), Values)) 342 if (LHS == RHS) 343 return LHS; 344 } 345 } 346 return 0; 347} 348 349/// GatherValueComparisons - If the specified Cond is an 'and' or 'or' of a 350/// bunch of comparisons of one value against constants, return the value and 351/// the constants being compared. 352bool SimplifyCFGOpt::GatherValueComparisons(Instruction *Cond, Value *&CompVal, 353 std::vector<ConstantInt*> &Values) { 354 if (Cond->getOpcode() == Instruction::Or) { 355 CompVal = GatherConstantSetEQs(Cond, Values); 356 357 // Return true to indicate that the condition is true if the CompVal is 358 // equal to one of the constants. 359 return true; 360 } else if (Cond->getOpcode() == Instruction::And) { 361 CompVal = GatherConstantSetNEs(Cond, Values); 362 363 // Return false to indicate that the condition is false if the CompVal is 364 // equal to one of the constants. 365 return false; 366 } 367 return false; 368} 369 370static void EraseTerminatorInstAndDCECond(TerminatorInst *TI) { 371 Instruction* Cond = 0; 372 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) { 373 Cond = dyn_cast<Instruction>(SI->getCondition()); 374 } else if (BranchInst *BI = dyn_cast<BranchInst>(TI)) { 375 if (BI->isConditional()) 376 Cond = dyn_cast<Instruction>(BI->getCondition()); 377 } 378 379 TI->eraseFromParent(); 380 if (Cond) RecursivelyDeleteTriviallyDeadInstructions(Cond); 381} 382 383/// isValueEqualityComparison - Return true if the specified terminator checks 384/// to see if a value is equal to constant integer value. 385Value *SimplifyCFGOpt::isValueEqualityComparison(TerminatorInst *TI) { 386 Value *CV = 0; 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 CV = SI->getCondition(); 393 } else if (BranchInst *BI = dyn_cast<BranchInst>(TI)) 394 if (BI->isConditional() && BI->getCondition()->hasOneUse()) 395 if (ICmpInst *ICI = dyn_cast<ICmpInst>(BI->getCondition())) 396 if ((ICI->getPredicate() == ICmpInst::ICMP_EQ || 397 ICI->getPredicate() == ICmpInst::ICMP_NE) && 398 GetConstantInt(ICI->getOperand(1))) 399 CV = ICI->getOperand(0); 400 401 // Unwrap any lossless ptrtoint cast. 402 if (TD && CV && CV->getType() == TD->getIntPtrType(CV->getContext())) 403 if (PtrToIntInst *PTII = dyn_cast<PtrToIntInst>(CV)) 404 CV = PTII->getOperand(0); 405 return CV; 406} 407 408/// GetValueEqualityComparisonCases - Given a value comparison instruction, 409/// decode all of the 'cases' that it represents and return the 'default' block. 410BasicBlock *SimplifyCFGOpt:: 411GetValueEqualityComparisonCases(TerminatorInst *TI, 412 std::vector<std::pair<ConstantInt*, 413 BasicBlock*> > &Cases) { 414 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) { 415 Cases.reserve(SI->getNumCases()); 416 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) 417 Cases.push_back(std::make_pair(SI->getCaseValue(i), SI->getSuccessor(i))); 418 return SI->getDefaultDest(); 419 } 420 421 BranchInst *BI = cast<BranchInst>(TI); 422 ICmpInst *ICI = cast<ICmpInst>(BI->getCondition()); 423 Cases.push_back(std::make_pair(GetConstantInt(ICI->getOperand(1)), 424 BI->getSuccessor(ICI->getPredicate() == 425 ICmpInst::ICMP_NE))); 426 return BI->getSuccessor(ICI->getPredicate() == ICmpInst::ICMP_EQ); 427} 428 429 430/// EliminateBlockCases - Given a vector of bb/value pairs, remove any entries 431/// in the list that match the specified block. 432static void EliminateBlockCases(BasicBlock *BB, 433 std::vector<std::pair<ConstantInt*, BasicBlock*> > &Cases) { 434 for (unsigned i = 0, e = Cases.size(); i != e; ++i) 435 if (Cases[i].second == BB) { 436 Cases.erase(Cases.begin()+i); 437 --i; --e; 438 } 439} 440 441/// ValuesOverlap - Return true if there are any keys in C1 that exist in C2 as 442/// well. 443static bool 444ValuesOverlap(std::vector<std::pair<ConstantInt*, BasicBlock*> > &C1, 445 std::vector<std::pair<ConstantInt*, BasicBlock*> > &C2) { 446 std::vector<std::pair<ConstantInt*, BasicBlock*> > *V1 = &C1, *V2 = &C2; 447 448 // Make V1 be smaller than V2. 449 if (V1->size() > V2->size()) 450 std::swap(V1, V2); 451 452 if (V1->size() == 0) return false; 453 if (V1->size() == 1) { 454 // Just scan V2. 455 ConstantInt *TheVal = (*V1)[0].first; 456 for (unsigned i = 0, e = V2->size(); i != e; ++i) 457 if (TheVal == (*V2)[i].first) 458 return true; 459 } 460 461 // Otherwise, just sort both lists and compare element by element. 462 std::sort(V1->begin(), V1->end()); 463 std::sort(V2->begin(), V2->end()); 464 unsigned i1 = 0, i2 = 0, e1 = V1->size(), e2 = V2->size(); 465 while (i1 != e1 && i2 != e2) { 466 if ((*V1)[i1].first == (*V2)[i2].first) 467 return true; 468 if ((*V1)[i1].first < (*V2)[i2].first) 469 ++i1; 470 else 471 ++i2; 472 } 473 return false; 474} 475 476/// SimplifyEqualityComparisonWithOnlyPredecessor - If TI is known to be a 477/// terminator instruction and its block is known to only have a single 478/// predecessor block, check to see if that predecessor is also a value 479/// comparison with the same value, and if that comparison determines the 480/// outcome of this comparison. If so, simplify TI. This does a very limited 481/// form of jump threading. 482bool SimplifyCFGOpt:: 483SimplifyEqualityComparisonWithOnlyPredecessor(TerminatorInst *TI, 484 BasicBlock *Pred) { 485 Value *PredVal = isValueEqualityComparison(Pred->getTerminator()); 486 if (!PredVal) return false; // Not a value comparison in predecessor. 487 488 Value *ThisVal = isValueEqualityComparison(TI); 489 assert(ThisVal && "This isn't a value comparison!!"); 490 if (ThisVal != PredVal) return false; // Different predicates. 491 492 // Find out information about when control will move from Pred to TI's block. 493 std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases; 494 BasicBlock *PredDef = GetValueEqualityComparisonCases(Pred->getTerminator(), 495 PredCases); 496 EliminateBlockCases(PredDef, PredCases); // Remove default from cases. 497 498 // Find information about how control leaves this block. 499 std::vector<std::pair<ConstantInt*, BasicBlock*> > ThisCases; 500 BasicBlock *ThisDef = GetValueEqualityComparisonCases(TI, ThisCases); 501 EliminateBlockCases(ThisDef, ThisCases); // Remove default from cases. 502 503 // If TI's block is the default block from Pred's comparison, potentially 504 // simplify TI based on this knowledge. 505 if (PredDef == TI->getParent()) { 506 // If we are here, we know that the value is none of those cases listed in 507 // PredCases. If there are any cases in ThisCases that are in PredCases, we 508 // can simplify TI. 509 if (ValuesOverlap(PredCases, ThisCases)) { 510 if (isa<BranchInst>(TI)) { 511 // Okay, one of the successors of this condbr is dead. Convert it to a 512 // uncond br. 513 assert(ThisCases.size() == 1 && "Branch can only have one case!"); 514 // Insert the new branch. 515 Instruction *NI = BranchInst::Create(ThisDef, TI); 516 (void) NI; 517 518 // Remove PHI node entries for the dead edge. 519 ThisCases[0].second->removePredecessor(TI->getParent()); 520 521 DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator() 522 << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n"); 523 524 EraseTerminatorInstAndDCECond(TI); 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 SmallPtrSet<Constant*, 16> DeadCases; 531 for (unsigned i = 0, e = PredCases.size(); i != e; ++i) 532 DeadCases.insert(PredCases[i].first); 533 534 DEBUG(dbgs() << "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(dbgs() << "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 } 560 assert(TIV && "No edge from pred to succ?"); 561 562 // Okay, we found the one constant that our value can be if we get into TI's 563 // BB. Find out which successor will unconditionally be branched to. 564 BasicBlock *TheRealDest = 0; 565 for (unsigned i = 0, e = ThisCases.size(); i != e; ++i) 566 if (ThisCases[i].first == TIV) { 567 TheRealDest = ThisCases[i].second; 568 break; 569 } 570 571 // If not handled by any explicit cases, it is handled by the default case. 572 if (TheRealDest == 0) TheRealDest = ThisDef; 573 574 // Remove PHI node entries for dead edges. 575 BasicBlock *CheckEdge = TheRealDest; 576 for (succ_iterator SI = succ_begin(TIBB), e = succ_end(TIBB); SI != e; ++SI) 577 if (*SI != CheckEdge) 578 (*SI)->removePredecessor(TIBB); 579 else 580 CheckEdge = 0; 581 582 // Insert the new branch. 583 Instruction *NI = BranchInst::Create(TheRealDest, TI); 584 (void) NI; 585 586 DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator() 587 << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n"); 588 589 EraseTerminatorInstAndDCECond(TI); 590 return true; 591 } 592 return false; 593} 594 595namespace { 596 /// ConstantIntOrdering - This class implements a stable ordering of constant 597 /// integers that does not depend on their address. This is important for 598 /// applications that sort ConstantInt's to ensure uniqueness. 599 struct ConstantIntOrdering { 600 bool operator()(const ConstantInt *LHS, const ConstantInt *RHS) const { 601 return LHS->getValue().ult(RHS->getValue()); 602 } 603 }; 604} 605 606/// FoldValueComparisonIntoPredecessors - The specified terminator is a value 607/// equality comparison instruction (either a switch or a branch on "X == c"). 608/// See if any of the predecessors of the terminator block are value comparisons 609/// on the same value. If so, and if safe to do so, fold them together. 610bool SimplifyCFGOpt::FoldValueComparisonIntoPredecessors(TerminatorInst *TI) { 611 BasicBlock *BB = TI->getParent(); 612 Value *CV = isValueEqualityComparison(TI); // CondVal 613 assert(CV && "Not a comparison?"); 614 bool Changed = false; 615 616 SmallVector<BasicBlock*, 16> Preds(pred_begin(BB), pred_end(BB)); 617 while (!Preds.empty()) { 618 BasicBlock *Pred = Preds.pop_back_val(); 619 620 // See if the predecessor is a comparison with the same value. 621 TerminatorInst *PTI = Pred->getTerminator(); 622 Value *PCV = isValueEqualityComparison(PTI); // PredCondVal 623 624 if (PCV == CV && SafeToMergeTerminators(TI, PTI)) { 625 // Figure out which 'cases' to copy from SI to PSI. 626 std::vector<std::pair<ConstantInt*, BasicBlock*> > BBCases; 627 BasicBlock *BBDefault = GetValueEqualityComparisonCases(TI, BBCases); 628 629 std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases; 630 BasicBlock *PredDefault = GetValueEqualityComparisonCases(PTI, PredCases); 631 632 // Based on whether the default edge from PTI goes to BB or not, fill in 633 // PredCases and PredDefault with the new switch cases we would like to 634 // build. 635 SmallVector<BasicBlock*, 8> NewSuccessors; 636 637 if (PredDefault == BB) { 638 // If this is the default destination from PTI, only the edges in TI 639 // that don't occur in PTI, or that branch to BB will be activated. 640 std::set<ConstantInt*, ConstantIntOrdering> PTIHandled; 641 for (unsigned i = 0, e = PredCases.size(); i != e; ++i) 642 if (PredCases[i].second != BB) 643 PTIHandled.insert(PredCases[i].first); 644 else { 645 // The default destination is BB, we don't need explicit targets. 646 std::swap(PredCases[i], PredCases.back()); 647 PredCases.pop_back(); 648 --i; --e; 649 } 650 651 // Reconstruct the new switch statement we will be building. 652 if (PredDefault != BBDefault) { 653 PredDefault->removePredecessor(Pred); 654 PredDefault = BBDefault; 655 NewSuccessors.push_back(BBDefault); 656 } 657 for (unsigned i = 0, e = BBCases.size(); i != e; ++i) 658 if (!PTIHandled.count(BBCases[i].first) && 659 BBCases[i].second != BBDefault) { 660 PredCases.push_back(BBCases[i]); 661 NewSuccessors.push_back(BBCases[i].second); 662 } 663 664 } else { 665 // If this is not the default destination from PSI, only the edges 666 // in SI that occur in PSI with a destination of BB will be 667 // activated. 668 std::set<ConstantInt*, ConstantIntOrdering> PTIHandled; 669 for (unsigned i = 0, e = PredCases.size(); i != e; ++i) 670 if (PredCases[i].second == BB) { 671 PTIHandled.insert(PredCases[i].first); 672 std::swap(PredCases[i], PredCases.back()); 673 PredCases.pop_back(); 674 --i; --e; 675 } 676 677 // Okay, now we know which constants were sent to BB from the 678 // predecessor. Figure out where they will all go now. 679 for (unsigned i = 0, e = BBCases.size(); i != e; ++i) 680 if (PTIHandled.count(BBCases[i].first)) { 681 // If this is one we are capable of getting... 682 PredCases.push_back(BBCases[i]); 683 NewSuccessors.push_back(BBCases[i].second); 684 PTIHandled.erase(BBCases[i].first);// This constant is taken care of 685 } 686 687 // If there are any constants vectored to BB that TI doesn't handle, 688 // they must go to the default destination of TI. 689 for (std::set<ConstantInt*, ConstantIntOrdering>::iterator I = 690 PTIHandled.begin(), 691 E = PTIHandled.end(); I != E; ++I) { 692 PredCases.push_back(std::make_pair(*I, BBDefault)); 693 NewSuccessors.push_back(BBDefault); 694 } 695 } 696 697 // Okay, at this point, we know which new successor Pred will get. Make 698 // sure we update the number of entries in the PHI nodes for these 699 // successors. 700 for (unsigned i = 0, e = NewSuccessors.size(); i != e; ++i) 701 AddPredecessorToBlock(NewSuccessors[i], Pred, BB); 702 703 // Convert pointer to int before we switch. 704 if (isa<PointerType>(CV->getType())) { 705 assert(TD && "Cannot switch on pointer without TargetData"); 706 CV = new PtrToIntInst(CV, TD->getIntPtrType(CV->getContext()), 707 "magicptr", PTI); 708 } 709 710 // Now that the successors are updated, create the new Switch instruction. 711 SwitchInst *NewSI = SwitchInst::Create(CV, PredDefault, 712 PredCases.size(), PTI); 713 for (unsigned i = 0, e = PredCases.size(); i != e; ++i) 714 NewSI->addCase(PredCases[i].first, PredCases[i].second); 715 716 EraseTerminatorInstAndDCECond(PTI); 717 718 // Okay, last check. If BB is still a successor of PSI, then we must 719 // have an infinite loop case. If so, add an infinitely looping block 720 // to handle the case to preserve the behavior of the code. 721 BasicBlock *InfLoopBlock = 0; 722 for (unsigned i = 0, e = NewSI->getNumSuccessors(); i != e; ++i) 723 if (NewSI->getSuccessor(i) == BB) { 724 if (InfLoopBlock == 0) { 725 // Insert it at the end of the function, because it's either code, 726 // or it won't matter if it's hot. :) 727 InfLoopBlock = BasicBlock::Create(BB->getContext(), 728 "infloop", BB->getParent()); 729 BranchInst::Create(InfLoopBlock, InfLoopBlock); 730 } 731 NewSI->setSuccessor(i, InfLoopBlock); 732 } 733 734 Changed = true; 735 } 736 } 737 return Changed; 738} 739 740// isSafeToHoistInvoke - If we would need to insert a select that uses the 741// value of this invoke (comments in HoistThenElseCodeToIf explain why we 742// would need to do this), we can't hoist the invoke, as there is nowhere 743// to put the select in this case. 744static bool isSafeToHoistInvoke(BasicBlock *BB1, BasicBlock *BB2, 745 Instruction *I1, Instruction *I2) { 746 for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) { 747 PHINode *PN; 748 for (BasicBlock::iterator BBI = SI->begin(); 749 (PN = dyn_cast<PHINode>(BBI)); ++BBI) { 750 Value *BB1V = PN->getIncomingValueForBlock(BB1); 751 Value *BB2V = PN->getIncomingValueForBlock(BB2); 752 if (BB1V != BB2V && (BB1V==I1 || BB2V==I2)) { 753 return false; 754 } 755 } 756 } 757 return true; 758} 759 760/// HoistThenElseCodeToIf - Given a conditional branch that goes to BB1 and 761/// BB2, hoist any common code in the two blocks up into the branch block. The 762/// caller of this function guarantees that BI's block dominates BB1 and BB2. 763static bool HoistThenElseCodeToIf(BranchInst *BI) { 764 // This does very trivial matching, with limited scanning, to find identical 765 // instructions in the two blocks. In particular, we don't want to get into 766 // O(M*N) situations here where M and N are the sizes of BB1 and BB2. As 767 // such, we currently just scan for obviously identical instructions in an 768 // identical order. 769 BasicBlock *BB1 = BI->getSuccessor(0); // The true destination. 770 BasicBlock *BB2 = BI->getSuccessor(1); // The false destination 771 772 BasicBlock::iterator BB1_Itr = BB1->begin(); 773 BasicBlock::iterator BB2_Itr = BB2->begin(); 774 775 Instruction *I1 = BB1_Itr++, *I2 = BB2_Itr++; 776 while (isa<DbgInfoIntrinsic>(I1)) 777 I1 = BB1_Itr++; 778 while (isa<DbgInfoIntrinsic>(I2)) 779 I2 = BB2_Itr++; 780 if (I1->getOpcode() != I2->getOpcode() || isa<PHINode>(I1) || 781 !I1->isIdenticalToWhenDefined(I2) || 782 (isa<InvokeInst>(I1) && !isSafeToHoistInvoke(BB1, BB2, I1, I2))) 783 return false; 784 785 // If we get here, we can hoist at least one instruction. 786 BasicBlock *BIParent = BI->getParent(); 787 788 do { 789 // If we are hoisting the terminator instruction, don't move one (making a 790 // broken BB), instead clone it, and remove BI. 791 if (isa<TerminatorInst>(I1)) 792 goto HoistTerminator; 793 794 // For a normal instruction, we just move one to right before the branch, 795 // then replace all uses of the other with the first. Finally, we remove 796 // the now redundant second instruction. 797 BIParent->getInstList().splice(BI, BB1->getInstList(), I1); 798 if (!I2->use_empty()) 799 I2->replaceAllUsesWith(I1); 800 I1->intersectOptionalDataWith(I2); 801 BB2->getInstList().erase(I2); 802 803 I1 = BB1_Itr++; 804 while (isa<DbgInfoIntrinsic>(I1)) 805 I1 = BB1_Itr++; 806 I2 = BB2_Itr++; 807 while (isa<DbgInfoIntrinsic>(I2)) 808 I2 = BB2_Itr++; 809 } while (I1->getOpcode() == I2->getOpcode() && 810 I1->isIdenticalToWhenDefined(I2)); 811 812 return true; 813 814HoistTerminator: 815 // It may not be possible to hoist an invoke. 816 if (isa<InvokeInst>(I1) && !isSafeToHoistInvoke(BB1, BB2, I1, I2)) 817 return true; 818 819 // Okay, it is safe to hoist the terminator. 820 Instruction *NT = I1->clone(); 821 BIParent->getInstList().insert(BI, NT); 822 if (!NT->getType()->isVoidTy()) { 823 I1->replaceAllUsesWith(NT); 824 I2->replaceAllUsesWith(NT); 825 NT->takeName(I1); 826 } 827 828 // Hoisting one of the terminators from our successor is a great thing. 829 // Unfortunately, the successors of the if/else blocks may have PHI nodes in 830 // them. If they do, all PHI entries for BB1/BB2 must agree for all PHI 831 // nodes, so we insert select instruction to compute the final result. 832 std::map<std::pair<Value*,Value*>, SelectInst*> InsertedSelects; 833 for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) { 834 PHINode *PN; 835 for (BasicBlock::iterator BBI = SI->begin(); 836 (PN = dyn_cast<PHINode>(BBI)); ++BBI) { 837 Value *BB1V = PN->getIncomingValueForBlock(BB1); 838 Value *BB2V = PN->getIncomingValueForBlock(BB2); 839 if (BB1V != BB2V) { 840 // These values do not agree. Insert a select instruction before NT 841 // that determines the right value. 842 SelectInst *&SI = InsertedSelects[std::make_pair(BB1V, BB2V)]; 843 if (SI == 0) 844 SI = SelectInst::Create(BI->getCondition(), BB1V, BB2V, 845 BB1V->getName()+"."+BB2V->getName(), NT); 846 // Make the PHI node use the select for all incoming values for BB1/BB2 847 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 848 if (PN->getIncomingBlock(i) == BB1 || PN->getIncomingBlock(i) == BB2) 849 PN->setIncomingValue(i, SI); 850 } 851 } 852 } 853 854 // Update any PHI nodes in our new successors. 855 for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) 856 AddPredecessorToBlock(*SI, BIParent, BB1); 857 858 EraseTerminatorInstAndDCECond(BI); 859 return true; 860} 861 862/// SpeculativelyExecuteBB - Given a conditional branch that goes to BB1 863/// and an BB2 and the only successor of BB1 is BB2, hoist simple code 864/// (for now, restricted to a single instruction that's side effect free) from 865/// the BB1 into the branch block to speculatively execute it. 866static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *BB1) { 867 // Only speculatively execution a single instruction (not counting the 868 // terminator) for now. 869 Instruction *HInst = NULL; 870 Instruction *Term = BB1->getTerminator(); 871 for (BasicBlock::iterator BBI = BB1->begin(), BBE = BB1->end(); 872 BBI != BBE; ++BBI) { 873 Instruction *I = BBI; 874 // Skip debug info. 875 if (isa<DbgInfoIntrinsic>(I)) continue; 876 if (I == Term) break; 877 878 if (!HInst) 879 HInst = I; 880 else 881 return false; 882 } 883 if (!HInst) 884 return false; 885 886 // Be conservative for now. FP select instruction can often be expensive. 887 Value *BrCond = BI->getCondition(); 888 if (isa<Instruction>(BrCond) && 889 cast<Instruction>(BrCond)->getOpcode() == Instruction::FCmp) 890 return false; 891 892 // If BB1 is actually on the false edge of the conditional branch, remember 893 // to swap the select operands later. 894 bool Invert = false; 895 if (BB1 != BI->getSuccessor(0)) { 896 assert(BB1 == BI->getSuccessor(1) && "No edge from 'if' block?"); 897 Invert = true; 898 } 899 900 // Turn 901 // BB: 902 // %t1 = icmp 903 // br i1 %t1, label %BB1, label %BB2 904 // BB1: 905 // %t3 = add %t2, c 906 // br label BB2 907 // BB2: 908 // => 909 // BB: 910 // %t1 = icmp 911 // %t4 = add %t2, c 912 // %t3 = select i1 %t1, %t2, %t3 913 switch (HInst->getOpcode()) { 914 default: return false; // Not safe / profitable to hoist. 915 case Instruction::Add: 916 case Instruction::Sub: 917 // Not worth doing for vector ops. 918 if (isa<VectorType>(HInst->getType())) 919 return false; 920 break; 921 case Instruction::And: 922 case Instruction::Or: 923 case Instruction::Xor: 924 case Instruction::Shl: 925 case Instruction::LShr: 926 case Instruction::AShr: 927 // Don't mess with vector operations. 928 if (isa<VectorType>(HInst->getType())) 929 return false; 930 break; // These are all cheap and non-trapping instructions. 931 } 932 933 // If the instruction is obviously dead, don't try to predicate it. 934 if (HInst->use_empty()) { 935 HInst->eraseFromParent(); 936 return true; 937 } 938 939 // Can we speculatively execute the instruction? And what is the value 940 // if the condition is false? Consider the phi uses, if the incoming value 941 // from the "if" block are all the same V, then V is the value of the 942 // select if the condition is false. 943 BasicBlock *BIParent = BI->getParent(); 944 SmallVector<PHINode*, 4> PHIUses; 945 Value *FalseV = NULL; 946 947 BasicBlock *BB2 = BB1->getTerminator()->getSuccessor(0); 948 for (Value::use_iterator UI = HInst->use_begin(), E = HInst->use_end(); 949 UI != E; ++UI) { 950 // Ignore any user that is not a PHI node in BB2. These can only occur in 951 // unreachable blocks, because they would not be dominated by the instr. 952 PHINode *PN = dyn_cast<PHINode>(UI); 953 if (!PN || PN->getParent() != BB2) 954 return false; 955 PHIUses.push_back(PN); 956 957 Value *PHIV = PN->getIncomingValueForBlock(BIParent); 958 if (!FalseV) 959 FalseV = PHIV; 960 else if (FalseV != PHIV) 961 return false; // Inconsistent value when condition is false. 962 } 963 964 assert(FalseV && "Must have at least one user, and it must be a PHI"); 965 966 // Do not hoist the instruction if any of its operands are defined but not 967 // used in this BB. The transformation will prevent the operand from 968 // being sunk into the use block. 969 for (User::op_iterator i = HInst->op_begin(), e = HInst->op_end(); 970 i != e; ++i) { 971 Instruction *OpI = dyn_cast<Instruction>(*i); 972 if (OpI && OpI->getParent() == BIParent && 973 !OpI->isUsedInBasicBlock(BIParent)) 974 return false; 975 } 976 977 // If we get here, we can hoist the instruction. Try to place it 978 // before the icmp instruction preceding the conditional branch. 979 BasicBlock::iterator InsertPos = BI; 980 if (InsertPos != BIParent->begin()) 981 --InsertPos; 982 // Skip debug info between condition and branch. 983 while (InsertPos != BIParent->begin() && isa<DbgInfoIntrinsic>(InsertPos)) 984 --InsertPos; 985 if (InsertPos == BrCond && !isa<PHINode>(BrCond)) { 986 SmallPtrSet<Instruction *, 4> BB1Insns; 987 for(BasicBlock::iterator BB1I = BB1->begin(), BB1E = BB1->end(); 988 BB1I != BB1E; ++BB1I) 989 BB1Insns.insert(BB1I); 990 for(Value::use_iterator UI = BrCond->use_begin(), UE = BrCond->use_end(); 991 UI != UE; ++UI) { 992 Instruction *Use = cast<Instruction>(*UI); 993 if (BB1Insns.count(Use)) { 994 // If BrCond uses the instruction that place it just before 995 // branch instruction. 996 InsertPos = BI; 997 break; 998 } 999 } 1000 } else 1001 InsertPos = BI; 1002 BIParent->getInstList().splice(InsertPos, BB1->getInstList(), HInst); 1003 1004 // Create a select whose true value is the speculatively executed value and 1005 // false value is the previously determined FalseV. 1006 SelectInst *SI; 1007 if (Invert) 1008 SI = SelectInst::Create(BrCond, FalseV, HInst, 1009 FalseV->getName() + "." + HInst->getName(), BI); 1010 else 1011 SI = SelectInst::Create(BrCond, HInst, FalseV, 1012 HInst->getName() + "." + FalseV->getName(), BI); 1013 1014 // Make the PHI node use the select for all incoming values for "then" and 1015 // "if" blocks. 1016 for (unsigned i = 0, e = PHIUses.size(); i != e; ++i) { 1017 PHINode *PN = PHIUses[i]; 1018 for (unsigned j = 0, ee = PN->getNumIncomingValues(); j != ee; ++j) 1019 if (PN->getIncomingBlock(j) == BB1 || 1020 PN->getIncomingBlock(j) == BIParent) 1021 PN->setIncomingValue(j, SI); 1022 } 1023 1024 ++NumSpeculations; 1025 return true; 1026} 1027 1028/// BlockIsSimpleEnoughToThreadThrough - Return true if we can thread a branch 1029/// across this block. 1030static bool BlockIsSimpleEnoughToThreadThrough(BasicBlock *BB) { 1031 BranchInst *BI = cast<BranchInst>(BB->getTerminator()); 1032 unsigned Size = 0; 1033 1034 for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI) { 1035 if (isa<DbgInfoIntrinsic>(BBI)) 1036 continue; 1037 if (Size > 10) return false; // Don't clone large BB's. 1038 ++Size; 1039 1040 // We can only support instructions that do not define values that are 1041 // live outside of the current basic block. 1042 for (Value::use_iterator UI = BBI->use_begin(), E = BBI->use_end(); 1043 UI != E; ++UI) { 1044 Instruction *U = cast<Instruction>(*UI); 1045 if (U->getParent() != BB || isa<PHINode>(U)) return false; 1046 } 1047 1048 // Looks ok, continue checking. 1049 } 1050 1051 return true; 1052} 1053 1054/// FoldCondBranchOnPHI - If we have a conditional branch on a PHI node value 1055/// that is defined in the same block as the branch and if any PHI entries are 1056/// constants, thread edges corresponding to that entry to be branches to their 1057/// ultimate destination. 1058static bool FoldCondBranchOnPHI(BranchInst *BI) { 1059 BasicBlock *BB = BI->getParent(); 1060 PHINode *PN = dyn_cast<PHINode>(BI->getCondition()); 1061 // NOTE: we currently cannot transform this case if the PHI node is used 1062 // outside of the block. 1063 if (!PN || PN->getParent() != BB || !PN->hasOneUse()) 1064 return false; 1065 1066 // Degenerate case of a single entry PHI. 1067 if (PN->getNumIncomingValues() == 1) { 1068 FoldSingleEntryPHINodes(PN->getParent()); 1069 return true; 1070 } 1071 1072 // Now we know that this block has multiple preds and two succs. 1073 if (!BlockIsSimpleEnoughToThreadThrough(BB)) return false; 1074 1075 // Okay, this is a simple enough basic block. See if any phi values are 1076 // constants. 1077 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 1078 ConstantInt *CB; 1079 if ((CB = dyn_cast<ConstantInt>(PN->getIncomingValue(i))) && 1080 CB->getType()->isIntegerTy(1)) { 1081 // Okay, we now know that all edges from PredBB should be revectored to 1082 // branch to RealDest. 1083 BasicBlock *PredBB = PN->getIncomingBlock(i); 1084 BasicBlock *RealDest = BI->getSuccessor(!CB->getZExtValue()); 1085 1086 if (RealDest == BB) continue; // Skip self loops. 1087 1088 // The dest block might have PHI nodes, other predecessors and other 1089 // difficult cases. Instead of being smart about this, just insert a new 1090 // block that jumps to the destination block, effectively splitting 1091 // the edge we are about to create. 1092 BasicBlock *EdgeBB = BasicBlock::Create(BB->getContext(), 1093 RealDest->getName()+".critedge", 1094 RealDest->getParent(), RealDest); 1095 BranchInst::Create(RealDest, EdgeBB); 1096 PHINode *PN; 1097 for (BasicBlock::iterator BBI = RealDest->begin(); 1098 (PN = dyn_cast<PHINode>(BBI)); ++BBI) { 1099 Value *V = PN->getIncomingValueForBlock(BB); 1100 PN->addIncoming(V, EdgeBB); 1101 } 1102 1103 // BB may have instructions that are being threaded over. Clone these 1104 // instructions into EdgeBB. We know that there will be no uses of the 1105 // cloned instructions outside of EdgeBB. 1106 BasicBlock::iterator InsertPt = EdgeBB->begin(); 1107 std::map<Value*, Value*> TranslateMap; // Track translated values. 1108 for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI) { 1109 if (PHINode *PN = dyn_cast<PHINode>(BBI)) { 1110 TranslateMap[PN] = PN->getIncomingValueForBlock(PredBB); 1111 } else { 1112 // Clone the instruction. 1113 Instruction *N = BBI->clone(); 1114 if (BBI->hasName()) N->setName(BBI->getName()+".c"); 1115 1116 // Update operands due to translation. 1117 for (User::op_iterator i = N->op_begin(), e = N->op_end(); 1118 i != e; ++i) { 1119 std::map<Value*, Value*>::iterator PI = 1120 TranslateMap.find(*i); 1121 if (PI != TranslateMap.end()) 1122 *i = PI->second; 1123 } 1124 1125 // Check for trivial simplification. 1126 if (Constant *C = ConstantFoldInstruction(N)) { 1127 TranslateMap[BBI] = C; 1128 delete N; // Constant folded away, don't need actual inst 1129 } else { 1130 // Insert the new instruction into its new home. 1131 EdgeBB->getInstList().insert(InsertPt, N); 1132 if (!BBI->use_empty()) 1133 TranslateMap[BBI] = N; 1134 } 1135 } 1136 } 1137 1138 // Loop over all of the edges from PredBB to BB, changing them to branch 1139 // to EdgeBB instead. 1140 TerminatorInst *PredBBTI = PredBB->getTerminator(); 1141 for (unsigned i = 0, e = PredBBTI->getNumSuccessors(); i != e; ++i) 1142 if (PredBBTI->getSuccessor(i) == BB) { 1143 BB->removePredecessor(PredBB); 1144 PredBBTI->setSuccessor(i, EdgeBB); 1145 } 1146 1147 // Recurse, simplifying any other constants. 1148 return FoldCondBranchOnPHI(BI) | true; 1149 } 1150 } 1151 1152 return false; 1153} 1154 1155/// FoldTwoEntryPHINode - Given a BB that starts with the specified two-entry 1156/// PHI node, see if we can eliminate it. 1157static bool FoldTwoEntryPHINode(PHINode *PN) { 1158 // Ok, this is a two entry PHI node. Check to see if this is a simple "if 1159 // statement", which has a very simple dominance structure. Basically, we 1160 // are trying to find the condition that is being branched on, which 1161 // subsequently causes this merge to happen. We really want control 1162 // dependence information for this check, but simplifycfg can't keep it up 1163 // to date, and this catches most of the cases we care about anyway. 1164 // 1165 BasicBlock *BB = PN->getParent(); 1166 BasicBlock *IfTrue, *IfFalse; 1167 Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse); 1168 if (!IfCond) return false; 1169 1170 // Okay, we found that we can merge this two-entry phi node into a select. 1171 // Doing so would require us to fold *all* two entry phi nodes in this block. 1172 // At some point this becomes non-profitable (particularly if the target 1173 // doesn't support cmov's). Only do this transformation if there are two or 1174 // fewer PHI nodes in this block. 1175 unsigned NumPhis = 0; 1176 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++NumPhis, ++I) 1177 if (NumPhis > 2) 1178 return false; 1179 1180 DEBUG(dbgs() << "FOUND IF CONDITION! " << *IfCond << " T: " 1181 << IfTrue->getName() << " F: " << IfFalse->getName() << "\n"); 1182 1183 // Loop over the PHI's seeing if we can promote them all to select 1184 // instructions. While we are at it, keep track of the instructions 1185 // that need to be moved to the dominating block. 1186 std::set<Instruction*> AggressiveInsts; 1187 1188 BasicBlock::iterator AfterPHIIt = BB->begin(); 1189 while (isa<PHINode>(AfterPHIIt)) { 1190 PHINode *PN = cast<PHINode>(AfterPHIIt++); 1191 if (PN->getIncomingValue(0) == PN->getIncomingValue(1)) { 1192 if (PN->getIncomingValue(0) != PN) 1193 PN->replaceAllUsesWith(PN->getIncomingValue(0)); 1194 else 1195 PN->replaceAllUsesWith(UndefValue::get(PN->getType())); 1196 } else if (!DominatesMergePoint(PN->getIncomingValue(0), BB, 1197 &AggressiveInsts) || 1198 !DominatesMergePoint(PN->getIncomingValue(1), BB, 1199 &AggressiveInsts)) { 1200 return false; 1201 } 1202 } 1203 1204 // If we all PHI nodes are promotable, check to make sure that all 1205 // instructions in the predecessor blocks can be promoted as well. If 1206 // not, we won't be able to get rid of the control flow, so it's not 1207 // worth promoting to select instructions. 1208 BasicBlock *DomBlock = 0, *IfBlock1 = 0, *IfBlock2 = 0; 1209 PN = cast<PHINode>(BB->begin()); 1210 BasicBlock *Pred = PN->getIncomingBlock(0); 1211 if (cast<BranchInst>(Pred->getTerminator())->isUnconditional()) { 1212 IfBlock1 = Pred; 1213 DomBlock = *pred_begin(Pred); 1214 for (BasicBlock::iterator I = Pred->begin(); 1215 !isa<TerminatorInst>(I); ++I) 1216 if (!AggressiveInsts.count(I) && !isa<DbgInfoIntrinsic>(I)) { 1217 // This is not an aggressive instruction that we can promote. 1218 // Because of this, we won't be able to get rid of the control 1219 // flow, so the xform is not worth it. 1220 return false; 1221 } 1222 } 1223 1224 Pred = PN->getIncomingBlock(1); 1225 if (cast<BranchInst>(Pred->getTerminator())->isUnconditional()) { 1226 IfBlock2 = Pred; 1227 DomBlock = *pred_begin(Pred); 1228 for (BasicBlock::iterator I = Pred->begin(); 1229 !isa<TerminatorInst>(I); ++I) 1230 if (!AggressiveInsts.count(I) && !isa<DbgInfoIntrinsic>(I)) { 1231 // This is not an aggressive instruction that we can promote. 1232 // Because of this, we won't be able to get rid of the control 1233 // flow, so the xform is not worth it. 1234 return false; 1235 } 1236 } 1237 1238 // If we can still promote the PHI nodes after this gauntlet of tests, 1239 // do all of the PHI's now. 1240 1241 // Move all 'aggressive' instructions, which are defined in the 1242 // conditional parts of the if's up to the dominating block. 1243 if (IfBlock1) { 1244 DomBlock->getInstList().splice(DomBlock->getTerminator(), 1245 IfBlock1->getInstList(), 1246 IfBlock1->begin(), 1247 IfBlock1->getTerminator()); 1248 } 1249 if (IfBlock2) { 1250 DomBlock->getInstList().splice(DomBlock->getTerminator(), 1251 IfBlock2->getInstList(), 1252 IfBlock2->begin(), 1253 IfBlock2->getTerminator()); 1254 } 1255 1256 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) { 1257 // Change the PHI node into a select instruction. 1258 Value *TrueVal = 1259 PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse); 1260 Value *FalseVal = 1261 PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue); 1262 1263 Value *NV = SelectInst::Create(IfCond, TrueVal, FalseVal, "", AfterPHIIt); 1264 PN->replaceAllUsesWith(NV); 1265 NV->takeName(PN); 1266 1267 BB->getInstList().erase(PN); 1268 } 1269 return true; 1270} 1271 1272/// isTerminatorFirstRelevantInsn - Return true if Term is very first 1273/// instruction ignoring Phi nodes and dbg intrinsics. 1274static bool isTerminatorFirstRelevantInsn(BasicBlock *BB, Instruction *Term) { 1275 BasicBlock::iterator BBI = Term; 1276 while (BBI != BB->begin()) { 1277 --BBI; 1278 if (!isa<DbgInfoIntrinsic>(BBI)) 1279 break; 1280 } 1281 1282 if (isa<PHINode>(BBI) || &*BBI == Term || isa<DbgInfoIntrinsic>(BBI)) 1283 return true; 1284 return false; 1285} 1286 1287/// SimplifyCondBranchToTwoReturns - If we found a conditional branch that goes 1288/// to two returning blocks, try to merge them together into one return, 1289/// introducing a select if the return values disagree. 1290static bool SimplifyCondBranchToTwoReturns(BranchInst *BI) { 1291 assert(BI->isConditional() && "Must be a conditional branch"); 1292 BasicBlock *TrueSucc = BI->getSuccessor(0); 1293 BasicBlock *FalseSucc = BI->getSuccessor(1); 1294 ReturnInst *TrueRet = cast<ReturnInst>(TrueSucc->getTerminator()); 1295 ReturnInst *FalseRet = cast<ReturnInst>(FalseSucc->getTerminator()); 1296 1297 // Check to ensure both blocks are empty (just a return) or optionally empty 1298 // with PHI nodes. If there are other instructions, merging would cause extra 1299 // computation on one path or the other. 1300 if (!isTerminatorFirstRelevantInsn(TrueSucc, TrueRet)) 1301 return false; 1302 if (!isTerminatorFirstRelevantInsn(FalseSucc, FalseRet)) 1303 return false; 1304 1305 // Okay, we found a branch that is going to two return nodes. If 1306 // there is no return value for this function, just change the 1307 // branch into a return. 1308 if (FalseRet->getNumOperands() == 0) { 1309 TrueSucc->removePredecessor(BI->getParent()); 1310 FalseSucc->removePredecessor(BI->getParent()); 1311 ReturnInst::Create(BI->getContext(), 0, BI); 1312 EraseTerminatorInstAndDCECond(BI); 1313 return true; 1314 } 1315 1316 // Otherwise, figure out what the true and false return values are 1317 // so we can insert a new select instruction. 1318 Value *TrueValue = TrueRet->getReturnValue(); 1319 Value *FalseValue = FalseRet->getReturnValue(); 1320 1321 // Unwrap any PHI nodes in the return blocks. 1322 if (PHINode *TVPN = dyn_cast_or_null<PHINode>(TrueValue)) 1323 if (TVPN->getParent() == TrueSucc) 1324 TrueValue = TVPN->getIncomingValueForBlock(BI->getParent()); 1325 if (PHINode *FVPN = dyn_cast_or_null<PHINode>(FalseValue)) 1326 if (FVPN->getParent() == FalseSucc) 1327 FalseValue = FVPN->getIncomingValueForBlock(BI->getParent()); 1328 1329 // In order for this transformation to be safe, we must be able to 1330 // unconditionally execute both operands to the return. This is 1331 // normally the case, but we could have a potentially-trapping 1332 // constant expression that prevents this transformation from being 1333 // safe. 1334 if (ConstantExpr *TCV = dyn_cast_or_null<ConstantExpr>(TrueValue)) 1335 if (TCV->canTrap()) 1336 return false; 1337 if (ConstantExpr *FCV = dyn_cast_or_null<ConstantExpr>(FalseValue)) 1338 if (FCV->canTrap()) 1339 return false; 1340 1341 // Okay, we collected all the mapped values and checked them for sanity, and 1342 // defined to really do this transformation. First, update the CFG. 1343 TrueSucc->removePredecessor(BI->getParent()); 1344 FalseSucc->removePredecessor(BI->getParent()); 1345 1346 // Insert select instructions where needed. 1347 Value *BrCond = BI->getCondition(); 1348 if (TrueValue) { 1349 // Insert a select if the results differ. 1350 if (TrueValue == FalseValue || isa<UndefValue>(FalseValue)) { 1351 } else if (isa<UndefValue>(TrueValue)) { 1352 TrueValue = FalseValue; 1353 } else { 1354 TrueValue = SelectInst::Create(BrCond, TrueValue, 1355 FalseValue, "retval", BI); 1356 } 1357 } 1358 1359 Value *RI = !TrueValue ? 1360 ReturnInst::Create(BI->getContext(), BI) : 1361 ReturnInst::Create(BI->getContext(), TrueValue, BI); 1362 (void) RI; 1363 1364 DEBUG(dbgs() << "\nCHANGING BRANCH TO TWO RETURNS INTO SELECT:" 1365 << "\n " << *BI << "NewRet = " << *RI 1366 << "TRUEBLOCK: " << *TrueSucc << "FALSEBLOCK: "<< *FalseSucc); 1367 1368 EraseTerminatorInstAndDCECond(BI); 1369 1370 return true; 1371} 1372 1373/// FoldBranchToCommonDest - If this basic block is ONLY a setcc and a branch, 1374/// and if a predecessor branches to us and one of our successors, fold the 1375/// setcc into the predecessor and use logical operations to pick the right 1376/// destination. 1377bool llvm::FoldBranchToCommonDest(BranchInst *BI) { 1378 BasicBlock *BB = BI->getParent(); 1379 Instruction *Cond = dyn_cast<Instruction>(BI->getCondition()); 1380 if (Cond == 0) return false; 1381 1382 1383 // Only allow this if the condition is a simple instruction that can be 1384 // executed unconditionally. It must be in the same block as the branch, and 1385 // must be at the front of the block. 1386 BasicBlock::iterator FrontIt = BB->front(); 1387 // Ignore dbg intrinsics. 1388 while(isa<DbgInfoIntrinsic>(FrontIt)) 1389 ++FrontIt; 1390 if ((!isa<CmpInst>(Cond) && !isa<BinaryOperator>(Cond)) || 1391 Cond->getParent() != BB || &*FrontIt != Cond || !Cond->hasOneUse()) { 1392 return false; 1393 } 1394 1395 // Make sure the instruction after the condition is the cond branch. 1396 BasicBlock::iterator CondIt = Cond; ++CondIt; 1397 // Ingore dbg intrinsics. 1398 while(isa<DbgInfoIntrinsic>(CondIt)) 1399 ++CondIt; 1400 if (&*CondIt != BI) { 1401 assert (!isa<DbgInfoIntrinsic>(CondIt) && "Hey do not forget debug info!"); 1402 return false; 1403 } 1404 1405 // Cond is known to be a compare or binary operator. Check to make sure that 1406 // neither operand is a potentially-trapping constant expression. 1407 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Cond->getOperand(0))) 1408 if (CE->canTrap()) 1409 return false; 1410 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Cond->getOperand(1))) 1411 if (CE->canTrap()) 1412 return false; 1413 1414 1415 // Finally, don't infinitely unroll conditional loops. 1416 BasicBlock *TrueDest = BI->getSuccessor(0); 1417 BasicBlock *FalseDest = BI->getSuccessor(1); 1418 if (TrueDest == BB || FalseDest == BB) 1419 return false; 1420 1421 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { 1422 BasicBlock *PredBlock = *PI; 1423 BranchInst *PBI = dyn_cast<BranchInst>(PredBlock->getTerminator()); 1424 1425 // Check that we have two conditional branches. If there is a PHI node in 1426 // the common successor, verify that the same value flows in from both 1427 // blocks. 1428 if (PBI == 0 || PBI->isUnconditional() || 1429 !SafeToMergeTerminators(BI, PBI)) 1430 continue; 1431 1432 Instruction::BinaryOps Opc; 1433 bool InvertPredCond = false; 1434 1435 if (PBI->getSuccessor(0) == TrueDest) 1436 Opc = Instruction::Or; 1437 else if (PBI->getSuccessor(1) == FalseDest) 1438 Opc = Instruction::And; 1439 else if (PBI->getSuccessor(0) == FalseDest) 1440 Opc = Instruction::And, InvertPredCond = true; 1441 else if (PBI->getSuccessor(1) == TrueDest) 1442 Opc = Instruction::Or, InvertPredCond = true; 1443 else 1444 continue; 1445 1446 DEBUG(dbgs() << "FOLDING BRANCH TO COMMON DEST:\n" << *PBI << *BB); 1447 1448 // If we need to invert the condition in the pred block to match, do so now. 1449 if (InvertPredCond) { 1450 Value *NewCond = 1451 BinaryOperator::CreateNot(PBI->getCondition(), 1452 PBI->getCondition()->getName()+".not", PBI); 1453 PBI->setCondition(NewCond); 1454 BasicBlock *OldTrue = PBI->getSuccessor(0); 1455 BasicBlock *OldFalse = PBI->getSuccessor(1); 1456 PBI->setSuccessor(0, OldFalse); 1457 PBI->setSuccessor(1, OldTrue); 1458 } 1459 1460 // Clone Cond into the predecessor basic block, and or/and the 1461 // two conditions together. 1462 Instruction *New = Cond->clone(); 1463 PredBlock->getInstList().insert(PBI, New); 1464 New->takeName(Cond); 1465 Cond->setName(New->getName()+".old"); 1466 1467 Value *NewCond = BinaryOperator::Create(Opc, PBI->getCondition(), 1468 New, "or.cond", PBI); 1469 PBI->setCondition(NewCond); 1470 if (PBI->getSuccessor(0) == BB) { 1471 AddPredecessorToBlock(TrueDest, PredBlock, BB); 1472 PBI->setSuccessor(0, TrueDest); 1473 } 1474 if (PBI->getSuccessor(1) == BB) { 1475 AddPredecessorToBlock(FalseDest, PredBlock, BB); 1476 PBI->setSuccessor(1, FalseDest); 1477 } 1478 return true; 1479 } 1480 return false; 1481} 1482 1483/// SimplifyCondBranchToCondBranch - If we have a conditional branch as a 1484/// predecessor of another block, this function tries to simplify it. We know 1485/// that PBI and BI are both conditional branches, and BI is in one of the 1486/// successor blocks of PBI - PBI branches to BI. 1487static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI) { 1488 assert(PBI->isConditional() && BI->isConditional()); 1489 BasicBlock *BB = BI->getParent(); 1490 1491 // If this block ends with a branch instruction, and if there is a 1492 // predecessor that ends on a branch of the same condition, make 1493 // this conditional branch redundant. 1494 if (PBI->getCondition() == BI->getCondition() && 1495 PBI->getSuccessor(0) != PBI->getSuccessor(1)) { 1496 // Okay, the outcome of this conditional branch is statically 1497 // knowable. If this block had a single pred, handle specially. 1498 if (BB->getSinglePredecessor()) { 1499 // Turn this into a branch on constant. 1500 bool CondIsTrue = PBI->getSuccessor(0) == BB; 1501 BI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()), 1502 CondIsTrue)); 1503 return true; // Nuke the branch on constant. 1504 } 1505 1506 // Otherwise, if there are multiple predecessors, insert a PHI that merges 1507 // in the constant and simplify the block result. Subsequent passes of 1508 // simplifycfg will thread the block. 1509 if (BlockIsSimpleEnoughToThreadThrough(BB)) { 1510 PHINode *NewPN = PHINode::Create(Type::getInt1Ty(BB->getContext()), 1511 BI->getCondition()->getName() + ".pr", 1512 BB->begin()); 1513 // Okay, we're going to insert the PHI node. Since PBI is not the only 1514 // predecessor, compute the PHI'd conditional value for all of the preds. 1515 // Any predecessor where the condition is not computable we keep symbolic. 1516 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) 1517 if ((PBI = dyn_cast<BranchInst>((*PI)->getTerminator())) && 1518 PBI != BI && PBI->isConditional() && 1519 PBI->getCondition() == BI->getCondition() && 1520 PBI->getSuccessor(0) != PBI->getSuccessor(1)) { 1521 bool CondIsTrue = PBI->getSuccessor(0) == BB; 1522 NewPN->addIncoming(ConstantInt::get(Type::getInt1Ty(BB->getContext()), 1523 CondIsTrue), *PI); 1524 } else { 1525 NewPN->addIncoming(BI->getCondition(), *PI); 1526 } 1527 1528 BI->setCondition(NewPN); 1529 return true; 1530 } 1531 } 1532 1533 // If this is a conditional branch in an empty block, and if any 1534 // predecessors is a conditional branch to one of our destinations, 1535 // fold the conditions into logical ops and one cond br. 1536 BasicBlock::iterator BBI = BB->begin(); 1537 // Ignore dbg intrinsics. 1538 while (isa<DbgInfoIntrinsic>(BBI)) 1539 ++BBI; 1540 if (&*BBI != BI) 1541 return false; 1542 1543 1544 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(BI->getCondition())) 1545 if (CE->canTrap()) 1546 return false; 1547 1548 int PBIOp, BIOp; 1549 if (PBI->getSuccessor(0) == BI->getSuccessor(0)) 1550 PBIOp = BIOp = 0; 1551 else if (PBI->getSuccessor(0) == BI->getSuccessor(1)) 1552 PBIOp = 0, BIOp = 1; 1553 else if (PBI->getSuccessor(1) == BI->getSuccessor(0)) 1554 PBIOp = 1, BIOp = 0; 1555 else if (PBI->getSuccessor(1) == BI->getSuccessor(1)) 1556 PBIOp = BIOp = 1; 1557 else 1558 return false; 1559 1560 // Check to make sure that the other destination of this branch 1561 // isn't BB itself. If so, this is an infinite loop that will 1562 // keep getting unwound. 1563 if (PBI->getSuccessor(PBIOp) == BB) 1564 return false; 1565 1566 // Do not perform this transformation if it would require 1567 // insertion of a large number of select instructions. For targets 1568 // without predication/cmovs, this is a big pessimization. 1569 BasicBlock *CommonDest = PBI->getSuccessor(PBIOp); 1570 1571 unsigned NumPhis = 0; 1572 for (BasicBlock::iterator II = CommonDest->begin(); 1573 isa<PHINode>(II); ++II, ++NumPhis) 1574 if (NumPhis > 2) // Disable this xform. 1575 return false; 1576 1577 // Finally, if everything is ok, fold the branches to logical ops. 1578 BasicBlock *OtherDest = BI->getSuccessor(BIOp ^ 1); 1579 1580 DEBUG(dbgs() << "FOLDING BRs:" << *PBI->getParent() 1581 << "AND: " << *BI->getParent()); 1582 1583 1584 // If OtherDest *is* BB, then BB is a basic block with a single conditional 1585 // branch in it, where one edge (OtherDest) goes back to itself but the other 1586 // exits. We don't *know* that the program avoids the infinite loop 1587 // (even though that seems likely). If we do this xform naively, we'll end up 1588 // recursively unpeeling the loop. Since we know that (after the xform is 1589 // done) that the block *is* infinite if reached, we just make it an obviously 1590 // infinite loop with no cond branch. 1591 if (OtherDest == BB) { 1592 // Insert it at the end of the function, because it's either code, 1593 // or it won't matter if it's hot. :) 1594 BasicBlock *InfLoopBlock = BasicBlock::Create(BB->getContext(), 1595 "infloop", BB->getParent()); 1596 BranchInst::Create(InfLoopBlock, InfLoopBlock); 1597 OtherDest = InfLoopBlock; 1598 } 1599 1600 DEBUG(dbgs() << *PBI->getParent()->getParent()); 1601 1602 // BI may have other predecessors. Because of this, we leave 1603 // it alone, but modify PBI. 1604 1605 // Make sure we get to CommonDest on True&True directions. 1606 Value *PBICond = PBI->getCondition(); 1607 if (PBIOp) 1608 PBICond = BinaryOperator::CreateNot(PBICond, 1609 PBICond->getName()+".not", 1610 PBI); 1611 Value *BICond = BI->getCondition(); 1612 if (BIOp) 1613 BICond = BinaryOperator::CreateNot(BICond, 1614 BICond->getName()+".not", 1615 PBI); 1616 // Merge the conditions. 1617 Value *Cond = BinaryOperator::CreateOr(PBICond, BICond, "brmerge", PBI); 1618 1619 // Modify PBI to branch on the new condition to the new dests. 1620 PBI->setCondition(Cond); 1621 PBI->setSuccessor(0, CommonDest); 1622 PBI->setSuccessor(1, OtherDest); 1623 1624 // OtherDest may have phi nodes. If so, add an entry from PBI's 1625 // block that are identical to the entries for BI's block. 1626 PHINode *PN; 1627 for (BasicBlock::iterator II = OtherDest->begin(); 1628 (PN = dyn_cast<PHINode>(II)); ++II) { 1629 Value *V = PN->getIncomingValueForBlock(BB); 1630 PN->addIncoming(V, PBI->getParent()); 1631 } 1632 1633 // We know that the CommonDest already had an edge from PBI to 1634 // it. If it has PHIs though, the PHIs may have different 1635 // entries for BB and PBI's BB. If so, insert a select to make 1636 // them agree. 1637 for (BasicBlock::iterator II = CommonDest->begin(); 1638 (PN = dyn_cast<PHINode>(II)); ++II) { 1639 Value *BIV = PN->getIncomingValueForBlock(BB); 1640 unsigned PBBIdx = PN->getBasicBlockIndex(PBI->getParent()); 1641 Value *PBIV = PN->getIncomingValue(PBBIdx); 1642 if (BIV != PBIV) { 1643 // Insert a select in PBI to pick the right value. 1644 Value *NV = SelectInst::Create(PBICond, PBIV, BIV, 1645 PBIV->getName()+".mux", PBI); 1646 PN->setIncomingValue(PBBIdx, NV); 1647 } 1648 } 1649 1650 DEBUG(dbgs() << "INTO: " << *PBI->getParent()); 1651 DEBUG(dbgs() << *PBI->getParent()->getParent()); 1652 1653 // This basic block is probably dead. We know it has at least 1654 // one fewer predecessor. 1655 return true; 1656} 1657 1658bool SimplifyCFGOpt::run(BasicBlock *BB) { 1659 bool Changed = false; 1660 Function *M = BB->getParent(); 1661 1662 assert(BB && BB->getParent() && "Block not embedded in function!"); 1663 assert(BB->getTerminator() && "Degenerate basic block encountered!"); 1664 assert(&BB->getParent()->getEntryBlock() != BB && 1665 "Can't Simplify entry block!"); 1666 1667 // Remove basic blocks that have no predecessors... or that just have themself 1668 // as a predecessor. These are unreachable. 1669 if (pred_begin(BB) == pred_end(BB) || BB->getSinglePredecessor() == BB) { 1670 DEBUG(dbgs() << "Removing BB: \n" << *BB); 1671 DeleteDeadBlock(BB); 1672 return true; 1673 } 1674 1675 // Check to see if we can constant propagate this terminator instruction 1676 // away... 1677 Changed |= ConstantFoldTerminator(BB); 1678 1679 // Check for and eliminate duplicate PHI nodes in this block. 1680 Changed |= EliminateDuplicatePHINodes(BB); 1681 1682 // If there is a trivial two-entry PHI node in this basic block, and we can 1683 // eliminate it, do so now. 1684 if (PHINode *PN = dyn_cast<PHINode>(BB->begin())) 1685 if (PN->getNumIncomingValues() == 2) 1686 Changed |= FoldTwoEntryPHINode(PN); 1687 1688 // If this is a returning block with only PHI nodes in it, fold the return 1689 // instruction into any unconditional branch predecessors. 1690 // 1691 // If any predecessor is a conditional branch that just selects among 1692 // different return values, fold the replace the branch/return with a select 1693 // and return. 1694 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) { 1695 if (isTerminatorFirstRelevantInsn(BB, BB->getTerminator())) { 1696 // Find predecessors that end with branches. 1697 SmallVector<BasicBlock*, 8> UncondBranchPreds; 1698 SmallVector<BranchInst*, 8> CondBranchPreds; 1699 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { 1700 TerminatorInst *PTI = (*PI)->getTerminator(); 1701 if (BranchInst *BI = dyn_cast<BranchInst>(PTI)) { 1702 if (BI->isUnconditional()) 1703 UncondBranchPreds.push_back(*PI); 1704 else 1705 CondBranchPreds.push_back(BI); 1706 } 1707 } 1708 1709 // If we found some, do the transformation! 1710 if (!UncondBranchPreds.empty()) { 1711 while (!UncondBranchPreds.empty()) { 1712 BasicBlock *Pred = UncondBranchPreds.pop_back_val(); 1713 DEBUG(dbgs() << "FOLDING: " << *BB 1714 << "INTO UNCOND BRANCH PRED: " << *Pred); 1715 Instruction *UncondBranch = Pred->getTerminator(); 1716 // Clone the return and add it to the end of the predecessor. 1717 Instruction *NewRet = RI->clone(); 1718 Pred->getInstList().push_back(NewRet); 1719 1720 // If the return instruction returns a value, and if the value was a 1721 // PHI node in "BB", propagate the right value into the return. 1722 for (User::op_iterator i = NewRet->op_begin(), e = NewRet->op_end(); 1723 i != e; ++i) 1724 if (PHINode *PN = dyn_cast<PHINode>(*i)) 1725 if (PN->getParent() == BB) 1726 *i = PN->getIncomingValueForBlock(Pred); 1727 1728 // Update any PHI nodes in the returning block to realize that we no 1729 // longer branch to them. 1730 BB->removePredecessor(Pred); 1731 Pred->getInstList().erase(UncondBranch); 1732 } 1733 1734 // If we eliminated all predecessors of the block, delete the block now. 1735 if (pred_begin(BB) == pred_end(BB)) 1736 // We know there are no successors, so just nuke the block. 1737 M->getBasicBlockList().erase(BB); 1738 1739 return true; 1740 } 1741 1742 // Check out all of the conditional branches going to this return 1743 // instruction. If any of them just select between returns, change the 1744 // branch itself into a select/return pair. 1745 while (!CondBranchPreds.empty()) { 1746 BranchInst *BI = CondBranchPreds.pop_back_val(); 1747 1748 // Check to see if the non-BB successor is also a return block. 1749 if (isa<ReturnInst>(BI->getSuccessor(0)->getTerminator()) && 1750 isa<ReturnInst>(BI->getSuccessor(1)->getTerminator()) && 1751 SimplifyCondBranchToTwoReturns(BI)) 1752 return true; 1753 } 1754 } 1755 } else if (isa<UnwindInst>(BB->begin())) { 1756 // Check to see if the first instruction in this block is just an unwind. 1757 // If so, replace any invoke instructions which use this as an exception 1758 // destination with call instructions. 1759 // 1760 SmallVector<BasicBlock*, 8> Preds(pred_begin(BB), pred_end(BB)); 1761 while (!Preds.empty()) { 1762 BasicBlock *Pred = Preds.back(); 1763 if (InvokeInst *II = dyn_cast<InvokeInst>(Pred->getTerminator())) 1764 if (II->getUnwindDest() == BB) { 1765 // Insert a new branch instruction before the invoke, because this 1766 // is now a fall through. 1767 BranchInst *BI = BranchInst::Create(II->getNormalDest(), II); 1768 Pred->getInstList().remove(II); // Take out of symbol table 1769 1770 // Insert the call now. 1771 SmallVector<Value*,8> Args(II->op_begin()+3, II->op_end()); 1772 CallInst *CI = CallInst::Create(II->getCalledValue(), 1773 Args.begin(), Args.end(), 1774 II->getName(), BI); 1775 CI->setCallingConv(II->getCallingConv()); 1776 CI->setAttributes(II->getAttributes()); 1777 // If the invoke produced a value, the Call now does instead. 1778 II->replaceAllUsesWith(CI); 1779 delete II; 1780 Changed = true; 1781 } 1782 1783 Preds.pop_back(); 1784 } 1785 1786 // If this block is now dead, remove it. 1787 if (pred_begin(BB) == pred_end(BB)) { 1788 // We know there are no successors, so just nuke the block. 1789 M->getBasicBlockList().erase(BB); 1790 return true; 1791 } 1792 1793 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) { 1794 if (isValueEqualityComparison(SI)) { 1795 // If we only have one predecessor, and if it is a branch on this value, 1796 // see if that predecessor totally determines the outcome of this switch. 1797 if (BasicBlock *OnlyPred = BB->getSinglePredecessor()) 1798 if (SimplifyEqualityComparisonWithOnlyPredecessor(SI, OnlyPred)) 1799 return SimplifyCFG(BB) || 1; 1800 1801 // If the block only contains the switch, see if we can fold the block 1802 // away into any preds. 1803 BasicBlock::iterator BBI = BB->begin(); 1804 // Ignore dbg intrinsics. 1805 while (isa<DbgInfoIntrinsic>(BBI)) 1806 ++BBI; 1807 if (SI == &*BBI) 1808 if (FoldValueComparisonIntoPredecessors(SI)) 1809 return SimplifyCFG(BB) || 1; 1810 } 1811 } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) { 1812 if (BI->isUnconditional()) { 1813 BasicBlock::iterator BBI = BB->getFirstNonPHI(); 1814 1815 // Ignore dbg intrinsics. 1816 while (isa<DbgInfoIntrinsic>(BBI)) 1817 ++BBI; 1818 if (BBI->isTerminator()) // Terminator is the only non-phi instruction! 1819 if (TryToSimplifyUncondBranchFromEmptyBlock(BB)) 1820 return true; 1821 1822 } else { // Conditional branch 1823 if (isValueEqualityComparison(BI)) { 1824 // If we only have one predecessor, and if it is a branch on this value, 1825 // see if that predecessor totally determines the outcome of this 1826 // switch. 1827 if (BasicBlock *OnlyPred = BB->getSinglePredecessor()) 1828 if (SimplifyEqualityComparisonWithOnlyPredecessor(BI, OnlyPred)) 1829 return SimplifyCFG(BB) || 1; 1830 1831 // This block must be empty, except for the setcond inst, if it exists. 1832 // Ignore dbg intrinsics. 1833 BasicBlock::iterator I = BB->begin(); 1834 // Ignore dbg intrinsics. 1835 while (isa<DbgInfoIntrinsic>(I)) 1836 ++I; 1837 if (&*I == BI) { 1838 if (FoldValueComparisonIntoPredecessors(BI)) 1839 return SimplifyCFG(BB) | true; 1840 } else if (&*I == cast<Instruction>(BI->getCondition())){ 1841 ++I; 1842 // Ignore dbg intrinsics. 1843 while (isa<DbgInfoIntrinsic>(I)) 1844 ++I; 1845 if(&*I == BI) { 1846 if (FoldValueComparisonIntoPredecessors(BI)) 1847 return SimplifyCFG(BB) | true; 1848 } 1849 } 1850 } 1851 1852 // If this is a branch on a phi node in the current block, thread control 1853 // through this block if any PHI node entries are constants. 1854 if (PHINode *PN = dyn_cast<PHINode>(BI->getCondition())) 1855 if (PN->getParent() == BI->getParent()) 1856 if (FoldCondBranchOnPHI(BI)) 1857 return SimplifyCFG(BB) | true; 1858 1859 // If this basic block is ONLY a setcc and a branch, and if a predecessor 1860 // branches to us and one of our successors, fold the setcc into the 1861 // predecessor and use logical operations to pick the right destination. 1862 if (FoldBranchToCommonDest(BI)) 1863 return SimplifyCFG(BB) | 1; 1864 1865 1866 // Scan predecessor blocks for conditional branches. 1867 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) 1868 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator())) 1869 if (PBI != BI && PBI->isConditional()) 1870 if (SimplifyCondBranchToCondBranch(PBI, BI)) 1871 return SimplifyCFG(BB) | true; 1872 } 1873 } else if (isa<UnreachableInst>(BB->getTerminator())) { 1874 // If there are any instructions immediately before the unreachable that can 1875 // be removed, do so. 1876 Instruction *Unreachable = BB->getTerminator(); 1877 while (Unreachable != BB->begin()) { 1878 BasicBlock::iterator BBI = Unreachable; 1879 --BBI; 1880 // Do not delete instructions that can have side effects, like calls 1881 // (which may never return) and volatile loads and stores. 1882 if (isa<CallInst>(BBI) && !isa<DbgInfoIntrinsic>(BBI)) break; 1883 1884 if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) 1885 if (SI->isVolatile()) 1886 break; 1887 1888 if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) 1889 if (LI->isVolatile()) 1890 break; 1891 1892 // Delete this instruction 1893 BB->getInstList().erase(BBI); 1894 Changed = true; 1895 } 1896 1897 // If the unreachable instruction is the first in the block, take a gander 1898 // at all of the predecessors of this instruction, and simplify them. 1899 if (&BB->front() == Unreachable) { 1900 SmallVector<BasicBlock*, 8> Preds(pred_begin(BB), pred_end(BB)); 1901 for (unsigned i = 0, e = Preds.size(); i != e; ++i) { 1902 TerminatorInst *TI = Preds[i]->getTerminator(); 1903 1904 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) { 1905 if (BI->isUnconditional()) { 1906 if (BI->getSuccessor(0) == BB) { 1907 new UnreachableInst(TI->getContext(), TI); 1908 TI->eraseFromParent(); 1909 Changed = true; 1910 } 1911 } else { 1912 if (BI->getSuccessor(0) == BB) { 1913 BranchInst::Create(BI->getSuccessor(1), BI); 1914 EraseTerminatorInstAndDCECond(BI); 1915 } else if (BI->getSuccessor(1) == BB) { 1916 BranchInst::Create(BI->getSuccessor(0), BI); 1917 EraseTerminatorInstAndDCECond(BI); 1918 Changed = true; 1919 } 1920 } 1921 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) { 1922 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) 1923 if (SI->getSuccessor(i) == BB) { 1924 BB->removePredecessor(SI->getParent()); 1925 SI->removeCase(i); 1926 --i; --e; 1927 Changed = true; 1928 } 1929 // If the default value is unreachable, figure out the most popular 1930 // destination and make it the default. 1931 if (SI->getSuccessor(0) == BB) { 1932 std::map<BasicBlock*, unsigned> Popularity; 1933 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) 1934 Popularity[SI->getSuccessor(i)]++; 1935 1936 // Find the most popular block. 1937 unsigned MaxPop = 0; 1938 BasicBlock *MaxBlock = 0; 1939 for (std::map<BasicBlock*, unsigned>::iterator 1940 I = Popularity.begin(), E = Popularity.end(); I != E; ++I) { 1941 if (I->second > MaxPop) { 1942 MaxPop = I->second; 1943 MaxBlock = I->first; 1944 } 1945 } 1946 if (MaxBlock) { 1947 // Make this the new default, allowing us to delete any explicit 1948 // edges to it. 1949 SI->setSuccessor(0, MaxBlock); 1950 Changed = true; 1951 1952 // If MaxBlock has phinodes in it, remove MaxPop-1 entries from 1953 // it. 1954 if (isa<PHINode>(MaxBlock->begin())) 1955 for (unsigned i = 0; i != MaxPop-1; ++i) 1956 MaxBlock->removePredecessor(SI->getParent()); 1957 1958 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) 1959 if (SI->getSuccessor(i) == MaxBlock) { 1960 SI->removeCase(i); 1961 --i; --e; 1962 } 1963 } 1964 } 1965 } else if (InvokeInst *II = dyn_cast<InvokeInst>(TI)) { 1966 if (II->getUnwindDest() == BB) { 1967 // Convert the invoke to a call instruction. This would be a good 1968 // place to note that the call does not throw though. 1969 BranchInst *BI = BranchInst::Create(II->getNormalDest(), II); 1970 II->removeFromParent(); // Take out of symbol table 1971 1972 // Insert the call now... 1973 SmallVector<Value*, 8> Args(II->op_begin()+3, II->op_end()); 1974 CallInst *CI = CallInst::Create(II->getCalledValue(), 1975 Args.begin(), Args.end(), 1976 II->getName(), BI); 1977 CI->setCallingConv(II->getCallingConv()); 1978 CI->setAttributes(II->getAttributes()); 1979 // If the invoke produced a value, the Call does now instead. 1980 II->replaceAllUsesWith(CI); 1981 delete II; 1982 Changed = true; 1983 } 1984 } 1985 } 1986 1987 // If this block is now dead, remove it. 1988 if (pred_begin(BB) == pred_end(BB)) { 1989 // We know there are no successors, so just nuke the block. 1990 M->getBasicBlockList().erase(BB); 1991 return true; 1992 } 1993 } 1994 } 1995 1996 // Merge basic blocks into their predecessor if there is only one distinct 1997 // pred, and if there is only one distinct successor of the predecessor, and 1998 // if there are no PHI nodes. 1999 // 2000 if (MergeBlockIntoPredecessor(BB)) 2001 return true; 2002 2003 // Otherwise, if this block only has a single predecessor, and if that block 2004 // is a conditional branch, see if we can hoist any code from this block up 2005 // into our predecessor. 2006 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB)); 2007 BasicBlock *OnlyPred = *PI++; 2008 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same 2009 if (*PI != OnlyPred) { 2010 OnlyPred = 0; // There are multiple different predecessors... 2011 break; 2012 } 2013 2014 if (OnlyPred) 2015 if (BranchInst *BI = dyn_cast<BranchInst>(OnlyPred->getTerminator())) 2016 if (BI->isConditional()) { 2017 // Get the other block. 2018 BasicBlock *OtherBB = BI->getSuccessor(BI->getSuccessor(0) == BB); 2019 PI = pred_begin(OtherBB); 2020 ++PI; 2021 2022 if (PI == pred_end(OtherBB)) { 2023 // We have a conditional branch to two blocks that are only reachable 2024 // from the condbr. We know that the condbr dominates the two blocks, 2025 // so see if there is any identical code in the "then" and "else" 2026 // blocks. If so, we can hoist it up to the branching block. 2027 Changed |= HoistThenElseCodeToIf(BI); 2028 } else { 2029 BasicBlock* OnlySucc = NULL; 2030 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); 2031 SI != SE; ++SI) { 2032 if (!OnlySucc) 2033 OnlySucc = *SI; 2034 else if (*SI != OnlySucc) { 2035 OnlySucc = 0; // There are multiple distinct successors! 2036 break; 2037 } 2038 } 2039 2040 if (OnlySucc == OtherBB) { 2041 // If BB's only successor is the other successor of the predecessor, 2042 // i.e. a triangle, see if we can hoist any code from this block up 2043 // to the "if" block. 2044 Changed |= SpeculativelyExecuteBB(BI, BB); 2045 } 2046 } 2047 } 2048 2049 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) 2050 if (BranchInst *BI = dyn_cast<BranchInst>((*PI)->getTerminator())) 2051 // Change br (X == 0 | X == 1), T, F into a switch instruction. 2052 if (BI->isConditional() && isa<Instruction>(BI->getCondition())) { 2053 Instruction *Cond = cast<Instruction>(BI->getCondition()); 2054 // If this is a bunch of seteq's or'd together, or if it's a bunch of 2055 // 'setne's and'ed together, collect them. 2056 Value *CompVal = 0; 2057 std::vector<ConstantInt*> Values; 2058 bool TrueWhenEqual = GatherValueComparisons(Cond, CompVal, Values); 2059 if (CompVal) { 2060 // There might be duplicate constants in the list, which the switch 2061 // instruction can't handle, remove them now. 2062 std::sort(Values.begin(), Values.end(), ConstantIntOrdering()); 2063 Values.erase(std::unique(Values.begin(), Values.end()), Values.end()); 2064 2065 // Figure out which block is which destination. 2066 BasicBlock *DefaultBB = BI->getSuccessor(1); 2067 BasicBlock *EdgeBB = BI->getSuccessor(0); 2068 if (!TrueWhenEqual) std::swap(DefaultBB, EdgeBB); 2069 2070 // Convert pointer to int before we switch. 2071 if (isa<PointerType>(CompVal->getType())) { 2072 assert(TD && "Cannot switch on pointer without TargetData"); 2073 CompVal = new PtrToIntInst(CompVal, 2074 TD->getIntPtrType(CompVal->getContext()), 2075 "magicptr", BI); 2076 } 2077 2078 // Create the new switch instruction now. 2079 SwitchInst *New = SwitchInst::Create(CompVal, DefaultBB, 2080 Values.size(), BI); 2081 2082 // Add all of the 'cases' to the switch instruction. 2083 for (unsigned i = 0, e = Values.size(); i != e; ++i) 2084 New->addCase(Values[i], EdgeBB); 2085 2086 // We added edges from PI to the EdgeBB. As such, if there were any 2087 // PHI nodes in EdgeBB, they need entries to be added corresponding to 2088 // the number of edges added. 2089 for (BasicBlock::iterator BBI = EdgeBB->begin(); 2090 isa<PHINode>(BBI); ++BBI) { 2091 PHINode *PN = cast<PHINode>(BBI); 2092 Value *InVal = PN->getIncomingValueForBlock(*PI); 2093 for (unsigned i = 0, e = Values.size()-1; i != e; ++i) 2094 PN->addIncoming(InVal, *PI); 2095 } 2096 2097 // Erase the old branch instruction. 2098 EraseTerminatorInstAndDCECond(BI); 2099 return true; 2100 } 2101 } 2102 2103 return Changed; 2104} 2105 2106/// SimplifyCFG - This function is used to do simplification of a CFG. For 2107/// example, it adjusts branches to branches to eliminate the extra hop, it 2108/// eliminates unreachable basic blocks, and does other "peephole" optimization 2109/// of the CFG. It returns true if a modification was made. 2110/// 2111/// WARNING: The entry node of a function may not be simplified. 2112/// 2113bool llvm::SimplifyCFG(BasicBlock *BB, const TargetData *TD) { 2114 return SimplifyCFGOpt(TD).run(BB); 2115} 2116