SimplifyCFG.cpp revision 0d45a096cff7de5b487f7f7aac17684945dd0b93
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 "llvm/Transforms/Utils/BasicBlockUtils.h" 22#include <algorithm> 23#include <functional> 24#include <set> 25#include <map> 26using namespace llvm; 27 28/// SafeToMergeTerminators - Return true if it is safe to merge these two 29/// terminator instructions together. 30/// 31static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) { 32 if (SI1 == SI2) return false; // Can't merge with self! 33 34 // It is not safe to merge these two switch instructions if they have a common 35 // successor, and if that successor has a PHI node, and if *that* PHI node has 36 // conflicting incoming values from the two switch blocks. 37 BasicBlock *SI1BB = SI1->getParent(); 38 BasicBlock *SI2BB = SI2->getParent(); 39 std::set<BasicBlock*> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB)); 40 41 for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I) 42 if (SI1Succs.count(*I)) 43 for (BasicBlock::iterator BBI = (*I)->begin(); 44 isa<PHINode>(BBI); ++BBI) { 45 PHINode *PN = cast<PHINode>(BBI); 46 if (PN->getIncomingValueForBlock(SI1BB) != 47 PN->getIncomingValueForBlock(SI2BB)) 48 return false; 49 } 50 51 return true; 52} 53 54/// AddPredecessorToBlock - Update PHI nodes in Succ to indicate that there will 55/// now be entries in it from the 'NewPred' block. The values that will be 56/// flowing into the PHI nodes will be the same as those coming in from 57/// ExistPred, an existing predecessor of Succ. 58static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred, 59 BasicBlock *ExistPred) { 60 assert(std::find(succ_begin(ExistPred), succ_end(ExistPred), Succ) != 61 succ_end(ExistPred) && "ExistPred is not a predecessor of Succ!"); 62 if (!isa<PHINode>(Succ->begin())) return; // Quick exit if nothing to do 63 64 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) { 65 PHINode *PN = cast<PHINode>(I); 66 Value *V = PN->getIncomingValueForBlock(ExistPred); 67 PN->addIncoming(V, NewPred); 68 } 69} 70 71// CanPropagatePredecessorsForPHIs - Return true if we can fold BB, an 72// almost-empty BB ending in an unconditional branch to Succ, into succ. 73// 74// Assumption: Succ is the single successor for BB. 75// 76static bool CanPropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) { 77 assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!"); 78 79 // Check to see if one of the predecessors of BB is already a predecessor of 80 // Succ. If so, we cannot do the transformation if there are any PHI nodes 81 // with incompatible values coming in from the two edges! 82 // 83 if (isa<PHINode>(Succ->front())) { 84 std::set<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB)); 85 for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ); 86 PI != PE; ++PI) 87 if (std::find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end()) { 88 // Loop over all of the PHI nodes checking to see if there are 89 // incompatible values coming in. 90 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) { 91 PHINode *PN = cast<PHINode>(I); 92 // Loop up the entries in the PHI node for BB and for *PI if the 93 // values coming in are non-equal, we cannot merge these two blocks 94 // (instead we should insert a conditional move or something, then 95 // merge the blocks). 96 if (PN->getIncomingValueForBlock(BB) != 97 PN->getIncomingValueForBlock(*PI)) 98 return false; // Values are not equal... 99 } 100 } 101 } 102 103 // Finally, if BB has PHI nodes that are used by things other than the PHIs in 104 // Succ and Succ has predecessors that are not Succ and not Pred, we cannot 105 // fold these blocks, as we don't know whether BB dominates Succ or not to 106 // update the PHI nodes correctly. 107 if (!isa<PHINode>(BB->begin()) || Succ->getSinglePredecessor()) return true; 108 109 // If the predecessors of Succ are only BB and Succ itself, we can handle this. 110 bool IsSafe = true; 111 for (pred_iterator PI = pred_begin(Succ), E = pred_end(Succ); PI != E; ++PI) 112 if (*PI != Succ && *PI != BB) { 113 IsSafe = false; 114 break; 115 } 116 if (IsSafe) return true; 117 118 // If the PHI nodes in BB are only used by instructions in Succ, we are ok if 119 // BB and Succ have no common predecessors. 120 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I) { 121 PHINode *PN = cast<PHINode>(I); 122 for (Value::use_iterator UI = PN->use_begin(), E = PN->use_end(); UI != E; 123 ++UI) 124 if (cast<Instruction>(*UI)->getParent() != Succ) 125 return false; 126 } 127 128 // Scan the predecessor sets of BB and Succ, making sure there are no common 129 // predecessors. Common predecessors would cause us to build a phi node with 130 // differing incoming values, which is not legal. 131 std::set<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB)); 132 for (pred_iterator PI = pred_begin(Succ), E = pred_end(Succ); PI != E; ++PI) 133 if (BBPreds.count(*PI)) 134 return false; 135 136 return true; 137} 138 139/// TryToSimplifyUncondBranchFromEmptyBlock - BB contains an unconditional 140/// branch to Succ, and contains no instructions other than PHI nodes and the 141/// branch. If possible, eliminate BB. 142static bool TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB, 143 BasicBlock *Succ) { 144 // If our successor has PHI nodes, then we need to update them to include 145 // entries for BB's predecessors, not for BB itself. Be careful though, 146 // if this transformation fails (returns true) then we cannot do this 147 // transformation! 148 // 149 if (!CanPropagatePredecessorsForPHIs(BB, Succ)) return false; 150 151 DOUT << "Killing Trivial BB: \n" << *BB; 152 153 if (isa<PHINode>(Succ->begin())) { 154 // If there is more than one pred of succ, and there are PHI nodes in 155 // the successor, then we need to add incoming edges for the PHI nodes 156 // 157 const std::vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB)); 158 159 // Loop over all of the PHI nodes in the successor of BB. 160 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) { 161 PHINode *PN = cast<PHINode>(I); 162 Value *OldVal = PN->removeIncomingValue(BB, false); 163 assert(OldVal && "No entry in PHI for Pred BB!"); 164 165 // If this incoming value is one of the PHI nodes in BB, the new entries 166 // in the PHI node are the entries from the old PHI. 167 if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) { 168 PHINode *OldValPN = cast<PHINode>(OldVal); 169 for (unsigned i = 0, e = OldValPN->getNumIncomingValues(); i != e; ++i) 170 PN->addIncoming(OldValPN->getIncomingValue(i), 171 OldValPN->getIncomingBlock(i)); 172 } else { 173 for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(), 174 End = BBPreds.end(); PredI != End; ++PredI) { 175 // Add an incoming value for each of the new incoming values... 176 PN->addIncoming(OldVal, *PredI); 177 } 178 } 179 } 180 } 181 182 if (isa<PHINode>(&BB->front())) { 183 std::vector<BasicBlock*> 184 OldSuccPreds(pred_begin(Succ), pred_end(Succ)); 185 186 // Move all PHI nodes in BB to Succ if they are alive, otherwise 187 // delete them. 188 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) 189 if (PN->use_empty()) { 190 // Just remove the dead phi. This happens if Succ's PHIs were the only 191 // users of the PHI nodes. 192 PN->eraseFromParent(); 193 } else { 194 // The instruction is alive, so this means that Succ must have 195 // *ONLY* had BB as a predecessor, and the PHI node is still valid 196 // now. Simply move it into Succ, because we know that BB 197 // strictly dominated Succ. 198 Succ->getInstList().splice(Succ->begin(), 199 BB->getInstList(), BB->begin()); 200 201 // We need to add new entries for the PHI node to account for 202 // predecessors of Succ that the PHI node does not take into 203 // account. At this point, since we know that BB dominated succ, 204 // this means that we should any newly added incoming edges should 205 // use the PHI node as the value for these edges, because they are 206 // loop back edges. 207 for (unsigned i = 0, e = OldSuccPreds.size(); i != e; ++i) 208 if (OldSuccPreds[i] != BB) 209 PN->addIncoming(PN, OldSuccPreds[i]); 210 } 211 } 212 213 // Everything that jumped to BB now goes to Succ. 214 std::string OldName = BB->getName(); 215 BB->replaceAllUsesWith(Succ); 216 BB->eraseFromParent(); // Delete the old basic block. 217 218 if (!OldName.empty() && !Succ->hasName()) // Transfer name if we can 219 Succ->setName(OldName); 220 return true; 221} 222 223/// GetIfCondition - Given a basic block (BB) with two predecessors (and 224/// presumably PHI nodes in it), check to see if the merge at this block is due 225/// to an "if condition". If so, return the boolean condition that determines 226/// which entry into BB will be taken. Also, return by references the block 227/// that will be entered from if the condition is true, and the block that will 228/// be entered if the condition is false. 229/// 230/// 231static Value *GetIfCondition(BasicBlock *BB, 232 BasicBlock *&IfTrue, BasicBlock *&IfFalse) { 233 assert(std::distance(pred_begin(BB), pred_end(BB)) == 2 && 234 "Function can only handle blocks with 2 predecessors!"); 235 BasicBlock *Pred1 = *pred_begin(BB); 236 BasicBlock *Pred2 = *++pred_begin(BB); 237 238 // We can only handle branches. Other control flow will be lowered to 239 // branches if possible anyway. 240 if (!isa<BranchInst>(Pred1->getTerminator()) || 241 !isa<BranchInst>(Pred2->getTerminator())) 242 return 0; 243 BranchInst *Pred1Br = cast<BranchInst>(Pred1->getTerminator()); 244 BranchInst *Pred2Br = cast<BranchInst>(Pred2->getTerminator()); 245 246 // Eliminate code duplication by ensuring that Pred1Br is conditional if 247 // either are. 248 if (Pred2Br->isConditional()) { 249 // If both branches are conditional, we don't have an "if statement". In 250 // reality, we could transform this case, but since the condition will be 251 // required anyway, we stand no chance of eliminating it, so the xform is 252 // probably not profitable. 253 if (Pred1Br->isConditional()) 254 return 0; 255 256 std::swap(Pred1, Pred2); 257 std::swap(Pred1Br, Pred2Br); 258 } 259 260 if (Pred1Br->isConditional()) { 261 // If we found a conditional branch predecessor, make sure that it branches 262 // to BB and Pred2Br. If it doesn't, this isn't an "if statement". 263 if (Pred1Br->getSuccessor(0) == BB && 264 Pred1Br->getSuccessor(1) == Pred2) { 265 IfTrue = Pred1; 266 IfFalse = Pred2; 267 } else if (Pred1Br->getSuccessor(0) == Pred2 && 268 Pred1Br->getSuccessor(1) == BB) { 269 IfTrue = Pred2; 270 IfFalse = Pred1; 271 } else { 272 // We know that one arm of the conditional goes to BB, so the other must 273 // go somewhere unrelated, and this must not be an "if statement". 274 return 0; 275 } 276 277 // The only thing we have to watch out for here is to make sure that Pred2 278 // doesn't have incoming edges from other blocks. If it does, the condition 279 // doesn't dominate BB. 280 if (++pred_begin(Pred2) != pred_end(Pred2)) 281 return 0; 282 283 return Pred1Br->getCondition(); 284 } 285 286 // Ok, if we got here, both predecessors end with an unconditional branch to 287 // BB. Don't panic! If both blocks only have a single (identical) 288 // predecessor, and THAT is a conditional branch, then we're all ok! 289 if (pred_begin(Pred1) == pred_end(Pred1) || 290 ++pred_begin(Pred1) != pred_end(Pred1) || 291 pred_begin(Pred2) == pred_end(Pred2) || 292 ++pred_begin(Pred2) != pred_end(Pred2) || 293 *pred_begin(Pred1) != *pred_begin(Pred2)) 294 return 0; 295 296 // Otherwise, if this is a conditional branch, then we can use it! 297 BasicBlock *CommonPred = *pred_begin(Pred1); 298 if (BranchInst *BI = dyn_cast<BranchInst>(CommonPred->getTerminator())) { 299 assert(BI->isConditional() && "Two successors but not conditional?"); 300 if (BI->getSuccessor(0) == Pred1) { 301 IfTrue = Pred1; 302 IfFalse = Pred2; 303 } else { 304 IfTrue = Pred2; 305 IfFalse = Pred1; 306 } 307 return BI->getCondition(); 308 } 309 return 0; 310} 311 312 313// If we have a merge point of an "if condition" as accepted above, return true 314// if the specified value dominates the block. We don't handle the true 315// generality of domination here, just a special case which works well enough 316// for us. 317// 318// If AggressiveInsts is non-null, and if V does not dominate BB, we check to 319// see if V (which must be an instruction) is cheap to compute and is 320// non-trapping. If both are true, the instruction is inserted into the set and 321// true is returned. 322static bool DominatesMergePoint(Value *V, BasicBlock *BB, 323 std::set<Instruction*> *AggressiveInsts) { 324 Instruction *I = dyn_cast<Instruction>(V); 325 if (!I) { 326 // Non-instructions all dominate instructions, but not all constantexprs 327 // can be executed unconditionally. 328 if (ConstantExpr *C = dyn_cast<ConstantExpr>(V)) 329 if (C->canTrap()) 330 return false; 331 return true; 332 } 333 BasicBlock *PBB = I->getParent(); 334 335 // We don't want to allow weird loops that might have the "if condition" in 336 // the bottom of this block. 337 if (PBB == BB) return false; 338 339 // If this instruction is defined in a block that contains an unconditional 340 // branch to BB, then it must be in the 'conditional' part of the "if 341 // statement". 342 if (BranchInst *BI = dyn_cast<BranchInst>(PBB->getTerminator())) 343 if (BI->isUnconditional() && BI->getSuccessor(0) == BB) { 344 if (!AggressiveInsts) return false; 345 // Okay, it looks like the instruction IS in the "condition". Check to 346 // see if its a cheap instruction to unconditionally compute, and if it 347 // only uses stuff defined outside of the condition. If so, hoist it out. 348 switch (I->getOpcode()) { 349 default: return false; // Cannot hoist this out safely. 350 case Instruction::Load: 351 // We can hoist loads that are non-volatile and obviously cannot trap. 352 if (cast<LoadInst>(I)->isVolatile()) 353 return false; 354 if (!isa<AllocaInst>(I->getOperand(0)) && 355 !isa<Constant>(I->getOperand(0))) 356 return false; 357 358 // Finally, we have to check to make sure there are no instructions 359 // before the load in its basic block, as we are going to hoist the loop 360 // out to its predecessor. 361 if (PBB->begin() != BasicBlock::iterator(I)) 362 return false; 363 break; 364 case Instruction::Add: 365 case Instruction::Sub: 366 case Instruction::And: 367 case Instruction::Or: 368 case Instruction::Xor: 369 case Instruction::Shl: 370 case Instruction::LShr: 371 case Instruction::AShr: 372 case Instruction::SetEQ: 373 case Instruction::SetNE: 374 case Instruction::SetLT: 375 case Instruction::SetGT: 376 case Instruction::SetLE: 377 case Instruction::SetGE: 378 break; // These are all cheap and non-trapping instructions. 379 } 380 381 // Okay, we can only really hoist these out if their operands are not 382 // defined in the conditional region. 383 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) 384 if (!DominatesMergePoint(I->getOperand(i), BB, 0)) 385 return false; 386 // Okay, it's safe to do this! Remember this instruction. 387 AggressiveInsts->insert(I); 388 } 389 390 return true; 391} 392 393// GatherConstantSetEQs - Given a potentially 'or'd together collection of seteq 394// instructions that compare a value against a constant, return the value being 395// compared, and stick the constant into the Values vector. 396static Value *GatherConstantSetEQs(Value *V, std::vector<ConstantInt*> &Values){ 397 if (Instruction *Inst = dyn_cast<Instruction>(V)) 398 if (Inst->getOpcode() == Instruction::SetEQ) { 399 if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(1))) { 400 Values.push_back(C); 401 return Inst->getOperand(0); 402 } else if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(0))) { 403 Values.push_back(C); 404 return Inst->getOperand(1); 405 } 406 } else if (Inst->getOpcode() == Instruction::Or) { 407 if (Value *LHS = GatherConstantSetEQs(Inst->getOperand(0), Values)) 408 if (Value *RHS = GatherConstantSetEQs(Inst->getOperand(1), Values)) 409 if (LHS == RHS) 410 return LHS; 411 } 412 return 0; 413} 414 415// GatherConstantSetNEs - Given a potentially 'and'd together collection of 416// setne instructions that compare a value against a constant, return the value 417// being compared, and stick the constant into the Values vector. 418static Value *GatherConstantSetNEs(Value *V, std::vector<ConstantInt*> &Values){ 419 if (Instruction *Inst = dyn_cast<Instruction>(V)) 420 if (Inst->getOpcode() == Instruction::SetNE) { 421 if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(1))) { 422 Values.push_back(C); 423 return Inst->getOperand(0); 424 } else if (ConstantInt *C = dyn_cast<ConstantInt>(Inst->getOperand(0))) { 425 Values.push_back(C); 426 return Inst->getOperand(1); 427 } 428 } else if (Inst->getOpcode() == Instruction::Cast) { 429 // Cast of X to bool is really a comparison against zero. 430 assert(Inst->getType() == Type::BoolTy && "Can only handle bool values!"); 431 Values.push_back(ConstantInt::get(Inst->getOperand(0)->getType(), 0)); 432 return Inst->getOperand(0); 433 } else if (Inst->getOpcode() == Instruction::And) { 434 if (Value *LHS = GatherConstantSetNEs(Inst->getOperand(0), Values)) 435 if (Value *RHS = GatherConstantSetNEs(Inst->getOperand(1), Values)) 436 if (LHS == RHS) 437 return LHS; 438 } 439 return 0; 440} 441 442 443 444/// GatherValueComparisons - If the specified Cond is an 'and' or 'or' of a 445/// bunch of comparisons of one value against constants, return the value and 446/// the constants being compared. 447static bool GatherValueComparisons(Instruction *Cond, Value *&CompVal, 448 std::vector<ConstantInt*> &Values) { 449 if (Cond->getOpcode() == Instruction::Or) { 450 CompVal = GatherConstantSetEQs(Cond, Values); 451 452 // Return true to indicate that the condition is true if the CompVal is 453 // equal to one of the constants. 454 return true; 455 } else if (Cond->getOpcode() == Instruction::And) { 456 CompVal = GatherConstantSetNEs(Cond, Values); 457 458 // Return false to indicate that the condition is false if the CompVal is 459 // equal to one of the constants. 460 return false; 461 } 462 return false; 463} 464 465/// ErasePossiblyDeadInstructionTree - If the specified instruction is dead and 466/// has no side effects, nuke it. If it uses any instructions that become dead 467/// because the instruction is now gone, nuke them too. 468static void ErasePossiblyDeadInstructionTree(Instruction *I) { 469 if (!isInstructionTriviallyDead(I)) return; 470 471 std::vector<Instruction*> InstrsToInspect; 472 InstrsToInspect.push_back(I); 473 474 while (!InstrsToInspect.empty()) { 475 I = InstrsToInspect.back(); 476 InstrsToInspect.pop_back(); 477 478 if (!isInstructionTriviallyDead(I)) continue; 479 480 // If I is in the work list multiple times, remove previous instances. 481 for (unsigned i = 0, e = InstrsToInspect.size(); i != e; ++i) 482 if (InstrsToInspect[i] == I) { 483 InstrsToInspect.erase(InstrsToInspect.begin()+i); 484 --i, --e; 485 } 486 487 // Add operands of dead instruction to worklist. 488 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) 489 if (Instruction *OpI = dyn_cast<Instruction>(I->getOperand(i))) 490 InstrsToInspect.push_back(OpI); 491 492 // Remove dead instruction. 493 I->eraseFromParent(); 494 } 495} 496 497// isValueEqualityComparison - Return true if the specified terminator checks to 498// see if a value is equal to constant integer value. 499static Value *isValueEqualityComparison(TerminatorInst *TI) { 500 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) { 501 // Do not permit merging of large switch instructions into their 502 // predecessors unless there is only one predecessor. 503 if (SI->getNumSuccessors() * std::distance(pred_begin(SI->getParent()), 504 pred_end(SI->getParent())) > 128) 505 return 0; 506 507 return SI->getCondition(); 508 } 509 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) 510 if (BI->isConditional() && BI->getCondition()->hasOneUse()) 511 if (SetCondInst *SCI = dyn_cast<SetCondInst>(BI->getCondition())) 512 if ((SCI->getOpcode() == Instruction::SetEQ || 513 SCI->getOpcode() == Instruction::SetNE) && 514 isa<ConstantInt>(SCI->getOperand(1))) 515 return SCI->getOperand(0); 516 return 0; 517} 518 519// Given a value comparison instruction, decode all of the 'cases' that it 520// represents and return the 'default' block. 521static BasicBlock * 522GetValueEqualityComparisonCases(TerminatorInst *TI, 523 std::vector<std::pair<ConstantInt*, 524 BasicBlock*> > &Cases) { 525 if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) { 526 Cases.reserve(SI->getNumCases()); 527 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) 528 Cases.push_back(std::make_pair(SI->getCaseValue(i), SI->getSuccessor(i))); 529 return SI->getDefaultDest(); 530 } 531 532 BranchInst *BI = cast<BranchInst>(TI); 533 SetCondInst *SCI = cast<SetCondInst>(BI->getCondition()); 534 Cases.push_back(std::make_pair(cast<ConstantInt>(SCI->getOperand(1)), 535 BI->getSuccessor(SCI->getOpcode() == 536 Instruction::SetNE))); 537 return BI->getSuccessor(SCI->getOpcode() == Instruction::SetEQ); 538} 539 540 541// EliminateBlockCases - Given an vector of bb/value pairs, remove any entries 542// in the list that match the specified block. 543static void EliminateBlockCases(BasicBlock *BB, 544 std::vector<std::pair<ConstantInt*, BasicBlock*> > &Cases) { 545 for (unsigned i = 0, e = Cases.size(); i != e; ++i) 546 if (Cases[i].second == BB) { 547 Cases.erase(Cases.begin()+i); 548 --i; --e; 549 } 550} 551 552// ValuesOverlap - Return true if there are any keys in C1 that exist in C2 as 553// well. 554static bool 555ValuesOverlap(std::vector<std::pair<ConstantInt*, BasicBlock*> > &C1, 556 std::vector<std::pair<ConstantInt*, BasicBlock*> > &C2) { 557 std::vector<std::pair<ConstantInt*, BasicBlock*> > *V1 = &C1, *V2 = &C2; 558 559 // Make V1 be smaller than V2. 560 if (V1->size() > V2->size()) 561 std::swap(V1, V2); 562 563 if (V1->size() == 0) return false; 564 if (V1->size() == 1) { 565 // Just scan V2. 566 ConstantInt *TheVal = (*V1)[0].first; 567 for (unsigned i = 0, e = V2->size(); i != e; ++i) 568 if (TheVal == (*V2)[i].first) 569 return true; 570 } 571 572 // Otherwise, just sort both lists and compare element by element. 573 std::sort(V1->begin(), V1->end()); 574 std::sort(V2->begin(), V2->end()); 575 unsigned i1 = 0, i2 = 0, e1 = V1->size(), e2 = V2->size(); 576 while (i1 != e1 && i2 != e2) { 577 if ((*V1)[i1].first == (*V2)[i2].first) 578 return true; 579 if ((*V1)[i1].first < (*V2)[i2].first) 580 ++i1; 581 else 582 ++i2; 583 } 584 return false; 585} 586 587// SimplifyEqualityComparisonWithOnlyPredecessor - If TI is known to be a 588// terminator instruction and its block is known to only have a single 589// predecessor block, check to see if that predecessor is also a value 590// comparison with the same value, and if that comparison determines the outcome 591// of this comparison. If so, simplify TI. This does a very limited form of 592// jump threading. 593static bool SimplifyEqualityComparisonWithOnlyPredecessor(TerminatorInst *TI, 594 BasicBlock *Pred) { 595 Value *PredVal = isValueEqualityComparison(Pred->getTerminator()); 596 if (!PredVal) return false; // Not a value comparison in predecessor. 597 598 Value *ThisVal = isValueEqualityComparison(TI); 599 assert(ThisVal && "This isn't a value comparison!!"); 600 if (ThisVal != PredVal) return false; // Different predicates. 601 602 // Find out information about when control will move from Pred to TI's block. 603 std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases; 604 BasicBlock *PredDef = GetValueEqualityComparisonCases(Pred->getTerminator(), 605 PredCases); 606 EliminateBlockCases(PredDef, PredCases); // Remove default from cases. 607 608 // Find information about how control leaves this block. 609 std::vector<std::pair<ConstantInt*, BasicBlock*> > ThisCases; 610 BasicBlock *ThisDef = GetValueEqualityComparisonCases(TI, ThisCases); 611 EliminateBlockCases(ThisDef, ThisCases); // Remove default from cases. 612 613 // If TI's block is the default block from Pred's comparison, potentially 614 // simplify TI based on this knowledge. 615 if (PredDef == TI->getParent()) { 616 // If we are here, we know that the value is none of those cases listed in 617 // PredCases. If there are any cases in ThisCases that are in PredCases, we 618 // can simplify TI. 619 if (ValuesOverlap(PredCases, ThisCases)) { 620 if (BranchInst *BTI = dyn_cast<BranchInst>(TI)) { 621 // Okay, one of the successors of this condbr is dead. Convert it to a 622 // uncond br. 623 assert(ThisCases.size() == 1 && "Branch can only have one case!"); 624 Value *Cond = BTI->getCondition(); 625 // Insert the new branch. 626 Instruction *NI = new BranchInst(ThisDef, TI); 627 628 // Remove PHI node entries for the dead edge. 629 ThisCases[0].second->removePredecessor(TI->getParent()); 630 631 DOUT << "Threading pred instr: " << *Pred->getTerminator() 632 << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n"; 633 634 TI->eraseFromParent(); // Nuke the old one. 635 // If condition is now dead, nuke it. 636 if (Instruction *CondI = dyn_cast<Instruction>(Cond)) 637 ErasePossiblyDeadInstructionTree(CondI); 638 return true; 639 640 } else { 641 SwitchInst *SI = cast<SwitchInst>(TI); 642 // Okay, TI has cases that are statically dead, prune them away. 643 std::set<Constant*> DeadCases; 644 for (unsigned i = 0, e = PredCases.size(); i != e; ++i) 645 DeadCases.insert(PredCases[i].first); 646 647 DOUT << "Threading pred instr: " << *Pred->getTerminator() 648 << "Through successor TI: " << *TI; 649 650 for (unsigned i = SI->getNumCases()-1; i != 0; --i) 651 if (DeadCases.count(SI->getCaseValue(i))) { 652 SI->getSuccessor(i)->removePredecessor(TI->getParent()); 653 SI->removeCase(i); 654 } 655 656 DOUT << "Leaving: " << *TI << "\n"; 657 return true; 658 } 659 } 660 661 } else { 662 // Otherwise, TI's block must correspond to some matched value. Find out 663 // which value (or set of values) this is. 664 ConstantInt *TIV = 0; 665 BasicBlock *TIBB = TI->getParent(); 666 for (unsigned i = 0, e = PredCases.size(); i != e; ++i) 667 if (PredCases[i].second == TIBB) 668 if (TIV == 0) 669 TIV = PredCases[i].first; 670 else 671 return false; // Cannot handle multiple values coming to this block. 672 assert(TIV && "No edge from pred to succ?"); 673 674 // Okay, we found the one constant that our value can be if we get into TI's 675 // BB. Find out which successor will unconditionally be branched to. 676 BasicBlock *TheRealDest = 0; 677 for (unsigned i = 0, e = ThisCases.size(); i != e; ++i) 678 if (ThisCases[i].first == TIV) { 679 TheRealDest = ThisCases[i].second; 680 break; 681 } 682 683 // If not handled by any explicit cases, it is handled by the default case. 684 if (TheRealDest == 0) TheRealDest = ThisDef; 685 686 // Remove PHI node entries for dead edges. 687 BasicBlock *CheckEdge = TheRealDest; 688 for (succ_iterator SI = succ_begin(TIBB), e = succ_end(TIBB); SI != e; ++SI) 689 if (*SI != CheckEdge) 690 (*SI)->removePredecessor(TIBB); 691 else 692 CheckEdge = 0; 693 694 // Insert the new branch. 695 Instruction *NI = new BranchInst(TheRealDest, TI); 696 697 DOUT << "Threading pred instr: " << *Pred->getTerminator() 698 << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n"; 699 Instruction *Cond = 0; 700 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) 701 Cond = dyn_cast<Instruction>(BI->getCondition()); 702 TI->eraseFromParent(); // Nuke the old one. 703 704 if (Cond) ErasePossiblyDeadInstructionTree(Cond); 705 return true; 706 } 707 return false; 708} 709 710// FoldValueComparisonIntoPredecessors - The specified terminator is a value 711// equality comparison instruction (either a switch or a branch on "X == c"). 712// See if any of the predecessors of the terminator block are value comparisons 713// on the same value. If so, and if safe to do so, fold them together. 714static bool FoldValueComparisonIntoPredecessors(TerminatorInst *TI) { 715 BasicBlock *BB = TI->getParent(); 716 Value *CV = isValueEqualityComparison(TI); // CondVal 717 assert(CV && "Not a comparison?"); 718 bool Changed = false; 719 720 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB)); 721 while (!Preds.empty()) { 722 BasicBlock *Pred = Preds.back(); 723 Preds.pop_back(); 724 725 // See if the predecessor is a comparison with the same value. 726 TerminatorInst *PTI = Pred->getTerminator(); 727 Value *PCV = isValueEqualityComparison(PTI); // PredCondVal 728 729 if (PCV == CV && SafeToMergeTerminators(TI, PTI)) { 730 // Figure out which 'cases' to copy from SI to PSI. 731 std::vector<std::pair<ConstantInt*, BasicBlock*> > BBCases; 732 BasicBlock *BBDefault = GetValueEqualityComparisonCases(TI, BBCases); 733 734 std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases; 735 BasicBlock *PredDefault = GetValueEqualityComparisonCases(PTI, PredCases); 736 737 // Based on whether the default edge from PTI goes to BB or not, fill in 738 // PredCases and PredDefault with the new switch cases we would like to 739 // build. 740 std::vector<BasicBlock*> NewSuccessors; 741 742 if (PredDefault == BB) { 743 // If this is the default destination from PTI, only the edges in TI 744 // that don't occur in PTI, or that branch to BB will be activated. 745 std::set<ConstantInt*> PTIHandled; 746 for (unsigned i = 0, e = PredCases.size(); i != e; ++i) 747 if (PredCases[i].second != BB) 748 PTIHandled.insert(PredCases[i].first); 749 else { 750 // The default destination is BB, we don't need explicit targets. 751 std::swap(PredCases[i], PredCases.back()); 752 PredCases.pop_back(); 753 --i; --e; 754 } 755 756 // Reconstruct the new switch statement we will be building. 757 if (PredDefault != BBDefault) { 758 PredDefault->removePredecessor(Pred); 759 PredDefault = BBDefault; 760 NewSuccessors.push_back(BBDefault); 761 } 762 for (unsigned i = 0, e = BBCases.size(); i != e; ++i) 763 if (!PTIHandled.count(BBCases[i].first) && 764 BBCases[i].second != BBDefault) { 765 PredCases.push_back(BBCases[i]); 766 NewSuccessors.push_back(BBCases[i].second); 767 } 768 769 } else { 770 // If this is not the default destination from PSI, only the edges 771 // in SI that occur in PSI with a destination of BB will be 772 // activated. 773 std::set<ConstantInt*> PTIHandled; 774 for (unsigned i = 0, e = PredCases.size(); i != e; ++i) 775 if (PredCases[i].second == BB) { 776 PTIHandled.insert(PredCases[i].first); 777 std::swap(PredCases[i], PredCases.back()); 778 PredCases.pop_back(); 779 --i; --e; 780 } 781 782 // Okay, now we know which constants were sent to BB from the 783 // predecessor. Figure out where they will all go now. 784 for (unsigned i = 0, e = BBCases.size(); i != e; ++i) 785 if (PTIHandled.count(BBCases[i].first)) { 786 // If this is one we are capable of getting... 787 PredCases.push_back(BBCases[i]); 788 NewSuccessors.push_back(BBCases[i].second); 789 PTIHandled.erase(BBCases[i].first);// This constant is taken care of 790 } 791 792 // If there are any constants vectored to BB that TI doesn't handle, 793 // they must go to the default destination of TI. 794 for (std::set<ConstantInt*>::iterator I = PTIHandled.begin(), 795 E = PTIHandled.end(); I != E; ++I) { 796 PredCases.push_back(std::make_pair(*I, BBDefault)); 797 NewSuccessors.push_back(BBDefault); 798 } 799 } 800 801 // Okay, at this point, we know which new successor Pred will get. Make 802 // sure we update the number of entries in the PHI nodes for these 803 // successors. 804 for (unsigned i = 0, e = NewSuccessors.size(); i != e; ++i) 805 AddPredecessorToBlock(NewSuccessors[i], Pred, BB); 806 807 // Now that the successors are updated, create the new Switch instruction. 808 SwitchInst *NewSI = new SwitchInst(CV, PredDefault, PredCases.size(),PTI); 809 for (unsigned i = 0, e = PredCases.size(); i != e; ++i) 810 NewSI->addCase(PredCases[i].first, PredCases[i].second); 811 812 Instruction *DeadCond = 0; 813 if (BranchInst *BI = dyn_cast<BranchInst>(PTI)) 814 // If PTI is a branch, remember the condition. 815 DeadCond = dyn_cast<Instruction>(BI->getCondition()); 816 Pred->getInstList().erase(PTI); 817 818 // If the condition is dead now, remove the instruction tree. 819 if (DeadCond) ErasePossiblyDeadInstructionTree(DeadCond); 820 821 // Okay, last check. If BB is still a successor of PSI, then we must 822 // have an infinite loop case. If so, add an infinitely looping block 823 // to handle the case to preserve the behavior of the code. 824 BasicBlock *InfLoopBlock = 0; 825 for (unsigned i = 0, e = NewSI->getNumSuccessors(); i != e; ++i) 826 if (NewSI->getSuccessor(i) == BB) { 827 if (InfLoopBlock == 0) { 828 // Insert it at the end of the loop, because it's either code, 829 // or it won't matter if it's hot. :) 830 InfLoopBlock = new BasicBlock("infloop", BB->getParent()); 831 new BranchInst(InfLoopBlock, InfLoopBlock); 832 } 833 NewSI->setSuccessor(i, InfLoopBlock); 834 } 835 836 Changed = true; 837 } 838 } 839 return Changed; 840} 841 842/// HoistThenElseCodeToIf - Given a conditional branch that goes to BB1 and 843/// BB2, hoist any common code in the two blocks up into the branch block. The 844/// caller of this function guarantees that BI's block dominates BB1 and BB2. 845static bool HoistThenElseCodeToIf(BranchInst *BI) { 846 // This does very trivial matching, with limited scanning, to find identical 847 // instructions in the two blocks. In particular, we don't want to get into 848 // O(M*N) situations here where M and N are the sizes of BB1 and BB2. As 849 // such, we currently just scan for obviously identical instructions in an 850 // identical order. 851 BasicBlock *BB1 = BI->getSuccessor(0); // The true destination. 852 BasicBlock *BB2 = BI->getSuccessor(1); // The false destination 853 854 Instruction *I1 = BB1->begin(), *I2 = BB2->begin(); 855 if (I1->getOpcode() != I2->getOpcode() || !I1->isIdenticalTo(I2) || 856 isa<PHINode>(I1) || isa<InvokeInst>(I1)) 857 return false; 858 859 // If we get here, we can hoist at least one instruction. 860 BasicBlock *BIParent = BI->getParent(); 861 862 do { 863 // If we are hoisting the terminator instruction, don't move one (making a 864 // broken BB), instead clone it, and remove BI. 865 if (isa<TerminatorInst>(I1)) 866 goto HoistTerminator; 867 868 // For a normal instruction, we just move one to right before the branch, 869 // then replace all uses of the other with the first. Finally, we remove 870 // the now redundant second instruction. 871 BIParent->getInstList().splice(BI, BB1->getInstList(), I1); 872 if (!I2->use_empty()) 873 I2->replaceAllUsesWith(I1); 874 BB2->getInstList().erase(I2); 875 876 I1 = BB1->begin(); 877 I2 = BB2->begin(); 878 } while (I1->getOpcode() == I2->getOpcode() && I1->isIdenticalTo(I2)); 879 880 return true; 881 882HoistTerminator: 883 // Okay, it is safe to hoist the terminator. 884 Instruction *NT = I1->clone(); 885 BIParent->getInstList().insert(BI, NT); 886 if (NT->getType() != Type::VoidTy) { 887 I1->replaceAllUsesWith(NT); 888 I2->replaceAllUsesWith(NT); 889 NT->setName(I1->getName()); 890 } 891 892 // Hoisting one of the terminators from our successor is a great thing. 893 // Unfortunately, the successors of the if/else blocks may have PHI nodes in 894 // them. If they do, all PHI entries for BB1/BB2 must agree for all PHI 895 // nodes, so we insert select instruction to compute the final result. 896 std::map<std::pair<Value*,Value*>, SelectInst*> InsertedSelects; 897 for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) { 898 PHINode *PN; 899 for (BasicBlock::iterator BBI = SI->begin(); 900 (PN = dyn_cast<PHINode>(BBI)); ++BBI) { 901 Value *BB1V = PN->getIncomingValueForBlock(BB1); 902 Value *BB2V = PN->getIncomingValueForBlock(BB2); 903 if (BB1V != BB2V) { 904 // These values do not agree. Insert a select instruction before NT 905 // that determines the right value. 906 SelectInst *&SI = InsertedSelects[std::make_pair(BB1V, BB2V)]; 907 if (SI == 0) 908 SI = new SelectInst(BI->getCondition(), BB1V, BB2V, 909 BB1V->getName()+"."+BB2V->getName(), NT); 910 // Make the PHI node use the select for all incoming values for BB1/BB2 911 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 912 if (PN->getIncomingBlock(i) == BB1 || PN->getIncomingBlock(i) == BB2) 913 PN->setIncomingValue(i, SI); 914 } 915 } 916 } 917 918 // Update any PHI nodes in our new successors. 919 for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) 920 AddPredecessorToBlock(*SI, BIParent, BB1); 921 922 BI->eraseFromParent(); 923 return true; 924} 925 926/// BlockIsSimpleEnoughToThreadThrough - Return true if we can thread a branch 927/// across this block. 928static bool BlockIsSimpleEnoughToThreadThrough(BasicBlock *BB) { 929 BranchInst *BI = cast<BranchInst>(BB->getTerminator()); 930 unsigned Size = 0; 931 932 // If this basic block contains anything other than a PHI (which controls the 933 // branch) and branch itself, bail out. FIXME: improve this in the future. 934 for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI, ++Size) { 935 if (Size > 10) return false; // Don't clone large BB's. 936 937 // We can only support instructions that are do not define values that are 938 // live outside of the current basic block. 939 for (Value::use_iterator UI = BBI->use_begin(), E = BBI->use_end(); 940 UI != E; ++UI) { 941 Instruction *U = cast<Instruction>(*UI); 942 if (U->getParent() != BB || isa<PHINode>(U)) return false; 943 } 944 945 // Looks ok, continue checking. 946 } 947 948 return true; 949} 950 951/// FoldCondBranchOnPHI - If we have a conditional branch on a PHI node value 952/// that is defined in the same block as the branch and if any PHI entries are 953/// constants, thread edges corresponding to that entry to be branches to their 954/// ultimate destination. 955static bool FoldCondBranchOnPHI(BranchInst *BI) { 956 BasicBlock *BB = BI->getParent(); 957 PHINode *PN = dyn_cast<PHINode>(BI->getCondition()); 958 // NOTE: we currently cannot transform this case if the PHI node is used 959 // outside of the block. 960 if (!PN || PN->getParent() != BB || !PN->hasOneUse()) 961 return false; 962 963 // Degenerate case of a single entry PHI. 964 if (PN->getNumIncomingValues() == 1) { 965 if (PN->getIncomingValue(0) != PN) 966 PN->replaceAllUsesWith(PN->getIncomingValue(0)); 967 else 968 PN->replaceAllUsesWith(UndefValue::get(PN->getType())); 969 PN->eraseFromParent(); 970 return true; 971 } 972 973 // Now we know that this block has multiple preds and two succs. 974 if (!BlockIsSimpleEnoughToThreadThrough(BB)) return false; 975 976 // Okay, this is a simple enough basic block. See if any phi values are 977 // constants. 978 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 979 if (ConstantBool *CB = dyn_cast<ConstantBool>(PN->getIncomingValue(i))) { 980 // Okay, we now know that all edges from PredBB should be revectored to 981 // branch to RealDest. 982 BasicBlock *PredBB = PN->getIncomingBlock(i); 983 BasicBlock *RealDest = BI->getSuccessor(!CB->getValue()); 984 985 if (RealDest == BB) continue; // Skip self loops. 986 987 // The dest block might have PHI nodes, other predecessors and other 988 // difficult cases. Instead of being smart about this, just insert a new 989 // block that jumps to the destination block, effectively splitting 990 // the edge we are about to create. 991 BasicBlock *EdgeBB = new BasicBlock(RealDest->getName()+".critedge", 992 RealDest->getParent(), RealDest); 993 new BranchInst(RealDest, EdgeBB); 994 PHINode *PN; 995 for (BasicBlock::iterator BBI = RealDest->begin(); 996 (PN = dyn_cast<PHINode>(BBI)); ++BBI) { 997 Value *V = PN->getIncomingValueForBlock(BB); 998 PN->addIncoming(V, EdgeBB); 999 } 1000 1001 // BB may have instructions that are being threaded over. Clone these 1002 // instructions into EdgeBB. We know that there will be no uses of the 1003 // cloned instructions outside of EdgeBB. 1004 BasicBlock::iterator InsertPt = EdgeBB->begin(); 1005 std::map<Value*, Value*> TranslateMap; // Track translated values. 1006 for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI) { 1007 if (PHINode *PN = dyn_cast<PHINode>(BBI)) { 1008 TranslateMap[PN] = PN->getIncomingValueForBlock(PredBB); 1009 } else { 1010 // Clone the instruction. 1011 Instruction *N = BBI->clone(); 1012 if (BBI->hasName()) N->setName(BBI->getName()+".c"); 1013 1014 // Update operands due to translation. 1015 for (unsigned i = 0, e = N->getNumOperands(); i != e; ++i) { 1016 std::map<Value*, Value*>::iterator PI = 1017 TranslateMap.find(N->getOperand(i)); 1018 if (PI != TranslateMap.end()) 1019 N->setOperand(i, PI->second); 1020 } 1021 1022 // Check for trivial simplification. 1023 if (Constant *C = ConstantFoldInstruction(N)) { 1024 TranslateMap[BBI] = C; 1025 delete N; // Constant folded away, don't need actual inst 1026 } else { 1027 // Insert the new instruction into its new home. 1028 EdgeBB->getInstList().insert(InsertPt, N); 1029 if (!BBI->use_empty()) 1030 TranslateMap[BBI] = N; 1031 } 1032 } 1033 } 1034 1035 // Loop over all of the edges from PredBB to BB, changing them to branch 1036 // to EdgeBB instead. 1037 TerminatorInst *PredBBTI = PredBB->getTerminator(); 1038 for (unsigned i = 0, e = PredBBTI->getNumSuccessors(); i != e; ++i) 1039 if (PredBBTI->getSuccessor(i) == BB) { 1040 BB->removePredecessor(PredBB); 1041 PredBBTI->setSuccessor(i, EdgeBB); 1042 } 1043 1044 // Recurse, simplifying any other constants. 1045 return FoldCondBranchOnPHI(BI) | true; 1046 } 1047 1048 return false; 1049} 1050 1051/// FoldTwoEntryPHINode - Given a BB that starts with the specified two-entry 1052/// PHI node, see if we can eliminate it. 1053static bool FoldTwoEntryPHINode(PHINode *PN) { 1054 // Ok, this is a two entry PHI node. Check to see if this is a simple "if 1055 // statement", which has a very simple dominance structure. Basically, we 1056 // are trying to find the condition that is being branched on, which 1057 // subsequently causes this merge to happen. We really want control 1058 // dependence information for this check, but simplifycfg can't keep it up 1059 // to date, and this catches most of the cases we care about anyway. 1060 // 1061 BasicBlock *BB = PN->getParent(); 1062 BasicBlock *IfTrue, *IfFalse; 1063 Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse); 1064 if (!IfCond) return false; 1065 1066 // Okay, we found that we can merge this two-entry phi node into a select. 1067 // Doing so would require us to fold *all* two entry phi nodes in this block. 1068 // At some point this becomes non-profitable (particularly if the target 1069 // doesn't support cmov's). Only do this transformation if there are two or 1070 // fewer PHI nodes in this block. 1071 unsigned NumPhis = 0; 1072 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++NumPhis, ++I) 1073 if (NumPhis > 2) 1074 return false; 1075 1076 DOUT << "FOUND IF CONDITION! " << *IfCond << " T: " 1077 << IfTrue->getName() << " F: " << IfFalse->getName() << "\n"; 1078 1079 // Loop over the PHI's seeing if we can promote them all to select 1080 // instructions. While we are at it, keep track of the instructions 1081 // that need to be moved to the dominating block. 1082 std::set<Instruction*> AggressiveInsts; 1083 1084 BasicBlock::iterator AfterPHIIt = BB->begin(); 1085 while (isa<PHINode>(AfterPHIIt)) { 1086 PHINode *PN = cast<PHINode>(AfterPHIIt++); 1087 if (PN->getIncomingValue(0) == PN->getIncomingValue(1)) { 1088 if (PN->getIncomingValue(0) != PN) 1089 PN->replaceAllUsesWith(PN->getIncomingValue(0)); 1090 else 1091 PN->replaceAllUsesWith(UndefValue::get(PN->getType())); 1092 } else if (!DominatesMergePoint(PN->getIncomingValue(0), BB, 1093 &AggressiveInsts) || 1094 !DominatesMergePoint(PN->getIncomingValue(1), BB, 1095 &AggressiveInsts)) { 1096 return false; 1097 } 1098 } 1099 1100 // If we all PHI nodes are promotable, check to make sure that all 1101 // instructions in the predecessor blocks can be promoted as well. If 1102 // not, we won't be able to get rid of the control flow, so it's not 1103 // worth promoting to select instructions. 1104 BasicBlock *DomBlock = 0, *IfBlock1 = 0, *IfBlock2 = 0; 1105 PN = cast<PHINode>(BB->begin()); 1106 BasicBlock *Pred = PN->getIncomingBlock(0); 1107 if (cast<BranchInst>(Pred->getTerminator())->isUnconditional()) { 1108 IfBlock1 = Pred; 1109 DomBlock = *pred_begin(Pred); 1110 for (BasicBlock::iterator I = Pred->begin(); 1111 !isa<TerminatorInst>(I); ++I) 1112 if (!AggressiveInsts.count(I)) { 1113 // This is not an aggressive instruction that we can promote. 1114 // Because of this, we won't be able to get rid of the control 1115 // flow, so the xform is not worth it. 1116 return false; 1117 } 1118 } 1119 1120 Pred = PN->getIncomingBlock(1); 1121 if (cast<BranchInst>(Pred->getTerminator())->isUnconditional()) { 1122 IfBlock2 = Pred; 1123 DomBlock = *pred_begin(Pred); 1124 for (BasicBlock::iterator I = Pred->begin(); 1125 !isa<TerminatorInst>(I); ++I) 1126 if (!AggressiveInsts.count(I)) { 1127 // This is not an aggressive instruction that we can promote. 1128 // Because of this, we won't be able to get rid of the control 1129 // flow, so the xform is not worth it. 1130 return false; 1131 } 1132 } 1133 1134 // If we can still promote the PHI nodes after this gauntlet of tests, 1135 // do all of the PHI's now. 1136 1137 // Move all 'aggressive' instructions, which are defined in the 1138 // conditional parts of the if's up to the dominating block. 1139 if (IfBlock1) { 1140 DomBlock->getInstList().splice(DomBlock->getTerminator(), 1141 IfBlock1->getInstList(), 1142 IfBlock1->begin(), 1143 IfBlock1->getTerminator()); 1144 } 1145 if (IfBlock2) { 1146 DomBlock->getInstList().splice(DomBlock->getTerminator(), 1147 IfBlock2->getInstList(), 1148 IfBlock2->begin(), 1149 IfBlock2->getTerminator()); 1150 } 1151 1152 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) { 1153 // Change the PHI node into a select instruction. 1154 Value *TrueVal = 1155 PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse); 1156 Value *FalseVal = 1157 PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue); 1158 1159 std::string Name = PN->getName(); PN->setName(""); 1160 PN->replaceAllUsesWith(new SelectInst(IfCond, TrueVal, FalseVal, 1161 Name, AfterPHIIt)); 1162 BB->getInstList().erase(PN); 1163 } 1164 return true; 1165} 1166 1167namespace { 1168 /// ConstantIntOrdering - This class implements a stable ordering of constant 1169 /// integers that does not depend on their address. This is important for 1170 /// applications that sort ConstantInt's to ensure uniqueness. 1171 struct ConstantIntOrdering { 1172 bool operator()(const ConstantInt *LHS, const ConstantInt *RHS) const { 1173 return LHS->getZExtValue() < RHS->getZExtValue(); 1174 } 1175 }; 1176} 1177 1178// SimplifyCFG - This function is used to do simplification of a CFG. For 1179// example, it adjusts branches to branches to eliminate the extra hop, it 1180// eliminates unreachable basic blocks, and does other "peephole" optimization 1181// of the CFG. It returns true if a modification was made. 1182// 1183// WARNING: The entry node of a function may not be simplified. 1184// 1185bool llvm::SimplifyCFG(BasicBlock *BB) { 1186 bool Changed = false; 1187 Function *M = BB->getParent(); 1188 1189 assert(BB && BB->getParent() && "Block not embedded in function!"); 1190 assert(BB->getTerminator() && "Degenerate basic block encountered!"); 1191 assert(&BB->getParent()->front() != BB && "Can't Simplify entry block!"); 1192 1193 // Remove basic blocks that have no predecessors... which are unreachable. 1194 if (pred_begin(BB) == pred_end(BB) || 1195 *pred_begin(BB) == BB && ++pred_begin(BB) == pred_end(BB)) { 1196 DOUT << "Removing BB: \n" << *BB; 1197 1198 // Loop through all of our successors and make sure they know that one 1199 // of their predecessors is going away. 1200 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) 1201 SI->removePredecessor(BB); 1202 1203 while (!BB->empty()) { 1204 Instruction &I = BB->back(); 1205 // If this instruction is used, replace uses with an arbitrary 1206 // value. Because control flow can't get here, we don't care 1207 // what we replace the value with. Note that since this block is 1208 // unreachable, and all values contained within it must dominate their 1209 // uses, that all uses will eventually be removed. 1210 if (!I.use_empty()) 1211 // Make all users of this instruction use undef instead 1212 I.replaceAllUsesWith(UndefValue::get(I.getType())); 1213 1214 // Remove the instruction from the basic block 1215 BB->getInstList().pop_back(); 1216 } 1217 M->getBasicBlockList().erase(BB); 1218 return true; 1219 } 1220 1221 // Check to see if we can constant propagate this terminator instruction 1222 // away... 1223 Changed |= ConstantFoldTerminator(BB); 1224 1225 // If this is a returning block with only PHI nodes in it, fold the return 1226 // instruction into any unconditional branch predecessors. 1227 // 1228 // If any predecessor is a conditional branch that just selects among 1229 // different return values, fold the replace the branch/return with a select 1230 // and return. 1231 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) { 1232 BasicBlock::iterator BBI = BB->getTerminator(); 1233 if (BBI == BB->begin() || isa<PHINode>(--BBI)) { 1234 // Find predecessors that end with branches. 1235 std::vector<BasicBlock*> UncondBranchPreds; 1236 std::vector<BranchInst*> CondBranchPreds; 1237 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { 1238 TerminatorInst *PTI = (*PI)->getTerminator(); 1239 if (BranchInst *BI = dyn_cast<BranchInst>(PTI)) 1240 if (BI->isUnconditional()) 1241 UncondBranchPreds.push_back(*PI); 1242 else 1243 CondBranchPreds.push_back(BI); 1244 } 1245 1246 // If we found some, do the transformation! 1247 if (!UncondBranchPreds.empty()) { 1248 while (!UncondBranchPreds.empty()) { 1249 BasicBlock *Pred = UncondBranchPreds.back(); 1250 DOUT << "FOLDING: " << *BB 1251 << "INTO UNCOND BRANCH PRED: " << *Pred; 1252 UncondBranchPreds.pop_back(); 1253 Instruction *UncondBranch = Pred->getTerminator(); 1254 // Clone the return and add it to the end of the predecessor. 1255 Instruction *NewRet = RI->clone(); 1256 Pred->getInstList().push_back(NewRet); 1257 1258 // If the return instruction returns a value, and if the value was a 1259 // PHI node in "BB", propagate the right value into the return. 1260 if (NewRet->getNumOperands() == 1) 1261 if (PHINode *PN = dyn_cast<PHINode>(NewRet->getOperand(0))) 1262 if (PN->getParent() == BB) 1263 NewRet->setOperand(0, PN->getIncomingValueForBlock(Pred)); 1264 // Update any PHI nodes in the returning block to realize that we no 1265 // longer branch to them. 1266 BB->removePredecessor(Pred); 1267 Pred->getInstList().erase(UncondBranch); 1268 } 1269 1270 // If we eliminated all predecessors of the block, delete the block now. 1271 if (pred_begin(BB) == pred_end(BB)) 1272 // We know there are no successors, so just nuke the block. 1273 M->getBasicBlockList().erase(BB); 1274 1275 return true; 1276 } 1277 1278 // Check out all of the conditional branches going to this return 1279 // instruction. If any of them just select between returns, change the 1280 // branch itself into a select/return pair. 1281 while (!CondBranchPreds.empty()) { 1282 BranchInst *BI = CondBranchPreds.back(); 1283 CondBranchPreds.pop_back(); 1284 BasicBlock *TrueSucc = BI->getSuccessor(0); 1285 BasicBlock *FalseSucc = BI->getSuccessor(1); 1286 BasicBlock *OtherSucc = TrueSucc == BB ? FalseSucc : TrueSucc; 1287 1288 // Check to see if the non-BB successor is also a return block. 1289 if (isa<ReturnInst>(OtherSucc->getTerminator())) { 1290 // Check to see if there are only PHI instructions in this block. 1291 BasicBlock::iterator OSI = OtherSucc->getTerminator(); 1292 if (OSI == OtherSucc->begin() || isa<PHINode>(--OSI)) { 1293 // Okay, we found a branch that is going to two return nodes. If 1294 // there is no return value for this function, just change the 1295 // branch into a return. 1296 if (RI->getNumOperands() == 0) { 1297 TrueSucc->removePredecessor(BI->getParent()); 1298 FalseSucc->removePredecessor(BI->getParent()); 1299 new ReturnInst(0, BI); 1300 BI->getParent()->getInstList().erase(BI); 1301 return true; 1302 } 1303 1304 // Otherwise, figure out what the true and false return values are 1305 // so we can insert a new select instruction. 1306 Value *TrueValue = TrueSucc->getTerminator()->getOperand(0); 1307 Value *FalseValue = FalseSucc->getTerminator()->getOperand(0); 1308 1309 // Unwrap any PHI nodes in the return blocks. 1310 if (PHINode *TVPN = dyn_cast<PHINode>(TrueValue)) 1311 if (TVPN->getParent() == TrueSucc) 1312 TrueValue = TVPN->getIncomingValueForBlock(BI->getParent()); 1313 if (PHINode *FVPN = dyn_cast<PHINode>(FalseValue)) 1314 if (FVPN->getParent() == FalseSucc) 1315 FalseValue = FVPN->getIncomingValueForBlock(BI->getParent()); 1316 1317 // In order for this transformation to be safe, we must be able to 1318 // unconditionally execute both operands to the return. This is 1319 // normally the case, but we could have a potentially-trapping 1320 // constant expression that prevents this transformation from being 1321 // safe. 1322 if ((!isa<ConstantExpr>(TrueValue) || 1323 !cast<ConstantExpr>(TrueValue)->canTrap()) && 1324 (!isa<ConstantExpr>(TrueValue) || 1325 !cast<ConstantExpr>(TrueValue)->canTrap())) { 1326 TrueSucc->removePredecessor(BI->getParent()); 1327 FalseSucc->removePredecessor(BI->getParent()); 1328 1329 // Insert a new select instruction. 1330 Value *NewRetVal; 1331 Value *BrCond = BI->getCondition(); 1332 if (TrueValue != FalseValue) 1333 NewRetVal = new SelectInst(BrCond, TrueValue, 1334 FalseValue, "retval", BI); 1335 else 1336 NewRetVal = TrueValue; 1337 1338 DOUT << "\nCHANGING BRANCH TO TWO RETURNS INTO SELECT:" 1339 << "\n " << *BI << "Select = " << *NewRetVal 1340 << "TRUEBLOCK: " << *TrueSucc << "FALSEBLOCK: "<< *FalseSucc; 1341 1342 new ReturnInst(NewRetVal, BI); 1343 BI->eraseFromParent(); 1344 if (Instruction *BrCondI = dyn_cast<Instruction>(BrCond)) 1345 if (isInstructionTriviallyDead(BrCondI)) 1346 BrCondI->eraseFromParent(); 1347 return true; 1348 } 1349 } 1350 } 1351 } 1352 } 1353 } else if (isa<UnwindInst>(BB->begin())) { 1354 // Check to see if the first instruction in this block is just an unwind. 1355 // If so, replace any invoke instructions which use this as an exception 1356 // destination with call instructions, and any unconditional branch 1357 // predecessor with an unwind. 1358 // 1359 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB)); 1360 while (!Preds.empty()) { 1361 BasicBlock *Pred = Preds.back(); 1362 if (BranchInst *BI = dyn_cast<BranchInst>(Pred->getTerminator())) { 1363 if (BI->isUnconditional()) { 1364 Pred->getInstList().pop_back(); // nuke uncond branch 1365 new UnwindInst(Pred); // Use unwind. 1366 Changed = true; 1367 } 1368 } else if (InvokeInst *II = dyn_cast<InvokeInst>(Pred->getTerminator())) 1369 if (II->getUnwindDest() == BB) { 1370 // Insert a new branch instruction before the invoke, because this 1371 // is now a fall through... 1372 BranchInst *BI = new BranchInst(II->getNormalDest(), II); 1373 Pred->getInstList().remove(II); // Take out of symbol table 1374 1375 // Insert the call now... 1376 std::vector<Value*> Args(II->op_begin()+3, II->op_end()); 1377 CallInst *CI = new CallInst(II->getCalledValue(), Args, 1378 II->getName(), BI); 1379 CI->setCallingConv(II->getCallingConv()); 1380 // If the invoke produced a value, the Call now does instead 1381 II->replaceAllUsesWith(CI); 1382 delete II; 1383 Changed = true; 1384 } 1385 1386 Preds.pop_back(); 1387 } 1388 1389 // If this block is now dead, remove it. 1390 if (pred_begin(BB) == pred_end(BB)) { 1391 // We know there are no successors, so just nuke the block. 1392 M->getBasicBlockList().erase(BB); 1393 return true; 1394 } 1395 1396 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) { 1397 if (isValueEqualityComparison(SI)) { 1398 // If we only have one predecessor, and if it is a branch on this value, 1399 // see if that predecessor totally determines the outcome of this switch. 1400 if (BasicBlock *OnlyPred = BB->getSinglePredecessor()) 1401 if (SimplifyEqualityComparisonWithOnlyPredecessor(SI, OnlyPred)) 1402 return SimplifyCFG(BB) || 1; 1403 1404 // If the block only contains the switch, see if we can fold the block 1405 // away into any preds. 1406 if (SI == &BB->front()) 1407 if (FoldValueComparisonIntoPredecessors(SI)) 1408 return SimplifyCFG(BB) || 1; 1409 } 1410 } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) { 1411 if (BI->isUnconditional()) { 1412 BasicBlock::iterator BBI = BB->begin(); // Skip over phi nodes... 1413 while (isa<PHINode>(*BBI)) ++BBI; 1414 1415 BasicBlock *Succ = BI->getSuccessor(0); 1416 if (BBI->isTerminator() && // Terminator is the only non-phi instruction! 1417 Succ != BB) // Don't hurt infinite loops! 1418 if (TryToSimplifyUncondBranchFromEmptyBlock(BB, Succ)) 1419 return 1; 1420 1421 } else { // Conditional branch 1422 if (isValueEqualityComparison(BI)) { 1423 // If we only have one predecessor, and if it is a branch on this value, 1424 // see if that predecessor totally determines the outcome of this 1425 // switch. 1426 if (BasicBlock *OnlyPred = BB->getSinglePredecessor()) 1427 if (SimplifyEqualityComparisonWithOnlyPredecessor(BI, OnlyPred)) 1428 return SimplifyCFG(BB) || 1; 1429 1430 // This block must be empty, except for the setcond inst, if it exists. 1431 BasicBlock::iterator I = BB->begin(); 1432 if (&*I == BI || 1433 (&*I == cast<Instruction>(BI->getCondition()) && 1434 &*++I == BI)) 1435 if (FoldValueComparisonIntoPredecessors(BI)) 1436 return SimplifyCFG(BB) | true; 1437 } 1438 1439 // If this is a branch on a phi node in the current block, thread control 1440 // through this block if any PHI node entries are constants. 1441 if (PHINode *PN = dyn_cast<PHINode>(BI->getCondition())) 1442 if (PN->getParent() == BI->getParent()) 1443 if (FoldCondBranchOnPHI(BI)) 1444 return SimplifyCFG(BB) | true; 1445 1446 // If this basic block is ONLY a setcc and a branch, and if a predecessor 1447 // branches to us and one of our successors, fold the setcc into the 1448 // predecessor and use logical operations to pick the right destination. 1449 BasicBlock *TrueDest = BI->getSuccessor(0); 1450 BasicBlock *FalseDest = BI->getSuccessor(1); 1451 if (BinaryOperator *Cond = dyn_cast<BinaryOperator>(BI->getCondition())) 1452 if (Cond->getParent() == BB && &BB->front() == Cond && 1453 Cond->getNext() == BI && Cond->hasOneUse() && 1454 TrueDest != BB && FalseDest != BB) 1455 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI!=E; ++PI) 1456 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator())) 1457 if (PBI->isConditional() && SafeToMergeTerminators(BI, PBI)) { 1458 BasicBlock *PredBlock = *PI; 1459 if (PBI->getSuccessor(0) == FalseDest || 1460 PBI->getSuccessor(1) == TrueDest) { 1461 // Invert the predecessors condition test (xor it with true), 1462 // which allows us to write this code once. 1463 Value *NewCond = 1464 BinaryOperator::createNot(PBI->getCondition(), 1465 PBI->getCondition()->getName()+".not", PBI); 1466 PBI->setCondition(NewCond); 1467 BasicBlock *OldTrue = PBI->getSuccessor(0); 1468 BasicBlock *OldFalse = PBI->getSuccessor(1); 1469 PBI->setSuccessor(0, OldFalse); 1470 PBI->setSuccessor(1, OldTrue); 1471 } 1472 1473 if ((PBI->getSuccessor(0) == TrueDest && FalseDest != BB) || 1474 (PBI->getSuccessor(1) == FalseDest && TrueDest != BB)) { 1475 // Clone Cond into the predecessor basic block, and or/and the 1476 // two conditions together. 1477 Instruction *New = Cond->clone(); 1478 New->setName(Cond->getName()); 1479 Cond->setName(Cond->getName()+".old"); 1480 PredBlock->getInstList().insert(PBI, New); 1481 Instruction::BinaryOps Opcode = 1482 PBI->getSuccessor(0) == TrueDest ? 1483 Instruction::Or : Instruction::And; 1484 Value *NewCond = 1485 BinaryOperator::create(Opcode, PBI->getCondition(), 1486 New, "bothcond", PBI); 1487 PBI->setCondition(NewCond); 1488 if (PBI->getSuccessor(0) == BB) { 1489 AddPredecessorToBlock(TrueDest, PredBlock, BB); 1490 PBI->setSuccessor(0, TrueDest); 1491 } 1492 if (PBI->getSuccessor(1) == BB) { 1493 AddPredecessorToBlock(FalseDest, PredBlock, BB); 1494 PBI->setSuccessor(1, FalseDest); 1495 } 1496 return SimplifyCFG(BB) | 1; 1497 } 1498 } 1499 1500 // Scan predessor blocks for conditional branchs. 1501 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) 1502 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator())) 1503 if (PBI != BI && PBI->isConditional()) { 1504 1505 // If this block ends with a branch instruction, and if there is a 1506 // predecessor that ends on a branch of the same condition, make this 1507 // conditional branch redundant. 1508 if (PBI->getCondition() == BI->getCondition() && 1509 PBI->getSuccessor(0) != PBI->getSuccessor(1)) { 1510 // Okay, the outcome of this conditional branch is statically 1511 // knowable. If this block had a single pred, handle specially. 1512 if (BB->getSinglePredecessor()) { 1513 // Turn this into a branch on constant. 1514 bool CondIsTrue = PBI->getSuccessor(0) == BB; 1515 BI->setCondition(ConstantBool::get(CondIsTrue)); 1516 return SimplifyCFG(BB); // Nuke the branch on constant. 1517 } 1518 1519 // Otherwise, if there are multiple predecessors, insert a PHI that 1520 // merges in the constant and simplify the block result. 1521 if (BlockIsSimpleEnoughToThreadThrough(BB)) { 1522 PHINode *NewPN = new PHINode(Type::BoolTy, 1523 BI->getCondition()->getName()+".pr", 1524 BB->begin()); 1525 for (PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) 1526 if ((PBI = dyn_cast<BranchInst>((*PI)->getTerminator())) && 1527 PBI != BI && PBI->isConditional() && 1528 PBI->getCondition() == BI->getCondition() && 1529 PBI->getSuccessor(0) != PBI->getSuccessor(1)) { 1530 bool CondIsTrue = PBI->getSuccessor(0) == BB; 1531 NewPN->addIncoming(ConstantBool::get(CondIsTrue), *PI); 1532 } else { 1533 NewPN->addIncoming(BI->getCondition(), *PI); 1534 } 1535 1536 BI->setCondition(NewPN); 1537 // This will thread the branch. 1538 return SimplifyCFG(BB) | true; 1539 } 1540 } 1541 1542 // If this is a conditional branch in an empty block, and if any 1543 // predecessors is a conditional branch to one of our destinations, 1544 // fold the conditions into logical ops and one cond br. 1545 if (&BB->front() == BI) { 1546 int PBIOp, BIOp; 1547 if (PBI->getSuccessor(0) == BI->getSuccessor(0)) { 1548 PBIOp = BIOp = 0; 1549 } else if (PBI->getSuccessor(0) == BI->getSuccessor(1)) { 1550 PBIOp = 0; BIOp = 1; 1551 } else if (PBI->getSuccessor(1) == BI->getSuccessor(0)) { 1552 PBIOp = 1; BIOp = 0; 1553 } else if (PBI->getSuccessor(1) == BI->getSuccessor(1)) { 1554 PBIOp = BIOp = 1; 1555 } else { 1556 PBIOp = BIOp = -1; 1557 } 1558 1559 // Check to make sure that the other destination of this branch 1560 // isn't BB itself. If so, this is an infinite loop that will 1561 // keep getting unwound. 1562 if (PBIOp != -1 && PBI->getSuccessor(PBIOp) == BB) 1563 PBIOp = BIOp = -1; 1564 1565 // Do not perform this transformation if it would require 1566 // insertion of a large number of select instructions. For targets 1567 // without predication/cmovs, this is a big pessimization. 1568 if (PBIOp != -1) { 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) { 1575 // Disable this xform. 1576 PBIOp = -1; 1577 break; 1578 } 1579 } 1580 } 1581 1582 // Finally, if everything is ok, fold the branches to logical ops. 1583 if (PBIOp != -1) { 1584 BasicBlock *CommonDest = PBI->getSuccessor(PBIOp); 1585 BasicBlock *OtherDest = BI->getSuccessor(BIOp ^ 1); 1586 1587 // If OtherDest *is* BB, then this is a basic block with just 1588 // a conditional branch in it, where one edge (OtherDesg) goes 1589 // back to the block. We know that the program doesn't get 1590 // stuck in the infinite loop, so the condition must be such 1591 // that OtherDest isn't branched through. Forward to CommonDest, 1592 // and avoid an infinite loop at optimizer time. 1593 if (OtherDest == BB) 1594 OtherDest = CommonDest; 1595 1596 DOUT << "FOLDING BRs:" << *PBI->getParent() 1597 << "AND: " << *BI->getParent(); 1598 1599 // BI may have other predecessors. Because of this, we leave 1600 // it alone, but modify PBI. 1601 1602 // Make sure we get to CommonDest on True&True directions. 1603 Value *PBICond = PBI->getCondition(); 1604 if (PBIOp) 1605 PBICond = BinaryOperator::createNot(PBICond, 1606 PBICond->getName()+".not", 1607 PBI); 1608 Value *BICond = BI->getCondition(); 1609 if (BIOp) 1610 BICond = BinaryOperator::createNot(BICond, 1611 BICond->getName()+".not", 1612 PBI); 1613 // Merge the conditions. 1614 Value *Cond = 1615 BinaryOperator::createOr(PBICond, BICond, "brmerge", PBI); 1616 1617 // Modify PBI to branch on the new condition to the new dests. 1618 PBI->setCondition(Cond); 1619 PBI->setSuccessor(0, CommonDest); 1620 PBI->setSuccessor(1, OtherDest); 1621 1622 // OtherDest may have phi nodes. If so, add an entry from PBI's 1623 // block that are identical to the entries for BI's block. 1624 PHINode *PN; 1625 for (BasicBlock::iterator II = OtherDest->begin(); 1626 (PN = dyn_cast<PHINode>(II)); ++II) { 1627 Value *V = PN->getIncomingValueForBlock(BB); 1628 PN->addIncoming(V, PBI->getParent()); 1629 } 1630 1631 // We know that the CommonDest already had an edge from PBI to 1632 // it. If it has PHIs though, the PHIs may have different 1633 // entries for BB and PBI's BB. If so, insert a select to make 1634 // them agree. 1635 for (BasicBlock::iterator II = CommonDest->begin(); 1636 (PN = dyn_cast<PHINode>(II)); ++II) { 1637 Value * BIV = PN->getIncomingValueForBlock(BB); 1638 unsigned PBBIdx = PN->getBasicBlockIndex(PBI->getParent()); 1639 Value *PBIV = PN->getIncomingValue(PBBIdx); 1640 if (BIV != PBIV) { 1641 // Insert a select in PBI to pick the right value. 1642 Value *NV = new SelectInst(PBICond, PBIV, BIV, 1643 PBIV->getName()+".mux", PBI); 1644 PN->setIncomingValue(PBBIdx, NV); 1645 } 1646 } 1647 1648 DOUT << "INTO: " << *PBI->getParent(); 1649 1650 // This basic block is probably dead. We know it has at least 1651 // one fewer predecessor. 1652 return SimplifyCFG(BB) | true; 1653 } 1654 } 1655 } 1656 } 1657 } else if (isa<UnreachableInst>(BB->getTerminator())) { 1658 // If there are any instructions immediately before the unreachable that can 1659 // be removed, do so. 1660 Instruction *Unreachable = BB->getTerminator(); 1661 while (Unreachable != BB->begin()) { 1662 BasicBlock::iterator BBI = Unreachable; 1663 --BBI; 1664 if (isa<CallInst>(BBI)) break; 1665 // Delete this instruction 1666 BB->getInstList().erase(BBI); 1667 Changed = true; 1668 } 1669 1670 // If the unreachable instruction is the first in the block, take a gander 1671 // at all of the predecessors of this instruction, and simplify them. 1672 if (&BB->front() == Unreachable) { 1673 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB)); 1674 for (unsigned i = 0, e = Preds.size(); i != e; ++i) { 1675 TerminatorInst *TI = Preds[i]->getTerminator(); 1676 1677 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) { 1678 if (BI->isUnconditional()) { 1679 if (BI->getSuccessor(0) == BB) { 1680 new UnreachableInst(TI); 1681 TI->eraseFromParent(); 1682 Changed = true; 1683 } 1684 } else { 1685 if (BI->getSuccessor(0) == BB) { 1686 new BranchInst(BI->getSuccessor(1), BI); 1687 BI->eraseFromParent(); 1688 } else if (BI->getSuccessor(1) == BB) { 1689 new BranchInst(BI->getSuccessor(0), BI); 1690 BI->eraseFromParent(); 1691 Changed = true; 1692 } 1693 } 1694 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) { 1695 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) 1696 if (SI->getSuccessor(i) == BB) { 1697 BB->removePredecessor(SI->getParent()); 1698 SI->removeCase(i); 1699 --i; --e; 1700 Changed = true; 1701 } 1702 // If the default value is unreachable, figure out the most popular 1703 // destination and make it the default. 1704 if (SI->getSuccessor(0) == BB) { 1705 std::map<BasicBlock*, unsigned> Popularity; 1706 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) 1707 Popularity[SI->getSuccessor(i)]++; 1708 1709 // Find the most popular block. 1710 unsigned MaxPop = 0; 1711 BasicBlock *MaxBlock = 0; 1712 for (std::map<BasicBlock*, unsigned>::iterator 1713 I = Popularity.begin(), E = Popularity.end(); I != E; ++I) { 1714 if (I->second > MaxPop) { 1715 MaxPop = I->second; 1716 MaxBlock = I->first; 1717 } 1718 } 1719 if (MaxBlock) { 1720 // Make this the new default, allowing us to delete any explicit 1721 // edges to it. 1722 SI->setSuccessor(0, MaxBlock); 1723 Changed = true; 1724 1725 // If MaxBlock has phinodes in it, remove MaxPop-1 entries from 1726 // it. 1727 if (isa<PHINode>(MaxBlock->begin())) 1728 for (unsigned i = 0; i != MaxPop-1; ++i) 1729 MaxBlock->removePredecessor(SI->getParent()); 1730 1731 for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) 1732 if (SI->getSuccessor(i) == MaxBlock) { 1733 SI->removeCase(i); 1734 --i; --e; 1735 } 1736 } 1737 } 1738 } else if (InvokeInst *II = dyn_cast<InvokeInst>(TI)) { 1739 if (II->getUnwindDest() == BB) { 1740 // Convert the invoke to a call instruction. This would be a good 1741 // place to note that the call does not throw though. 1742 BranchInst *BI = new BranchInst(II->getNormalDest(), II); 1743 II->removeFromParent(); // Take out of symbol table 1744 1745 // Insert the call now... 1746 std::vector<Value*> Args(II->op_begin()+3, II->op_end()); 1747 CallInst *CI = new CallInst(II->getCalledValue(), Args, 1748 II->getName(), BI); 1749 CI->setCallingConv(II->getCallingConv()); 1750 // If the invoke produced a value, the Call does now instead. 1751 II->replaceAllUsesWith(CI); 1752 delete II; 1753 Changed = true; 1754 } 1755 } 1756 } 1757 1758 // If this block is now dead, remove it. 1759 if (pred_begin(BB) == pred_end(BB)) { 1760 // We know there are no successors, so just nuke the block. 1761 M->getBasicBlockList().erase(BB); 1762 return true; 1763 } 1764 } 1765 } 1766 1767 // Merge basic blocks into their predecessor if there is only one distinct 1768 // pred, and if there is only one distinct successor of the predecessor, and 1769 // if there are no PHI nodes. 1770 // 1771 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB)); 1772 BasicBlock *OnlyPred = *PI++; 1773 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same 1774 if (*PI != OnlyPred) { 1775 OnlyPred = 0; // There are multiple different predecessors... 1776 break; 1777 } 1778 1779 BasicBlock *OnlySucc = 0; 1780 if (OnlyPred && OnlyPred != BB && // Don't break self loops 1781 OnlyPred->getTerminator()->getOpcode() != Instruction::Invoke) { 1782 // Check to see if there is only one distinct successor... 1783 succ_iterator SI(succ_begin(OnlyPred)), SE(succ_end(OnlyPred)); 1784 OnlySucc = BB; 1785 for (; SI != SE; ++SI) 1786 if (*SI != OnlySucc) { 1787 OnlySucc = 0; // There are multiple distinct successors! 1788 break; 1789 } 1790 } 1791 1792 if (OnlySucc) { 1793 DOUT << "Merging: " << *BB << "into: " << *OnlyPred; 1794 1795 // Resolve any PHI nodes at the start of the block. They are all 1796 // guaranteed to have exactly one entry if they exist, unless there are 1797 // multiple duplicate (but guaranteed to be equal) entries for the 1798 // incoming edges. This occurs when there are multiple edges from 1799 // OnlyPred to OnlySucc. 1800 // 1801 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) { 1802 PN->replaceAllUsesWith(PN->getIncomingValue(0)); 1803 BB->getInstList().pop_front(); // Delete the phi node... 1804 } 1805 1806 // Delete the unconditional branch from the predecessor... 1807 OnlyPred->getInstList().pop_back(); 1808 1809 // Move all definitions in the successor to the predecessor... 1810 OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList()); 1811 1812 // Make all PHI nodes that referred to BB now refer to Pred as their 1813 // source... 1814 BB->replaceAllUsesWith(OnlyPred); 1815 1816 std::string OldName = BB->getName(); 1817 1818 // Erase basic block from the function... 1819 M->getBasicBlockList().erase(BB); 1820 1821 // Inherit predecessors name if it exists... 1822 if (!OldName.empty() && !OnlyPred->hasName()) 1823 OnlyPred->setName(OldName); 1824 1825 return true; 1826 } 1827 1828 // Otherwise, if this block only has a single predecessor, and if that block 1829 // is a conditional branch, see if we can hoist any code from this block up 1830 // into our predecessor. 1831 if (OnlyPred) 1832 if (BranchInst *BI = dyn_cast<BranchInst>(OnlyPred->getTerminator())) 1833 if (BI->isConditional()) { 1834 // Get the other block. 1835 BasicBlock *OtherBB = BI->getSuccessor(BI->getSuccessor(0) == BB); 1836 PI = pred_begin(OtherBB); 1837 ++PI; 1838 if (PI == pred_end(OtherBB)) { 1839 // We have a conditional branch to two blocks that are only reachable 1840 // from the condbr. We know that the condbr dominates the two blocks, 1841 // so see if there is any identical code in the "then" and "else" 1842 // blocks. If so, we can hoist it up to the branching block. 1843 Changed |= HoistThenElseCodeToIf(BI); 1844 } 1845 } 1846 1847 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) 1848 if (BranchInst *BI = dyn_cast<BranchInst>((*PI)->getTerminator())) 1849 // Change br (X == 0 | X == 1), T, F into a switch instruction. 1850 if (BI->isConditional() && isa<Instruction>(BI->getCondition())) { 1851 Instruction *Cond = cast<Instruction>(BI->getCondition()); 1852 // If this is a bunch of seteq's or'd together, or if it's a bunch of 1853 // 'setne's and'ed together, collect them. 1854 Value *CompVal = 0; 1855 std::vector<ConstantInt*> Values; 1856 bool TrueWhenEqual = GatherValueComparisons(Cond, CompVal, Values); 1857 if (CompVal && CompVal->getType()->isInteger()) { 1858 // There might be duplicate constants in the list, which the switch 1859 // instruction can't handle, remove them now. 1860 std::sort(Values.begin(), Values.end(), ConstantIntOrdering()); 1861 Values.erase(std::unique(Values.begin(), Values.end()), Values.end()); 1862 1863 // Figure out which block is which destination. 1864 BasicBlock *DefaultBB = BI->getSuccessor(1); 1865 BasicBlock *EdgeBB = BI->getSuccessor(0); 1866 if (!TrueWhenEqual) std::swap(DefaultBB, EdgeBB); 1867 1868 // Create the new switch instruction now. 1869 SwitchInst *New = new SwitchInst(CompVal, DefaultBB,Values.size(),BI); 1870 1871 // Add all of the 'cases' to the switch instruction. 1872 for (unsigned i = 0, e = Values.size(); i != e; ++i) 1873 New->addCase(Values[i], EdgeBB); 1874 1875 // We added edges from PI to the EdgeBB. As such, if there were any 1876 // PHI nodes in EdgeBB, they need entries to be added corresponding to 1877 // the number of edges added. 1878 for (BasicBlock::iterator BBI = EdgeBB->begin(); 1879 isa<PHINode>(BBI); ++BBI) { 1880 PHINode *PN = cast<PHINode>(BBI); 1881 Value *InVal = PN->getIncomingValueForBlock(*PI); 1882 for (unsigned i = 0, e = Values.size()-1; i != e; ++i) 1883 PN->addIncoming(InVal, *PI); 1884 } 1885 1886 // Erase the old branch instruction. 1887 (*PI)->getInstList().erase(BI); 1888 1889 // Erase the potentially condition tree that was used to computed the 1890 // branch condition. 1891 ErasePossiblyDeadInstructionTree(Cond); 1892 return true; 1893 } 1894 } 1895 1896 // If there is a trivial two-entry PHI node in this basic block, and we can 1897 // eliminate it, do so now. 1898 if (PHINode *PN = dyn_cast<PHINode>(BB->begin())) 1899 if (PN->getNumIncomingValues() == 2) 1900 Changed |= FoldTwoEntryPHINode(PN); 1901 1902 return Changed; 1903} 1904