JumpThreading.cpp revision eede65ce6cb29c8f3a701be8606e95c9a213efff
1//===- JumpThreading.cpp - Thread control through conditional blocks ------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file is distributed under the University of Illinois Open Source 6// License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// This file implements the Jump Threading pass. 11// 12//===----------------------------------------------------------------------===// 13 14#define DEBUG_TYPE "jump-threading" 15#include "llvm/Transforms/Scalar.h" 16#include "llvm/IntrinsicInst.h" 17#include "llvm/Pass.h" 18#include "llvm/ADT/DenseMap.h" 19#include "llvm/ADT/Statistic.h" 20#include "llvm/Transforms/Utils/BasicBlockUtils.h" 21#include "llvm/Transforms/Utils/Local.h" 22#include "llvm/Support/CommandLine.h" 23#include "llvm/Support/Compiler.h" 24#include "llvm/Support/Debug.h" 25using namespace llvm; 26 27STATISTIC(NumThreads, "Number of jumps threaded"); 28STATISTIC(NumFolds, "Number of terminators folded"); 29 30static cl::opt<unsigned> 31Threshold("jump-threading-threshold", 32 cl::desc("Max block size to duplicate for jump threading"), 33 cl::init(6), cl::Hidden); 34 35namespace { 36 /// This pass performs 'jump threading', which looks at blocks that have 37 /// multiple predecessors and multiple successors. If one or more of the 38 /// predecessors of the block can be proven to always jump to one of the 39 /// successors, we forward the edge from the predecessor to the successor by 40 /// duplicating the contents of this block. 41 /// 42 /// An example of when this can occur is code like this: 43 /// 44 /// if () { ... 45 /// X = 4; 46 /// } 47 /// if (X < 3) { 48 /// 49 /// In this case, the unconditional branch at the end of the first if can be 50 /// revectored to the false side of the second if. 51 /// 52 class VISIBILITY_HIDDEN JumpThreading : public FunctionPass { 53 public: 54 static char ID; // Pass identification 55 JumpThreading() : FunctionPass((intptr_t)&ID) {} 56 57 bool runOnFunction(Function &F); 58 bool ThreadBlock(BasicBlock *BB); 59 void ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BasicBlock *SuccBB); 60 BasicBlock *FactorCommonPHIPreds(PHINode *PN, Constant *CstVal); 61 62 bool ProcessJumpOnPHI(PHINode *PN); 63 bool ProcessBranchOnLogical(Value *V, BasicBlock *BB, bool isAnd); 64 bool ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB); 65 }; 66 char JumpThreading::ID = 0; 67 RegisterPass<JumpThreading> X("jump-threading", "Jump Threading"); 68} 69 70// Public interface to the Jump Threading pass 71FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); } 72 73/// runOnFunction - Top level algorithm. 74/// 75bool JumpThreading::runOnFunction(Function &F) { 76 DOUT << "Jump threading on function '" << F.getNameStart() << "'\n"; 77 78 bool AnotherIteration = true, EverChanged = false; 79 while (AnotherIteration) { 80 AnotherIteration = false; 81 bool Changed = false; 82 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) 83 while (ThreadBlock(I)) 84 Changed = true; 85 AnotherIteration = Changed; 86 EverChanged |= Changed; 87 } 88 return EverChanged; 89} 90 91/// FactorCommonPHIPreds - If there are multiple preds with the same incoming 92/// value for the PHI, factor them together so we get one block to thread for 93/// the whole group. 94/// This is important for things like "phi i1 [true, true, false, true, x]" 95/// where we only need to clone the block for the true blocks once. 96/// 97BasicBlock *JumpThreading::FactorCommonPHIPreds(PHINode *PN, Constant *CstVal) { 98 SmallVector<BasicBlock*, 16> CommonPreds; 99 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 100 if (PN->getIncomingValue(i) == CstVal) 101 CommonPreds.push_back(PN->getIncomingBlock(i)); 102 103 if (CommonPreds.size() == 1) 104 return CommonPreds[0]; 105 106 DOUT << " Factoring out " << CommonPreds.size() 107 << " common predecessors.\n"; 108 return SplitBlockPredecessors(PN->getParent(), 109 &CommonPreds[0], CommonPreds.size(), 110 ".thr_comm", this); 111} 112 113 114/// getJumpThreadDuplicationCost - Return the cost of duplicating this block to 115/// thread across it. 116static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) { 117 BasicBlock::const_iterator I = BB->begin(); 118 /// Ignore PHI nodes, these will be flattened when duplication happens. 119 while (isa<PHINode>(*I)) ++I; 120 121 // Sum up the cost of each instruction until we get to the terminator. Don't 122 // include the terminator because the copy won't include it. 123 unsigned Size = 0; 124 for (; !isa<TerminatorInst>(I); ++I) { 125 // Debugger intrinsics don't incur code size. 126 if (isa<DbgInfoIntrinsic>(I)) continue; 127 128 // If this is a pointer->pointer bitcast, it is free. 129 if (isa<BitCastInst>(I) && isa<PointerType>(I->getType())) 130 continue; 131 132 // All other instructions count for at least one unit. 133 ++Size; 134 135 // Calls are more expensive. If they are non-intrinsic calls, we model them 136 // as having cost of 4. If they are a non-vector intrinsic, we model them 137 // as having cost of 2 total, and if they are a vector intrinsic, we model 138 // them as having cost 1. 139 if (const CallInst *CI = dyn_cast<CallInst>(I)) { 140 if (!isa<IntrinsicInst>(CI)) 141 Size += 3; 142 else if (isa<VectorType>(CI->getType())) 143 Size += 1; 144 } 145 } 146 147 // Threading through a switch statement is particularly profitable. If this 148 // block ends in a switch, decrease its cost to make it more likely to happen. 149 if (isa<SwitchInst>(I)) 150 Size = Size > 6 ? Size-6 : 0; 151 152 return Size; 153} 154 155 156/// ThreadBlock - If there are any predecessors whose control can be threaded 157/// through to a successor, transform them now. 158bool JumpThreading::ThreadBlock(BasicBlock *BB) { 159 // See if this block ends with a branch of switch. If so, see if the 160 // condition is a phi node. If so, and if an entry of the phi node is a 161 // constant, we can thread the block. 162 Value *Condition; 163 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) { 164 // Can't thread an unconditional jump. 165 if (BI->isUnconditional()) return false; 166 Condition = BI->getCondition(); 167 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) 168 Condition = SI->getCondition(); 169 else 170 return false; // Must be an invoke. 171 172 // If the terminator of this block is branching on a constant, simplify the 173 // terminator to an unconditional branch. This can occur due to threading in 174 // other blocks. 175 if (isa<ConstantInt>(Condition)) { 176 DOUT << " In block '" << BB->getNameStart() 177 << "' folding terminator: " << *BB->getTerminator(); 178 ++NumFolds; 179 ConstantFoldTerminator(BB); 180 return true; 181 } 182 183 // If there is only a single predecessor of this block, nothing to fold. 184 if (BB->getSinglePredecessor()) 185 return false; 186 187 // See if this is a phi node in the current block. 188 PHINode *PN = dyn_cast<PHINode>(Condition); 189 if (PN && PN->getParent() == BB) 190 return ProcessJumpOnPHI(PN); 191 192 // If this is a conditional branch whose condition is and/or of a phi, try to 193 // simplify it. 194 if (BinaryOperator *CondI = dyn_cast<BinaryOperator>(Condition)) { 195 if ((CondI->getOpcode() == Instruction::And || 196 CondI->getOpcode() == Instruction::Or) && 197 isa<BranchInst>(BB->getTerminator()) && 198 ProcessBranchOnLogical(CondI, BB, 199 CondI->getOpcode() == Instruction::And)) 200 return true; 201 } 202 203 // If we have "br (phi != 42)" and the phi node has any constant values as 204 // operands, we can thread through this block. 205 if (CmpInst *CondCmp = dyn_cast<CmpInst>(Condition)) 206 if (isa<PHINode>(CondCmp->getOperand(0)) && 207 isa<Constant>(CondCmp->getOperand(1)) && 208 ProcessBranchOnCompare(CondCmp, BB)) 209 return true; 210 211 return false; 212} 213 214/// ProcessJumpOnPHI - We have a conditional branch of switch on a PHI node in 215/// the current block. See if there are any simplifications we can do based on 216/// inputs to the phi node. 217/// 218bool JumpThreading::ProcessJumpOnPHI(PHINode *PN) { 219 // See if the phi node has any constant values. If so, we can determine where 220 // the corresponding predecessor will branch. 221 ConstantInt *PredCst = 0; 222 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 223 if ((PredCst = dyn_cast<ConstantInt>(PN->getIncomingValue(i)))) 224 break; 225 226 // If no incoming value has a constant, we don't know the destination of any 227 // predecessors. 228 if (PredCst == 0) 229 return false; 230 231 // See if the cost of duplicating this block is low enough. 232 BasicBlock *BB = PN->getParent(); 233 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); 234 if (JumpThreadCost > Threshold) { 235 DOUT << " Not threading BB '" << BB->getNameStart() 236 << "' - Cost is too high: " << JumpThreadCost << "\n"; 237 return false; 238 } 239 240 // If so, we can actually do this threading. Merge any common predecessors 241 // that will act the same. 242 BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst); 243 244 // Next, figure out which successor we are threading to. 245 BasicBlock *SuccBB; 246 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) 247 SuccBB = BI->getSuccessor(PredCst == ConstantInt::getFalse()); 248 else { 249 SwitchInst *SI = cast<SwitchInst>(BB->getTerminator()); 250 SuccBB = SI->getSuccessor(SI->findCaseValue(PredCst)); 251 } 252 253 // If threading to the same block as we come from, we would infinite loop. 254 if (SuccBB == BB) { 255 DOUT << " Not threading BB '" << BB->getNameStart() 256 << "' - would thread to self!\n"; 257 return false; 258 } 259 260 // And finally, do it! 261 DOUT << " Threading edge from '" << PredBB->getNameStart() << "' to '" 262 << SuccBB->getNameStart() << "' with cost: " << JumpThreadCost 263 << ", across block:\n " 264 << *BB << "\n"; 265 266 ThreadEdge(BB, PredBB, SuccBB); 267 ++NumThreads; 268 return true; 269} 270 271/// ProcessJumpOnLogicalPHI - PN's basic block contains a conditional branch 272/// whose condition is an AND/OR where one side is PN. If PN has constant 273/// operands that permit us to evaluate the condition for some operand, thread 274/// through the block. For example with: 275/// br (and X, phi(Y, Z, false)) 276/// the predecessor corresponding to the 'false' will always jump to the false 277/// destination of the branch. 278/// 279bool JumpThreading::ProcessBranchOnLogical(Value *V, BasicBlock *BB, 280 bool isAnd) { 281 // If this is a binary operator tree of the same AND/OR opcode, check the 282 // LHS/RHS. 283 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(V)) 284 if (isAnd && BO->getOpcode() == Instruction::And || 285 !isAnd && BO->getOpcode() == Instruction::Or) { 286 if (ProcessBranchOnLogical(BO->getOperand(0), BB, isAnd)) 287 return true; 288 if (ProcessBranchOnLogical(BO->getOperand(1), BB, isAnd)) 289 return true; 290 } 291 292 // If this isn't a PHI node, we can't handle it. 293 PHINode *PN = dyn_cast<PHINode>(V); 294 if (!PN || PN->getParent() != BB) return false; 295 296 // We can only do the simplification for phi nodes of 'false' with AND or 297 // 'true' with OR. See if we have any entries in the phi for this. 298 unsigned PredNo = ~0U; 299 ConstantInt *PredCst = ConstantInt::get(Type::Int1Ty, !isAnd); 300 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 301 if (PN->getIncomingValue(i) == PredCst) { 302 PredNo = i; 303 break; 304 } 305 } 306 307 // If no match, bail out. 308 if (PredNo == ~0U) 309 return false; 310 311 // See if the cost of duplicating this block is low enough. 312 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); 313 if (JumpThreadCost > Threshold) { 314 DOUT << " Not threading BB '" << BB->getNameStart() 315 << "' - Cost is too high: " << JumpThreadCost << "\n"; 316 return false; 317 } 318 319 // If so, we can actually do this threading. Merge any common predecessors 320 // that will act the same. 321 BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst); 322 323 // Next, figure out which successor we are threading to. If this was an AND, 324 // the constant must be FALSE, and we must be targeting the 'false' block. 325 // If this is an OR, the constant must be TRUE, and we must be targeting the 326 // 'true' block. 327 BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(isAnd); 328 329 // If threading to the same block as we come from, we would infinite loop. 330 if (SuccBB == BB) { 331 DOUT << " Not threading BB '" << BB->getNameStart() 332 << "' - would thread to self!\n"; 333 return false; 334 } 335 336 // And finally, do it! 337 DOUT << " Threading edge through bool from '" << PredBB->getNameStart() 338 << "' to '" << SuccBB->getNameStart() << "' with cost: " 339 << JumpThreadCost << ", across block:\n " 340 << *BB << "\n"; 341 342 ThreadEdge(BB, PredBB, SuccBB); 343 ++NumThreads; 344 return true; 345} 346 347/// ProcessBranchOnCompare - We found a branch on a comparison between a phi 348/// node and a constant. If the PHI node contains any constants as inputs, we 349/// can fold the compare for that edge and thread through it. 350bool JumpThreading::ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB) { 351 PHINode *PN = cast<PHINode>(Cmp->getOperand(0)); 352 Constant *RHS = cast<Constant>(Cmp->getOperand(1)); 353 354 // If the phi isn't in the current block, an incoming edge to this block 355 // doesn't control the destination. 356 if (PN->getParent() != BB) 357 return false; 358 359 // We can do this simplification if any comparisons fold to true or false. 360 // See if any do. 361 Constant *PredCst = 0; 362 bool TrueDirection = false; 363 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { 364 PredCst = dyn_cast<Constant>(PN->getIncomingValue(i)); 365 if (PredCst == 0) continue; 366 367 Constant *Res; 368 if (ICmpInst *ICI = dyn_cast<ICmpInst>(Cmp)) 369 Res = ConstantExpr::getICmp(ICI->getPredicate(), PredCst, RHS); 370 else 371 Res = ConstantExpr::getFCmp(cast<FCmpInst>(Cmp)->getPredicate(), 372 PredCst, RHS); 373 // If this folded to a constant expr, we can't do anything. 374 if (ConstantInt *ResC = dyn_cast<ConstantInt>(Res)) { 375 TrueDirection = ResC->getZExtValue(); 376 break; 377 } 378 // If this folded to undef, just go the false way. 379 if (isa<UndefValue>(Res)) { 380 TrueDirection = false; 381 break; 382 } 383 384 // Otherwise, we can't fold this input. 385 PredCst = 0; 386 } 387 388 // If no match, bail out. 389 if (PredCst == 0) 390 return false; 391 392 // See if the cost of duplicating this block is low enough. 393 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); 394 if (JumpThreadCost > Threshold) { 395 DOUT << " Not threading BB '" << BB->getNameStart() 396 << "' - Cost is too high: " << JumpThreadCost << "\n"; 397 return false; 398 } 399 400 // If so, we can actually do this threading. Merge any common predecessors 401 // that will act the same. 402 BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst); 403 404 // Next, get our successor. 405 BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(!TrueDirection); 406 407 // If threading to the same block as we come from, we would infinite loop. 408 if (SuccBB == BB) { 409 DOUT << " Not threading BB '" << BB->getNameStart() 410 << "' - would thread to self!\n"; 411 return false; 412 } 413 414 415 // And finally, do it! 416 DOUT << " Threading edge through bool from '" << PredBB->getNameStart() 417 << "' to '" << SuccBB->getNameStart() << "' with cost: " 418 << JumpThreadCost << ", across block:\n " 419 << *BB << "\n"; 420 421 ThreadEdge(BB, PredBB, SuccBB); 422 ++NumThreads; 423 return true; 424} 425 426 427/// ThreadEdge - We have decided that it is safe and profitable to thread an 428/// edge from PredBB to SuccBB across BB. Transform the IR to reflect this 429/// change. 430void JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, 431 BasicBlock *SuccBB) { 432 433 // Jump Threading can not update SSA properties correctly if the values 434 // defined in the duplicated block are used outside of the block itself. For 435 // this reason, we spill all values that are used outside of BB to the stack. 436 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) 437 if (I->isUsedOutsideOfBlock(BB)) { 438 // We found a use of I outside of BB. Create a new stack slot to 439 // break this inter-block usage pattern. 440 DemoteRegToStack(*I); 441 } 442 443 // We are going to have to map operands from the original BB block to the new 444 // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to 445 // account for entry from PredBB. 446 DenseMap<Instruction*, Value*> ValueMapping; 447 448 BasicBlock *NewBB = 449 BasicBlock::Create(BB->getName()+".thread", BB->getParent(), BB); 450 NewBB->moveAfter(PredBB); 451 452 BasicBlock::iterator BI = BB->begin(); 453 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) 454 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB); 455 456 // Clone the non-phi instructions of BB into NewBB, keeping track of the 457 // mapping and using it to remap operands in the cloned instructions. 458 for (; !isa<TerminatorInst>(BI); ++BI) { 459 Instruction *New = BI->clone(); 460 New->setName(BI->getNameStart()); 461 NewBB->getInstList().push_back(New); 462 ValueMapping[BI] = New; 463 464 // Remap operands to patch up intra-block references. 465 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i) 466 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) 467 if (Value *Remapped = ValueMapping[Inst]) 468 New->setOperand(i, Remapped); 469 } 470 471 // We didn't copy the terminator from BB over to NewBB, because there is now 472 // an unconditional jump to SuccBB. Insert the unconditional jump. 473 BranchInst::Create(SuccBB, NewBB); 474 475 // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the 476 // PHI nodes for NewBB now. 477 for (BasicBlock::iterator PNI = SuccBB->begin(); isa<PHINode>(PNI); ++PNI) { 478 PHINode *PN = cast<PHINode>(PNI); 479 // Ok, we have a PHI node. Figure out what the incoming value was for the 480 // DestBlock. 481 Value *IV = PN->getIncomingValueForBlock(BB); 482 483 // Remap the value if necessary. 484 if (Instruction *Inst = dyn_cast<Instruction>(IV)) 485 if (Value *MappedIV = ValueMapping[Inst]) 486 IV = MappedIV; 487 PN->addIncoming(IV, NewBB); 488 } 489 490 // Finally, NewBB is good to go. Update the terminator of PredBB to jump to 491 // NewBB instead of BB. This eliminates predecessors from BB, which requires 492 // us to simplify any PHI nodes in BB. 493 TerminatorInst *PredTerm = PredBB->getTerminator(); 494 for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i) 495 if (PredTerm->getSuccessor(i) == BB) { 496 BB->removePredecessor(PredBB); 497 PredTerm->setSuccessor(i, NewBB); 498 } 499} 500