LICM.cpp revision 1c0af0ed251af3d2ef795903133513656e5c369d
1//===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file is distributed under the University of Illinois Open Source 6// License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// This pass performs loop invariant code motion, attempting to remove as much 11// code from the body of a loop as possible. It does this by either hoisting 12// code into the preheader block, or by sinking code to the exit blocks if it is 13// safe. This pass also promotes must-aliased memory locations in the loop to 14// live in registers, thus hoisting and sinking "invariant" loads and stores. 15// 16// This pass uses alias analysis for two purposes: 17// 18// 1. Moving loop invariant loads and calls out of loops. If we can determine 19// that a load or call inside of a loop never aliases anything stored to, 20// we can hoist it or sink it like any other instruction. 21// 2. Scalar Promotion of Memory - If there is a store instruction inside of 22// the loop, we try to move the store to happen AFTER the loop instead of 23// inside of the loop. This can only happen if a few conditions are true: 24// A. The pointer stored through is loop invariant 25// B. There are no stores or loads in the loop which _may_ alias the 26// pointer. There are no calls in the loop which mod/ref the pointer. 27// If these conditions are true, we can promote the loads and stores in the 28// loop of the pointer to use a temporary alloca'd variable. We then use 29// the SSAUpdater to construct the appropriate SSA form for the value. 30// 31//===----------------------------------------------------------------------===// 32 33#define DEBUG_TYPE "licm" 34#include "llvm/Transforms/Scalar.h" 35#include "llvm/Constants.h" 36#include "llvm/DerivedTypes.h" 37#include "llvm/IntrinsicInst.h" 38#include "llvm/Instructions.h" 39#include "llvm/LLVMContext.h" 40#include "llvm/Analysis/AliasAnalysis.h" 41#include "llvm/Analysis/AliasSetTracker.h" 42#include "llvm/Analysis/ConstantFolding.h" 43#include "llvm/Analysis/LoopInfo.h" 44#include "llvm/Analysis/LoopPass.h" 45#include "llvm/Analysis/Dominators.h" 46#include "llvm/Transforms/Utils/Local.h" 47#include "llvm/Transforms/Utils/SSAUpdater.h" 48#include "llvm/Support/CFG.h" 49#include "llvm/Support/CommandLine.h" 50#include "llvm/Support/raw_ostream.h" 51#include "llvm/Support/Debug.h" 52#include "llvm/ADT/Statistic.h" 53#include <algorithm> 54using namespace llvm; 55 56STATISTIC(NumSunk , "Number of instructions sunk out of loop"); 57STATISTIC(NumHoisted , "Number of instructions hoisted out of loop"); 58STATISTIC(NumMovedLoads, "Number of load insts hoisted or sunk"); 59STATISTIC(NumMovedCalls, "Number of call insts hoisted or sunk"); 60STATISTIC(NumPromoted , "Number of memory locations promoted to registers"); 61 62static cl::opt<bool> 63DisablePromotion("disable-licm-promotion", cl::Hidden, 64 cl::desc("Disable memory promotion in LICM pass")); 65 66namespace { 67 struct LICM : public LoopPass { 68 static char ID; // Pass identification, replacement for typeid 69 LICM() : LoopPass(ID) { 70 initializeLICMPass(*PassRegistry::getPassRegistry()); 71 } 72 73 virtual bool runOnLoop(Loop *L, LPPassManager &LPM); 74 75 /// This transformation requires natural loop information & requires that 76 /// loop preheaders be inserted into the CFG... 77 /// 78 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 79 AU.setPreservesCFG(); 80 AU.addRequired<DominatorTree>(); 81 AU.addRequired<LoopInfo>(); 82 AU.addRequiredID(LoopSimplifyID); 83 AU.addRequired<AliasAnalysis>(); 84 AU.addPreserved<AliasAnalysis>(); 85 AU.addPreserved("scalar-evolution"); 86 AU.addPreservedID(LoopSimplifyID); 87 } 88 89 bool doFinalization() { 90 assert(LoopToAliasSetMap.empty() && "Didn't free loop alias sets"); 91 return false; 92 } 93 94 private: 95 AliasAnalysis *AA; // Current AliasAnalysis information 96 LoopInfo *LI; // Current LoopInfo 97 DominatorTree *DT; // Dominator Tree for the current Loop. 98 99 // State that is updated as we process loops. 100 bool Changed; // Set to true when we change anything. 101 BasicBlock *Preheader; // The preheader block of the current loop... 102 Loop *CurLoop; // The current loop we are working on... 103 AliasSetTracker *CurAST; // AliasSet information for the current loop... 104 DenseMap<Loop*, AliasSetTracker*> LoopToAliasSetMap; 105 106 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info. 107 void cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L); 108 109 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias 110 /// set. 111 void deleteAnalysisValue(Value *V, Loop *L); 112 113 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks 114 /// dominated by the specified block, and that are in the current loop) in 115 /// reverse depth first order w.r.t the DominatorTree. This allows us to 116 /// visit uses before definitions, allowing us to sink a loop body in one 117 /// pass without iteration. 118 /// 119 void SinkRegion(DomTreeNode *N); 120 121 /// HoistRegion - Walk the specified region of the CFG (defined by all 122 /// blocks dominated by the specified block, and that are in the current 123 /// loop) in depth first order w.r.t the DominatorTree. This allows us to 124 /// visit definitions before uses, allowing us to hoist a loop body in one 125 /// pass without iteration. 126 /// 127 void HoistRegion(DomTreeNode *N); 128 129 /// inSubLoop - Little predicate that returns true if the specified basic 130 /// block is in a subloop of the current one, not the current one itself. 131 /// 132 bool inSubLoop(BasicBlock *BB) { 133 assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop"); 134 for (Loop::iterator I = CurLoop->begin(), E = CurLoop->end(); I != E; ++I) 135 if ((*I)->contains(BB)) 136 return true; // A subloop actually contains this block! 137 return false; 138 } 139 140 /// isExitBlockDominatedByBlockInLoop - This method checks to see if the 141 /// specified exit block of the loop is dominated by the specified block 142 /// that is in the body of the loop. We use these constraints to 143 /// dramatically limit the amount of the dominator tree that needs to be 144 /// searched. 145 bool isExitBlockDominatedByBlockInLoop(BasicBlock *ExitBlock, 146 BasicBlock *BlockInLoop) const { 147 // If the block in the loop is the loop header, it must be dominated! 148 BasicBlock *LoopHeader = CurLoop->getHeader(); 149 if (BlockInLoop == LoopHeader) 150 return true; 151 152 DomTreeNode *BlockInLoopNode = DT->getNode(BlockInLoop); 153 DomTreeNode *IDom = DT->getNode(ExitBlock); 154 155 // Because the exit block is not in the loop, we know we have to get _at 156 // least_ its immediate dominator. 157 IDom = IDom->getIDom(); 158 159 while (IDom && IDom != BlockInLoopNode) { 160 // If we have got to the header of the loop, then the instructions block 161 // did not dominate the exit node, so we can't hoist it. 162 if (IDom->getBlock() == LoopHeader) 163 return false; 164 165 // Get next Immediate Dominator. 166 IDom = IDom->getIDom(); 167 }; 168 169 return true; 170 } 171 172 /// sink - When an instruction is found to only be used outside of the loop, 173 /// this function moves it to the exit blocks and patches up SSA form as 174 /// needed. 175 /// 176 void sink(Instruction &I); 177 178 /// hoist - When an instruction is found to only use loop invariant operands 179 /// that is safe to hoist, this instruction is called to do the dirty work. 180 /// 181 void hoist(Instruction &I); 182 183 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it 184 /// is not a trapping instruction or if it is a trapping instruction and is 185 /// guaranteed to execute. 186 /// 187 bool isSafeToExecuteUnconditionally(Instruction &I); 188 189 /// pointerInvalidatedByLoop - Return true if the body of this loop may 190 /// store into the memory location pointed to by V. 191 /// 192 bool pointerInvalidatedByLoop(Value *V, uint64_t Size, 193 const MDNode *TBAAInfo) { 194 // Check to see if any of the basic blocks in CurLoop invalidate *V. 195 return CurAST->getAliasSetForPointer(V, Size, TBAAInfo).isMod(); 196 } 197 198 bool canSinkOrHoistInst(Instruction &I); 199 bool isNotUsedInLoop(Instruction &I); 200 201 void PromoteAliasSet(AliasSet &AS); 202 }; 203} 204 205char LICM::ID = 0; 206INITIALIZE_PASS_BEGIN(LICM, "licm", "Loop Invariant Code Motion", false, false) 207INITIALIZE_PASS_DEPENDENCY(DominatorTree) 208INITIALIZE_PASS_DEPENDENCY(LoopInfo) 209INITIALIZE_PASS_DEPENDENCY(LoopSimplify) 210INITIALIZE_AG_DEPENDENCY(AliasAnalysis) 211INITIALIZE_PASS_END(LICM, "licm", "Loop Invariant Code Motion", false, false) 212 213Pass *llvm::createLICMPass() { return new LICM(); } 214 215/// Hoist expressions out of the specified loop. Note, alias info for inner 216/// loop is not preserved so it is not a good idea to run LICM multiple 217/// times on one loop. 218/// 219bool LICM::runOnLoop(Loop *L, LPPassManager &LPM) { 220 Changed = false; 221 222 // Get our Loop and Alias Analysis information... 223 LI = &getAnalysis<LoopInfo>(); 224 AA = &getAnalysis<AliasAnalysis>(); 225 DT = &getAnalysis<DominatorTree>(); 226 227 CurAST = new AliasSetTracker(*AA); 228 // Collect Alias info from subloops. 229 for (Loop::iterator LoopItr = L->begin(), LoopItrE = L->end(); 230 LoopItr != LoopItrE; ++LoopItr) { 231 Loop *InnerL = *LoopItr; 232 AliasSetTracker *InnerAST = LoopToAliasSetMap[InnerL]; 233 assert(InnerAST && "Where is my AST?"); 234 235 // What if InnerLoop was modified by other passes ? 236 CurAST->add(*InnerAST); 237 238 // Once we've incorporated the inner loop's AST into ours, we don't need the 239 // subloop's anymore. 240 delete InnerAST; 241 LoopToAliasSetMap.erase(InnerL); 242 } 243 244 CurLoop = L; 245 246 // Get the preheader block to move instructions into... 247 Preheader = L->getLoopPreheader(); 248 249 // Loop over the body of this loop, looking for calls, invokes, and stores. 250 // Because subloops have already been incorporated into AST, we skip blocks in 251 // subloops. 252 // 253 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 254 I != E; ++I) { 255 BasicBlock *BB = *I; 256 if (LI->getLoopFor(BB) == L) // Ignore blocks in subloops. 257 CurAST->add(*BB); // Incorporate the specified basic block 258 } 259 260 // We want to visit all of the instructions in this loop... that are not parts 261 // of our subloops (they have already had their invariants hoisted out of 262 // their loop, into this loop, so there is no need to process the BODIES of 263 // the subloops). 264 // 265 // Traverse the body of the loop in depth first order on the dominator tree so 266 // that we are guaranteed to see definitions before we see uses. This allows 267 // us to sink instructions in one pass, without iteration. After sinking 268 // instructions, we perform another pass to hoist them out of the loop. 269 // 270 if (L->hasDedicatedExits()) 271 SinkRegion(DT->getNode(L->getHeader())); 272 if (Preheader) 273 HoistRegion(DT->getNode(L->getHeader())); 274 275 // Now that all loop invariants have been removed from the loop, promote any 276 // memory references to scalars that we can. 277 if (!DisablePromotion && Preheader && L->hasDedicatedExits()) { 278 // Loop over all of the alias sets in the tracker object. 279 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end(); 280 I != E; ++I) 281 PromoteAliasSet(*I); 282 } 283 284 // Clear out loops state information for the next iteration 285 CurLoop = 0; 286 Preheader = 0; 287 288 // If this loop is nested inside of another one, save the alias information 289 // for when we process the outer loop. 290 if (L->getParentLoop()) 291 LoopToAliasSetMap[L] = CurAST; 292 else 293 delete CurAST; 294 return Changed; 295} 296 297/// SinkRegion - Walk the specified region of the CFG (defined by all blocks 298/// dominated by the specified block, and that are in the current loop) in 299/// reverse depth first order w.r.t the DominatorTree. This allows us to visit 300/// uses before definitions, allowing us to sink a loop body in one pass without 301/// iteration. 302/// 303void LICM::SinkRegion(DomTreeNode *N) { 304 assert(N != 0 && "Null dominator tree node?"); 305 BasicBlock *BB = N->getBlock(); 306 307 // If this subregion is not in the top level loop at all, exit. 308 if (!CurLoop->contains(BB)) return; 309 310 // We are processing blocks in reverse dfo, so process children first. 311 const std::vector<DomTreeNode*> &Children = N->getChildren(); 312 for (unsigned i = 0, e = Children.size(); i != e; ++i) 313 SinkRegion(Children[i]); 314 315 // Only need to process the contents of this block if it is not part of a 316 // subloop (which would already have been processed). 317 if (inSubLoop(BB)) return; 318 319 for (BasicBlock::iterator II = BB->end(); II != BB->begin(); ) { 320 Instruction &I = *--II; 321 322 // If the instruction is dead, we would try to sink it because it isn't used 323 // in the loop, instead, just delete it. 324 if (isInstructionTriviallyDead(&I)) { 325 DEBUG(dbgs() << "LICM deleting dead inst: " << I << '\n'); 326 ++II; 327 CurAST->deleteValue(&I); 328 I.eraseFromParent(); 329 Changed = true; 330 continue; 331 } 332 333 // Check to see if we can sink this instruction to the exit blocks 334 // of the loop. We can do this if the all users of the instruction are 335 // outside of the loop. In this case, it doesn't even matter if the 336 // operands of the instruction are loop invariant. 337 // 338 if (isNotUsedInLoop(I) && canSinkOrHoistInst(I)) { 339 ++II; 340 sink(I); 341 } 342 } 343} 344 345/// HoistRegion - Walk the specified region of the CFG (defined by all blocks 346/// dominated by the specified block, and that are in the current loop) in depth 347/// first order w.r.t the DominatorTree. This allows us to visit definitions 348/// before uses, allowing us to hoist a loop body in one pass without iteration. 349/// 350void LICM::HoistRegion(DomTreeNode *N) { 351 assert(N != 0 && "Null dominator tree node?"); 352 BasicBlock *BB = N->getBlock(); 353 354 // If this subregion is not in the top level loop at all, exit. 355 if (!CurLoop->contains(BB)) return; 356 357 // Only need to process the contents of this block if it is not part of a 358 // subloop (which would already have been processed). 359 if (!inSubLoop(BB)) 360 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ) { 361 Instruction &I = *II++; 362 363 // Try constant folding this instruction. If all the operands are 364 // constants, it is technically hoistable, but it would be better to just 365 // fold it. 366 if (Constant *C = ConstantFoldInstruction(&I)) { 367 DEBUG(dbgs() << "LICM folding inst: " << I << " --> " << *C << '\n'); 368 CurAST->copyValue(&I, C); 369 CurAST->deleteValue(&I); 370 I.replaceAllUsesWith(C); 371 I.eraseFromParent(); 372 continue; 373 } 374 375 // Try hoisting the instruction out to the preheader. We can only do this 376 // if all of the operands of the instruction are loop invariant and if it 377 // is safe to hoist the instruction. 378 // 379 if (CurLoop->hasLoopInvariantOperands(&I) && canSinkOrHoistInst(I) && 380 isSafeToExecuteUnconditionally(I)) 381 hoist(I); 382 } 383 384 const std::vector<DomTreeNode*> &Children = N->getChildren(); 385 for (unsigned i = 0, e = Children.size(); i != e; ++i) 386 HoistRegion(Children[i]); 387} 388 389/// canSinkOrHoistInst - Return true if the hoister and sinker can handle this 390/// instruction. 391/// 392bool LICM::canSinkOrHoistInst(Instruction &I) { 393 // Loads have extra constraints we have to verify before we can hoist them. 394 if (LoadInst *LI = dyn_cast<LoadInst>(&I)) { 395 if (LI->isVolatile()) 396 return false; // Don't hoist volatile loads! 397 398 // Loads from constant memory are always safe to move, even if they end up 399 // in the same alias set as something that ends up being modified. 400 if (AA->pointsToConstantMemory(LI->getOperand(0))) 401 return true; 402 403 // Don't hoist loads which have may-aliased stores in loop. 404 uint64_t Size = 0; 405 if (LI->getType()->isSized()) 406 Size = AA->getTypeStoreSize(LI->getType()); 407 return !pointerInvalidatedByLoop(LI->getOperand(0), Size, 408 LI->getMetadata(LLVMContext::MD_tbaa)); 409 } else if (CallInst *CI = dyn_cast<CallInst>(&I)) { 410 // Handle obvious cases efficiently. 411 AliasAnalysis::ModRefBehavior Behavior = AA->getModRefBehavior(CI); 412 if (Behavior == AliasAnalysis::DoesNotAccessMemory) 413 return true; 414 if (AliasAnalysis::onlyReadsMemory(Behavior)) { 415 // If this call only reads from memory and there are no writes to memory 416 // in the loop, we can hoist or sink the call as appropriate. 417 bool FoundMod = false; 418 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end(); 419 I != E; ++I) { 420 AliasSet &AS = *I; 421 if (!AS.isForwardingAliasSet() && AS.isMod()) { 422 FoundMod = true; 423 break; 424 } 425 } 426 if (!FoundMod) return true; 427 } 428 429 // FIXME: This should use mod/ref information to see if we can hoist or sink 430 // the call. 431 432 return false; 433 } 434 435 // Otherwise these instructions are hoistable/sinkable 436 return isa<BinaryOperator>(I) || isa<CastInst>(I) || 437 isa<SelectInst>(I) || isa<GetElementPtrInst>(I) || isa<CmpInst>(I) || 438 isa<InsertElementInst>(I) || isa<ExtractElementInst>(I) || 439 isa<ShuffleVectorInst>(I); 440} 441 442/// isNotUsedInLoop - Return true if the only users of this instruction are 443/// outside of the loop. If this is true, we can sink the instruction to the 444/// exit blocks of the loop. 445/// 446bool LICM::isNotUsedInLoop(Instruction &I) { 447 for (Value::use_iterator UI = I.use_begin(), E = I.use_end(); UI != E; ++UI) { 448 Instruction *User = cast<Instruction>(*UI); 449 if (PHINode *PN = dyn_cast<PHINode>(User)) { 450 // PHI node uses occur in predecessor blocks! 451 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 452 if (PN->getIncomingValue(i) == &I) 453 if (CurLoop->contains(PN->getIncomingBlock(i))) 454 return false; 455 } else if (CurLoop->contains(User)) { 456 return false; 457 } 458 } 459 return true; 460} 461 462 463/// sink - When an instruction is found to only be used outside of the loop, 464/// this function moves it to the exit blocks and patches up SSA form as needed. 465/// This method is guaranteed to remove the original instruction from its 466/// position, and may either delete it or move it to outside of the loop. 467/// 468void LICM::sink(Instruction &I) { 469 DEBUG(dbgs() << "LICM sinking instruction: " << I << "\n"); 470 471 SmallVector<BasicBlock*, 8> ExitBlocks; 472 CurLoop->getUniqueExitBlocks(ExitBlocks); 473 474 if (isa<LoadInst>(I)) ++NumMovedLoads; 475 else if (isa<CallInst>(I)) ++NumMovedCalls; 476 ++NumSunk; 477 Changed = true; 478 479 // The case where there is only a single exit node of this loop is common 480 // enough that we handle it as a special (more efficient) case. It is more 481 // efficient to handle because there are no PHI nodes that need to be placed. 482 if (ExitBlocks.size() == 1) { 483 if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[0], I.getParent())) { 484 // Instruction is not used, just delete it. 485 CurAST->deleteValue(&I); 486 // If I has users in unreachable blocks, eliminate. 487 // If I is not void type then replaceAllUsesWith undef. 488 // This allows ValueHandlers and custom metadata to adjust itself. 489 if (!I.use_empty()) 490 I.replaceAllUsesWith(UndefValue::get(I.getType())); 491 I.eraseFromParent(); 492 } else { 493 // Move the instruction to the start of the exit block, after any PHI 494 // nodes in it. 495 I.moveBefore(ExitBlocks[0]->getFirstNonPHI()); 496 497 // This instruction is no longer in the AST for the current loop, because 498 // we just sunk it out of the loop. If we just sunk it into an outer 499 // loop, we will rediscover the operation when we process it. 500 CurAST->deleteValue(&I); 501 } 502 return; 503 } 504 505 if (ExitBlocks.empty()) { 506 // The instruction is actually dead if there ARE NO exit blocks. 507 CurAST->deleteValue(&I); 508 // If I has users in unreachable blocks, eliminate. 509 // If I is not void type then replaceAllUsesWith undef. 510 // This allows ValueHandlers and custom metadata to adjust itself. 511 if (!I.use_empty()) 512 I.replaceAllUsesWith(UndefValue::get(I.getType())); 513 I.eraseFromParent(); 514 return; 515 } 516 517 // Otherwise, if we have multiple exits, use the SSAUpdater to do all of the 518 // hard work of inserting PHI nodes as necessary. 519 SmallVector<PHINode*, 8> NewPHIs; 520 SSAUpdater SSA(&NewPHIs); 521 522 if (!I.use_empty()) 523 SSA.Initialize(I.getType(), I.getName()); 524 525 // Insert a copy of the instruction in each exit block of the loop that is 526 // dominated by the instruction. Each exit block is known to only be in the 527 // ExitBlocks list once. 528 BasicBlock *InstOrigBB = I.getParent(); 529 unsigned NumInserted = 0; 530 531 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { 532 BasicBlock *ExitBlock = ExitBlocks[i]; 533 534 if (!isExitBlockDominatedByBlockInLoop(ExitBlock, InstOrigBB)) 535 continue; 536 537 // Insert the code after the last PHI node. 538 BasicBlock::iterator InsertPt = ExitBlock->getFirstNonPHI(); 539 540 // If this is the first exit block processed, just move the original 541 // instruction, otherwise clone the original instruction and insert 542 // the copy. 543 Instruction *New; 544 if (NumInserted++ == 0) { 545 I.moveBefore(InsertPt); 546 New = &I; 547 } else { 548 New = I.clone(); 549 if (!I.getName().empty()) 550 New->setName(I.getName()+".le"); 551 ExitBlock->getInstList().insert(InsertPt, New); 552 } 553 554 // Now that we have inserted the instruction, inform SSAUpdater. 555 if (!I.use_empty()) 556 SSA.AddAvailableValue(ExitBlock, New); 557 } 558 559 // If the instruction doesn't dominate any exit blocks, it must be dead. 560 if (NumInserted == 0) { 561 CurAST->deleteValue(&I); 562 if (!I.use_empty()) 563 I.replaceAllUsesWith(UndefValue::get(I.getType())); 564 I.eraseFromParent(); 565 return; 566 } 567 568 // Next, rewrite uses of the instruction, inserting PHI nodes as needed. 569 for (Value::use_iterator UI = I.use_begin(), UE = I.use_end(); UI != UE; ) { 570 // Grab the use before incrementing the iterator. 571 Use &U = UI.getUse(); 572 // Increment the iterator before removing the use from the list. 573 ++UI; 574 SSA.RewriteUseAfterInsertions(U); 575 } 576 577 // Update CurAST for NewPHIs if I had pointer type. 578 if (I.getType()->isPointerTy()) 579 for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i) 580 CurAST->copyValue(&I, NewPHIs[i]); 581 582 // Finally, remove the instruction from CurAST. It is no longer in the loop. 583 CurAST->deleteValue(&I); 584} 585 586/// hoist - When an instruction is found to only use loop invariant operands 587/// that is safe to hoist, this instruction is called to do the dirty work. 588/// 589void LICM::hoist(Instruction &I) { 590 DEBUG(dbgs() << "LICM hoisting to " << Preheader->getName() << ": " 591 << I << "\n"); 592 593 // Move the new node to the Preheader, before its terminator. 594 I.moveBefore(Preheader->getTerminator()); 595 596 if (isa<LoadInst>(I)) ++NumMovedLoads; 597 else if (isa<CallInst>(I)) ++NumMovedCalls; 598 ++NumHoisted; 599 Changed = true; 600} 601 602/// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it is 603/// not a trapping instruction or if it is a trapping instruction and is 604/// guaranteed to execute. 605/// 606bool LICM::isSafeToExecuteUnconditionally(Instruction &Inst) { 607 // If it is not a trapping instruction, it is always safe to hoist. 608 if (Inst.isSafeToSpeculativelyExecute()) 609 return true; 610 611 // Otherwise we have to check to make sure that the instruction dominates all 612 // of the exit blocks. If it doesn't, then there is a path out of the loop 613 // which does not execute this instruction, so we can't hoist it. 614 615 // If the instruction is in the header block for the loop (which is very 616 // common), it is always guaranteed to dominate the exit blocks. Since this 617 // is a common case, and can save some work, check it now. 618 if (Inst.getParent() == CurLoop->getHeader()) 619 return true; 620 621 // Get the exit blocks for the current loop. 622 SmallVector<BasicBlock*, 8> ExitBlocks; 623 CurLoop->getExitBlocks(ExitBlocks); 624 625 // For each exit block, get the DT node and walk up the DT until the 626 // instruction's basic block is found or we exit the loop. 627 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) 628 if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[i], Inst.getParent())) 629 return false; 630 631 return true; 632} 633 634/// PromoteAliasSet - Try to promote memory values to scalars by sinking 635/// stores out of the loop and moving loads to before the loop. We do this by 636/// looping over the stores in the loop, looking for stores to Must pointers 637/// which are loop invariant. 638/// 639void LICM::PromoteAliasSet(AliasSet &AS) { 640 // We can promote this alias set if it has a store, if it is a "Must" alias 641 // set, if the pointer is loop invariant, and if we are not eliminating any 642 // volatile loads or stores. 643 if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() || 644 AS.isVolatile() || !CurLoop->isLoopInvariant(AS.begin()->getValue())) 645 return; 646 647 assert(!AS.empty() && 648 "Must alias set should have at least one pointer element in it!"); 649 Value *SomePtr = AS.begin()->getValue(); 650 651 // It isn't safe to promote a load/store from the loop if the load/store is 652 // conditional. For example, turning: 653 // 654 // for () { if (c) *P += 1; } 655 // 656 // into: 657 // 658 // tmp = *P; for () { if (c) tmp +=1; } *P = tmp; 659 // 660 // is not safe, because *P may only be valid to access if 'c' is true. 661 // 662 // It is safe to promote P if all uses are direct load/stores and if at 663 // least one is guaranteed to be executed. 664 bool GuaranteedToExecute = false; 665 666 SmallVector<Instruction*, 64> LoopUses; 667 SmallPtrSet<Value*, 4> PointerMustAliases; 668 669 // Check that all of the pointers in the alias set have the same type. We 670 // cannot (yet) promote a memory location that is loaded and stored in 671 // different sizes. 672 for (AliasSet::iterator ASI = AS.begin(), E = AS.end(); ASI != E; ++ASI) { 673 Value *ASIV = ASI->getValue(); 674 PointerMustAliases.insert(ASIV); 675 676 // Check that all of the pointers in the alias set have the same type. We 677 // cannot (yet) promote a memory location that is loaded and stored in 678 // different sizes. 679 if (SomePtr->getType() != ASIV->getType()) 680 return; 681 682 for (Value::use_iterator UI = ASIV->use_begin(), UE = ASIV->use_end(); 683 UI != UE; ++UI) { 684 // Ignore instructions that are outside the loop. 685 Instruction *Use = dyn_cast<Instruction>(*UI); 686 if (!Use || !CurLoop->contains(Use)) 687 continue; 688 689 // If there is an non-load/store instruction in the loop, we can't promote 690 // it. 691 if (isa<LoadInst>(Use)) 692 assert(!cast<LoadInst>(Use)->isVolatile() && "AST broken"); 693 else if (isa<StoreInst>(Use)) { 694 if (Use->getOperand(0) == ASIV) return; 695 assert(!cast<StoreInst>(Use)->isVolatile() && "AST broken"); 696 } else 697 return; // Not a load or store. 698 699 if (!GuaranteedToExecute) 700 GuaranteedToExecute = isSafeToExecuteUnconditionally(*Use); 701 702 LoopUses.push_back(Use); 703 } 704 } 705 706 // If there isn't a guaranteed-to-execute instruction, we can't promote. 707 if (!GuaranteedToExecute) 708 return; 709 710 // Otherwise, this is safe to promote, lets do it! 711 DEBUG(dbgs() << "LICM: Promoting value stored to in loop: " <<*SomePtr<<'\n'); 712 Changed = true; 713 ++NumPromoted; 714 715 // We use the SSAUpdater interface to insert phi nodes as required. 716 SmallVector<PHINode*, 16> NewPHIs; 717 SSAUpdater SSA(&NewPHIs); 718 719 // It wants to know some value of the same type as what we'll be inserting. 720 Value *SomeValue; 721 if (isa<LoadInst>(LoopUses[0])) 722 SomeValue = LoopUses[0]; 723 else 724 SomeValue = cast<StoreInst>(LoopUses[0])->getOperand(0); 725 SSA.Initialize(SomeValue->getType(), SomeValue->getName()); 726 727 // First step: bucket up uses of the pointers by the block they occur in. 728 // This is important because we have to handle multiple defs/uses in a block 729 // ourselves: SSAUpdater is purely for cross-block references. 730 // FIXME: Want a TinyVector<Instruction*> since there is usually 0/1 element. 731 DenseMap<BasicBlock*, std::vector<Instruction*> > UsesByBlock; 732 for (unsigned i = 0, e = LoopUses.size(); i != e; ++i) { 733 Instruction *User = LoopUses[i]; 734 UsesByBlock[User->getParent()].push_back(User); 735 } 736 737 // Okay, now we can iterate over all the blocks in the loop with uses, 738 // processing them. Keep track of which loads are loading a live-in value. 739 SmallVector<LoadInst*, 32> LiveInLoads; 740 DenseMap<Value*, Value*> ReplacedLoads; 741 742 for (unsigned LoopUse = 0, e = LoopUses.size(); LoopUse != e; ++LoopUse) { 743 Instruction *User = LoopUses[LoopUse]; 744 std::vector<Instruction*> &BlockUses = UsesByBlock[User->getParent()]; 745 746 // If this block has already been processed, ignore this repeat use. 747 if (BlockUses.empty()) continue; 748 749 // Okay, this is the first use in the block. If this block just has a 750 // single user in it, we can rewrite it trivially. 751 if (BlockUses.size() == 1) { 752 // If it is a store, it is a trivial def of the value in the block. 753 if (isa<StoreInst>(User)) { 754 SSA.AddAvailableValue(User->getParent(), 755 cast<StoreInst>(User)->getOperand(0)); 756 } else { 757 // Otherwise it is a load, queue it to rewrite as a live-in load. 758 LiveInLoads.push_back(cast<LoadInst>(User)); 759 } 760 BlockUses.clear(); 761 continue; 762 } 763 764 // Otherwise, check to see if this block is all loads. If so, we can queue 765 // them all as live in loads. 766 bool HasStore = false; 767 for (unsigned i = 0, e = BlockUses.size(); i != e; ++i) { 768 if (isa<StoreInst>(BlockUses[i])) { 769 HasStore = true; 770 break; 771 } 772 } 773 774 if (!HasStore) { 775 for (unsigned i = 0, e = BlockUses.size(); i != e; ++i) 776 LiveInLoads.push_back(cast<LoadInst>(BlockUses[i])); 777 BlockUses.clear(); 778 continue; 779 } 780 781 // Otherwise, we have mixed loads and stores (or just a bunch of stores). 782 // Since SSAUpdater is purely for cross-block values, we need to determine 783 // the order of these instructions in the block. If the first use in the 784 // block is a load, then it uses the live in value. The last store defines 785 // the live out value. We handle this by doing a linear scan of the block. 786 BasicBlock *BB = User->getParent(); 787 Value *StoredValue = 0; 788 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) { 789 if (LoadInst *L = dyn_cast<LoadInst>(II)) { 790 // If this is a load from an unrelated pointer, ignore it. 791 if (!PointerMustAliases.count(L->getOperand(0))) continue; 792 793 // If we haven't seen a store yet, this is a live in use, otherwise 794 // use the stored value. 795 if (StoredValue) { 796 L->replaceAllUsesWith(StoredValue); 797 ReplacedLoads[L] = StoredValue; 798 } else { 799 LiveInLoads.push_back(L); 800 } 801 continue; 802 } 803 804 if (StoreInst *S = dyn_cast<StoreInst>(II)) { 805 // If this is a store to an unrelated pointer, ignore it. 806 if (!PointerMustAliases.count(S->getOperand(1))) continue; 807 808 // Remember that this is the active value in the block. 809 StoredValue = S->getOperand(0); 810 } 811 } 812 813 // The last stored value that happened is the live-out for the block. 814 assert(StoredValue && "Already checked that there is a store in block"); 815 SSA.AddAvailableValue(BB, StoredValue); 816 BlockUses.clear(); 817 } 818 819 // Now that all the intra-loop values are classified, set up the preheader. 820 // It gets a load of the pointer we're promoting, and it is the live-out value 821 // from the preheader. 822 LoadInst *PreheaderLoad = new LoadInst(SomePtr,SomePtr->getName()+".promoted", 823 Preheader->getTerminator()); 824 SSA.AddAvailableValue(Preheader, PreheaderLoad); 825 826 // Now that the preheader is good to go, set up the exit blocks. Each exit 827 // block gets a store of the live-out values that feed them. Since we've 828 // already told the SSA updater about the defs in the loop and the preheader 829 // definition, it is all set and we can start using it. 830 SmallVector<BasicBlock*, 8> ExitBlocks; 831 CurLoop->getUniqueExitBlocks(ExitBlocks); 832 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { 833 BasicBlock *ExitBlock = ExitBlocks[i]; 834 Value *LiveInValue = SSA.GetValueInMiddleOfBlock(ExitBlock); 835 Instruction *InsertPos = ExitBlock->getFirstNonPHI(); 836 new StoreInst(LiveInValue, SomePtr, InsertPos); 837 } 838 839 // Okay, now we rewrite all loads that use live-in values in the loop, 840 // inserting PHI nodes as necessary. 841 for (unsigned i = 0, e = LiveInLoads.size(); i != e; ++i) { 842 LoadInst *ALoad = LiveInLoads[i]; 843 Value *NewVal = SSA.GetValueInMiddleOfBlock(ALoad->getParent()); 844 ALoad->replaceAllUsesWith(NewVal); 845 CurAST->copyValue(ALoad, NewVal); 846 ReplacedLoads[ALoad] = NewVal; 847 } 848 849 // If the preheader load is itself a pointer, we need to tell alias analysis 850 // about the new pointer we created in the preheader block and about any PHI 851 // nodes that just got inserted. 852 if (PreheaderLoad->getType()->isPointerTy()) { 853 // Copy any value stored to or loaded from a must-alias of the pointer. 854 CurAST->copyValue(SomeValue, PreheaderLoad); 855 856 for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i) 857 CurAST->copyValue(SomeValue, NewPHIs[i]); 858 } 859 860 // Now that everything is rewritten, delete the old instructions from the body 861 // of the loop. They should all be dead now. 862 for (unsigned i = 0, e = LoopUses.size(); i != e; ++i) { 863 Instruction *User = LoopUses[i]; 864 865 // If this is a load that still has uses, then the load must have been added 866 // as a live value in the SSAUpdate data structure for a block (e.g. because 867 // the loaded value was stored later). In this case, we need to recursively 868 // propagate the updates until we get to the real value. 869 if (!User->use_empty()) { 870 Value *NewVal = ReplacedLoads[User]; 871 assert(NewVal && "not a replaced load?"); 872 873 // Propagate down to the ultimate replacee. The intermediately loads 874 // could theoretically already have been deleted, so we don't want to 875 // dereference the Value*'s. 876 DenseMap<Value*, Value*>::iterator RLI = ReplacedLoads.find(NewVal); 877 while (RLI != ReplacedLoads.end()) { 878 NewVal = RLI->second; 879 RLI = ReplacedLoads.find(NewVal); 880 } 881 882 User->replaceAllUsesWith(NewVal); 883 CurAST->copyValue(User, NewVal); 884 } 885 886 CurAST->deleteValue(User); 887 User->eraseFromParent(); 888 } 889 890 // fwew, we're done! 891} 892 893 894/// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info. 895void LICM::cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L) { 896 AliasSetTracker *AST = LoopToAliasSetMap.lookup(L); 897 if (!AST) 898 return; 899 900 AST->copyValue(From, To); 901} 902 903/// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias 904/// set. 905void LICM::deleteAnalysisValue(Value *V, Loop *L) { 906 AliasSetTracker *AST = LoopToAliasSetMap.lookup(L); 907 if (!AST) 908 return; 909 910 AST->deleteValue(V); 911} 912