LICM.cpp revision 44e2bd31f13da68bca451e4210560db3644b8208
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/Target/TargetData.h" 40#include "llvm/Analysis/LoopInfo.h" 41#include "llvm/Analysis/LoopPass.h" 42#include "llvm/Analysis/AliasAnalysis.h" 43#include "llvm/Analysis/AliasSetTracker.h" 44#include "llvm/Analysis/Dominators.h" 45#include "llvm/Analysis/ScalarEvolution.h" 46#include "llvm/Transforms/Utils/SSAUpdater.h" 47#include "llvm/Support/CFG.h" 48#include "llvm/Support/CommandLine.h" 49#include "llvm/Support/raw_ostream.h" 50#include "llvm/Support/Debug.h" 51#include "llvm/ADT/Statistic.h" 52#include <algorithm> 53using namespace llvm; 54 55STATISTIC(NumSunk , "Number of instructions sunk out of loop"); 56STATISTIC(NumHoisted , "Number of instructions hoisted out of loop"); 57STATISTIC(NumMovedLoads, "Number of load insts hoisted or sunk"); 58STATISTIC(NumMovedCalls, "Number of call insts hoisted or sunk"); 59STATISTIC(NumPromoted , "Number of memory locations promoted to registers"); 60 61static cl::opt<bool> 62DisablePromotion("disable-licm-promotion", cl::Hidden, 63 cl::desc("Disable memory promotion in LICM pass")); 64 65namespace { 66 struct LICM : public LoopPass { 67 static char ID; // Pass identification, replacement for typeid 68 LICM() : LoopPass(ID) {} 69 70 virtual bool runOnLoop(Loop *L, LPPassManager &LPM); 71 72 /// This transformation requires natural loop information & requires that 73 /// loop preheaders be inserted into the CFG... 74 /// 75 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 76 AU.setPreservesCFG(); 77 AU.addRequired<DominatorTree>(); 78 AU.addRequired<LoopInfo>(); 79 AU.addRequiredID(LoopSimplifyID); 80 AU.addRequired<AliasAnalysis>(); 81 AU.addPreserved<ScalarEvolution>(); 82 AU.addPreserved<DominanceFrontier>(); 83 AU.addPreservedID(LoopSimplifyID); 84 } 85 86 bool doFinalization() { 87 // Free the values stored in the map 88 for (std::map<Loop *, AliasSetTracker *>::iterator 89 I = LoopToAliasMap.begin(), E = LoopToAliasMap.end(); I != E; ++I) 90 delete I->second; 91 92 LoopToAliasMap.clear(); 93 return false; 94 } 95 96 private: 97 // Various analyses that we use... 98 AliasAnalysis *AA; // Current AliasAnalysis information 99 LoopInfo *LI; // Current LoopInfo 100 DominatorTree *DT; // Dominator Tree for the current Loop. 101 102 // State that is updated as we process loops 103 bool Changed; // Set to true when we change anything. 104 BasicBlock *Preheader; // The preheader block of the current loop... 105 Loop *CurLoop; // The current loop we are working on... 106 AliasSetTracker *CurAST; // AliasSet information for the current loop... 107 std::map<Loop *, AliasSetTracker *> LoopToAliasMap; 108 109 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info. 110 void cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L); 111 112 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias 113 /// set. 114 void deleteAnalysisValue(Value *V, Loop *L); 115 116 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks 117 /// dominated by the specified block, and that are in the current loop) in 118 /// reverse depth first order w.r.t the DominatorTree. This allows us to 119 /// visit uses before definitions, allowing us to sink a loop body in one 120 /// pass without iteration. 121 /// 122 void SinkRegion(DomTreeNode *N); 123 124 /// HoistRegion - Walk the specified region of the CFG (defined by all 125 /// blocks dominated by the specified block, and that are in the current 126 /// loop) in depth first order w.r.t the DominatorTree. This allows us to 127 /// visit definitions before uses, allowing us to hoist a loop body in one 128 /// pass without iteration. 129 /// 130 void HoistRegion(DomTreeNode *N); 131 132 /// inSubLoop - Little predicate that returns true if the specified basic 133 /// block is in a subloop of the current one, not the current one itself. 134 /// 135 bool inSubLoop(BasicBlock *BB) { 136 assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop"); 137 for (Loop::iterator I = CurLoop->begin(), E = CurLoop->end(); I != E; ++I) 138 if ((*I)->contains(BB)) 139 return true; // A subloop actually contains this block! 140 return false; 141 } 142 143 /// isExitBlockDominatedByBlockInLoop - This method checks to see if the 144 /// specified exit block of the loop is dominated by the specified block 145 /// that is in the body of the loop. We use these constraints to 146 /// dramatically limit the amount of the dominator tree that needs to be 147 /// searched. 148 bool isExitBlockDominatedByBlockInLoop(BasicBlock *ExitBlock, 149 BasicBlock *BlockInLoop) const { 150 // If the block in the loop is the loop header, it must be dominated! 151 BasicBlock *LoopHeader = CurLoop->getHeader(); 152 if (BlockInLoop == LoopHeader) 153 return true; 154 155 DomTreeNode *BlockInLoopNode = DT->getNode(BlockInLoop); 156 DomTreeNode *IDom = DT->getNode(ExitBlock); 157 158 // Because the exit block is not in the loop, we know we have to get _at 159 // least_ its immediate dominator. 160 IDom = IDom->getIDom(); 161 162 while (IDom && IDom != BlockInLoopNode) { 163 // If we have got to the header of the loop, then the instructions block 164 // did not dominate the exit node, so we can't hoist it. 165 if (IDom->getBlock() == LoopHeader) 166 return false; 167 168 // Get next Immediate Dominator. 169 IDom = IDom->getIDom(); 170 }; 171 172 return true; 173 } 174 175 /// sink - When an instruction is found to only be used outside of the loop, 176 /// this function moves it to the exit blocks and patches up SSA form as 177 /// needed. 178 /// 179 void sink(Instruction &I); 180 181 /// hoist - When an instruction is found to only use loop invariant operands 182 /// that is safe to hoist, this instruction is called to do the dirty work. 183 /// 184 void hoist(Instruction &I); 185 186 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it 187 /// is not a trapping instruction or if it is a trapping instruction and is 188 /// guaranteed to execute. 189 /// 190 bool isSafeToExecuteUnconditionally(Instruction &I); 191 192 /// pointerInvalidatedByLoop - Return true if the body of this loop may 193 /// store into the memory location pointed to by V. 194 /// 195 bool pointerInvalidatedByLoop(Value *V, unsigned Size) { 196 // Check to see if any of the basic blocks in CurLoop invalidate *V. 197 return CurAST->getAliasSetForPointer(V, Size).isMod(); 198 } 199 200 bool canSinkOrHoistInst(Instruction &I); 201 bool isLoopInvariantInst(Instruction &I); 202 bool isNotUsedInLoop(Instruction &I); 203 204 void PromoteAliasSet(AliasSet &AS); 205 }; 206} 207 208char LICM::ID = 0; 209INITIALIZE_PASS(LICM, "licm", "Loop Invariant Code Motion", false, false); 210 211Pass *llvm::createLICMPass() { return new LICM(); } 212 213/// Hoist expressions out of the specified loop. Note, alias info for inner 214/// loop is not preserved so it is not a good idea to run LICM multiple 215/// times on one loop. 216/// 217bool LICM::runOnLoop(Loop *L, LPPassManager &LPM) { 218 Changed = false; 219 220 // Get our Loop and Alias Analysis information... 221 LI = &getAnalysis<LoopInfo>(); 222 AA = &getAnalysis<AliasAnalysis>(); 223 DT = &getAnalysis<DominatorTree>(); 224 225 CurAST = new AliasSetTracker(*AA); 226 // Collect Alias info from subloops 227 for (Loop::iterator LoopItr = L->begin(), LoopItrE = L->end(); 228 LoopItr != LoopItrE; ++LoopItr) { 229 Loop *InnerL = *LoopItr; 230 AliasSetTracker *InnerAST = LoopToAliasMap[InnerL]; 231 assert (InnerAST && "Where is my AST?"); 232 233 // What if InnerLoop was modified by other passes ? 234 CurAST->add(*InnerAST); 235 } 236 237 CurLoop = L; 238 239 // Get the preheader block to move instructions into... 240 Preheader = L->getLoopPreheader(); 241 242 // Loop over the body of this loop, looking for calls, invokes, and stores. 243 // Because subloops have already been incorporated into AST, we skip blocks in 244 // subloops. 245 // 246 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); 247 I != E; ++I) { 248 BasicBlock *BB = *I; 249 if (LI->getLoopFor(BB) == L) // Ignore blocks in subloops... 250 CurAST->add(*BB); // Incorporate the specified basic block 251 } 252 253 // We want to visit all of the instructions in this loop... that are not parts 254 // of our subloops (they have already had their invariants hoisted out of 255 // their loop, into this loop, so there is no need to process the BODIES of 256 // the subloops). 257 // 258 // Traverse the body of the loop in depth first order on the dominator tree so 259 // that we are guaranteed to see definitions before we see uses. This allows 260 // us to sink instructions in one pass, without iteration. After sinking 261 // instructions, we perform another pass to hoist them out of the loop. 262 // 263 if (L->hasDedicatedExits()) 264 SinkRegion(DT->getNode(L->getHeader())); 265 if (Preheader) 266 HoistRegion(DT->getNode(L->getHeader())); 267 268 // Now that all loop invariants have been removed from the loop, promote any 269 // memory references to scalars that we can. 270 if (!DisablePromotion && Preheader && L->hasDedicatedExits()) { 271 // Loop over all of the alias sets in the tracker object. 272 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end(); 273 I != E; ++I) 274 PromoteAliasSet(*I); 275 } 276 277 // Clear out loops state information for the next iteration 278 CurLoop = 0; 279 Preheader = 0; 280 281 LoopToAliasMap[L] = CurAST; 282 return Changed; 283} 284 285/// SinkRegion - Walk the specified region of the CFG (defined by all blocks 286/// dominated by the specified block, and that are in the current loop) in 287/// reverse depth first order w.r.t the DominatorTree. This allows us to visit 288/// uses before definitions, allowing us to sink a loop body in one pass without 289/// iteration. 290/// 291void LICM::SinkRegion(DomTreeNode *N) { 292 assert(N != 0 && "Null dominator tree node?"); 293 BasicBlock *BB = N->getBlock(); 294 295 // If this subregion is not in the top level loop at all, exit. 296 if (!CurLoop->contains(BB)) return; 297 298 // We are processing blocks in reverse dfo, so process children first... 299 const std::vector<DomTreeNode*> &Children = N->getChildren(); 300 for (unsigned i = 0, e = Children.size(); i != e; ++i) 301 SinkRegion(Children[i]); 302 303 // Only need to process the contents of this block if it is not part of a 304 // subloop (which would already have been processed). 305 if (inSubLoop(BB)) return; 306 307 for (BasicBlock::iterator II = BB->end(); II != BB->begin(); ) { 308 Instruction &I = *--II; 309 310 // Check to see if we can sink this instruction to the exit blocks 311 // of the loop. We can do this if the all users of the instruction are 312 // outside of the loop. In this case, it doesn't even matter if the 313 // operands of the instruction are loop invariant. 314 // 315 if (isNotUsedInLoop(I) && canSinkOrHoistInst(I)) { 316 ++II; 317 sink(I); 318 } 319 } 320} 321 322/// HoistRegion - Walk the specified region of the CFG (defined by all blocks 323/// dominated by the specified block, and that are in the current loop) in depth 324/// first order w.r.t the DominatorTree. This allows us to visit definitions 325/// before uses, allowing us to hoist a loop body in one pass without iteration. 326/// 327void LICM::HoistRegion(DomTreeNode *N) { 328 assert(N != 0 && "Null dominator tree node?"); 329 BasicBlock *BB = N->getBlock(); 330 331 // If this subregion is not in the top level loop at all, exit. 332 if (!CurLoop->contains(BB)) return; 333 334 // Only need to process the contents of this block if it is not part of a 335 // subloop (which would already have been processed). 336 if (!inSubLoop(BB)) 337 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ) { 338 Instruction &I = *II++; 339 340 // Try hoisting the instruction out to the preheader. We can only do this 341 // if all of the operands of the instruction are loop invariant and if it 342 // is safe to hoist the instruction. 343 // 344 if (isLoopInvariantInst(I) && canSinkOrHoistInst(I) && 345 isSafeToExecuteUnconditionally(I)) 346 hoist(I); 347 } 348 349 const std::vector<DomTreeNode*> &Children = N->getChildren(); 350 for (unsigned i = 0, e = Children.size(); i != e; ++i) 351 HoistRegion(Children[i]); 352} 353 354/// canSinkOrHoistInst - Return true if the hoister and sinker can handle this 355/// instruction. 356/// 357bool LICM::canSinkOrHoistInst(Instruction &I) { 358 // Loads have extra constraints we have to verify before we can hoist them. 359 if (LoadInst *LI = dyn_cast<LoadInst>(&I)) { 360 if (LI->isVolatile()) 361 return false; // Don't hoist volatile loads! 362 363 // Loads from constant memory are always safe to move, even if they end up 364 // in the same alias set as something that ends up being modified. 365 if (AA->pointsToConstantMemory(LI->getOperand(0))) 366 return true; 367 368 // Don't hoist loads which have may-aliased stores in loop. 369 unsigned Size = 0; 370 if (LI->getType()->isSized()) 371 Size = AA->getTypeStoreSize(LI->getType()); 372 return !pointerInvalidatedByLoop(LI->getOperand(0), Size); 373 } else if (CallInst *CI = dyn_cast<CallInst>(&I)) { 374 // Handle obvious cases efficiently. 375 AliasAnalysis::ModRefBehavior Behavior = AA->getModRefBehavior(CI); 376 if (Behavior == AliasAnalysis::DoesNotAccessMemory) 377 return true; 378 else if (Behavior == AliasAnalysis::OnlyReadsMemory) { 379 // If this call only reads from memory and there are no writes to memory 380 // in the loop, we can hoist or sink the call as appropriate. 381 bool FoundMod = false; 382 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end(); 383 I != E; ++I) { 384 AliasSet &AS = *I; 385 if (!AS.isForwardingAliasSet() && AS.isMod()) { 386 FoundMod = true; 387 break; 388 } 389 } 390 if (!FoundMod) return true; 391 } 392 393 // FIXME: This should use mod/ref information to see if we can hoist or sink 394 // the call. 395 396 return false; 397 } 398 399 // Otherwise these instructions are hoistable/sinkable 400 return isa<BinaryOperator>(I) || isa<CastInst>(I) || 401 isa<SelectInst>(I) || isa<GetElementPtrInst>(I) || isa<CmpInst>(I) || 402 isa<InsertElementInst>(I) || isa<ExtractElementInst>(I) || 403 isa<ShuffleVectorInst>(I); 404} 405 406/// isNotUsedInLoop - Return true if the only users of this instruction are 407/// outside of the loop. If this is true, we can sink the instruction to the 408/// exit blocks of the loop. 409/// 410bool LICM::isNotUsedInLoop(Instruction &I) { 411 for (Value::use_iterator UI = I.use_begin(), E = I.use_end(); UI != E; ++UI) { 412 Instruction *User = cast<Instruction>(*UI); 413 if (PHINode *PN = dyn_cast<PHINode>(User)) { 414 // PHI node uses occur in predecessor blocks! 415 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 416 if (PN->getIncomingValue(i) == &I) 417 if (CurLoop->contains(PN->getIncomingBlock(i))) 418 return false; 419 } else if (CurLoop->contains(User)) { 420 return false; 421 } 422 } 423 return true; 424} 425 426 427/// isLoopInvariantInst - Return true if all operands of this instruction are 428/// loop invariant. We also filter out non-hoistable instructions here just for 429/// efficiency. 430/// 431bool LICM::isLoopInvariantInst(Instruction &I) { 432 // The instruction is loop invariant if all of its operands are loop-invariant 433 for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i) 434 if (!CurLoop->isLoopInvariant(I.getOperand(i))) 435 return false; 436 437 // If we got this far, the instruction is loop invariant! 438 return true; 439} 440 441/// sink - When an instruction is found to only be used outside of the loop, 442/// this function moves it to the exit blocks and patches up SSA form as needed. 443/// This method is guaranteed to remove the original instruction from its 444/// position, and may either delete it or move it to outside of the loop. 445/// 446void LICM::sink(Instruction &I) { 447 DEBUG(dbgs() << "LICM sinking instruction: " << I << "\n"); 448 449 SmallVector<BasicBlock*, 8> ExitBlocks; 450 CurLoop->getUniqueExitBlocks(ExitBlocks); 451 452 if (isa<LoadInst>(I)) ++NumMovedLoads; 453 else if (isa<CallInst>(I)) ++NumMovedCalls; 454 ++NumSunk; 455 Changed = true; 456 457 // The case where there is only a single exit node of this loop is common 458 // enough that we handle it as a special (more efficient) case. It is more 459 // efficient to handle because there are no PHI nodes that need to be placed. 460 if (ExitBlocks.size() == 1) { 461 if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[0], I.getParent())) { 462 // Instruction is not used, just delete it. 463 CurAST->deleteValue(&I); 464 // If I has users in unreachable blocks, eliminate. 465 // If I is not void type then replaceAllUsesWith undef. 466 // This allows ValueHandlers and custom metadata to adjust itself. 467 if (!I.use_empty()) 468 I.replaceAllUsesWith(UndefValue::get(I.getType())); 469 I.eraseFromParent(); 470 } else { 471 // Move the instruction to the start of the exit block, after any PHI 472 // nodes in it. 473 I.removeFromParent(); 474 BasicBlock::iterator InsertPt = ExitBlocks[0]->getFirstNonPHI(); 475 ExitBlocks[0]->getInstList().insert(InsertPt, &I); 476 } 477 return; 478 } 479 480 if (ExitBlocks.empty()) { 481 // The instruction is actually dead if there ARE NO exit blocks. 482 CurAST->deleteValue(&I); 483 // If I has users in unreachable blocks, eliminate. 484 // If I is not void type then replaceAllUsesWith undef. 485 // This allows ValueHandlers and custom metadata to adjust itself. 486 if (!I.use_empty()) 487 I.replaceAllUsesWith(UndefValue::get(I.getType())); 488 I.eraseFromParent(); 489 return; 490 } 491 492 // Otherwise, if we have multiple exits, use the SSAUpdater to do all of the 493 // hard work of inserting PHI nodes as necessary. 494 SmallVector<PHINode*, 8> NewPHIs; 495 SSAUpdater SSA(&NewPHIs); 496 497 if (!I.use_empty()) 498 SSA.Initialize(&I); 499 500 // Insert a copy of the instruction in each exit block of the loop that is 501 // dominated by the instruction. Each exit block is known to only be in the 502 // ExitBlocks list once. 503 BasicBlock *InstOrigBB = I.getParent(); 504 unsigned NumInserted = 0; 505 506 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { 507 BasicBlock *ExitBlock = ExitBlocks[i]; 508 509 if (!isExitBlockDominatedByBlockInLoop(ExitBlock, InstOrigBB)) 510 continue; 511 512 // Insert the code after the last PHI node. 513 BasicBlock::iterator InsertPt = ExitBlock->getFirstNonPHI(); 514 515 // If this is the first exit block processed, just move the original 516 // instruction, otherwise clone the original instruction and insert 517 // the copy. 518 Instruction *New; 519 if (NumInserted++ == 0) { 520 I.moveBefore(InsertPt); 521 New = &I; 522 } else { 523 New = I.clone(); 524 CurAST->copyValue(&I, New); 525 if (!I.getName().empty()) 526 New->setName(I.getName()+".le"); 527 ExitBlock->getInstList().insert(InsertPt, New); 528 } 529 530 // Now that we have inserted the instruction, inform SSAUpdater. 531 if (!I.use_empty()) 532 SSA.AddAvailableValue(ExitBlock, New); 533 } 534 535 // If the instruction doesn't dominate any exit blocks, it must be dead. 536 if (NumInserted == 0) { 537 CurAST->deleteValue(&I); 538 if (!I.use_empty()) 539 I.replaceAllUsesWith(UndefValue::get(I.getType())); 540 I.eraseFromParent(); 541 return; 542 } 543 544 // Next, rewrite uses of the instruction, inserting PHI nodes as needed. 545 for (Value::use_iterator UI = I.use_begin(), UE = I.use_end(); UI != UE; ) { 546 // Grab the use before incrementing the iterator. 547 Use &U = UI.getUse(); 548 // Increment the iterator before removing the use from the list. 549 ++UI; 550 SSA.RewriteUseAfterInsertions(U); 551 } 552 553 // Update CurAST for NewPHIs if I had pointer type. 554 if (I.getType()->isPointerTy()) 555 for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i) 556 CurAST->copyValue(NewPHIs[i], &I); 557} 558 559/// hoist - When an instruction is found to only use loop invariant operands 560/// that is safe to hoist, this instruction is called to do the dirty work. 561/// 562void LICM::hoist(Instruction &I) { 563 DEBUG(dbgs() << "LICM hoisting to " << Preheader->getName() << ": " 564 << I << "\n"); 565 566 // Remove the instruction from its current basic block... but don't delete the 567 // instruction. 568 I.removeFromParent(); 569 570 // Insert the new node in Preheader, before the terminator. 571 Preheader->getInstList().insert(Preheader->getTerminator(), &I); 572 573 if (isa<LoadInst>(I)) ++NumMovedLoads; 574 else if (isa<CallInst>(I)) ++NumMovedCalls; 575 ++NumHoisted; 576 Changed = true; 577} 578 579/// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it is 580/// not a trapping instruction or if it is a trapping instruction and is 581/// guaranteed to execute. 582/// 583bool LICM::isSafeToExecuteUnconditionally(Instruction &Inst) { 584 // If it is not a trapping instruction, it is always safe to hoist. 585 if (Inst.isSafeToSpeculativelyExecute()) 586 return true; 587 588 // Otherwise we have to check to make sure that the instruction dominates all 589 // of the exit blocks. If it doesn't, then there is a path out of the loop 590 // which does not execute this instruction, so we can't hoist it. 591 592 // If the instruction is in the header block for the loop (which is very 593 // common), it is always guaranteed to dominate the exit blocks. Since this 594 // is a common case, and can save some work, check it now. 595 if (Inst.getParent() == CurLoop->getHeader()) 596 return true; 597 598 // Get the exit blocks for the current loop. 599 SmallVector<BasicBlock*, 8> ExitBlocks; 600 CurLoop->getExitBlocks(ExitBlocks); 601 602 // For each exit block, get the DT node and walk up the DT until the 603 // instruction's basic block is found or we exit the loop. 604 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) 605 if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[i], Inst.getParent())) 606 return false; 607 608 return true; 609} 610 611/// PromoteAliasSet - Try to promote memory values to scalars by sinking 612/// stores out of the loop and moving loads to before the loop. We do this by 613/// looping over the stores in the loop, looking for stores to Must pointers 614/// which are loop invariant. 615/// 616void LICM::PromoteAliasSet(AliasSet &AS) { 617 // We can promote this alias set if it has a store, if it is a "Must" alias 618 // set, if the pointer is loop invariant, and if we are not eliminating any 619 // volatile loads or stores. 620 if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() || 621 AS.isVolatile() || !CurLoop->isLoopInvariant(AS.begin()->getValue())) 622 return; 623 624 assert(!AS.empty() && 625 "Must alias set should have at least one pointer element in it!"); 626 Value *SomePtr = AS.begin()->getValue(); 627 628 // It isn't safe to promote a load/store from the loop if the load/store is 629 // conditional. For example, turning: 630 // 631 // for () { if (c) *P += 1; } 632 // 633 // into: 634 // 635 // tmp = *P; for () { if (c) tmp +=1; } *P = tmp; 636 // 637 // is not safe, because *P may only be valid to access if 'c' is true. 638 // 639 // It is safe to promote P if all uses are direct load/stores and if at 640 // least one is guaranteed to be executed. 641 bool GuaranteedToExecute = false; 642 643 SmallVector<Instruction*, 64> LoopUses; 644 SmallPtrSet<Value*, 4> PointerMustAliases; 645 646 // Check that all of the pointers in the alias set have the same type. We 647 // cannot (yet) promote a memory location that is loaded and stored in 648 // different sizes. 649 for (AliasSet::iterator ASI = AS.begin(), E = AS.end(); ASI != E; ++ASI) { 650 Value *ASIV = ASI->getValue(); 651 PointerMustAliases.insert(ASIV); 652 653 // Check that all of the pointers in the alias set have the same type. We 654 // cannot (yet) promote a memory location that is loaded and stored in 655 // different sizes. 656 if (SomePtr->getType() != ASIV->getType()) 657 return; 658 659 for (Value::use_iterator UI = ASIV->use_begin(), UE = ASIV->use_end(); 660 UI != UE; ++UI) { 661 // Ignore instructions that are outside the loop. 662 Instruction *Use = dyn_cast<Instruction>(*UI); 663 if (!Use || !CurLoop->contains(Use)) 664 continue; 665 666 // If there is an non-load/store instruction in the loop, we can't promote 667 // it. 668 if (isa<LoadInst>(Use)) 669 assert(!cast<LoadInst>(Use)->isVolatile() && "AST broken"); 670 else if (isa<StoreInst>(Use)) 671 assert(!cast<StoreInst>(Use)->isVolatile() && 672 Use->getOperand(0) != ASIV && "AST broken"); 673 else 674 return; // Not a load or store. 675 676 if (!GuaranteedToExecute) 677 GuaranteedToExecute = isSafeToExecuteUnconditionally(*Use); 678 679 LoopUses.push_back(Use); 680 } 681 } 682 683 // If there isn't a guaranteed-to-execute instruction, we can't promote. 684 if (!GuaranteedToExecute) 685 return; 686 687 // Otherwise, this is safe to promote, lets do it! 688 DEBUG(dbgs() << "LICM: Promoting value stored to in loop: " <<*SomePtr<<'\n'); 689 Changed = true; 690 ++NumPromoted; 691 692 // We use the SSAUpdater interface to insert phi nodes as required. 693 SmallVector<PHINode*, 16> NewPHIs; 694 SSAUpdater SSA(&NewPHIs); 695 696 // It wants to know some value of the same type as what we'll be inserting. 697 Value *SomeValue; 698 if (isa<LoadInst>(LoopUses[0])) 699 SomeValue = LoopUses[0]; 700 else 701 SomeValue = cast<StoreInst>(LoopUses[0])->getOperand(0); 702 SSA.Initialize(SomeValue); 703 704 // First step: bucket up uses of the pointers by the block they occur in. 705 // This is important because we have to handle multiple defs/uses in a block 706 // ourselves: SSAUpdater is purely for cross-block references. 707 // FIXME: Want a TinyVector<Instruction*> since there is usually 0/1 element. 708 DenseMap<BasicBlock*, std::vector<Instruction*> > UsesByBlock; 709 for (unsigned i = 0, e = LoopUses.size(); i != e; ++i) { 710 Instruction *User = LoopUses[i]; 711 UsesByBlock[User->getParent()].push_back(User); 712 } 713 714 // Okay, now we can iterate over all the blocks in the loop with uses, 715 // processing them. Keep track of which loads are loading a live-in value. 716 SmallVector<LoadInst*, 32> LiveInLoads; 717 718 for (unsigned LoopUse = 0, e = LoopUses.size(); LoopUse != e; ++LoopUse) { 719 Instruction *User = LoopUses[LoopUse]; 720 std::vector<Instruction*> &BlockUses = UsesByBlock[User->getParent()]; 721 722 // If this block has already been processed, ignore this repeat use. 723 if (BlockUses.empty()) continue; 724 725 // Okay, this is the first use in the block. If this block just has a 726 // single user in it, we can rewrite it trivially. 727 if (BlockUses.size() == 1) { 728 // If it is a store, it is a trivial def of the value in the block. 729 if (isa<StoreInst>(User)) { 730 SSA.AddAvailableValue(User->getParent(), 731 cast<StoreInst>(User)->getOperand(0)); 732 } else { 733 // Otherwise it is a load, queue it to rewrite as a live-in load. 734 LiveInLoads.push_back(cast<LoadInst>(User)); 735 } 736 BlockUses.clear(); 737 continue; 738 } 739 740 // Otherwise, check to see if this block is all loads. If so, we can queue 741 // them all as live in loads. 742 bool HasStore = false; 743 for (unsigned i = 0, e = BlockUses.size(); i != e; ++i) { 744 if (isa<StoreInst>(BlockUses[i])) { 745 HasStore = true; 746 break; 747 } 748 } 749 750 if (!HasStore) { 751 for (unsigned i = 0, e = BlockUses.size(); i != e; ++i) 752 LiveInLoads.push_back(cast<LoadInst>(BlockUses[i])); 753 BlockUses.clear(); 754 continue; 755 } 756 757 // Otherwise, we have mixed loads and stores (or just a bunch of stores). 758 // Since SSAUpdater is purely for cross-block values, we need to determine 759 // the order of these instructions in the block. If the first use in the 760 // block is a load, then it uses the live in value. The last store defines 761 // the live out value. We handle this by doing a linear scan of the block. 762 BasicBlock *BB = User->getParent(); 763 Value *StoredValue = 0; 764 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) { 765 if (LoadInst *L = dyn_cast<LoadInst>(II)) { 766 // If this is a load to an unrelated pointer, ignore it. 767 if (!PointerMustAliases.count(L->getOperand(0))) continue; 768 769 // If we haven't seen a store yet, this is a live in use, otherwise 770 // use the stored value. 771 if (StoredValue) 772 L->replaceAllUsesWith(StoredValue); 773 else 774 LiveInLoads.push_back(L); 775 continue; 776 } 777 778 if (StoreInst *S = dyn_cast<StoreInst>(II)) { 779 // If this is a load to an unrelated pointer, ignore it. 780 if (!PointerMustAliases.count(S->getOperand(1))) continue; 781 782 // Remember that this is the active value in the block. 783 StoredValue = S->getOperand(0); 784 } 785 } 786 787 // The last stored value that happened is the live-out for the block. 788 assert(StoredValue && "Already checked that there is a store in block"); 789 SSA.AddAvailableValue(BB, StoredValue); 790 BlockUses.clear(); 791 } 792 793 // Now that all the intra-loop values are classified, set up the preheader. 794 // It gets a load of the pointer we're promoting, and it is the live-out value 795 // from the preheader. 796 LoadInst *PreheaderLoad = new LoadInst(SomePtr,SomePtr->getName()+".promoted", 797 Preheader->getTerminator()); 798 SSA.AddAvailableValue(Preheader, PreheaderLoad); 799 800 // Now that the preheader is good to go, set up the exit blocks. Each exit 801 // block gets a store of the live-out values that feed them. Since we've 802 // already told the SSA updater about the defs in the loop and the preheader 803 // definition, it is all set and we can start using it. 804 SmallVector<BasicBlock*, 8> ExitBlocks; 805 CurLoop->getUniqueExitBlocks(ExitBlocks); 806 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { 807 BasicBlock *ExitBlock = ExitBlocks[i]; 808 Value *LiveInValue = SSA.GetValueInMiddleOfBlock(ExitBlock); 809 Instruction *InsertPos = ExitBlock->getFirstNonPHI(); 810 new StoreInst(LiveInValue, SomePtr, InsertPos); 811 } 812 813 // Okay, now we rewrite all loads that use live-in values in the loop, 814 // inserting PHI nodes as necessary. 815 for (unsigned i = 0, e = LiveInLoads.size(); i != e; ++i) { 816 LoadInst *ALoad = LiveInLoads[i]; 817 ALoad->replaceAllUsesWith(SSA.GetValueInMiddleOfBlock(ALoad->getParent())); 818 } 819 820 // Now that everything is rewritten, delete the old instructions from the body 821 // of the loop. They should all be dead now. 822 for (unsigned i = 0, e = LoopUses.size(); i != e; ++i) { 823 Instruction *User = LoopUses[i]; 824 CurAST->deleteValue(User); 825 User->eraseFromParent(); 826 } 827 828 // If the preheader load is itself a pointer, we need to tell alias analysis 829 // about the new pointer we created in the preheader block and about any PHI 830 // nodes that just got inserted. 831 if (PreheaderLoad->getType()->isPointerTy()) { 832 // Copy any value stored to or loaded from a must-alias of the pointer. 833 CurAST->copyValue(SomeValue, PreheaderLoad); 834 835 for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i) 836 CurAST->copyValue(SomeValue, NewPHIs[i]); 837 } 838 839 // fwew, we're done! 840} 841 842 843/// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info. 844void LICM::cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L) { 845 AliasSetTracker *AST = LoopToAliasMap[L]; 846 if (!AST) 847 return; 848 849 AST->copyValue(From, To); 850} 851 852/// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias 853/// set. 854void LICM::deleteAnalysisValue(Value *V, Loop *L) { 855 AliasSetTracker *AST = LoopToAliasMap[L]; 856 if (!AST) 857 return; 858 859 AST->deleteValue(V); 860} 861