LICM.cpp revision 2741c971044d2165be572749b94398043caccfeb
1//===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===// 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// 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 mem2reg functionality to construct the appropriate SSA form for the 30// variable. 31// 32//===----------------------------------------------------------------------===// 33 34#include "llvm/Transforms/Scalar.h" 35#include "llvm/DerivedTypes.h" 36#include "llvm/Instructions.h" 37#include "llvm/Target/TargetData.h" 38#include "llvm/Analysis/LoopInfo.h" 39#include "llvm/Analysis/AliasAnalysis.h" 40#include "llvm/Analysis/AliasSetTracker.h" 41#include "llvm/Analysis/Dominators.h" 42#include "llvm/Support/CFG.h" 43#include "llvm/Transforms/Utils/PromoteMemToReg.h" 44#include "llvm/Transforms/Utils/Local.h" 45#include "Support/CommandLine.h" 46#include "Support/Debug.h" 47#include "Support/Statistic.h" 48#include <algorithm> 49using namespace llvm; 50 51namespace { 52 cl::opt<bool> 53 DisablePromotion("disable-licm-promotion", cl::Hidden, 54 cl::desc("Disable memory promotion in LICM pass")); 55 56 Statistic<> NumSunk("licm", "Number of instructions sunk out of loop"); 57 Statistic<> NumHoisted("licm", "Number of instructions hoisted out of loop"); 58 Statistic<> NumMovedLoads("licm", "Number of load insts hoisted or sunk"); 59 Statistic<> NumMovedCalls("licm", "Number of call insts hoisted or sunk"); 60 Statistic<> NumPromoted("licm", 61 "Number of memory locations promoted to registers"); 62 63 struct LICM : public FunctionPass { 64 virtual bool runOnFunction(Function &F); 65 66 /// This transformation requires natural loop information & requires that 67 /// loop preheaders be inserted into the CFG... 68 /// 69 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 70 AU.setPreservesCFG(); 71 AU.addRequiredID(LoopSimplifyID); 72 AU.addRequired<LoopInfo>(); 73 AU.addRequired<DominatorTree>(); 74 AU.addRequired<DominanceFrontier>(); // For scalar promotion (mem2reg) 75 AU.addRequired<AliasAnalysis>(); 76 } 77 78 private: 79 // Various analyses that we use... 80 AliasAnalysis *AA; // Current AliasAnalysis information 81 LoopInfo *LI; // Current LoopInfo 82 DominatorTree *DT; // Dominator Tree for the current Loop... 83 DominanceFrontier *DF; // Current Dominance Frontier 84 85 // State that is updated as we process loops 86 bool Changed; // Set to true when we change anything. 87 BasicBlock *Preheader; // The preheader block of the current loop... 88 Loop *CurLoop; // The current loop we are working on... 89 AliasSetTracker *CurAST; // AliasSet information for the current loop... 90 91 /// visitLoop - Hoist expressions out of the specified loop... 92 /// 93 void visitLoop(Loop *L, AliasSetTracker &AST); 94 95 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks 96 /// dominated by the specified block, and that are in the current loop) in 97 /// reverse depth first order w.r.t the DominatorTree. This allows us to 98 /// visit uses before definitions, allowing us to sink a loop body in one 99 /// pass without iteration. 100 /// 101 void SinkRegion(DominatorTree::Node *N); 102 103 /// HoistRegion - Walk the specified region of the CFG (defined by all 104 /// blocks dominated by the specified block, and that are in the current 105 /// loop) in depth first order w.r.t the DominatorTree. This allows us to 106 /// visit definitions before uses, allowing us to hoist a loop body in one 107 /// pass without iteration. 108 /// 109 void HoistRegion(DominatorTree::Node *N); 110 111 /// inSubLoop - Little predicate that returns true if the specified basic 112 /// block is in a subloop of the current one, not the current one itself. 113 /// 114 bool inSubLoop(BasicBlock *BB) { 115 assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop"); 116 for (Loop::iterator I = CurLoop->begin(), E = CurLoop->end(); I != E; ++I) 117 if ((*I)->contains(BB)) 118 return true; // A subloop actually contains this block! 119 return false; 120 } 121 122 /// isExitBlockDominatedByBlockInLoop - This method checks to see if the 123 /// specified exit block of the loop is dominated by the specified block 124 /// that is in the body of the loop. We use these constraints to 125 /// dramatically limit the amount of the dominator tree that needs to be 126 /// searched. 127 bool isExitBlockDominatedByBlockInLoop(BasicBlock *ExitBlock, 128 BasicBlock *BlockInLoop) const { 129 // If the block in the loop is the loop header, it must be dominated! 130 BasicBlock *LoopHeader = CurLoop->getHeader(); 131 if (BlockInLoop == LoopHeader) 132 return true; 133 134 DominatorTree::Node *BlockInLoopNode = DT->getNode(BlockInLoop); 135 DominatorTree::Node *IDom = DT->getNode(ExitBlock); 136 137 // Because the exit block is not in the loop, we know we have to get _at 138 // least_ its immediate dominator. 139 do { 140 // Get next Immediate Dominator. 141 IDom = IDom->getIDom(); 142 143 // If we have got to the header of the loop, then the instructions block 144 // did not dominate the exit node, so we can't hoist it. 145 if (IDom->getBlock() == LoopHeader) 146 return false; 147 148 } while (IDom != BlockInLoopNode); 149 150 return true; 151 } 152 153 /// sink - When an instruction is found to only be used outside of the loop, 154 /// this function moves it to the exit blocks and patches up SSA form as 155 /// needed. 156 /// 157 void sink(Instruction &I); 158 159 /// hoist - When an instruction is found to only use loop invariant operands 160 /// that is safe to hoist, this instruction is called to do the dirty work. 161 /// 162 void hoist(Instruction &I); 163 164 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it 165 /// is not a trapping instruction or if it is a trapping instruction and is 166 /// guaranteed to execute. 167 /// 168 bool isSafeToExecuteUnconditionally(Instruction &I); 169 170 /// pointerInvalidatedByLoop - Return true if the body of this loop may 171 /// store into the memory location pointed to by V. 172 /// 173 bool pointerInvalidatedByLoop(Value *V) { 174 // Check to see if any of the basic blocks in CurLoop invalidate *V. 175 return CurAST->getAliasSetForPointer(V, 0).isMod(); 176 } 177 178 bool canSinkOrHoistInst(Instruction &I); 179 bool isLoopInvariantInst(Instruction &I); 180 bool isNotUsedInLoop(Instruction &I); 181 182 /// PromoteValuesInLoop - Look at the stores in the loop and promote as many 183 /// to scalars as we can. 184 /// 185 void PromoteValuesInLoop(); 186 187 /// findPromotableValuesInLoop - Check the current loop for stores to 188 /// definite pointers, which are not loaded and stored through may aliases. 189 /// If these are found, create an alloca for the value, add it to the 190 /// PromotedValues list, and keep track of the mapping from value to 191 /// alloca... 192 /// 193 void findPromotableValuesInLoop( 194 std::vector<std::pair<AllocaInst*, Value*> > &PromotedValues, 195 std::map<Value*, AllocaInst*> &Val2AlMap); 196 }; 197 198 RegisterOpt<LICM> X("licm", "Loop Invariant Code Motion"); 199} 200 201FunctionPass *llvm::createLICMPass() { return new LICM(); } 202 203/// runOnFunction - For LICM, this simply traverses the loop structure of the 204/// function, hoisting expressions out of loops if possible. 205/// 206bool LICM::runOnFunction(Function &) { 207 Changed = false; 208 209 // Get our Loop and Alias Analysis information... 210 LI = &getAnalysis<LoopInfo>(); 211 AA = &getAnalysis<AliasAnalysis>(); 212 DF = &getAnalysis<DominanceFrontier>(); 213 DT = &getAnalysis<DominatorTree>(); 214 215 // Hoist expressions out of all of the top-level loops. 216 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I) { 217 AliasSetTracker AST(*AA); 218 visitLoop(*I, AST); 219 } 220 return Changed; 221} 222 223 224/// visitLoop - Hoist expressions out of the specified loop... 225/// 226void LICM::visitLoop(Loop *L, AliasSetTracker &AST) { 227 // Recurse through all subloops before we process this loop... 228 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I) { 229 AliasSetTracker SubAST(*AA); 230 visitLoop(*I, SubAST); 231 232 // Incorporate information about the subloops into this loop... 233 AST.add(SubAST); 234 } 235 CurLoop = L; 236 CurAST = &AST; 237 238 // Get the preheader block to move instructions into... 239 Preheader = L->getLoopPreheader(); 240 assert(Preheader&&"Preheader insertion pass guarantees we have a preheader!"); 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 (std::vector<BasicBlock*>::const_iterator I = L->getBlocks().begin(), 247 E = L->getBlocks().end(); I != E; ++I) 248 if (LI->getLoopFor(*I) == L) // Ignore blocks in subloops... 249 AST.add(**I); // Incorporate the specified basic block 250 251 // We want to visit all of the instructions in this loop... that are not parts 252 // of our subloops (they have already had their invariants hoisted out of 253 // their loop, into this loop, so there is no need to process the BODIES of 254 // the subloops). 255 // 256 // Traverse the body of the loop in depth first order on the dominator tree so 257 // that we are guaranteed to see definitions before we see uses. This allows 258 // us to sink instructions in one pass, without iteration. AFter sinking 259 // instructions, we perform another pass to hoist them out of the loop. 260 // 261 SinkRegion(DT->getNode(L->getHeader())); 262 HoistRegion(DT->getNode(L->getHeader())); 263 264 // Now that all loop invariants have been removed from the loop, promote any 265 // memory references to scalars that we can... 266 if (!DisablePromotion) 267 PromoteValuesInLoop(); 268 269 // Clear out loops state information for the next iteration 270 CurLoop = 0; 271 Preheader = 0; 272} 273 274/// SinkRegion - Walk the specified region of the CFG (defined by all blocks 275/// dominated by the specified block, and that are in the current loop) in 276/// reverse depth first order w.r.t the DominatorTree. This allows us to visit 277/// uses before definitions, allowing us to sink a loop body in one pass without 278/// iteration. 279/// 280void LICM::SinkRegion(DominatorTree::Node *N) { 281 assert(N != 0 && "Null dominator tree node?"); 282 BasicBlock *BB = N->getBlock(); 283 284 // If this subregion is not in the top level loop at all, exit. 285 if (!CurLoop->contains(BB)) return; 286 287 // We are processing blocks in reverse dfo, so process children first... 288 const std::vector<DominatorTree::Node*> &Children = N->getChildren(); 289 for (unsigned i = 0, e = Children.size(); i != e; ++i) 290 SinkRegion(Children[i]); 291 292 // Only need to process the contents of this block if it is not part of a 293 // subloop (which would already have been processed). 294 if (inSubLoop(BB)) return; 295 296 for (BasicBlock::iterator II = BB->end(); II != BB->begin(); ) { 297 Instruction &I = *--II; 298 299 // Check to see if we can sink this instruction to the exit blocks 300 // of the loop. We can do this if the all users of the instruction are 301 // outside of the loop. In this case, it doesn't even matter if the 302 // operands of the instruction are loop invariant. 303 // 304 if (canSinkOrHoistInst(I) && isNotUsedInLoop(I)) { 305 ++II; 306 sink(I); 307 } 308 } 309} 310 311 312/// HoistRegion - Walk the specified region of the CFG (defined by all blocks 313/// dominated by the specified block, and that are in the current loop) in depth 314/// first order w.r.t the DominatorTree. This allows us to visit definitions 315/// before uses, allowing us to hoist a loop body in one pass without iteration. 316/// 317void LICM::HoistRegion(DominatorTree::Node *N) { 318 assert(N != 0 && "Null dominator tree node?"); 319 BasicBlock *BB = N->getBlock(); 320 321 // If this subregion is not in the top level loop at all, exit. 322 if (!CurLoop->contains(BB)) return; 323 324 // Only need to process the contents of this block if it is not part of a 325 // subloop (which would already have been processed). 326 if (!inSubLoop(BB)) 327 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ) { 328 Instruction &I = *II++; 329 330 // Try hoisting the instruction out to the preheader. We can only do this 331 // if all of the operands of the instruction are loop invariant and if it 332 // is safe to hoist the instruction. 333 // 334 if (isLoopInvariantInst(I) && canSinkOrHoistInst(I) && 335 isSafeToExecuteUnconditionally(I)) 336 hoist(I); 337 } 338 339 const std::vector<DominatorTree::Node*> &Children = N->getChildren(); 340 for (unsigned i = 0, e = Children.size(); i != e; ++i) 341 HoistRegion(Children[i]); 342} 343 344/// canSinkOrHoistInst - Return true if the hoister and sinker can handle this 345/// instruction. 346/// 347bool LICM::canSinkOrHoistInst(Instruction &I) { 348 // Loads have extra constraints we have to verify before we can hoist them. 349 if (LoadInst *LI = dyn_cast<LoadInst>(&I)) { 350 if (LI->isVolatile()) 351 return false; // Don't hoist volatile loads! 352 353 // Don't hoist loads which have may-aliased stores in loop. 354 return !pointerInvalidatedByLoop(LI->getOperand(0)); 355 } else if (CallInst *CI = dyn_cast<CallInst>(&I)) { 356 // Handle obvious cases efficiently. 357 if (Function *Callee = CI->getCalledFunction()) { 358 if (AA->doesNotAccessMemory(Callee)) 359 return true; 360 else if (AA->onlyReadsMemory(Callee)) { 361 // If this call only reads from memory and there are no writes to memory 362 // in the loop, we can hoist or sink the call as appropriate. 363 bool FoundMod = false; 364 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end(); 365 I != E; ++I) { 366 AliasSet &AS = *I; 367 if (!AS.isForwardingAliasSet() && AS.isMod()) { 368 FoundMod = true; 369 break; 370 } 371 } 372 if (!FoundMod) return true; 373 } 374 } 375 376 // FIXME: This should use mod/ref information to see if we can hoist or sink 377 // the call. 378 379 return false; 380 } 381 382 return isa<BinaryOperator>(I) || isa<ShiftInst>(I) || isa<CastInst>(I) || 383 isa<SelectInst>(I) || 384 isa<GetElementPtrInst>(I) || isa<VANextInst>(I) || isa<VAArgInst>(I); 385} 386 387/// isNotUsedInLoop - Return true if the only users of this instruction are 388/// outside of the loop. If this is true, we can sink the instruction to the 389/// exit blocks of the loop. 390/// 391bool LICM::isNotUsedInLoop(Instruction &I) { 392 for (Value::use_iterator UI = I.use_begin(), E = I.use_end(); UI != E; ++UI) { 393 Instruction *User = cast<Instruction>(*UI); 394 if (PHINode *PN = dyn_cast<PHINode>(User)) { 395 // PHI node uses occur in predecessor blocks! 396 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 397 if (PN->getIncomingValue(i) == &I) 398 if (CurLoop->contains(PN->getIncomingBlock(i))) 399 return false; 400 } else if (CurLoop->contains(User->getParent())) { 401 return false; 402 } 403 } 404 return true; 405} 406 407 408/// isLoopInvariantInst - Return true if all operands of this instruction are 409/// loop invariant. We also filter out non-hoistable instructions here just for 410/// efficiency. 411/// 412bool LICM::isLoopInvariantInst(Instruction &I) { 413 // The instruction is loop invariant if all of its operands are loop-invariant 414 for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i) 415 if (!CurLoop->isLoopInvariant(I.getOperand(i))) 416 return false; 417 418 // If we got this far, the instruction is loop invariant! 419 return true; 420} 421 422/// sink - When an instruction is found to only be used outside of the loop, 423/// this function moves it to the exit blocks and patches up SSA form as needed. 424/// This method is guaranteed to remove the original instruction from its 425/// position, and may either delete it or move it to outside of the loop. 426/// 427void LICM::sink(Instruction &I) { 428 DEBUG(std::cerr << "LICM sinking instruction: " << I); 429 430 std::vector<BasicBlock*> ExitBlocks; 431 CurLoop->getExitBlocks(ExitBlocks); 432 433 if (isa<LoadInst>(I)) ++NumMovedLoads; 434 else if (isa<CallInst>(I)) ++NumMovedCalls; 435 ++NumSunk; 436 Changed = true; 437 438 // The case where there is only a single exit node of this loop is common 439 // enough that we handle it as a special (more efficient) case. It is more 440 // efficient to handle because there are no PHI nodes that need to be placed. 441 if (ExitBlocks.size() == 1) { 442 if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[0], I.getParent())) { 443 // Instruction is not used, just delete it. 444 CurAST->deleteValue(&I); 445 I.getParent()->getInstList().erase(&I); 446 } else { 447 // Move the instruction to the start of the exit block, after any PHI 448 // nodes in it. 449 I.getParent()->getInstList().remove(&I); 450 451 BasicBlock::iterator InsertPt = ExitBlocks[0]->begin(); 452 while (isa<PHINode>(InsertPt)) ++InsertPt; 453 ExitBlocks[0]->getInstList().insert(InsertPt, &I); 454 } 455 } else if (ExitBlocks.size() == 0) { 456 // The instruction is actually dead if there ARE NO exit blocks. 457 CurAST->deleteValue(&I); 458 I.getParent()->getInstList().erase(&I); 459 } else { 460 // Otherwise, if we have multiple exits, use the PromoteMem2Reg function to 461 // do all of the hard work of inserting PHI nodes as necessary. We convert 462 // the value into a stack object to get it to do this. 463 464 // Firstly, we create a stack object to hold the value... 465 AllocaInst *AI = new AllocaInst(I.getType(), 0, I.getName(), 466 I.getParent()->getParent()->front().begin()); 467 468 // Secondly, insert load instructions for each use of the instruction 469 // outside of the loop. 470 while (!I.use_empty()) { 471 Instruction *U = cast<Instruction>(I.use_back()); 472 473 // If the user is a PHI Node, we actually have to insert load instructions 474 // in all predecessor blocks, not in the PHI block itself! 475 if (PHINode *UPN = dyn_cast<PHINode>(U)) { 476 // Only insert into each predecessor once, so that we don't have 477 // different incoming values from the same block! 478 std::map<BasicBlock*, Value*> InsertedBlocks; 479 for (unsigned i = 0, e = UPN->getNumIncomingValues(); i != e; ++i) 480 if (UPN->getIncomingValue(i) == &I) { 481 BasicBlock *Pred = UPN->getIncomingBlock(i); 482 Value *&PredVal = InsertedBlocks[Pred]; 483 if (!PredVal) { 484 // Insert a new load instruction right before the terminator in 485 // the predecessor block. 486 PredVal = new LoadInst(AI, "", Pred->getTerminator()); 487 } 488 489 UPN->setIncomingValue(i, PredVal); 490 } 491 492 } else { 493 LoadInst *L = new LoadInst(AI, "", U); 494 U->replaceUsesOfWith(&I, L); 495 } 496 } 497 498 // Thirdly, insert a copy of the instruction in each exit block of the loop 499 // that is dominated by the instruction, storing the result into the memory 500 // location. Be careful not to insert the instruction into any particular 501 // basic block more than once. 502 std::set<BasicBlock*> InsertedBlocks; 503 BasicBlock *InstOrigBB = I.getParent(); 504 505 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) { 506 BasicBlock *ExitBlock = ExitBlocks[i]; 507 508 if (isExitBlockDominatedByBlockInLoop(ExitBlock, InstOrigBB)) { 509 // If we haven't already processed this exit block, do so now. 510 if (InsertedBlocks.insert(ExitBlock).second) { 511 // Insert the code after the last PHI node... 512 BasicBlock::iterator InsertPt = ExitBlock->begin(); 513 while (isa<PHINode>(InsertPt)) ++InsertPt; 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 (InsertedBlocks.size() == 1) { 520 I.getParent()->getInstList().remove(&I); 521 ExitBlock->getInstList().insert(InsertPt, &I); 522 New = &I; 523 } else { 524 New = I.clone(); 525 New->setName(I.getName()+".le"); 526 ExitBlock->getInstList().insert(InsertPt, New); 527 } 528 529 // Now that we have inserted the instruction, store it into the alloca 530 new StoreInst(New, AI, InsertPt); 531 } 532 } 533 } 534 535 // If the instruction doesn't dominate any exit blocks, it must be dead. 536 if (InsertedBlocks.empty()) { 537 CurAST->deleteValue(&I); 538 I.getParent()->getInstList().erase(&I); 539 } 540 541 // Finally, promote the fine value to SSA form. 542 std::vector<AllocaInst*> Allocas; 543 Allocas.push_back(AI); 544 PromoteMemToReg(Allocas, *DT, *DF, AA->getTargetData()); 545 } 546} 547 548/// hoist - When an instruction is found to only use loop invariant operands 549/// that is safe to hoist, this instruction is called to do the dirty work. 550/// 551void LICM::hoist(Instruction &I) { 552 DEBUG(std::cerr << "LICM hoisting to" << Preheader->getName() 553 << ": " << I); 554 555 // Remove the instruction from its current basic block... but don't delete the 556 // instruction. 557 I.getParent()->getInstList().remove(&I); 558 559 // Insert the new node in Preheader, before the terminator. 560 Preheader->getInstList().insert(Preheader->getTerminator(), &I); 561 562 if (isa<LoadInst>(I)) ++NumMovedLoads; 563 else if (isa<CallInst>(I)) ++NumMovedCalls; 564 ++NumHoisted; 565 Changed = true; 566} 567 568/// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it is 569/// not a trapping instruction or if it is a trapping instruction and is 570/// guaranteed to execute. 571/// 572bool LICM::isSafeToExecuteUnconditionally(Instruction &Inst) { 573 // If it is not a trapping instruction, it is always safe to hoist. 574 if (!Inst.isTrapping()) return true; 575 576 // Otherwise we have to check to make sure that the instruction dominates all 577 // of the exit blocks. If it doesn't, then there is a path out of the loop 578 // which does not execute this instruction, so we can't hoist it. 579 580 // If the instruction is in the header block for the loop (which is very 581 // common), it is always guaranteed to dominate the exit blocks. Since this 582 // is a common case, and can save some work, check it now. 583 if (Inst.getParent() == CurLoop->getHeader()) 584 return true; 585 586 // Get the exit blocks for the current loop. 587 std::vector<BasicBlock*> ExitBlocks; 588 CurLoop->getExitBlocks(ExitBlocks); 589 590 // For each exit block, get the DT node and walk up the DT until the 591 // instruction's basic block is found or we exit the loop. 592 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) 593 if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[i], Inst.getParent())) 594 return false; 595 596 return true; 597} 598 599 600/// PromoteValuesInLoop - Try to promote memory values to scalars by sinking 601/// stores out of the loop and moving loads to before the loop. We do this by 602/// looping over the stores in the loop, looking for stores to Must pointers 603/// which are loop invariant. We promote these memory locations to use allocas 604/// instead. These allocas can easily be raised to register values by the 605/// PromoteMem2Reg functionality. 606/// 607void LICM::PromoteValuesInLoop() { 608 // PromotedValues - List of values that are promoted out of the loop. Each 609 // value has an alloca instruction for it, and a canonical version of the 610 // pointer. 611 std::vector<std::pair<AllocaInst*, Value*> > PromotedValues; 612 std::map<Value*, AllocaInst*> ValueToAllocaMap; // Map of ptr to alloca 613 614 findPromotableValuesInLoop(PromotedValues, ValueToAllocaMap); 615 if (ValueToAllocaMap.empty()) return; // If there are values to promote... 616 617 Changed = true; 618 NumPromoted += PromotedValues.size(); 619 620 // Emit a copy from the value into the alloca'd value in the loop preheader 621 TerminatorInst *LoopPredInst = Preheader->getTerminator(); 622 for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) { 623 // Load from the memory we are promoting... 624 LoadInst *LI = new LoadInst(PromotedValues[i].second, 625 PromotedValues[i].second->getName()+".promoted", 626 LoopPredInst); 627 // Store into the temporary alloca... 628 new StoreInst(LI, PromotedValues[i].first, LoopPredInst); 629 } 630 631 // Scan the basic blocks in the loop, replacing uses of our pointers with 632 // uses of the allocas in question. 633 // 634 const std::vector<BasicBlock*> &LoopBBs = CurLoop->getBlocks(); 635 for (std::vector<BasicBlock*>::const_iterator I = LoopBBs.begin(), 636 E = LoopBBs.end(); I != E; ++I) { 637 // Rewrite all loads and stores in the block of the pointer... 638 for (BasicBlock::iterator II = (*I)->begin(), E = (*I)->end(); 639 II != E; ++II) { 640 if (LoadInst *L = dyn_cast<LoadInst>(II)) { 641 std::map<Value*, AllocaInst*>::iterator 642 I = ValueToAllocaMap.find(L->getOperand(0)); 643 if (I != ValueToAllocaMap.end()) 644 L->setOperand(0, I->second); // Rewrite load instruction... 645 } else if (StoreInst *S = dyn_cast<StoreInst>(II)) { 646 std::map<Value*, AllocaInst*>::iterator 647 I = ValueToAllocaMap.find(S->getOperand(1)); 648 if (I != ValueToAllocaMap.end()) 649 S->setOperand(1, I->second); // Rewrite store instruction... 650 } 651 } 652 } 653 654 // Now that the body of the loop uses the allocas instead of the original 655 // memory locations, insert code to copy the alloca value back into the 656 // original memory location on all exits from the loop. Note that we only 657 // want to insert one copy of the code in each exit block, though the loop may 658 // exit to the same block more than once. 659 // 660 std::set<BasicBlock*> ProcessedBlocks; 661 662 std::vector<BasicBlock*> ExitBlocks; 663 CurLoop->getExitBlocks(ExitBlocks); 664 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) 665 if (ProcessedBlocks.insert(ExitBlocks[i]).second) { 666 // Copy all of the allocas into their memory locations... 667 BasicBlock::iterator BI = ExitBlocks[i]->begin(); 668 while (isa<PHINode>(*BI)) 669 ++BI; // Skip over all of the phi nodes in the block... 670 Instruction *InsertPos = BI; 671 for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) { 672 // Load from the alloca... 673 LoadInst *LI = new LoadInst(PromotedValues[i].first, "", InsertPos); 674 // Store into the memory we promoted... 675 new StoreInst(LI, PromotedValues[i].second, InsertPos); 676 } 677 } 678 679 // Now that we have done the deed, use the mem2reg functionality to promote 680 // all of the new allocas we just created into real SSA registers... 681 // 682 std::vector<AllocaInst*> PromotedAllocas; 683 PromotedAllocas.reserve(PromotedValues.size()); 684 for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) 685 PromotedAllocas.push_back(PromotedValues[i].first); 686 PromoteMemToReg(PromotedAllocas, *DT, *DF, AA->getTargetData()); 687} 688 689/// findPromotableValuesInLoop - Check the current loop for stores to definite 690/// pointers, which are not loaded and stored through may aliases. If these are 691/// found, create an alloca for the value, add it to the PromotedValues list, 692/// and keep track of the mapping from value to alloca... 693/// 694void LICM::findPromotableValuesInLoop( 695 std::vector<std::pair<AllocaInst*, Value*> > &PromotedValues, 696 std::map<Value*, AllocaInst*> &ValueToAllocaMap) { 697 Instruction *FnStart = CurLoop->getHeader()->getParent()->begin()->begin(); 698 699 // Loop over all of the alias sets in the tracker object... 700 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end(); 701 I != E; ++I) { 702 AliasSet &AS = *I; 703 // We can promote this alias set if it has a store, if it is a "Must" alias 704 // set, if the pointer is loop invariant, if if we are not eliminating any 705 // volatile loads or stores. 706 if (!AS.isForwardingAliasSet() && AS.isMod() && AS.isMustAlias() && 707 !AS.isVolatile() && CurLoop->isLoopInvariant(AS.begin()->first)) { 708 assert(AS.begin() != AS.end() && 709 "Must alias set should have at least one pointer element in it!"); 710 Value *V = AS.begin()->first; 711 712 // Check that all of the pointers in the alias set have the same type. We 713 // cannot (yet) promote a memory location that is loaded and stored in 714 // different sizes. 715 bool PointerOk = true; 716 for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I) 717 if (V->getType() != I->first->getType()) { 718 PointerOk = false; 719 break; 720 } 721 722 if (PointerOk) { 723 const Type *Ty = cast<PointerType>(V->getType())->getElementType(); 724 AllocaInst *AI = new AllocaInst(Ty, 0, V->getName()+".tmp", FnStart); 725 PromotedValues.push_back(std::make_pair(AI, V)); 726 727 for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I) 728 ValueToAllocaMap.insert(std::make_pair(I->first, AI)); 729 730 DEBUG(std::cerr << "LICM: Promoting value: " << *V << "\n"); 731 } 732 } 733 } 734} 735