1 //===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
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.
16 // This pass uses alias analysis for two purposes:
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.
31 //===----------------------------------------------------------------------===//
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"
56 STATISTIC(NumSunk , "Number of instructions sunk out of loop");
57 STATISTIC(NumHoisted , "Number of instructions hoisted out of loop");
58 STATISTIC(NumMovedLoads, "Number of load insts hoisted or sunk");
59 STATISTIC(NumMovedCalls, "Number of call insts hoisted or sunk");
60 STATISTIC(NumPromoted , "Number of memory locations promoted to registers");
63 DisablePromotion("disable-licm-promotion", cl::Hidden,
64 cl::desc("Disable memory promotion in LICM pass"));
67 struct LICM : public LoopPass {
68 static char ID; // Pass identification, replacement for typeid
69 LICM() : LoopPass(ID) {
70 initializeLICMPass(*PassRegistry::getPassRegistry());
73 virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
75 /// This transformation requires natural loop information & requires that
76 /// loop preheaders be inserted into the CFG...
78 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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);
89 bool doFinalization() {
90 assert(LoopToAliasSetMap.empty() && "Didn't free loop alias sets");
95 AliasAnalysis *AA; // Current AliasAnalysis information
96 LoopInfo *LI; // Current LoopInfo
97 DominatorTree *DT; // Dominator Tree for the current Loop.
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;
106 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
107 void cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L);
109 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
111 void deleteAnalysisValue(Value *V, Loop *L);
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.
119 void SinkRegion(DomTreeNode *N);
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.
127 void HoistRegion(DomTreeNode *N);
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.
132 bool inSubLoop(BasicBlock *BB) {
133 assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
134 return LI->getLoopFor(BB) != CurLoop;
137 /// sink - When an instruction is found to only be used outside of the loop,
138 /// this function moves it to the exit blocks and patches up SSA form as
141 void sink(Instruction &I);
143 /// hoist - When an instruction is found to only use loop invariant operands
144 /// that is safe to hoist, this instruction is called to do the dirty work.
146 void hoist(Instruction &I);
148 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it
149 /// is not a trapping instruction or if it is a trapping instruction and is
150 /// guaranteed to execute.
152 bool isSafeToExecuteUnconditionally(Instruction &I);
154 /// pointerInvalidatedByLoop - Return true if the body of this loop may
155 /// store into the memory location pointed to by V.
157 bool pointerInvalidatedByLoop(Value *V, uint64_t Size,
158 const MDNode *TBAAInfo) {
159 // Check to see if any of the basic blocks in CurLoop invalidate *V.
160 return CurAST->getAliasSetForPointer(V, Size, TBAAInfo).isMod();
163 bool canSinkOrHoistInst(Instruction &I);
164 bool isNotUsedInLoop(Instruction &I);
166 void PromoteAliasSet(AliasSet &AS);
171 INITIALIZE_PASS_BEGIN(LICM, "licm", "Loop Invariant Code Motion", false, false)
172 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
173 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
174 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
175 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
176 INITIALIZE_PASS_END(LICM, "licm", "Loop Invariant Code Motion", false, false)
178 Pass *llvm::createLICMPass() { return new LICM(); }
180 /// Hoist expressions out of the specified loop. Note, alias info for inner
181 /// loop is not preserved so it is not a good idea to run LICM multiple
182 /// times on one loop.
184 bool LICM::runOnLoop(Loop *L, LPPassManager &LPM) {
187 // Get our Loop and Alias Analysis information...
188 LI = &getAnalysis<LoopInfo>();
189 AA = &getAnalysis<AliasAnalysis>();
190 DT = &getAnalysis<DominatorTree>();
192 CurAST = new AliasSetTracker(*AA);
193 // Collect Alias info from subloops.
194 for (Loop::iterator LoopItr = L->begin(), LoopItrE = L->end();
195 LoopItr != LoopItrE; ++LoopItr) {
196 Loop *InnerL = *LoopItr;
197 AliasSetTracker *InnerAST = LoopToAliasSetMap[InnerL];
198 assert(InnerAST && "Where is my AST?");
200 // What if InnerLoop was modified by other passes ?
201 CurAST->add(*InnerAST);
203 // Once we've incorporated the inner loop's AST into ours, we don't need the
204 // subloop's anymore.
206 LoopToAliasSetMap.erase(InnerL);
211 // Get the preheader block to move instructions into...
212 Preheader = L->getLoopPreheader();
214 // Loop over the body of this loop, looking for calls, invokes, and stores.
215 // Because subloops have already been incorporated into AST, we skip blocks in
218 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
221 if (LI->getLoopFor(BB) == L) // Ignore blocks in subloops.
222 CurAST->add(*BB); // Incorporate the specified basic block
225 // We want to visit all of the instructions in this loop... that are not parts
226 // of our subloops (they have already had their invariants hoisted out of
227 // their loop, into this loop, so there is no need to process the BODIES of
230 // Traverse the body of the loop in depth first order on the dominator tree so
231 // that we are guaranteed to see definitions before we see uses. This allows
232 // us to sink instructions in one pass, without iteration. After sinking
233 // instructions, we perform another pass to hoist them out of the loop.
235 if (L->hasDedicatedExits())
236 SinkRegion(DT->getNode(L->getHeader()));
238 HoistRegion(DT->getNode(L->getHeader()));
240 // Now that all loop invariants have been removed from the loop, promote any
241 // memory references to scalars that we can.
242 if (!DisablePromotion && Preheader && L->hasDedicatedExits()) {
243 // Loop over all of the alias sets in the tracker object.
244 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
249 // Clear out loops state information for the next iteration
253 // If this loop is nested inside of another one, save the alias information
254 // for when we process the outer loop.
255 if (L->getParentLoop())
256 LoopToAliasSetMap[L] = CurAST;
262 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks
263 /// dominated by the specified block, and that are in the current loop) in
264 /// reverse depth first order w.r.t the DominatorTree. This allows us to visit
265 /// uses before definitions, allowing us to sink a loop body in one pass without
268 void LICM::SinkRegion(DomTreeNode *N) {
269 assert(N != 0 && "Null dominator tree node?");
270 BasicBlock *BB = N->getBlock();
272 // If this subregion is not in the top level loop at all, exit.
273 if (!CurLoop->contains(BB)) return;
275 // We are processing blocks in reverse dfo, so process children first.
276 const std::vector<DomTreeNode*> &Children = N->getChildren();
277 for (unsigned i = 0, e = Children.size(); i != e; ++i)
278 SinkRegion(Children[i]);
280 // Only need to process the contents of this block if it is not part of a
281 // subloop (which would already have been processed).
282 if (inSubLoop(BB)) return;
284 for (BasicBlock::iterator II = BB->end(); II != BB->begin(); ) {
285 Instruction &I = *--II;
287 // If the instruction is dead, we would try to sink it because it isn't used
288 // in the loop, instead, just delete it.
289 if (isInstructionTriviallyDead(&I)) {
290 DEBUG(dbgs() << "LICM deleting dead inst: " << I << '\n');
292 CurAST->deleteValue(&I);
298 // Check to see if we can sink this instruction to the exit blocks
299 // of the loop. We can do this if the all users of the instruction are
300 // outside of the loop. In this case, it doesn't even matter if the
301 // operands of the instruction are loop invariant.
303 if (isNotUsedInLoop(I) && canSinkOrHoistInst(I)) {
310 /// HoistRegion - Walk the specified region of the CFG (defined by all blocks
311 /// dominated by the specified block, and that are in the current loop) in depth
312 /// first order w.r.t the DominatorTree. This allows us to visit definitions
313 /// before uses, allowing us to hoist a loop body in one pass without iteration.
315 void LICM::HoistRegion(DomTreeNode *N) {
316 assert(N != 0 && "Null dominator tree node?");
317 BasicBlock *BB = N->getBlock();
319 // If this subregion is not in the top level loop at all, exit.
320 if (!CurLoop->contains(BB)) return;
322 // Only need to process the contents of this block if it is not part of a
323 // subloop (which would already have been processed).
325 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ) {
326 Instruction &I = *II++;
328 // Try constant folding this instruction. If all the operands are
329 // constants, it is technically hoistable, but it would be better to just
331 if (Constant *C = ConstantFoldInstruction(&I)) {
332 DEBUG(dbgs() << "LICM folding inst: " << I << " --> " << *C << '\n');
333 CurAST->copyValue(&I, C);
334 CurAST->deleteValue(&I);
335 I.replaceAllUsesWith(C);
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.
344 if (CurLoop->hasLoopInvariantOperands(&I) && canSinkOrHoistInst(I) &&
345 isSafeToExecuteUnconditionally(I))
349 const std::vector<DomTreeNode*> &Children = N->getChildren();
350 for (unsigned i = 0, e = Children.size(); i != e; ++i)
351 HoistRegion(Children[i]);
354 /// canSinkOrHoistInst - Return true if the hoister and sinker can handle this
357 bool 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!
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)))
368 // Don't hoist loads which have may-aliased stores in loop.
370 if (LI->getType()->isSized())
371 Size = AA->getTypeStoreSize(LI->getType());
372 return !pointerInvalidatedByLoop(LI->getOperand(0), Size,
373 LI->getMetadata(LLVMContext::MD_tbaa));
374 } else if (CallInst *CI = dyn_cast<CallInst>(&I)) {
375 // Don't sink or hoist dbg info; it's legal, but not useful.
376 if (isa<DbgInfoIntrinsic>(I))
379 // Handle simple cases by querying alias analysis.
380 AliasAnalysis::ModRefBehavior Behavior = AA->getModRefBehavior(CI);
381 if (Behavior == AliasAnalysis::DoesNotAccessMemory)
383 if (AliasAnalysis::onlyReadsMemory(Behavior)) {
384 // If this call only reads from memory and there are no writes to memory
385 // in the loop, we can hoist or sink the call as appropriate.
386 bool FoundMod = false;
387 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
390 if (!AS.isForwardingAliasSet() && AS.isMod()) {
395 if (!FoundMod) return true;
398 // FIXME: This should use mod/ref information to see if we can hoist or sink
404 // Otherwise these instructions are hoistable/sinkable
405 return isa<BinaryOperator>(I) || isa<CastInst>(I) ||
406 isa<SelectInst>(I) || isa<GetElementPtrInst>(I) || isa<CmpInst>(I) ||
407 isa<InsertElementInst>(I) || isa<ExtractElementInst>(I) ||
408 isa<ShuffleVectorInst>(I);
411 /// isNotUsedInLoop - Return true if the only users of this instruction are
412 /// outside of the loop. If this is true, we can sink the instruction to the
413 /// exit blocks of the loop.
415 bool LICM::isNotUsedInLoop(Instruction &I) {
416 for (Value::use_iterator UI = I.use_begin(), E = I.use_end(); UI != E; ++UI) {
417 Instruction *User = cast<Instruction>(*UI);
418 if (PHINode *PN = dyn_cast<PHINode>(User)) {
419 // PHI node uses occur in predecessor blocks!
420 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
421 if (PN->getIncomingValue(i) == &I)
422 if (CurLoop->contains(PN->getIncomingBlock(i)))
424 } else if (CurLoop->contains(User)) {
432 /// sink - When an instruction is found to only be used outside of the loop,
433 /// this function moves it to the exit blocks and patches up SSA form as needed.
434 /// This method is guaranteed to remove the original instruction from its
435 /// position, and may either delete it or move it to outside of the loop.
437 void LICM::sink(Instruction &I) {
438 DEBUG(dbgs() << "LICM sinking instruction: " << I << "\n");
440 SmallVector<BasicBlock*, 8> ExitBlocks;
441 CurLoop->getUniqueExitBlocks(ExitBlocks);
443 if (isa<LoadInst>(I)) ++NumMovedLoads;
444 else if (isa<CallInst>(I)) ++NumMovedCalls;
448 // The case where there is only a single exit node of this loop is common
449 // enough that we handle it as a special (more efficient) case. It is more
450 // efficient to handle because there are no PHI nodes that need to be placed.
451 if (ExitBlocks.size() == 1) {
452 if (!isa<DbgInfoIntrinsic>(I) &&
453 !DT->dominates(I.getParent(), ExitBlocks[0])) {
454 // Instruction is not used, just delete it.
455 CurAST->deleteValue(&I);
456 // If I has users in unreachable blocks, eliminate.
457 // If I is not void type then replaceAllUsesWith undef.
458 // This allows ValueHandlers and custom metadata to adjust itself.
460 I.replaceAllUsesWith(UndefValue::get(I.getType()));
463 // Move the instruction to the start of the exit block, after any PHI
465 I.moveBefore(ExitBlocks[0]->getFirstNonPHI());
467 // This instruction is no longer in the AST for the current loop, because
468 // we just sunk it out of the loop. If we just sunk it into an outer
469 // loop, we will rediscover the operation when we process it.
470 CurAST->deleteValue(&I);
475 if (ExitBlocks.empty()) {
476 // The instruction is actually dead if there ARE NO exit blocks.
477 CurAST->deleteValue(&I);
478 // If I has users in unreachable blocks, eliminate.
479 // If I is not void type then replaceAllUsesWith undef.
480 // This allows ValueHandlers and custom metadata to adjust itself.
482 I.replaceAllUsesWith(UndefValue::get(I.getType()));
487 // Otherwise, if we have multiple exits, use the SSAUpdater to do all of the
488 // hard work of inserting PHI nodes as necessary.
489 SmallVector<PHINode*, 8> NewPHIs;
490 SSAUpdater SSA(&NewPHIs);
493 SSA.Initialize(I.getType(), I.getName());
495 // Insert a copy of the instruction in each exit block of the loop that is
496 // dominated by the instruction. Each exit block is known to only be in the
497 // ExitBlocks list once.
498 BasicBlock *InstOrigBB = I.getParent();
499 unsigned NumInserted = 0;
501 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
502 BasicBlock *ExitBlock = ExitBlocks[i];
504 if (!DT->dominates(InstOrigBB, ExitBlock))
507 // Insert the code after the last PHI node.
508 BasicBlock::iterator InsertPt = ExitBlock->getFirstNonPHI();
510 // If this is the first exit block processed, just move the original
511 // instruction, otherwise clone the original instruction and insert
514 if (NumInserted++ == 0) {
515 I.moveBefore(InsertPt);
519 if (!I.getName().empty())
520 New->setName(I.getName()+".le");
521 ExitBlock->getInstList().insert(InsertPt, New);
524 // Now that we have inserted the instruction, inform SSAUpdater.
526 SSA.AddAvailableValue(ExitBlock, New);
529 assert(NumInserted && "We shouldn't see dead instructions here!");
531 // Next, rewrite uses of the instruction, inserting PHI nodes as needed.
532 for (Value::use_iterator UI = I.use_begin(), UE = I.use_end(); UI != UE; ) {
533 // Grab the use before incrementing the iterator.
534 Use &U = UI.getUse();
535 // Increment the iterator before removing the use from the list.
537 SSA.RewriteUseAfterInsertions(U);
540 // Update CurAST for NewPHIs if I had pointer type.
541 if (I.getType()->isPointerTy())
542 for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i)
543 CurAST->copyValue(&I, NewPHIs[i]);
545 // Finally, remove the instruction from CurAST. It is no longer in the loop.
546 CurAST->deleteValue(&I);
549 /// hoist - When an instruction is found to only use loop invariant operands
550 /// that is safe to hoist, this instruction is called to do the dirty work.
552 void LICM::hoist(Instruction &I) {
553 DEBUG(dbgs() << "LICM hoisting to " << Preheader->getName() << ": "
556 // Move the new node to the Preheader, before its terminator.
557 I.moveBefore(Preheader->getTerminator());
559 if (isa<LoadInst>(I)) ++NumMovedLoads;
560 else if (isa<CallInst>(I)) ++NumMovedCalls;
565 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it is
566 /// not a trapping instruction or if it is a trapping instruction and is
567 /// guaranteed to execute.
569 bool LICM::isSafeToExecuteUnconditionally(Instruction &Inst) {
570 // If it is not a trapping instruction, it is always safe to hoist.
571 if (Inst.isSafeToSpeculativelyExecute())
574 // Otherwise we have to check to make sure that the instruction dominates all
575 // of the exit blocks. If it doesn't, then there is a path out of the loop
576 // which does not execute this instruction, so we can't hoist it.
578 // If the instruction is in the header block for the loop (which is very
579 // common), it is always guaranteed to dominate the exit blocks. Since this
580 // is a common case, and can save some work, check it now.
581 if (Inst.getParent() == CurLoop->getHeader())
584 // Get the exit blocks for the current loop.
585 SmallVector<BasicBlock*, 8> ExitBlocks;
586 CurLoop->getExitBlocks(ExitBlocks);
588 // Verify that the block dominates each of the exit blocks of the loop.
589 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
590 if (!DT->dominates(Inst.getParent(), ExitBlocks[i]))
597 class LoopPromoter : public LoadAndStorePromoter {
598 Value *SomePtr; // Designated pointer to store to.
599 SmallPtrSet<Value*, 4> &PointerMustAliases;
600 SmallVectorImpl<BasicBlock*> &LoopExitBlocks;
601 AliasSetTracker &AST;
603 LoopPromoter(Value *SP,
604 const SmallVectorImpl<Instruction*> &Insts, SSAUpdater &S,
605 SmallPtrSet<Value*, 4> &PMA,
606 SmallVectorImpl<BasicBlock*> &LEB, AliasSetTracker &ast)
607 : LoadAndStorePromoter(Insts, S, 0, 0), SomePtr(SP),
608 PointerMustAliases(PMA), LoopExitBlocks(LEB), AST(ast) {}
610 virtual bool isInstInList(Instruction *I,
611 const SmallVectorImpl<Instruction*> &) const {
613 if (LoadInst *LI = dyn_cast<LoadInst>(I))
614 Ptr = LI->getOperand(0);
616 Ptr = cast<StoreInst>(I)->getPointerOperand();
617 return PointerMustAliases.count(Ptr);
620 virtual void doExtraRewritesBeforeFinalDeletion() const {
621 // Insert stores after in the loop exit blocks. Each exit block gets a
622 // store of the live-out values that feed them. Since we've already told
623 // the SSA updater about the defs in the loop and the preheader
624 // definition, it is all set and we can start using it.
625 for (unsigned i = 0, e = LoopExitBlocks.size(); i != e; ++i) {
626 BasicBlock *ExitBlock = LoopExitBlocks[i];
627 Value *LiveInValue = SSA.GetValueInMiddleOfBlock(ExitBlock);
628 Instruction *InsertPos = ExitBlock->getFirstNonPHI();
629 new StoreInst(LiveInValue, SomePtr, InsertPos);
633 virtual void replaceLoadWithValue(LoadInst *LI, Value *V) const {
634 // Update alias analysis.
635 AST.copyValue(LI, V);
637 virtual void instructionDeleted(Instruction *I) const {
641 } // end anon namespace
643 /// PromoteAliasSet - Try to promote memory values to scalars by sinking
644 /// stores out of the loop and moving loads to before the loop. We do this by
645 /// looping over the stores in the loop, looking for stores to Must pointers
646 /// which are loop invariant.
648 void LICM::PromoteAliasSet(AliasSet &AS) {
649 // We can promote this alias set if it has a store, if it is a "Must" alias
650 // set, if the pointer is loop invariant, and if we are not eliminating any
651 // volatile loads or stores.
652 if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() ||
653 AS.isVolatile() || !CurLoop->isLoopInvariant(AS.begin()->getValue()))
656 assert(!AS.empty() &&
657 "Must alias set should have at least one pointer element in it!");
658 Value *SomePtr = AS.begin()->getValue();
660 // It isn't safe to promote a load/store from the loop if the load/store is
661 // conditional. For example, turning:
663 // for () { if (c) *P += 1; }
667 // tmp = *P; for () { if (c) tmp +=1; } *P = tmp;
669 // is not safe, because *P may only be valid to access if 'c' is true.
671 // It is safe to promote P if all uses are direct load/stores and if at
672 // least one is guaranteed to be executed.
673 bool GuaranteedToExecute = false;
675 SmallVector<Instruction*, 64> LoopUses;
676 SmallPtrSet<Value*, 4> PointerMustAliases;
678 // Check that all of the pointers in the alias set have the same type. We
679 // cannot (yet) promote a memory location that is loaded and stored in
681 for (AliasSet::iterator ASI = AS.begin(), E = AS.end(); ASI != E; ++ASI) {
682 Value *ASIV = ASI->getValue();
683 PointerMustAliases.insert(ASIV);
685 // Check that all of the pointers in the alias set have the same type. We
686 // cannot (yet) promote a memory location that is loaded and stored in
688 if (SomePtr->getType() != ASIV->getType())
691 for (Value::use_iterator UI = ASIV->use_begin(), UE = ASIV->use_end();
693 // Ignore instructions that are outside the loop.
694 Instruction *Use = dyn_cast<Instruction>(*UI);
695 if (!Use || !CurLoop->contains(Use))
698 // If there is an non-load/store instruction in the loop, we can't promote
700 if (isa<LoadInst>(Use))
701 assert(!cast<LoadInst>(Use)->isVolatile() && "AST broken");
702 else if (isa<StoreInst>(Use)) {
703 // Stores *of* the pointer are not interesting, only stores *to* the
705 if (Use->getOperand(1) != ASIV)
707 assert(!cast<StoreInst>(Use)->isVolatile() && "AST broken");
709 return; // Not a load or store.
711 if (!GuaranteedToExecute)
712 GuaranteedToExecute = isSafeToExecuteUnconditionally(*Use);
714 LoopUses.push_back(Use);
718 // If there isn't a guaranteed-to-execute instruction, we can't promote.
719 if (!GuaranteedToExecute)
722 // Otherwise, this is safe to promote, lets do it!
723 DEBUG(dbgs() << "LICM: Promoting value stored to in loop: " <<*SomePtr<<'\n');
727 SmallVector<BasicBlock*, 8> ExitBlocks;
728 CurLoop->getUniqueExitBlocks(ExitBlocks);
730 // We use the SSAUpdater interface to insert phi nodes as required.
731 SmallVector<PHINode*, 16> NewPHIs;
732 SSAUpdater SSA(&NewPHIs);
733 LoopPromoter Promoter(SomePtr, LoopUses, SSA, PointerMustAliases, ExitBlocks,
736 // Set up the preheader to have a definition of the value. It is the live-out
737 // value from the preheader that uses in the loop will use.
738 LoadInst *PreheaderLoad =
739 new LoadInst(SomePtr, SomePtr->getName()+".promoted",
740 Preheader->getTerminator());
741 SSA.AddAvailableValue(Preheader, PreheaderLoad);
743 // Rewrite all the loads in the loop and remember all the definitions from
744 // stores in the loop.
745 Promoter.run(LoopUses);
747 // If the SSAUpdater didn't use the load in the preheader, just zap it now.
748 if (PreheaderLoad->use_empty())
749 PreheaderLoad->eraseFromParent();
753 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
754 void LICM::cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L) {
755 AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);
759 AST->copyValue(From, To);
762 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
764 void LICM::deleteAnalysisValue(Value *V, Loop *L) {
765 AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);