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/Target/TargetData.h"
49 #include "llvm/Target/TargetLibraryInfo.h"
50 #include "llvm/Support/CFG.h"
51 #include "llvm/Support/CommandLine.h"
52 #include "llvm/Support/raw_ostream.h"
53 #include "llvm/Support/Debug.h"
54 #include "llvm/ADT/Statistic.h"
58 STATISTIC(NumSunk , "Number of instructions sunk out of loop");
59 STATISTIC(NumHoisted , "Number of instructions hoisted out of loop");
60 STATISTIC(NumMovedLoads, "Number of load insts hoisted or sunk");
61 STATISTIC(NumMovedCalls, "Number of call insts hoisted or sunk");
62 STATISTIC(NumPromoted , "Number of memory locations promoted to registers");
65 DisablePromotion("disable-licm-promotion", cl::Hidden,
66 cl::desc("Disable memory promotion in LICM pass"));
69 struct LICM : public LoopPass {
70 static char ID; // Pass identification, replacement for typeid
71 LICM() : LoopPass(ID) {
72 initializeLICMPass(*PassRegistry::getPassRegistry());
75 virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
77 /// This transformation requires natural loop information & requires that
78 /// loop preheaders be inserted into the CFG...
80 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
82 AU.addRequired<DominatorTree>();
83 AU.addRequired<LoopInfo>();
84 AU.addRequiredID(LoopSimplifyID);
85 AU.addRequired<AliasAnalysis>();
86 AU.addPreserved<AliasAnalysis>();
87 AU.addPreserved("scalar-evolution");
88 AU.addPreservedID(LoopSimplifyID);
89 AU.addRequired<TargetLibraryInfo>();
92 bool doFinalization() {
93 assert(LoopToAliasSetMap.empty() && "Didn't free loop alias sets");
98 AliasAnalysis *AA; // Current AliasAnalysis information
99 LoopInfo *LI; // Current LoopInfo
100 DominatorTree *DT; // Dominator Tree for the current Loop.
102 TargetData *TD; // TargetData for constant folding.
103 TargetLibraryInfo *TLI; // TargetLibraryInfo for constant folding.
105 // State that is updated as we process loops.
106 bool Changed; // Set to true when we change anything.
107 BasicBlock *Preheader; // The preheader block of the current loop...
108 Loop *CurLoop; // The current loop we are working on...
109 AliasSetTracker *CurAST; // AliasSet information for the current loop...
110 DenseMap<Loop*, AliasSetTracker*> LoopToAliasSetMap;
112 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
113 void cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L);
115 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
117 void deleteAnalysisValue(Value *V, Loop *L);
119 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks
120 /// dominated by the specified block, and that are in the current loop) in
121 /// reverse depth first order w.r.t the DominatorTree. This allows us to
122 /// visit uses before definitions, allowing us to sink a loop body in one
123 /// pass without iteration.
125 void SinkRegion(DomTreeNode *N);
127 /// HoistRegion - Walk the specified region of the CFG (defined by all
128 /// blocks dominated by the specified block, and that are in the current
129 /// loop) in depth first order w.r.t the DominatorTree. This allows us to
130 /// visit definitions before uses, allowing us to hoist a loop body in one
131 /// pass without iteration.
133 void HoistRegion(DomTreeNode *N);
135 /// inSubLoop - Little predicate that returns true if the specified basic
136 /// block is in a subloop of the current one, not the current one itself.
138 bool inSubLoop(BasicBlock *BB) {
139 assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
140 return LI->getLoopFor(BB) != CurLoop;
143 /// sink - When an instruction is found to only be used outside of the loop,
144 /// this function moves it to the exit blocks and patches up SSA form as
147 void sink(Instruction &I);
149 /// hoist - When an instruction is found to only use loop invariant operands
150 /// that is safe to hoist, this instruction is called to do the dirty work.
152 void hoist(Instruction &I);
154 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it
155 /// is not a trapping instruction or if it is a trapping instruction and is
156 /// guaranteed to execute.
158 bool isSafeToExecuteUnconditionally(Instruction &I);
160 /// isGuaranteedToExecute - Check that the instruction is guaranteed to
163 bool isGuaranteedToExecute(Instruction &I);
165 /// pointerInvalidatedByLoop - Return true if the body of this loop may
166 /// store into the memory location pointed to by V.
168 bool pointerInvalidatedByLoop(Value *V, uint64_t Size,
169 const MDNode *TBAAInfo) {
170 // Check to see if any of the basic blocks in CurLoop invalidate *V.
171 return CurAST->getAliasSetForPointer(V, Size, TBAAInfo).isMod();
174 bool canSinkOrHoistInst(Instruction &I);
175 bool isNotUsedInLoop(Instruction &I);
177 void PromoteAliasSet(AliasSet &AS);
182 INITIALIZE_PASS_BEGIN(LICM, "licm", "Loop Invariant Code Motion", false, false)
183 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
184 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
185 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
186 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
187 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
188 INITIALIZE_PASS_END(LICM, "licm", "Loop Invariant Code Motion", false, false)
190 Pass *llvm::createLICMPass() { return new LICM(); }
192 /// Hoist expressions out of the specified loop. Note, alias info for inner
193 /// loop is not preserved so it is not a good idea to run LICM multiple
194 /// times on one loop.
196 bool LICM::runOnLoop(Loop *L, LPPassManager &LPM) {
199 // Get our Loop and Alias Analysis information...
200 LI = &getAnalysis<LoopInfo>();
201 AA = &getAnalysis<AliasAnalysis>();
202 DT = &getAnalysis<DominatorTree>();
204 TD = getAnalysisIfAvailable<TargetData>();
205 TLI = &getAnalysis<TargetLibraryInfo>();
207 CurAST = new AliasSetTracker(*AA);
208 // Collect Alias info from subloops.
209 for (Loop::iterator LoopItr = L->begin(), LoopItrE = L->end();
210 LoopItr != LoopItrE; ++LoopItr) {
211 Loop *InnerL = *LoopItr;
212 AliasSetTracker *InnerAST = LoopToAliasSetMap[InnerL];
213 assert(InnerAST && "Where is my AST?");
215 // What if InnerLoop was modified by other passes ?
216 CurAST->add(*InnerAST);
218 // Once we've incorporated the inner loop's AST into ours, we don't need the
219 // subloop's anymore.
221 LoopToAliasSetMap.erase(InnerL);
226 // Get the preheader block to move instructions into...
227 Preheader = L->getLoopPreheader();
229 // Loop over the body of this loop, looking for calls, invokes, and stores.
230 // Because subloops have already been incorporated into AST, we skip blocks in
233 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
236 if (LI->getLoopFor(BB) == L) // Ignore blocks in subloops.
237 CurAST->add(*BB); // Incorporate the specified basic block
240 // We want to visit all of the instructions in this loop... that are not parts
241 // of our subloops (they have already had their invariants hoisted out of
242 // their loop, into this loop, so there is no need to process the BODIES of
245 // Traverse the body of the loop in depth first order on the dominator tree so
246 // that we are guaranteed to see definitions before we see uses. This allows
247 // us to sink instructions in one pass, without iteration. After sinking
248 // instructions, we perform another pass to hoist them out of the loop.
250 if (L->hasDedicatedExits())
251 SinkRegion(DT->getNode(L->getHeader()));
253 HoistRegion(DT->getNode(L->getHeader()));
255 // Now that all loop invariants have been removed from the loop, promote any
256 // memory references to scalars that we can.
257 if (!DisablePromotion && Preheader && L->hasDedicatedExits()) {
258 // Loop over all of the alias sets in the tracker object.
259 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
264 // Clear out loops state information for the next iteration
268 // If this loop is nested inside of another one, save the alias information
269 // for when we process the outer loop.
270 if (L->getParentLoop())
271 LoopToAliasSetMap[L] = CurAST;
277 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks
278 /// dominated by the specified block, and that are in the current loop) in
279 /// reverse depth first order w.r.t the DominatorTree. This allows us to visit
280 /// uses before definitions, allowing us to sink a loop body in one pass without
283 void LICM::SinkRegion(DomTreeNode *N) {
284 assert(N != 0 && "Null dominator tree node?");
285 BasicBlock *BB = N->getBlock();
287 // If this subregion is not in the top level loop at all, exit.
288 if (!CurLoop->contains(BB)) return;
290 // We are processing blocks in reverse dfo, so process children first.
291 const std::vector<DomTreeNode*> &Children = N->getChildren();
292 for (unsigned i = 0, e = Children.size(); i != e; ++i)
293 SinkRegion(Children[i]);
295 // Only need to process the contents of this block if it is not part of a
296 // subloop (which would already have been processed).
297 if (inSubLoop(BB)) return;
299 for (BasicBlock::iterator II = BB->end(); II != BB->begin(); ) {
300 Instruction &I = *--II;
302 // If the instruction is dead, we would try to sink it because it isn't used
303 // in the loop, instead, just delete it.
304 if (isInstructionTriviallyDead(&I)) {
305 DEBUG(dbgs() << "LICM deleting dead inst: " << I << '\n');
307 CurAST->deleteValue(&I);
313 // Check to see if we can sink this instruction to the exit blocks
314 // of the loop. We can do this if the all users of the instruction are
315 // outside of the loop. In this case, it doesn't even matter if the
316 // operands of the instruction are loop invariant.
318 if (isNotUsedInLoop(I) && canSinkOrHoistInst(I)) {
325 /// HoistRegion - Walk the specified region of the CFG (defined by all blocks
326 /// dominated by the specified block, and that are in the current loop) in depth
327 /// first order w.r.t the DominatorTree. This allows us to visit definitions
328 /// before uses, allowing us to hoist a loop body in one pass without iteration.
330 void LICM::HoistRegion(DomTreeNode *N) {
331 assert(N != 0 && "Null dominator tree node?");
332 BasicBlock *BB = N->getBlock();
334 // If this subregion is not in the top level loop at all, exit.
335 if (!CurLoop->contains(BB)) return;
337 // Only need to process the contents of this block if it is not part of a
338 // subloop (which would already have been processed).
340 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ) {
341 Instruction &I = *II++;
343 // Try constant folding this instruction. If all the operands are
344 // constants, it is technically hoistable, but it would be better to just
346 if (Constant *C = ConstantFoldInstruction(&I, TD, TLI)) {
347 DEBUG(dbgs() << "LICM folding inst: " << I << " --> " << *C << '\n');
348 CurAST->copyValue(&I, C);
349 CurAST->deleteValue(&I);
350 I.replaceAllUsesWith(C);
355 // Try hoisting the instruction out to the preheader. We can only do this
356 // if all of the operands of the instruction are loop invariant and if it
357 // is safe to hoist the instruction.
359 if (CurLoop->hasLoopInvariantOperands(&I) && canSinkOrHoistInst(I) &&
360 isSafeToExecuteUnconditionally(I))
364 const std::vector<DomTreeNode*> &Children = N->getChildren();
365 for (unsigned i = 0, e = Children.size(); i != e; ++i)
366 HoistRegion(Children[i]);
369 /// canSinkOrHoistInst - Return true if the hoister and sinker can handle this
372 bool LICM::canSinkOrHoistInst(Instruction &I) {
373 // Loads have extra constraints we have to verify before we can hoist them.
374 if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
375 if (!LI->isUnordered())
376 return false; // Don't hoist volatile/atomic loads!
378 // Loads from constant memory are always safe to move, even if they end up
379 // in the same alias set as something that ends up being modified.
380 if (AA->pointsToConstantMemory(LI->getOperand(0)))
382 if (LI->getMetadata(LI->getContext().getMDKindID("invariant.load")))
385 // Don't hoist loads which have may-aliased stores in loop.
387 if (LI->getType()->isSized())
388 Size = AA->getTypeStoreSize(LI->getType());
389 return !pointerInvalidatedByLoop(LI->getOperand(0), Size,
390 LI->getMetadata(LLVMContext::MD_tbaa));
391 } else if (CallInst *CI = dyn_cast<CallInst>(&I)) {
392 // Don't sink or hoist dbg info; it's legal, but not useful.
393 if (isa<DbgInfoIntrinsic>(I))
396 // Handle simple cases by querying alias analysis.
397 AliasAnalysis::ModRefBehavior Behavior = AA->getModRefBehavior(CI);
398 if (Behavior == AliasAnalysis::DoesNotAccessMemory)
400 if (AliasAnalysis::onlyReadsMemory(Behavior)) {
401 // If this call only reads from memory and there are no writes to memory
402 // in the loop, we can hoist or sink the call as appropriate.
403 bool FoundMod = false;
404 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
407 if (!AS.isForwardingAliasSet() && AS.isMod()) {
412 if (!FoundMod) return true;
415 // FIXME: This should use mod/ref information to see if we can hoist or sink
421 // Otherwise these instructions are hoistable/sinkable
422 return isa<BinaryOperator>(I) || isa<CastInst>(I) ||
423 isa<SelectInst>(I) || isa<GetElementPtrInst>(I) || isa<CmpInst>(I) ||
424 isa<InsertElementInst>(I) || isa<ExtractElementInst>(I) ||
425 isa<ShuffleVectorInst>(I);
428 /// isNotUsedInLoop - Return true if the only users of this instruction are
429 /// outside of the loop. If this is true, we can sink the instruction to the
430 /// exit blocks of the loop.
432 bool LICM::isNotUsedInLoop(Instruction &I) {
433 for (Value::use_iterator UI = I.use_begin(), E = I.use_end(); UI != E; ++UI) {
434 Instruction *User = cast<Instruction>(*UI);
435 if (PHINode *PN = dyn_cast<PHINode>(User)) {
436 // PHI node uses occur in predecessor blocks!
437 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
438 if (PN->getIncomingValue(i) == &I)
439 if (CurLoop->contains(PN->getIncomingBlock(i)))
441 } else if (CurLoop->contains(User)) {
449 /// sink - When an instruction is found to only be used outside of the loop,
450 /// this function moves it to the exit blocks and patches up SSA form as needed.
451 /// This method is guaranteed to remove the original instruction from its
452 /// position, and may either delete it or move it to outside of the loop.
454 void LICM::sink(Instruction &I) {
455 DEBUG(dbgs() << "LICM sinking instruction: " << I << "\n");
457 SmallVector<BasicBlock*, 8> ExitBlocks;
458 CurLoop->getUniqueExitBlocks(ExitBlocks);
460 if (isa<LoadInst>(I)) ++NumMovedLoads;
461 else if (isa<CallInst>(I)) ++NumMovedCalls;
465 // The case where there is only a single exit node of this loop is common
466 // enough that we handle it as a special (more efficient) case. It is more
467 // efficient to handle because there are no PHI nodes that need to be placed.
468 if (ExitBlocks.size() == 1) {
469 if (!DT->dominates(I.getParent(), ExitBlocks[0])) {
470 // Instruction is not used, just delete it.
471 CurAST->deleteValue(&I);
472 // If I has users in unreachable blocks, eliminate.
473 // If I is not void type then replaceAllUsesWith undef.
474 // This allows ValueHandlers and custom metadata to adjust itself.
476 I.replaceAllUsesWith(UndefValue::get(I.getType()));
479 // Move the instruction to the start of the exit block, after any PHI
481 I.moveBefore(ExitBlocks[0]->getFirstInsertionPt());
483 // This instruction is no longer in the AST for the current loop, because
484 // we just sunk it out of the loop. If we just sunk it into an outer
485 // loop, we will rediscover the operation when we process it.
486 CurAST->deleteValue(&I);
491 if (ExitBlocks.empty()) {
492 // The instruction is actually dead if there ARE NO exit blocks.
493 CurAST->deleteValue(&I);
494 // If I has users in unreachable blocks, eliminate.
495 // If I is not void type then replaceAllUsesWith undef.
496 // This allows ValueHandlers and custom metadata to adjust itself.
498 I.replaceAllUsesWith(UndefValue::get(I.getType()));
503 // Otherwise, if we have multiple exits, use the SSAUpdater to do all of the
504 // hard work of inserting PHI nodes as necessary.
505 SmallVector<PHINode*, 8> NewPHIs;
506 SSAUpdater SSA(&NewPHIs);
509 SSA.Initialize(I.getType(), I.getName());
511 // Insert a copy of the instruction in each exit block of the loop that is
512 // dominated by the instruction. Each exit block is known to only be in the
513 // ExitBlocks list once.
514 BasicBlock *InstOrigBB = I.getParent();
515 unsigned NumInserted = 0;
517 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
518 BasicBlock *ExitBlock = ExitBlocks[i];
520 if (!DT->dominates(InstOrigBB, ExitBlock))
523 // Insert the code after the last PHI node.
524 BasicBlock::iterator InsertPt = ExitBlock->getFirstInsertionPt();
526 // If this is the first exit block processed, just move the original
527 // instruction, otherwise clone the original instruction and insert
530 if (NumInserted++ == 0) {
531 I.moveBefore(InsertPt);
535 if (!I.getName().empty())
536 New->setName(I.getName()+".le");
537 ExitBlock->getInstList().insert(InsertPt, New);
540 // Now that we have inserted the instruction, inform SSAUpdater.
542 SSA.AddAvailableValue(ExitBlock, New);
545 // If the instruction doesn't dominate any exit blocks, it must be dead.
546 if (NumInserted == 0) {
547 CurAST->deleteValue(&I);
549 I.replaceAllUsesWith(UndefValue::get(I.getType()));
554 // Next, rewrite uses of the instruction, inserting PHI nodes as needed.
555 for (Value::use_iterator UI = I.use_begin(), UE = I.use_end(); UI != UE; ) {
556 // Grab the use before incrementing the iterator.
557 Use &U = UI.getUse();
558 // Increment the iterator before removing the use from the list.
560 SSA.RewriteUseAfterInsertions(U);
563 // Update CurAST for NewPHIs if I had pointer type.
564 if (I.getType()->isPointerTy())
565 for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i)
566 CurAST->copyValue(&I, NewPHIs[i]);
568 // Finally, remove the instruction from CurAST. It is no longer in the loop.
569 CurAST->deleteValue(&I);
572 /// hoist - When an instruction is found to only use loop invariant operands
573 /// that is safe to hoist, this instruction is called to do the dirty work.
575 void LICM::hoist(Instruction &I) {
576 DEBUG(dbgs() << "LICM hoisting to " << Preheader->getName() << ": "
579 // Move the new node to the Preheader, before its terminator.
580 I.moveBefore(Preheader->getTerminator());
582 if (isa<LoadInst>(I)) ++NumMovedLoads;
583 else if (isa<CallInst>(I)) ++NumMovedCalls;
588 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it is
589 /// not a trapping instruction or if it is a trapping instruction and is
590 /// guaranteed to execute.
592 bool LICM::isSafeToExecuteUnconditionally(Instruction &Inst) {
593 // If it is not a trapping instruction, it is always safe to hoist.
594 if (Inst.isSafeToSpeculativelyExecute())
597 return isGuaranteedToExecute(Inst);
600 bool LICM::isGuaranteedToExecute(Instruction &Inst) {
601 // Otherwise we have to check to make sure that the instruction dominates all
602 // of the exit blocks. If it doesn't, then there is a path out of the loop
603 // which does not execute this instruction, so we can't hoist it.
605 // If the instruction is in the header block for the loop (which is very
606 // common), it is always guaranteed to dominate the exit blocks. Since this
607 // is a common case, and can save some work, check it now.
608 if (Inst.getParent() == CurLoop->getHeader())
611 // Get the exit blocks for the current loop.
612 SmallVector<BasicBlock*, 8> ExitBlocks;
613 CurLoop->getExitBlocks(ExitBlocks);
615 // Verify that the block dominates each of the exit blocks of the loop.
616 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
617 if (!DT->dominates(Inst.getParent(), ExitBlocks[i]))
624 class LoopPromoter : public LoadAndStorePromoter {
625 Value *SomePtr; // Designated pointer to store to.
626 SmallPtrSet<Value*, 4> &PointerMustAliases;
627 SmallVectorImpl<BasicBlock*> &LoopExitBlocks;
628 AliasSetTracker &AST;
632 LoopPromoter(Value *SP,
633 const SmallVectorImpl<Instruction*> &Insts, SSAUpdater &S,
634 SmallPtrSet<Value*, 4> &PMA,
635 SmallVectorImpl<BasicBlock*> &LEB, AliasSetTracker &ast,
636 DebugLoc dl, int alignment)
637 : LoadAndStorePromoter(Insts, S), SomePtr(SP),
638 PointerMustAliases(PMA), LoopExitBlocks(LEB), AST(ast), DL(dl),
639 Alignment(alignment) {}
641 virtual bool isInstInList(Instruction *I,
642 const SmallVectorImpl<Instruction*> &) const {
644 if (LoadInst *LI = dyn_cast<LoadInst>(I))
645 Ptr = LI->getOperand(0);
647 Ptr = cast<StoreInst>(I)->getPointerOperand();
648 return PointerMustAliases.count(Ptr);
651 virtual void doExtraRewritesBeforeFinalDeletion() const {
652 // Insert stores after in the loop exit blocks. Each exit block gets a
653 // store of the live-out values that feed them. Since we've already told
654 // the SSA updater about the defs in the loop and the preheader
655 // definition, it is all set and we can start using it.
656 for (unsigned i = 0, e = LoopExitBlocks.size(); i != e; ++i) {
657 BasicBlock *ExitBlock = LoopExitBlocks[i];
658 Value *LiveInValue = SSA.GetValueInMiddleOfBlock(ExitBlock);
659 Instruction *InsertPos = ExitBlock->getFirstInsertionPt();
660 StoreInst *NewSI = new StoreInst(LiveInValue, SomePtr, InsertPos);
661 NewSI->setAlignment(Alignment);
662 NewSI->setDebugLoc(DL);
666 virtual void replaceLoadWithValue(LoadInst *LI, Value *V) const {
667 // Update alias analysis.
668 AST.copyValue(LI, V);
670 virtual void instructionDeleted(Instruction *I) const {
674 } // end anon namespace
676 /// PromoteAliasSet - Try to promote memory values to scalars by sinking
677 /// stores out of the loop and moving loads to before the loop. We do this by
678 /// looping over the stores in the loop, looking for stores to Must pointers
679 /// which are loop invariant.
681 void LICM::PromoteAliasSet(AliasSet &AS) {
682 // We can promote this alias set if it has a store, if it is a "Must" alias
683 // set, if the pointer is loop invariant, and if we are not eliminating any
684 // volatile loads or stores.
685 if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() ||
686 AS.isVolatile() || !CurLoop->isLoopInvariant(AS.begin()->getValue()))
689 assert(!AS.empty() &&
690 "Must alias set should have at least one pointer element in it!");
691 Value *SomePtr = AS.begin()->getValue();
693 // It isn't safe to promote a load/store from the loop if the load/store is
694 // conditional. For example, turning:
696 // for () { if (c) *P += 1; }
700 // tmp = *P; for () { if (c) tmp +=1; } *P = tmp;
702 // is not safe, because *P may only be valid to access if 'c' is true.
704 // It is safe to promote P if all uses are direct load/stores and if at
705 // least one is guaranteed to be executed.
706 bool GuaranteedToExecute = false;
708 SmallVector<Instruction*, 64> LoopUses;
709 SmallPtrSet<Value*, 4> PointerMustAliases;
711 // We start with an alignment of one and try to find instructions that allow
712 // us to prove better alignment.
713 unsigned Alignment = 1;
715 // Check that all of the pointers in the alias set have the same type. We
716 // cannot (yet) promote a memory location that is loaded and stored in
718 for (AliasSet::iterator ASI = AS.begin(), E = AS.end(); ASI != E; ++ASI) {
719 Value *ASIV = ASI->getValue();
720 PointerMustAliases.insert(ASIV);
722 // Check that all of the pointers in the alias set have the same type. We
723 // cannot (yet) promote a memory location that is loaded and stored in
725 if (SomePtr->getType() != ASIV->getType())
728 for (Value::use_iterator UI = ASIV->use_begin(), UE = ASIV->use_end();
730 // Ignore instructions that are outside the loop.
731 Instruction *Use = dyn_cast<Instruction>(*UI);
732 if (!Use || !CurLoop->contains(Use))
735 // If there is an non-load/store instruction in the loop, we can't promote
737 if (LoadInst *load = dyn_cast<LoadInst>(Use)) {
738 assert(!load->isVolatile() && "AST broken");
739 if (!load->isSimple())
741 } else if (StoreInst *store = dyn_cast<StoreInst>(Use)) {
742 // Stores *of* the pointer are not interesting, only stores *to* the
744 if (Use->getOperand(1) != ASIV)
746 assert(!store->isVolatile() && "AST broken");
747 if (!store->isSimple())
750 // Note that we only check GuaranteedToExecute inside the store case
751 // so that we do not introduce stores where they did not exist before
752 // (which would break the LLVM concurrency model).
754 // If the alignment of this instruction allows us to specify a more
755 // restrictive (and performant) alignment and if we are sure this
756 // instruction will be executed, update the alignment.
757 // Larger is better, with the exception of 0 being the best alignment.
758 unsigned InstAlignment = store->getAlignment();
759 if ((InstAlignment > Alignment || InstAlignment == 0)
761 if (isGuaranteedToExecute(*Use)) {
762 GuaranteedToExecute = true;
763 Alignment = InstAlignment;
766 if (!GuaranteedToExecute)
767 GuaranteedToExecute = isGuaranteedToExecute(*Use);
770 return; // Not a load or store.
772 LoopUses.push_back(Use);
776 // If there isn't a guaranteed-to-execute instruction, we can't promote.
777 if (!GuaranteedToExecute)
780 // Otherwise, this is safe to promote, lets do it!
781 DEBUG(dbgs() << "LICM: Promoting value stored to in loop: " <<*SomePtr<<'\n');
785 // Grab a debug location for the inserted loads/stores; given that the
786 // inserted loads/stores have little relation to the original loads/stores,
787 // this code just arbitrarily picks a location from one, since any debug
788 // location is better than none.
789 DebugLoc DL = LoopUses[0]->getDebugLoc();
791 SmallVector<BasicBlock*, 8> ExitBlocks;
792 CurLoop->getUniqueExitBlocks(ExitBlocks);
794 // We use the SSAUpdater interface to insert phi nodes as required.
795 SmallVector<PHINode*, 16> NewPHIs;
796 SSAUpdater SSA(&NewPHIs);
797 LoopPromoter Promoter(SomePtr, LoopUses, SSA, PointerMustAliases, ExitBlocks,
798 *CurAST, DL, Alignment);
800 // Set up the preheader to have a definition of the value. It is the live-out
801 // value from the preheader that uses in the loop will use.
802 LoadInst *PreheaderLoad =
803 new LoadInst(SomePtr, SomePtr->getName()+".promoted",
804 Preheader->getTerminator());
805 PreheaderLoad->setAlignment(Alignment);
806 PreheaderLoad->setDebugLoc(DL);
807 SSA.AddAvailableValue(Preheader, PreheaderLoad);
809 // Rewrite all the loads in the loop and remember all the definitions from
810 // stores in the loop.
811 Promoter.run(LoopUses);
813 // If the SSAUpdater didn't use the load in the preheader, just zap it now.
814 if (PreheaderLoad->use_empty())
815 PreheaderLoad->eraseFromParent();
819 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
820 void LICM::cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L) {
821 AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);
825 AST->copyValue(From, To);
828 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
830 void LICM::deleteAnalysisValue(Value *V, Loop *L) {
831 AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);