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 for (Loop::iterator I = CurLoop->begin(), E = CurLoop->end(); I != E; ++I)
135 if ((*I)->contains(BB))
136 return true; // A subloop actually contains this block!
140 /// sink - When an instruction is found to only be used outside of the loop,
141 /// this function moves it to the exit blocks and patches up SSA form as
144 void sink(Instruction &I);
146 /// hoist - When an instruction is found to only use loop invariant operands
147 /// that is safe to hoist, this instruction is called to do the dirty work.
149 void hoist(Instruction &I);
151 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it
152 /// is not a trapping instruction or if it is a trapping instruction and is
153 /// guaranteed to execute.
155 bool isSafeToExecuteUnconditionally(Instruction &I);
157 /// pointerInvalidatedByLoop - Return true if the body of this loop may
158 /// store into the memory location pointed to by V.
160 bool pointerInvalidatedByLoop(Value *V, uint64_t Size,
161 const MDNode *TBAAInfo) {
162 // Check to see if any of the basic blocks in CurLoop invalidate *V.
163 return CurAST->getAliasSetForPointer(V, Size, TBAAInfo).isMod();
166 bool canSinkOrHoistInst(Instruction &I);
167 bool isNotUsedInLoop(Instruction &I);
169 void PromoteAliasSet(AliasSet &AS);
174 INITIALIZE_PASS_BEGIN(LICM, "licm", "Loop Invariant Code Motion", false, false)
175 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
176 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
177 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
178 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
179 INITIALIZE_PASS_END(LICM, "licm", "Loop Invariant Code Motion", false, false)
181 Pass *llvm::createLICMPass() { return new LICM(); }
183 /// Hoist expressions out of the specified loop. Note, alias info for inner
184 /// loop is not preserved so it is not a good idea to run LICM multiple
185 /// times on one loop.
187 bool LICM::runOnLoop(Loop *L, LPPassManager &LPM) {
190 // Get our Loop and Alias Analysis information...
191 LI = &getAnalysis<LoopInfo>();
192 AA = &getAnalysis<AliasAnalysis>();
193 DT = &getAnalysis<DominatorTree>();
195 CurAST = new AliasSetTracker(*AA);
196 // Collect Alias info from subloops.
197 for (Loop::iterator LoopItr = L->begin(), LoopItrE = L->end();
198 LoopItr != LoopItrE; ++LoopItr) {
199 Loop *InnerL = *LoopItr;
200 AliasSetTracker *InnerAST = LoopToAliasSetMap[InnerL];
201 assert(InnerAST && "Where is my AST?");
203 // What if InnerLoop was modified by other passes ?
204 CurAST->add(*InnerAST);
206 // Once we've incorporated the inner loop's AST into ours, we don't need the
207 // subloop's anymore.
209 LoopToAliasSetMap.erase(InnerL);
214 // Get the preheader block to move instructions into...
215 Preheader = L->getLoopPreheader();
217 // Loop over the body of this loop, looking for calls, invokes, and stores.
218 // Because subloops have already been incorporated into AST, we skip blocks in
221 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
224 if (LI->getLoopFor(BB) == L) // Ignore blocks in subloops.
225 CurAST->add(*BB); // Incorporate the specified basic block
228 // We want to visit all of the instructions in this loop... that are not parts
229 // of our subloops (they have already had their invariants hoisted out of
230 // their loop, into this loop, so there is no need to process the BODIES of
233 // Traverse the body of the loop in depth first order on the dominator tree so
234 // that we are guaranteed to see definitions before we see uses. This allows
235 // us to sink instructions in one pass, without iteration. After sinking
236 // instructions, we perform another pass to hoist them out of the loop.
238 if (L->hasDedicatedExits())
239 SinkRegion(DT->getNode(L->getHeader()));
241 HoistRegion(DT->getNode(L->getHeader()));
243 // Now that all loop invariants have been removed from the loop, promote any
244 // memory references to scalars that we can.
245 if (!DisablePromotion && Preheader && L->hasDedicatedExits()) {
246 // Loop over all of the alias sets in the tracker object.
247 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
252 // Clear out loops state information for the next iteration
256 // If this loop is nested inside of another one, save the alias information
257 // for when we process the outer loop.
258 if (L->getParentLoop())
259 LoopToAliasSetMap[L] = CurAST;
265 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks
266 /// dominated by the specified block, and that are in the current loop) in
267 /// reverse depth first order w.r.t the DominatorTree. This allows us to visit
268 /// uses before definitions, allowing us to sink a loop body in one pass without
271 void LICM::SinkRegion(DomTreeNode *N) {
272 assert(N != 0 && "Null dominator tree node?");
273 BasicBlock *BB = N->getBlock();
275 // If this subregion is not in the top level loop at all, exit.
276 if (!CurLoop->contains(BB)) return;
278 // We are processing blocks in reverse dfo, so process children first.
279 const std::vector<DomTreeNode*> &Children = N->getChildren();
280 for (unsigned i = 0, e = Children.size(); i != e; ++i)
281 SinkRegion(Children[i]);
283 // Only need to process the contents of this block if it is not part of a
284 // subloop (which would already have been processed).
285 if (inSubLoop(BB)) return;
287 for (BasicBlock::iterator II = BB->end(); II != BB->begin(); ) {
288 Instruction &I = *--II;
290 // If the instruction is dead, we would try to sink it because it isn't used
291 // in the loop, instead, just delete it.
292 if (isInstructionTriviallyDead(&I)) {
293 DEBUG(dbgs() << "LICM deleting dead inst: " << I << '\n');
295 CurAST->deleteValue(&I);
301 // Check to see if we can sink this instruction to the exit blocks
302 // of the loop. We can do this if the all users of the instruction are
303 // outside of the loop. In this case, it doesn't even matter if the
304 // operands of the instruction are loop invariant.
306 if (isNotUsedInLoop(I) && canSinkOrHoistInst(I)) {
313 /// HoistRegion - Walk the specified region of the CFG (defined by all blocks
314 /// dominated by the specified block, and that are in the current loop) in depth
315 /// first order w.r.t the DominatorTree. This allows us to visit definitions
316 /// before uses, allowing us to hoist a loop body in one pass without iteration.
318 void LICM::HoistRegion(DomTreeNode *N) {
319 assert(N != 0 && "Null dominator tree node?");
320 BasicBlock *BB = N->getBlock();
322 // If this subregion is not in the top level loop at all, exit.
323 if (!CurLoop->contains(BB)) return;
325 // Only need to process the contents of this block if it is not part of a
326 // subloop (which would already have been processed).
328 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ) {
329 Instruction &I = *II++;
331 // Try constant folding this instruction. If all the operands are
332 // constants, it is technically hoistable, but it would be better to just
334 if (Constant *C = ConstantFoldInstruction(&I)) {
335 DEBUG(dbgs() << "LICM folding inst: " << I << " --> " << *C << '\n');
336 CurAST->copyValue(&I, C);
337 CurAST->deleteValue(&I);
338 I.replaceAllUsesWith(C);
343 // Try hoisting the instruction out to the preheader. We can only do this
344 // if all of the operands of the instruction are loop invariant and if it
345 // is safe to hoist the instruction.
347 if (CurLoop->hasLoopInvariantOperands(&I) && canSinkOrHoistInst(I) &&
348 isSafeToExecuteUnconditionally(I))
352 const std::vector<DomTreeNode*> &Children = N->getChildren();
353 for (unsigned i = 0, e = Children.size(); i != e; ++i)
354 HoistRegion(Children[i]);
357 /// canSinkOrHoistInst - Return true if the hoister and sinker can handle this
360 bool LICM::canSinkOrHoistInst(Instruction &I) {
361 // Loads have extra constraints we have to verify before we can hoist them.
362 if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
363 if (LI->isVolatile())
364 return false; // Don't hoist volatile loads!
366 // Loads from constant memory are always safe to move, even if they end up
367 // in the same alias set as something that ends up being modified.
368 if (AA->pointsToConstantMemory(LI->getOperand(0)))
371 // Don't hoist loads which have may-aliased stores in loop.
373 if (LI->getType()->isSized())
374 Size = AA->getTypeStoreSize(LI->getType());
375 return !pointerInvalidatedByLoop(LI->getOperand(0), Size,
376 LI->getMetadata(LLVMContext::MD_tbaa));
377 } else if (CallInst *CI = dyn_cast<CallInst>(&I)) {
378 // Handle obvious cases efficiently.
379 AliasAnalysis::ModRefBehavior Behavior = AA->getModRefBehavior(CI);
380 if (Behavior == AliasAnalysis::DoesNotAccessMemory)
382 if (AliasAnalysis::onlyReadsMemory(Behavior)) {
383 // If this call only reads from memory and there are no writes to memory
384 // in the loop, we can hoist or sink the call as appropriate.
385 bool FoundMod = false;
386 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
389 if (!AS.isForwardingAliasSet() && AS.isMod()) {
394 if (!FoundMod) return true;
397 // FIXME: This should use mod/ref information to see if we can hoist or sink
403 // Otherwise these instructions are hoistable/sinkable
404 return isa<BinaryOperator>(I) || isa<CastInst>(I) ||
405 isa<SelectInst>(I) || isa<GetElementPtrInst>(I) || isa<CmpInst>(I) ||
406 isa<InsertElementInst>(I) || isa<ExtractElementInst>(I) ||
407 isa<ShuffleVectorInst>(I);
410 /// isNotUsedInLoop - Return true if the only users of this instruction are
411 /// outside of the loop. If this is true, we can sink the instruction to the
412 /// exit blocks of the loop.
414 bool LICM::isNotUsedInLoop(Instruction &I) {
415 for (Value::use_iterator UI = I.use_begin(), E = I.use_end(); UI != E; ++UI) {
416 Instruction *User = cast<Instruction>(*UI);
417 if (PHINode *PN = dyn_cast<PHINode>(User)) {
418 // PHI node uses occur in predecessor blocks!
419 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
420 if (PN->getIncomingValue(i) == &I)
421 if (CurLoop->contains(PN->getIncomingBlock(i)))
423 } else if (CurLoop->contains(User)) {
431 /// sink - When an instruction is found to only be used outside of the loop,
432 /// this function moves it to the exit blocks and patches up SSA form as needed.
433 /// This method is guaranteed to remove the original instruction from its
434 /// position, and may either delete it or move it to outside of the loop.
436 void LICM::sink(Instruction &I) {
437 DEBUG(dbgs() << "LICM sinking instruction: " << I << "\n");
439 SmallVector<BasicBlock*, 8> ExitBlocks;
440 CurLoop->getUniqueExitBlocks(ExitBlocks);
442 if (isa<LoadInst>(I)) ++NumMovedLoads;
443 else if (isa<CallInst>(I)) ++NumMovedCalls;
447 // The case where there is only a single exit node of this loop is common
448 // enough that we handle it as a special (more efficient) case. It is more
449 // efficient to handle because there are no PHI nodes that need to be placed.
450 if (ExitBlocks.size() == 1) {
451 if (!DT->dominates(I.getParent(), ExitBlocks[0])) {
452 // Instruction is not used, just delete it.
453 CurAST->deleteValue(&I);
454 // If I has users in unreachable blocks, eliminate.
455 // If I is not void type then replaceAllUsesWith undef.
456 // This allows ValueHandlers and custom metadata to adjust itself.
458 I.replaceAllUsesWith(UndefValue::get(I.getType()));
461 // Move the instruction to the start of the exit block, after any PHI
463 I.moveBefore(ExitBlocks[0]->getFirstNonPHI());
465 // This instruction is no longer in the AST for the current loop, because
466 // we just sunk it out of the loop. If we just sunk it into an outer
467 // loop, we will rediscover the operation when we process it.
468 CurAST->deleteValue(&I);
473 if (ExitBlocks.empty()) {
474 // The instruction is actually dead if there ARE NO exit blocks.
475 CurAST->deleteValue(&I);
476 // If I has users in unreachable blocks, eliminate.
477 // If I is not void type then replaceAllUsesWith undef.
478 // This allows ValueHandlers and custom metadata to adjust itself.
480 I.replaceAllUsesWith(UndefValue::get(I.getType()));
485 // Otherwise, if we have multiple exits, use the SSAUpdater to do all of the
486 // hard work of inserting PHI nodes as necessary.
487 SmallVector<PHINode*, 8> NewPHIs;
488 SSAUpdater SSA(&NewPHIs);
491 SSA.Initialize(I.getType(), I.getName());
493 // Insert a copy of the instruction in each exit block of the loop that is
494 // dominated by the instruction. Each exit block is known to only be in the
495 // ExitBlocks list once.
496 BasicBlock *InstOrigBB = I.getParent();
497 unsigned NumInserted = 0;
499 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
500 BasicBlock *ExitBlock = ExitBlocks[i];
502 if (!DT->dominates(InstOrigBB, ExitBlock))
505 // Insert the code after the last PHI node.
506 BasicBlock::iterator InsertPt = ExitBlock->getFirstNonPHI();
508 // If this is the first exit block processed, just move the original
509 // instruction, otherwise clone the original instruction and insert
512 if (NumInserted++ == 0) {
513 I.moveBefore(InsertPt);
517 if (!I.getName().empty())
518 New->setName(I.getName()+".le");
519 ExitBlock->getInstList().insert(InsertPt, New);
522 // Now that we have inserted the instruction, inform SSAUpdater.
524 SSA.AddAvailableValue(ExitBlock, New);
527 // If the instruction doesn't dominate any exit blocks, it must be dead.
528 if (NumInserted == 0) {
529 CurAST->deleteValue(&I);
531 I.replaceAllUsesWith(UndefValue::get(I.getType()));
536 // Next, rewrite uses of the instruction, inserting PHI nodes as needed.
537 for (Value::use_iterator UI = I.use_begin(), UE = I.use_end(); UI != UE; ) {
538 // Grab the use before incrementing the iterator.
539 Use &U = UI.getUse();
540 // Increment the iterator before removing the use from the list.
542 SSA.RewriteUseAfterInsertions(U);
545 // Update CurAST for NewPHIs if I had pointer type.
546 if (I.getType()->isPointerTy())
547 for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i)
548 CurAST->copyValue(&I, NewPHIs[i]);
550 // Finally, remove the instruction from CurAST. It is no longer in the loop.
551 CurAST->deleteValue(&I);
554 /// hoist - When an instruction is found to only use loop invariant operands
555 /// that is safe to hoist, this instruction is called to do the dirty work.
557 void LICM::hoist(Instruction &I) {
558 DEBUG(dbgs() << "LICM hoisting to " << Preheader->getName() << ": "
561 // Move the new node to the Preheader, before its terminator.
562 I.moveBefore(Preheader->getTerminator());
564 if (isa<LoadInst>(I)) ++NumMovedLoads;
565 else if (isa<CallInst>(I)) ++NumMovedCalls;
570 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it is
571 /// not a trapping instruction or if it is a trapping instruction and is
572 /// guaranteed to execute.
574 bool LICM::isSafeToExecuteUnconditionally(Instruction &Inst) {
575 // If it is not a trapping instruction, it is always safe to hoist.
576 if (Inst.isSafeToSpeculativelyExecute())
579 // Otherwise we have to check to make sure that the instruction dominates all
580 // of the exit blocks. If it doesn't, then there is a path out of the loop
581 // which does not execute this instruction, so we can't hoist it.
583 // If the instruction is in the header block for the loop (which is very
584 // common), it is always guaranteed to dominate the exit blocks. Since this
585 // is a common case, and can save some work, check it now.
586 if (Inst.getParent() == CurLoop->getHeader())
589 // Get the exit blocks for the current loop.
590 SmallVector<BasicBlock*, 8> ExitBlocks;
591 CurLoop->getExitBlocks(ExitBlocks);
593 // Verify that the block dominates each of the exit blocks of the loop.
594 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
595 if (!DT->dominates(Inst.getParent(), ExitBlocks[i]))
601 /// PromoteAliasSet - Try to promote memory values to scalars by sinking
602 /// stores out of the loop and moving loads to before the loop. We do this by
603 /// looping over the stores in the loop, looking for stores to Must pointers
604 /// which are loop invariant.
606 void LICM::PromoteAliasSet(AliasSet &AS) {
607 // We can promote this alias set if it has a store, if it is a "Must" alias
608 // set, if the pointer is loop invariant, and if we are not eliminating any
609 // volatile loads or stores.
610 if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() ||
611 AS.isVolatile() || !CurLoop->isLoopInvariant(AS.begin()->getValue()))
614 assert(!AS.empty() &&
615 "Must alias set should have at least one pointer element in it!");
616 Value *SomePtr = AS.begin()->getValue();
618 // It isn't safe to promote a load/store from the loop if the load/store is
619 // conditional. For example, turning:
621 // for () { if (c) *P += 1; }
625 // tmp = *P; for () { if (c) tmp +=1; } *P = tmp;
627 // is not safe, because *P may only be valid to access if 'c' is true.
629 // It is safe to promote P if all uses are direct load/stores and if at
630 // least one is guaranteed to be executed.
631 bool GuaranteedToExecute = false;
633 SmallVector<Instruction*, 64> LoopUses;
634 SmallPtrSet<Value*, 4> PointerMustAliases;
636 // Check that all of the pointers in the alias set have the same type. We
637 // cannot (yet) promote a memory location that is loaded and stored in
639 for (AliasSet::iterator ASI = AS.begin(), E = AS.end(); ASI != E; ++ASI) {
640 Value *ASIV = ASI->getValue();
641 PointerMustAliases.insert(ASIV);
643 // Check that all of the pointers in the alias set have the same type. We
644 // cannot (yet) promote a memory location that is loaded and stored in
646 if (SomePtr->getType() != ASIV->getType())
649 for (Value::use_iterator UI = ASIV->use_begin(), UE = ASIV->use_end();
651 // Ignore instructions that are outside the loop.
652 Instruction *Use = dyn_cast<Instruction>(*UI);
653 if (!Use || !CurLoop->contains(Use))
656 // If there is an non-load/store instruction in the loop, we can't promote
658 if (isa<LoadInst>(Use))
659 assert(!cast<LoadInst>(Use)->isVolatile() && "AST broken");
660 else if (isa<StoreInst>(Use)) {
661 // Stores *of* the pointer are not interesting, only stores *to* the
663 if (Use->getOperand(1) != ASIV)
665 assert(!cast<StoreInst>(Use)->isVolatile() && "AST broken");
667 return; // Not a load or store.
669 if (!GuaranteedToExecute)
670 GuaranteedToExecute = isSafeToExecuteUnconditionally(*Use);
672 LoopUses.push_back(Use);
676 // If there isn't a guaranteed-to-execute instruction, we can't promote.
677 if (!GuaranteedToExecute)
680 // Otherwise, this is safe to promote, lets do it!
681 DEBUG(dbgs() << "LICM: Promoting value stored to in loop: " <<*SomePtr<<'\n');
685 // We use the SSAUpdater interface to insert phi nodes as required.
686 SmallVector<PHINode*, 16> NewPHIs;
687 SSAUpdater SSA(&NewPHIs);
689 // It wants to know some value of the same type as what we'll be inserting.
691 if (isa<LoadInst>(LoopUses[0]))
692 SomeValue = LoopUses[0];
694 SomeValue = cast<StoreInst>(LoopUses[0])->getOperand(0);
695 SSA.Initialize(SomeValue->getType(), SomeValue->getName());
697 // First step: bucket up uses of the pointers by the block they occur in.
698 // This is important because we have to handle multiple defs/uses in a block
699 // ourselves: SSAUpdater is purely for cross-block references.
700 // FIXME: Want a TinyVector<Instruction*> since there is usually 0/1 element.
701 DenseMap<BasicBlock*, std::vector<Instruction*> > UsesByBlock;
702 for (unsigned i = 0, e = LoopUses.size(); i != e; ++i) {
703 Instruction *User = LoopUses[i];
704 UsesByBlock[User->getParent()].push_back(User);
707 // Okay, now we can iterate over all the blocks in the loop with uses,
708 // processing them. Keep track of which loads are loading a live-in value.
709 SmallVector<LoadInst*, 32> LiveInLoads;
710 DenseMap<Value*, Value*> ReplacedLoads;
712 for (unsigned LoopUse = 0, e = LoopUses.size(); LoopUse != e; ++LoopUse) {
713 Instruction *User = LoopUses[LoopUse];
714 std::vector<Instruction*> &BlockUses = UsesByBlock[User->getParent()];
716 // If this block has already been processed, ignore this repeat use.
717 if (BlockUses.empty()) continue;
719 // Okay, this is the first use in the block. If this block just has a
720 // single user in it, we can rewrite it trivially.
721 if (BlockUses.size() == 1) {
722 // If it is a store, it is a trivial def of the value in the block.
723 if (isa<StoreInst>(User)) {
724 SSA.AddAvailableValue(User->getParent(),
725 cast<StoreInst>(User)->getOperand(0));
727 // Otherwise it is a load, queue it to rewrite as a live-in load.
728 LiveInLoads.push_back(cast<LoadInst>(User));
734 // Otherwise, check to see if this block is all loads. If so, we can queue
735 // them all as live in loads.
736 bool HasStore = false;
737 for (unsigned i = 0, e = BlockUses.size(); i != e; ++i) {
738 if (isa<StoreInst>(BlockUses[i])) {
745 for (unsigned i = 0, e = BlockUses.size(); i != e; ++i)
746 LiveInLoads.push_back(cast<LoadInst>(BlockUses[i]));
751 // Otherwise, we have mixed loads and stores (or just a bunch of stores).
752 // Since SSAUpdater is purely for cross-block values, we need to determine
753 // the order of these instructions in the block. If the first use in the
754 // block is a load, then it uses the live in value. The last store defines
755 // the live out value. We handle this by doing a linear scan of the block.
756 BasicBlock *BB = User->getParent();
757 Value *StoredValue = 0;
758 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
759 if (LoadInst *L = dyn_cast<LoadInst>(II)) {
760 // If this is a load from an unrelated pointer, ignore it.
761 if (!PointerMustAliases.count(L->getOperand(0))) continue;
763 // If we haven't seen a store yet, this is a live in use, otherwise
764 // use the stored value.
766 L->replaceAllUsesWith(StoredValue);
767 ReplacedLoads[L] = StoredValue;
769 LiveInLoads.push_back(L);
774 if (StoreInst *S = dyn_cast<StoreInst>(II)) {
775 // If this is a store to an unrelated pointer, ignore it.
776 if (!PointerMustAliases.count(S->getOperand(1))) continue;
778 // Remember that this is the active value in the block.
779 StoredValue = S->getOperand(0);
783 // The last stored value that happened is the live-out for the block.
784 assert(StoredValue && "Already checked that there is a store in block");
785 SSA.AddAvailableValue(BB, StoredValue);
789 // Now that all the intra-loop values are classified, set up the preheader.
790 // It gets a load of the pointer we're promoting, and it is the live-out value
791 // from the preheader.
792 LoadInst *PreheaderLoad = new LoadInst(SomePtr,SomePtr->getName()+".promoted",
793 Preheader->getTerminator());
794 SSA.AddAvailableValue(Preheader, PreheaderLoad);
796 // Now that the preheader is good to go, set up the exit blocks. Each exit
797 // block gets a store of the live-out values that feed them. Since we've
798 // already told the SSA updater about the defs in the loop and the preheader
799 // definition, it is all set and we can start using it.
800 SmallVector<BasicBlock*, 8> ExitBlocks;
801 CurLoop->getUniqueExitBlocks(ExitBlocks);
802 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
803 BasicBlock *ExitBlock = ExitBlocks[i];
804 Value *LiveInValue = SSA.GetValueInMiddleOfBlock(ExitBlock);
805 Instruction *InsertPos = ExitBlock->getFirstNonPHI();
806 new StoreInst(LiveInValue, SomePtr, InsertPos);
809 // Okay, now we rewrite all loads that use live-in values in the loop,
810 // inserting PHI nodes as necessary.
811 for (unsigned i = 0, e = LiveInLoads.size(); i != e; ++i) {
812 LoadInst *ALoad = LiveInLoads[i];
813 Value *NewVal = SSA.GetValueInMiddleOfBlock(ALoad->getParent());
814 ALoad->replaceAllUsesWith(NewVal);
815 CurAST->copyValue(ALoad, NewVal);
816 ReplacedLoads[ALoad] = NewVal;
819 // If the preheader load is itself a pointer, we need to tell alias analysis
820 // about the new pointer we created in the preheader block and about any PHI
821 // nodes that just got inserted.
822 if (PreheaderLoad->getType()->isPointerTy()) {
823 // Copy any value stored to or loaded from a must-alias of the pointer.
824 CurAST->copyValue(SomeValue, PreheaderLoad);
826 for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i)
827 CurAST->copyValue(SomeValue, NewPHIs[i]);
830 // Now that everything is rewritten, delete the old instructions from the body
831 // of the loop. They should all be dead now.
832 for (unsigned i = 0, e = LoopUses.size(); i != e; ++i) {
833 Instruction *User = LoopUses[i];
835 // If this is a load that still has uses, then the load must have been added
836 // as a live value in the SSAUpdate data structure for a block (e.g. because
837 // the loaded value was stored later). In this case, we need to recursively
838 // propagate the updates until we get to the real value.
839 if (!User->use_empty()) {
840 Value *NewVal = ReplacedLoads[User];
841 assert(NewVal && "not a replaced load?");
843 // Propagate down to the ultimate replacee. The intermediately loads
844 // could theoretically already have been deleted, so we don't want to
845 // dereference the Value*'s.
846 DenseMap<Value*, Value*>::iterator RLI = ReplacedLoads.find(NewVal);
847 while (RLI != ReplacedLoads.end()) {
848 NewVal = RLI->second;
849 RLI = ReplacedLoads.find(NewVal);
852 User->replaceAllUsesWith(NewVal);
853 CurAST->copyValue(User, NewVal);
856 CurAST->deleteValue(User);
857 User->eraseFromParent();
864 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
865 void LICM::cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L) {
866 AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);
870 AST->copyValue(From, To);
873 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
875 void LICM::deleteAnalysisValue(Value *V, Loop *L) {
876 AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);