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/Analysis/ScalarEvolution.h"
47 #include "llvm/Transforms/Utils/Local.h"
48 #include "llvm/Transforms/Utils/SSAUpdater.h"
49 #include "llvm/Support/CFG.h"
50 #include "llvm/Support/CommandLine.h"
51 #include "llvm/Support/raw_ostream.h"
52 #include "llvm/Support/Debug.h"
53 #include "llvm/ADT/Statistic.h"
57 STATISTIC(NumSunk , "Number of instructions sunk out of loop");
58 STATISTIC(NumHoisted , "Number of instructions hoisted out of loop");
59 STATISTIC(NumMovedLoads, "Number of load insts hoisted or sunk");
60 STATISTIC(NumMovedCalls, "Number of call insts hoisted or sunk");
61 STATISTIC(NumPromoted , "Number of memory locations promoted to registers");
64 DisablePromotion("disable-licm-promotion", cl::Hidden,
65 cl::desc("Disable memory promotion in LICM pass"));
68 struct LICM : public LoopPass {
69 static char ID; // Pass identification, replacement for typeid
70 LICM() : LoopPass(ID) {
71 initializeLICMPass(*PassRegistry::getPassRegistry());
74 virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
76 /// This transformation requires natural loop information & requires that
77 /// loop preheaders be inserted into the CFG...
79 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
81 AU.addRequired<DominatorTree>();
82 AU.addRequired<LoopInfo>();
83 AU.addRequiredID(LoopSimplifyID);
84 AU.addRequired<AliasAnalysis>();
85 AU.addPreserved<AliasAnalysis>();
86 AU.addPreserved<ScalarEvolution>();
87 AU.addPreservedID(LoopSimplifyID);
90 bool doFinalization() {
91 assert(LoopToAliasSetMap.empty() && "Didn't free loop alias sets");
96 AliasAnalysis *AA; // Current AliasAnalysis information
97 LoopInfo *LI; // Current LoopInfo
98 DominatorTree *DT; // Dominator Tree for the current Loop.
100 // State that is updated as we process loops.
101 bool Changed; // Set to true when we change anything.
102 BasicBlock *Preheader; // The preheader block of the current loop...
103 Loop *CurLoop; // The current loop we are working on...
104 AliasSetTracker *CurAST; // AliasSet information for the current loop...
105 DenseMap<Loop*, AliasSetTracker*> LoopToAliasSetMap;
107 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
108 void cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L);
110 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
112 void deleteAnalysisValue(Value *V, Loop *L);
114 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks
115 /// dominated by the specified block, and that are in the current loop) in
116 /// reverse depth first order w.r.t the DominatorTree. This allows us to
117 /// visit uses before definitions, allowing us to sink a loop body in one
118 /// pass without iteration.
120 void SinkRegion(DomTreeNode *N);
122 /// HoistRegion - Walk the specified region of the CFG (defined by all
123 /// blocks dominated by the specified block, and that are in the current
124 /// loop) in depth first order w.r.t the DominatorTree. This allows us to
125 /// visit definitions before uses, allowing us to hoist a loop body in one
126 /// pass without iteration.
128 void HoistRegion(DomTreeNode *N);
130 /// inSubLoop - Little predicate that returns true if the specified basic
131 /// block is in a subloop of the current one, not the current one itself.
133 bool inSubLoop(BasicBlock *BB) {
134 assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
135 for (Loop::iterator I = CurLoop->begin(), E = CurLoop->end(); I != E; ++I)
136 if ((*I)->contains(BB))
137 return true; // A subloop actually contains this block!
141 /// isExitBlockDominatedByBlockInLoop - This method checks to see if the
142 /// specified exit block of the loop is dominated by the specified block
143 /// that is in the body of the loop. We use these constraints to
144 /// dramatically limit the amount of the dominator tree that needs to be
146 bool isExitBlockDominatedByBlockInLoop(BasicBlock *ExitBlock,
147 BasicBlock *BlockInLoop) const {
148 // If the block in the loop is the loop header, it must be dominated!
149 BasicBlock *LoopHeader = CurLoop->getHeader();
150 if (BlockInLoop == LoopHeader)
153 DomTreeNode *BlockInLoopNode = DT->getNode(BlockInLoop);
154 DomTreeNode *IDom = DT->getNode(ExitBlock);
156 // Because the exit block is not in the loop, we know we have to get _at
157 // least_ its immediate dominator.
158 IDom = IDom->getIDom();
160 while (IDom && IDom != BlockInLoopNode) {
161 // If we have got to the header of the loop, then the instructions block
162 // did not dominate the exit node, so we can't hoist it.
163 if (IDom->getBlock() == LoopHeader)
166 // Get next Immediate Dominator.
167 IDom = IDom->getIDom();
173 /// sink - When an instruction is found to only be used outside of the loop,
174 /// this function moves it to the exit blocks and patches up SSA form as
177 void sink(Instruction &I);
179 /// hoist - When an instruction is found to only use loop invariant operands
180 /// that is safe to hoist, this instruction is called to do the dirty work.
182 void hoist(Instruction &I);
184 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it
185 /// is not a trapping instruction or if it is a trapping instruction and is
186 /// guaranteed to execute.
188 bool isSafeToExecuteUnconditionally(Instruction &I);
190 /// pointerInvalidatedByLoop - Return true if the body of this loop may
191 /// store into the memory location pointed to by V.
193 bool pointerInvalidatedByLoop(Value *V, unsigned Size,
194 const MDNode *TBAAInfo) {
195 // Check to see if any of the basic blocks in CurLoop invalidate *V.
196 return CurAST->getAliasSetForPointer(V, Size, TBAAInfo).isMod();
199 bool canSinkOrHoistInst(Instruction &I);
200 bool isNotUsedInLoop(Instruction &I);
202 void PromoteAliasSet(AliasSet &AS);
207 INITIALIZE_PASS_BEGIN(LICM, "licm", "Loop Invariant Code Motion", false, false)
208 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
209 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
210 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
211 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
212 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
213 INITIALIZE_PASS_END(LICM, "licm", "Loop Invariant Code Motion", false, false)
215 Pass *llvm::createLICMPass() { return new LICM(); }
217 /// Hoist expressions out of the specified loop. Note, alias info for inner
218 /// loop is not preserved so it is not a good idea to run LICM multiple
219 /// times on one loop.
221 bool LICM::runOnLoop(Loop *L, LPPassManager &LPM) {
224 // Get our Loop and Alias Analysis information...
225 LI = &getAnalysis<LoopInfo>();
226 AA = &getAnalysis<AliasAnalysis>();
227 DT = &getAnalysis<DominatorTree>();
229 CurAST = new AliasSetTracker(*AA);
230 // Collect Alias info from subloops.
231 for (Loop::iterator LoopItr = L->begin(), LoopItrE = L->end();
232 LoopItr != LoopItrE; ++LoopItr) {
233 Loop *InnerL = *LoopItr;
234 AliasSetTracker *InnerAST = LoopToAliasSetMap[InnerL];
235 assert(InnerAST && "Where is my AST?");
237 // What if InnerLoop was modified by other passes ?
238 CurAST->add(*InnerAST);
240 // Once we've incorporated the inner loop's AST into ours, we don't need the
241 // subloop's anymore.
243 LoopToAliasSetMap.erase(InnerL);
248 // Get the preheader block to move instructions into...
249 Preheader = L->getLoopPreheader();
251 // Loop over the body of this loop, looking for calls, invokes, and stores.
252 // Because subloops have already been incorporated into AST, we skip blocks in
255 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
258 if (LI->getLoopFor(BB) == L) // Ignore blocks in subloops.
259 CurAST->add(*BB); // Incorporate the specified basic block
262 // We want to visit all of the instructions in this loop... that are not parts
263 // of our subloops (they have already had their invariants hoisted out of
264 // their loop, into this loop, so there is no need to process the BODIES of
267 // Traverse the body of the loop in depth first order on the dominator tree so
268 // that we are guaranteed to see definitions before we see uses. This allows
269 // us to sink instructions in one pass, without iteration. After sinking
270 // instructions, we perform another pass to hoist them out of the loop.
272 if (L->hasDedicatedExits())
273 SinkRegion(DT->getNode(L->getHeader()));
275 HoistRegion(DT->getNode(L->getHeader()));
277 // Now that all loop invariants have been removed from the loop, promote any
278 // memory references to scalars that we can.
279 if (!DisablePromotion && Preheader && L->hasDedicatedExits()) {
280 // Loop over all of the alias sets in the tracker object.
281 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
286 // Clear out loops state information for the next iteration
290 // If this loop is nested inside of another one, save the alias information
291 // for when we process the outer loop.
292 if (L->getParentLoop())
293 LoopToAliasSetMap[L] = CurAST;
299 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks
300 /// dominated by the specified block, and that are in the current loop) in
301 /// reverse depth first order w.r.t the DominatorTree. This allows us to visit
302 /// uses before definitions, allowing us to sink a loop body in one pass without
305 void LICM::SinkRegion(DomTreeNode *N) {
306 assert(N != 0 && "Null dominator tree node?");
307 BasicBlock *BB = N->getBlock();
309 // If this subregion is not in the top level loop at all, exit.
310 if (!CurLoop->contains(BB)) return;
312 // We are processing blocks in reverse dfo, so process children first.
313 const std::vector<DomTreeNode*> &Children = N->getChildren();
314 for (unsigned i = 0, e = Children.size(); i != e; ++i)
315 SinkRegion(Children[i]);
317 // Only need to process the contents of this block if it is not part of a
318 // subloop (which would already have been processed).
319 if (inSubLoop(BB)) return;
321 for (BasicBlock::iterator II = BB->end(); II != BB->begin(); ) {
322 Instruction &I = *--II;
324 // If the instruction is dead, we would try to sink it because it isn't used
325 // in the loop, instead, just delete it.
326 if (isInstructionTriviallyDead(&I)) {
327 DEBUG(dbgs() << "LICM deleting dead inst: " << I << '\n');
329 CurAST->deleteValue(&I);
335 // Check to see if we can sink this instruction to the exit blocks
336 // of the loop. We can do this if the all users of the instruction are
337 // outside of the loop. In this case, it doesn't even matter if the
338 // operands of the instruction are loop invariant.
340 if (isNotUsedInLoop(I) && canSinkOrHoistInst(I)) {
347 /// HoistRegion - Walk the specified region of the CFG (defined by all blocks
348 /// dominated by the specified block, and that are in the current loop) in depth
349 /// first order w.r.t the DominatorTree. This allows us to visit definitions
350 /// before uses, allowing us to hoist a loop body in one pass without iteration.
352 void LICM::HoistRegion(DomTreeNode *N) {
353 assert(N != 0 && "Null dominator tree node?");
354 BasicBlock *BB = N->getBlock();
356 // If this subregion is not in the top level loop at all, exit.
357 if (!CurLoop->contains(BB)) return;
359 // Only need to process the contents of this block if it is not part of a
360 // subloop (which would already have been processed).
362 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ) {
363 Instruction &I = *II++;
365 // Try constant folding this instruction. If all the operands are
366 // constants, it is technically hoistable, but it would be better to just
368 if (Constant *C = ConstantFoldInstruction(&I)) {
369 DEBUG(dbgs() << "LICM folding inst: " << I << " --> " << *C << '\n');
370 CurAST->copyValue(&I, C);
371 CurAST->deleteValue(&I);
372 I.replaceAllUsesWith(C);
377 // Try hoisting the instruction out to the preheader. We can only do this
378 // if all of the operands of the instruction are loop invariant and if it
379 // is safe to hoist the instruction.
381 if (CurLoop->hasLoopInvariantOperands(&I) && canSinkOrHoistInst(I) &&
382 isSafeToExecuteUnconditionally(I))
386 const std::vector<DomTreeNode*> &Children = N->getChildren();
387 for (unsigned i = 0, e = Children.size(); i != e; ++i)
388 HoistRegion(Children[i]);
391 /// canSinkOrHoistInst - Return true if the hoister and sinker can handle this
394 bool LICM::canSinkOrHoistInst(Instruction &I) {
395 // Loads have extra constraints we have to verify before we can hoist them.
396 if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
397 if (LI->isVolatile())
398 return false; // Don't hoist volatile loads!
400 // Loads from constant memory are always safe to move, even if they end up
401 // in the same alias set as something that ends up being modified.
402 if (AA->pointsToConstantMemory(LI->getOperand(0)))
405 // Don't hoist loads which have may-aliased stores in loop.
407 if (LI->getType()->isSized())
408 Size = AA->getTypeStoreSize(LI->getType());
409 return !pointerInvalidatedByLoop(LI->getOperand(0), Size,
410 LI->getMetadata(LLVMContext::MD_tbaa));
411 } else if (CallInst *CI = dyn_cast<CallInst>(&I)) {
412 // Handle obvious cases efficiently.
413 AliasAnalysis::ModRefBehavior Behavior = AA->getModRefBehavior(CI);
414 if (Behavior == AliasAnalysis::DoesNotAccessMemory)
416 else if (Behavior == AliasAnalysis::OnlyReadsMemory) {
417 // If this call only reads from memory and there are no writes to memory
418 // in the loop, we can hoist or sink the call as appropriate.
419 bool FoundMod = false;
420 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
423 if (!AS.isForwardingAliasSet() && AS.isMod()) {
428 if (!FoundMod) return true;
431 // FIXME: This should use mod/ref information to see if we can hoist or sink
437 // Otherwise these instructions are hoistable/sinkable
438 return isa<BinaryOperator>(I) || isa<CastInst>(I) ||
439 isa<SelectInst>(I) || isa<GetElementPtrInst>(I) || isa<CmpInst>(I) ||
440 isa<InsertElementInst>(I) || isa<ExtractElementInst>(I) ||
441 isa<ShuffleVectorInst>(I);
444 /// isNotUsedInLoop - Return true if the only users of this instruction are
445 /// outside of the loop. If this is true, we can sink the instruction to the
446 /// exit blocks of the loop.
448 bool LICM::isNotUsedInLoop(Instruction &I) {
449 for (Value::use_iterator UI = I.use_begin(), E = I.use_end(); UI != E; ++UI) {
450 Instruction *User = cast<Instruction>(*UI);
451 if (PHINode *PN = dyn_cast<PHINode>(User)) {
452 // PHI node uses occur in predecessor blocks!
453 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
454 if (PN->getIncomingValue(i) == &I)
455 if (CurLoop->contains(PN->getIncomingBlock(i)))
457 } else if (CurLoop->contains(User)) {
465 /// sink - When an instruction is found to only be used outside of the loop,
466 /// this function moves it to the exit blocks and patches up SSA form as needed.
467 /// This method is guaranteed to remove the original instruction from its
468 /// position, and may either delete it or move it to outside of the loop.
470 void LICM::sink(Instruction &I) {
471 DEBUG(dbgs() << "LICM sinking instruction: " << I << "\n");
473 SmallVector<BasicBlock*, 8> ExitBlocks;
474 CurLoop->getUniqueExitBlocks(ExitBlocks);
476 if (isa<LoadInst>(I)) ++NumMovedLoads;
477 else if (isa<CallInst>(I)) ++NumMovedCalls;
481 // The case where there is only a single exit node of this loop is common
482 // enough that we handle it as a special (more efficient) case. It is more
483 // efficient to handle because there are no PHI nodes that need to be placed.
484 if (ExitBlocks.size() == 1) {
485 if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[0], I.getParent())) {
486 // Instruction is not used, just delete it.
487 CurAST->deleteValue(&I);
488 // If I has users in unreachable blocks, eliminate.
489 // If I is not void type then replaceAllUsesWith undef.
490 // This allows ValueHandlers and custom metadata to adjust itself.
492 I.replaceAllUsesWith(UndefValue::get(I.getType()));
495 // Move the instruction to the start of the exit block, after any PHI
497 I.moveBefore(ExitBlocks[0]->getFirstNonPHI());
499 // This instruction is no longer in the AST for the current loop, because
500 // we just sunk it out of the loop. If we just sunk it into an outer
501 // loop, we will rediscover the operation when we process it.
502 CurAST->deleteValue(&I);
507 if (ExitBlocks.empty()) {
508 // The instruction is actually dead if there ARE NO exit blocks.
509 CurAST->deleteValue(&I);
510 // If I has users in unreachable blocks, eliminate.
511 // If I is not void type then replaceAllUsesWith undef.
512 // This allows ValueHandlers and custom metadata to adjust itself.
514 I.replaceAllUsesWith(UndefValue::get(I.getType()));
519 // Otherwise, if we have multiple exits, use the SSAUpdater to do all of the
520 // hard work of inserting PHI nodes as necessary.
521 SmallVector<PHINode*, 8> NewPHIs;
522 SSAUpdater SSA(&NewPHIs);
525 SSA.Initialize(I.getType(), I.getName());
527 // Insert a copy of the instruction in each exit block of the loop that is
528 // dominated by the instruction. Each exit block is known to only be in the
529 // ExitBlocks list once.
530 BasicBlock *InstOrigBB = I.getParent();
531 unsigned NumInserted = 0;
533 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
534 BasicBlock *ExitBlock = ExitBlocks[i];
536 if (!isExitBlockDominatedByBlockInLoop(ExitBlock, InstOrigBB))
539 // Insert the code after the last PHI node.
540 BasicBlock::iterator InsertPt = ExitBlock->getFirstNonPHI();
542 // If this is the first exit block processed, just move the original
543 // instruction, otherwise clone the original instruction and insert
546 if (NumInserted++ == 0) {
547 I.moveBefore(InsertPt);
551 if (!I.getName().empty())
552 New->setName(I.getName()+".le");
553 ExitBlock->getInstList().insert(InsertPt, New);
556 // Now that we have inserted the instruction, inform SSAUpdater.
558 SSA.AddAvailableValue(ExitBlock, New);
561 // If the instruction doesn't dominate any exit blocks, it must be dead.
562 if (NumInserted == 0) {
563 CurAST->deleteValue(&I);
565 I.replaceAllUsesWith(UndefValue::get(I.getType()));
570 // Next, rewrite uses of the instruction, inserting PHI nodes as needed.
571 for (Value::use_iterator UI = I.use_begin(), UE = I.use_end(); UI != UE; ) {
572 // Grab the use before incrementing the iterator.
573 Use &U = UI.getUse();
574 // Increment the iterator before removing the use from the list.
576 SSA.RewriteUseAfterInsertions(U);
579 // Update CurAST for NewPHIs if I had pointer type.
580 if (I.getType()->isPointerTy())
581 for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i)
582 CurAST->copyValue(&I, NewPHIs[i]);
584 // Finally, remove the instruction from CurAST. It is no longer in the loop.
585 CurAST->deleteValue(&I);
588 /// hoist - When an instruction is found to only use loop invariant operands
589 /// that is safe to hoist, this instruction is called to do the dirty work.
591 void LICM::hoist(Instruction &I) {
592 DEBUG(dbgs() << "LICM hoisting to " << Preheader->getName() << ": "
595 // Move the new node to the Preheader, before its terminator.
596 I.moveBefore(Preheader->getTerminator());
598 if (isa<LoadInst>(I)) ++NumMovedLoads;
599 else if (isa<CallInst>(I)) ++NumMovedCalls;
604 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it is
605 /// not a trapping instruction or if it is a trapping instruction and is
606 /// guaranteed to execute.
608 bool LICM::isSafeToExecuteUnconditionally(Instruction &Inst) {
609 // If it is not a trapping instruction, it is always safe to hoist.
610 if (Inst.isSafeToSpeculativelyExecute())
613 // Otherwise we have to check to make sure that the instruction dominates all
614 // of the exit blocks. If it doesn't, then there is a path out of the loop
615 // which does not execute this instruction, so we can't hoist it.
617 // If the instruction is in the header block for the loop (which is very
618 // common), it is always guaranteed to dominate the exit blocks. Since this
619 // is a common case, and can save some work, check it now.
620 if (Inst.getParent() == CurLoop->getHeader())
623 // Get the exit blocks for the current loop.
624 SmallVector<BasicBlock*, 8> ExitBlocks;
625 CurLoop->getExitBlocks(ExitBlocks);
627 // For each exit block, get the DT node and walk up the DT until the
628 // instruction's basic block is found or we exit the loop.
629 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
630 if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[i], Inst.getParent()))
636 /// PromoteAliasSet - Try to promote memory values to scalars by sinking
637 /// stores out of the loop and moving loads to before the loop. We do this by
638 /// looping over the stores in the loop, looking for stores to Must pointers
639 /// which are loop invariant.
641 void LICM::PromoteAliasSet(AliasSet &AS) {
642 // We can promote this alias set if it has a store, if it is a "Must" alias
643 // set, if the pointer is loop invariant, and if we are not eliminating any
644 // volatile loads or stores.
645 if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() ||
646 AS.isVolatile() || !CurLoop->isLoopInvariant(AS.begin()->getValue()))
649 assert(!AS.empty() &&
650 "Must alias set should have at least one pointer element in it!");
651 Value *SomePtr = AS.begin()->getValue();
653 // It isn't safe to promote a load/store from the loop if the load/store is
654 // conditional. For example, turning:
656 // for () { if (c) *P += 1; }
660 // tmp = *P; for () { if (c) tmp +=1; } *P = tmp;
662 // is not safe, because *P may only be valid to access if 'c' is true.
664 // It is safe to promote P if all uses are direct load/stores and if at
665 // least one is guaranteed to be executed.
666 bool GuaranteedToExecute = false;
668 SmallVector<Instruction*, 64> LoopUses;
669 SmallPtrSet<Value*, 4> PointerMustAliases;
671 // Check that all of the pointers in the alias set have the same type. We
672 // cannot (yet) promote a memory location that is loaded and stored in
674 for (AliasSet::iterator ASI = AS.begin(), E = AS.end(); ASI != E; ++ASI) {
675 Value *ASIV = ASI->getValue();
676 PointerMustAliases.insert(ASIV);
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 if (SomePtr->getType() != ASIV->getType())
684 for (Value::use_iterator UI = ASIV->use_begin(), UE = ASIV->use_end();
686 // Ignore instructions that are outside the loop.
687 Instruction *Use = dyn_cast<Instruction>(*UI);
688 if (!Use || !CurLoop->contains(Use))
691 // If there is an non-load/store instruction in the loop, we can't promote
693 if (isa<LoadInst>(Use))
694 assert(!cast<LoadInst>(Use)->isVolatile() && "AST broken");
695 else if (isa<StoreInst>(Use)) {
696 assert(!cast<StoreInst>(Use)->isVolatile() && "AST broken");
697 if (Use->getOperand(0) == ASIV) return;
699 return; // Not a load or store.
701 if (!GuaranteedToExecute)
702 GuaranteedToExecute = isSafeToExecuteUnconditionally(*Use);
704 LoopUses.push_back(Use);
708 // If there isn't a guaranteed-to-execute instruction, we can't promote.
709 if (!GuaranteedToExecute)
712 // Otherwise, this is safe to promote, lets do it!
713 DEBUG(dbgs() << "LICM: Promoting value stored to in loop: " <<*SomePtr<<'\n');
717 // We use the SSAUpdater interface to insert phi nodes as required.
718 SmallVector<PHINode*, 16> NewPHIs;
719 SSAUpdater SSA(&NewPHIs);
721 // It wants to know some value of the same type as what we'll be inserting.
723 if (isa<LoadInst>(LoopUses[0]))
724 SomeValue = LoopUses[0];
726 SomeValue = cast<StoreInst>(LoopUses[0])->getOperand(0);
727 SSA.Initialize(SomeValue->getType(), SomeValue->getName());
729 // First step: bucket up uses of the pointers by the block they occur in.
730 // This is important because we have to handle multiple defs/uses in a block
731 // ourselves: SSAUpdater is purely for cross-block references.
732 // FIXME: Want a TinyVector<Instruction*> since there is usually 0/1 element.
733 DenseMap<BasicBlock*, std::vector<Instruction*> > UsesByBlock;
734 for (unsigned i = 0, e = LoopUses.size(); i != e; ++i) {
735 Instruction *User = LoopUses[i];
736 UsesByBlock[User->getParent()].push_back(User);
739 // Okay, now we can iterate over all the blocks in the loop with uses,
740 // processing them. Keep track of which loads are loading a live-in value.
741 SmallVector<LoadInst*, 32> LiveInLoads;
742 DenseMap<Value*, Value*> ReplacedLoads;
744 for (unsigned LoopUse = 0, e = LoopUses.size(); LoopUse != e; ++LoopUse) {
745 Instruction *User = LoopUses[LoopUse];
746 std::vector<Instruction*> &BlockUses = UsesByBlock[User->getParent()];
748 // If this block has already been processed, ignore this repeat use.
749 if (BlockUses.empty()) continue;
751 // Okay, this is the first use in the block. If this block just has a
752 // single user in it, we can rewrite it trivially.
753 if (BlockUses.size() == 1) {
754 // If it is a store, it is a trivial def of the value in the block.
755 if (isa<StoreInst>(User)) {
756 SSA.AddAvailableValue(User->getParent(),
757 cast<StoreInst>(User)->getOperand(0));
759 // Otherwise it is a load, queue it to rewrite as a live-in load.
760 LiveInLoads.push_back(cast<LoadInst>(User));
766 // Otherwise, check to see if this block is all loads. If so, we can queue
767 // them all as live in loads.
768 bool HasStore = false;
769 for (unsigned i = 0, e = BlockUses.size(); i != e; ++i) {
770 if (isa<StoreInst>(BlockUses[i])) {
777 for (unsigned i = 0, e = BlockUses.size(); i != e; ++i)
778 LiveInLoads.push_back(cast<LoadInst>(BlockUses[i]));
783 // Otherwise, we have mixed loads and stores (or just a bunch of stores).
784 // Since SSAUpdater is purely for cross-block values, we need to determine
785 // the order of these instructions in the block. If the first use in the
786 // block is a load, then it uses the live in value. The last store defines
787 // the live out value. We handle this by doing a linear scan of the block.
788 BasicBlock *BB = User->getParent();
789 Value *StoredValue = 0;
790 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
791 if (LoadInst *L = dyn_cast<LoadInst>(II)) {
792 // If this is a load from an unrelated pointer, ignore it.
793 if (!PointerMustAliases.count(L->getOperand(0))) continue;
795 // If we haven't seen a store yet, this is a live in use, otherwise
796 // use the stored value.
798 L->replaceAllUsesWith(StoredValue);
799 ReplacedLoads[L] = StoredValue;
801 LiveInLoads.push_back(L);
806 if (StoreInst *S = dyn_cast<StoreInst>(II)) {
807 // If this is a store to an unrelated pointer, ignore it.
808 if (!PointerMustAliases.count(S->getOperand(1))) continue;
810 // Remember that this is the active value in the block.
811 StoredValue = S->getOperand(0);
815 // The last stored value that happened is the live-out for the block.
816 assert(StoredValue && "Already checked that there is a store in block");
817 SSA.AddAvailableValue(BB, StoredValue);
821 // Now that all the intra-loop values are classified, set up the preheader.
822 // It gets a load of the pointer we're promoting, and it is the live-out value
823 // from the preheader.
824 LoadInst *PreheaderLoad = new LoadInst(SomePtr,SomePtr->getName()+".promoted",
825 Preheader->getTerminator());
826 SSA.AddAvailableValue(Preheader, PreheaderLoad);
828 // Now that the preheader is good to go, set up the exit blocks. Each exit
829 // block gets a store of the live-out values that feed them. Since we've
830 // already told the SSA updater about the defs in the loop and the preheader
831 // definition, it is all set and we can start using it.
832 SmallVector<BasicBlock*, 8> ExitBlocks;
833 CurLoop->getUniqueExitBlocks(ExitBlocks);
834 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
835 BasicBlock *ExitBlock = ExitBlocks[i];
836 Value *LiveInValue = SSA.GetValueInMiddleOfBlock(ExitBlock);
837 Instruction *InsertPos = ExitBlock->getFirstNonPHI();
838 new StoreInst(LiveInValue, SomePtr, InsertPos);
841 // Okay, now we rewrite all loads that use live-in values in the loop,
842 // inserting PHI nodes as necessary.
843 for (unsigned i = 0, e = LiveInLoads.size(); i != e; ++i) {
844 LoadInst *ALoad = LiveInLoads[i];
845 Value *NewVal = SSA.GetValueInMiddleOfBlock(ALoad->getParent());
846 ALoad->replaceAllUsesWith(NewVal);
847 CurAST->copyValue(ALoad, NewVal);
848 ReplacedLoads[ALoad] = NewVal;
851 // If the preheader load is itself a pointer, we need to tell alias analysis
852 // about the new pointer we created in the preheader block and about any PHI
853 // nodes that just got inserted.
854 if (PreheaderLoad->getType()->isPointerTy()) {
855 // Copy any value stored to or loaded from a must-alias of the pointer.
856 CurAST->copyValue(SomeValue, PreheaderLoad);
858 for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i)
859 CurAST->copyValue(SomeValue, NewPHIs[i]);
862 // Now that everything is rewritten, delete the old instructions from the body
863 // of the loop. They should all be dead now.
864 for (unsigned i = 0, e = LoopUses.size(); i != e; ++i) {
865 Instruction *User = LoopUses[i];
867 // If this is a load that still has uses, then the load must have been added
868 // as a live value in the SSAUpdate data structure for a block (e.g. because
869 // the loaded value was stored later). In this case, we need to recursively
870 // propagate the updates until we get to the real value.
871 if (!User->use_empty()) {
872 Value *NewVal = ReplacedLoads[User];
873 assert(NewVal && "not a replaced load?");
875 // Propagate down to the ultimate replacee. The intermediately loads
876 // could theoretically already have been deleted, so we don't want to
877 // dereference the Value*'s.
878 DenseMap<Value*, Value*>::iterator RLI = ReplacedLoads.find(NewVal);
879 while (RLI != ReplacedLoads.end()) {
880 NewVal = RLI->second;
881 RLI = ReplacedLoads.find(NewVal);
884 User->replaceAllUsesWith(NewVal);
885 CurAST->copyValue(User, NewVal);
888 CurAST->deleteValue(User);
889 User->eraseFromParent();
896 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
897 void LICM::cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L) {
898 AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);
902 AST->copyValue(From, To);
905 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
907 void LICM::deleteAnalysisValue(Value *V, Loop *L) {
908 AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);