1 //===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass performs loop invariant code motion, attempting to remove as much
11 // code from the body of a loop as possible. It does this by either hoisting
12 // code into the preheader block, or by sinking code to the exit blocks if it is
13 // safe. This pass also promotes must-aliased memory locations in the loop to
14 // live in registers, thus hoisting and sinking "invariant" loads and stores.
16 // This pass uses alias analysis for two purposes:
18 // 1. Moving loop invariant loads and calls out of loops. If we can determine
19 // that a load or call inside of a loop never aliases anything stored to,
20 // we can hoist it or sink it like any other instruction.
21 // 2. Scalar Promotion of Memory - If there is a store instruction inside of
22 // the loop, we try to move the store to happen AFTER the loop instead of
23 // inside of the loop. This can only happen if a few conditions are true:
24 // A. The pointer stored through is loop invariant
25 // B. There are no stores or loads in the loop which _may_ alias the
26 // pointer. There are no calls in the loop which mod/ref the pointer.
27 // If these conditions are true, we can promote the loads and stores in the
28 // loop of the pointer to use a temporary alloca'd variable. We then use
29 // the SSAUpdater to construct the appropriate SSA form for the value.
31 //===----------------------------------------------------------------------===//
33 #define DEBUG_TYPE "licm"
34 #include "llvm/Transforms/Scalar.h"
35 #include "llvm/Constants.h"
36 #include "llvm/DerivedTypes.h"
37 #include "llvm/IntrinsicInst.h"
38 #include "llvm/Instructions.h"
39 #include "llvm/LLVMContext.h"
40 #include "llvm/Analysis/AliasAnalysis.h"
41 #include "llvm/Analysis/AliasSetTracker.h"
42 #include "llvm/Analysis/ConstantFolding.h"
43 #include "llvm/Analysis/LoopInfo.h"
44 #include "llvm/Analysis/LoopPass.h"
45 #include "llvm/Analysis/Dominators.h"
46 #include "llvm/Transforms/Utils/Local.h"
47 #include "llvm/Transforms/Utils/SSAUpdater.h"
48 #include "llvm/Support/CFG.h"
49 #include "llvm/Support/CommandLine.h"
50 #include "llvm/Support/raw_ostream.h"
51 #include "llvm/Support/Debug.h"
52 #include "llvm/ADT/Statistic.h"
56 STATISTIC(NumSunk , "Number of instructions sunk out of loop");
57 STATISTIC(NumHoisted , "Number of instructions hoisted out of loop");
58 STATISTIC(NumMovedLoads, "Number of load insts hoisted or sunk");
59 STATISTIC(NumMovedCalls, "Number of call insts hoisted or sunk");
60 STATISTIC(NumPromoted , "Number of memory locations promoted to registers");
63 DisablePromotion("disable-licm-promotion", cl::Hidden,
64 cl::desc("Disable memory promotion in LICM pass"));
67 struct LICM : public LoopPass {
68 static char ID; // Pass identification, replacement for typeid
69 LICM() : LoopPass(ID) {
70 initializeLICMPass(*PassRegistry::getPassRegistry());
73 virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
75 /// This transformation requires natural loop information & requires that
76 /// loop preheaders be inserted into the CFG...
78 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
80 AU.addRequired<DominatorTree>();
81 AU.addRequired<LoopInfo>();
82 AU.addRequiredID(LoopSimplifyID);
83 AU.addRequired<AliasAnalysis>();
84 AU.addPreserved<AliasAnalysis>();
85 AU.addPreserved("scalar-evolution");
86 AU.addPreservedID(LoopSimplifyID);
89 bool doFinalization() {
90 assert(LoopToAliasSetMap.empty() && "Didn't free loop alias sets");
95 AliasAnalysis *AA; // Current AliasAnalysis information
96 LoopInfo *LI; // Current LoopInfo
97 DominatorTree *DT; // Dominator Tree for the current Loop.
99 // State that is updated as we process loops.
100 bool Changed; // Set to true when we change anything.
101 BasicBlock *Preheader; // The preheader block of the current loop...
102 Loop *CurLoop; // The current loop we are working on...
103 AliasSetTracker *CurAST; // AliasSet information for the current loop...
104 DenseMap<Loop*, AliasSetTracker*> LoopToAliasSetMap;
106 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
107 void cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L);
109 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
111 void deleteAnalysisValue(Value *V, Loop *L);
113 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks
114 /// dominated by the specified block, and that are in the current loop) in
115 /// reverse depth first order w.r.t the DominatorTree. This allows us to
116 /// visit uses before definitions, allowing us to sink a loop body in one
117 /// pass without iteration.
119 void SinkRegion(DomTreeNode *N);
121 /// HoistRegion - Walk the specified region of the CFG (defined by all
122 /// blocks dominated by the specified block, and that are in the current
123 /// loop) in depth first order w.r.t the DominatorTree. This allows us to
124 /// visit definitions before uses, allowing us to hoist a loop body in one
125 /// pass without iteration.
127 void HoistRegion(DomTreeNode *N);
129 /// inSubLoop - Little predicate that returns true if the specified basic
130 /// block is in a subloop of the current one, not the current one itself.
132 bool inSubLoop(BasicBlock *BB) {
133 assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
134 return LI->getLoopFor(BB) != CurLoop;
137 /// sink - When an instruction is found to only be used outside of the loop,
138 /// this function moves it to the exit blocks and patches up SSA form as
141 void sink(Instruction &I);
143 /// hoist - When an instruction is found to only use loop invariant operands
144 /// that is safe to hoist, this instruction is called to do the dirty work.
146 void hoist(Instruction &I);
148 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it
149 /// is not a trapping instruction or if it is a trapping instruction and is
150 /// guaranteed to execute.
152 bool isSafeToExecuteUnconditionally(Instruction &I);
154 /// isGuaranteedToExecute - Check that the instruction is guaranteed to
157 bool isGuaranteedToExecute(Instruction &I);
159 /// pointerInvalidatedByLoop - Return true if the body of this loop may
160 /// store into the memory location pointed to by V.
162 bool pointerInvalidatedByLoop(Value *V, uint64_t Size,
163 const MDNode *TBAAInfo) {
164 // Check to see if any of the basic blocks in CurLoop invalidate *V.
165 return CurAST->getAliasSetForPointer(V, Size, TBAAInfo).isMod();
168 bool canSinkOrHoistInst(Instruction &I);
169 bool isNotUsedInLoop(Instruction &I);
171 void PromoteAliasSet(AliasSet &AS);
176 INITIALIZE_PASS_BEGIN(LICM, "licm", "Loop Invariant Code Motion", false, false)
177 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
178 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
179 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
180 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
181 INITIALIZE_PASS_END(LICM, "licm", "Loop Invariant Code Motion", false, false)
183 Pass *llvm::createLICMPass() { return new LICM(); }
185 /// Hoist expressions out of the specified loop. Note, alias info for inner
186 /// loop is not preserved so it is not a good idea to run LICM multiple
187 /// times on one loop.
189 bool LICM::runOnLoop(Loop *L, LPPassManager &LPM) {
192 // Get our Loop and Alias Analysis information...
193 LI = &getAnalysis<LoopInfo>();
194 AA = &getAnalysis<AliasAnalysis>();
195 DT = &getAnalysis<DominatorTree>();
197 CurAST = new AliasSetTracker(*AA);
198 // Collect Alias info from subloops.
199 for (Loop::iterator LoopItr = L->begin(), LoopItrE = L->end();
200 LoopItr != LoopItrE; ++LoopItr) {
201 Loop *InnerL = *LoopItr;
202 AliasSetTracker *InnerAST = LoopToAliasSetMap[InnerL];
203 assert(InnerAST && "Where is my AST?");
205 // What if InnerLoop was modified by other passes ?
206 CurAST->add(*InnerAST);
208 // Once we've incorporated the inner loop's AST into ours, we don't need the
209 // subloop's anymore.
211 LoopToAliasSetMap.erase(InnerL);
216 // Get the preheader block to move instructions into...
217 Preheader = L->getLoopPreheader();
219 // Loop over the body of this loop, looking for calls, invokes, and stores.
220 // Because subloops have already been incorporated into AST, we skip blocks in
223 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
226 if (LI->getLoopFor(BB) == L) // Ignore blocks in subloops.
227 CurAST->add(*BB); // Incorporate the specified basic block
230 // We want to visit all of the instructions in this loop... that are not parts
231 // of our subloops (they have already had their invariants hoisted out of
232 // their loop, into this loop, so there is no need to process the BODIES of
235 // Traverse the body of the loop in depth first order on the dominator tree so
236 // that we are guaranteed to see definitions before we see uses. This allows
237 // us to sink instructions in one pass, without iteration. After sinking
238 // instructions, we perform another pass to hoist them out of the loop.
240 if (L->hasDedicatedExits())
241 SinkRegion(DT->getNode(L->getHeader()));
243 HoistRegion(DT->getNode(L->getHeader()));
245 // Now that all loop invariants have been removed from the loop, promote any
246 // memory references to scalars that we can.
247 if (!DisablePromotion && Preheader && L->hasDedicatedExits()) {
248 // Loop over all of the alias sets in the tracker object.
249 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
254 // Clear out loops state information for the next iteration
258 // If this loop is nested inside of another one, save the alias information
259 // for when we process the outer loop.
260 if (L->getParentLoop())
261 LoopToAliasSetMap[L] = CurAST;
267 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks
268 /// dominated by the specified block, and that are in the current loop) in
269 /// reverse depth first order w.r.t the DominatorTree. This allows us to visit
270 /// uses before definitions, allowing us to sink a loop body in one pass without
273 void LICM::SinkRegion(DomTreeNode *N) {
274 assert(N != 0 && "Null dominator tree node?");
275 BasicBlock *BB = N->getBlock();
277 // If this subregion is not in the top level loop at all, exit.
278 if (!CurLoop->contains(BB)) return;
280 // We are processing blocks in reverse dfo, so process children first.
281 const std::vector<DomTreeNode*> &Children = N->getChildren();
282 for (unsigned i = 0, e = Children.size(); i != e; ++i)
283 SinkRegion(Children[i]);
285 // Only need to process the contents of this block if it is not part of a
286 // subloop (which would already have been processed).
287 if (inSubLoop(BB)) return;
289 for (BasicBlock::iterator II = BB->end(); II != BB->begin(); ) {
290 Instruction &I = *--II;
292 // If the instruction is dead, we would try to sink it because it isn't used
293 // in the loop, instead, just delete it.
294 if (isInstructionTriviallyDead(&I)) {
295 DEBUG(dbgs() << "LICM deleting dead inst: " << I << '\n');
297 CurAST->deleteValue(&I);
303 // Check to see if we can sink this instruction to the exit blocks
304 // of the loop. We can do this if the all users of the instruction are
305 // outside of the loop. In this case, it doesn't even matter if the
306 // operands of the instruction are loop invariant.
308 if (isNotUsedInLoop(I) && canSinkOrHoistInst(I)) {
315 /// HoistRegion - Walk the specified region of the CFG (defined by all blocks
316 /// dominated by the specified block, and that are in the current loop) in depth
317 /// first order w.r.t the DominatorTree. This allows us to visit definitions
318 /// before uses, allowing us to hoist a loop body in one pass without iteration.
320 void LICM::HoistRegion(DomTreeNode *N) {
321 assert(N != 0 && "Null dominator tree node?");
322 BasicBlock *BB = N->getBlock();
324 // If this subregion is not in the top level loop at all, exit.
325 if (!CurLoop->contains(BB)) return;
327 // Only need to process the contents of this block if it is not part of a
328 // subloop (which would already have been processed).
330 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ) {
331 Instruction &I = *II++;
333 // Try constant folding this instruction. If all the operands are
334 // constants, it is technically hoistable, but it would be better to just
336 if (Constant *C = ConstantFoldInstruction(&I)) {
337 DEBUG(dbgs() << "LICM folding inst: " << I << " --> " << *C << '\n');
338 CurAST->copyValue(&I, C);
339 CurAST->deleteValue(&I);
340 I.replaceAllUsesWith(C);
345 // Try hoisting the instruction out to the preheader. We can only do this
346 // if all of the operands of the instruction are loop invariant and if it
347 // is safe to hoist the instruction.
349 if (CurLoop->hasLoopInvariantOperands(&I) && canSinkOrHoistInst(I) &&
350 isSafeToExecuteUnconditionally(I))
354 const std::vector<DomTreeNode*> &Children = N->getChildren();
355 for (unsigned i = 0, e = Children.size(); i != e; ++i)
356 HoistRegion(Children[i]);
359 /// canSinkOrHoistInst - Return true if the hoister and sinker can handle this
362 bool LICM::canSinkOrHoistInst(Instruction &I) {
363 // Loads have extra constraints we have to verify before we can hoist them.
364 if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
365 if (!LI->isUnordered())
366 return false; // Don't hoist volatile/atomic loads!
368 // Loads from constant memory are always safe to move, even if they end up
369 // in the same alias set as something that ends up being modified.
370 if (AA->pointsToConstantMemory(LI->getOperand(0)))
372 if (LI->getMetadata(LI->getContext().getMDKindID("invariant.load")))
375 // Don't hoist loads which have may-aliased stores in loop.
377 if (LI->getType()->isSized())
378 Size = AA->getTypeStoreSize(LI->getType());
379 return !pointerInvalidatedByLoop(LI->getOperand(0), Size,
380 LI->getMetadata(LLVMContext::MD_tbaa));
381 } else if (CallInst *CI = dyn_cast<CallInst>(&I)) {
382 // Don't sink or hoist dbg info; it's legal, but not useful.
383 if (isa<DbgInfoIntrinsic>(I))
386 // Handle simple cases by querying alias analysis.
387 AliasAnalysis::ModRefBehavior Behavior = AA->getModRefBehavior(CI);
388 if (Behavior == AliasAnalysis::DoesNotAccessMemory)
390 if (AliasAnalysis::onlyReadsMemory(Behavior)) {
391 // If this call only reads from memory and there are no writes to memory
392 // in the loop, we can hoist or sink the call as appropriate.
393 bool FoundMod = false;
394 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
397 if (!AS.isForwardingAliasSet() && AS.isMod()) {
402 if (!FoundMod) return true;
405 // FIXME: This should use mod/ref information to see if we can hoist or sink
411 // Otherwise these instructions are hoistable/sinkable
412 return isa<BinaryOperator>(I) || isa<CastInst>(I) ||
413 isa<SelectInst>(I) || isa<GetElementPtrInst>(I) || isa<CmpInst>(I) ||
414 isa<InsertElementInst>(I) || isa<ExtractElementInst>(I) ||
415 isa<ShuffleVectorInst>(I);
418 /// isNotUsedInLoop - Return true if the only users of this instruction are
419 /// outside of the loop. If this is true, we can sink the instruction to the
420 /// exit blocks of the loop.
422 bool LICM::isNotUsedInLoop(Instruction &I) {
423 for (Value::use_iterator UI = I.use_begin(), E = I.use_end(); UI != E; ++UI) {
424 Instruction *User = cast<Instruction>(*UI);
425 if (PHINode *PN = dyn_cast<PHINode>(User)) {
426 // PHI node uses occur in predecessor blocks!
427 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
428 if (PN->getIncomingValue(i) == &I)
429 if (CurLoop->contains(PN->getIncomingBlock(i)))
431 } else if (CurLoop->contains(User)) {
439 /// sink - When an instruction is found to only be used outside of the loop,
440 /// this function moves it to the exit blocks and patches up SSA form as needed.
441 /// This method is guaranteed to remove the original instruction from its
442 /// position, and may either delete it or move it to outside of the loop.
444 void LICM::sink(Instruction &I) {
445 DEBUG(dbgs() << "LICM sinking instruction: " << I << "\n");
447 SmallVector<BasicBlock*, 8> ExitBlocks;
448 CurLoop->getUniqueExitBlocks(ExitBlocks);
450 if (isa<LoadInst>(I)) ++NumMovedLoads;
451 else if (isa<CallInst>(I)) ++NumMovedCalls;
455 // The case where there is only a single exit node of this loop is common
456 // enough that we handle it as a special (more efficient) case. It is more
457 // efficient to handle because there are no PHI nodes that need to be placed.
458 if (ExitBlocks.size() == 1) {
459 if (!DT->dominates(I.getParent(), ExitBlocks[0])) {
460 // Instruction is not used, just delete it.
461 CurAST->deleteValue(&I);
462 // If I has users in unreachable blocks, eliminate.
463 // If I is not void type then replaceAllUsesWith undef.
464 // This allows ValueHandlers and custom metadata to adjust itself.
466 I.replaceAllUsesWith(UndefValue::get(I.getType()));
469 // Move the instruction to the start of the exit block, after any PHI
471 I.moveBefore(ExitBlocks[0]->getFirstInsertionPt());
473 // This instruction is no longer in the AST for the current loop, because
474 // we just sunk it out of the loop. If we just sunk it into an outer
475 // loop, we will rediscover the operation when we process it.
476 CurAST->deleteValue(&I);
481 if (ExitBlocks.empty()) {
482 // The instruction is actually dead if there ARE NO exit blocks.
483 CurAST->deleteValue(&I);
484 // If I has users in unreachable blocks, eliminate.
485 // If I is not void type then replaceAllUsesWith undef.
486 // This allows ValueHandlers and custom metadata to adjust itself.
488 I.replaceAllUsesWith(UndefValue::get(I.getType()));
493 // Otherwise, if we have multiple exits, use the SSAUpdater to do all of the
494 // hard work of inserting PHI nodes as necessary.
495 SmallVector<PHINode*, 8> NewPHIs;
496 SSAUpdater SSA(&NewPHIs);
499 SSA.Initialize(I.getType(), I.getName());
501 // Insert a copy of the instruction in each exit block of the loop that is
502 // dominated by the instruction. Each exit block is known to only be in the
503 // ExitBlocks list once.
504 BasicBlock *InstOrigBB = I.getParent();
505 unsigned NumInserted = 0;
507 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
508 BasicBlock *ExitBlock = ExitBlocks[i];
510 if (!DT->dominates(InstOrigBB, ExitBlock))
513 // Insert the code after the last PHI node.
514 BasicBlock::iterator InsertPt = ExitBlock->getFirstInsertionPt();
516 // If this is the first exit block processed, just move the original
517 // instruction, otherwise clone the original instruction and insert
520 if (NumInserted++ == 0) {
521 I.moveBefore(InsertPt);
525 if (!I.getName().empty())
526 New->setName(I.getName()+".le");
527 ExitBlock->getInstList().insert(InsertPt, New);
530 // Now that we have inserted the instruction, inform SSAUpdater.
532 SSA.AddAvailableValue(ExitBlock, New);
535 // If the instruction doesn't dominate any exit blocks, it must be dead.
536 if (NumInserted == 0) {
537 CurAST->deleteValue(&I);
539 I.replaceAllUsesWith(UndefValue::get(I.getType()));
544 // Next, rewrite uses of the instruction, inserting PHI nodes as needed.
545 for (Value::use_iterator UI = I.use_begin(), UE = I.use_end(); UI != UE; ) {
546 // Grab the use before incrementing the iterator.
547 Use &U = UI.getUse();
548 // Increment the iterator before removing the use from the list.
550 SSA.RewriteUseAfterInsertions(U);
553 // Update CurAST for NewPHIs if I had pointer type.
554 if (I.getType()->isPointerTy())
555 for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i)
556 CurAST->copyValue(&I, NewPHIs[i]);
558 // Finally, remove the instruction from CurAST. It is no longer in the loop.
559 CurAST->deleteValue(&I);
562 /// hoist - When an instruction is found to only use loop invariant operands
563 /// that is safe to hoist, this instruction is called to do the dirty work.
565 void LICM::hoist(Instruction &I) {
566 DEBUG(dbgs() << "LICM hoisting to " << Preheader->getName() << ": "
569 // Move the new node to the Preheader, before its terminator.
570 I.moveBefore(Preheader->getTerminator());
572 if (isa<LoadInst>(I)) ++NumMovedLoads;
573 else if (isa<CallInst>(I)) ++NumMovedCalls;
578 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it is
579 /// not a trapping instruction or if it is a trapping instruction and is
580 /// guaranteed to execute.
582 bool LICM::isSafeToExecuteUnconditionally(Instruction &Inst) {
583 // If it is not a trapping instruction, it is always safe to hoist.
584 if (Inst.isSafeToSpeculativelyExecute())
587 return isGuaranteedToExecute(Inst);
590 bool LICM::isGuaranteedToExecute(Instruction &Inst) {
591 // Otherwise we have to check to make sure that the instruction dominates all
592 // of the exit blocks. If it doesn't, then there is a path out of the loop
593 // which does not execute this instruction, so we can't hoist it.
595 // If the instruction is in the header block for the loop (which is very
596 // common), it is always guaranteed to dominate the exit blocks. Since this
597 // is a common case, and can save some work, check it now.
598 if (Inst.getParent() == CurLoop->getHeader())
601 // Get the exit blocks for the current loop.
602 SmallVector<BasicBlock*, 8> ExitBlocks;
603 CurLoop->getExitBlocks(ExitBlocks);
605 // Verify that the block dominates each of the exit blocks of the loop.
606 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
607 if (!DT->dominates(Inst.getParent(), ExitBlocks[i]))
614 class LoopPromoter : public LoadAndStorePromoter {
615 Value *SomePtr; // Designated pointer to store to.
616 SmallPtrSet<Value*, 4> &PointerMustAliases;
617 SmallVectorImpl<BasicBlock*> &LoopExitBlocks;
618 AliasSetTracker &AST;
622 LoopPromoter(Value *SP,
623 const SmallVectorImpl<Instruction*> &Insts, SSAUpdater &S,
624 SmallPtrSet<Value*, 4> &PMA,
625 SmallVectorImpl<BasicBlock*> &LEB, AliasSetTracker &ast,
626 DebugLoc dl, int alignment)
627 : LoadAndStorePromoter(Insts, S), SomePtr(SP),
628 PointerMustAliases(PMA), LoopExitBlocks(LEB), AST(ast), DL(dl),
629 Alignment(alignment) {}
631 virtual bool isInstInList(Instruction *I,
632 const SmallVectorImpl<Instruction*> &) const {
634 if (LoadInst *LI = dyn_cast<LoadInst>(I))
635 Ptr = LI->getOperand(0);
637 Ptr = cast<StoreInst>(I)->getPointerOperand();
638 return PointerMustAliases.count(Ptr);
641 virtual void doExtraRewritesBeforeFinalDeletion() const {
642 // Insert stores after in the loop exit blocks. Each exit block gets a
643 // store of the live-out values that feed them. Since we've already told
644 // the SSA updater about the defs in the loop and the preheader
645 // definition, it is all set and we can start using it.
646 for (unsigned i = 0, e = LoopExitBlocks.size(); i != e; ++i) {
647 BasicBlock *ExitBlock = LoopExitBlocks[i];
648 Value *LiveInValue = SSA.GetValueInMiddleOfBlock(ExitBlock);
649 Instruction *InsertPos = ExitBlock->getFirstInsertionPt();
650 StoreInst *NewSI = new StoreInst(LiveInValue, SomePtr, InsertPos);
651 NewSI->setAlignment(Alignment);
652 NewSI->setDebugLoc(DL);
656 virtual void replaceLoadWithValue(LoadInst *LI, Value *V) const {
657 // Update alias analysis.
658 AST.copyValue(LI, V);
660 virtual void instructionDeleted(Instruction *I) const {
664 } // end anon namespace
666 /// PromoteAliasSet - Try to promote memory values to scalars by sinking
667 /// stores out of the loop and moving loads to before the loop. We do this by
668 /// looping over the stores in the loop, looking for stores to Must pointers
669 /// which are loop invariant.
671 void LICM::PromoteAliasSet(AliasSet &AS) {
672 // We can promote this alias set if it has a store, if it is a "Must" alias
673 // set, if the pointer is loop invariant, and if we are not eliminating any
674 // volatile loads or stores.
675 if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() ||
676 AS.isVolatile() || !CurLoop->isLoopInvariant(AS.begin()->getValue()))
679 assert(!AS.empty() &&
680 "Must alias set should have at least one pointer element in it!");
681 Value *SomePtr = AS.begin()->getValue();
683 // It isn't safe to promote a load/store from the loop if the load/store is
684 // conditional. For example, turning:
686 // for () { if (c) *P += 1; }
690 // tmp = *P; for () { if (c) tmp +=1; } *P = tmp;
692 // is not safe, because *P may only be valid to access if 'c' is true.
694 // It is safe to promote P if all uses are direct load/stores and if at
695 // least one is guaranteed to be executed.
696 bool GuaranteedToExecute = false;
698 SmallVector<Instruction*, 64> LoopUses;
699 SmallPtrSet<Value*, 4> PointerMustAliases;
701 // We start with an alignment of one and try to find instructions that allow
702 // us to prove better alignment.
703 unsigned Alignment = 1;
705 // Check that all of the pointers in the alias set have the same type. We
706 // cannot (yet) promote a memory location that is loaded and stored in
708 for (AliasSet::iterator ASI = AS.begin(), E = AS.end(); ASI != E; ++ASI) {
709 Value *ASIV = ASI->getValue();
710 PointerMustAliases.insert(ASIV);
712 // Check that all of the pointers in the alias set have the same type. We
713 // cannot (yet) promote a memory location that is loaded and stored in
715 if (SomePtr->getType() != ASIV->getType())
718 for (Value::use_iterator UI = ASIV->use_begin(), UE = ASIV->use_end();
720 // Ignore instructions that are outside the loop.
721 Instruction *Use = dyn_cast<Instruction>(*UI);
722 if (!Use || !CurLoop->contains(Use))
725 // If there is an non-load/store instruction in the loop, we can't promote
727 if (LoadInst *load = dyn_cast<LoadInst>(Use)) {
728 assert(!load->isVolatile() && "AST broken");
729 if (!load->isSimple())
731 } else if (StoreInst *store = dyn_cast<StoreInst>(Use)) {
732 // Stores *of* the pointer are not interesting, only stores *to* the
734 if (Use->getOperand(1) != ASIV)
736 assert(!store->isVolatile() && "AST broken");
737 if (!store->isSimple())
740 // Note that we only check GuaranteedToExecute inside the store case
741 // so that we do not introduce stores where they did not exist before
742 // (which would break the LLVM concurrency model).
744 // If the alignment of this instruction allows us to specify a more
745 // restrictive (and performant) alignment and if we are sure this
746 // instruction will be executed, update the alignment.
747 // Larger is better, with the exception of 0 being the best alignment.
748 unsigned InstAlignment = store->getAlignment();
749 if ((InstAlignment > Alignment || InstAlignment == 0)
751 if (isGuaranteedToExecute(*Use)) {
752 GuaranteedToExecute = true;
753 Alignment = InstAlignment;
756 if (!GuaranteedToExecute)
757 GuaranteedToExecute = isGuaranteedToExecute(*Use);
760 return; // Not a load or store.
762 LoopUses.push_back(Use);
766 // If there isn't a guaranteed-to-execute instruction, we can't promote.
767 if (!GuaranteedToExecute)
770 // Otherwise, this is safe to promote, lets do it!
771 DEBUG(dbgs() << "LICM: Promoting value stored to in loop: " <<*SomePtr<<'\n');
775 // Grab a debug location for the inserted loads/stores; given that the
776 // inserted loads/stores have little relation to the original loads/stores,
777 // this code just arbitrarily picks a location from one, since any debug
778 // location is better than none.
779 DebugLoc DL = LoopUses[0]->getDebugLoc();
781 SmallVector<BasicBlock*, 8> ExitBlocks;
782 CurLoop->getUniqueExitBlocks(ExitBlocks);
784 // We use the SSAUpdater interface to insert phi nodes as required.
785 SmallVector<PHINode*, 16> NewPHIs;
786 SSAUpdater SSA(&NewPHIs);
787 LoopPromoter Promoter(SomePtr, LoopUses, SSA, PointerMustAliases, ExitBlocks,
788 *CurAST, DL, Alignment);
790 // Set up the preheader to have a definition of the value. It is the live-out
791 // value from the preheader that uses in the loop will use.
792 LoadInst *PreheaderLoad =
793 new LoadInst(SomePtr, SomePtr->getName()+".promoted",
794 Preheader->getTerminator());
795 PreheaderLoad->setAlignment(Alignment);
796 PreheaderLoad->setDebugLoc(DL);
797 SSA.AddAvailableValue(Preheader, PreheaderLoad);
799 // Rewrite all the loads in the loop and remember all the definitions from
800 // stores in the loop.
801 Promoter.run(LoopUses);
803 // If the SSAUpdater didn't use the load in the preheader, just zap it now.
804 if (PreheaderLoad->use_empty())
805 PreheaderLoad->eraseFromParent();
809 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
810 void LICM::cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L) {
811 AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);
815 AST->copyValue(From, To);
818 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
820 void LICM::deleteAnalysisValue(Value *V, Loop *L) {
821 AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);