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/ValueTracking.h"
47 #include "llvm/Transforms/Utils/Local.h"
48 #include "llvm/Transforms/Utils/SSAUpdater.h"
49 #include "llvm/Target/TargetData.h"
50 #include "llvm/Target/TargetLibraryInfo.h"
51 #include "llvm/Support/CFG.h"
52 #include "llvm/Support/CommandLine.h"
53 #include "llvm/Support/raw_ostream.h"
54 #include "llvm/Support/Debug.h"
55 #include "llvm/ADT/Statistic.h"
59 STATISTIC(NumSunk , "Number of instructions sunk out of loop");
60 STATISTIC(NumHoisted , "Number of instructions hoisted out of loop");
61 STATISTIC(NumMovedLoads, "Number of load insts hoisted or sunk");
62 STATISTIC(NumMovedCalls, "Number of call insts hoisted or sunk");
63 STATISTIC(NumPromoted , "Number of memory locations promoted to registers");
66 DisablePromotion("disable-licm-promotion", cl::Hidden,
67 cl::desc("Disable memory promotion in LICM pass"));
70 struct LICM : public LoopPass {
71 static char ID; // Pass identification, replacement for typeid
72 LICM() : LoopPass(ID) {
73 initializeLICMPass(*PassRegistry::getPassRegistry());
76 virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
78 /// This transformation requires natural loop information & requires that
79 /// loop preheaders be inserted into the CFG...
81 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
83 AU.addRequired<DominatorTree>();
84 AU.addRequired<LoopInfo>();
85 AU.addRequiredID(LoopSimplifyID);
86 AU.addRequired<AliasAnalysis>();
87 AU.addPreserved<AliasAnalysis>();
88 AU.addPreserved("scalar-evolution");
89 AU.addPreservedID(LoopSimplifyID);
90 AU.addRequired<TargetLibraryInfo>();
93 bool doFinalization() {
94 assert(LoopToAliasSetMap.empty() && "Didn't free loop alias sets");
99 AliasAnalysis *AA; // Current AliasAnalysis information
100 LoopInfo *LI; // Current LoopInfo
101 DominatorTree *DT; // Dominator Tree for the current Loop.
103 TargetData *TD; // TargetData for constant folding.
104 TargetLibraryInfo *TLI; // TargetLibraryInfo for constant folding.
106 // State that is updated as we process loops.
107 bool Changed; // Set to true when we change anything.
108 BasicBlock *Preheader; // The preheader block of the current loop...
109 Loop *CurLoop; // The current loop we are working on...
110 AliasSetTracker *CurAST; // AliasSet information for the current loop...
111 DenseMap<Loop*, AliasSetTracker*> LoopToAliasSetMap;
113 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
114 void cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L);
116 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
118 void deleteAnalysisValue(Value *V, Loop *L);
120 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks
121 /// dominated by the specified block, and that are in the current loop) in
122 /// reverse depth first order w.r.t the DominatorTree. This allows us to
123 /// visit uses before definitions, allowing us to sink a loop body in one
124 /// pass without iteration.
126 void SinkRegion(DomTreeNode *N);
128 /// HoistRegion - Walk the specified region of the CFG (defined by all
129 /// blocks dominated by the specified block, and that are in the current
130 /// loop) in depth first order w.r.t the DominatorTree. This allows us to
131 /// visit definitions before uses, allowing us to hoist a loop body in one
132 /// pass without iteration.
134 void HoistRegion(DomTreeNode *N);
136 /// inSubLoop - Little predicate that returns true if the specified basic
137 /// block is in a subloop of the current one, not the current one itself.
139 bool inSubLoop(BasicBlock *BB) {
140 assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
141 return LI->getLoopFor(BB) != CurLoop;
144 /// sink - When an instruction is found to only be used outside of the loop,
145 /// this function moves it to the exit blocks and patches up SSA form as
148 void sink(Instruction &I);
150 /// hoist - When an instruction is found to only use loop invariant operands
151 /// that is safe to hoist, this instruction is called to do the dirty work.
153 void hoist(Instruction &I);
155 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it
156 /// is not a trapping instruction or if it is a trapping instruction and is
157 /// guaranteed to execute.
159 bool isSafeToExecuteUnconditionally(Instruction &I);
161 /// isGuaranteedToExecute - Check that the instruction is guaranteed to
164 bool isGuaranteedToExecute(Instruction &I);
166 /// pointerInvalidatedByLoop - Return true if the body of this loop may
167 /// store into the memory location pointed to by V.
169 bool pointerInvalidatedByLoop(Value *V, uint64_t Size,
170 const MDNode *TBAAInfo) {
171 // Check to see if any of the basic blocks in CurLoop invalidate *V.
172 return CurAST->getAliasSetForPointer(V, Size, TBAAInfo).isMod();
175 bool canSinkOrHoistInst(Instruction &I);
176 bool isNotUsedInLoop(Instruction &I);
178 void PromoteAliasSet(AliasSet &AS,
179 SmallVectorImpl<BasicBlock*> &ExitBlocks,
180 SmallVectorImpl<Instruction*> &InsertPts);
185 INITIALIZE_PASS_BEGIN(LICM, "licm", "Loop Invariant Code Motion", false, false)
186 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
187 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
188 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
189 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
190 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
191 INITIALIZE_PASS_END(LICM, "licm", "Loop Invariant Code Motion", false, false)
193 Pass *llvm::createLICMPass() { return new LICM(); }
195 /// Hoist expressions out of the specified loop. Note, alias info for inner
196 /// loop is not preserved so it is not a good idea to run LICM multiple
197 /// times on one loop.
199 bool LICM::runOnLoop(Loop *L, LPPassManager &LPM) {
202 // Get our Loop and Alias Analysis information...
203 LI = &getAnalysis<LoopInfo>();
204 AA = &getAnalysis<AliasAnalysis>();
205 DT = &getAnalysis<DominatorTree>();
207 TD = getAnalysisIfAvailable<TargetData>();
208 TLI = &getAnalysis<TargetLibraryInfo>();
210 CurAST = new AliasSetTracker(*AA);
211 // Collect Alias info from subloops.
212 for (Loop::iterator LoopItr = L->begin(), LoopItrE = L->end();
213 LoopItr != LoopItrE; ++LoopItr) {
214 Loop *InnerL = *LoopItr;
215 AliasSetTracker *InnerAST = LoopToAliasSetMap[InnerL];
216 assert(InnerAST && "Where is my AST?");
218 // What if InnerLoop was modified by other passes ?
219 CurAST->add(*InnerAST);
221 // Once we've incorporated the inner loop's AST into ours, we don't need the
222 // subloop's anymore.
224 LoopToAliasSetMap.erase(InnerL);
229 // Get the preheader block to move instructions into...
230 Preheader = L->getLoopPreheader();
232 // Loop over the body of this loop, looking for calls, invokes, and stores.
233 // Because subloops have already been incorporated into AST, we skip blocks in
236 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
239 if (LI->getLoopFor(BB) == L) // Ignore blocks in subloops.
240 CurAST->add(*BB); // Incorporate the specified basic block
243 // We want to visit all of the instructions in this loop... that are not parts
244 // of our subloops (they have already had their invariants hoisted out of
245 // their loop, into this loop, so there is no need to process the BODIES of
248 // Traverse the body of the loop in depth first order on the dominator tree so
249 // that we are guaranteed to see definitions before we see uses. This allows
250 // us to sink instructions in one pass, without iteration. After sinking
251 // instructions, we perform another pass to hoist them out of the loop.
253 if (L->hasDedicatedExits())
254 SinkRegion(DT->getNode(L->getHeader()));
256 HoistRegion(DT->getNode(L->getHeader()));
258 // Now that all loop invariants have been removed from the loop, promote any
259 // memory references to scalars that we can.
260 if (!DisablePromotion && Preheader && L->hasDedicatedExits()) {
261 SmallVector<BasicBlock *, 8> ExitBlocks;
262 SmallVector<Instruction *, 8> InsertPts;
264 // Loop over all of the alias sets in the tracker object.
265 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
267 PromoteAliasSet(*I, ExitBlocks, InsertPts);
270 // Clear out loops state information for the next iteration
274 // If this loop is nested inside of another one, save the alias information
275 // for when we process the outer loop.
276 if (L->getParentLoop())
277 LoopToAliasSetMap[L] = CurAST;
283 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks
284 /// dominated by the specified block, and that are in the current loop) in
285 /// reverse depth first order w.r.t the DominatorTree. This allows us to visit
286 /// uses before definitions, allowing us to sink a loop body in one pass without
289 void LICM::SinkRegion(DomTreeNode *N) {
290 assert(N != 0 && "Null dominator tree node?");
291 BasicBlock *BB = N->getBlock();
293 // If this subregion is not in the top level loop at all, exit.
294 if (!CurLoop->contains(BB)) return;
296 // We are processing blocks in reverse dfo, so process children first.
297 const std::vector<DomTreeNode*> &Children = N->getChildren();
298 for (unsigned i = 0, e = Children.size(); i != e; ++i)
299 SinkRegion(Children[i]);
301 // Only need to process the contents of this block if it is not part of a
302 // subloop (which would already have been processed).
303 if (inSubLoop(BB)) return;
305 for (BasicBlock::iterator II = BB->end(); II != BB->begin(); ) {
306 Instruction &I = *--II;
308 // If the instruction is dead, we would try to sink it because it isn't used
309 // in the loop, instead, just delete it.
310 if (isInstructionTriviallyDead(&I, TLI)) {
311 DEBUG(dbgs() << "LICM deleting dead inst: " << I << '\n');
313 CurAST->deleteValue(&I);
319 // Check to see if we can sink this instruction to the exit blocks
320 // of the loop. We can do this if the all users of the instruction are
321 // outside of the loop. In this case, it doesn't even matter if the
322 // operands of the instruction are loop invariant.
324 if (isNotUsedInLoop(I) && canSinkOrHoistInst(I)) {
331 /// HoistRegion - Walk the specified region of the CFG (defined by all blocks
332 /// dominated by the specified block, and that are in the current loop) in depth
333 /// first order w.r.t the DominatorTree. This allows us to visit definitions
334 /// before uses, allowing us to hoist a loop body in one pass without iteration.
336 void LICM::HoistRegion(DomTreeNode *N) {
337 assert(N != 0 && "Null dominator tree node?");
338 BasicBlock *BB = N->getBlock();
340 // If this subregion is not in the top level loop at all, exit.
341 if (!CurLoop->contains(BB)) return;
343 // Only need to process the contents of this block if it is not part of a
344 // subloop (which would already have been processed).
346 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ) {
347 Instruction &I = *II++;
349 // Try constant folding this instruction. If all the operands are
350 // constants, it is technically hoistable, but it would be better to just
352 if (Constant *C = ConstantFoldInstruction(&I, TD, TLI)) {
353 DEBUG(dbgs() << "LICM folding inst: " << I << " --> " << *C << '\n');
354 CurAST->copyValue(&I, C);
355 CurAST->deleteValue(&I);
356 I.replaceAllUsesWith(C);
361 // Try hoisting the instruction out to the preheader. We can only do this
362 // if all of the operands of the instruction are loop invariant and if it
363 // is safe to hoist the instruction.
365 if (CurLoop->hasLoopInvariantOperands(&I) && canSinkOrHoistInst(I) &&
366 isSafeToExecuteUnconditionally(I))
370 const std::vector<DomTreeNode*> &Children = N->getChildren();
371 for (unsigned i = 0, e = Children.size(); i != e; ++i)
372 HoistRegion(Children[i]);
375 /// canSinkOrHoistInst - Return true if the hoister and sinker can handle this
378 bool LICM::canSinkOrHoistInst(Instruction &I) {
379 // Loads have extra constraints we have to verify before we can hoist them.
380 if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
381 if (!LI->isUnordered())
382 return false; // Don't hoist volatile/atomic loads!
384 // Loads from constant memory are always safe to move, even if they end up
385 // in the same alias set as something that ends up being modified.
386 if (AA->pointsToConstantMemory(LI->getOperand(0)))
388 if (LI->getMetadata("invariant.load"))
391 // Don't hoist loads which have may-aliased stores in loop.
393 if (LI->getType()->isSized())
394 Size = AA->getTypeStoreSize(LI->getType());
395 return !pointerInvalidatedByLoop(LI->getOperand(0), Size,
396 LI->getMetadata(LLVMContext::MD_tbaa));
397 } else if (CallInst *CI = dyn_cast<CallInst>(&I)) {
398 // Don't sink or hoist dbg info; it's legal, but not useful.
399 if (isa<DbgInfoIntrinsic>(I))
402 // Handle simple cases by querying alias analysis.
403 AliasAnalysis::ModRefBehavior Behavior = AA->getModRefBehavior(CI);
404 if (Behavior == AliasAnalysis::DoesNotAccessMemory)
406 if (AliasAnalysis::onlyReadsMemory(Behavior)) {
407 // If this call only reads from memory and there are no writes to memory
408 // in the loop, we can hoist or sink the call as appropriate.
409 bool FoundMod = false;
410 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
413 if (!AS.isForwardingAliasSet() && AS.isMod()) {
418 if (!FoundMod) return true;
421 // FIXME: This should use mod/ref information to see if we can hoist or sink
427 // Otherwise these instructions are hoistable/sinkable
428 return isa<BinaryOperator>(I) || isa<CastInst>(I) ||
429 isa<SelectInst>(I) || isa<GetElementPtrInst>(I) || isa<CmpInst>(I) ||
430 isa<InsertElementInst>(I) || isa<ExtractElementInst>(I) ||
431 isa<ShuffleVectorInst>(I);
434 /// isNotUsedInLoop - Return true if the only users of this instruction are
435 /// outside of the loop. If this is true, we can sink the instruction to the
436 /// exit blocks of the loop.
438 bool LICM::isNotUsedInLoop(Instruction &I) {
439 for (Value::use_iterator UI = I.use_begin(), E = I.use_end(); UI != E; ++UI) {
440 Instruction *User = cast<Instruction>(*UI);
441 if (PHINode *PN = dyn_cast<PHINode>(User)) {
442 // PHI node uses occur in predecessor blocks!
443 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
444 if (PN->getIncomingValue(i) == &I)
445 if (CurLoop->contains(PN->getIncomingBlock(i)))
447 } else if (CurLoop->contains(User)) {
455 /// sink - When an instruction is found to only be used outside of the loop,
456 /// this function moves it to the exit blocks and patches up SSA form as needed.
457 /// This method is guaranteed to remove the original instruction from its
458 /// position, and may either delete it or move it to outside of the loop.
460 void LICM::sink(Instruction &I) {
461 DEBUG(dbgs() << "LICM sinking instruction: " << I << "\n");
463 SmallVector<BasicBlock*, 8> ExitBlocks;
464 CurLoop->getUniqueExitBlocks(ExitBlocks);
466 if (isa<LoadInst>(I)) ++NumMovedLoads;
467 else if (isa<CallInst>(I)) ++NumMovedCalls;
471 // The case where there is only a single exit node of this loop is common
472 // enough that we handle it as a special (more efficient) case. It is more
473 // efficient to handle because there are no PHI nodes that need to be placed.
474 if (ExitBlocks.size() == 1) {
475 if (!DT->dominates(I.getParent(), ExitBlocks[0])) {
476 // Instruction is not used, just delete it.
477 CurAST->deleteValue(&I);
478 // If I has users in unreachable blocks, eliminate.
479 // If I is not void type then replaceAllUsesWith undef.
480 // This allows ValueHandlers and custom metadata to adjust itself.
482 I.replaceAllUsesWith(UndefValue::get(I.getType()));
485 // Move the instruction to the start of the exit block, after any PHI
487 I.moveBefore(ExitBlocks[0]->getFirstInsertionPt());
489 // This instruction is no longer in the AST for the current loop, because
490 // we just sunk it out of the loop. If we just sunk it into an outer
491 // loop, we will rediscover the operation when we process it.
492 CurAST->deleteValue(&I);
497 if (ExitBlocks.empty()) {
498 // The instruction is actually dead if there ARE NO exit blocks.
499 CurAST->deleteValue(&I);
500 // If I has users in unreachable blocks, eliminate.
501 // If I is not void type then replaceAllUsesWith undef.
502 // This allows ValueHandlers and custom metadata to adjust itself.
504 I.replaceAllUsesWith(UndefValue::get(I.getType()));
509 // Otherwise, if we have multiple exits, use the SSAUpdater to do all of the
510 // hard work of inserting PHI nodes as necessary.
511 SmallVector<PHINode*, 8> NewPHIs;
512 SSAUpdater SSA(&NewPHIs);
515 SSA.Initialize(I.getType(), I.getName());
517 // Insert a copy of the instruction in each exit block of the loop that is
518 // dominated by the instruction. Each exit block is known to only be in the
519 // ExitBlocks list once.
520 BasicBlock *InstOrigBB = I.getParent();
521 unsigned NumInserted = 0;
523 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
524 BasicBlock *ExitBlock = ExitBlocks[i];
526 if (!DT->dominates(InstOrigBB, ExitBlock))
529 // Insert the code after the last PHI node.
530 BasicBlock::iterator InsertPt = ExitBlock->getFirstInsertionPt();
532 // If this is the first exit block processed, just move the original
533 // instruction, otherwise clone the original instruction and insert
536 if (NumInserted++ == 0) {
537 I.moveBefore(InsertPt);
541 if (!I.getName().empty())
542 New->setName(I.getName()+".le");
543 ExitBlock->getInstList().insert(InsertPt, New);
546 // Now that we have inserted the instruction, inform SSAUpdater.
548 SSA.AddAvailableValue(ExitBlock, New);
551 // If the instruction doesn't dominate any exit blocks, it must be dead.
552 if (NumInserted == 0) {
553 CurAST->deleteValue(&I);
555 I.replaceAllUsesWith(UndefValue::get(I.getType()));
560 // Next, rewrite uses of the instruction, inserting PHI nodes as needed.
561 for (Value::use_iterator UI = I.use_begin(), UE = I.use_end(); UI != UE; ) {
562 // Grab the use before incrementing the iterator.
563 Use &U = UI.getUse();
564 // Increment the iterator before removing the use from the list.
566 SSA.RewriteUseAfterInsertions(U);
569 // Update CurAST for NewPHIs if I had pointer type.
570 if (I.getType()->isPointerTy())
571 for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i)
572 CurAST->copyValue(&I, NewPHIs[i]);
574 // Finally, remove the instruction from CurAST. It is no longer in the loop.
575 CurAST->deleteValue(&I);
578 /// hoist - When an instruction is found to only use loop invariant operands
579 /// that is safe to hoist, this instruction is called to do the dirty work.
581 void LICM::hoist(Instruction &I) {
582 DEBUG(dbgs() << "LICM hoisting to " << Preheader->getName() << ": "
585 // Move the new node to the Preheader, before its terminator.
586 I.moveBefore(Preheader->getTerminator());
588 if (isa<LoadInst>(I)) ++NumMovedLoads;
589 else if (isa<CallInst>(I)) ++NumMovedCalls;
594 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it is
595 /// not a trapping instruction or if it is a trapping instruction and is
596 /// guaranteed to execute.
598 bool LICM::isSafeToExecuteUnconditionally(Instruction &Inst) {
599 // If it is not a trapping instruction, it is always safe to hoist.
600 if (isSafeToSpeculativelyExecute(&Inst))
603 return isGuaranteedToExecute(Inst);
606 bool LICM::isGuaranteedToExecute(Instruction &Inst) {
607 // Otherwise we have to check to make sure that the instruction dominates all
608 // of the exit blocks. If it doesn't, then there is a path out of the loop
609 // which does not execute this instruction, so we can't hoist it.
611 // If the instruction is in the header block for the loop (which is very
612 // common), it is always guaranteed to dominate the exit blocks. Since this
613 // is a common case, and can save some work, check it now.
614 if (Inst.getParent() == CurLoop->getHeader())
617 // Get the exit blocks for the current loop.
618 SmallVector<BasicBlock*, 8> ExitBlocks;
619 CurLoop->getExitBlocks(ExitBlocks);
621 // Verify that the block dominates each of the exit blocks of the loop.
622 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
623 if (!DT->dominates(Inst.getParent(), ExitBlocks[i]))
626 // As a degenerate case, if the loop is statically infinite then we haven't
627 // proven anything since there are no exit blocks.
628 if (ExitBlocks.empty())
635 class LoopPromoter : public LoadAndStorePromoter {
636 Value *SomePtr; // Designated pointer to store to.
637 SmallPtrSet<Value*, 4> &PointerMustAliases;
638 SmallVectorImpl<BasicBlock*> &LoopExitBlocks;
639 SmallVectorImpl<Instruction*> &LoopInsertPts;
640 AliasSetTracker &AST;
644 LoopPromoter(Value *SP,
645 const SmallVectorImpl<Instruction*> &Insts, SSAUpdater &S,
646 SmallPtrSet<Value*, 4> &PMA,
647 SmallVectorImpl<BasicBlock*> &LEB,
648 SmallVectorImpl<Instruction*> &LIP,
649 AliasSetTracker &ast, DebugLoc dl, int alignment)
650 : LoadAndStorePromoter(Insts, S), SomePtr(SP),
651 PointerMustAliases(PMA), LoopExitBlocks(LEB), LoopInsertPts(LIP),
652 AST(ast), DL(dl), Alignment(alignment) {}
654 virtual bool isInstInList(Instruction *I,
655 const SmallVectorImpl<Instruction*> &) const {
657 if (LoadInst *LI = dyn_cast<LoadInst>(I))
658 Ptr = LI->getOperand(0);
660 Ptr = cast<StoreInst>(I)->getPointerOperand();
661 return PointerMustAliases.count(Ptr);
664 virtual void doExtraRewritesBeforeFinalDeletion() const {
665 // Insert stores after in the loop exit blocks. Each exit block gets a
666 // store of the live-out values that feed them. Since we've already told
667 // the SSA updater about the defs in the loop and the preheader
668 // definition, it is all set and we can start using it.
669 for (unsigned i = 0, e = LoopExitBlocks.size(); i != e; ++i) {
670 BasicBlock *ExitBlock = LoopExitBlocks[i];
671 Value *LiveInValue = SSA.GetValueInMiddleOfBlock(ExitBlock);
672 Instruction *InsertPos = LoopInsertPts[i];
673 StoreInst *NewSI = new StoreInst(LiveInValue, SomePtr, InsertPos);
674 NewSI->setAlignment(Alignment);
675 NewSI->setDebugLoc(DL);
679 virtual void replaceLoadWithValue(LoadInst *LI, Value *V) const {
680 // Update alias analysis.
681 AST.copyValue(LI, V);
683 virtual void instructionDeleted(Instruction *I) const {
687 } // end anon namespace
689 /// PromoteAliasSet - Try to promote memory values to scalars by sinking
690 /// stores out of the loop and moving loads to before the loop. We do this by
691 /// looping over the stores in the loop, looking for stores to Must pointers
692 /// which are loop invariant.
694 void LICM::PromoteAliasSet(AliasSet &AS,
695 SmallVectorImpl<BasicBlock*> &ExitBlocks,
696 SmallVectorImpl<Instruction*> &InsertPts) {
697 // We can promote this alias set if it has a store, if it is a "Must" alias
698 // set, if the pointer is loop invariant, and if we are not eliminating any
699 // volatile loads or stores.
700 if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() ||
701 AS.isVolatile() || !CurLoop->isLoopInvariant(AS.begin()->getValue()))
704 assert(!AS.empty() &&
705 "Must alias set should have at least one pointer element in it!");
706 Value *SomePtr = AS.begin()->getValue();
708 // It isn't safe to promote a load/store from the loop if the load/store is
709 // conditional. For example, turning:
711 // for () { if (c) *P += 1; }
715 // tmp = *P; for () { if (c) tmp +=1; } *P = tmp;
717 // is not safe, because *P may only be valid to access if 'c' is true.
719 // It is safe to promote P if all uses are direct load/stores and if at
720 // least one is guaranteed to be executed.
721 bool GuaranteedToExecute = false;
723 SmallVector<Instruction*, 64> LoopUses;
724 SmallPtrSet<Value*, 4> PointerMustAliases;
726 // We start with an alignment of one and try to find instructions that allow
727 // us to prove better alignment.
728 unsigned Alignment = 1;
730 // Check that all of the pointers in the alias set have the same type. We
731 // cannot (yet) promote a memory location that is loaded and stored in
733 for (AliasSet::iterator ASI = AS.begin(), E = AS.end(); ASI != E; ++ASI) {
734 Value *ASIV = ASI->getValue();
735 PointerMustAliases.insert(ASIV);
737 // Check that all of the pointers in the alias set have the same type. We
738 // cannot (yet) promote a memory location that is loaded and stored in
740 if (SomePtr->getType() != ASIV->getType())
743 for (Value::use_iterator UI = ASIV->use_begin(), UE = ASIV->use_end();
745 // Ignore instructions that are outside the loop.
746 Instruction *Use = dyn_cast<Instruction>(*UI);
747 if (!Use || !CurLoop->contains(Use))
750 // If there is an non-load/store instruction in the loop, we can't promote
752 if (LoadInst *load = dyn_cast<LoadInst>(Use)) {
753 assert(!load->isVolatile() && "AST broken");
754 if (!load->isSimple())
756 } else if (StoreInst *store = dyn_cast<StoreInst>(Use)) {
757 // Stores *of* the pointer are not interesting, only stores *to* the
759 if (Use->getOperand(1) != ASIV)
761 assert(!store->isVolatile() && "AST broken");
762 if (!store->isSimple())
765 // Note that we only check GuaranteedToExecute inside the store case
766 // so that we do not introduce stores where they did not exist before
767 // (which would break the LLVM concurrency model).
769 // If the alignment of this instruction allows us to specify a more
770 // restrictive (and performant) alignment and if we are sure this
771 // instruction will be executed, update the alignment.
772 // Larger is better, with the exception of 0 being the best alignment.
773 unsigned InstAlignment = store->getAlignment();
774 if ((InstAlignment > Alignment || InstAlignment == 0)
776 if (isGuaranteedToExecute(*Use)) {
777 GuaranteedToExecute = true;
778 Alignment = InstAlignment;
781 if (!GuaranteedToExecute)
782 GuaranteedToExecute = isGuaranteedToExecute(*Use);
785 return; // Not a load or store.
787 LoopUses.push_back(Use);
791 // If there isn't a guaranteed-to-execute instruction, we can't promote.
792 if (!GuaranteedToExecute)
795 // Otherwise, this is safe to promote, lets do it!
796 DEBUG(dbgs() << "LICM: Promoting value stored to in loop: " <<*SomePtr<<'\n');
800 // Grab a debug location for the inserted loads/stores; given that the
801 // inserted loads/stores have little relation to the original loads/stores,
802 // this code just arbitrarily picks a location from one, since any debug
803 // location is better than none.
804 DebugLoc DL = LoopUses[0]->getDebugLoc();
806 // Figure out the loop exits and their insertion points, if this is the
808 if (ExitBlocks.empty()) {
809 CurLoop->getUniqueExitBlocks(ExitBlocks);
810 InsertPts.resize(ExitBlocks.size());
811 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
812 InsertPts[i] = ExitBlocks[i]->getFirstInsertionPt();
815 // We use the SSAUpdater interface to insert phi nodes as required.
816 SmallVector<PHINode*, 16> NewPHIs;
817 SSAUpdater SSA(&NewPHIs);
818 LoopPromoter Promoter(SomePtr, LoopUses, SSA, PointerMustAliases, ExitBlocks,
819 InsertPts, *CurAST, DL, Alignment);
821 // Set up the preheader to have a definition of the value. It is the live-out
822 // value from the preheader that uses in the loop will use.
823 LoadInst *PreheaderLoad =
824 new LoadInst(SomePtr, SomePtr->getName()+".promoted",
825 Preheader->getTerminator());
826 PreheaderLoad->setAlignment(Alignment);
827 PreheaderLoad->setDebugLoc(DL);
828 SSA.AddAvailableValue(Preheader, PreheaderLoad);
830 // Rewrite all the loads in the loop and remember all the definitions from
831 // stores in the loop.
832 Promoter.run(LoopUses);
834 // If the SSAUpdater didn't use the load in the preheader, just zap it now.
835 if (PreheaderLoad->use_empty())
836 PreheaderLoad->eraseFromParent();
840 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
841 void LICM::cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L) {
842 AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);
846 AST->copyValue(From, To);
849 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
851 void LICM::deleteAnalysisValue(Value *V, Loop *L) {
852 AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);