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/DataLayout.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 DataLayout *TD; // DataLayout 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 bool MayThrow; // The current loop contains an instruction which
112 // may throw, thus preventing code motion of
113 // instructions with side effects.
114 DenseMap<Loop*, AliasSetTracker*> LoopToAliasSetMap;
116 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
117 void cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L);
119 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
121 void deleteAnalysisValue(Value *V, Loop *L);
123 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks
124 /// dominated by the specified block, and that are in the current loop) in
125 /// reverse depth first order w.r.t the DominatorTree. This allows us to
126 /// visit uses before definitions, allowing us to sink a loop body in one
127 /// pass without iteration.
129 void SinkRegion(DomTreeNode *N);
131 /// HoistRegion - Walk the specified region of the CFG (defined by all
132 /// blocks dominated by the specified block, and that are in the current
133 /// loop) in depth first order w.r.t the DominatorTree. This allows us to
134 /// visit definitions before uses, allowing us to hoist a loop body in one
135 /// pass without iteration.
137 void HoistRegion(DomTreeNode *N);
139 /// inSubLoop - Little predicate that returns true if the specified basic
140 /// block is in a subloop of the current one, not the current one itself.
142 bool inSubLoop(BasicBlock *BB) {
143 assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
144 return LI->getLoopFor(BB) != CurLoop;
147 /// sink - When an instruction is found to only be used outside of the loop,
148 /// this function moves it to the exit blocks and patches up SSA form as
151 void sink(Instruction &I);
153 /// hoist - When an instruction is found to only use loop invariant operands
154 /// that is safe to hoist, this instruction is called to do the dirty work.
156 void hoist(Instruction &I);
158 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it
159 /// is not a trapping instruction or if it is a trapping instruction and is
160 /// guaranteed to execute.
162 bool isSafeToExecuteUnconditionally(Instruction &I);
164 /// isGuaranteedToExecute - Check that the instruction is guaranteed to
167 bool isGuaranteedToExecute(Instruction &I);
169 /// pointerInvalidatedByLoop - Return true if the body of this loop may
170 /// store into the memory location pointed to by V.
172 bool pointerInvalidatedByLoop(Value *V, uint64_t Size,
173 const MDNode *TBAAInfo) {
174 // Check to see if any of the basic blocks in CurLoop invalidate *V.
175 return CurAST->getAliasSetForPointer(V, Size, TBAAInfo).isMod();
178 bool canSinkOrHoistInst(Instruction &I);
179 bool isNotUsedInLoop(Instruction &I);
181 void PromoteAliasSet(AliasSet &AS,
182 SmallVectorImpl<BasicBlock*> &ExitBlocks,
183 SmallVectorImpl<Instruction*> &InsertPts);
188 INITIALIZE_PASS_BEGIN(LICM, "licm", "Loop Invariant Code Motion", false, false)
189 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
190 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
191 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
192 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
193 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
194 INITIALIZE_PASS_END(LICM, "licm", "Loop Invariant Code Motion", false, false)
196 Pass *llvm::createLICMPass() { return new LICM(); }
198 /// Hoist expressions out of the specified loop. Note, alias info for inner
199 /// loop is not preserved so it is not a good idea to run LICM multiple
200 /// times on one loop.
202 bool LICM::runOnLoop(Loop *L, LPPassManager &LPM) {
205 // Get our Loop and Alias Analysis information...
206 LI = &getAnalysis<LoopInfo>();
207 AA = &getAnalysis<AliasAnalysis>();
208 DT = &getAnalysis<DominatorTree>();
210 TD = getAnalysisIfAvailable<DataLayout>();
211 TLI = &getAnalysis<TargetLibraryInfo>();
213 CurAST = new AliasSetTracker(*AA);
214 // Collect Alias info from subloops.
215 for (Loop::iterator LoopItr = L->begin(), LoopItrE = L->end();
216 LoopItr != LoopItrE; ++LoopItr) {
217 Loop *InnerL = *LoopItr;
218 AliasSetTracker *InnerAST = LoopToAliasSetMap[InnerL];
219 assert(InnerAST && "Where is my AST?");
221 // What if InnerLoop was modified by other passes ?
222 CurAST->add(*InnerAST);
224 // Once we've incorporated the inner loop's AST into ours, we don't need the
225 // subloop's anymore.
227 LoopToAliasSetMap.erase(InnerL);
232 // Get the preheader block to move instructions into...
233 Preheader = L->getLoopPreheader();
235 // Loop over the body of this loop, looking for calls, invokes, and stores.
236 // Because subloops have already been incorporated into AST, we skip blocks in
239 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
242 if (LI->getLoopFor(BB) == L) // Ignore blocks in subloops.
243 CurAST->add(*BB); // Incorporate the specified basic block
247 // TODO: We've already searched for instructions which may throw in subloops.
248 // We may want to reuse this information.
249 for (Loop::block_iterator BB = L->block_begin(), BBE = L->block_end();
250 (BB != BBE) && !MayThrow ; ++BB)
251 for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end();
252 (I != E) && !MayThrow; ++I)
253 MayThrow |= I->mayThrow();
255 // We want to visit all of the instructions in this loop... that are not parts
256 // of our subloops (they have already had their invariants hoisted out of
257 // their loop, into this loop, so there is no need to process the BODIES of
260 // Traverse the body of the loop in depth first order on the dominator tree so
261 // that we are guaranteed to see definitions before we see uses. This allows
262 // us to sink instructions in one pass, without iteration. After sinking
263 // instructions, we perform another pass to hoist them out of the loop.
265 if (L->hasDedicatedExits())
266 SinkRegion(DT->getNode(L->getHeader()));
268 HoistRegion(DT->getNode(L->getHeader()));
270 // Now that all loop invariants have been removed from the loop, promote any
271 // memory references to scalars that we can.
272 if (!DisablePromotion && Preheader && L->hasDedicatedExits()) {
273 SmallVector<BasicBlock *, 8> ExitBlocks;
274 SmallVector<Instruction *, 8> InsertPts;
276 // Loop over all of the alias sets in the tracker object.
277 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
279 PromoteAliasSet(*I, ExitBlocks, InsertPts);
282 // Clear out loops state information for the next iteration
286 // If this loop is nested inside of another one, save the alias information
287 // for when we process the outer loop.
288 if (L->getParentLoop())
289 LoopToAliasSetMap[L] = CurAST;
295 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks
296 /// dominated by the specified block, and that are in the current loop) in
297 /// reverse depth first order w.r.t the DominatorTree. This allows us to visit
298 /// uses before definitions, allowing us to sink a loop body in one pass without
301 void LICM::SinkRegion(DomTreeNode *N) {
302 assert(N != 0 && "Null dominator tree node?");
303 BasicBlock *BB = N->getBlock();
305 // If this subregion is not in the top level loop at all, exit.
306 if (!CurLoop->contains(BB)) return;
308 // We are processing blocks in reverse dfo, so process children first.
309 const std::vector<DomTreeNode*> &Children = N->getChildren();
310 for (unsigned i = 0, e = Children.size(); i != e; ++i)
311 SinkRegion(Children[i]);
313 // Only need to process the contents of this block if it is not part of a
314 // subloop (which would already have been processed).
315 if (inSubLoop(BB)) return;
317 for (BasicBlock::iterator II = BB->end(); II != BB->begin(); ) {
318 Instruction &I = *--II;
320 // If the instruction is dead, we would try to sink it because it isn't used
321 // in the loop, instead, just delete it.
322 if (isInstructionTriviallyDead(&I, TLI)) {
323 DEBUG(dbgs() << "LICM deleting dead inst: " << I << '\n');
325 CurAST->deleteValue(&I);
331 // Check to see if we can sink this instruction to the exit blocks
332 // of the loop. We can do this if the all users of the instruction are
333 // outside of the loop. In this case, it doesn't even matter if the
334 // operands of the instruction are loop invariant.
336 if (isNotUsedInLoop(I) && canSinkOrHoistInst(I)) {
343 /// HoistRegion - Walk the specified region of the CFG (defined by all blocks
344 /// dominated by the specified block, and that are in the current loop) in depth
345 /// first order w.r.t the DominatorTree. This allows us to visit definitions
346 /// before uses, allowing us to hoist a loop body in one pass without iteration.
348 void LICM::HoistRegion(DomTreeNode *N) {
349 assert(N != 0 && "Null dominator tree node?");
350 BasicBlock *BB = N->getBlock();
352 // If this subregion is not in the top level loop at all, exit.
353 if (!CurLoop->contains(BB)) return;
355 // Only need to process the contents of this block if it is not part of a
356 // subloop (which would already have been processed).
358 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ) {
359 Instruction &I = *II++;
361 // Try constant folding this instruction. If all the operands are
362 // constants, it is technically hoistable, but it would be better to just
364 if (Constant *C = ConstantFoldInstruction(&I, TD, TLI)) {
365 DEBUG(dbgs() << "LICM folding inst: " << I << " --> " << *C << '\n');
366 CurAST->copyValue(&I, C);
367 CurAST->deleteValue(&I);
368 I.replaceAllUsesWith(C);
373 // Try hoisting the instruction out to the preheader. We can only do this
374 // if all of the operands of the instruction are loop invariant and if it
375 // is safe to hoist the instruction.
377 if (CurLoop->hasLoopInvariantOperands(&I) && canSinkOrHoistInst(I) &&
378 isSafeToExecuteUnconditionally(I))
382 const std::vector<DomTreeNode*> &Children = N->getChildren();
383 for (unsigned i = 0, e = Children.size(); i != e; ++i)
384 HoistRegion(Children[i]);
387 /// canSinkOrHoistInst - Return true if the hoister and sinker can handle this
390 bool LICM::canSinkOrHoistInst(Instruction &I) {
391 // Loads have extra constraints we have to verify before we can hoist them.
392 if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
393 if (!LI->isUnordered())
394 return false; // Don't hoist volatile/atomic loads!
396 // Loads from constant memory are always safe to move, even if they end up
397 // in the same alias set as something that ends up being modified.
398 if (AA->pointsToConstantMemory(LI->getOperand(0)))
400 if (LI->getMetadata("invariant.load"))
403 // Don't hoist loads which have may-aliased stores in loop.
405 if (LI->getType()->isSized())
406 Size = AA->getTypeStoreSize(LI->getType());
407 return !pointerInvalidatedByLoop(LI->getOperand(0), Size,
408 LI->getMetadata(LLVMContext::MD_tbaa));
409 } else if (CallInst *CI = dyn_cast<CallInst>(&I)) {
410 // Don't sink or hoist dbg info; it's legal, but not useful.
411 if (isa<DbgInfoIntrinsic>(I))
414 // Handle simple cases by querying alias analysis.
415 AliasAnalysis::ModRefBehavior Behavior = AA->getModRefBehavior(CI);
416 if (Behavior == AliasAnalysis::DoesNotAccessMemory)
418 if (AliasAnalysis::onlyReadsMemory(Behavior)) {
419 // If this call only reads from memory and there are no writes to memory
420 // in the loop, we can hoist or sink the call as appropriate.
421 bool FoundMod = false;
422 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
425 if (!AS.isForwardingAliasSet() && AS.isMod()) {
430 if (!FoundMod) return true;
433 // FIXME: This should use mod/ref information to see if we can hoist or
439 // Only these instructions are hoistable/sinkable.
440 bool HoistableKind = (isa<BinaryOperator>(I) || isa<CastInst>(I) ||
441 isa<SelectInst>(I) || isa<GetElementPtrInst>(I) ||
442 isa<CmpInst>(I) || isa<InsertElementInst>(I) ||
443 isa<ExtractElementInst>(I) ||
444 isa<ShuffleVectorInst>(I));
448 return isSafeToExecuteUnconditionally(I);
451 /// isNotUsedInLoop - Return true if the only users of this instruction are
452 /// outside of the loop. If this is true, we can sink the instruction to the
453 /// exit blocks of the loop.
455 bool LICM::isNotUsedInLoop(Instruction &I) {
456 for (Value::use_iterator UI = I.use_begin(), E = I.use_end(); UI != E; ++UI) {
457 Instruction *User = cast<Instruction>(*UI);
458 if (PHINode *PN = dyn_cast<PHINode>(User)) {
459 // PHI node uses occur in predecessor blocks!
460 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
461 if (PN->getIncomingValue(i) == &I)
462 if (CurLoop->contains(PN->getIncomingBlock(i)))
464 } else if (CurLoop->contains(User)) {
472 /// sink - When an instruction is found to only be used outside of the loop,
473 /// this function moves it to the exit blocks and patches up SSA form as needed.
474 /// This method is guaranteed to remove the original instruction from its
475 /// position, and may either delete it or move it to outside of the loop.
477 void LICM::sink(Instruction &I) {
478 DEBUG(dbgs() << "LICM sinking instruction: " << I << "\n");
480 SmallVector<BasicBlock*, 8> ExitBlocks;
481 CurLoop->getUniqueExitBlocks(ExitBlocks);
483 if (isa<LoadInst>(I)) ++NumMovedLoads;
484 else if (isa<CallInst>(I)) ++NumMovedCalls;
488 // The case where there is only a single exit node of this loop is common
489 // enough that we handle it as a special (more efficient) case. It is more
490 // efficient to handle because there are no PHI nodes that need to be placed.
491 if (ExitBlocks.size() == 1) {
492 if (!DT->dominates(I.getParent(), ExitBlocks[0])) {
493 // Instruction is not used, just delete it.
494 CurAST->deleteValue(&I);
495 // If I has users in unreachable blocks, eliminate.
496 // If I is not void type then replaceAllUsesWith undef.
497 // This allows ValueHandlers and custom metadata to adjust itself.
499 I.replaceAllUsesWith(UndefValue::get(I.getType()));
502 // Move the instruction to the start of the exit block, after any PHI
504 I.moveBefore(ExitBlocks[0]->getFirstInsertionPt());
506 // This instruction is no longer in the AST for the current loop, because
507 // we just sunk it out of the loop. If we just sunk it into an outer
508 // loop, we will rediscover the operation when we process it.
509 CurAST->deleteValue(&I);
514 if (ExitBlocks.empty()) {
515 // The instruction is actually dead if there ARE NO exit blocks.
516 CurAST->deleteValue(&I);
517 // If I has users in unreachable blocks, eliminate.
518 // If I is not void type then replaceAllUsesWith undef.
519 // This allows ValueHandlers and custom metadata to adjust itself.
521 I.replaceAllUsesWith(UndefValue::get(I.getType()));
526 // Otherwise, if we have multiple exits, use the SSAUpdater to do all of the
527 // hard work of inserting PHI nodes as necessary.
528 SmallVector<PHINode*, 8> NewPHIs;
529 SSAUpdater SSA(&NewPHIs);
532 SSA.Initialize(I.getType(), I.getName());
534 // Insert a copy of the instruction in each exit block of the loop that is
535 // dominated by the instruction. Each exit block is known to only be in the
536 // ExitBlocks list once.
537 BasicBlock *InstOrigBB = I.getParent();
538 unsigned NumInserted = 0;
540 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
541 BasicBlock *ExitBlock = ExitBlocks[i];
543 if (!DT->dominates(InstOrigBB, ExitBlock))
546 // Insert the code after the last PHI node.
547 BasicBlock::iterator InsertPt = ExitBlock->getFirstInsertionPt();
549 // If this is the first exit block processed, just move the original
550 // instruction, otherwise clone the original instruction and insert
553 if (NumInserted++ == 0) {
554 I.moveBefore(InsertPt);
558 if (!I.getName().empty())
559 New->setName(I.getName()+".le");
560 ExitBlock->getInstList().insert(InsertPt, New);
563 // Now that we have inserted the instruction, inform SSAUpdater.
565 SSA.AddAvailableValue(ExitBlock, New);
568 // If the instruction doesn't dominate any exit blocks, it must be dead.
569 if (NumInserted == 0) {
570 CurAST->deleteValue(&I);
572 I.replaceAllUsesWith(UndefValue::get(I.getType()));
577 // Next, rewrite uses of the instruction, inserting PHI nodes as needed.
578 for (Value::use_iterator UI = I.use_begin(), UE = I.use_end(); UI != UE; ) {
579 // Grab the use before incrementing the iterator.
580 Use &U = UI.getUse();
581 // Increment the iterator before removing the use from the list.
583 SSA.RewriteUseAfterInsertions(U);
586 // Update CurAST for NewPHIs if I had pointer type.
587 if (I.getType()->isPointerTy())
588 for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i)
589 CurAST->copyValue(&I, NewPHIs[i]);
591 // Finally, remove the instruction from CurAST. It is no longer in the loop.
592 CurAST->deleteValue(&I);
595 /// hoist - When an instruction is found to only use loop invariant operands
596 /// that is safe to hoist, this instruction is called to do the dirty work.
598 void LICM::hoist(Instruction &I) {
599 DEBUG(dbgs() << "LICM hoisting to " << Preheader->getName() << ": "
602 // Move the new node to the Preheader, before its terminator.
603 I.moveBefore(Preheader->getTerminator());
605 if (isa<LoadInst>(I)) ++NumMovedLoads;
606 else if (isa<CallInst>(I)) ++NumMovedCalls;
611 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it is
612 /// not a trapping instruction or if it is a trapping instruction and is
613 /// guaranteed to execute.
615 bool LICM::isSafeToExecuteUnconditionally(Instruction &Inst) {
616 // If it is not a trapping instruction, it is always safe to hoist.
617 if (isSafeToSpeculativelyExecute(&Inst))
620 return isGuaranteedToExecute(Inst);
623 bool LICM::isGuaranteedToExecute(Instruction &Inst) {
625 // Somewhere in this loop there is an instruction which may throw and make us
630 // Otherwise we have to check to make sure that the instruction dominates all
631 // of the exit blocks. If it doesn't, then there is a path out of the loop
632 // which does not execute this instruction, so we can't hoist it.
634 // If the instruction is in the header block for the loop (which is very
635 // common), it is always guaranteed to dominate the exit blocks. Since this
636 // is a common case, and can save some work, check it now.
637 if (Inst.getParent() == CurLoop->getHeader())
640 // Get the exit blocks for the current loop.
641 SmallVector<BasicBlock*, 8> ExitBlocks;
642 CurLoop->getExitBlocks(ExitBlocks);
644 // Verify that the block dominates each of the exit blocks of the loop.
645 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
646 if (!DT->dominates(Inst.getParent(), ExitBlocks[i]))
649 // As a degenerate case, if the loop is statically infinite then we haven't
650 // proven anything since there are no exit blocks.
651 if (ExitBlocks.empty())
658 class LoopPromoter : public LoadAndStorePromoter {
659 Value *SomePtr; // Designated pointer to store to.
660 SmallPtrSet<Value*, 4> &PointerMustAliases;
661 SmallVectorImpl<BasicBlock*> &LoopExitBlocks;
662 SmallVectorImpl<Instruction*> &LoopInsertPts;
663 AliasSetTracker &AST;
667 LoopPromoter(Value *SP,
668 const SmallVectorImpl<Instruction*> &Insts, SSAUpdater &S,
669 SmallPtrSet<Value*, 4> &PMA,
670 SmallVectorImpl<BasicBlock*> &LEB,
671 SmallVectorImpl<Instruction*> &LIP,
672 AliasSetTracker &ast, DebugLoc dl, int alignment)
673 : LoadAndStorePromoter(Insts, S), SomePtr(SP),
674 PointerMustAliases(PMA), LoopExitBlocks(LEB), LoopInsertPts(LIP),
675 AST(ast), DL(dl), Alignment(alignment) {}
677 virtual bool isInstInList(Instruction *I,
678 const SmallVectorImpl<Instruction*> &) const {
680 if (LoadInst *LI = dyn_cast<LoadInst>(I))
681 Ptr = LI->getOperand(0);
683 Ptr = cast<StoreInst>(I)->getPointerOperand();
684 return PointerMustAliases.count(Ptr);
687 virtual void doExtraRewritesBeforeFinalDeletion() const {
688 // Insert stores after in the loop exit blocks. Each exit block gets a
689 // store of the live-out values that feed them. Since we've already told
690 // the SSA updater about the defs in the loop and the preheader
691 // definition, it is all set and we can start using it.
692 for (unsigned i = 0, e = LoopExitBlocks.size(); i != e; ++i) {
693 BasicBlock *ExitBlock = LoopExitBlocks[i];
694 Value *LiveInValue = SSA.GetValueInMiddleOfBlock(ExitBlock);
695 Instruction *InsertPos = LoopInsertPts[i];
696 StoreInst *NewSI = new StoreInst(LiveInValue, SomePtr, InsertPos);
697 NewSI->setAlignment(Alignment);
698 NewSI->setDebugLoc(DL);
702 virtual void replaceLoadWithValue(LoadInst *LI, Value *V) const {
703 // Update alias analysis.
704 AST.copyValue(LI, V);
706 virtual void instructionDeleted(Instruction *I) const {
710 } // end anon namespace
712 /// PromoteAliasSet - Try to promote memory values to scalars by sinking
713 /// stores out of the loop and moving loads to before the loop. We do this by
714 /// looping over the stores in the loop, looking for stores to Must pointers
715 /// which are loop invariant.
717 void LICM::PromoteAliasSet(AliasSet &AS,
718 SmallVectorImpl<BasicBlock*> &ExitBlocks,
719 SmallVectorImpl<Instruction*> &InsertPts) {
720 // We can promote this alias set if it has a store, if it is a "Must" alias
721 // set, if the pointer is loop invariant, and if we are not eliminating any
722 // volatile loads or stores.
723 if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() ||
724 AS.isVolatile() || !CurLoop->isLoopInvariant(AS.begin()->getValue()))
727 assert(!AS.empty() &&
728 "Must alias set should have at least one pointer element in it!");
729 Value *SomePtr = AS.begin()->getValue();
731 // It isn't safe to promote a load/store from the loop if the load/store is
732 // conditional. For example, turning:
734 // for () { if (c) *P += 1; }
738 // tmp = *P; for () { if (c) tmp +=1; } *P = tmp;
740 // is not safe, because *P may only be valid to access if 'c' is true.
742 // It is safe to promote P if all uses are direct load/stores and if at
743 // least one is guaranteed to be executed.
744 bool GuaranteedToExecute = false;
746 SmallVector<Instruction*, 64> LoopUses;
747 SmallPtrSet<Value*, 4> PointerMustAliases;
749 // We start with an alignment of one and try to find instructions that allow
750 // us to prove better alignment.
751 unsigned Alignment = 1;
753 // Check that all of the pointers in the alias set have the same type. We
754 // cannot (yet) promote a memory location that is loaded and stored in
756 for (AliasSet::iterator ASI = AS.begin(), E = AS.end(); ASI != E; ++ASI) {
757 Value *ASIV = ASI->getValue();
758 PointerMustAliases.insert(ASIV);
760 // Check that all of the pointers in the alias set have the same type. We
761 // cannot (yet) promote a memory location that is loaded and stored in
763 if (SomePtr->getType() != ASIV->getType())
766 for (Value::use_iterator UI = ASIV->use_begin(), UE = ASIV->use_end();
768 // Ignore instructions that are outside the loop.
769 Instruction *Use = dyn_cast<Instruction>(*UI);
770 if (!Use || !CurLoop->contains(Use))
773 // If there is an non-load/store instruction in the loop, we can't promote
775 if (LoadInst *load = dyn_cast<LoadInst>(Use)) {
776 assert(!load->isVolatile() && "AST broken");
777 if (!load->isSimple())
779 } else if (StoreInst *store = dyn_cast<StoreInst>(Use)) {
780 // Stores *of* the pointer are not interesting, only stores *to* the
782 if (Use->getOperand(1) != ASIV)
784 assert(!store->isVolatile() && "AST broken");
785 if (!store->isSimple())
788 // Note that we only check GuaranteedToExecute inside the store case
789 // so that we do not introduce stores where they did not exist before
790 // (which would break the LLVM concurrency model).
792 // If the alignment of this instruction allows us to specify a more
793 // restrictive (and performant) alignment and if we are sure this
794 // instruction will be executed, update the alignment.
795 // Larger is better, with the exception of 0 being the best alignment.
796 unsigned InstAlignment = store->getAlignment();
797 if ((InstAlignment > Alignment || InstAlignment == 0)
799 if (isGuaranteedToExecute(*Use)) {
800 GuaranteedToExecute = true;
801 Alignment = InstAlignment;
804 if (!GuaranteedToExecute)
805 GuaranteedToExecute = isGuaranteedToExecute(*Use);
808 return; // Not a load or store.
810 LoopUses.push_back(Use);
814 // If there isn't a guaranteed-to-execute instruction, we can't promote.
815 if (!GuaranteedToExecute)
818 // Otherwise, this is safe to promote, lets do it!
819 DEBUG(dbgs() << "LICM: Promoting value stored to in loop: " <<*SomePtr<<'\n');
823 // Grab a debug location for the inserted loads/stores; given that the
824 // inserted loads/stores have little relation to the original loads/stores,
825 // this code just arbitrarily picks a location from one, since any debug
826 // location is better than none.
827 DebugLoc DL = LoopUses[0]->getDebugLoc();
829 // Figure out the loop exits and their insertion points, if this is the
831 if (ExitBlocks.empty()) {
832 CurLoop->getUniqueExitBlocks(ExitBlocks);
833 InsertPts.resize(ExitBlocks.size());
834 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
835 InsertPts[i] = ExitBlocks[i]->getFirstInsertionPt();
838 // We use the SSAUpdater interface to insert phi nodes as required.
839 SmallVector<PHINode*, 16> NewPHIs;
840 SSAUpdater SSA(&NewPHIs);
841 LoopPromoter Promoter(SomePtr, LoopUses, SSA, PointerMustAliases, ExitBlocks,
842 InsertPts, *CurAST, DL, Alignment);
844 // Set up the preheader to have a definition of the value. It is the live-out
845 // value from the preheader that uses in the loop will use.
846 LoadInst *PreheaderLoad =
847 new LoadInst(SomePtr, SomePtr->getName()+".promoted",
848 Preheader->getTerminator());
849 PreheaderLoad->setAlignment(Alignment);
850 PreheaderLoad->setDebugLoc(DL);
851 SSA.AddAvailableValue(Preheader, PreheaderLoad);
853 // Rewrite all the loads in the loop and remember all the definitions from
854 // stores in the loop.
855 Promoter.run(LoopUses);
857 // If the SSAUpdater didn't use the load in the preheader, just zap it now.
858 if (PreheaderLoad->use_empty())
859 PreheaderLoad->eraseFromParent();
863 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
864 void LICM::cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L) {
865 AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);
869 AST->copyValue(From, To);
872 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
874 void LICM::deleteAnalysisValue(Value *V, Loop *L) {
875 AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);