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 #include "llvm/Transforms/Scalar.h"
34 #include "llvm/ADT/Statistic.h"
35 #include "llvm/Analysis/AliasAnalysis.h"
36 #include "llvm/Analysis/AliasSetTracker.h"
37 #include "llvm/Analysis/ConstantFolding.h"
38 #include "llvm/Analysis/LoopInfo.h"
39 #include "llvm/Analysis/LoopPass.h"
40 #include "llvm/Analysis/ScalarEvolution.h"
41 #include "llvm/Analysis/ValueTracking.h"
42 #include "llvm/IR/CFG.h"
43 #include "llvm/IR/Constants.h"
44 #include "llvm/IR/DataLayout.h"
45 #include "llvm/IR/DerivedTypes.h"
46 #include "llvm/IR/Dominators.h"
47 #include "llvm/IR/Instructions.h"
48 #include "llvm/IR/IntrinsicInst.h"
49 #include "llvm/IR/LLVMContext.h"
50 #include "llvm/IR/Metadata.h"
51 #include "llvm/IR/PredIteratorCache.h"
52 #include "llvm/Support/CommandLine.h"
53 #include "llvm/Support/Debug.h"
54 #include "llvm/Support/raw_ostream.h"
55 #include "llvm/Analysis/TargetLibraryInfo.h"
56 #include "llvm/Transforms/Utils/Local.h"
57 #include "llvm/Transforms/Utils/LoopUtils.h"
58 #include "llvm/Transforms/Utils/SSAUpdater.h"
62 #define DEBUG_TYPE "licm"
64 STATISTIC(NumSunk , "Number of instructions sunk out of loop");
65 STATISTIC(NumHoisted , "Number of instructions hoisted out of loop");
66 STATISTIC(NumMovedLoads, "Number of load insts hoisted or sunk");
67 STATISTIC(NumMovedCalls, "Number of call insts hoisted or sunk");
68 STATISTIC(NumPromoted , "Number of memory locations promoted to registers");
71 DisablePromotion("disable-licm-promotion", cl::Hidden,
72 cl::desc("Disable memory promotion in LICM pass"));
74 static bool inSubLoop(BasicBlock *BB, Loop *CurLoop, LoopInfo *LI);
75 static bool isNotUsedInLoop(Instruction &I, Loop *CurLoop);
76 static bool hoist(Instruction &I, BasicBlock *Preheader);
77 static bool sink(Instruction &I, LoopInfo *LI, DominatorTree *DT,
78 Loop *CurLoop, AliasSetTracker *CurAST );
79 static bool isGuaranteedToExecute(Instruction &Inst, DominatorTree *DT,
80 Loop *CurLoop, LICMSafetyInfo *SafetyInfo);
81 static bool isSafeToExecuteUnconditionally(Instruction &Inst, DominatorTree *DT,
83 LICMSafetyInfo *SafetyInfo);
84 static bool pointerInvalidatedByLoop(Value *V, uint64_t Size,
85 const AAMDNodes &AAInfo,
86 AliasSetTracker *CurAST);
87 static Instruction *CloneInstructionInExitBlock(Instruction &I,
88 BasicBlock &ExitBlock,
89 PHINode &PN, LoopInfo *LI);
90 static bool canSinkOrHoistInst(Instruction &I, AliasAnalysis *AA,
91 DominatorTree *DT, Loop *CurLoop,
92 AliasSetTracker *CurAST,
93 LICMSafetyInfo *SafetyInfo);
96 struct LICM : public LoopPass {
97 static char ID; // Pass identification, replacement for typeid
98 LICM() : LoopPass(ID) {
99 initializeLICMPass(*PassRegistry::getPassRegistry());
102 bool runOnLoop(Loop *L, LPPassManager &LPM) override;
104 /// This transformation requires natural loop information & requires that
105 /// loop preheaders be inserted into the CFG...
107 void getAnalysisUsage(AnalysisUsage &AU) const override {
108 AU.setPreservesCFG();
109 AU.addRequired<DominatorTreeWrapperPass>();
110 AU.addRequired<LoopInfoWrapperPass>();
111 AU.addRequiredID(LoopSimplifyID);
112 AU.addPreservedID(LoopSimplifyID);
113 AU.addRequiredID(LCSSAID);
114 AU.addPreservedID(LCSSAID);
115 AU.addRequired<AliasAnalysis>();
116 AU.addPreserved<AliasAnalysis>();
117 AU.addPreserved<ScalarEvolution>();
118 AU.addRequired<TargetLibraryInfoWrapperPass>();
121 using llvm::Pass::doFinalization;
123 bool doFinalization() override {
124 assert(LoopToAliasSetMap.empty() && "Didn't free loop alias sets");
129 AliasAnalysis *AA; // Current AliasAnalysis information
130 LoopInfo *LI; // Current LoopInfo
131 DominatorTree *DT; // Dominator Tree for the current Loop.
133 TargetLibraryInfo *TLI; // TargetLibraryInfo for constant folding.
135 // State that is updated as we process loops.
136 bool Changed; // Set to true when we change anything.
137 BasicBlock *Preheader; // The preheader block of the current loop...
138 Loop *CurLoop; // The current loop we are working on...
139 AliasSetTracker *CurAST; // AliasSet information for the current loop...
140 DenseMap<Loop*, AliasSetTracker*> LoopToAliasSetMap;
142 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
143 void cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To,
146 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
148 void deleteAnalysisValue(Value *V, Loop *L) override;
150 /// Simple Analysis hook. Delete loop L from alias set map.
151 void deleteAnalysisLoop(Loop *L) override;
156 INITIALIZE_PASS_BEGIN(LICM, "licm", "Loop Invariant Code Motion", false, false)
157 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
158 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
159 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
160 INITIALIZE_PASS_DEPENDENCY(LCSSA)
161 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
162 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
163 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
164 INITIALIZE_PASS_END(LICM, "licm", "Loop Invariant Code Motion", false, false)
166 Pass *llvm::createLICMPass() { return new LICM(); }
168 /// Hoist expressions out of the specified loop. Note, alias info for inner
169 /// loop is not preserved so it is not a good idea to run LICM multiple
170 /// times on one loop.
172 bool LICM::runOnLoop(Loop *L, LPPassManager &LPM) {
173 if (skipOptnoneFunction(L))
178 // Get our Loop and Alias Analysis information...
179 LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
180 AA = &getAnalysis<AliasAnalysis>();
181 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
183 TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
185 assert(L->isLCSSAForm(*DT) && "Loop is not in LCSSA form.");
187 CurAST = new AliasSetTracker(*AA);
188 // Collect Alias info from subloops.
189 for (Loop::iterator LoopItr = L->begin(), LoopItrE = L->end();
190 LoopItr != LoopItrE; ++LoopItr) {
191 Loop *InnerL = *LoopItr;
192 AliasSetTracker *InnerAST = LoopToAliasSetMap[InnerL];
193 assert(InnerAST && "Where is my AST?");
195 // What if InnerLoop was modified by other passes ?
196 CurAST->add(*InnerAST);
198 // Once we've incorporated the inner loop's AST into ours, we don't need the
199 // subloop's anymore.
201 LoopToAliasSetMap.erase(InnerL);
206 // Get the preheader block to move instructions into...
207 Preheader = L->getLoopPreheader();
209 // Loop over the body of this loop, looking for calls, invokes, and stores.
210 // Because subloops have already been incorporated into AST, we skip blocks in
213 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
216 if (LI->getLoopFor(BB) == L) // Ignore blocks in subloops.
217 CurAST->add(*BB); // Incorporate the specified basic block
220 // Compute loop safety information.
221 LICMSafetyInfo SafetyInfo;
222 computeLICMSafetyInfo(&SafetyInfo, CurLoop);
224 // We want to visit all of the instructions in this loop... that are not parts
225 // of our subloops (they have already had their invariants hoisted out of
226 // their loop, into this loop, so there is no need to process the BODIES of
229 // Traverse the body of the loop in depth first order on the dominator tree so
230 // that we are guaranteed to see definitions before we see uses. This allows
231 // us to sink instructions in one pass, without iteration. After sinking
232 // instructions, we perform another pass to hoist them out of the loop.
234 if (L->hasDedicatedExits())
235 Changed |= sinkRegion(DT->getNode(L->getHeader()), AA, LI, DT, TLI, CurLoop,
236 CurAST, &SafetyInfo);
238 Changed |= hoistRegion(DT->getNode(L->getHeader()), AA, LI, DT, TLI,
239 CurLoop, CurAST, &SafetyInfo);
241 // Now that all loop invariants have been removed from the loop, promote any
242 // memory references to scalars that we can.
243 if (!DisablePromotion && (Preheader || L->hasDedicatedExits())) {
244 SmallVector<BasicBlock *, 8> ExitBlocks;
245 SmallVector<Instruction *, 8> InsertPts;
246 PredIteratorCache PIC;
248 // Loop over all of the alias sets in the tracker object.
249 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
251 Changed |= promoteLoopAccessesToScalars(*I, ExitBlocks, InsertPts,
252 PIC, LI, DT, CurLoop,
253 CurAST, &SafetyInfo);
255 // Once we have promoted values across the loop body we have to recursively
256 // reform LCSSA as any nested loop may now have values defined within the
257 // loop used in the outer loop.
258 // FIXME: This is really heavy handed. It would be a bit better to use an
259 // SSAUpdater strategy during promotion that was LCSSA aware and reformed
262 formLCSSARecursively(*L, *DT, LI,
263 getAnalysisIfAvailable<ScalarEvolution>());
266 // Check that neither this loop nor its parent have had LCSSA broken. LICM is
267 // specifically moving instructions across the loop boundary and so it is
268 // especially in need of sanity checking here.
269 assert(L->isLCSSAForm(*DT) && "Loop not left in LCSSA form after LICM!");
270 assert((!L->getParentLoop() || L->getParentLoop()->isLCSSAForm(*DT)) &&
271 "Parent loop not left in LCSSA form after LICM!");
273 // Clear out loops state information for the next iteration
277 // If this loop is nested inside of another one, save the alias information
278 // for when we process the outer loop.
279 if (L->getParentLoop())
280 LoopToAliasSetMap[L] = CurAST;
286 /// Walk the specified region of the CFG (defined by all blocks dominated by
287 /// the specified block, and that are in the current loop) in reverse depth
288 /// first order w.r.t the DominatorTree. This allows us to visit uses before
289 /// definitions, allowing us to sink a loop body in one pass without iteration.
291 bool llvm::sinkRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
292 DominatorTree *DT, TargetLibraryInfo *TLI, Loop *CurLoop,
293 AliasSetTracker *CurAST, LICMSafetyInfo *SafetyInfo) {
296 assert(N != nullptr && AA != nullptr && LI != nullptr &&
297 DT != nullptr && CurLoop != nullptr && CurAST != nullptr &&
298 SafetyInfo != nullptr && "Unexpected input to sinkRegion");
300 // Set changed as false.
301 bool Changed = false;
303 BasicBlock *BB = N->getBlock();
304 // If this subregion is not in the top level loop at all, exit.
305 if (!CurLoop->contains(BB)) return Changed;
307 // We are processing blocks in reverse dfo, so process children first.
308 const std::vector<DomTreeNode*> &Children = N->getChildren();
309 for (unsigned i = 0, e = Children.size(); i != e; ++i)
311 sinkRegion(Children[i], AA, LI, DT, TLI, CurLoop, CurAST, SafetyInfo);
312 // Only need to process the contents of this block if it is not part of a
313 // subloop (which would already have been processed).
314 if (inSubLoop(BB,CurLoop,LI)) return Changed;
316 for (BasicBlock::iterator II = BB->end(); II != BB->begin(); ) {
317 Instruction &I = *--II;
319 // If the instruction is dead, we would try to sink it because it isn't used
320 // in the loop, instead, just delete it.
321 if (isInstructionTriviallyDead(&I, TLI)) {
322 DEBUG(dbgs() << "LICM deleting dead inst: " << I << '\n');
324 CurAST->deleteValue(&I);
330 // Check to see if we can sink this instruction to the exit blocks
331 // of the loop. We can do this if the all users of the instruction are
332 // outside of the loop. In this case, it doesn't even matter if the
333 // operands of the instruction are loop invariant.
335 if (isNotUsedInLoop(I, CurLoop) &&
336 canSinkOrHoistInst(I, AA, DT, CurLoop, CurAST, SafetyInfo)) {
338 Changed |= sink(I, LI, DT, CurLoop, CurAST);
344 /// Walk the specified region of the CFG (defined by all blocks dominated by
345 /// the specified block, and that are in the current loop) in depth first
346 /// order w.r.t the DominatorTree. This allows us to visit definitions before
347 /// uses, allowing us to hoist a loop body in one pass without iteration.
349 bool llvm::hoistRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
350 DominatorTree *DT, TargetLibraryInfo *TLI, Loop *CurLoop,
351 AliasSetTracker *CurAST, LICMSafetyInfo *SafetyInfo) {
353 assert(N != nullptr && AA != nullptr && LI != nullptr &&
354 DT != nullptr && CurLoop != nullptr && CurAST != nullptr &&
355 SafetyInfo != nullptr && "Unexpected input to hoistRegion");
356 // Set changed as false.
357 bool Changed = false;
359 BasicBlock *BB = N->getBlock();
360 // If this subregion is not in the top level loop at all, exit.
361 if (!CurLoop->contains(BB)) return Changed;
362 // Only need to process the contents of this block if it is not part of a
363 // subloop (which would already have been processed).
364 if (!inSubLoop(BB, CurLoop, LI))
365 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ) {
366 Instruction &I = *II++;
367 // Try constant folding this instruction. If all the operands are
368 // constants, it is technically hoistable, but it would be better to just
370 if (Constant *C = ConstantFoldInstruction(
371 &I, I.getModule()->getDataLayout(), TLI)) {
372 DEBUG(dbgs() << "LICM folding inst: " << I << " --> " << *C << '\n');
373 CurAST->copyValue(&I, C);
374 CurAST->deleteValue(&I);
375 I.replaceAllUsesWith(C);
380 // Try hoisting the instruction out to the preheader. We can only do this
381 // if all of the operands of the instruction are loop invariant and if it
382 // is safe to hoist the instruction.
384 if (CurLoop->hasLoopInvariantOperands(&I) &&
385 canSinkOrHoistInst(I, AA, DT, CurLoop, CurAST, SafetyInfo) &&
386 isSafeToExecuteUnconditionally(I, DT, CurLoop, SafetyInfo))
387 Changed |= hoist(I, CurLoop->getLoopPreheader());
390 const std::vector<DomTreeNode*> &Children = N->getChildren();
391 for (unsigned i = 0, e = Children.size(); i != e; ++i)
393 hoistRegion(Children[i], AA, LI, DT, TLI, CurLoop, CurAST, SafetyInfo);
397 /// Computes loop safety information, checks loop body & header
398 /// for the possiblity of may throw exception.
400 void llvm::computeLICMSafetyInfo(LICMSafetyInfo * SafetyInfo, Loop * CurLoop) {
401 assert(CurLoop != nullptr && "CurLoop cant be null");
402 BasicBlock *Header = CurLoop->getHeader();
403 // Setting default safety values.
404 SafetyInfo->MayThrow = false;
405 SafetyInfo->HeaderMayThrow = false;
406 // Iterate over header and compute dafety info.
407 for (BasicBlock::iterator I = Header->begin(), E = Header->end();
408 (I != E) && !SafetyInfo->HeaderMayThrow; ++I)
409 SafetyInfo->HeaderMayThrow |= I->mayThrow();
411 SafetyInfo->MayThrow = SafetyInfo->HeaderMayThrow;
412 // Iterate over loop instructions and compute safety info.
413 for (Loop::block_iterator BB = CurLoop->block_begin(),
414 BBE = CurLoop->block_end(); (BB != BBE) && !SafetyInfo->MayThrow ; ++BB)
415 for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end();
416 (I != E) && !SafetyInfo->MayThrow; ++I)
417 SafetyInfo->MayThrow |= I->mayThrow();
420 /// canSinkOrHoistInst - Return true if the hoister and sinker can handle this
423 bool canSinkOrHoistInst(Instruction &I, AliasAnalysis *AA, DominatorTree *DT,
424 Loop *CurLoop, AliasSetTracker *CurAST,
425 LICMSafetyInfo *SafetyInfo) {
426 // Loads have extra constraints we have to verify before we can hoist them.
427 if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
428 if (!LI->isUnordered())
429 return false; // Don't hoist volatile/atomic loads!
431 // Loads from constant memory are always safe to move, even if they end up
432 // in the same alias set as something that ends up being modified.
433 if (AA->pointsToConstantMemory(LI->getOperand(0)))
435 if (LI->getMetadata(LLVMContext::MD_invariant_load))
438 // Don't hoist loads which have may-aliased stores in loop.
440 if (LI->getType()->isSized())
441 Size = AA->getTypeStoreSize(LI->getType());
444 LI->getAAMetadata(AAInfo);
446 return !pointerInvalidatedByLoop(LI->getOperand(0), Size, AAInfo, CurAST);
447 } else if (CallInst *CI = dyn_cast<CallInst>(&I)) {
448 // Don't sink or hoist dbg info; it's legal, but not useful.
449 if (isa<DbgInfoIntrinsic>(I))
452 // Handle simple cases by querying alias analysis.
453 AliasAnalysis::ModRefBehavior Behavior = AA->getModRefBehavior(CI);
454 if (Behavior == AliasAnalysis::DoesNotAccessMemory)
456 if (AliasAnalysis::onlyReadsMemory(Behavior)) {
457 // If this call only reads from memory and there are no writes to memory
458 // in the loop, we can hoist or sink the call as appropriate.
459 bool FoundMod = false;
460 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
463 if (!AS.isForwardingAliasSet() && AS.isMod()) {
468 if (!FoundMod) return true;
471 // FIXME: This should use mod/ref information to see if we can hoist or
477 // Only these instructions are hoistable/sinkable.
478 if (!isa<BinaryOperator>(I) && !isa<CastInst>(I) && !isa<SelectInst>(I) &&
479 !isa<GetElementPtrInst>(I) && !isa<CmpInst>(I) &&
480 !isa<InsertElementInst>(I) && !isa<ExtractElementInst>(I) &&
481 !isa<ShuffleVectorInst>(I) && !isa<ExtractValueInst>(I) &&
482 !isa<InsertValueInst>(I))
485 return isSafeToExecuteUnconditionally(I, DT, CurLoop, SafetyInfo);
488 /// Returns true if a PHINode is a trivially replaceable with an
490 /// This is true when all incoming values are that instruction.
491 /// This pattern occurs most often with LCSSA PHI nodes.
493 static bool isTriviallyReplacablePHI(PHINode &PN, Instruction &I) {
494 for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i)
495 if (PN.getIncomingValue(i) != &I)
501 /// Return true if the only users of this instruction are outside of
502 /// the loop. If this is true, we can sink the instruction to the exit
503 /// blocks of the loop.
505 static bool isNotUsedInLoop(Instruction &I, Loop *CurLoop) {
506 for (User *U : I.users()) {
507 Instruction *UI = cast<Instruction>(U);
508 if (PHINode *PN = dyn_cast<PHINode>(UI)) {
509 // A PHI node where all of the incoming values are this instruction are
510 // special -- they can just be RAUW'ed with the instruction and thus
511 // don't require a use in the predecessor. This is a particular important
512 // special case because it is the pattern found in LCSSA form.
513 if (isTriviallyReplacablePHI(*PN, I)) {
514 if (CurLoop->contains(PN))
520 // Otherwise, PHI node uses occur in predecessor blocks if the incoming
521 // values. Check for such a use being inside the loop.
522 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
523 if (PN->getIncomingValue(i) == &I)
524 if (CurLoop->contains(PN->getIncomingBlock(i)))
530 if (CurLoop->contains(UI))
536 static Instruction *CloneInstructionInExitBlock(Instruction &I,
537 BasicBlock &ExitBlock,
538 PHINode &PN, LoopInfo *LI) {
539 Instruction *New = I.clone();
540 ExitBlock.getInstList().insert(ExitBlock.getFirstInsertionPt(), New);
541 if (!I.getName().empty()) New->setName(I.getName() + ".le");
543 // Build LCSSA PHI nodes for any in-loop operands. Note that this is
544 // particularly cheap because we can rip off the PHI node that we're
545 // replacing for the number and blocks of the predecessors.
546 // OPT: If this shows up in a profile, we can instead finish sinking all
547 // invariant instructions, and then walk their operands to re-establish
548 // LCSSA. That will eliminate creating PHI nodes just to nuke them when
549 // sinking bottom-up.
550 for (User::op_iterator OI = New->op_begin(), OE = New->op_end(); OI != OE;
552 if (Instruction *OInst = dyn_cast<Instruction>(*OI))
553 if (Loop *OLoop = LI->getLoopFor(OInst->getParent()))
554 if (!OLoop->contains(&PN)) {
556 PHINode::Create(OInst->getType(), PN.getNumIncomingValues(),
557 OInst->getName() + ".lcssa", ExitBlock.begin());
558 for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i)
559 OpPN->addIncoming(OInst, PN.getIncomingBlock(i));
565 /// When an instruction is found to only be used outside of the loop, this
566 /// function moves it to the exit blocks and patches up SSA form as needed.
567 /// This method is guaranteed to remove the original instruction from its
568 /// position, and may either delete it or move it to outside of the loop.
570 static bool sink(Instruction &I, LoopInfo *LI, DominatorTree *DT,
571 Loop *CurLoop, AliasSetTracker *CurAST ) {
572 DEBUG(dbgs() << "LICM sinking instruction: " << I << "\n");
573 bool Changed = false;
574 if (isa<LoadInst>(I)) ++NumMovedLoads;
575 else if (isa<CallInst>(I)) ++NumMovedCalls;
580 SmallVector<BasicBlock *, 32> ExitBlocks;
581 CurLoop->getUniqueExitBlocks(ExitBlocks);
582 SmallPtrSet<BasicBlock *, 32> ExitBlockSet(ExitBlocks.begin(),
586 // Clones of this instruction. Don't create more than one per exit block!
587 SmallDenseMap<BasicBlock *, Instruction *, 32> SunkCopies;
589 // If this instruction is only used outside of the loop, then all users are
590 // PHI nodes in exit blocks due to LCSSA form. Just RAUW them with clones of
592 while (!I.use_empty()) {
593 Instruction *User = I.user_back();
594 if (!DT->isReachableFromEntry(User->getParent())) {
595 User->replaceUsesOfWith(&I, UndefValue::get(I.getType()));
598 // The user must be a PHI node.
599 PHINode *PN = cast<PHINode>(User);
601 BasicBlock *ExitBlock = PN->getParent();
602 assert(ExitBlockSet.count(ExitBlock) &&
603 "The LCSSA PHI is not in an exit block!");
606 auto It = SunkCopies.find(ExitBlock);
607 if (It != SunkCopies.end())
610 New = SunkCopies[ExitBlock] =
611 CloneInstructionInExitBlock(I, *ExitBlock, *PN, LI);
613 PN->replaceAllUsesWith(New);
614 PN->eraseFromParent();
617 CurAST->deleteValue(&I);
622 /// When an instruction is found to only use loop invariant operands that
623 /// is safe to hoist, this instruction is called to do the dirty work.
625 static bool hoist(Instruction &I, BasicBlock *Preheader) {
626 DEBUG(dbgs() << "LICM hoisting to " << Preheader->getName() << ": "
628 // Move the new node to the Preheader, before its terminator.
629 I.moveBefore(Preheader->getTerminator());
631 if (isa<LoadInst>(I)) ++NumMovedLoads;
632 else if (isa<CallInst>(I)) ++NumMovedCalls;
637 /// Only sink or hoist an instruction if it is not a trapping instruction
638 /// or if it is a trapping instruction and is guaranteed to execute.
640 static bool isSafeToExecuteUnconditionally(Instruction &Inst, DominatorTree *DT,
642 LICMSafetyInfo *SafetyInfo) {
643 // If it is not a trapping instruction, it is always safe to hoist.
644 if (isSafeToSpeculativelyExecute(&Inst))
647 return isGuaranteedToExecute(Inst, DT, CurLoop, SafetyInfo);
650 static bool isGuaranteedToExecute(Instruction &Inst, DominatorTree *DT,
651 Loop *CurLoop, LICMSafetyInfo * SafetyInfo) {
653 // We have to check to make sure that the instruction dominates all
654 // of the exit blocks. If it doesn't, then there is a path out of the loop
655 // which does not execute this instruction, so we can't hoist it.
657 // If the instruction is in the header block for the loop (which is very
658 // common), it is always guaranteed to dominate the exit blocks. Since this
659 // is a common case, and can save some work, check it now.
660 if (Inst.getParent() == CurLoop->getHeader())
661 // If there's a throw in the header block, we can't guarantee we'll reach
663 return !SafetyInfo->HeaderMayThrow;
665 // Somewhere in this loop there is an instruction which may throw and make us
667 if (SafetyInfo->MayThrow)
670 // Get the exit blocks for the current loop.
671 SmallVector<BasicBlock*, 8> ExitBlocks;
672 CurLoop->getExitBlocks(ExitBlocks);
674 // Verify that the block dominates each of the exit blocks of the loop.
675 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
676 if (!DT->dominates(Inst.getParent(), ExitBlocks[i]))
679 // As a degenerate case, if the loop is statically infinite then we haven't
680 // proven anything since there are no exit blocks.
681 if (ExitBlocks.empty())
688 class LoopPromoter : public LoadAndStorePromoter {
689 Value *SomePtr; // Designated pointer to store to.
690 SmallPtrSetImpl<Value*> &PointerMustAliases;
691 SmallVectorImpl<BasicBlock*> &LoopExitBlocks;
692 SmallVectorImpl<Instruction*> &LoopInsertPts;
693 PredIteratorCache &PredCache;
694 AliasSetTracker &AST;
700 Value *maybeInsertLCSSAPHI(Value *V, BasicBlock *BB) const {
701 if (Instruction *I = dyn_cast<Instruction>(V))
702 if (Loop *L = LI.getLoopFor(I->getParent()))
703 if (!L->contains(BB)) {
704 // We need to create an LCSSA PHI node for the incoming value and
706 PHINode *PN = PHINode::Create(
707 I->getType(), PredCache.GetNumPreds(BB),
708 I->getName() + ".lcssa", BB->begin());
709 for (BasicBlock **PI = PredCache.GetPreds(BB); *PI; ++PI)
710 PN->addIncoming(I, *PI);
717 LoopPromoter(Value *SP, const SmallVectorImpl<Instruction *> &Insts,
718 SSAUpdater &S, SmallPtrSetImpl<Value *> &PMA,
719 SmallVectorImpl<BasicBlock *> &LEB,
720 SmallVectorImpl<Instruction *> &LIP, PredIteratorCache &PIC,
721 AliasSetTracker &ast, LoopInfo &li, DebugLoc dl, int alignment,
722 const AAMDNodes &AATags)
723 : LoadAndStorePromoter(Insts, S), SomePtr(SP), PointerMustAliases(PMA),
724 LoopExitBlocks(LEB), LoopInsertPts(LIP), PredCache(PIC), AST(ast),
725 LI(li), DL(dl), Alignment(alignment), AATags(AATags) {}
727 bool isInstInList(Instruction *I,
728 const SmallVectorImpl<Instruction*> &) const override {
730 if (LoadInst *LI = dyn_cast<LoadInst>(I))
731 Ptr = LI->getOperand(0);
733 Ptr = cast<StoreInst>(I)->getPointerOperand();
734 return PointerMustAliases.count(Ptr);
737 void doExtraRewritesBeforeFinalDeletion() const override {
738 // Insert stores after in the loop exit blocks. Each exit block gets a
739 // store of the live-out values that feed them. Since we've already told
740 // the SSA updater about the defs in the loop and the preheader
741 // definition, it is all set and we can start using it.
742 for (unsigned i = 0, e = LoopExitBlocks.size(); i != e; ++i) {
743 BasicBlock *ExitBlock = LoopExitBlocks[i];
744 Value *LiveInValue = SSA.GetValueInMiddleOfBlock(ExitBlock);
745 LiveInValue = maybeInsertLCSSAPHI(LiveInValue, ExitBlock);
746 Value *Ptr = maybeInsertLCSSAPHI(SomePtr, ExitBlock);
747 Instruction *InsertPos = LoopInsertPts[i];
748 StoreInst *NewSI = new StoreInst(LiveInValue, Ptr, InsertPos);
749 NewSI->setAlignment(Alignment);
750 NewSI->setDebugLoc(DL);
751 if (AATags) NewSI->setAAMetadata(AATags);
755 void replaceLoadWithValue(LoadInst *LI, Value *V) const override {
756 // Update alias analysis.
757 AST.copyValue(LI, V);
759 void instructionDeleted(Instruction *I) const override {
763 } // end anon namespace
765 /// Try to promote memory values to scalars by sinking stores out of the
766 /// loop and moving loads to before the loop. We do this by looping over
767 /// the stores in the loop, looking for stores to Must pointers which are
770 bool llvm::promoteLoopAccessesToScalars(AliasSet &AS,
771 SmallVectorImpl<BasicBlock*>&ExitBlocks,
772 SmallVectorImpl<Instruction*>&InsertPts,
773 PredIteratorCache &PIC, LoopInfo *LI,
774 DominatorTree *DT, Loop *CurLoop,
775 AliasSetTracker *CurAST,
776 LICMSafetyInfo * SafetyInfo) {
778 assert(LI != nullptr && DT != nullptr &&
779 CurLoop != nullptr && CurAST != nullptr &&
780 SafetyInfo != nullptr &&
781 "Unexpected Input to promoteLoopAccessesToScalars");
782 // Initially set Changed status to false.
783 bool Changed = false;
784 // We can promote this alias set if it has a store, if it is a "Must" alias
785 // set, if the pointer is loop invariant, and if we are not eliminating any
786 // volatile loads or stores.
787 if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() ||
788 AS.isVolatile() || !CurLoop->isLoopInvariant(AS.begin()->getValue()))
791 assert(!AS.empty() &&
792 "Must alias set should have at least one pointer element in it!");
794 Value *SomePtr = AS.begin()->getValue();
795 BasicBlock * Preheader = CurLoop->getLoopPreheader();
797 // It isn't safe to promote a load/store from the loop if the load/store is
798 // conditional. For example, turning:
800 // for () { if (c) *P += 1; }
804 // tmp = *P; for () { if (c) tmp +=1; } *P = tmp;
806 // is not safe, because *P may only be valid to access if 'c' is true.
808 // It is safe to promote P if all uses are direct load/stores and if at
809 // least one is guaranteed to be executed.
810 bool GuaranteedToExecute = false;
812 SmallVector<Instruction*, 64> LoopUses;
813 SmallPtrSet<Value*, 4> PointerMustAliases;
815 // We start with an alignment of one and try to find instructions that allow
816 // us to prove better alignment.
817 unsigned Alignment = 1;
819 bool HasDedicatedExits = CurLoop->hasDedicatedExits();
821 // Check that all of the pointers in the alias set have the same type. We
822 // cannot (yet) promote a memory location that is loaded and stored in
823 // different sizes. While we are at it, collect alignment and AA info.
824 for (AliasSet::iterator ASI = AS.begin(), E = AS.end(); ASI != E; ++ASI) {
825 Value *ASIV = ASI->getValue();
826 PointerMustAliases.insert(ASIV);
828 // Check that all of the pointers in the alias set have the same type. We
829 // cannot (yet) promote a memory location that is loaded and stored in
831 if (SomePtr->getType() != ASIV->getType())
834 for (User *U : ASIV->users()) {
835 // Ignore instructions that are outside the loop.
836 Instruction *UI = dyn_cast<Instruction>(U);
837 if (!UI || !CurLoop->contains(UI))
840 // If there is an non-load/store instruction in the loop, we can't promote
842 if (LoadInst *load = dyn_cast<LoadInst>(UI)) {
843 assert(!load->isVolatile() && "AST broken");
844 if (!load->isSimple())
846 } else if (StoreInst *store = dyn_cast<StoreInst>(UI)) {
847 // Stores *of* the pointer are not interesting, only stores *to* the
849 if (UI->getOperand(1) != ASIV)
851 assert(!store->isVolatile() && "AST broken");
852 if (!store->isSimple())
854 // Don't sink stores from loops without dedicated block exits. Exits
855 // containing indirect branches are not transformed by loop simplify,
856 // make sure we catch that. An additional load may be generated in the
857 // preheader for SSA updater, so also avoid sinking when no preheader
859 if (!HasDedicatedExits || !Preheader)
862 // Note that we only check GuaranteedToExecute inside the store case
863 // so that we do not introduce stores where they did not exist before
864 // (which would break the LLVM concurrency model).
866 // If the alignment of this instruction allows us to specify a more
867 // restrictive (and performant) alignment and if we are sure this
868 // instruction will be executed, update the alignment.
869 // Larger is better, with the exception of 0 being the best alignment.
870 unsigned InstAlignment = store->getAlignment();
871 if ((InstAlignment > Alignment || InstAlignment == 0) && Alignment != 0)
872 if (isGuaranteedToExecute(*UI, DT, CurLoop, SafetyInfo)) {
873 GuaranteedToExecute = true;
874 Alignment = InstAlignment;
877 if (!GuaranteedToExecute)
878 GuaranteedToExecute = isGuaranteedToExecute(*UI, DT,
879 CurLoop, SafetyInfo);
882 return Changed; // Not a load or store.
884 // Merge the AA tags.
885 if (LoopUses.empty()) {
886 // On the first load/store, just take its AA tags.
887 UI->getAAMetadata(AATags);
889 UI->getAAMetadata(AATags, /* Merge = */ true);
892 LoopUses.push_back(UI);
896 // If there isn't a guaranteed-to-execute instruction, we can't promote.
897 if (!GuaranteedToExecute)
900 // Otherwise, this is safe to promote, lets do it!
901 DEBUG(dbgs() << "LICM: Promoting value stored to in loop: " <<*SomePtr<<'\n');
905 // Grab a debug location for the inserted loads/stores; given that the
906 // inserted loads/stores have little relation to the original loads/stores,
907 // this code just arbitrarily picks a location from one, since any debug
908 // location is better than none.
909 DebugLoc DL = LoopUses[0]->getDebugLoc();
911 // Figure out the loop exits and their insertion points, if this is the
913 if (ExitBlocks.empty()) {
914 CurLoop->getUniqueExitBlocks(ExitBlocks);
915 InsertPts.resize(ExitBlocks.size());
916 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
917 InsertPts[i] = ExitBlocks[i]->getFirstInsertionPt();
920 // We use the SSAUpdater interface to insert phi nodes as required.
921 SmallVector<PHINode*, 16> NewPHIs;
922 SSAUpdater SSA(&NewPHIs);
923 LoopPromoter Promoter(SomePtr, LoopUses, SSA, PointerMustAliases, ExitBlocks,
924 InsertPts, PIC, *CurAST, *LI, DL, Alignment, AATags);
926 // Set up the preheader to have a definition of the value. It is the live-out
927 // value from the preheader that uses in the loop will use.
928 LoadInst *PreheaderLoad =
929 new LoadInst(SomePtr, SomePtr->getName()+".promoted",
930 Preheader->getTerminator());
931 PreheaderLoad->setAlignment(Alignment);
932 PreheaderLoad->setDebugLoc(DL);
933 if (AATags) PreheaderLoad->setAAMetadata(AATags);
934 SSA.AddAvailableValue(Preheader, PreheaderLoad);
936 // Rewrite all the loads in the loop and remember all the definitions from
937 // stores in the loop.
938 Promoter.run(LoopUses);
940 // If the SSAUpdater didn't use the load in the preheader, just zap it now.
941 if (PreheaderLoad->use_empty())
942 PreheaderLoad->eraseFromParent();
947 /// Simple Analysis hook. Clone alias set info.
949 void LICM::cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L) {
950 AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);
954 AST->copyValue(From, To);
957 /// Simple Analysis hook. Delete value V from alias set
959 void LICM::deleteAnalysisValue(Value *V, Loop *L) {
960 AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);
967 /// Simple Analysis hook. Delete value L from alias set map.
969 void LICM::deleteAnalysisLoop(Loop *L) {
970 AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);
975 LoopToAliasSetMap.erase(L);
979 /// Return true if the body of this loop may store into the memory
980 /// location pointed to by V.
982 static bool pointerInvalidatedByLoop(Value *V, uint64_t Size,
983 const AAMDNodes &AAInfo,
984 AliasSetTracker *CurAST) {
985 // Check to see if any of the basic blocks in CurLoop invalidate *V.
986 return CurAST->getAliasSetForPointer(V, Size, AAInfo).isMod();
989 /// Little predicate that returns true if the specified basic block is in
990 /// a subloop of the current one, not the current one itself.
992 static bool inSubLoop(BasicBlock *BB, Loop *CurLoop, LoopInfo *LI) {
993 assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
994 return LI->getLoopFor(BB) != CurLoop;