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/Target/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"));
75 struct LICM : public LoopPass {
76 static char ID; // Pass identification, replacement for typeid
77 LICM() : LoopPass(ID) {
78 initializeLICMPass(*PassRegistry::getPassRegistry());
81 bool runOnLoop(Loop *L, LPPassManager &LPM) override;
83 /// This transformation requires natural loop information & requires that
84 /// loop preheaders be inserted into the CFG...
86 void getAnalysisUsage(AnalysisUsage &AU) const override {
88 AU.addRequired<DominatorTreeWrapperPass>();
89 AU.addRequired<LoopInfo>();
90 AU.addRequiredID(LoopSimplifyID);
91 AU.addPreservedID(LoopSimplifyID);
92 AU.addRequiredID(LCSSAID);
93 AU.addPreservedID(LCSSAID);
94 AU.addRequired<AliasAnalysis>();
95 AU.addPreserved<AliasAnalysis>();
96 AU.addPreserved<ScalarEvolution>();
97 AU.addRequired<TargetLibraryInfo>();
100 using llvm::Pass::doFinalization;
102 bool doFinalization() override {
103 assert(LoopToAliasSetMap.empty() && "Didn't free loop alias sets");
108 AliasAnalysis *AA; // Current AliasAnalysis information
109 LoopInfo *LI; // Current LoopInfo
110 DominatorTree *DT; // Dominator Tree for the current Loop.
112 const DataLayout *DL; // DataLayout for constant folding.
113 TargetLibraryInfo *TLI; // TargetLibraryInfo for constant folding.
115 // State that is updated as we process loops.
116 bool Changed; // Set to true when we change anything.
117 BasicBlock *Preheader; // The preheader block of the current loop...
118 Loop *CurLoop; // The current loop we are working on...
119 AliasSetTracker *CurAST; // AliasSet information for the current loop...
120 bool MayThrow; // The current loop contains an instruction which
121 // may throw, thus preventing code motion of
122 // instructions with side effects.
123 DenseMap<Loop*, AliasSetTracker*> LoopToAliasSetMap;
125 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
126 void cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To,
129 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
131 void deleteAnalysisValue(Value *V, Loop *L) override;
133 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks
134 /// dominated by the specified block, and that are in the current loop) in
135 /// reverse depth first order w.r.t the DominatorTree. This allows us to
136 /// visit uses before definitions, allowing us to sink a loop body in one
137 /// pass without iteration.
139 void SinkRegion(DomTreeNode *N);
141 /// HoistRegion - Walk the specified region of the CFG (defined by all
142 /// blocks dominated by the specified block, and that are in the current
143 /// loop) in depth first order w.r.t the DominatorTree. This allows us to
144 /// visit definitions before uses, allowing us to hoist a loop body in one
145 /// pass without iteration.
147 void HoistRegion(DomTreeNode *N);
149 /// inSubLoop - Little predicate that returns true if the specified basic
150 /// block is in a subloop of the current one, not the current one itself.
152 bool inSubLoop(BasicBlock *BB) {
153 assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
154 return LI->getLoopFor(BB) != CurLoop;
157 /// sink - When an instruction is found to only be used outside of the loop,
158 /// this function moves it to the exit blocks and patches up SSA form as
161 void sink(Instruction &I);
163 /// hoist - When an instruction is found to only use loop invariant operands
164 /// that is safe to hoist, this instruction is called to do the dirty work.
166 void hoist(Instruction &I);
168 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it
169 /// is not a trapping instruction or if it is a trapping instruction and is
170 /// guaranteed to execute.
172 bool isSafeToExecuteUnconditionally(Instruction &I);
174 /// isGuaranteedToExecute - Check that the instruction is guaranteed to
177 bool isGuaranteedToExecute(Instruction &I);
179 /// pointerInvalidatedByLoop - Return true if the body of this loop may
180 /// store into the memory location pointed to by V.
182 bool pointerInvalidatedByLoop(Value *V, uint64_t Size,
183 const MDNode *TBAAInfo) {
184 // Check to see if any of the basic blocks in CurLoop invalidate *V.
185 return CurAST->getAliasSetForPointer(V, Size, TBAAInfo).isMod();
188 bool canSinkOrHoistInst(Instruction &I);
189 bool isNotUsedInLoop(Instruction &I);
191 void PromoteAliasSet(AliasSet &AS,
192 SmallVectorImpl<BasicBlock*> &ExitBlocks,
193 SmallVectorImpl<Instruction*> &InsertPts,
194 PredIteratorCache &PIC);
199 INITIALIZE_PASS_BEGIN(LICM, "licm", "Loop Invariant Code Motion", false, false)
200 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
201 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
202 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
203 INITIALIZE_PASS_DEPENDENCY(LCSSA)
204 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
205 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
206 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
207 INITIALIZE_PASS_END(LICM, "licm", "Loop Invariant Code Motion", false, false)
209 Pass *llvm::createLICMPass() { return new LICM(); }
211 /// Hoist expressions out of the specified loop. Note, alias info for inner
212 /// loop is not preserved so it is not a good idea to run LICM multiple
213 /// times on one loop.
215 bool LICM::runOnLoop(Loop *L, LPPassManager &LPM) {
216 if (skipOptnoneFunction(L))
221 // Get our Loop and Alias Analysis information...
222 LI = &getAnalysis<LoopInfo>();
223 AA = &getAnalysis<AliasAnalysis>();
224 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
226 DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
227 DL = DLP ? &DLP->getDataLayout() : nullptr;
228 TLI = &getAnalysis<TargetLibraryInfo>();
230 assert(L->isLCSSAForm(*DT) && "Loop is not in LCSSA form.");
232 CurAST = new AliasSetTracker(*AA);
233 // Collect Alias info from subloops.
234 for (Loop::iterator LoopItr = L->begin(), LoopItrE = L->end();
235 LoopItr != LoopItrE; ++LoopItr) {
236 Loop *InnerL = *LoopItr;
237 AliasSetTracker *InnerAST = LoopToAliasSetMap[InnerL];
238 assert(InnerAST && "Where is my AST?");
240 // What if InnerLoop was modified by other passes ?
241 CurAST->add(*InnerAST);
243 // Once we've incorporated the inner loop's AST into ours, we don't need the
244 // subloop's anymore.
246 LoopToAliasSetMap.erase(InnerL);
251 // Get the preheader block to move instructions into...
252 Preheader = L->getLoopPreheader();
254 // Loop over the body of this loop, looking for calls, invokes, and stores.
255 // Because subloops have already been incorporated into AST, we skip blocks in
258 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
261 if (LI->getLoopFor(BB) == L) // Ignore blocks in subloops.
262 CurAST->add(*BB); // Incorporate the specified basic block
266 // TODO: We've already searched for instructions which may throw in subloops.
267 // We may want to reuse this information.
268 for (Loop::block_iterator BB = L->block_begin(), BBE = L->block_end();
269 (BB != BBE) && !MayThrow ; ++BB)
270 for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end();
271 (I != E) && !MayThrow; ++I)
272 MayThrow |= I->mayThrow();
274 // We want to visit all of the instructions in this loop... that are not parts
275 // of our subloops (they have already had their invariants hoisted out of
276 // their loop, into this loop, so there is no need to process the BODIES of
279 // Traverse the body of the loop in depth first order on the dominator tree so
280 // that we are guaranteed to see definitions before we see uses. This allows
281 // us to sink instructions in one pass, without iteration. After sinking
282 // instructions, we perform another pass to hoist them out of the loop.
284 if (L->hasDedicatedExits())
285 SinkRegion(DT->getNode(L->getHeader()));
287 HoistRegion(DT->getNode(L->getHeader()));
289 // Now that all loop invariants have been removed from the loop, promote any
290 // memory references to scalars that we can.
291 if (!DisablePromotion && (Preheader || L->hasDedicatedExits())) {
292 SmallVector<BasicBlock *, 8> ExitBlocks;
293 SmallVector<Instruction *, 8> InsertPts;
294 PredIteratorCache PIC;
296 // Loop over all of the alias sets in the tracker object.
297 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
299 PromoteAliasSet(*I, ExitBlocks, InsertPts, PIC);
301 // Once we have promoted values across the loop body we have to recursively
302 // reform LCSSA as any nested loop may now have values defined within the
303 // loop used in the outer loop.
304 // FIXME: This is really heavy handed. It would be a bit better to use an
305 // SSAUpdater strategy during promotion that was LCSSA aware and reformed
308 formLCSSARecursively(*L, *DT, getAnalysisIfAvailable<ScalarEvolution>());
311 // Check that neither this loop nor its parent have had LCSSA broken. LICM is
312 // specifically moving instructions across the loop boundary and so it is
313 // especially in need of sanity checking here.
314 assert(L->isLCSSAForm(*DT) && "Loop not left in LCSSA form after LICM!");
315 assert((!L->getParentLoop() || L->getParentLoop()->isLCSSAForm(*DT)) &&
316 "Parent loop not left in LCSSA form after LICM!");
318 // Clear out loops state information for the next iteration
322 // If this loop is nested inside of another one, save the alias information
323 // for when we process the outer loop.
324 if (L->getParentLoop())
325 LoopToAliasSetMap[L] = CurAST;
331 /// SinkRegion - 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
333 /// reverse depth first order w.r.t the DominatorTree. This allows us to visit
334 /// uses before definitions, allowing us to sink a loop body in one pass without
337 void LICM::SinkRegion(DomTreeNode *N) {
338 assert(N != nullptr && "Null dominator tree node?");
339 BasicBlock *BB = N->getBlock();
341 // If this subregion is not in the top level loop at all, exit.
342 if (!CurLoop->contains(BB)) return;
344 // We are processing blocks in reverse dfo, so process children first.
345 const std::vector<DomTreeNode*> &Children = N->getChildren();
346 for (unsigned i = 0, e = Children.size(); i != e; ++i)
347 SinkRegion(Children[i]);
349 // Only need to process the contents of this block if it is not part of a
350 // subloop (which would already have been processed).
351 if (inSubLoop(BB)) return;
353 for (BasicBlock::iterator II = BB->end(); II != BB->begin(); ) {
354 Instruction &I = *--II;
356 // If the instruction is dead, we would try to sink it because it isn't used
357 // in the loop, instead, just delete it.
358 if (isInstructionTriviallyDead(&I, TLI)) {
359 DEBUG(dbgs() << "LICM deleting dead inst: " << I << '\n');
361 CurAST->deleteValue(&I);
367 // Check to see if we can sink this instruction to the exit blocks
368 // of the loop. We can do this if the all users of the instruction are
369 // outside of the loop. In this case, it doesn't even matter if the
370 // operands of the instruction are loop invariant.
372 if (isNotUsedInLoop(I) && canSinkOrHoistInst(I)) {
379 /// HoistRegion - Walk the specified region of the CFG (defined by all blocks
380 /// dominated by the specified block, and that are in the current loop) in depth
381 /// first order w.r.t the DominatorTree. This allows us to visit definitions
382 /// before uses, allowing us to hoist a loop body in one pass without iteration.
384 void LICM::HoistRegion(DomTreeNode *N) {
385 assert(N != nullptr && "Null dominator tree node?");
386 BasicBlock *BB = N->getBlock();
388 // If this subregion is not in the top level loop at all, exit.
389 if (!CurLoop->contains(BB)) return;
391 // Only need to process the contents of this block if it is not part of a
392 // subloop (which would already have been processed).
394 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ) {
395 Instruction &I = *II++;
397 // Try constant folding this instruction. If all the operands are
398 // constants, it is technically hoistable, but it would be better to just
400 if (Constant *C = ConstantFoldInstruction(&I, DL, TLI)) {
401 DEBUG(dbgs() << "LICM folding inst: " << I << " --> " << *C << '\n');
402 CurAST->copyValue(&I, C);
403 CurAST->deleteValue(&I);
404 I.replaceAllUsesWith(C);
409 // Try hoisting the instruction out to the preheader. We can only do this
410 // if all of the operands of the instruction are loop invariant and if it
411 // is safe to hoist the instruction.
413 if (CurLoop->hasLoopInvariantOperands(&I) && canSinkOrHoistInst(I) &&
414 isSafeToExecuteUnconditionally(I))
418 const std::vector<DomTreeNode*> &Children = N->getChildren();
419 for (unsigned i = 0, e = Children.size(); i != e; ++i)
420 HoistRegion(Children[i]);
423 /// canSinkOrHoistInst - Return true if the hoister and sinker can handle this
426 bool LICM::canSinkOrHoistInst(Instruction &I) {
427 // Loads have extra constraints we have to verify before we can hoist them.
428 if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
429 if (!LI->isUnordered())
430 return false; // Don't hoist volatile/atomic loads!
432 // Loads from constant memory are always safe to move, even if they end up
433 // in the same alias set as something that ends up being modified.
434 if (AA->pointsToConstantMemory(LI->getOperand(0)))
436 if (LI->getMetadata("invariant.load"))
439 // Don't hoist loads which have may-aliased stores in loop.
441 if (LI->getType()->isSized())
442 Size = AA->getTypeStoreSize(LI->getType());
443 return !pointerInvalidatedByLoop(LI->getOperand(0), Size,
444 LI->getMetadata(LLVMContext::MD_tbaa));
445 } else if (CallInst *CI = dyn_cast<CallInst>(&I)) {
446 // Don't sink or hoist dbg info; it's legal, but not useful.
447 if (isa<DbgInfoIntrinsic>(I))
450 // Handle simple cases by querying alias analysis.
451 AliasAnalysis::ModRefBehavior Behavior = AA->getModRefBehavior(CI);
452 if (Behavior == AliasAnalysis::DoesNotAccessMemory)
454 if (AliasAnalysis::onlyReadsMemory(Behavior)) {
455 // If this call only reads from memory and there are no writes to memory
456 // in the loop, we can hoist or sink the call as appropriate.
457 bool FoundMod = false;
458 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
461 if (!AS.isForwardingAliasSet() && AS.isMod()) {
466 if (!FoundMod) return true;
469 // FIXME: This should use mod/ref information to see if we can hoist or
475 // Only these instructions are hoistable/sinkable.
476 if (!isa<BinaryOperator>(I) && !isa<CastInst>(I) && !isa<SelectInst>(I) &&
477 !isa<GetElementPtrInst>(I) && !isa<CmpInst>(I) &&
478 !isa<InsertElementInst>(I) && !isa<ExtractElementInst>(I) &&
479 !isa<ShuffleVectorInst>(I) && !isa<ExtractValueInst>(I) &&
480 !isa<InsertValueInst>(I))
483 return isSafeToExecuteUnconditionally(I);
486 /// \brief Returns true if a PHINode is a trivially replaceable with an
489 /// This is true when all incoming values are that instruction. This pattern
490 /// occurs most often with LCSSA PHI nodes.
491 static bool isTriviallyReplacablePHI(PHINode &PN, Instruction &I) {
492 for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i)
493 if (PN.getIncomingValue(i) != &I)
499 /// isNotUsedInLoop - Return true if the only users of this instruction are
500 /// outside of the loop. If this is true, we can sink the instruction to the
501 /// exit blocks of the loop.
503 bool LICM::isNotUsedInLoop(Instruction &I) {
504 for (User *U : I.users()) {
505 Instruction *UI = cast<Instruction>(U);
506 if (PHINode *PN = dyn_cast<PHINode>(UI)) {
507 // A PHI node where all of the incoming values are this instruction are
508 // special -- they can just be RAUW'ed with the instruction and thus
509 // don't require a use in the predecessor. This is a particular important
510 // special case because it is the pattern found in LCSSA form.
511 if (isTriviallyReplacablePHI(*PN, I)) {
512 if (CurLoop->contains(PN))
518 // Otherwise, PHI node uses occur in predecessor blocks if the incoming
519 // values. Check for such a use being inside the loop.
520 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
521 if (PN->getIncomingValue(i) == &I)
522 if (CurLoop->contains(PN->getIncomingBlock(i)))
528 if (CurLoop->contains(UI))
534 /// sink - When an instruction is found to only be used outside of the loop,
535 /// this function moves it to the exit blocks and patches up SSA form as needed.
536 /// This method is guaranteed to remove the original instruction from its
537 /// position, and may either delete it or move it to outside of the loop.
539 void LICM::sink(Instruction &I) {
540 DEBUG(dbgs() << "LICM sinking instruction: " << I << "\n");
542 if (isa<LoadInst>(I)) ++NumMovedLoads;
543 else if (isa<CallInst>(I)) ++NumMovedCalls;
548 SmallVector<BasicBlock *, 32> ExitBlocks;
549 CurLoop->getUniqueExitBlocks(ExitBlocks);
550 SmallPtrSet<BasicBlock *, 32> ExitBlockSet(ExitBlocks.begin(), ExitBlocks.end());
553 // If this instruction is only used outside of the loop, then all users are
554 // PHI nodes in exit blocks due to LCSSA form. Just RAUW them with clones of
556 while (!I.use_empty()) {
557 // The user must be a PHI node.
558 PHINode *PN = cast<PHINode>(I.user_back());
560 BasicBlock *ExitBlock = PN->getParent();
561 assert(ExitBlockSet.count(ExitBlock) &&
562 "The LCSSA PHI is not in an exit block!");
564 Instruction *New = I.clone();
565 ExitBlock->getInstList().insert(ExitBlock->getFirstInsertionPt(), New);
566 if (!I.getName().empty())
567 New->setName(I.getName() + ".le");
569 // Build LCSSA PHI nodes for any in-loop operands. Note that this is
570 // particularly cheap because we can rip off the PHI node that we're
571 // replacing for the number and blocks of the predecessors.
572 // OPT: If this shows up in a profile, we can instead finish sinking all
573 // invariant instructions, and then walk their operands to re-establish
574 // LCSSA. That will eliminate creating PHI nodes just to nuke them when
575 // sinking bottom-up.
576 for (User::op_iterator OI = New->op_begin(), OE = New->op_end(); OI != OE;
578 if (Instruction *OInst = dyn_cast<Instruction>(*OI))
579 if (Loop *OLoop = LI->getLoopFor(OInst->getParent()))
580 if (!OLoop->contains(PN)) {
581 PHINode *OpPN = PHINode::Create(
582 OInst->getType(), PN->getNumIncomingValues(),
583 OInst->getName() + ".lcssa", ExitBlock->begin());
584 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
585 OpPN->addIncoming(OInst, PN->getIncomingBlock(i));
589 PN->replaceAllUsesWith(New);
590 PN->eraseFromParent();
593 CurAST->deleteValue(&I);
597 /// hoist - When an instruction is found to only use loop invariant operands
598 /// that is safe to hoist, this instruction is called to do the dirty work.
600 void LICM::hoist(Instruction &I) {
601 DEBUG(dbgs() << "LICM hoisting to " << Preheader->getName() << ": "
604 // Move the new node to the Preheader, before its terminator.
605 I.moveBefore(Preheader->getTerminator());
607 if (isa<LoadInst>(I)) ++NumMovedLoads;
608 else if (isa<CallInst>(I)) ++NumMovedCalls;
613 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it is
614 /// not a trapping instruction or if it is a trapping instruction and is
615 /// guaranteed to execute.
617 bool LICM::isSafeToExecuteUnconditionally(Instruction &Inst) {
618 // If it is not a trapping instruction, it is always safe to hoist.
619 if (isSafeToSpeculativelyExecute(&Inst))
622 return isGuaranteedToExecute(Inst);
625 bool LICM::isGuaranteedToExecute(Instruction &Inst) {
627 // Somewhere in this loop there is an instruction which may throw and make us
632 // Otherwise we have to check to make sure that the instruction dominates all
633 // of the exit blocks. If it doesn't, then there is a path out of the loop
634 // which does not execute this instruction, so we can't hoist it.
636 // If the instruction is in the header block for the loop (which is very
637 // common), it is always guaranteed to dominate the exit blocks. Since this
638 // is a common case, and can save some work, check it now.
639 if (Inst.getParent() == CurLoop->getHeader())
642 // Get the exit blocks for the current loop.
643 SmallVector<BasicBlock*, 8> ExitBlocks;
644 CurLoop->getExitBlocks(ExitBlocks);
646 // Verify that the block dominates each of the exit blocks of the loop.
647 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
648 if (!DT->dominates(Inst.getParent(), ExitBlocks[i]))
651 // As a degenerate case, if the loop is statically infinite then we haven't
652 // proven anything since there are no exit blocks.
653 if (ExitBlocks.empty())
660 class LoopPromoter : public LoadAndStorePromoter {
661 Value *SomePtr; // Designated pointer to store to.
662 SmallPtrSet<Value*, 4> &PointerMustAliases;
663 SmallVectorImpl<BasicBlock*> &LoopExitBlocks;
664 SmallVectorImpl<Instruction*> &LoopInsertPts;
665 PredIteratorCache &PredCache;
666 AliasSetTracker &AST;
672 Value *maybeInsertLCSSAPHI(Value *V, BasicBlock *BB) const {
673 if (Instruction *I = dyn_cast<Instruction>(V))
674 if (Loop *L = LI.getLoopFor(I->getParent()))
675 if (!L->contains(BB)) {
676 // We need to create an LCSSA PHI node for the incoming value and
678 PHINode *PN = PHINode::Create(
679 I->getType(), PredCache.GetNumPreds(BB),
680 I->getName() + ".lcssa", BB->begin());
681 for (BasicBlock **PI = PredCache.GetPreds(BB); *PI; ++PI)
682 PN->addIncoming(I, *PI);
689 LoopPromoter(Value *SP, const SmallVectorImpl<Instruction *> &Insts,
690 SSAUpdater &S, SmallPtrSet<Value *, 4> &PMA,
691 SmallVectorImpl<BasicBlock *> &LEB,
692 SmallVectorImpl<Instruction *> &LIP, PredIteratorCache &PIC,
693 AliasSetTracker &ast, LoopInfo &li, DebugLoc dl, int alignment,
695 : LoadAndStorePromoter(Insts, S), SomePtr(SP), PointerMustAliases(PMA),
696 LoopExitBlocks(LEB), LoopInsertPts(LIP), PredCache(PIC), AST(ast),
697 LI(li), DL(dl), Alignment(alignment), TBAATag(TBAATag) {}
699 bool isInstInList(Instruction *I,
700 const SmallVectorImpl<Instruction*> &) const override {
702 if (LoadInst *LI = dyn_cast<LoadInst>(I))
703 Ptr = LI->getOperand(0);
705 Ptr = cast<StoreInst>(I)->getPointerOperand();
706 return PointerMustAliases.count(Ptr);
709 void doExtraRewritesBeforeFinalDeletion() const override {
710 // Insert stores after in the loop exit blocks. Each exit block gets a
711 // store of the live-out values that feed them. Since we've already told
712 // the SSA updater about the defs in the loop and the preheader
713 // definition, it is all set and we can start using it.
714 for (unsigned i = 0, e = LoopExitBlocks.size(); i != e; ++i) {
715 BasicBlock *ExitBlock = LoopExitBlocks[i];
716 Value *LiveInValue = SSA.GetValueInMiddleOfBlock(ExitBlock);
717 LiveInValue = maybeInsertLCSSAPHI(LiveInValue, ExitBlock);
718 Value *Ptr = maybeInsertLCSSAPHI(SomePtr, ExitBlock);
719 Instruction *InsertPos = LoopInsertPts[i];
720 StoreInst *NewSI = new StoreInst(LiveInValue, Ptr, InsertPos);
721 NewSI->setAlignment(Alignment);
722 NewSI->setDebugLoc(DL);
723 if (TBAATag) NewSI->setMetadata(LLVMContext::MD_tbaa, TBAATag);
727 void replaceLoadWithValue(LoadInst *LI, Value *V) const override {
728 // Update alias analysis.
729 AST.copyValue(LI, V);
731 void instructionDeleted(Instruction *I) const override {
735 } // end anon namespace
737 /// PromoteAliasSet - Try to promote memory values to scalars by sinking
738 /// stores out of the loop and moving loads to before the loop. We do this by
739 /// looping over the stores in the loop, looking for stores to Must pointers
740 /// which are loop invariant.
742 void LICM::PromoteAliasSet(AliasSet &AS,
743 SmallVectorImpl<BasicBlock*> &ExitBlocks,
744 SmallVectorImpl<Instruction*> &InsertPts,
745 PredIteratorCache &PIC) {
746 // We can promote this alias set if it has a store, if it is a "Must" alias
747 // set, if the pointer is loop invariant, and if we are not eliminating any
748 // volatile loads or stores.
749 if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() ||
750 AS.isVolatile() || !CurLoop->isLoopInvariant(AS.begin()->getValue()))
753 assert(!AS.empty() &&
754 "Must alias set should have at least one pointer element in it!");
755 Value *SomePtr = AS.begin()->getValue();
757 // It isn't safe to promote a load/store from the loop if the load/store is
758 // conditional. For example, turning:
760 // for () { if (c) *P += 1; }
764 // tmp = *P; for () { if (c) tmp +=1; } *P = tmp;
766 // is not safe, because *P may only be valid to access if 'c' is true.
768 // It is safe to promote P if all uses are direct load/stores and if at
769 // least one is guaranteed to be executed.
770 bool GuaranteedToExecute = false;
772 SmallVector<Instruction*, 64> LoopUses;
773 SmallPtrSet<Value*, 4> PointerMustAliases;
775 // We start with an alignment of one and try to find instructions that allow
776 // us to prove better alignment.
777 unsigned Alignment = 1;
778 MDNode *TBAATag = nullptr;
780 // Check that all of the pointers in the alias set have the same type. We
781 // cannot (yet) promote a memory location that is loaded and stored in
782 // different sizes. While we are at it, collect alignment and TBAA info.
783 for (AliasSet::iterator ASI = AS.begin(), E = AS.end(); ASI != E; ++ASI) {
784 Value *ASIV = ASI->getValue();
785 PointerMustAliases.insert(ASIV);
787 // Check that all of the pointers in the alias set have the same type. We
788 // cannot (yet) promote a memory location that is loaded and stored in
790 if (SomePtr->getType() != ASIV->getType())
793 for (User *U : ASIV->users()) {
794 // Ignore instructions that are outside the loop.
795 Instruction *UI = dyn_cast<Instruction>(U);
796 if (!UI || !CurLoop->contains(UI))
799 // If there is an non-load/store instruction in the loop, we can't promote
801 if (LoadInst *load = dyn_cast<LoadInst>(UI)) {
802 assert(!load->isVolatile() && "AST broken");
803 if (!load->isSimple())
805 } else if (StoreInst *store = dyn_cast<StoreInst>(UI)) {
806 // Stores *of* the pointer are not interesting, only stores *to* the
808 if (UI->getOperand(1) != ASIV)
810 assert(!store->isVolatile() && "AST broken");
811 if (!store->isSimple())
814 // Note that we only check GuaranteedToExecute inside the store case
815 // so that we do not introduce stores where they did not exist before
816 // (which would break the LLVM concurrency model).
818 // If the alignment of this instruction allows us to specify a more
819 // restrictive (and performant) alignment and if we are sure this
820 // instruction will be executed, update the alignment.
821 // Larger is better, with the exception of 0 being the best alignment.
822 unsigned InstAlignment = store->getAlignment();
823 if ((InstAlignment > Alignment || InstAlignment == 0) && Alignment != 0)
824 if (isGuaranteedToExecute(*UI)) {
825 GuaranteedToExecute = true;
826 Alignment = InstAlignment;
829 if (!GuaranteedToExecute)
830 GuaranteedToExecute = isGuaranteedToExecute(*UI);
833 return; // Not a load or store.
835 // Merge the TBAA tags.
836 if (LoopUses.empty()) {
837 // On the first load/store, just take its TBAA tag.
838 TBAATag = UI->getMetadata(LLVMContext::MD_tbaa);
839 } else if (TBAATag) {
840 TBAATag = MDNode::getMostGenericTBAA(TBAATag,
841 UI->getMetadata(LLVMContext::MD_tbaa));
844 LoopUses.push_back(UI);
848 // If there isn't a guaranteed-to-execute instruction, we can't promote.
849 if (!GuaranteedToExecute)
852 // Otherwise, this is safe to promote, lets do it!
853 DEBUG(dbgs() << "LICM: Promoting value stored to in loop: " <<*SomePtr<<'\n');
857 // Grab a debug location for the inserted loads/stores; given that the
858 // inserted loads/stores have little relation to the original loads/stores,
859 // this code just arbitrarily picks a location from one, since any debug
860 // location is better than none.
861 DebugLoc DL = LoopUses[0]->getDebugLoc();
863 // Figure out the loop exits and their insertion points, if this is the
865 if (ExitBlocks.empty()) {
866 CurLoop->getUniqueExitBlocks(ExitBlocks);
867 InsertPts.resize(ExitBlocks.size());
868 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
869 InsertPts[i] = ExitBlocks[i]->getFirstInsertionPt();
872 // We use the SSAUpdater interface to insert phi nodes as required.
873 SmallVector<PHINode*, 16> NewPHIs;
874 SSAUpdater SSA(&NewPHIs);
875 LoopPromoter Promoter(SomePtr, LoopUses, SSA, PointerMustAliases, ExitBlocks,
876 InsertPts, PIC, *CurAST, *LI, DL, Alignment, TBAATag);
878 // Set up the preheader to have a definition of the value. It is the live-out
879 // value from the preheader that uses in the loop will use.
880 LoadInst *PreheaderLoad =
881 new LoadInst(SomePtr, SomePtr->getName()+".promoted",
882 Preheader->getTerminator());
883 PreheaderLoad->setAlignment(Alignment);
884 PreheaderLoad->setDebugLoc(DL);
885 if (TBAATag) PreheaderLoad->setMetadata(LLVMContext::MD_tbaa, TBAATag);
886 SSA.AddAvailableValue(Preheader, PreheaderLoad);
888 // Rewrite all the loads in the loop and remember all the definitions from
889 // stores in the loop.
890 Promoter.run(LoopUses);
892 // If the SSAUpdater didn't use the load in the preheader, just zap it now.
893 if (PreheaderLoad->use_empty())
894 PreheaderLoad->eraseFromParent();
898 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
899 void LICM::cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L) {
900 AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);
904 AST->copyValue(From, To);
907 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
909 void LICM::deleteAnalysisValue(Value *V, Loop *L) {
910 AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);