1 //===- PromoteMemoryToRegister.cpp - Convert allocas to registers ---------===//
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
7 //===----------------------------------------------------------------------===//
9 // This file promotes memory references to be register references. It promotes
10 // alloca instructions which only have loads and stores as uses. An alloca is
11 // transformed by using iterated dominator frontiers to place PHI nodes, then
12 // traversing the function in depth-first order to rewrite loads and stores as
15 //===----------------------------------------------------------------------===//
17 #include "llvm/ADT/ArrayRef.h"
18 #include "llvm/ADT/DenseMap.h"
19 #include "llvm/ADT/STLExtras.h"
20 #include "llvm/ADT/SmallPtrSet.h"
21 #include "llvm/ADT/SmallVector.h"
22 #include "llvm/ADT/Statistic.h"
23 #include "llvm/ADT/TinyPtrVector.h"
24 #include "llvm/ADT/Twine.h"
25 #include "llvm/Analysis/AssumptionCache.h"
26 #include "llvm/Analysis/InstructionSimplify.h"
27 #include "llvm/Analysis/IteratedDominanceFrontier.h"
28 #include "llvm/Transforms/Utils/Local.h"
29 #include "llvm/Analysis/ValueTracking.h"
30 #include "llvm/IR/BasicBlock.h"
31 #include "llvm/IR/CFG.h"
32 #include "llvm/IR/Constant.h"
33 #include "llvm/IR/Constants.h"
34 #include "llvm/IR/DIBuilder.h"
35 #include "llvm/IR/DerivedTypes.h"
36 #include "llvm/IR/Dominators.h"
37 #include "llvm/IR/Function.h"
38 #include "llvm/IR/InstrTypes.h"
39 #include "llvm/IR/Instruction.h"
40 #include "llvm/IR/Instructions.h"
41 #include "llvm/IR/IntrinsicInst.h"
42 #include "llvm/IR/Intrinsics.h"
43 #include "llvm/IR/LLVMContext.h"
44 #include "llvm/IR/Module.h"
45 #include "llvm/IR/Type.h"
46 #include "llvm/IR/User.h"
47 #include "llvm/Support/Casting.h"
48 #include "llvm/Transforms/Utils/PromoteMemToReg.h"
57 #define DEBUG_TYPE "mem2reg"
59 STATISTIC(NumLocalPromoted, "Number of alloca's promoted within one block");
60 STATISTIC(NumSingleStore, "Number of alloca's promoted with a single store");
61 STATISTIC(NumDeadAlloca, "Number of dead alloca's removed");
62 STATISTIC(NumPHIInsert, "Number of PHI nodes inserted");
64 bool llvm::isAllocaPromotable(const AllocaInst *AI) {
65 // FIXME: If the memory unit is of pointer or integer type, we can permit
66 // assignments to subsections of the memory unit.
67 unsigned AS = AI->getType()->getAddressSpace();
69 // Only allow direct and non-volatile loads and stores...
70 for (const User *U : AI->users()) {
71 if (const LoadInst *LI = dyn_cast<LoadInst>(U)) {
72 // Note that atomic loads can be transformed; atomic semantics do
73 // not have any meaning for a local alloca.
76 } else if (const StoreInst *SI = dyn_cast<StoreInst>(U)) {
77 if (SI->getOperand(0) == AI)
78 return false; // Don't allow a store OF the AI, only INTO the AI.
79 // Note that atomic stores can be transformed; atomic semantics do
80 // not have any meaning for a local alloca.
83 } else if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(U)) {
84 if (!II->isLifetimeStartOrEnd())
86 } else if (const BitCastInst *BCI = dyn_cast<BitCastInst>(U)) {
87 if (BCI->getType() != Type::getInt8PtrTy(U->getContext(), AS))
89 if (!onlyUsedByLifetimeMarkers(BCI))
91 } else if (const GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(U)) {
92 if (GEPI->getType() != Type::getInt8PtrTy(U->getContext(), AS))
94 if (!GEPI->hasAllZeroIndices())
96 if (!onlyUsedByLifetimeMarkers(GEPI))
109 SmallVector<BasicBlock *, 32> DefiningBlocks;
110 SmallVector<BasicBlock *, 32> UsingBlocks;
112 StoreInst *OnlyStore;
113 BasicBlock *OnlyBlock;
114 bool OnlyUsedInOneBlock;
116 TinyPtrVector<DbgVariableIntrinsic *> DbgDeclares;
119 DefiningBlocks.clear();
123 OnlyUsedInOneBlock = true;
127 /// Scan the uses of the specified alloca, filling in the AllocaInfo used
128 /// by the rest of the pass to reason about the uses of this alloca.
129 void AnalyzeAlloca(AllocaInst *AI) {
132 // As we scan the uses of the alloca instruction, keep track of stores,
133 // and decide whether all of the loads and stores to the alloca are within
134 // the same basic block.
135 for (auto UI = AI->user_begin(), E = AI->user_end(); UI != E;) {
136 Instruction *User = cast<Instruction>(*UI++);
138 if (StoreInst *SI = dyn_cast<StoreInst>(User)) {
139 // Remember the basic blocks which define new values for the alloca
140 DefiningBlocks.push_back(SI->getParent());
143 LoadInst *LI = cast<LoadInst>(User);
144 // Otherwise it must be a load instruction, keep track of variable
146 UsingBlocks.push_back(LI->getParent());
149 if (OnlyUsedInOneBlock) {
151 OnlyBlock = User->getParent();
152 else if (OnlyBlock != User->getParent())
153 OnlyUsedInOneBlock = false;
157 DbgDeclares = FindDbgAddrUses(AI);
161 /// Data package used by RenamePass().
162 struct RenamePassData {
163 using ValVector = std::vector<Value *>;
164 using LocationVector = std::vector<DebugLoc>;
166 RenamePassData(BasicBlock *B, BasicBlock *P, ValVector V, LocationVector L)
167 : BB(B), Pred(P), Values(std::move(V)), Locations(std::move(L)) {}
172 LocationVector Locations;
175 /// This assigns and keeps a per-bb relative ordering of load/store
176 /// instructions in the block that directly load or store an alloca.
178 /// This functionality is important because it avoids scanning large basic
179 /// blocks multiple times when promoting many allocas in the same block.
180 class LargeBlockInfo {
181 /// For each instruction that we track, keep the index of the
184 /// The index starts out as the number of the instruction from the start of
186 DenseMap<const Instruction *, unsigned> InstNumbers;
190 /// This code only looks at accesses to allocas.
191 static bool isInterestingInstruction(const Instruction *I) {
192 return (isa<LoadInst>(I) && isa<AllocaInst>(I->getOperand(0))) ||
193 (isa<StoreInst>(I) && isa<AllocaInst>(I->getOperand(1)));
196 /// Get or calculate the index of the specified instruction.
197 unsigned getInstructionIndex(const Instruction *I) {
198 assert(isInterestingInstruction(I) &&
199 "Not a load/store to/from an alloca?");
201 // If we already have this instruction number, return it.
202 DenseMap<const Instruction *, unsigned>::iterator It = InstNumbers.find(I);
203 if (It != InstNumbers.end())
206 // Scan the whole block to get the instruction. This accumulates
207 // information for every interesting instruction in the block, in order to
208 // avoid gratuitus rescans.
209 const BasicBlock *BB = I->getParent();
211 for (const Instruction &BBI : *BB)
212 if (isInterestingInstruction(&BBI))
213 InstNumbers[&BBI] = InstNo++;
214 It = InstNumbers.find(I);
216 assert(It != InstNumbers.end() && "Didn't insert instruction?");
220 void deleteValue(const Instruction *I) { InstNumbers.erase(I); }
222 void clear() { InstNumbers.clear(); }
225 struct PromoteMem2Reg {
226 /// The alloca instructions being promoted.
227 std::vector<AllocaInst *> Allocas;
232 /// A cache of @llvm.assume intrinsics used by SimplifyInstruction.
235 const SimplifyQuery SQ;
237 /// Reverse mapping of Allocas.
238 DenseMap<AllocaInst *, unsigned> AllocaLookup;
240 /// The PhiNodes we're adding.
242 /// That map is used to simplify some Phi nodes as we iterate over it, so
243 /// it should have deterministic iterators. We could use a MapVector, but
244 /// since we already maintain a map from BasicBlock* to a stable numbering
245 /// (BBNumbers), the DenseMap is more efficient (also supports removal).
246 DenseMap<std::pair<unsigned, unsigned>, PHINode *> NewPhiNodes;
248 /// For each PHI node, keep track of which entry in Allocas it corresponds
250 DenseMap<PHINode *, unsigned> PhiToAllocaMap;
252 /// For each alloca, we keep track of the dbg.declare intrinsic that
253 /// describes it, if any, so that we can convert it to a dbg.value
254 /// intrinsic if the alloca gets promoted.
255 SmallVector<TinyPtrVector<DbgVariableIntrinsic *>, 8> AllocaDbgDeclares;
257 /// The set of basic blocks the renamer has already visited.
258 SmallPtrSet<BasicBlock *, 16> Visited;
260 /// Contains a stable numbering of basic blocks to avoid non-determinstic
262 DenseMap<BasicBlock *, unsigned> BBNumbers;
264 /// Lazily compute the number of predecessors a block has.
265 DenseMap<const BasicBlock *, unsigned> BBNumPreds;
268 PromoteMem2Reg(ArrayRef<AllocaInst *> Allocas, DominatorTree &DT,
270 : Allocas(Allocas.begin(), Allocas.end()), DT(DT),
271 DIB(*DT.getRoot()->getParent()->getParent(), /*AllowUnresolved*/ false),
272 AC(AC), SQ(DT.getRoot()->getParent()->getParent()->getDataLayout(),
278 void RemoveFromAllocasList(unsigned &AllocaIdx) {
279 Allocas[AllocaIdx] = Allocas.back();
284 unsigned getNumPreds(const BasicBlock *BB) {
285 unsigned &NP = BBNumPreds[BB];
287 NP = pred_size(BB) + 1;
291 void ComputeLiveInBlocks(AllocaInst *AI, AllocaInfo &Info,
292 const SmallPtrSetImpl<BasicBlock *> &DefBlocks,
293 SmallPtrSetImpl<BasicBlock *> &LiveInBlocks);
294 void RenamePass(BasicBlock *BB, BasicBlock *Pred,
295 RenamePassData::ValVector &IncVals,
296 RenamePassData::LocationVector &IncLocs,
297 std::vector<RenamePassData> &Worklist);
298 bool QueuePhiNode(BasicBlock *BB, unsigned AllocaIdx, unsigned &Version);
301 } // end anonymous namespace
303 /// Given a LoadInst LI this adds assume(LI != null) after it.
304 static void addAssumeNonNull(AssumptionCache *AC, LoadInst *LI) {
305 Function *AssumeIntrinsic =
306 Intrinsic::getDeclaration(LI->getModule(), Intrinsic::assume);
307 ICmpInst *LoadNotNull = new ICmpInst(ICmpInst::ICMP_NE, LI,
308 Constant::getNullValue(LI->getType()));
309 LoadNotNull->insertAfter(LI);
310 CallInst *CI = CallInst::Create(AssumeIntrinsic, {LoadNotNull});
311 CI->insertAfter(LoadNotNull);
312 AC->registerAssumption(CI);
315 static void removeLifetimeIntrinsicUsers(AllocaInst *AI) {
316 // Knowing that this alloca is promotable, we know that it's safe to kill all
317 // instructions except for load and store.
319 for (auto UI = AI->user_begin(), UE = AI->user_end(); UI != UE;) {
320 Instruction *I = cast<Instruction>(*UI);
322 if (isa<LoadInst>(I) || isa<StoreInst>(I))
325 if (!I->getType()->isVoidTy()) {
326 // The only users of this bitcast/GEP instruction are lifetime intrinsics.
327 // Follow the use/def chain to erase them now instead of leaving it for
328 // dead code elimination later.
329 for (auto UUI = I->user_begin(), UUE = I->user_end(); UUI != UUE;) {
330 Instruction *Inst = cast<Instruction>(*UUI);
332 Inst->eraseFromParent();
335 I->eraseFromParent();
339 /// Rewrite as many loads as possible given a single store.
341 /// When there is only a single store, we can use the domtree to trivially
342 /// replace all of the dominated loads with the stored value. Do so, and return
343 /// true if this has successfully promoted the alloca entirely. If this returns
344 /// false there were some loads which were not dominated by the single store
345 /// and thus must be phi-ed with undef. We fall back to the standard alloca
346 /// promotion algorithm in that case.
347 static bool rewriteSingleStoreAlloca(AllocaInst *AI, AllocaInfo &Info,
348 LargeBlockInfo &LBI, const DataLayout &DL,
349 DominatorTree &DT, AssumptionCache *AC) {
350 StoreInst *OnlyStore = Info.OnlyStore;
351 bool StoringGlobalVal = !isa<Instruction>(OnlyStore->getOperand(0));
352 BasicBlock *StoreBB = OnlyStore->getParent();
355 // Clear out UsingBlocks. We will reconstruct it here if needed.
356 Info.UsingBlocks.clear();
358 for (auto UI = AI->user_begin(), E = AI->user_end(); UI != E;) {
359 Instruction *UserInst = cast<Instruction>(*UI++);
360 if (UserInst == OnlyStore)
362 LoadInst *LI = cast<LoadInst>(UserInst);
364 // Okay, if we have a load from the alloca, we want to replace it with the
365 // only value stored to the alloca. We can do this if the value is
366 // dominated by the store. If not, we use the rest of the mem2reg machinery
367 // to insert the phi nodes as needed.
368 if (!StoringGlobalVal) { // Non-instructions are always dominated.
369 if (LI->getParent() == StoreBB) {
370 // If we have a use that is in the same block as the store, compare the
371 // indices of the two instructions to see which one came first. If the
372 // load came before the store, we can't handle it.
373 if (StoreIndex == -1)
374 StoreIndex = LBI.getInstructionIndex(OnlyStore);
376 if (unsigned(StoreIndex) > LBI.getInstructionIndex(LI)) {
377 // Can't handle this load, bail out.
378 Info.UsingBlocks.push_back(StoreBB);
381 } else if (!DT.dominates(StoreBB, LI->getParent())) {
382 // If the load and store are in different blocks, use BB dominance to
383 // check their relationships. If the store doesn't dom the use, bail
385 Info.UsingBlocks.push_back(LI->getParent());
390 // Otherwise, we *can* safely rewrite this load.
391 Value *ReplVal = OnlyStore->getOperand(0);
392 // If the replacement value is the load, this must occur in unreachable
395 ReplVal = UndefValue::get(LI->getType());
397 // If the load was marked as nonnull we don't want to lose
398 // that information when we erase this Load. So we preserve
399 // it with an assume.
400 if (AC && LI->getMetadata(LLVMContext::MD_nonnull) &&
401 !isKnownNonZero(ReplVal, DL, 0, AC, LI, &DT))
402 addAssumeNonNull(AC, LI);
404 LI->replaceAllUsesWith(ReplVal);
405 LI->eraseFromParent();
409 // Finally, after the scan, check to see if the store is all that is left.
410 if (!Info.UsingBlocks.empty())
411 return false; // If not, we'll have to fall back for the remainder.
413 // Record debuginfo for the store and remove the declaration's
415 for (DbgVariableIntrinsic *DII : Info.DbgDeclares) {
416 DIBuilder DIB(*AI->getModule(), /*AllowUnresolved*/ false);
417 ConvertDebugDeclareToDebugValue(DII, Info.OnlyStore, DIB);
418 DII->eraseFromParent();
420 // Remove the (now dead) store and alloca.
421 Info.OnlyStore->eraseFromParent();
422 LBI.deleteValue(Info.OnlyStore);
424 AI->eraseFromParent();
428 /// Many allocas are only used within a single basic block. If this is the
429 /// case, avoid traversing the CFG and inserting a lot of potentially useless
430 /// PHI nodes by just performing a single linear pass over the basic block
431 /// using the Alloca.
433 /// If we cannot promote this alloca (because it is read before it is written),
434 /// return false. This is necessary in cases where, due to control flow, the
435 /// alloca is undefined only on some control flow paths. e.g. code like
436 /// this is correct in LLVM IR:
437 /// // A is an alloca with no stores so far
440 /// if (!first_iteration)
444 static bool promoteSingleBlockAlloca(AllocaInst *AI, const AllocaInfo &Info,
446 const DataLayout &DL,
448 AssumptionCache *AC) {
449 // The trickiest case to handle is when we have large blocks. Because of this,
450 // this code is optimized assuming that large blocks happen. This does not
451 // significantly pessimize the small block case. This uses LargeBlockInfo to
452 // make it efficient to get the index of various operations in the block.
454 // Walk the use-def list of the alloca, getting the locations of all stores.
455 using StoresByIndexTy = SmallVector<std::pair<unsigned, StoreInst *>, 64>;
456 StoresByIndexTy StoresByIndex;
458 for (User *U : AI->users())
459 if (StoreInst *SI = dyn_cast<StoreInst>(U))
460 StoresByIndex.push_back(std::make_pair(LBI.getInstructionIndex(SI), SI));
462 // Sort the stores by their index, making it efficient to do a lookup with a
464 llvm::sort(StoresByIndex, less_first());
466 // Walk all of the loads from this alloca, replacing them with the nearest
467 // store above them, if any.
468 for (auto UI = AI->user_begin(), E = AI->user_end(); UI != E;) {
469 LoadInst *LI = dyn_cast<LoadInst>(*UI++);
473 unsigned LoadIdx = LBI.getInstructionIndex(LI);
475 // Find the nearest store that has a lower index than this load.
476 StoresByIndexTy::iterator I = llvm::lower_bound(
478 std::make_pair(LoadIdx, static_cast<StoreInst *>(nullptr)),
480 if (I == StoresByIndex.begin()) {
481 if (StoresByIndex.empty())
482 // If there are no stores, the load takes the undef value.
483 LI->replaceAllUsesWith(UndefValue::get(LI->getType()));
485 // There is no store before this load, bail out (load may be affected
486 // by the following stores - see main comment).
489 // Otherwise, there was a store before this load, the load takes its value.
490 // Note, if the load was marked as nonnull we don't want to lose that
491 // information when we erase it. So we preserve it with an assume.
492 Value *ReplVal = std::prev(I)->second->getOperand(0);
493 if (AC && LI->getMetadata(LLVMContext::MD_nonnull) &&
494 !isKnownNonZero(ReplVal, DL, 0, AC, LI, &DT))
495 addAssumeNonNull(AC, LI);
497 // If the replacement value is the load, this must occur in unreachable
500 ReplVal = UndefValue::get(LI->getType());
502 LI->replaceAllUsesWith(ReplVal);
505 LI->eraseFromParent();
509 // Remove the (now dead) stores and alloca.
510 while (!AI->use_empty()) {
511 StoreInst *SI = cast<StoreInst>(AI->user_back());
512 // Record debuginfo for the store before removing it.
513 for (DbgVariableIntrinsic *DII : Info.DbgDeclares) {
514 DIBuilder DIB(*AI->getModule(), /*AllowUnresolved*/ false);
515 ConvertDebugDeclareToDebugValue(DII, SI, DIB);
517 SI->eraseFromParent();
521 AI->eraseFromParent();
523 // The alloca's debuginfo can be removed as well.
524 for (DbgVariableIntrinsic *DII : Info.DbgDeclares)
525 DII->eraseFromParent();
531 void PromoteMem2Reg::run() {
532 Function &F = *DT.getRoot()->getParent();
534 AllocaDbgDeclares.resize(Allocas.size());
538 ForwardIDFCalculator IDF(DT);
540 for (unsigned AllocaNum = 0; AllocaNum != Allocas.size(); ++AllocaNum) {
541 AllocaInst *AI = Allocas[AllocaNum];
543 assert(isAllocaPromotable(AI) && "Cannot promote non-promotable alloca!");
544 assert(AI->getParent()->getParent() == &F &&
545 "All allocas should be in the same function, which is same as DF!");
547 removeLifetimeIntrinsicUsers(AI);
549 if (AI->use_empty()) {
550 // If there are no uses of the alloca, just delete it now.
551 AI->eraseFromParent();
553 // Remove the alloca from the Allocas list, since it has been processed
554 RemoveFromAllocasList(AllocaNum);
559 // Calculate the set of read and write-locations for each alloca. This is
560 // analogous to finding the 'uses' and 'definitions' of each variable.
561 Info.AnalyzeAlloca(AI);
563 // If there is only a single store to this value, replace any loads of
564 // it that are directly dominated by the definition with the value stored.
565 if (Info.DefiningBlocks.size() == 1) {
566 if (rewriteSingleStoreAlloca(AI, Info, LBI, SQ.DL, DT, AC)) {
567 // The alloca has been processed, move on.
568 RemoveFromAllocasList(AllocaNum);
574 // If the alloca is only read and written in one basic block, just perform a
575 // linear sweep over the block to eliminate it.
576 if (Info.OnlyUsedInOneBlock &&
577 promoteSingleBlockAlloca(AI, Info, LBI, SQ.DL, DT, AC)) {
578 // The alloca has been processed, move on.
579 RemoveFromAllocasList(AllocaNum);
583 // If we haven't computed a numbering for the BB's in the function, do so
585 if (BBNumbers.empty()) {
588 BBNumbers[&BB] = ID++;
591 // Remember the dbg.declare intrinsic describing this alloca, if any.
592 if (!Info.DbgDeclares.empty())
593 AllocaDbgDeclares[AllocaNum] = Info.DbgDeclares;
595 // Keep the reverse mapping of the 'Allocas' array for the rename pass.
596 AllocaLookup[Allocas[AllocaNum]] = AllocaNum;
598 // At this point, we're committed to promoting the alloca using IDF's, and
599 // the standard SSA construction algorithm. Determine which blocks need PHI
600 // nodes and see if we can optimize out some work by avoiding insertion of
603 // Unique the set of defining blocks for efficient lookup.
604 SmallPtrSet<BasicBlock *, 32> DefBlocks(Info.DefiningBlocks.begin(),
605 Info.DefiningBlocks.end());
607 // Determine which blocks the value is live in. These are blocks which lead
609 SmallPtrSet<BasicBlock *, 32> LiveInBlocks;
610 ComputeLiveInBlocks(AI, Info, DefBlocks, LiveInBlocks);
612 // At this point, we're committed to promoting the alloca using IDF's, and
613 // the standard SSA construction algorithm. Determine which blocks need phi
614 // nodes and see if we can optimize out some work by avoiding insertion of
616 IDF.setLiveInBlocks(LiveInBlocks);
617 IDF.setDefiningBlocks(DefBlocks);
618 SmallVector<BasicBlock *, 32> PHIBlocks;
619 IDF.calculate(PHIBlocks);
620 llvm::sort(PHIBlocks, [this](BasicBlock *A, BasicBlock *B) {
621 return BBNumbers.find(A)->second < BBNumbers.find(B)->second;
624 unsigned CurrentVersion = 0;
625 for (BasicBlock *BB : PHIBlocks)
626 QueuePhiNode(BB, AllocaNum, CurrentVersion);
630 return; // All of the allocas must have been trivial!
634 // Set the incoming values for the basic block to be null values for all of
635 // the alloca's. We do this in case there is a load of a value that has not
636 // been stored yet. In this case, it will get this null value.
637 RenamePassData::ValVector Values(Allocas.size());
638 for (unsigned i = 0, e = Allocas.size(); i != e; ++i)
639 Values[i] = UndefValue::get(Allocas[i]->getAllocatedType());
641 // When handling debug info, treat all incoming values as if they have unknown
642 // locations until proven otherwise.
643 RenamePassData::LocationVector Locations(Allocas.size());
645 // Walks all basic blocks in the function performing the SSA rename algorithm
646 // and inserting the phi nodes we marked as necessary
647 std::vector<RenamePassData> RenamePassWorkList;
648 RenamePassWorkList.emplace_back(&F.front(), nullptr, std::move(Values),
649 std::move(Locations));
651 RenamePassData RPD = std::move(RenamePassWorkList.back());
652 RenamePassWorkList.pop_back();
653 // RenamePass may add new worklist entries.
654 RenamePass(RPD.BB, RPD.Pred, RPD.Values, RPD.Locations, RenamePassWorkList);
655 } while (!RenamePassWorkList.empty());
657 // The renamer uses the Visited set to avoid infinite loops. Clear it now.
660 // Remove the allocas themselves from the function.
661 for (Instruction *A : Allocas) {
662 // If there are any uses of the alloca instructions left, they must be in
663 // unreachable basic blocks that were not processed by walking the dominator
664 // tree. Just delete the users now.
666 A->replaceAllUsesWith(UndefValue::get(A->getType()));
667 A->eraseFromParent();
670 // Remove alloca's dbg.declare instrinsics from the function.
671 for (auto &Declares : AllocaDbgDeclares)
672 for (auto *DII : Declares)
673 DII->eraseFromParent();
675 // Loop over all of the PHI nodes and see if there are any that we can get
676 // rid of because they merge all of the same incoming values. This can
677 // happen due to undef values coming into the PHI nodes. This process is
678 // iterative, because eliminating one PHI node can cause others to be removed.
679 bool EliminatedAPHI = true;
680 while (EliminatedAPHI) {
681 EliminatedAPHI = false;
683 // Iterating over NewPhiNodes is deterministic, so it is safe to try to
684 // simplify and RAUW them as we go. If it was not, we could add uses to
685 // the values we replace with in a non-deterministic order, thus creating
686 // non-deterministic def->use chains.
687 for (DenseMap<std::pair<unsigned, unsigned>, PHINode *>::iterator
688 I = NewPhiNodes.begin(),
689 E = NewPhiNodes.end();
691 PHINode *PN = I->second;
693 // If this PHI node merges one value and/or undefs, get the value.
694 if (Value *V = SimplifyInstruction(PN, SQ)) {
695 PN->replaceAllUsesWith(V);
696 PN->eraseFromParent();
697 NewPhiNodes.erase(I++);
698 EliminatedAPHI = true;
705 // At this point, the renamer has added entries to PHI nodes for all reachable
706 // code. Unfortunately, there may be unreachable blocks which the renamer
707 // hasn't traversed. If this is the case, the PHI nodes may not
708 // have incoming values for all predecessors. Loop over all PHI nodes we have
709 // created, inserting undef values if they are missing any incoming values.
710 for (DenseMap<std::pair<unsigned, unsigned>, PHINode *>::iterator
711 I = NewPhiNodes.begin(),
712 E = NewPhiNodes.end();
714 // We want to do this once per basic block. As such, only process a block
715 // when we find the PHI that is the first entry in the block.
716 PHINode *SomePHI = I->second;
717 BasicBlock *BB = SomePHI->getParent();
718 if (&BB->front() != SomePHI)
721 // Only do work here if there the PHI nodes are missing incoming values. We
722 // know that all PHI nodes that were inserted in a block will have the same
723 // number of incoming values, so we can just check any of them.
724 if (SomePHI->getNumIncomingValues() == getNumPreds(BB))
727 // Get the preds for BB.
728 SmallVector<BasicBlock *, 16> Preds(pred_begin(BB), pred_end(BB));
730 // Ok, now we know that all of the PHI nodes are missing entries for some
731 // basic blocks. Start by sorting the incoming predecessors for efficient
733 auto CompareBBNumbers = [this](BasicBlock *A, BasicBlock *B) {
734 return BBNumbers.find(A)->second < BBNumbers.find(B)->second;
736 llvm::sort(Preds, CompareBBNumbers);
738 // Now we loop through all BB's which have entries in SomePHI and remove
739 // them from the Preds list.
740 for (unsigned i = 0, e = SomePHI->getNumIncomingValues(); i != e; ++i) {
741 // Do a log(n) search of the Preds list for the entry we want.
742 SmallVectorImpl<BasicBlock *>::iterator EntIt = llvm::lower_bound(
743 Preds, SomePHI->getIncomingBlock(i), CompareBBNumbers);
744 assert(EntIt != Preds.end() && *EntIt == SomePHI->getIncomingBlock(i) &&
745 "PHI node has entry for a block which is not a predecessor!");
751 // At this point, the blocks left in the preds list must have dummy
752 // entries inserted into every PHI nodes for the block. Update all the phi
753 // nodes in this block that we are inserting (there could be phis before
755 unsigned NumBadPreds = SomePHI->getNumIncomingValues();
756 BasicBlock::iterator BBI = BB->begin();
757 while ((SomePHI = dyn_cast<PHINode>(BBI++)) &&
758 SomePHI->getNumIncomingValues() == NumBadPreds) {
759 Value *UndefVal = UndefValue::get(SomePHI->getType());
760 for (BasicBlock *Pred : Preds)
761 SomePHI->addIncoming(UndefVal, Pred);
768 /// Determine which blocks the value is live in.
770 /// These are blocks which lead to uses. Knowing this allows us to avoid
771 /// inserting PHI nodes into blocks which don't lead to uses (thus, the
772 /// inserted phi nodes would be dead).
773 void PromoteMem2Reg::ComputeLiveInBlocks(
774 AllocaInst *AI, AllocaInfo &Info,
775 const SmallPtrSetImpl<BasicBlock *> &DefBlocks,
776 SmallPtrSetImpl<BasicBlock *> &LiveInBlocks) {
777 // To determine liveness, we must iterate through the predecessors of blocks
778 // where the def is live. Blocks are added to the worklist if we need to
779 // check their predecessors. Start with all the using blocks.
780 SmallVector<BasicBlock *, 64> LiveInBlockWorklist(Info.UsingBlocks.begin(),
781 Info.UsingBlocks.end());
783 // If any of the using blocks is also a definition block, check to see if the
784 // definition occurs before or after the use. If it happens before the use,
785 // the value isn't really live-in.
786 for (unsigned i = 0, e = LiveInBlockWorklist.size(); i != e; ++i) {
787 BasicBlock *BB = LiveInBlockWorklist[i];
788 if (!DefBlocks.count(BB))
791 // Okay, this is a block that both uses and defines the value. If the first
792 // reference to the alloca is a def (store), then we know it isn't live-in.
793 for (BasicBlock::iterator I = BB->begin();; ++I) {
794 if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
795 if (SI->getOperand(1) != AI)
798 // We found a store to the alloca before a load. The alloca is not
799 // actually live-in here.
800 LiveInBlockWorklist[i] = LiveInBlockWorklist.back();
801 LiveInBlockWorklist.pop_back();
807 if (LoadInst *LI = dyn_cast<LoadInst>(I))
808 // Okay, we found a load before a store to the alloca. It is actually
809 // live into this block.
810 if (LI->getOperand(0) == AI)
815 // Now that we have a set of blocks where the phi is live-in, recursively add
816 // their predecessors until we find the full region the value is live.
817 while (!LiveInBlockWorklist.empty()) {
818 BasicBlock *BB = LiveInBlockWorklist.pop_back_val();
820 // The block really is live in here, insert it into the set. If already in
821 // the set, then it has already been processed.
822 if (!LiveInBlocks.insert(BB).second)
825 // Since the value is live into BB, it is either defined in a predecessor or
826 // live into it to. Add the preds to the worklist unless they are a
828 for (BasicBlock *P : predecessors(BB)) {
829 // The value is not live into a predecessor if it defines the value.
830 if (DefBlocks.count(P))
833 // Otherwise it is, add to the worklist.
834 LiveInBlockWorklist.push_back(P);
839 /// Queue a phi-node to be added to a basic-block for a specific Alloca.
841 /// Returns true if there wasn't already a phi-node for that variable
842 bool PromoteMem2Reg::QueuePhiNode(BasicBlock *BB, unsigned AllocaNo,
844 // Look up the basic-block in question.
845 PHINode *&PN = NewPhiNodes[std::make_pair(BBNumbers[BB], AllocaNo)];
847 // If the BB already has a phi node added for the i'th alloca then we're done!
851 // Create a PhiNode using the dereferenced type... and add the phi-node to the
853 PN = PHINode::Create(Allocas[AllocaNo]->getAllocatedType(), getNumPreds(BB),
854 Allocas[AllocaNo]->getName() + "." + Twine(Version++),
857 PhiToAllocaMap[PN] = AllocaNo;
861 /// Update the debug location of a phi. \p ApplyMergedLoc indicates whether to
862 /// create a merged location incorporating \p DL, or to set \p DL directly.
863 static void updateForIncomingValueLocation(PHINode *PN, DebugLoc DL,
864 bool ApplyMergedLoc) {
866 PN->applyMergedLocation(PN->getDebugLoc(), DL);
871 /// Recursively traverse the CFG of the function, renaming loads and
872 /// stores to the allocas which we are promoting.
874 /// IncomingVals indicates what value each Alloca contains on exit from the
875 /// predecessor block Pred.
876 void PromoteMem2Reg::RenamePass(BasicBlock *BB, BasicBlock *Pred,
877 RenamePassData::ValVector &IncomingVals,
878 RenamePassData::LocationVector &IncomingLocs,
879 std::vector<RenamePassData> &Worklist) {
881 // If we are inserting any phi nodes into this BB, they will already be in the
883 if (PHINode *APN = dyn_cast<PHINode>(BB->begin())) {
884 // If we have PHI nodes to update, compute the number of edges from Pred to
886 if (PhiToAllocaMap.count(APN)) {
887 // We want to be able to distinguish between PHI nodes being inserted by
888 // this invocation of mem2reg from those phi nodes that already existed in
889 // the IR before mem2reg was run. We determine that APN is being inserted
890 // because it is missing incoming edges. All other PHI nodes being
891 // inserted by this pass of mem2reg will have the same number of incoming
892 // operands so far. Remember this count.
893 unsigned NewPHINumOperands = APN->getNumOperands();
895 unsigned NumEdges = std::count(succ_begin(Pred), succ_end(Pred), BB);
896 assert(NumEdges && "Must be at least one edge from Pred to BB!");
898 // Add entries for all the phis.
899 BasicBlock::iterator PNI = BB->begin();
901 unsigned AllocaNo = PhiToAllocaMap[APN];
903 // Update the location of the phi node.
904 updateForIncomingValueLocation(APN, IncomingLocs[AllocaNo],
905 APN->getNumIncomingValues() > 0);
907 // Add N incoming values to the PHI node.
908 for (unsigned i = 0; i != NumEdges; ++i)
909 APN->addIncoming(IncomingVals[AllocaNo], Pred);
911 // The currently active variable for this block is now the PHI.
912 IncomingVals[AllocaNo] = APN;
913 for (DbgVariableIntrinsic *DII : AllocaDbgDeclares[AllocaNo])
914 ConvertDebugDeclareToDebugValue(DII, APN, DIB);
916 // Get the next phi node.
918 APN = dyn_cast<PHINode>(PNI);
922 // Verify that it is missing entries. If not, it is not being inserted
923 // by this mem2reg invocation so we want to ignore it.
924 } while (APN->getNumOperands() == NewPHINumOperands);
928 // Don't revisit blocks.
929 if (!Visited.insert(BB).second)
932 for (BasicBlock::iterator II = BB->begin(); !II->isTerminator();) {
933 Instruction *I = &*II++; // get the instruction, increment iterator
935 if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
936 AllocaInst *Src = dyn_cast<AllocaInst>(LI->getPointerOperand());
940 DenseMap<AllocaInst *, unsigned>::iterator AI = AllocaLookup.find(Src);
941 if (AI == AllocaLookup.end())
944 Value *V = IncomingVals[AI->second];
946 // If the load was marked as nonnull we don't want to lose
947 // that information when we erase this Load. So we preserve
948 // it with an assume.
949 if (AC && LI->getMetadata(LLVMContext::MD_nonnull) &&
950 !isKnownNonZero(V, SQ.DL, 0, AC, LI, &DT))
951 addAssumeNonNull(AC, LI);
953 // Anything using the load now uses the current value.
954 LI->replaceAllUsesWith(V);
955 BB->getInstList().erase(LI);
956 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
957 // Delete this instruction and mark the name as the current holder of the
959 AllocaInst *Dest = dyn_cast<AllocaInst>(SI->getPointerOperand());
963 DenseMap<AllocaInst *, unsigned>::iterator ai = AllocaLookup.find(Dest);
964 if (ai == AllocaLookup.end())
967 // what value were we writing?
968 unsigned AllocaNo = ai->second;
969 IncomingVals[AllocaNo] = SI->getOperand(0);
971 // Record debuginfo for the store before removing it.
972 IncomingLocs[AllocaNo] = SI->getDebugLoc();
973 for (DbgVariableIntrinsic *DII : AllocaDbgDeclares[ai->second])
974 ConvertDebugDeclareToDebugValue(DII, SI, DIB);
975 BB->getInstList().erase(SI);
979 // 'Recurse' to our successors.
980 succ_iterator I = succ_begin(BB), E = succ_end(BB);
984 // Keep track of the successors so we don't visit the same successor twice
985 SmallPtrSet<BasicBlock *, 8> VisitedSuccs;
987 // Handle the first successor without using the worklist.
988 VisitedSuccs.insert(*I);
994 if (VisitedSuccs.insert(*I).second)
995 Worklist.emplace_back(*I, Pred, IncomingVals, IncomingLocs);
1000 void llvm::PromoteMemToReg(ArrayRef<AllocaInst *> Allocas, DominatorTree &DT,
1001 AssumptionCache *AC) {
1002 // If there is nothing to do, bail out...
1003 if (Allocas.empty())
1006 PromoteMem2Reg(Allocas, DT, AC).run();