1 //===- PromoteMemoryToRegister.cpp - Convert allocas to registers ---------===//
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 file promotes memory references to be register references. It promotes
11 // alloca instructions which only have loads and stores as uses. An alloca is
12 // transformed by using iterated dominator frontiers to place PHI nodes, then
13 // traversing the function in depth-first order to rewrite loads and stores as
16 // The algorithm used here is based on:
18 // Sreedhar and Gao. A linear time algorithm for placing phi-nodes.
19 // In Proceedings of the 22nd ACM SIGPLAN-SIGACT Symposium on Principles of
20 // Programming Languages
21 // POPL '95. ACM, New York, NY, 62-73.
23 // It has been modified to not explicitly use the DJ graph data structure and to
24 // directly compute pruned SSA using per-variable liveness information.
26 //===----------------------------------------------------------------------===//
28 #define DEBUG_TYPE "mem2reg"
29 #include "llvm/Transforms/Utils/PromoteMemToReg.h"
30 #include "llvm/ADT/ArrayRef.h"
31 #include "llvm/ADT/DenseMap.h"
32 #include "llvm/ADT/STLExtras.h"
33 #include "llvm/ADT/SmallPtrSet.h"
34 #include "llvm/ADT/SmallVector.h"
35 #include "llvm/ADT/Statistic.h"
36 #include "llvm/Analysis/AliasSetTracker.h"
37 #include "llvm/Analysis/InstructionSimplify.h"
38 #include "llvm/Analysis/ValueTracking.h"
39 #include "llvm/IR/CFG.h"
40 #include "llvm/IR/Constants.h"
41 #include "llvm/IR/DIBuilder.h"
42 #include "llvm/IR/DebugInfo.h"
43 #include "llvm/IR/DerivedTypes.h"
44 #include "llvm/IR/Dominators.h"
45 #include "llvm/IR/Function.h"
46 #include "llvm/IR/Instructions.h"
47 #include "llvm/IR/IntrinsicInst.h"
48 #include "llvm/IR/Metadata.h"
49 #include "llvm/Transforms/Utils/Local.h"
54 STATISTIC(NumLocalPromoted, "Number of alloca's promoted within one block");
55 STATISTIC(NumSingleStore, "Number of alloca's promoted with a single store");
56 STATISTIC(NumDeadAlloca, "Number of dead alloca's removed");
57 STATISTIC(NumPHIInsert, "Number of PHI nodes inserted");
59 bool llvm::isAllocaPromotable(const AllocaInst *AI) {
60 // FIXME: If the memory unit is of pointer or integer type, we can permit
61 // assignments to subsections of the memory unit.
63 // Only allow direct and non-volatile loads and stores...
64 for (const User *U : AI->users()) {
65 if (const LoadInst *LI = dyn_cast<LoadInst>(U)) {
66 // Note that atomic loads can be transformed; atomic semantics do
67 // not have any meaning for a local alloca.
70 } else if (const StoreInst *SI = dyn_cast<StoreInst>(U)) {
71 if (SI->getOperand(0) == AI)
72 return false; // Don't allow a store OF the AI, only INTO the AI.
73 // Note that atomic stores can be transformed; atomic semantics do
74 // not have any meaning for a local alloca.
77 } else if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(U)) {
78 if (II->getIntrinsicID() != Intrinsic::lifetime_start &&
79 II->getIntrinsicID() != Intrinsic::lifetime_end)
81 } else if (const BitCastInst *BCI = dyn_cast<BitCastInst>(U)) {
82 if (BCI->getType() != Type::getInt8PtrTy(U->getContext()))
84 if (!onlyUsedByLifetimeMarkers(BCI))
86 } else if (const GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(U)) {
87 if (GEPI->getType() != Type::getInt8PtrTy(U->getContext()))
89 if (!GEPI->hasAllZeroIndices())
91 if (!onlyUsedByLifetimeMarkers(GEPI))
104 SmallVector<BasicBlock *, 32> DefiningBlocks;
105 SmallVector<BasicBlock *, 32> UsingBlocks;
107 StoreInst *OnlyStore;
108 BasicBlock *OnlyBlock;
109 bool OnlyUsedInOneBlock;
111 Value *AllocaPointerVal;
112 DbgDeclareInst *DbgDeclare;
115 DefiningBlocks.clear();
119 OnlyUsedInOneBlock = true;
120 AllocaPointerVal = 0;
124 /// Scan the uses of the specified alloca, filling in the AllocaInfo used
125 /// by the rest of the pass to reason about the uses of this alloca.
126 void AnalyzeAlloca(AllocaInst *AI) {
129 // As we scan the uses of the alloca instruction, keep track of stores,
130 // and decide whether all of the loads and stores to the alloca are within
131 // the same basic block.
132 for (auto UI = AI->user_begin(), E = AI->user_end(); UI != E;) {
133 Instruction *User = cast<Instruction>(*UI++);
135 if (StoreInst *SI = dyn_cast<StoreInst>(User)) {
136 // Remember the basic blocks which define new values for the alloca
137 DefiningBlocks.push_back(SI->getParent());
138 AllocaPointerVal = SI->getOperand(0);
141 LoadInst *LI = cast<LoadInst>(User);
142 // Otherwise it must be a load instruction, keep track of variable
144 UsingBlocks.push_back(LI->getParent());
145 AllocaPointerVal = LI;
148 if (OnlyUsedInOneBlock) {
150 OnlyBlock = User->getParent();
151 else if (OnlyBlock != User->getParent())
152 OnlyUsedInOneBlock = false;
156 DbgDeclare = FindAllocaDbgDeclare(AI);
160 // Data package used by RenamePass()
161 class RenamePassData {
163 typedef std::vector<Value *> ValVector;
165 RenamePassData() : BB(NULL), Pred(NULL), Values() {}
166 RenamePassData(BasicBlock *B, BasicBlock *P, const ValVector &V)
167 : BB(B), Pred(P), Values(V) {}
172 void swap(RenamePassData &RHS) {
173 std::swap(BB, RHS.BB);
174 std::swap(Pred, RHS.Pred);
175 Values.swap(RHS.Values);
179 /// \brief This assigns and keeps a per-bb relative ordering of load/store
180 /// instructions in the block that directly load or store an alloca.
182 /// This functionality is important because it avoids scanning large basic
183 /// blocks multiple times when promoting many allocas in the same block.
184 class LargeBlockInfo {
185 /// \brief For each instruction that we track, keep the index of the
188 /// The index starts out as the number of the instruction from the start of
190 DenseMap<const Instruction *, unsigned> InstNumbers;
194 /// This code only looks at accesses to allocas.
195 static bool isInterestingInstruction(const Instruction *I) {
196 return (isa<LoadInst>(I) && isa<AllocaInst>(I->getOperand(0))) ||
197 (isa<StoreInst>(I) && isa<AllocaInst>(I->getOperand(1)));
200 /// Get or calculate the index of the specified instruction.
201 unsigned getInstructionIndex(const Instruction *I) {
202 assert(isInterestingInstruction(I) &&
203 "Not a load/store to/from an alloca?");
205 // If we already have this instruction number, return it.
206 DenseMap<const Instruction *, unsigned>::iterator It = InstNumbers.find(I);
207 if (It != InstNumbers.end())
210 // Scan the whole block to get the instruction. This accumulates
211 // information for every interesting instruction in the block, in order to
212 // avoid gratuitus rescans.
213 const BasicBlock *BB = I->getParent();
215 for (BasicBlock::const_iterator BBI = BB->begin(), E = BB->end(); BBI != E;
217 if (isInterestingInstruction(BBI))
218 InstNumbers[BBI] = InstNo++;
219 It = InstNumbers.find(I);
221 assert(It != InstNumbers.end() && "Didn't insert instruction?");
225 void deleteValue(const Instruction *I) { InstNumbers.erase(I); }
227 void clear() { InstNumbers.clear(); }
230 struct PromoteMem2Reg {
231 /// The alloca instructions being promoted.
232 std::vector<AllocaInst *> Allocas;
236 /// An AliasSetTracker object to update. If null, don't update it.
237 AliasSetTracker *AST;
239 /// Reverse mapping of Allocas.
240 DenseMap<AllocaInst *, unsigned> AllocaLookup;
242 /// \brief The PhiNodes we're adding.
244 /// That map is used to simplify some Phi nodes as we iterate over it, so
245 /// it should have deterministic iterators. We could use a MapVector, but
246 /// since we already maintain a map from BasicBlock* to a stable numbering
247 /// (BBNumbers), the DenseMap is more efficient (also supports removal).
248 DenseMap<std::pair<unsigned, unsigned>, PHINode *> NewPhiNodes;
250 /// For each PHI node, keep track of which entry in Allocas it corresponds
252 DenseMap<PHINode *, unsigned> PhiToAllocaMap;
254 /// If we are updating an AliasSetTracker, then for each alloca that is of
255 /// pointer type, we keep track of what to copyValue to the inserted PHI
257 std::vector<Value *> PointerAllocaValues;
259 /// For each alloca, we keep track of the dbg.declare intrinsic that
260 /// describes it, if any, so that we can convert it to a dbg.value
261 /// intrinsic if the alloca gets promoted.
262 SmallVector<DbgDeclareInst *, 8> AllocaDbgDeclares;
264 /// The set of basic blocks the renamer has already visited.
266 SmallPtrSet<BasicBlock *, 16> Visited;
268 /// Contains a stable numbering of basic blocks to avoid non-determinstic
270 DenseMap<BasicBlock *, unsigned> BBNumbers;
272 /// Maps DomTreeNodes to their level in the dominator tree.
273 DenseMap<DomTreeNode *, unsigned> DomLevels;
275 /// Lazily compute the number of predecessors a block has.
276 DenseMap<const BasicBlock *, unsigned> BBNumPreds;
279 PromoteMem2Reg(ArrayRef<AllocaInst *> Allocas, DominatorTree &DT,
280 AliasSetTracker *AST)
281 : Allocas(Allocas.begin(), Allocas.end()), DT(DT),
282 DIB(*DT.getRoot()->getParent()->getParent()), AST(AST) {}
287 void RemoveFromAllocasList(unsigned &AllocaIdx) {
288 Allocas[AllocaIdx] = Allocas.back();
293 unsigned getNumPreds(const BasicBlock *BB) {
294 unsigned &NP = BBNumPreds[BB];
296 NP = std::distance(pred_begin(BB), pred_end(BB)) + 1;
300 void DetermineInsertionPoint(AllocaInst *AI, unsigned AllocaNum,
302 void ComputeLiveInBlocks(AllocaInst *AI, AllocaInfo &Info,
303 const SmallPtrSet<BasicBlock *, 32> &DefBlocks,
304 SmallPtrSet<BasicBlock *, 32> &LiveInBlocks);
305 void RenamePass(BasicBlock *BB, BasicBlock *Pred,
306 RenamePassData::ValVector &IncVals,
307 std::vector<RenamePassData> &Worklist);
308 bool QueuePhiNode(BasicBlock *BB, unsigned AllocaIdx, unsigned &Version);
311 } // end of anonymous namespace
313 static void removeLifetimeIntrinsicUsers(AllocaInst *AI) {
314 // Knowing that this alloca is promotable, we know that it's safe to kill all
315 // instructions except for load and store.
317 for (auto UI = AI->user_begin(), UE = AI->user_end(); UI != UE;) {
318 Instruction *I = cast<Instruction>(*UI);
320 if (isa<LoadInst>(I) || isa<StoreInst>(I))
323 if (!I->getType()->isVoidTy()) {
324 // The only users of this bitcast/GEP instruction are lifetime intrinsics.
325 // Follow the use/def chain to erase them now instead of leaving it for
326 // dead code elimination later.
327 for (auto UUI = I->user_begin(), UUE = I->user_end(); UUI != UUE;) {
328 Instruction *Inst = cast<Instruction>(*UUI);
330 Inst->eraseFromParent();
333 I->eraseFromParent();
337 /// \brief Rewrite as many loads as possible given a single store.
339 /// When there is only a single store, we can use the domtree to trivially
340 /// replace all of the dominated loads with the stored value. Do so, and return
341 /// true if this has successfully promoted the alloca entirely. If this returns
342 /// false there were some loads which were not dominated by the single store
343 /// and thus must be phi-ed with undef. We fall back to the standard alloca
344 /// promotion algorithm in that case.
345 static bool rewriteSingleStoreAlloca(AllocaInst *AI, AllocaInfo &Info,
348 AliasSetTracker *AST) {
349 StoreInst *OnlyStore = Info.OnlyStore;
350 bool StoringGlobalVal = !isa<Instruction>(OnlyStore->getOperand(0));
351 BasicBlock *StoreBB = OnlyStore->getParent();
354 // Clear out UsingBlocks. We will reconstruct it here if needed.
355 Info.UsingBlocks.clear();
357 for (auto UI = AI->user_begin(), E = AI->user_end(); UI != E;) {
358 Instruction *UserInst = cast<Instruction>(*UI++);
359 if (!isa<LoadInst>(UserInst)) {
360 assert(UserInst == OnlyStore && "Should only have load/stores");
363 LoadInst *LI = cast<LoadInst>(UserInst);
365 // Okay, if we have a load from the alloca, we want to replace it with the
366 // only value stored to the alloca. We can do this if the value is
367 // dominated by the store. If not, we use the rest of the mem2reg machinery
368 // to insert the phi nodes as needed.
369 if (!StoringGlobalVal) { // Non-instructions are always dominated.
370 if (LI->getParent() == StoreBB) {
371 // If we have a use that is in the same block as the store, compare the
372 // indices of the two instructions to see which one came first. If the
373 // load came before the store, we can't handle it.
374 if (StoreIndex == -1)
375 StoreIndex = LBI.getInstructionIndex(OnlyStore);
377 if (unsigned(StoreIndex) > LBI.getInstructionIndex(LI)) {
378 // Can't handle this load, bail out.
379 Info.UsingBlocks.push_back(StoreBB);
383 } else if (LI->getParent() != StoreBB &&
384 !DT.dominates(StoreBB, LI->getParent())) {
385 // If the load and store are in different blocks, use BB dominance to
386 // check their relationships. If the store doesn't dom the use, bail
388 Info.UsingBlocks.push_back(LI->getParent());
393 // Otherwise, we *can* safely rewrite this load.
394 Value *ReplVal = OnlyStore->getOperand(0);
395 // If the replacement value is the load, this must occur in unreachable
398 ReplVal = UndefValue::get(LI->getType());
399 LI->replaceAllUsesWith(ReplVal);
400 if (AST && LI->getType()->isPointerTy())
401 AST->deleteValue(LI);
402 LI->eraseFromParent();
406 // Finally, after the scan, check to see if the store is all that is left.
407 if (!Info.UsingBlocks.empty())
408 return false; // If not, we'll have to fall back for the remainder.
410 // Record debuginfo for the store and remove the declaration's
412 if (DbgDeclareInst *DDI = Info.DbgDeclare) {
413 DIBuilder DIB(*AI->getParent()->getParent()->getParent());
414 ConvertDebugDeclareToDebugValue(DDI, Info.OnlyStore, DIB);
415 DDI->eraseFromParent();
416 LBI.deleteValue(DDI);
418 // Remove the (now dead) store and alloca.
419 Info.OnlyStore->eraseFromParent();
420 LBI.deleteValue(Info.OnlyStore);
423 AST->deleteValue(AI);
424 AI->eraseFromParent();
429 /// Many allocas are only used within a single basic block. If this is the
430 /// case, avoid traversing the CFG and inserting a lot of potentially useless
431 /// PHI nodes by just performing a single linear pass over the basic block
432 /// using the Alloca.
434 /// If we cannot promote this alloca (because it is read before it is written),
435 /// return true. This is necessary in cases where, due to control flow, the
436 /// alloca is potentially undefined on some control flow paths. e.g. code like
437 /// this is potentially correct:
439 /// for (...) { if (c) { A = undef; undef = B; } }
441 /// ... so long as A is not used before undef is set.
442 static void promoteSingleBlockAlloca(AllocaInst *AI, const AllocaInfo &Info,
444 AliasSetTracker *AST) {
445 // The trickiest case to handle is when we have large blocks. Because of this,
446 // this code is optimized assuming that large blocks happen. This does not
447 // significantly pessimize the small block case. This uses LargeBlockInfo to
448 // make it efficient to get the index of various operations in the block.
450 // Walk the use-def list of the alloca, getting the locations of all stores.
451 typedef SmallVector<std::pair<unsigned, StoreInst *>, 64> StoresByIndexTy;
452 StoresByIndexTy StoresByIndex;
454 for (User *U : AI->users())
455 if (StoreInst *SI = dyn_cast<StoreInst>(U))
456 StoresByIndex.push_back(std::make_pair(LBI.getInstructionIndex(SI), SI));
458 // Sort the stores by their index, making it efficient to do a lookup with a
460 std::sort(StoresByIndex.begin(), StoresByIndex.end(), less_first());
462 // Walk all of the loads from this alloca, replacing them with the nearest
463 // store above them, if any.
464 for (auto UI = AI->user_begin(), E = AI->user_end(); UI != E;) {
465 LoadInst *LI = dyn_cast<LoadInst>(*UI++);
469 unsigned LoadIdx = LBI.getInstructionIndex(LI);
471 // Find the nearest store that has a lower index than this load.
472 StoresByIndexTy::iterator I =
473 std::lower_bound(StoresByIndex.begin(), StoresByIndex.end(),
474 std::make_pair(LoadIdx, static_cast<StoreInst *>(0)),
477 if (I == StoresByIndex.begin())
478 // If there is no store before this load, the load takes the undef value.
479 LI->replaceAllUsesWith(UndefValue::get(LI->getType()));
481 // Otherwise, there was a store before this load, the load takes its value.
482 LI->replaceAllUsesWith(std::prev(I)->second->getOperand(0));
484 if (AST && LI->getType()->isPointerTy())
485 AST->deleteValue(LI);
486 LI->eraseFromParent();
490 // Remove the (now dead) stores and alloca.
491 while (!AI->use_empty()) {
492 StoreInst *SI = cast<StoreInst>(AI->user_back());
493 // Record debuginfo for the store before removing it.
494 if (DbgDeclareInst *DDI = Info.DbgDeclare) {
495 DIBuilder DIB(*AI->getParent()->getParent()->getParent());
496 ConvertDebugDeclareToDebugValue(DDI, SI, DIB);
498 SI->eraseFromParent();
503 AST->deleteValue(AI);
504 AI->eraseFromParent();
507 // The alloca's debuginfo can be removed as well.
508 if (DbgDeclareInst *DDI = Info.DbgDeclare) {
509 DDI->eraseFromParent();
510 LBI.deleteValue(DDI);
516 void PromoteMem2Reg::run() {
517 Function &F = *DT.getRoot()->getParent();
520 PointerAllocaValues.resize(Allocas.size());
521 AllocaDbgDeclares.resize(Allocas.size());
526 for (unsigned AllocaNum = 0; AllocaNum != Allocas.size(); ++AllocaNum) {
527 AllocaInst *AI = Allocas[AllocaNum];
529 assert(isAllocaPromotable(AI) && "Cannot promote non-promotable alloca!");
530 assert(AI->getParent()->getParent() == &F &&
531 "All allocas should be in the same function, which is same as DF!");
533 removeLifetimeIntrinsicUsers(AI);
535 if (AI->use_empty()) {
536 // If there are no uses of the alloca, just delete it now.
538 AST->deleteValue(AI);
539 AI->eraseFromParent();
541 // Remove the alloca from the Allocas list, since it has been processed
542 RemoveFromAllocasList(AllocaNum);
547 // Calculate the set of read and write-locations for each alloca. This is
548 // analogous to finding the 'uses' and 'definitions' of each variable.
549 Info.AnalyzeAlloca(AI);
551 // If there is only a single store to this value, replace any loads of
552 // it that are directly dominated by the definition with the value stored.
553 if (Info.DefiningBlocks.size() == 1) {
554 if (rewriteSingleStoreAlloca(AI, Info, LBI, DT, AST)) {
555 // The alloca has been processed, move on.
556 RemoveFromAllocasList(AllocaNum);
562 // If the alloca is only read and written in one basic block, just perform a
563 // linear sweep over the block to eliminate it.
564 if (Info.OnlyUsedInOneBlock) {
565 promoteSingleBlockAlloca(AI, Info, LBI, AST);
567 // The alloca has been processed, move on.
568 RemoveFromAllocasList(AllocaNum);
572 // If we haven't computed dominator tree levels, do so now.
573 if (DomLevels.empty()) {
574 SmallVector<DomTreeNode *, 32> Worklist;
576 DomTreeNode *Root = DT.getRootNode();
578 Worklist.push_back(Root);
580 while (!Worklist.empty()) {
581 DomTreeNode *Node = Worklist.pop_back_val();
582 unsigned ChildLevel = DomLevels[Node] + 1;
583 for (DomTreeNode::iterator CI = Node->begin(), CE = Node->end();
585 DomLevels[*CI] = ChildLevel;
586 Worklist.push_back(*CI);
591 // If we haven't computed a numbering for the BB's in the function, do so
593 if (BBNumbers.empty()) {
595 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
599 // If we have an AST to keep updated, remember some pointer value that is
600 // stored into the alloca.
602 PointerAllocaValues[AllocaNum] = Info.AllocaPointerVal;
604 // Remember the dbg.declare intrinsic describing this alloca, if any.
606 AllocaDbgDeclares[AllocaNum] = Info.DbgDeclare;
608 // Keep the reverse mapping of the 'Allocas' array for the rename pass.
609 AllocaLookup[Allocas[AllocaNum]] = AllocaNum;
611 // At this point, we're committed to promoting the alloca using IDF's, and
612 // the standard SSA construction algorithm. Determine which blocks need PHI
613 // nodes and see if we can optimize out some work by avoiding insertion of
615 DetermineInsertionPoint(AI, AllocaNum, Info);
619 return; // All of the allocas must have been trivial!
623 // Set the incoming values for the basic block to be null values for all of
624 // the alloca's. We do this in case there is a load of a value that has not
625 // been stored yet. In this case, it will get this null value.
627 RenamePassData::ValVector Values(Allocas.size());
628 for (unsigned i = 0, e = Allocas.size(); i != e; ++i)
629 Values[i] = UndefValue::get(Allocas[i]->getAllocatedType());
631 // Walks all basic blocks in the function performing the SSA rename algorithm
632 // and inserting the phi nodes we marked as necessary
634 std::vector<RenamePassData> RenamePassWorkList;
635 RenamePassWorkList.push_back(RenamePassData(F.begin(), 0, Values));
638 RPD.swap(RenamePassWorkList.back());
639 RenamePassWorkList.pop_back();
640 // RenamePass may add new worklist entries.
641 RenamePass(RPD.BB, RPD.Pred, RPD.Values, RenamePassWorkList);
642 } while (!RenamePassWorkList.empty());
644 // The renamer uses the Visited set to avoid infinite loops. Clear it now.
647 // Remove the allocas themselves from the function.
648 for (unsigned i = 0, e = Allocas.size(); i != e; ++i) {
649 Instruction *A = Allocas[i];
651 // If there are any uses of the alloca instructions left, they must be in
652 // unreachable basic blocks that were not processed by walking the dominator
653 // tree. Just delete the users now.
655 A->replaceAllUsesWith(UndefValue::get(A->getType()));
658 A->eraseFromParent();
661 // Remove alloca's dbg.declare instrinsics from the function.
662 for (unsigned i = 0, e = AllocaDbgDeclares.size(); i != e; ++i)
663 if (DbgDeclareInst *DDI = AllocaDbgDeclares[i])
664 DDI->eraseFromParent();
666 // Loop over all of the PHI nodes and see if there are any that we can get
667 // rid of because they merge all of the same incoming values. This can
668 // happen due to undef values coming into the PHI nodes. This process is
669 // iterative, because eliminating one PHI node can cause others to be removed.
670 bool EliminatedAPHI = true;
671 while (EliminatedAPHI) {
672 EliminatedAPHI = false;
674 // Iterating over NewPhiNodes is deterministic, so it is safe to try to
675 // simplify and RAUW them as we go. If it was not, we could add uses to
676 // the values we replace with in a non-deterministic order, thus creating
677 // non-deterministic def->use chains.
678 for (DenseMap<std::pair<unsigned, unsigned>, PHINode *>::iterator
679 I = NewPhiNodes.begin(),
680 E = NewPhiNodes.end();
682 PHINode *PN = I->second;
684 // If this PHI node merges one value and/or undefs, get the value.
685 if (Value *V = SimplifyInstruction(PN, 0, 0, &DT)) {
686 if (AST && PN->getType()->isPointerTy())
687 AST->deleteValue(PN);
688 PN->replaceAllUsesWith(V);
689 PN->eraseFromParent();
690 NewPhiNodes.erase(I++);
691 EliminatedAPHI = true;
698 // At this point, the renamer has added entries to PHI nodes for all reachable
699 // code. Unfortunately, there may be unreachable blocks which the renamer
700 // hasn't traversed. If this is the case, the PHI nodes may not
701 // have incoming values for all predecessors. Loop over all PHI nodes we have
702 // created, inserting undef values if they are missing any incoming values.
704 for (DenseMap<std::pair<unsigned, unsigned>, PHINode *>::iterator
705 I = NewPhiNodes.begin(),
706 E = NewPhiNodes.end();
708 // We want to do this once per basic block. As such, only process a block
709 // when we find the PHI that is the first entry in the block.
710 PHINode *SomePHI = I->second;
711 BasicBlock *BB = SomePHI->getParent();
712 if (&BB->front() != SomePHI)
715 // Only do work here if there the PHI nodes are missing incoming values. We
716 // know that all PHI nodes that were inserted in a block will have the same
717 // number of incoming values, so we can just check any of them.
718 if (SomePHI->getNumIncomingValues() == getNumPreds(BB))
721 // Get the preds for BB.
722 SmallVector<BasicBlock *, 16> Preds(pred_begin(BB), pred_end(BB));
724 // Ok, now we know that all of the PHI nodes are missing entries for some
725 // basic blocks. Start by sorting the incoming predecessors for efficient
727 std::sort(Preds.begin(), Preds.end());
729 // Now we loop through all BB's which have entries in SomePHI and remove
730 // them from the Preds list.
731 for (unsigned i = 0, e = SomePHI->getNumIncomingValues(); i != e; ++i) {
732 // Do a log(n) search of the Preds list for the entry we want.
733 SmallVectorImpl<BasicBlock *>::iterator EntIt = std::lower_bound(
734 Preds.begin(), Preds.end(), SomePHI->getIncomingBlock(i));
735 assert(EntIt != Preds.end() && *EntIt == SomePHI->getIncomingBlock(i) &&
736 "PHI node has entry for a block which is not a predecessor!");
742 // At this point, the blocks left in the preds list must have dummy
743 // entries inserted into every PHI nodes for the block. Update all the phi
744 // nodes in this block that we are inserting (there could be phis before
746 unsigned NumBadPreds = SomePHI->getNumIncomingValues();
747 BasicBlock::iterator BBI = BB->begin();
748 while ((SomePHI = dyn_cast<PHINode>(BBI++)) &&
749 SomePHI->getNumIncomingValues() == NumBadPreds) {
750 Value *UndefVal = UndefValue::get(SomePHI->getType());
751 for (unsigned pred = 0, e = Preds.size(); pred != e; ++pred)
752 SomePHI->addIncoming(UndefVal, Preds[pred]);
759 /// \brief Determine which blocks the value is live in.
761 /// These are blocks which lead to uses. Knowing this allows us to avoid
762 /// inserting PHI nodes into blocks which don't lead to uses (thus, the
763 /// inserted phi nodes would be dead).
764 void PromoteMem2Reg::ComputeLiveInBlocks(
765 AllocaInst *AI, AllocaInfo &Info,
766 const SmallPtrSet<BasicBlock *, 32> &DefBlocks,
767 SmallPtrSet<BasicBlock *, 32> &LiveInBlocks) {
769 // To determine liveness, we must iterate through the predecessors of blocks
770 // where the def is live. Blocks are added to the worklist if we need to
771 // check their predecessors. Start with all the using blocks.
772 SmallVector<BasicBlock *, 64> LiveInBlockWorklist(Info.UsingBlocks.begin(),
773 Info.UsingBlocks.end());
775 // If any of the using blocks is also a definition block, check to see if the
776 // definition occurs before or after the use. If it happens before the use,
777 // the value isn't really live-in.
778 for (unsigned i = 0, e = LiveInBlockWorklist.size(); i != e; ++i) {
779 BasicBlock *BB = LiveInBlockWorklist[i];
780 if (!DefBlocks.count(BB))
783 // Okay, this is a block that both uses and defines the value. If the first
784 // reference to the alloca is a def (store), then we know it isn't live-in.
785 for (BasicBlock::iterator I = BB->begin();; ++I) {
786 if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
787 if (SI->getOperand(1) != AI)
790 // We found a store to the alloca before a load. The alloca is not
791 // actually live-in here.
792 LiveInBlockWorklist[i] = LiveInBlockWorklist.back();
793 LiveInBlockWorklist.pop_back();
798 if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
799 if (LI->getOperand(0) != AI)
802 // Okay, we found a load before a store to the alloca. It is actually
803 // live into this block.
809 // Now that we have a set of blocks where the phi is live-in, recursively add
810 // their predecessors until we find the full region the value is live.
811 while (!LiveInBlockWorklist.empty()) {
812 BasicBlock *BB = LiveInBlockWorklist.pop_back_val();
814 // The block really is live in here, insert it into the set. If already in
815 // the set, then it has already been processed.
816 if (!LiveInBlocks.insert(BB))
819 // Since the value is live into BB, it is either defined in a predecessor or
820 // live into it to. Add the preds to the worklist unless they are a
822 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
825 // The value is not live into a predecessor if it defines the value.
826 if (DefBlocks.count(P))
829 // Otherwise it is, add to the worklist.
830 LiveInBlockWorklist.push_back(P);
835 /// At this point, we're committed to promoting the alloca using IDF's, and the
836 /// standard SSA construction algorithm. Determine which blocks need phi nodes
837 /// and see if we can optimize out some work by avoiding insertion of dead phi
839 void PromoteMem2Reg::DetermineInsertionPoint(AllocaInst *AI, unsigned AllocaNum,
841 // Unique the set of defining blocks for efficient lookup.
842 SmallPtrSet<BasicBlock *, 32> DefBlocks;
843 DefBlocks.insert(Info.DefiningBlocks.begin(), Info.DefiningBlocks.end());
845 // Determine which blocks the value is live in. These are blocks which lead
847 SmallPtrSet<BasicBlock *, 32> LiveInBlocks;
848 ComputeLiveInBlocks(AI, Info, DefBlocks, LiveInBlocks);
850 // Use a priority queue keyed on dominator tree level so that inserted nodes
851 // are handled from the bottom of the dominator tree upwards.
852 typedef std::pair<DomTreeNode *, unsigned> DomTreeNodePair;
853 typedef std::priority_queue<DomTreeNodePair, SmallVector<DomTreeNodePair, 32>,
854 less_second> IDFPriorityQueue;
857 for (SmallPtrSet<BasicBlock *, 32>::const_iterator I = DefBlocks.begin(),
860 if (DomTreeNode *Node = DT.getNode(*I))
861 PQ.push(std::make_pair(Node, DomLevels[Node]));
864 SmallVector<std::pair<unsigned, BasicBlock *>, 32> DFBlocks;
865 SmallPtrSet<DomTreeNode *, 32> Visited;
866 SmallVector<DomTreeNode *, 32> Worklist;
867 while (!PQ.empty()) {
868 DomTreeNodePair RootPair = PQ.top();
870 DomTreeNode *Root = RootPair.first;
871 unsigned RootLevel = RootPair.second;
873 // Walk all dominator tree children of Root, inspecting their CFG edges with
874 // targets elsewhere on the dominator tree. Only targets whose level is at
875 // most Root's level are added to the iterated dominance frontier of the
879 Worklist.push_back(Root);
881 while (!Worklist.empty()) {
882 DomTreeNode *Node = Worklist.pop_back_val();
883 BasicBlock *BB = Node->getBlock();
885 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE;
887 DomTreeNode *SuccNode = DT.getNode(*SI);
889 // Quickly skip all CFG edges that are also dominator tree edges instead
890 // of catching them below.
891 if (SuccNode->getIDom() == Node)
894 unsigned SuccLevel = DomLevels[SuccNode];
895 if (SuccLevel > RootLevel)
898 if (!Visited.insert(SuccNode))
901 BasicBlock *SuccBB = SuccNode->getBlock();
902 if (!LiveInBlocks.count(SuccBB))
905 DFBlocks.push_back(std::make_pair(BBNumbers[SuccBB], SuccBB));
906 if (!DefBlocks.count(SuccBB))
907 PQ.push(std::make_pair(SuccNode, SuccLevel));
910 for (DomTreeNode::iterator CI = Node->begin(), CE = Node->end(); CI != CE;
912 if (!Visited.count(*CI))
913 Worklist.push_back(*CI);
918 if (DFBlocks.size() > 1)
919 std::sort(DFBlocks.begin(), DFBlocks.end());
921 unsigned CurrentVersion = 0;
922 for (unsigned i = 0, e = DFBlocks.size(); i != e; ++i)
923 QueuePhiNode(DFBlocks[i].second, AllocaNum, CurrentVersion);
926 /// \brief Queue a phi-node to be added to a basic-block for a specific Alloca.
928 /// Returns true if there wasn't already a phi-node for that variable
929 bool PromoteMem2Reg::QueuePhiNode(BasicBlock *BB, unsigned AllocaNo,
931 // Look up the basic-block in question.
932 PHINode *&PN = NewPhiNodes[std::make_pair(BBNumbers[BB], AllocaNo)];
934 // If the BB already has a phi node added for the i'th alloca then we're done!
938 // Create a PhiNode using the dereferenced type... and add the phi-node to the
940 PN = PHINode::Create(Allocas[AllocaNo]->getAllocatedType(), getNumPreds(BB),
941 Allocas[AllocaNo]->getName() + "." + Twine(Version++),
944 PhiToAllocaMap[PN] = AllocaNo;
946 if (AST && PN->getType()->isPointerTy())
947 AST->copyValue(PointerAllocaValues[AllocaNo], PN);
952 /// \brief Recursively traverse the CFG of the function, renaming loads and
953 /// stores to the allocas which we are promoting.
955 /// IncomingVals indicates what value each Alloca contains on exit from the
956 /// predecessor block Pred.
957 void PromoteMem2Reg::RenamePass(BasicBlock *BB, BasicBlock *Pred,
958 RenamePassData::ValVector &IncomingVals,
959 std::vector<RenamePassData> &Worklist) {
961 // If we are inserting any phi nodes into this BB, they will already be in the
963 if (PHINode *APN = dyn_cast<PHINode>(BB->begin())) {
964 // If we have PHI nodes to update, compute the number of edges from Pred to
966 if (PhiToAllocaMap.count(APN)) {
967 // We want to be able to distinguish between PHI nodes being inserted by
968 // this invocation of mem2reg from those phi nodes that already existed in
969 // the IR before mem2reg was run. We determine that APN is being inserted
970 // because it is missing incoming edges. All other PHI nodes being
971 // inserted by this pass of mem2reg will have the same number of incoming
972 // operands so far. Remember this count.
973 unsigned NewPHINumOperands = APN->getNumOperands();
975 unsigned NumEdges = std::count(succ_begin(Pred), succ_end(Pred), BB);
976 assert(NumEdges && "Must be at least one edge from Pred to BB!");
978 // Add entries for all the phis.
979 BasicBlock::iterator PNI = BB->begin();
981 unsigned AllocaNo = PhiToAllocaMap[APN];
983 // Add N incoming values to the PHI node.
984 for (unsigned i = 0; i != NumEdges; ++i)
985 APN->addIncoming(IncomingVals[AllocaNo], Pred);
987 // The currently active variable for this block is now the PHI.
988 IncomingVals[AllocaNo] = APN;
990 // Get the next phi node.
992 APN = dyn_cast<PHINode>(PNI);
996 // Verify that it is missing entries. If not, it is not being inserted
997 // by this mem2reg invocation so we want to ignore it.
998 } while (APN->getNumOperands() == NewPHINumOperands);
1002 // Don't revisit blocks.
1003 if (!Visited.insert(BB))
1006 for (BasicBlock::iterator II = BB->begin(); !isa<TerminatorInst>(II);) {
1007 Instruction *I = II++; // get the instruction, increment iterator
1009 if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
1010 AllocaInst *Src = dyn_cast<AllocaInst>(LI->getPointerOperand());
1014 DenseMap<AllocaInst *, unsigned>::iterator AI = AllocaLookup.find(Src);
1015 if (AI == AllocaLookup.end())
1018 Value *V = IncomingVals[AI->second];
1020 // Anything using the load now uses the current value.
1021 LI->replaceAllUsesWith(V);
1022 if (AST && LI->getType()->isPointerTy())
1023 AST->deleteValue(LI);
1024 BB->getInstList().erase(LI);
1025 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
1026 // Delete this instruction and mark the name as the current holder of the
1028 AllocaInst *Dest = dyn_cast<AllocaInst>(SI->getPointerOperand());
1032 DenseMap<AllocaInst *, unsigned>::iterator ai = AllocaLookup.find(Dest);
1033 if (ai == AllocaLookup.end())
1036 // what value were we writing?
1037 IncomingVals[ai->second] = SI->getOperand(0);
1038 // Record debuginfo for the store before removing it.
1039 if (DbgDeclareInst *DDI = AllocaDbgDeclares[ai->second])
1040 ConvertDebugDeclareToDebugValue(DDI, SI, DIB);
1041 BB->getInstList().erase(SI);
1045 // 'Recurse' to our successors.
1046 succ_iterator I = succ_begin(BB), E = succ_end(BB);
1050 // Keep track of the successors so we don't visit the same successor twice
1051 SmallPtrSet<BasicBlock *, 8> VisitedSuccs;
1053 // Handle the first successor without using the worklist.
1054 VisitedSuccs.insert(*I);
1060 if (VisitedSuccs.insert(*I))
1061 Worklist.push_back(RenamePassData(*I, Pred, IncomingVals));
1066 void llvm::PromoteMemToReg(ArrayRef<AllocaInst *> Allocas, DominatorTree &DT,
1067 AliasSetTracker *AST) {
1068 // If there is nothing to do, bail out...
1069 if (Allocas.empty())
1072 PromoteMem2Reg(Allocas, DT, AST).run();