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 dominator frontiers to place PHI nodes, then traversing
13 // the function in depth-first order to rewrite loads and stores as appropriate.
14 // This is just the standard SSA construction algorithm to construct "pruned"
17 //===----------------------------------------------------------------------===//
19 #define DEBUG_TYPE "mem2reg"
20 #include "llvm/Transforms/Utils/PromoteMemToReg.h"
21 #include "llvm/Constants.h"
22 #include "llvm/DerivedTypes.h"
23 #include "llvm/Function.h"
24 #include "llvm/Instructions.h"
25 #include "llvm/IntrinsicInst.h"
26 #include "llvm/Analysis/Dominators.h"
27 #include "llvm/Analysis/AliasSetTracker.h"
28 #include "llvm/ADT/DenseMap.h"
29 #include "llvm/ADT/SmallPtrSet.h"
30 #include "llvm/ADT/SmallVector.h"
31 #include "llvm/ADT/Statistic.h"
32 #include "llvm/ADT/StringExtras.h"
33 #include "llvm/ADT/STLExtras.h"
34 #include "llvm/Support/CFG.h"
35 #include "llvm/Support/Compiler.h"
39 STATISTIC(NumLocalPromoted, "Number of alloca's promoted within one block");
40 STATISTIC(NumSingleStore, "Number of alloca's promoted with a single store");
41 STATISTIC(NumDeadAlloca, "Number of dead alloca's removed");
42 STATISTIC(NumPHIInsert, "Number of PHI nodes inserted");
44 // Provide DenseMapInfo for all pointers.
47 struct DenseMapInfo<std::pair<BasicBlock*, unsigned> > {
48 typedef std::pair<BasicBlock*, unsigned> EltTy;
49 static inline EltTy getEmptyKey() {
50 return EltTy(reinterpret_cast<BasicBlock*>(-1), ~0U);
52 static inline EltTy getTombstoneKey() {
53 return EltTy(reinterpret_cast<BasicBlock*>(-2), 0U);
55 static unsigned getHashValue(const std::pair<BasicBlock*, unsigned> &Val) {
56 return DenseMapInfo<void*>::getHashValue(Val.first) + Val.second*2;
58 static bool isEqual(const EltTy &LHS, const EltTy &RHS) {
61 static bool isPod() { return true; }
65 /// isAllocaPromotable - Return true if this alloca is legal for promotion.
66 /// This is true if there are only loads and stores to the alloca.
68 bool llvm::isAllocaPromotable(const AllocaInst *AI) {
69 // FIXME: If the memory unit is of pointer or integer type, we can permit
70 // assignments to subsections of the memory unit.
72 // Only allow direct and non-volatile loads and stores...
73 for (Value::use_const_iterator UI = AI->use_begin(), UE = AI->use_end();
74 UI != UE; ++UI) // Loop over all of the uses of the alloca
75 if (const LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
78 } else if (const StoreInst *SI = dyn_cast<StoreInst>(*UI)) {
79 if (SI->getOperand(0) == AI)
80 return false; // Don't allow a store OF the AI, only INTO the AI.
83 } else if (const BitCastInst *BC = dyn_cast<BitCastInst>(*UI)) {
84 // Uses by dbg info shouldn't inhibit promotion.
85 if (!BC->hasOneUse() || !isa<DbgInfoIntrinsic>(*BC->use_begin()))
97 // Data package used by RenamePass()
98 class VISIBILITY_HIDDEN RenamePassData {
100 typedef std::vector<Value *> ValVector;
103 RenamePassData(BasicBlock *B, BasicBlock *P,
104 const ValVector &V) : BB(B), Pred(P), Values(V) {}
109 void swap(RenamePassData &RHS) {
110 std::swap(BB, RHS.BB);
111 std::swap(Pred, RHS.Pred);
112 Values.swap(RHS.Values);
116 /// LargeBlockInfo - This assigns and keeps a per-bb relative ordering of
117 /// load/store instructions in the block that directly load or store an alloca.
119 /// This functionality is important because it avoids scanning large basic
120 /// blocks multiple times when promoting many allocas in the same block.
121 class VISIBILITY_HIDDEN LargeBlockInfo {
122 /// InstNumbers - For each instruction that we track, keep the index of the
123 /// instruction. The index starts out as the number of the instruction from
124 /// the start of the block.
125 DenseMap<const Instruction *, unsigned> InstNumbers;
128 /// isInterestingInstruction - This code only looks at accesses to allocas.
129 static bool isInterestingInstruction(const Instruction *I) {
130 return (isa<LoadInst>(I) && isa<AllocaInst>(I->getOperand(0))) ||
131 (isa<StoreInst>(I) && isa<AllocaInst>(I->getOperand(1)));
134 /// getInstructionIndex - Get or calculate the index of the specified
136 unsigned getInstructionIndex(const Instruction *I) {
137 assert(isInterestingInstruction(I) &&
138 "Not a load/store to/from an alloca?");
140 // If we already have this instruction number, return it.
141 DenseMap<const Instruction *, unsigned>::iterator It = InstNumbers.find(I);
142 if (It != InstNumbers.end()) return It->second;
144 // Scan the whole block to get the instruction. This accumulates
145 // information for every interesting instruction in the block, in order to
146 // avoid gratuitus rescans.
147 const BasicBlock *BB = I->getParent();
149 for (BasicBlock::const_iterator BBI = BB->begin(), E = BB->end();
151 if (isInterestingInstruction(BBI))
152 InstNumbers[BBI] = InstNo++;
153 It = InstNumbers.find(I);
155 assert(It != InstNumbers.end() && "Didn't insert instruction?");
159 void deleteValue(const Instruction *I) {
160 InstNumbers.erase(I);
168 struct VISIBILITY_HIDDEN PromoteMem2Reg {
169 /// Allocas - The alloca instructions being promoted.
171 std::vector<AllocaInst*> Allocas;
173 DominanceFrontier &DF;
175 /// AST - An AliasSetTracker object to update. If null, don't update it.
177 AliasSetTracker *AST;
179 /// AllocaLookup - Reverse mapping of Allocas.
181 std::map<AllocaInst*, unsigned> AllocaLookup;
183 /// NewPhiNodes - The PhiNodes we're adding.
185 DenseMap<std::pair<BasicBlock*, unsigned>, PHINode*> NewPhiNodes;
187 /// PhiToAllocaMap - For each PHI node, keep track of which entry in Allocas
188 /// it corresponds to.
189 DenseMap<PHINode*, unsigned> PhiToAllocaMap;
191 /// PointerAllocaValues - If we are updating an AliasSetTracker, then for
192 /// each alloca that is of pointer type, we keep track of what to copyValue
193 /// to the inserted PHI nodes here.
195 std::vector<Value*> PointerAllocaValues;
197 /// Visited - The set of basic blocks the renamer has already visited.
199 SmallPtrSet<BasicBlock*, 16> Visited;
201 /// BBNumbers - Contains a stable numbering of basic blocks to avoid
202 /// non-determinstic behavior.
203 DenseMap<BasicBlock*, unsigned> BBNumbers;
205 /// BBNumPreds - Lazily compute the number of predecessors a block has.
206 DenseMap<const BasicBlock*, unsigned> BBNumPreds;
208 PromoteMem2Reg(const std::vector<AllocaInst*> &A, DominatorTree &dt,
209 DominanceFrontier &df, AliasSetTracker *ast)
210 : Allocas(A), DT(dt), DF(df), AST(ast) {}
214 /// properlyDominates - Return true if I1 properly dominates I2.
216 bool properlyDominates(Instruction *I1, Instruction *I2) const {
217 if (InvokeInst *II = dyn_cast<InvokeInst>(I1))
218 I1 = II->getNormalDest()->begin();
219 return DT.properlyDominates(I1->getParent(), I2->getParent());
222 /// dominates - Return true if BB1 dominates BB2 using the DominatorTree.
224 bool dominates(BasicBlock *BB1, BasicBlock *BB2) const {
225 return DT.dominates(BB1, BB2);
229 void RemoveFromAllocasList(unsigned &AllocaIdx) {
230 Allocas[AllocaIdx] = Allocas.back();
235 unsigned getNumPreds(const BasicBlock *BB) {
236 unsigned &NP = BBNumPreds[BB];
238 NP = std::distance(pred_begin(BB), pred_end(BB))+1;
242 void DetermineInsertionPoint(AllocaInst *AI, unsigned AllocaNum,
244 void ComputeLiveInBlocks(AllocaInst *AI, AllocaInfo &Info,
245 const SmallPtrSet<BasicBlock*, 32> &DefBlocks,
246 SmallPtrSet<BasicBlock*, 32> &LiveInBlocks);
248 void RewriteSingleStoreAlloca(AllocaInst *AI, AllocaInfo &Info,
249 LargeBlockInfo &LBI);
250 void PromoteSingleBlockAlloca(AllocaInst *AI, AllocaInfo &Info,
251 LargeBlockInfo &LBI);
254 void RenamePass(BasicBlock *BB, BasicBlock *Pred,
255 RenamePassData::ValVector &IncVals,
256 std::vector<RenamePassData> &Worklist);
257 bool QueuePhiNode(BasicBlock *BB, unsigned AllocaIdx, unsigned &Version,
258 SmallPtrSet<PHINode*, 16> &InsertedPHINodes);
262 std::vector<BasicBlock*> DefiningBlocks;
263 std::vector<BasicBlock*> UsingBlocks;
265 StoreInst *OnlyStore;
266 BasicBlock *OnlyBlock;
267 bool OnlyUsedInOneBlock;
269 Value *AllocaPointerVal;
272 DefiningBlocks.clear();
276 OnlyUsedInOneBlock = true;
277 AllocaPointerVal = 0;
280 /// AnalyzeAlloca - Scan the uses of the specified alloca, filling in our
282 void AnalyzeAlloca(AllocaInst *AI) {
285 // As we scan the uses of the alloca instruction, keep track of stores,
286 // and decide whether all of the loads and stores to the alloca are within
287 // the same basic block.
288 for (Value::use_iterator U = AI->use_begin(), E = AI->use_end();
290 Instruction *User = cast<Instruction>(*U);
292 if (BitCastInst *BC = dyn_cast<BitCastInst>(User)) {
293 // Remove any uses of this alloca in DbgInfoInstrinsics.
294 assert(BC->hasOneUse() && "Unexpected alloca uses!");
295 DbgInfoIntrinsic *DI = cast<DbgInfoIntrinsic>(*BC->use_begin());
296 DI->eraseFromParent();
297 BC->eraseFromParent();
300 else if (StoreInst *SI = dyn_cast<StoreInst>(User)) {
301 // Remember the basic blocks which define new values for the alloca
302 DefiningBlocks.push_back(SI->getParent());
303 AllocaPointerVal = SI->getOperand(0);
306 LoadInst *LI = cast<LoadInst>(User);
307 // Otherwise it must be a load instruction, keep track of variable
309 UsingBlocks.push_back(LI->getParent());
310 AllocaPointerVal = LI;
313 if (OnlyUsedInOneBlock) {
315 OnlyBlock = User->getParent();
316 else if (OnlyBlock != User->getParent())
317 OnlyUsedInOneBlock = false;
322 } // end of anonymous namespace
325 void PromoteMem2Reg::run() {
326 Function &F = *DF.getRoot()->getParent();
328 if (AST) PointerAllocaValues.resize(Allocas.size());
333 for (unsigned AllocaNum = 0; AllocaNum != Allocas.size(); ++AllocaNum) {
334 AllocaInst *AI = Allocas[AllocaNum];
336 assert(isAllocaPromotable(AI) &&
337 "Cannot promote non-promotable alloca!");
338 assert(AI->getParent()->getParent() == &F &&
339 "All allocas should be in the same function, which is same as DF!");
341 if (AI->use_empty()) {
342 // If there are no uses of the alloca, just delete it now.
343 if (AST) AST->deleteValue(AI);
344 AI->eraseFromParent();
346 // Remove the alloca from the Allocas list, since it has been processed
347 RemoveFromAllocasList(AllocaNum);
352 // Calculate the set of read and write-locations for each alloca. This is
353 // analogous to finding the 'uses' and 'definitions' of each variable.
354 Info.AnalyzeAlloca(AI);
356 // If there is only a single store to this value, replace any loads of
357 // it that are directly dominated by the definition with the value stored.
358 if (Info.DefiningBlocks.size() == 1) {
359 RewriteSingleStoreAlloca(AI, Info, LBI);
361 // Finally, after the scan, check to see if the store is all that is left.
362 if (Info.UsingBlocks.empty()) {
363 // Remove the (now dead) store and alloca.
364 Info.OnlyStore->eraseFromParent();
365 LBI.deleteValue(Info.OnlyStore);
367 if (AST) AST->deleteValue(AI);
368 AI->eraseFromParent();
371 // The alloca has been processed, move on.
372 RemoveFromAllocasList(AllocaNum);
379 // If the alloca is only read and written in one basic block, just perform a
380 // linear sweep over the block to eliminate it.
381 if (Info.OnlyUsedInOneBlock) {
382 PromoteSingleBlockAlloca(AI, Info, LBI);
384 // Finally, after the scan, check to see if the stores are all that is
386 if (Info.UsingBlocks.empty()) {
388 // Remove the (now dead) stores and alloca.
389 while (!AI->use_empty()) {
390 StoreInst *SI = cast<StoreInst>(AI->use_back());
391 SI->eraseFromParent();
395 if (AST) AST->deleteValue(AI);
396 AI->eraseFromParent();
399 // The alloca has been processed, move on.
400 RemoveFromAllocasList(AllocaNum);
407 // If we haven't computed a numbering for the BB's in the function, do so
409 if (BBNumbers.empty()) {
411 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
415 // If we have an AST to keep updated, remember some pointer value that is
416 // stored into the alloca.
418 PointerAllocaValues[AllocaNum] = Info.AllocaPointerVal;
420 // Keep the reverse mapping of the 'Allocas' array for the rename pass.
421 AllocaLookup[Allocas[AllocaNum]] = AllocaNum;
423 // At this point, we're committed to promoting the alloca using IDF's, and
424 // the standard SSA construction algorithm. Determine which blocks need PHI
425 // nodes and see if we can optimize out some work by avoiding insertion of
427 DetermineInsertionPoint(AI, AllocaNum, Info);
431 return; // All of the allocas must have been trivial!
436 // Set the incoming values for the basic block to be null values for all of
437 // the alloca's. We do this in case there is a load of a value that has not
438 // been stored yet. In this case, it will get this null value.
440 RenamePassData::ValVector Values(Allocas.size());
441 for (unsigned i = 0, e = Allocas.size(); i != e; ++i)
442 Values[i] = UndefValue::get(Allocas[i]->getAllocatedType());
444 // Walks all basic blocks in the function performing the SSA rename algorithm
445 // and inserting the phi nodes we marked as necessary
447 std::vector<RenamePassData> RenamePassWorkList;
448 RenamePassWorkList.push_back(RenamePassData(F.begin(), 0, Values));
449 while (!RenamePassWorkList.empty()) {
451 RPD.swap(RenamePassWorkList.back());
452 RenamePassWorkList.pop_back();
453 // RenamePass may add new worklist entries.
454 RenamePass(RPD.BB, RPD.Pred, RPD.Values, RenamePassWorkList);
457 // The renamer uses the Visited set to avoid infinite loops. Clear it now.
460 // Remove the allocas themselves from the function.
461 for (unsigned i = 0, e = Allocas.size(); i != e; ++i) {
462 Instruction *A = Allocas[i];
464 // If there are any uses of the alloca instructions left, they must be in
465 // sections of dead code that were not processed on the dominance frontier.
466 // Just delete the users now.
469 A->replaceAllUsesWith(UndefValue::get(A->getType()));
470 if (AST) AST->deleteValue(A);
471 A->eraseFromParent();
475 // Loop over all of the PHI nodes and see if there are any that we can get
476 // rid of because they merge all of the same incoming values. This can
477 // happen due to undef values coming into the PHI nodes. This process is
478 // iterative, because eliminating one PHI node can cause others to be removed.
479 bool EliminatedAPHI = true;
480 while (EliminatedAPHI) {
481 EliminatedAPHI = false;
483 for (DenseMap<std::pair<BasicBlock*, unsigned>, PHINode*>::iterator I =
484 NewPhiNodes.begin(), E = NewPhiNodes.end(); I != E;) {
485 PHINode *PN = I->second;
487 // If this PHI node merges one value and/or undefs, get the value.
488 if (Value *V = PN->hasConstantValue(true)) {
489 if (!isa<Instruction>(V) ||
490 properlyDominates(cast<Instruction>(V), PN)) {
491 if (AST && isa<PointerType>(PN->getType()))
492 AST->deleteValue(PN);
493 PN->replaceAllUsesWith(V);
494 PN->eraseFromParent();
495 NewPhiNodes.erase(I++);
496 EliminatedAPHI = true;
504 // At this point, the renamer has added entries to PHI nodes for all reachable
505 // code. Unfortunately, there may be unreachable blocks which the renamer
506 // hasn't traversed. If this is the case, the PHI nodes may not
507 // have incoming values for all predecessors. Loop over all PHI nodes we have
508 // created, inserting undef values if they are missing any incoming values.
510 for (DenseMap<std::pair<BasicBlock*, unsigned>, PHINode*>::iterator I =
511 NewPhiNodes.begin(), E = NewPhiNodes.end(); I != E; ++I) {
512 // We want to do this once per basic block. As such, only process a block
513 // when we find the PHI that is the first entry in the block.
514 PHINode *SomePHI = I->second;
515 BasicBlock *BB = SomePHI->getParent();
516 if (&BB->front() != SomePHI)
519 // Only do work here if there the PHI nodes are missing incoming values. We
520 // know that all PHI nodes that were inserted in a block will have the same
521 // number of incoming values, so we can just check any of them.
522 if (SomePHI->getNumIncomingValues() == getNumPreds(BB))
525 // Get the preds for BB.
526 SmallVector<BasicBlock*, 16> Preds(pred_begin(BB), pred_end(BB));
528 // Ok, now we know that all of the PHI nodes are missing entries for some
529 // basic blocks. Start by sorting the incoming predecessors for efficient
531 std::sort(Preds.begin(), Preds.end());
533 // Now we loop through all BB's which have entries in SomePHI and remove
534 // them from the Preds list.
535 for (unsigned i = 0, e = SomePHI->getNumIncomingValues(); i != e; ++i) {
536 // Do a log(n) search of the Preds list for the entry we want.
537 SmallVector<BasicBlock*, 16>::iterator EntIt =
538 std::lower_bound(Preds.begin(), Preds.end(),
539 SomePHI->getIncomingBlock(i));
540 assert(EntIt != Preds.end() && *EntIt == SomePHI->getIncomingBlock(i)&&
541 "PHI node has entry for a block which is not a predecessor!");
547 // At this point, the blocks left in the preds list must have dummy
548 // entries inserted into every PHI nodes for the block. Update all the phi
549 // nodes in this block that we are inserting (there could be phis before
551 unsigned NumBadPreds = SomePHI->getNumIncomingValues();
552 BasicBlock::iterator BBI = BB->begin();
553 while ((SomePHI = dyn_cast<PHINode>(BBI++)) &&
554 SomePHI->getNumIncomingValues() == NumBadPreds) {
555 Value *UndefVal = UndefValue::get(SomePHI->getType());
556 for (unsigned pred = 0, e = Preds.size(); pred != e; ++pred)
557 SomePHI->addIncoming(UndefVal, Preds[pred]);
565 /// ComputeLiveInBlocks - Determine which blocks the value is live in. These
566 /// are blocks which lead to uses. Knowing this allows us to avoid inserting
567 /// PHI nodes into blocks which don't lead to uses (thus, the inserted phi nodes
569 void PromoteMem2Reg::
570 ComputeLiveInBlocks(AllocaInst *AI, AllocaInfo &Info,
571 const SmallPtrSet<BasicBlock*, 32> &DefBlocks,
572 SmallPtrSet<BasicBlock*, 32> &LiveInBlocks) {
574 // To determine liveness, we must iterate through the predecessors of blocks
575 // where the def is live. Blocks are added to the worklist if we need to
576 // check their predecessors. Start with all the using blocks.
577 SmallVector<BasicBlock*, 64> LiveInBlockWorklist;
578 LiveInBlockWorklist.insert(LiveInBlockWorklist.end(),
579 Info.UsingBlocks.begin(), Info.UsingBlocks.end());
581 // If any of the using blocks is also a definition block, check to see if the
582 // definition occurs before or after the use. If it happens before the use,
583 // the value isn't really live-in.
584 for (unsigned i = 0, e = LiveInBlockWorklist.size(); i != e; ++i) {
585 BasicBlock *BB = LiveInBlockWorklist[i];
586 if (!DefBlocks.count(BB)) continue;
588 // Okay, this is a block that both uses and defines the value. If the first
589 // reference to the alloca is a def (store), then we know it isn't live-in.
590 for (BasicBlock::iterator I = BB->begin(); ; ++I) {
591 if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
592 if (SI->getOperand(1) != AI) continue;
594 // We found a store to the alloca before a load. The alloca is not
595 // actually live-in here.
596 LiveInBlockWorklist[i] = LiveInBlockWorklist.back();
597 LiveInBlockWorklist.pop_back();
600 } else if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
601 if (LI->getOperand(0) != AI) continue;
603 // Okay, we found a load before a store to the alloca. It is actually
604 // live into this block.
610 // Now that we have a set of blocks where the phi is live-in, recursively add
611 // their predecessors until we find the full region the value is live.
612 while (!LiveInBlockWorklist.empty()) {
613 BasicBlock *BB = LiveInBlockWorklist.back();
614 LiveInBlockWorklist.pop_back();
616 // The block really is live in here, insert it into the set. If already in
617 // the set, then it has already been processed.
618 if (!LiveInBlocks.insert(BB))
621 // Since the value is live into BB, it is either defined in a predecessor or
622 // live into it to. Add the preds to the worklist unless they are a
624 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
627 // The value is not live into a predecessor if it defines the value.
628 if (DefBlocks.count(P))
631 // Otherwise it is, add to the worklist.
632 LiveInBlockWorklist.push_back(P);
637 /// DetermineInsertionPoint - At this point, we're committed to promoting the
638 /// alloca using IDF's, and the standard SSA construction algorithm. Determine
639 /// which blocks need phi nodes and see if we can optimize out some work by
640 /// avoiding insertion of dead phi nodes.
641 void PromoteMem2Reg::DetermineInsertionPoint(AllocaInst *AI, unsigned AllocaNum,
644 // Unique the set of defining blocks for efficient lookup.
645 SmallPtrSet<BasicBlock*, 32> DefBlocks;
646 DefBlocks.insert(Info.DefiningBlocks.begin(), Info.DefiningBlocks.end());
648 // Determine which blocks the value is live in. These are blocks which lead
650 SmallPtrSet<BasicBlock*, 32> LiveInBlocks;
651 ComputeLiveInBlocks(AI, Info, DefBlocks, LiveInBlocks);
653 // Compute the locations where PhiNodes need to be inserted. Look at the
654 // dominance frontier of EACH basic-block we have a write in.
655 unsigned CurrentVersion = 0;
656 SmallPtrSet<PHINode*, 16> InsertedPHINodes;
657 std::vector<std::pair<unsigned, BasicBlock*> > DFBlocks;
658 while (!Info.DefiningBlocks.empty()) {
659 BasicBlock *BB = Info.DefiningBlocks.back();
660 Info.DefiningBlocks.pop_back();
662 // Look up the DF for this write, add it to defining blocks.
663 DominanceFrontier::const_iterator it = DF.find(BB);
664 if (it == DF.end()) continue;
666 const DominanceFrontier::DomSetType &S = it->second;
668 // In theory we don't need the indirection through the DFBlocks vector.
669 // In practice, the order of calling QueuePhiNode would depend on the
670 // (unspecified) ordering of basic blocks in the dominance frontier,
671 // which would give PHI nodes non-determinstic subscripts. Fix this by
672 // processing blocks in order of the occurance in the function.
673 for (DominanceFrontier::DomSetType::const_iterator P = S.begin(),
674 PE = S.end(); P != PE; ++P) {
675 // If the frontier block is not in the live-in set for the alloca, don't
676 // bother processing it.
677 if (!LiveInBlocks.count(*P))
680 DFBlocks.push_back(std::make_pair(BBNumbers[*P], *P));
683 // Sort by which the block ordering in the function.
684 if (DFBlocks.size() > 1)
685 std::sort(DFBlocks.begin(), DFBlocks.end());
687 for (unsigned i = 0, e = DFBlocks.size(); i != e; ++i) {
688 BasicBlock *BB = DFBlocks[i].second;
689 if (QueuePhiNode(BB, AllocaNum, CurrentVersion, InsertedPHINodes))
690 Info.DefiningBlocks.push_back(BB);
696 /// RewriteSingleStoreAlloca - If there is only a single store to this value,
697 /// replace any loads of it that are directly dominated by the definition with
698 /// the value stored.
699 void PromoteMem2Reg::RewriteSingleStoreAlloca(AllocaInst *AI,
701 LargeBlockInfo &LBI) {
702 StoreInst *OnlyStore = Info.OnlyStore;
703 bool StoringGlobalVal = !isa<Instruction>(OnlyStore->getOperand(0));
704 BasicBlock *StoreBB = OnlyStore->getParent();
707 // Clear out UsingBlocks. We will reconstruct it here if needed.
708 Info.UsingBlocks.clear();
710 for (Value::use_iterator UI = AI->use_begin(), E = AI->use_end(); UI != E; ) {
711 Instruction *UserInst = cast<Instruction>(*UI++);
712 if (!isa<LoadInst>(UserInst)) {
713 assert(UserInst == OnlyStore && "Should only have load/stores");
716 LoadInst *LI = cast<LoadInst>(UserInst);
718 // Okay, if we have a load from the alloca, we want to replace it with the
719 // only value stored to the alloca. We can do this if the value is
720 // dominated by the store. If not, we use the rest of the mem2reg machinery
721 // to insert the phi nodes as needed.
722 if (!StoringGlobalVal) { // Non-instructions are always dominated.
723 if (LI->getParent() == StoreBB) {
724 // If we have a use that is in the same block as the store, compare the
725 // indices of the two instructions to see which one came first. If the
726 // load came before the store, we can't handle it.
727 if (StoreIndex == -1)
728 StoreIndex = LBI.getInstructionIndex(OnlyStore);
730 if (unsigned(StoreIndex) > LBI.getInstructionIndex(LI)) {
731 // Can't handle this load, bail out.
732 Info.UsingBlocks.push_back(StoreBB);
736 } else if (LI->getParent() != StoreBB &&
737 !dominates(StoreBB, LI->getParent())) {
738 // If the load and store are in different blocks, use BB dominance to
739 // check their relationships. If the store doesn't dom the use, bail
741 Info.UsingBlocks.push_back(LI->getParent());
746 // Otherwise, we *can* safely rewrite this load.
747 LI->replaceAllUsesWith(OnlyStore->getOperand(0));
748 if (AST && isa<PointerType>(LI->getType()))
749 AST->deleteValue(LI);
750 LI->eraseFromParent();
756 /// StoreIndexSearchPredicate - This is a helper predicate used to search by the
757 /// first element of a pair.
758 struct StoreIndexSearchPredicate {
759 bool operator()(const std::pair<unsigned, StoreInst*> &LHS,
760 const std::pair<unsigned, StoreInst*> &RHS) {
761 return LHS.first < RHS.first;
765 /// PromoteSingleBlockAlloca - Many allocas are only used within a single basic
766 /// block. If this is the case, avoid traversing the CFG and inserting a lot of
767 /// potentially useless PHI nodes by just performing a single linear pass over
768 /// the basic block using the Alloca.
770 /// If we cannot promote this alloca (because it is read before it is written),
771 /// return true. This is necessary in cases where, due to control flow, the
772 /// alloca is potentially undefined on some control flow paths. e.g. code like
773 /// this is potentially correct:
775 /// for (...) { if (c) { A = undef; undef = B; } }
777 /// ... so long as A is not used before undef is set.
779 void PromoteMem2Reg::PromoteSingleBlockAlloca(AllocaInst *AI, AllocaInfo &Info,
780 LargeBlockInfo &LBI) {
781 // The trickiest case to handle is when we have large blocks. Because of this,
782 // this code is optimized assuming that large blocks happen. This does not
783 // significantly pessimize the small block case. This uses LargeBlockInfo to
784 // make it efficient to get the index of various operations in the block.
786 // Clear out UsingBlocks. We will reconstruct it here if needed.
787 Info.UsingBlocks.clear();
789 // Walk the use-def list of the alloca, getting the locations of all stores.
790 typedef SmallVector<std::pair<unsigned, StoreInst*>, 64> StoresByIndexTy;
791 StoresByIndexTy StoresByIndex;
793 for (Value::use_iterator UI = AI->use_begin(), E = AI->use_end();
795 if (StoreInst *SI = dyn_cast<StoreInst>(*UI))
796 StoresByIndex.push_back(std::make_pair(LBI.getInstructionIndex(SI), SI));
798 // If there are no stores to the alloca, just replace any loads with undef.
799 if (StoresByIndex.empty()) {
800 for (Value::use_iterator UI = AI->use_begin(), E = AI->use_end(); UI != E;)
801 if (LoadInst *LI = dyn_cast<LoadInst>(*UI++)) {
802 LI->replaceAllUsesWith(UndefValue::get(LI->getType()));
803 if (AST && isa<PointerType>(LI->getType()))
804 AST->deleteValue(LI);
806 LI->eraseFromParent();
811 // Sort the stores by their index, making it efficient to do a lookup with a
813 std::sort(StoresByIndex.begin(), StoresByIndex.end());
815 // Walk all of the loads from this alloca, replacing them with the nearest
816 // store above them, if any.
817 for (Value::use_iterator UI = AI->use_begin(), E = AI->use_end(); UI != E;) {
818 LoadInst *LI = dyn_cast<LoadInst>(*UI++);
821 unsigned LoadIdx = LBI.getInstructionIndex(LI);
823 // Find the nearest store that has a lower than this load.
824 StoresByIndexTy::iterator I =
825 std::lower_bound(StoresByIndex.begin(), StoresByIndex.end(),
826 std::pair<unsigned, StoreInst*>(LoadIdx, 0),
827 StoreIndexSearchPredicate());
829 // If there is no store before this load, then we can't promote this load.
830 if (I == StoresByIndex.begin()) {
831 // Can't handle this load, bail out.
832 Info.UsingBlocks.push_back(LI->getParent());
836 // Otherwise, there was a store before this load, the load takes its value.
838 LI->replaceAllUsesWith(I->second->getOperand(0));
839 if (AST && isa<PointerType>(LI->getType()))
840 AST->deleteValue(LI);
841 LI->eraseFromParent();
847 // QueuePhiNode - queues a phi-node to be added to a basic-block for a specific
848 // Alloca returns true if there wasn't already a phi-node for that variable
850 bool PromoteMem2Reg::QueuePhiNode(BasicBlock *BB, unsigned AllocaNo,
852 SmallPtrSet<PHINode*, 16> &InsertedPHINodes) {
853 // Look up the basic-block in question.
854 PHINode *&PN = NewPhiNodes[std::make_pair(BB, AllocaNo)];
856 // If the BB already has a phi node added for the i'th alloca then we're done!
857 if (PN) return false;
859 // Create a PhiNode using the dereferenced type... and add the phi-node to the
861 PN = PHINode::Create(Allocas[AllocaNo]->getAllocatedType(),
862 Allocas[AllocaNo]->getName() + "." +
863 utostr(Version++), BB->begin());
865 PhiToAllocaMap[PN] = AllocaNo;
866 PN->reserveOperandSpace(getNumPreds(BB));
868 InsertedPHINodes.insert(PN);
870 if (AST && isa<PointerType>(PN->getType()))
871 AST->copyValue(PointerAllocaValues[AllocaNo], PN);
876 // RenamePass - Recursively traverse the CFG of the function, renaming loads and
877 // stores to the allocas which we are promoting. IncomingVals indicates what
878 // value each Alloca contains on exit from the predecessor block Pred.
880 void PromoteMem2Reg::RenamePass(BasicBlock *BB, BasicBlock *Pred,
881 RenamePassData::ValVector &IncomingVals,
882 std::vector<RenamePassData> &Worklist) {
884 // If we are inserting any phi nodes into this BB, they will already be in the
886 if (PHINode *APN = dyn_cast<PHINode>(BB->begin())) {
887 // Pred may have multiple edges to BB. If so, we want to add N incoming
888 // values to each PHI we are inserting on the first time we see the edge.
889 // Check to see if APN already has incoming values from Pred. This also
890 // prevents us from modifying PHI nodes that are not currently being
892 bool HasPredEntries = false;
893 for (unsigned i = 0, e = APN->getNumIncomingValues(); i != e; ++i) {
894 if (APN->getIncomingBlock(i) == Pred) {
895 HasPredEntries = true;
900 // If we have PHI nodes to update, compute the number of edges from Pred to
902 if (!HasPredEntries) {
903 // We want to be able to distinguish between PHI nodes being inserted by
904 // this invocation of mem2reg from those phi nodes that already existed in
905 // the IR before mem2reg was run. We determine that APN is being inserted
906 // because it is missing incoming edges. All other PHI nodes being
907 // inserted by this pass of mem2reg will have the same number of incoming
908 // operands so far. Remember this count.
909 unsigned NewPHINumOperands = APN->getNumOperands();
911 unsigned NumEdges = 0;
912 for (succ_iterator I = succ_begin(Pred), E = succ_end(Pred); I != E; ++I)
915 assert(NumEdges && "Must be at least one edge from Pred to BB!");
917 // Add entries for all the phis.
918 BasicBlock::iterator PNI = BB->begin();
920 unsigned AllocaNo = PhiToAllocaMap[APN];
922 // Add N incoming values to the PHI node.
923 for (unsigned i = 0; i != NumEdges; ++i)
924 APN->addIncoming(IncomingVals[AllocaNo], Pred);
926 // The currently active variable for this block is now the PHI.
927 IncomingVals[AllocaNo] = APN;
929 // Get the next phi node.
931 APN = dyn_cast<PHINode>(PNI);
934 // Verify that it is missing entries. If not, it is not being inserted
935 // by this mem2reg invocation so we want to ignore it.
936 } while (APN->getNumOperands() == NewPHINumOperands);
940 // Don't revisit blocks.
941 if (!Visited.insert(BB)) return;
943 for (BasicBlock::iterator II = BB->begin(); !isa<TerminatorInst>(II); ) {
944 Instruction *I = II++; // get the instruction, increment iterator
946 if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
947 AllocaInst *Src = dyn_cast<AllocaInst>(LI->getPointerOperand());
950 std::map<AllocaInst*, unsigned>::iterator AI = AllocaLookup.find(Src);
951 if (AI == AllocaLookup.end()) continue;
953 Value *V = IncomingVals[AI->second];
955 // Anything using the load now uses the current value.
956 LI->replaceAllUsesWith(V);
957 if (AST && isa<PointerType>(LI->getType()))
958 AST->deleteValue(LI);
959 BB->getInstList().erase(LI);
960 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
961 // Delete this instruction and mark the name as the current holder of the
963 AllocaInst *Dest = dyn_cast<AllocaInst>(SI->getPointerOperand());
966 std::map<AllocaInst *, unsigned>::iterator ai = AllocaLookup.find(Dest);
967 if (ai == AllocaLookup.end())
970 // what value were we writing?
971 IncomingVals[ai->second] = SI->getOperand(0);
972 BB->getInstList().erase(SI);
976 // 'Recurse' to our successors.
977 succ_iterator I = succ_begin(BB), E = succ_end(BB);
980 // Handle the last successor without using the worklist. This allows us to
981 // handle unconditional branches directly, for example.
984 Worklist.push_back(RenamePassData(*I, BB, IncomingVals));
991 /// PromoteMemToReg - Promote the specified list of alloca instructions into
992 /// scalar registers, inserting PHI nodes as appropriate. This function makes
993 /// use of DominanceFrontier information. This function does not modify the CFG
994 /// of the function at all. All allocas must be from the same function.
996 /// If AST is specified, the specified tracker is updated to reflect changes
999 void llvm::PromoteMemToReg(const std::vector<AllocaInst*> &Allocas,
1000 DominatorTree &DT, DominanceFrontier &DF,
1001 AliasSetTracker *AST) {
1002 // If there is nothing to do, bail out...
1003 if (Allocas.empty()) return;
1005 PromoteMem2Reg(Allocas, DT, DF, AST).run();