1 //===- PromoteMemoryToRegister.cpp - Convert allocas to registers ---------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file promote 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/Analysis/Dominators.h"
26 #include "llvm/Analysis/AliasSetTracker.h"
27 #include "llvm/ADT/DenseMap.h"
28 #include "llvm/ADT/SmallPtrSet.h"
29 #include "llvm/ADT/SmallVector.h"
30 #include "llvm/ADT/Statistic.h"
31 #include "llvm/ADT/StringExtras.h"
32 #include "llvm/Support/CFG.h"
33 #include "llvm/Support/Compiler.h"
37 STATISTIC(NumLocalPromoted, "Number of alloca's promoted within one block");
38 STATISTIC(NumSingleStore, "Number of alloca's promoted with a single store");
39 STATISTIC(NumDeadAlloca, "Number of dead alloca's removed");
41 // Provide DenseMapKeyInfo for all pointers.
44 struct DenseMapKeyInfo<std::pair<BasicBlock*, unsigned> > {
45 static inline std::pair<BasicBlock*, unsigned> getEmptyKey() {
46 return std::make_pair((BasicBlock*)-1, ~0U);
48 static inline std::pair<BasicBlock*, unsigned> getTombstoneKey() {
49 return std::make_pair((BasicBlock*)-2, 0U);
51 static unsigned getHashValue(const std::pair<BasicBlock*, unsigned> &Val) {
52 return DenseMapKeyInfo<void*>::getHashValue(Val.first) + Val.second*2;
54 static bool isPod() { return true; }
58 /// isAllocaPromotable - Return true if this alloca is legal for promotion.
59 /// This is true if there are only loads and stores to the alloca.
61 bool llvm::isAllocaPromotable(const AllocaInst *AI) {
62 // FIXME: If the memory unit is of pointer or integer type, we can permit
63 // assignments to subsections of the memory unit.
65 // Only allow direct loads and stores...
66 for (Value::use_const_iterator UI = AI->use_begin(), UE = AI->use_end();
67 UI != UE; ++UI) // Loop over all of the uses of the alloca
68 if (isa<LoadInst>(*UI)) {
70 } else if (const StoreInst *SI = dyn_cast<StoreInst>(*UI)) {
71 if (SI->getOperand(0) == AI)
72 return false; // Don't allow a store OF the AI, only INTO the AI.
74 return false; // Not a load or store.
83 // Data package used by RenamePass()
84 class VISIBILITY_HIDDEN RenamePassData {
86 typedef std::vector<Value *> ValVector;
89 RenamePassData(BasicBlock *B, BasicBlock *P,
90 const ValVector &V) : BB(B), Pred(P), Values(V) {}
95 void swap(RenamePassData &RHS) {
96 std::swap(BB, RHS.BB);
97 std::swap(Pred, RHS.Pred);
98 Values.swap(RHS.Values);
102 struct VISIBILITY_HIDDEN PromoteMem2Reg {
103 /// Allocas - The alloca instructions being promoted.
105 std::vector<AllocaInst*> Allocas;
106 SmallVector<AllocaInst*, 16> &RetryList;
108 DominanceFrontier &DF;
110 /// AST - An AliasSetTracker object to update. If null, don't update it.
112 AliasSetTracker *AST;
114 /// AllocaLookup - Reverse mapping of Allocas.
116 std::map<AllocaInst*, unsigned> AllocaLookup;
118 /// NewPhiNodes - The PhiNodes we're adding.
120 DenseMap<std::pair<BasicBlock*, unsigned>, PHINode*> NewPhiNodes;
122 /// PhiToAllocaMap - For each PHI node, keep track of which entry in Allocas
123 /// it corresponds to.
124 DenseMap<PHINode*, unsigned> PhiToAllocaMap;
126 /// PointerAllocaValues - If we are updating an AliasSetTracker, then for
127 /// each alloca that is of pointer type, we keep track of what to copyValue
128 /// to the inserted PHI nodes here.
130 std::vector<Value*> PointerAllocaValues;
132 /// Visited - The set of basic blocks the renamer has already visited.
134 SmallPtrSet<BasicBlock*, 16> Visited;
136 /// BBNumbers - Contains a stable numbering of basic blocks to avoid
137 /// non-determinstic behavior.
138 DenseMap<BasicBlock*, unsigned> BBNumbers;
141 PromoteMem2Reg(const std::vector<AllocaInst*> &A,
142 SmallVector<AllocaInst*, 16> &Retry, DominatorTree &dt,
143 DominanceFrontier &df, AliasSetTracker *ast)
144 : Allocas(A), RetryList(Retry), DT(dt), DF(df), AST(ast) {}
148 /// properlyDominates - Return true if I1 properly dominates I2.
150 bool properlyDominates(Instruction *I1, Instruction *I2) const {
151 if (InvokeInst *II = dyn_cast<InvokeInst>(I1))
152 I1 = II->getNormalDest()->begin();
153 return DT.properlyDominates(I1->getParent(), I2->getParent());
156 /// dominates - Return true if BB1 dominates BB2 using the DominatorTree.
158 bool dominates(BasicBlock *BB1, BasicBlock *BB2) const {
159 return DT.dominates(BB1, BB2);
163 void RemoveFromAllocasList(unsigned &AllocaIdx) {
164 Allocas[AllocaIdx] = Allocas.back();
169 void RewriteSingleStoreAlloca(AllocaInst *AI, AllocaInfo &Info);
171 void MarkDominatingPHILive(BasicBlock *BB, unsigned AllocaNum,
172 SmallPtrSet<PHINode*, 16> &DeadPHINodes);
173 bool PromoteLocallyUsedAlloca(BasicBlock *BB, AllocaInst *AI);
174 void PromoteLocallyUsedAllocas(BasicBlock *BB,
175 const std::vector<AllocaInst*> &AIs);
177 void RenamePass(BasicBlock *BB, BasicBlock *Pred,
178 RenamePassData::ValVector &IncVals,
179 std::vector<RenamePassData> &Worklist);
180 bool QueuePhiNode(BasicBlock *BB, unsigned AllocaIdx, unsigned &Version,
181 SmallPtrSet<PHINode*, 16> &InsertedPHINodes);
185 std::vector<BasicBlock*> DefiningBlocks;
186 std::vector<BasicBlock*> UsingBlocks;
188 StoreInst *OnlyStore;
189 BasicBlock *OnlyBlock;
190 bool OnlyUsedInOneBlock;
192 Value *AllocaPointerVal;
195 DefiningBlocks.clear();
199 OnlyUsedInOneBlock = true;
200 AllocaPointerVal = 0;
203 /// AnalyzeAlloca - Scan the uses of the specified alloca, filling in our
205 void AnalyzeAlloca(AllocaInst *AI) {
208 // As we scan the uses of the alloca instruction, keep track of stores,
209 // and decide whether all of the loads and stores to the alloca are within
210 // the same basic block.
211 for (Value::use_iterator U = AI->use_begin(), E = AI->use_end();
213 Instruction *User = cast<Instruction>(*U);
214 if (StoreInst *SI = dyn_cast<StoreInst>(User)) {
215 // Remember the basic blocks which define new values for the alloca
216 DefiningBlocks.push_back(SI->getParent());
217 AllocaPointerVal = SI->getOperand(0);
220 LoadInst *LI = cast<LoadInst>(User);
221 // Otherwise it must be a load instruction, keep track of variable reads
222 UsingBlocks.push_back(LI->getParent());
223 AllocaPointerVal = LI;
226 if (OnlyUsedInOneBlock) {
228 OnlyBlock = User->getParent();
229 else if (OnlyBlock != User->getParent())
230 OnlyUsedInOneBlock = false;
236 } // end of anonymous namespace
239 void PromoteMem2Reg::run() {
240 Function &F = *DF.getRoot()->getParent();
242 // LocallyUsedAllocas - Keep track of all of the alloca instructions which are
243 // only used in a single basic block. These instructions can be efficiently
244 // promoted by performing a single linear scan over that one block. Since
245 // individual basic blocks are sometimes large, we group together all allocas
246 // that are live in a single basic block by the basic block they are live in.
247 std::map<BasicBlock*, std::vector<AllocaInst*> > LocallyUsedAllocas;
249 if (AST) PointerAllocaValues.resize(Allocas.size());
253 for (unsigned AllocaNum = 0; AllocaNum != Allocas.size(); ++AllocaNum) {
254 AllocaInst *AI = Allocas[AllocaNum];
256 assert(isAllocaPromotable(AI) &&
257 "Cannot promote non-promotable alloca!");
258 assert(AI->getParent()->getParent() == &F &&
259 "All allocas should be in the same function, which is same as DF!");
261 if (AI->use_empty()) {
262 // If there are no uses of the alloca, just delete it now.
263 if (AST) AST->deleteValue(AI);
264 AI->eraseFromParent();
266 // Remove the alloca from the Allocas list, since it has been processed
267 RemoveFromAllocasList(AllocaNum);
272 // Calculate the set of read and write-locations for each alloca. This is
273 // analogous to finding the 'uses' and 'definitions' of each variable.
274 Info.AnalyzeAlloca(AI);
276 // If there is only a single store to this value, replace any loads of
277 // it that are directly dominated by the definition with the value stored.
278 if (Info.DefiningBlocks.size() == 1) {
279 RewriteSingleStoreAlloca(AI, Info);
281 // Finally, after the scan, check to see if the store is all that is left.
282 if (Info.UsingBlocks.empty()) {
283 // Remove the (now dead) store and alloca.
284 Info.OnlyStore->eraseFromParent();
285 if (AST) AST->deleteValue(AI);
286 AI->eraseFromParent();
288 // The alloca has been processed, move on.
289 RemoveFromAllocasList(AllocaNum);
296 // If the alloca is only read and written in one basic block, just perform a
297 // linear sweep over the block to eliminate it.
298 if (Info.OnlyUsedInOneBlock) {
299 LocallyUsedAllocas[Info.OnlyBlock].push_back(AI);
301 // Remove the alloca from the Allocas list, since it will be processed.
302 RemoveFromAllocasList(AllocaNum);
307 PointerAllocaValues[AllocaNum] = Info.AllocaPointerVal;
309 // If we haven't computed a numbering for the BB's in the function, do so
311 if (BBNumbers.empty()) {
313 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
317 // Compute the locations where PhiNodes need to be inserted. Look at the
318 // dominance frontier of EACH basic-block we have a write in.
320 unsigned CurrentVersion = 0;
321 SmallPtrSet<PHINode*, 16> InsertedPHINodes;
322 std::vector<std::pair<unsigned, BasicBlock*> > DFBlocks;
323 while (!Info.DefiningBlocks.empty()) {
324 BasicBlock *BB = Info.DefiningBlocks.back();
325 Info.DefiningBlocks.pop_back();
327 // Look up the DF for this write, add it to PhiNodes
328 DominanceFrontier::const_iterator it = DF.find(BB);
329 if (it != DF.end()) {
330 const DominanceFrontier::DomSetType &S = it->second;
332 // In theory we don't need the indirection through the DFBlocks vector.
333 // In practice, the order of calling QueuePhiNode would depend on the
334 // (unspecified) ordering of basic blocks in the dominance frontier,
335 // which would give PHI nodes non-determinstic subscripts. Fix this by
336 // processing blocks in order of the occurance in the function.
337 for (DominanceFrontier::DomSetType::const_iterator P = S.begin(),
338 PE = S.end(); P != PE; ++P)
339 DFBlocks.push_back(std::make_pair(BBNumbers[*P], *P));
341 // Sort by which the block ordering in the function.
342 std::sort(DFBlocks.begin(), DFBlocks.end());
344 for (unsigned i = 0, e = DFBlocks.size(); i != e; ++i) {
345 BasicBlock *BB = DFBlocks[i].second;
346 if (QueuePhiNode(BB, AllocaNum, CurrentVersion, InsertedPHINodes))
347 Info.DefiningBlocks.push_back(BB);
353 // Now that we have inserted PHI nodes along the Iterated Dominance Frontier
354 // of the writes to the variable, scan through the reads of the variable,
355 // marking PHI nodes which are actually necessary as alive (by removing them
356 // from the InsertedPHINodes set). This is not perfect: there may PHI
357 // marked alive because of loads which are dominated by stores, but there
358 // will be no unmarked PHI nodes which are actually used.
360 for (unsigned i = 0, e = Info.UsingBlocks.size(); i != e; ++i)
361 MarkDominatingPHILive(Info.UsingBlocks[i], AllocaNum, InsertedPHINodes);
362 Info.UsingBlocks.clear();
364 // If there are any PHI nodes which are now known to be dead, remove them!
365 for (SmallPtrSet<PHINode*, 16>::iterator I = InsertedPHINodes.begin(),
366 E = InsertedPHINodes.end(); I != E; ++I) {
368 bool Erased=NewPhiNodes.erase(std::make_pair(PN->getParent(), AllocaNum));
370 assert(Erased && "PHI already removed?");
372 if (AST && isa<PointerType>(PN->getType()))
373 AST->deleteValue(PN);
374 PN->eraseFromParent();
375 PhiToAllocaMap.erase(PN);
378 // Keep the reverse mapping of the 'Allocas' array.
379 AllocaLookup[Allocas[AllocaNum]] = AllocaNum;
382 // Process all allocas which are only used in a single basic block.
383 for (std::map<BasicBlock*, std::vector<AllocaInst*> >::iterator I =
384 LocallyUsedAllocas.begin(), E = LocallyUsedAllocas.end(); I != E; ++I){
385 const std::vector<AllocaInst*> &LocAllocas = I->second;
386 assert(!LocAllocas.empty() && "empty alloca list??");
388 // It's common for there to only be one alloca in the list. Handle it
390 if (LocAllocas.size() == 1) {
391 // If we can do the quick promotion pass, do so now.
392 if (PromoteLocallyUsedAlloca(I->first, LocAllocas[0]))
393 RetryList.push_back(LocAllocas[0]); // Failed, retry later.
395 // Locally promote anything possible. Note that if this is unable to
396 // promote a particular alloca, it puts the alloca onto the Allocas vector
397 // for global processing.
398 PromoteLocallyUsedAllocas(I->first, LocAllocas);
403 return; // All of the allocas must have been trivial!
405 // Set the incoming values for the basic block to be null values for all of
406 // the alloca's. We do this in case there is a load of a value that has not
407 // been stored yet. In this case, it will get this null value.
409 RenamePassData::ValVector Values(Allocas.size());
410 for (unsigned i = 0, e = Allocas.size(); i != e; ++i)
411 Values[i] = UndefValue::get(Allocas[i]->getAllocatedType());
413 // Walks all basic blocks in the function performing the SSA rename algorithm
414 // and inserting the phi nodes we marked as necessary
416 std::vector<RenamePassData> RenamePassWorkList;
417 RenamePassWorkList.push_back(RenamePassData(F.begin(), 0, Values));
418 while (!RenamePassWorkList.empty()) {
420 RPD.swap(RenamePassWorkList.back());
421 RenamePassWorkList.pop_back();
422 // RenamePass may add new worklist entries.
423 RenamePass(RPD.BB, RPD.Pred, RPD.Values, RenamePassWorkList);
426 // The renamer uses the Visited set to avoid infinite loops. Clear it now.
429 // Remove the allocas themselves from the function.
430 for (unsigned i = 0, e = Allocas.size(); i != e; ++i) {
431 Instruction *A = Allocas[i];
433 // If there are any uses of the alloca instructions left, they must be in
434 // sections of dead code that were not processed on the dominance frontier.
435 // Just delete the users now.
438 A->replaceAllUsesWith(UndefValue::get(A->getType()));
439 if (AST) AST->deleteValue(A);
440 A->eraseFromParent();
444 // Loop over all of the PHI nodes and see if there are any that we can get
445 // rid of because they merge all of the same incoming values. This can
446 // happen due to undef values coming into the PHI nodes. This process is
447 // iterative, because eliminating one PHI node can cause others to be removed.
448 bool EliminatedAPHI = true;
449 while (EliminatedAPHI) {
450 EliminatedAPHI = false;
452 for (DenseMap<std::pair<BasicBlock*, unsigned>, PHINode*>::iterator I =
453 NewPhiNodes.begin(), E = NewPhiNodes.end(); I != E;) {
454 PHINode *PN = I->second;
456 // If this PHI node merges one value and/or undefs, get the value.
457 if (Value *V = PN->hasConstantValue(true)) {
458 if (!isa<Instruction>(V) ||
459 properlyDominates(cast<Instruction>(V), PN)) {
460 if (AST && isa<PointerType>(PN->getType()))
461 AST->deleteValue(PN);
462 PN->replaceAllUsesWith(V);
463 PN->eraseFromParent();
464 NewPhiNodes.erase(I++);
465 EliminatedAPHI = true;
473 // At this point, the renamer has added entries to PHI nodes for all reachable
474 // code. Unfortunately, there may be unreachable blocks which the renamer
475 // hasn't traversed. If this is the case, the PHI nodes may not
476 // have incoming values for all predecessors. Loop over all PHI nodes we have
477 // created, inserting undef values if they are missing any incoming values.
479 for (DenseMap<std::pair<BasicBlock*, unsigned>, PHINode*>::iterator I =
480 NewPhiNodes.begin(), E = NewPhiNodes.end(); I != E; ++I) {
481 // We want to do this once per basic block. As such, only process a block
482 // when we find the PHI that is the first entry in the block.
483 PHINode *SomePHI = I->second;
484 BasicBlock *BB = SomePHI->getParent();
485 if (&BB->front() != SomePHI)
488 // Count the number of preds for BB.
489 SmallVector<BasicBlock*, 16> Preds(pred_begin(BB), pred_end(BB));
491 // Only do work here if there the PHI nodes are missing incoming values. We
492 // know that all PHI nodes that were inserted in a block will have the same
493 // number of incoming values, so we can just check any of them.
494 if (SomePHI->getNumIncomingValues() == Preds.size())
497 // Ok, now we know that all of the PHI nodes are missing entries for some
498 // basic blocks. Start by sorting the incoming predecessors for efficient
500 std::sort(Preds.begin(), Preds.end());
502 // Now we loop through all BB's which have entries in SomePHI and remove
503 // them from the Preds list.
504 for (unsigned i = 0, e = SomePHI->getNumIncomingValues(); i != e; ++i) {
505 // Do a log(n) search of the Preds list for the entry we want.
506 SmallVector<BasicBlock*, 16>::iterator EntIt =
507 std::lower_bound(Preds.begin(), Preds.end(),
508 SomePHI->getIncomingBlock(i));
509 assert(EntIt != Preds.end() && *EntIt == SomePHI->getIncomingBlock(i)&&
510 "PHI node has entry for a block which is not a predecessor!");
516 // At this point, the blocks left in the preds list must have dummy
517 // entries inserted into every PHI nodes for the block. Update all the phi
518 // nodes in this block that we are inserting (there could be phis before
520 unsigned NumBadPreds = SomePHI->getNumIncomingValues();
521 BasicBlock::iterator BBI = BB->begin();
522 while ((SomePHI = dyn_cast<PHINode>(BBI++)) &&
523 SomePHI->getNumIncomingValues() == NumBadPreds) {
524 Value *UndefVal = UndefValue::get(SomePHI->getType());
525 for (unsigned pred = 0, e = Preds.size(); pred != e; ++pred)
526 SomePHI->addIncoming(UndefVal, Preds[pred]);
534 /// RewriteSingleStoreAlloca - If there is only a single store to this value,
535 /// replace any loads of it that are directly dominated by the definition with
536 /// the value stored.
537 void PromoteMem2Reg::RewriteSingleStoreAlloca(AllocaInst *AI,
539 StoreInst *OnlyStore = Info.OnlyStore;
540 bool StoringGlobalVal = !isa<Instruction>(OnlyStore->getOperand(0));
542 // Be aware of loads before the store.
543 SmallPtrSet<BasicBlock*, 32> ProcessedBlocks;
544 for (unsigned i = 0, e = Info.UsingBlocks.size(); i != e; ++i) {
545 BasicBlock *UseBlock = Info.UsingBlocks[i];
547 // If we already processed this block, don't reprocess it.
548 if (!ProcessedBlocks.insert(UseBlock)) {
549 Info.UsingBlocks[i] = Info.UsingBlocks.back();
550 Info.UsingBlocks.pop_back();
555 // If the store dominates the block and if we haven't processed it yet,
556 // do so now. We can't handle the case where the store doesn't dominate a
557 // block because there may be a path between the store and the use, but we
558 // may need to insert phi nodes to handle dominance properly.
559 if (!StoringGlobalVal && !dominates(OnlyStore->getParent(), UseBlock))
562 // If the use and store are in the same block, do a quick scan to
563 // verify that there are no uses before the store.
564 if (UseBlock == OnlyStore->getParent()) {
565 BasicBlock::iterator I = UseBlock->begin();
566 for (; &*I != OnlyStore; ++I) { // scan block for store.
567 if (isa<LoadInst>(I) && I->getOperand(0) == AI)
570 if (&*I != OnlyStore)
571 continue; // Do not promote the uses of this in this block.
574 // Otherwise, if this is a different block or if all uses happen
575 // after the store, do a simple linear scan to replace loads with
577 for (BasicBlock::iterator I = UseBlock->begin(), E = UseBlock->end();
579 if (LoadInst *LI = dyn_cast<LoadInst>(I++)) {
580 if (LI->getOperand(0) == AI) {
581 LI->replaceAllUsesWith(OnlyStore->getOperand(0));
582 if (AST && isa<PointerType>(LI->getType()))
583 AST->deleteValue(LI);
584 LI->eraseFromParent();
589 // Finally, remove this block from the UsingBlock set.
590 Info.UsingBlocks[i] = Info.UsingBlocks.back();
591 Info.UsingBlocks.pop_back();
597 // MarkDominatingPHILive - Mem2Reg wants to construct "pruned" SSA form, not
598 // "minimal" SSA form. To do this, it inserts all of the PHI nodes on the IDF
599 // as usual (inserting the PHI nodes in the DeadPHINodes set), then processes
600 // each read of the variable. For each block that reads the variable, this
601 // function is called, which removes used PHI nodes from the DeadPHINodes set.
602 // After all of the reads have been processed, any PHI nodes left in the
603 // DeadPHINodes set are removed.
605 void PromoteMem2Reg::MarkDominatingPHILive(BasicBlock *BB, unsigned AllocaNum,
606 SmallPtrSet<PHINode*, 16> &DeadPHINodes) {
607 // Scan the immediate dominators of this block looking for a block which has a
608 // PHI node for Alloca num. If we find it, mark the PHI node as being alive!
609 DomTreeNode *IDomNode = DT.getNode(BB);
610 for (DomTreeNode *IDom = IDomNode; IDom; IDom = IDom->getIDom()) {
611 BasicBlock *DomBB = IDom->getBlock();
612 DenseMap<std::pair<BasicBlock*, unsigned>, PHINode*>::iterator
613 I = NewPhiNodes.find(std::make_pair(DomBB, AllocaNum));
614 if (I == NewPhiNodes.end()) continue;
616 // Ok, we found an inserted PHI node which dominates this value.
617 PHINode *DominatingPHI = I->second;
619 // Find out if we previously thought it was dead. If so, mark it as being
620 // live by removing it from the DeadPHINodes set.
621 if (!DeadPHINodes.erase(DominatingPHI))
624 // Now that we have marked the PHI node alive, also mark any PHI nodes
625 // which it might use as being alive as well.
626 for (pred_iterator PI = pred_begin(DomBB), PE = pred_end(DomBB);
628 MarkDominatingPHILive(*PI, AllocaNum, DeadPHINodes);
632 /// PromoteLocallyUsedAlloca - Many allocas are only used within a single basic
633 /// block. If this is the case, avoid traversing the CFG and inserting a lot of
634 /// potentially useless PHI nodes by just performing a single linear pass over
635 /// the basic block using the Alloca.
637 /// If we cannot promote this alloca (because it is read before it is written),
638 /// return true. This is necessary in cases where, due to control flow, the
639 /// alloca is potentially undefined on some control flow paths. e.g. code like
640 /// this is potentially correct:
642 /// for (...) { if (c) { A = undef; undef = B; } }
644 /// ... so long as A is not used before undef is set.
646 bool PromoteMem2Reg::PromoteLocallyUsedAlloca(BasicBlock *BB, AllocaInst *AI) {
647 assert(!AI->use_empty() && "There are no uses of the alloca!");
649 // Handle degenerate cases quickly.
650 if (AI->hasOneUse()) {
651 Instruction *U = cast<Instruction>(AI->use_back());
652 if (LoadInst *LI = dyn_cast<LoadInst>(U)) {
653 // Must be a load of uninitialized value.
654 LI->replaceAllUsesWith(UndefValue::get(AI->getAllocatedType()));
655 if (AST && isa<PointerType>(LI->getType()))
656 AST->deleteValue(LI);
658 // Otherwise it must be a store which is never read.
659 assert(isa<StoreInst>(U));
661 BB->getInstList().erase(U);
663 // Uses of the uninitialized memory location shall get undef.
666 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
667 Instruction *Inst = I++;
668 if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {
669 if (LI->getOperand(0) == AI) {
670 if (!CurVal) return true; // Could not locally promote!
672 // Loads just returns the "current value"...
673 LI->replaceAllUsesWith(CurVal);
674 if (AST && isa<PointerType>(LI->getType()))
675 AST->deleteValue(LI);
676 BB->getInstList().erase(LI);
678 } else if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
679 if (SI->getOperand(1) == AI) {
680 // Store updates the "current value"...
681 CurVal = SI->getOperand(0);
682 BB->getInstList().erase(SI);
688 // After traversing the basic block, there should be no more uses of the
689 // alloca: remove it now.
690 assert(AI->use_empty() && "Uses of alloca from more than one BB??");
691 if (AST) AST->deleteValue(AI);
692 AI->eraseFromParent();
698 /// PromoteLocallyUsedAllocas - This method is just like
699 /// PromoteLocallyUsedAlloca, except that it processes multiple alloca
700 /// instructions in parallel. This is important in cases where we have large
701 /// basic blocks, as we don't want to rescan the entire basic block for each
702 /// alloca which is locally used in it (which might be a lot).
703 void PromoteMem2Reg::
704 PromoteLocallyUsedAllocas(BasicBlock *BB, const std::vector<AllocaInst*> &AIs) {
705 DenseMap<AllocaInst*, Value*> CurValues;
706 for (unsigned i = 0, e = AIs.size(); i != e; ++i)
707 CurValues[AIs[i]] = 0; // Insert with null value
709 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
710 Instruction *Inst = I++;
711 if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {
712 // Is this a load of an alloca we are tracking?
713 if (AllocaInst *AI = dyn_cast<AllocaInst>(LI->getOperand(0))) {
714 DenseMap<AllocaInst*, Value*>::iterator AIt = CurValues.find(AI);
715 if (AIt != CurValues.end()) {
716 // If loading an uninitialized value, allow the inter-block case to
717 // handle it. Due to control flow, this might actually be ok.
718 if (AIt->second == 0) { // Use of locally uninitialized value??
719 RetryList.push_back(AI); // Retry elsewhere.
720 CurValues.erase(AIt); // Stop tracking this here.
721 if (CurValues.empty()) return;
723 // Loads just returns the "current value"...
724 LI->replaceAllUsesWith(AIt->second);
725 if (AST && isa<PointerType>(LI->getType()))
726 AST->deleteValue(LI);
727 BB->getInstList().erase(LI);
731 } else if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
732 if (AllocaInst *AI = dyn_cast<AllocaInst>(SI->getOperand(1))) {
733 DenseMap<AllocaInst*, Value*>::iterator AIt = CurValues.find(AI);
734 if (AIt != CurValues.end()) {
735 // Store updates the "current value"...
736 AIt->second = SI->getOperand(0);
737 SI->eraseFromParent();
743 // At the end of the block scan, all allocas in CurValues are dead.
744 for (DenseMap<AllocaInst*, Value*>::iterator I = CurValues.begin(),
745 E = CurValues.end(); I != E; ++I) {
746 AllocaInst *AI = I->first;
747 assert(AI->use_empty() && "Uses of alloca from more than one BB??");
748 if (AST) AST->deleteValue(AI);
749 AI->eraseFromParent();
752 NumLocalPromoted += CurValues.size();
757 // QueuePhiNode - queues a phi-node to be added to a basic-block for a specific
758 // Alloca returns true if there wasn't already a phi-node for that variable
760 bool PromoteMem2Reg::QueuePhiNode(BasicBlock *BB, unsigned AllocaNo,
762 SmallPtrSet<PHINode*, 16> &InsertedPHINodes) {
763 // Look up the basic-block in question.
764 PHINode *&PN = NewPhiNodes[std::make_pair(BB, AllocaNo)];
766 // If the BB already has a phi node added for the i'th alloca then we're done!
767 if (PN) return false;
769 // Create a PhiNode using the dereferenced type... and add the phi-node to the
771 PN = new PHINode(Allocas[AllocaNo]->getAllocatedType(),
772 Allocas[AllocaNo]->getName() + "." +
773 utostr(Version++), BB->begin());
774 PhiToAllocaMap[PN] = AllocaNo;
775 PN->reserveOperandSpace(std::distance(pred_begin(BB), pred_end(BB)));
777 InsertedPHINodes.insert(PN);
779 if (AST && isa<PointerType>(PN->getType()))
780 AST->copyValue(PointerAllocaValues[AllocaNo], PN);
786 // RenamePass - Recursively traverse the CFG of the function, renaming loads and
787 // stores to the allocas which we are promoting. IncomingVals indicates what
788 // value each Alloca contains on exit from the predecessor block Pred.
790 void PromoteMem2Reg::RenamePass(BasicBlock *BB, BasicBlock *Pred,
791 RenamePassData::ValVector &IncomingVals,
792 std::vector<RenamePassData> &Worklist) {
793 // If we are inserting any phi nodes into this BB, they will already be in the
795 if (PHINode *APN = dyn_cast<PHINode>(BB->begin())) {
796 // Pred may have multiple edges to BB. If so, we want to add N incoming
797 // values to each PHI we are inserting on the first time we see the edge.
798 // Check to see if APN already has incoming values from Pred. This also
799 // prevents us from modifying PHI nodes that are not currently being
801 bool HasPredEntries = false;
802 for (unsigned i = 0, e = APN->getNumIncomingValues(); i != e; ++i) {
803 if (APN->getIncomingBlock(i) == Pred) {
804 HasPredEntries = true;
809 // If we have PHI nodes to update, compute the number of edges from Pred to
811 if (!HasPredEntries) {
812 TerminatorInst *PredTerm = Pred->getTerminator();
813 unsigned NumEdges = 0;
814 for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i) {
815 if (PredTerm->getSuccessor(i) == BB)
818 assert(NumEdges && "Must be at least one edge from Pred to BB!");
820 // Add entries for all the phis.
821 BasicBlock::iterator PNI = BB->begin();
823 unsigned AllocaNo = PhiToAllocaMap[APN];
825 // Add N incoming values to the PHI node.
826 for (unsigned i = 0; i != NumEdges; ++i)
827 APN->addIncoming(IncomingVals[AllocaNo], Pred);
829 // The currently active variable for this block is now the PHI.
830 IncomingVals[AllocaNo] = APN;
832 // Get the next phi node.
834 APN = dyn_cast<PHINode>(PNI);
837 // Verify it doesn't already have entries for Pred. If it does, it is
838 // not being inserted by this mem2reg invocation.
839 HasPredEntries = false;
840 for (unsigned i = 0, e = APN->getNumIncomingValues(); i != e; ++i) {
841 if (APN->getIncomingBlock(i) == Pred) {
842 HasPredEntries = true;
846 } while (!HasPredEntries);
850 // Don't revisit blocks.
851 if (!Visited.insert(BB)) return;
853 for (BasicBlock::iterator II = BB->begin(); !isa<TerminatorInst>(II); ) {
854 Instruction *I = II++; // get the instruction, increment iterator
856 if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
857 if (AllocaInst *Src = dyn_cast<AllocaInst>(LI->getPointerOperand())) {
858 std::map<AllocaInst*, unsigned>::iterator AI = AllocaLookup.find(Src);
859 if (AI != AllocaLookup.end()) {
860 Value *V = IncomingVals[AI->second];
862 // walk the use list of this load and replace all uses with r
863 LI->replaceAllUsesWith(V);
864 if (AST && isa<PointerType>(LI->getType()))
865 AST->deleteValue(LI);
866 BB->getInstList().erase(LI);
869 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
870 // Delete this instruction and mark the name as the current holder of the
872 if (AllocaInst *Dest = dyn_cast<AllocaInst>(SI->getPointerOperand())) {
873 std::map<AllocaInst *, unsigned>::iterator ai = AllocaLookup.find(Dest);
874 if (ai != AllocaLookup.end()) {
875 // what value were we writing?
876 IncomingVals[ai->second] = SI->getOperand(0);
877 BB->getInstList().erase(SI);
883 // Recurse to our successors.
884 TerminatorInst *TI = BB->getTerminator();
885 for (unsigned i = 0; i != TI->getNumSuccessors(); i++)
886 Worklist.push_back(RenamePassData(TI->getSuccessor(i), BB, IncomingVals));
889 /// PromoteMemToReg - Promote the specified list of alloca instructions into
890 /// scalar registers, inserting PHI nodes as appropriate. This function makes
891 /// use of DominanceFrontier information. This function does not modify the CFG
892 /// of the function at all. All allocas must be from the same function.
894 /// If AST is specified, the specified tracker is updated to reflect changes
897 void llvm::PromoteMemToReg(const std::vector<AllocaInst*> &Allocas,
898 DominatorTree &DT, DominanceFrontier &DF,
899 AliasSetTracker *AST) {
900 // If there is nothing to do, bail out...
901 if (Allocas.empty()) return;
903 SmallVector<AllocaInst*, 16> RetryList;
904 PromoteMem2Reg(Allocas, RetryList, DT, DF, AST).run();
906 // PromoteMem2Reg may not have been able to promote all of the allocas in one
907 // pass, run it again if needed.
908 std::vector<AllocaInst*> NewAllocas;
909 while (!RetryList.empty()) {
910 // If we need to retry some allocas, this is due to there being no store
911 // before a read in a local block. To counteract this, insert a store of
912 // undef into the alloca right after the alloca itself.
913 for (unsigned i = 0, e = RetryList.size(); i != e; ++i) {
914 BasicBlock::iterator BBI = RetryList[i];
916 new StoreInst(UndefValue::get(RetryList[i]->getAllocatedType()),
917 RetryList[i], ++BBI);
920 NewAllocas.assign(RetryList.begin(), RetryList.end());
922 PromoteMem2Reg(NewAllocas, RetryList, DT, DF, AST).run();