1 //===- SSAUpdater.cpp - Unstructured SSA Update Tool ----------------------===//
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 implements the SSAUpdater class.
12 //===----------------------------------------------------------------------===//
14 #define DEBUG_TYPE "ssaupdater"
15 #include "llvm/Instructions.h"
16 #include "llvm/ADT/DenseMap.h"
17 #include "llvm/Analysis/InstructionSimplify.h"
18 #include "llvm/Support/AlignOf.h"
19 #include "llvm/Support/Allocator.h"
20 #include "llvm/Support/CFG.h"
21 #include "llvm/Support/Debug.h"
22 #include "llvm/Support/raw_ostream.h"
23 #include "llvm/Transforms/Utils/SSAUpdater.h"
24 #include "llvm/Transforms/Utils/SSAUpdaterImpl.h"
27 typedef DenseMap<BasicBlock*, Value*> AvailableValsTy;
28 static AvailableValsTy &getAvailableVals(void *AV) {
29 return *static_cast<AvailableValsTy*>(AV);
32 SSAUpdater::SSAUpdater(SmallVectorImpl<PHINode*> *NewPHI)
33 : AV(0), ProtoType(0), ProtoName(), InsertedPHIs(NewPHI) {}
35 SSAUpdater::~SSAUpdater() {
36 delete &getAvailableVals(AV);
39 /// Initialize - Reset this object to get ready for a new set of SSA
40 /// updates with type 'Ty'. PHI nodes get a name based on 'Name'.
41 void SSAUpdater::Initialize(const Type *Ty, StringRef Name) {
43 AV = new AvailableValsTy();
45 getAvailableVals(AV).clear();
50 /// HasValueForBlock - Return true if the SSAUpdater already has a value for
51 /// the specified block.
52 bool SSAUpdater::HasValueForBlock(BasicBlock *BB) const {
53 return getAvailableVals(AV).count(BB);
56 /// AddAvailableValue - Indicate that a rewritten value is available in the
57 /// specified block with the specified value.
58 void SSAUpdater::AddAvailableValue(BasicBlock *BB, Value *V) {
59 assert(ProtoType != 0 && "Need to initialize SSAUpdater");
60 assert(ProtoType == V->getType() &&
61 "All rewritten values must have the same type");
62 getAvailableVals(AV)[BB] = V;
65 /// IsEquivalentPHI - Check if PHI has the same incoming value as specified
66 /// in ValueMapping for each predecessor block.
67 static bool IsEquivalentPHI(PHINode *PHI,
68 DenseMap<BasicBlock*, Value*> &ValueMapping) {
69 unsigned PHINumValues = PHI->getNumIncomingValues();
70 if (PHINumValues != ValueMapping.size())
73 // Scan the phi to see if it matches.
74 for (unsigned i = 0, e = PHINumValues; i != e; ++i)
75 if (ValueMapping[PHI->getIncomingBlock(i)] !=
76 PHI->getIncomingValue(i)) {
83 /// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is
84 /// live at the end of the specified block.
85 Value *SSAUpdater::GetValueAtEndOfBlock(BasicBlock *BB) {
86 Value *Res = GetValueAtEndOfBlockInternal(BB);
90 /// GetValueInMiddleOfBlock - Construct SSA form, materializing a value that
91 /// is live in the middle of the specified block.
93 /// GetValueInMiddleOfBlock is the same as GetValueAtEndOfBlock except in one
94 /// important case: if there is a definition of the rewritten value after the
95 /// 'use' in BB. Consider code like this:
101 /// br Cond, SomeBB, OutBB
103 /// In this case, there are two values (X1 and X2) added to the AvailableVals
104 /// set by the client of the rewriter, and those values are both live out of
105 /// their respective blocks. However, the use of X happens in the *middle* of
106 /// a block. Because of this, we need to insert a new PHI node in SomeBB to
107 /// merge the appropriate values, and this value isn't live out of the block.
109 Value *SSAUpdater::GetValueInMiddleOfBlock(BasicBlock *BB) {
110 // If there is no definition of the renamed variable in this block, just use
111 // GetValueAtEndOfBlock to do our work.
112 if (!HasValueForBlock(BB))
113 return GetValueAtEndOfBlock(BB);
115 // Otherwise, we have the hard case. Get the live-in values for each
117 SmallVector<std::pair<BasicBlock*, Value*>, 8> PredValues;
118 Value *SingularValue = 0;
120 // We can get our predecessor info by walking the pred_iterator list, but it
121 // is relatively slow. If we already have PHI nodes in this block, walk one
122 // of them to get the predecessor list instead.
123 if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) {
124 for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) {
125 BasicBlock *PredBB = SomePhi->getIncomingBlock(i);
126 Value *PredVal = GetValueAtEndOfBlock(PredBB);
127 PredValues.push_back(std::make_pair(PredBB, PredVal));
129 // Compute SingularValue.
131 SingularValue = PredVal;
132 else if (PredVal != SingularValue)
136 bool isFirstPred = true;
137 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
138 BasicBlock *PredBB = *PI;
139 Value *PredVal = GetValueAtEndOfBlock(PredBB);
140 PredValues.push_back(std::make_pair(PredBB, PredVal));
142 // Compute SingularValue.
144 SingularValue = PredVal;
146 } else if (PredVal != SingularValue)
151 // If there are no predecessors, just return undef.
152 if (PredValues.empty())
153 return UndefValue::get(ProtoType);
155 // Otherwise, if all the merged values are the same, just use it.
156 if (SingularValue != 0)
157 return SingularValue;
159 // Otherwise, we do need a PHI: check to see if we already have one available
160 // in this block that produces the right value.
161 if (isa<PHINode>(BB->begin())) {
162 DenseMap<BasicBlock*, Value*> ValueMapping(PredValues.begin(),
165 for (BasicBlock::iterator It = BB->begin();
166 (SomePHI = dyn_cast<PHINode>(It)); ++It) {
167 if (IsEquivalentPHI(SomePHI, ValueMapping))
172 // Ok, we have no way out, insert a new one now.
173 PHINode *InsertedPHI = PHINode::Create(ProtoType, PredValues.size(),
174 ProtoName, &BB->front());
176 // Fill in all the predecessors of the PHI.
177 for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
178 InsertedPHI->addIncoming(PredValues[i].second, PredValues[i].first);
180 // See if the PHI node can be merged to a single value. This can happen in
181 // loop cases when we get a PHI of itself and one other value.
182 if (Value *V = SimplifyInstruction(InsertedPHI)) {
183 InsertedPHI->eraseFromParent();
187 // If the client wants to know about all new instructions, tell it.
188 if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);
190 DEBUG(dbgs() << " Inserted PHI: " << *InsertedPHI << "\n");
194 /// RewriteUse - Rewrite a use of the symbolic value. This handles PHI nodes,
195 /// which use their value in the corresponding predecessor.
196 void SSAUpdater::RewriteUse(Use &U) {
197 Instruction *User = cast<Instruction>(U.getUser());
200 if (PHINode *UserPN = dyn_cast<PHINode>(User))
201 V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U));
203 V = GetValueInMiddleOfBlock(User->getParent());
208 /// RewriteUseAfterInsertions - Rewrite a use, just like RewriteUse. However,
209 /// this version of the method can rewrite uses in the same block as a
210 /// definition, because it assumes that all uses of a value are below any
212 void SSAUpdater::RewriteUseAfterInsertions(Use &U) {
213 Instruction *User = cast<Instruction>(U.getUser());
216 if (PHINode *UserPN = dyn_cast<PHINode>(User))
217 V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U));
219 V = GetValueAtEndOfBlock(User->getParent());
224 /// PHIiter - Iterator for PHI operands. This is used for the PHI_iterator
225 /// in the SSAUpdaterImpl template.
233 explicit PHIiter(PHINode *P) // begin iterator
235 PHIiter(PHINode *P, bool) // end iterator
236 : PHI(P), idx(PHI->getNumIncomingValues()) {}
238 PHIiter &operator++() { ++idx; return *this; }
239 bool operator==(const PHIiter& x) const { return idx == x.idx; }
240 bool operator!=(const PHIiter& x) const { return !operator==(x); }
241 Value *getIncomingValue() { return PHI->getIncomingValue(idx); }
242 BasicBlock *getIncomingBlock() { return PHI->getIncomingBlock(idx); }
246 /// SSAUpdaterTraits<SSAUpdater> - Traits for the SSAUpdaterImpl template,
247 /// specialized for SSAUpdater.
250 class SSAUpdaterTraits<SSAUpdater> {
252 typedef BasicBlock BlkT;
254 typedef PHINode PhiT;
256 typedef succ_iterator BlkSucc_iterator;
257 static BlkSucc_iterator BlkSucc_begin(BlkT *BB) { return succ_begin(BB); }
258 static BlkSucc_iterator BlkSucc_end(BlkT *BB) { return succ_end(BB); }
260 typedef PHIiter PHI_iterator;
261 static inline PHI_iterator PHI_begin(PhiT *PHI) { return PHI_iterator(PHI); }
262 static inline PHI_iterator PHI_end(PhiT *PHI) {
263 return PHI_iterator(PHI, true);
266 /// FindPredecessorBlocks - Put the predecessors of Info->BB into the Preds
267 /// vector, set Info->NumPreds, and allocate space in Info->Preds.
268 static void FindPredecessorBlocks(BasicBlock *BB,
269 SmallVectorImpl<BasicBlock*> *Preds) {
270 // We can get our predecessor info by walking the pred_iterator list,
271 // but it is relatively slow. If we already have PHI nodes in this
272 // block, walk one of them to get the predecessor list instead.
273 if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) {
274 for (unsigned PI = 0, E = SomePhi->getNumIncomingValues(); PI != E; ++PI)
275 Preds->push_back(SomePhi->getIncomingBlock(PI));
277 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
278 Preds->push_back(*PI);
282 /// GetUndefVal - Get an undefined value of the same type as the value
284 static Value *GetUndefVal(BasicBlock *BB, SSAUpdater *Updater) {
285 return UndefValue::get(Updater->ProtoType);
288 /// CreateEmptyPHI - Create a new PHI instruction in the specified block.
289 /// Reserve space for the operands but do not fill them in yet.
290 static Value *CreateEmptyPHI(BasicBlock *BB, unsigned NumPreds,
291 SSAUpdater *Updater) {
292 PHINode *PHI = PHINode::Create(Updater->ProtoType, NumPreds,
293 Updater->ProtoName, &BB->front());
297 /// AddPHIOperand - Add the specified value as an operand of the PHI for
298 /// the specified predecessor block.
299 static void AddPHIOperand(PHINode *PHI, Value *Val, BasicBlock *Pred) {
300 PHI->addIncoming(Val, Pred);
303 /// InstrIsPHI - Check if an instruction is a PHI.
305 static PHINode *InstrIsPHI(Instruction *I) {
306 return dyn_cast<PHINode>(I);
309 /// ValueIsPHI - Check if a value is a PHI.
311 static PHINode *ValueIsPHI(Value *Val, SSAUpdater *Updater) {
312 return dyn_cast<PHINode>(Val);
315 /// ValueIsNewPHI - Like ValueIsPHI but also check if the PHI has no source
316 /// operands, i.e., it was just added.
317 static PHINode *ValueIsNewPHI(Value *Val, SSAUpdater *Updater) {
318 PHINode *PHI = ValueIsPHI(Val, Updater);
319 if (PHI && PHI->getNumIncomingValues() == 0)
324 /// GetPHIValue - For the specified PHI instruction, return the value
326 static Value *GetPHIValue(PHINode *PHI) {
331 } // End llvm namespace
333 /// GetValueAtEndOfBlockInternal - Check to see if AvailableVals has an entry
334 /// for the specified BB and if so, return it. If not, construct SSA form by
335 /// first calculating the required placement of PHIs and then inserting new
336 /// PHIs where needed.
337 Value *SSAUpdater::GetValueAtEndOfBlockInternal(BasicBlock *BB) {
338 AvailableValsTy &AvailableVals = getAvailableVals(AV);
339 if (Value *V = AvailableVals[BB])
342 SSAUpdaterImpl<SSAUpdater> Impl(this, &AvailableVals, InsertedPHIs);
343 return Impl.GetValue(BB);
346 //===----------------------------------------------------------------------===//
347 // LoadAndStorePromoter Implementation
348 //===----------------------------------------------------------------------===//
350 LoadAndStorePromoter::
351 LoadAndStorePromoter(const SmallVectorImpl<Instruction*> &Insts,
352 SSAUpdater &S, StringRef BaseName) : SSA(S) {
353 if (Insts.empty()) return;
356 if (LoadInst *LI = dyn_cast<LoadInst>(Insts[0]))
359 SomeVal = cast<StoreInst>(Insts[0])->getOperand(0);
361 if (BaseName.empty())
362 BaseName = SomeVal->getName();
363 SSA.Initialize(SomeVal->getType(), BaseName);
367 void LoadAndStorePromoter::
368 run(const SmallVectorImpl<Instruction*> &Insts) const {
370 // First step: bucket up uses of the alloca by the block they occur in.
371 // This is important because we have to handle multiple defs/uses in a block
372 // ourselves: SSAUpdater is purely for cross-block references.
373 // FIXME: Want a TinyVector<Instruction*> since there is often 0/1 element.
374 DenseMap<BasicBlock*, std::vector<Instruction*> > UsesByBlock;
376 for (unsigned i = 0, e = Insts.size(); i != e; ++i) {
377 Instruction *User = Insts[i];
378 UsesByBlock[User->getParent()].push_back(User);
381 // Okay, now we can iterate over all the blocks in the function with uses,
382 // processing them. Keep track of which loads are loading a live-in value.
383 // Walk the uses in the use-list order to be determinstic.
384 SmallVector<LoadInst*, 32> LiveInLoads;
385 DenseMap<Value*, Value*> ReplacedLoads;
387 for (unsigned i = 0, e = Insts.size(); i != e; ++i) {
388 Instruction *User = Insts[i];
389 BasicBlock *BB = User->getParent();
390 std::vector<Instruction*> &BlockUses = UsesByBlock[BB];
392 // If this block has already been processed, ignore this repeat use.
393 if (BlockUses.empty()) continue;
395 // Okay, this is the first use in the block. If this block just has a
396 // single user in it, we can rewrite it trivially.
397 if (BlockUses.size() == 1) {
398 // If it is a store, it is a trivial def of the value in the block.
399 if (StoreInst *SI = dyn_cast<StoreInst>(User))
400 SSA.AddAvailableValue(BB, SI->getOperand(0));
402 // Otherwise it is a load, queue it to rewrite as a live-in load.
403 LiveInLoads.push_back(cast<LoadInst>(User));
408 // Otherwise, check to see if this block is all loads.
409 bool HasStore = false;
410 for (unsigned i = 0, e = BlockUses.size(); i != e; ++i) {
411 if (isa<StoreInst>(BlockUses[i])) {
417 // If so, we can queue them all as live in loads. We don't have an
418 // efficient way to tell which on is first in the block and don't want to
419 // scan large blocks, so just add all loads as live ins.
421 for (unsigned i = 0, e = BlockUses.size(); i != e; ++i)
422 LiveInLoads.push_back(cast<LoadInst>(BlockUses[i]));
427 // Otherwise, we have mixed loads and stores (or just a bunch of stores).
428 // Since SSAUpdater is purely for cross-block values, we need to determine
429 // the order of these instructions in the block. If the first use in the
430 // block is a load, then it uses the live in value. The last store defines
431 // the live out value. We handle this by doing a linear scan of the block.
432 Value *StoredValue = 0;
433 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
434 if (LoadInst *L = dyn_cast<LoadInst>(II)) {
435 // If this is a load from an unrelated pointer, ignore it.
436 if (!isInstInList(L, Insts)) continue;
438 // If we haven't seen a store yet, this is a live in use, otherwise
439 // use the stored value.
441 replaceLoadWithValue(L, StoredValue);
442 L->replaceAllUsesWith(StoredValue);
443 ReplacedLoads[L] = StoredValue;
445 LiveInLoads.push_back(L);
450 if (StoreInst *S = dyn_cast<StoreInst>(II)) {
451 // If this is a store to an unrelated pointer, ignore it.
452 if (!isInstInList(S, Insts)) continue;
454 // Remember that this is the active value in the block.
455 StoredValue = S->getOperand(0);
459 // The last stored value that happened is the live-out for the block.
460 assert(StoredValue && "Already checked that there is a store in block");
461 SSA.AddAvailableValue(BB, StoredValue);
465 // Okay, now we rewrite all loads that use live-in values in the loop,
466 // inserting PHI nodes as necessary.
467 for (unsigned i = 0, e = LiveInLoads.size(); i != e; ++i) {
468 LoadInst *ALoad = LiveInLoads[i];
469 Value *NewVal = SSA.GetValueInMiddleOfBlock(ALoad->getParent());
470 replaceLoadWithValue(ALoad, NewVal);
472 // Avoid assertions in unreachable code.
473 if (NewVal == ALoad) NewVal = UndefValue::get(NewVal->getType());
474 ALoad->replaceAllUsesWith(NewVal);
475 ReplacedLoads[ALoad] = NewVal;
478 // Allow the client to do stuff before we start nuking things.
479 doExtraRewritesBeforeFinalDeletion();
481 // Now that everything is rewritten, delete the old instructions from the
482 // function. They should all be dead now.
483 for (unsigned i = 0, e = Insts.size(); i != e; ++i) {
484 Instruction *User = Insts[i];
486 // If this is a load that still has uses, then the load must have been added
487 // as a live value in the SSAUpdate data structure for a block (e.g. because
488 // the loaded value was stored later). In this case, we need to recursively
489 // propagate the updates until we get to the real value.
490 if (!User->use_empty()) {
491 Value *NewVal = ReplacedLoads[User];
492 assert(NewVal && "not a replaced load?");
494 // Propagate down to the ultimate replacee. The intermediately loads
495 // could theoretically already have been deleted, so we don't want to
496 // dereference the Value*'s.
497 DenseMap<Value*, Value*>::iterator RLI = ReplacedLoads.find(NewVal);
498 while (RLI != ReplacedLoads.end()) {
499 NewVal = RLI->second;
500 RLI = ReplacedLoads.find(NewVal);
503 replaceLoadWithValue(cast<LoadInst>(User), NewVal);
504 User->replaceAllUsesWith(NewVal);
507 instructionDeleted(User);
508 User->eraseFromParent();