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 #include "llvm/Transforms/Utils/SSAUpdater.h"
15 #include "llvm/Instructions.h"
16 #include "llvm/ADT/DenseMap.h"
17 #include "llvm/Support/CFG.h"
18 #include "llvm/Support/Debug.h"
19 #include "llvm/Support/ValueHandle.h"
20 #include "llvm/Support/raw_ostream.h"
23 typedef DenseMap<BasicBlock*, TrackingVH<Value> > AvailableValsTy;
24 typedef std::vector<std::pair<BasicBlock*, TrackingVH<Value> > >
27 static AvailableValsTy &getAvailableVals(void *AV) {
28 return *static_cast<AvailableValsTy*>(AV);
31 static IncomingPredInfoTy &getIncomingPredInfo(void *IPI) {
32 return *static_cast<IncomingPredInfoTy*>(IPI);
36 SSAUpdater::SSAUpdater(SmallVectorImpl<PHINode*> *NewPHI)
37 : AV(0), PrototypeValue(0), IPI(0), InsertedPHIs(NewPHI) {}
39 SSAUpdater::~SSAUpdater() {
40 delete &getAvailableVals(AV);
41 delete &getIncomingPredInfo(IPI);
44 /// Initialize - Reset this object to get ready for a new set of SSA
45 /// updates. ProtoValue is the value used to name PHI nodes.
46 void SSAUpdater::Initialize(Value *ProtoValue) {
48 AV = new AvailableValsTy();
50 getAvailableVals(AV).clear();
53 IPI = new IncomingPredInfoTy();
55 getIncomingPredInfo(IPI).clear();
56 PrototypeValue = ProtoValue;
59 /// AddAvailableValue - Indicate that a rewritten value is available in the
60 /// specified block with the specified value.
61 void SSAUpdater::AddAvailableValue(BasicBlock *BB, Value *V) {
62 assert(PrototypeValue != 0 && "Need to initialize SSAUpdater");
63 assert(PrototypeValue->getType() == V->getType() &&
64 "All rewritten values must have the same type");
65 getAvailableVals(AV)[BB] = V;
68 /// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is
69 /// live at the end of the specified block.
70 Value *SSAUpdater::GetValueAtEndOfBlock(BasicBlock *BB) {
71 assert(getIncomingPredInfo(IPI).empty() && "Unexpected Internal State");
72 Value *Res = GetValueAtEndOfBlockInternal(BB);
73 assert(getIncomingPredInfo(IPI).empty() && "Unexpected Internal State");
77 /// GetValueInMiddleOfBlock - Construct SSA form, materializing a value that
78 /// is live in the middle of the specified block.
80 /// GetValueInMiddleOfBlock is the same as GetValueAtEndOfBlock except in one
81 /// important case: if there is a definition of the rewritten value after the
82 /// 'use' in BB. Consider code like this:
88 /// br Cond, SomeBB, OutBB
90 /// In this case, there are two values (X1 and X2) added to the AvailableVals
91 /// set by the client of the rewriter, and those values are both live out of
92 /// their respective blocks. However, the use of X happens in the *middle* of
93 /// a block. Because of this, we need to insert a new PHI node in SomeBB to
94 /// merge the appropriate values, and this value isn't live out of the block.
96 Value *SSAUpdater::GetValueInMiddleOfBlock(BasicBlock *BB) {
97 // If there is no definition of the renamed variable in this block, just use
98 // GetValueAtEndOfBlock to do our work.
99 if (!getAvailableVals(AV).count(BB))
100 return GetValueAtEndOfBlock(BB);
102 // Otherwise, we have the hard case. Get the live-in values for each
104 SmallVector<std::pair<BasicBlock*, Value*>, 8> PredValues;
105 Value *SingularValue = 0;
107 // We can get our predecessor info by walking the pred_iterator list, but it
108 // is relatively slow. If we already have PHI nodes in this block, walk one
109 // of them to get the predecessor list instead.
110 if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) {
111 for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) {
112 BasicBlock *PredBB = SomePhi->getIncomingBlock(i);
113 Value *PredVal = GetValueAtEndOfBlock(PredBB);
114 PredValues.push_back(std::make_pair(PredBB, PredVal));
116 // Compute SingularValue.
118 SingularValue = PredVal;
119 else if (PredVal != SingularValue)
123 bool isFirstPred = true;
124 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
125 BasicBlock *PredBB = *PI;
126 Value *PredVal = GetValueAtEndOfBlock(PredBB);
127 PredValues.push_back(std::make_pair(PredBB, PredVal));
129 // Compute SingularValue.
131 SingularValue = PredVal;
133 } else if (PredVal != SingularValue)
138 // If there are no predecessors, just return undef.
139 if (PredValues.empty())
140 return UndefValue::get(PrototypeValue->getType());
142 // Otherwise, if all the merged values are the same, just use it.
143 if (SingularValue != 0)
144 return SingularValue;
146 // Otherwise, we do need a PHI: insert one now.
147 PHINode *InsertedPHI = PHINode::Create(PrototypeValue->getType(),
148 PrototypeValue->getName(),
150 InsertedPHI->reserveOperandSpace(PredValues.size());
152 // Fill in all the predecessors of the PHI.
153 for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
154 InsertedPHI->addIncoming(PredValues[i].second, PredValues[i].first);
156 // See if the PHI node can be merged to a single value. This can happen in
157 // loop cases when we get a PHI of itself and one other value.
158 if (Value *ConstVal = InsertedPHI->hasConstantValue()) {
159 InsertedPHI->eraseFromParent();
163 // If the client wants to know about all new instructions, tell it.
164 if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);
166 DEBUG(errs() << " Inserted PHI: " << *InsertedPHI << "\n");
170 /// RewriteUse - Rewrite a use of the symbolic value. This handles PHI nodes,
171 /// which use their value in the corresponding predecessor.
172 void SSAUpdater::RewriteUse(Use &U) {
173 Instruction *User = cast<Instruction>(U.getUser());
174 BasicBlock *UseBB = User->getParent();
175 if (PHINode *UserPN = dyn_cast<PHINode>(User))
176 UseBB = UserPN->getIncomingBlock(U);
178 U.set(GetValueInMiddleOfBlock(UseBB));
182 /// GetValueAtEndOfBlockInternal - Check to see if AvailableVals has an entry
183 /// for the specified BB and if so, return it. If not, construct SSA form by
184 /// walking predecessors inserting PHI nodes as needed until we get to a block
185 /// where the value is available.
187 Value *SSAUpdater::GetValueAtEndOfBlockInternal(BasicBlock *BB) {
188 AvailableValsTy &AvailableVals = getAvailableVals(AV);
190 // Query AvailableVals by doing an insertion of null.
191 std::pair<AvailableValsTy::iterator, bool> InsertRes =
192 AvailableVals.insert(std::make_pair(BB, WeakVH()));
194 // Handle the case when the insertion fails because we have already seen BB.
195 if (!InsertRes.second) {
196 // If the insertion failed, there are two cases. The first case is that the
197 // value is already available for the specified block. If we get this, just
199 if (InsertRes.first->second != 0)
200 return InsertRes.first->second;
202 // Otherwise, if the value we find is null, then this is the value is not
203 // known but it is being computed elsewhere in our recursion. This means
204 // that we have a cycle. Handle this by inserting a PHI node and returning
205 // it. When we get back to the first instance of the recursion we will fill
207 return InsertRes.first->second =
208 PHINode::Create(PrototypeValue->getType(), PrototypeValue->getName(),
212 // Okay, the value isn't in the map and we just inserted a null in the entry
213 // to indicate that we're processing the block. Since we have no idea what
214 // value is in this block, we have to recurse through our predecessors.
216 // While we're walking our predecessors, we keep track of them in a vector,
217 // then insert a PHI node in the end if we actually need one. We could use a
218 // smallvector here, but that would take a lot of stack space for every level
219 // of the recursion, just use IncomingPredInfo as an explicit stack.
220 IncomingPredInfoTy &IncomingPredInfo = getIncomingPredInfo(IPI);
221 unsigned FirstPredInfoEntry = IncomingPredInfo.size();
223 // As we're walking the predecessors, keep track of whether they are all
224 // producing the same value. If so, this value will capture it, if not, it
225 // will get reset to null. We distinguish the no-predecessor case explicitly
227 TrackingVH<Value> SingularValue;
229 // We can get our predecessor info by walking the pred_iterator list, but it
230 // is relatively slow. If we already have PHI nodes in this block, walk one
231 // of them to get the predecessor list instead.
232 if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) {
233 for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) {
234 BasicBlock *PredBB = SomePhi->getIncomingBlock(i);
235 Value *PredVal = GetValueAtEndOfBlockInternal(PredBB);
236 IncomingPredInfo.push_back(std::make_pair(PredBB, PredVal));
238 // Compute SingularValue.
240 SingularValue = PredVal;
241 else if (PredVal != SingularValue)
245 bool isFirstPred = true;
246 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
247 BasicBlock *PredBB = *PI;
248 Value *PredVal = GetValueAtEndOfBlockInternal(PredBB);
249 IncomingPredInfo.push_back(std::make_pair(PredBB, PredVal));
251 // Compute SingularValue.
253 SingularValue = PredVal;
255 } else if (PredVal != SingularValue)
260 // If there are no predecessors, then we must have found an unreachable block
261 // just return 'undef'. Since there are no predecessors, InsertRes must not
263 if (IncomingPredInfo.size() == FirstPredInfoEntry)
264 return InsertRes.first->second = UndefValue::get(PrototypeValue->getType());
266 /// Look up BB's entry in AvailableVals. 'InsertRes' may be invalidated. If
267 /// this block is involved in a loop, a no-entry PHI node will have been
268 /// inserted as InsertedVal. Otherwise, we'll still have the null we inserted
270 TrackingVH<Value> &InsertedVal = AvailableVals[BB];
272 // If all the predecessor values are the same then we don't need to insert a
273 // PHI. This is the simple and common case.
275 // If a PHI node got inserted, replace it with the singlar value and delete
278 PHINode *OldVal = cast<PHINode>(InsertedVal);
279 // Be careful about dead loops. These RAUW's also update InsertedVal.
280 if (InsertedVal != SingularValue)
281 OldVal->replaceAllUsesWith(SingularValue);
283 OldVal->replaceAllUsesWith(UndefValue::get(InsertedVal->getType()));
284 OldVal->eraseFromParent();
286 InsertedVal = SingularValue;
289 // Drop the entries we added in IncomingPredInfo to restore the stack.
290 IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry,
291 IncomingPredInfo.end());
295 // Otherwise, we do need a PHI: insert one now if we don't already have one.
296 if (InsertedVal == 0)
297 InsertedVal = PHINode::Create(PrototypeValue->getType(),
298 PrototypeValue->getName(), &BB->front());
300 PHINode *InsertedPHI = cast<PHINode>(InsertedVal);
301 InsertedPHI->reserveOperandSpace(IncomingPredInfo.size()-FirstPredInfoEntry);
303 // Fill in all the predecessors of the PHI.
304 for (IncomingPredInfoTy::iterator I =
305 IncomingPredInfo.begin()+FirstPredInfoEntry,
306 E = IncomingPredInfo.end(); I != E; ++I)
307 InsertedPHI->addIncoming(I->second, I->first);
309 // Drop the entries we added in IncomingPredInfo to restore the stack.
310 IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry,
311 IncomingPredInfo.end());
313 // See if the PHI node can be merged to a single value. This can happen in
314 // loop cases when we get a PHI of itself and one other value.
315 if (Value *ConstVal = InsertedPHI->hasConstantValue()) {
316 InsertedPHI->replaceAllUsesWith(ConstVal);
317 InsertedPHI->eraseFromParent();
318 InsertedVal = ConstVal;
320 DEBUG(errs() << " Inserted PHI: " << *InsertedPHI << "\n");
322 // If the client wants to know about all new instructions, tell it.
323 if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);