1//===- SSAUpdater.cpp - Unstructured SSA Update Tool ----------------------===//
2//
3// The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This file implements the SSAUpdater class.
11//
12//===----------------------------------------------------------------------===//
13
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"
21using namespace llvm;
22
23typedef DenseMap<BasicBlock*, TrackingVH<Value> > AvailableValsTy;
24typedef std::vector<std::pair<BasicBlock*, TrackingVH<Value> > >
25 IncomingPredInfoTy;
26
27static AvailableValsTy &getAvailableVals(void *AV) {
28 return *static_cast<AvailableValsTy*>(AV);
29}
30
31static IncomingPredInfoTy &getIncomingPredInfo(void *IPI) {
32 return *static_cast<IncomingPredInfoTy*>(IPI);
33}
34
35
36SSAUpdater::SSAUpdater(SmallVectorImpl<PHINode*> *NewPHI)
37 : AV(0), PrototypeValue(0), IPI(0), InsertedPHIs(NewPHI) {}
38
39SSAUpdater::~SSAUpdater() {
40 delete &getAvailableVals(AV);
41 delete &getIncomingPredInfo(IPI);
42}
43
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.
46void SSAUpdater::Initialize(Value *ProtoValue) {
47 if (AV == 0)
48 AV = new AvailableValsTy();
49 else
50 getAvailableVals(AV).clear();
51
52 if (IPI == 0)
53 IPI = new IncomingPredInfoTy();
54 else
55 getIncomingPredInfo(IPI).clear();
56 PrototypeValue = ProtoValue;
57}
58
59/// HasValueForBlock - Return true if the SSAUpdater already has a value for
60/// the specified block.
61bool SSAUpdater::HasValueForBlock(BasicBlock *BB) const {
62 return getAvailableVals(AV).count(BB);
63}
64
65/// AddAvailableValue - Indicate that a rewritten value is available in the
66/// specified block with the specified value.
67void SSAUpdater::AddAvailableValue(BasicBlock *BB, Value *V) {
68 assert(PrototypeValue != 0 && "Need to initialize SSAUpdater");
69 assert(PrototypeValue->getType() == V->getType() &&
70 "All rewritten values must have the same type");
71 getAvailableVals(AV)[BB] = V;
72}
73
74/// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is
75/// live at the end of the specified block.
76Value *SSAUpdater::GetValueAtEndOfBlock(BasicBlock *BB) {
77 assert(getIncomingPredInfo(IPI).empty() && "Unexpected Internal State");
78 Value *Res = GetValueAtEndOfBlockInternal(BB);
79 assert(getIncomingPredInfo(IPI).empty() && "Unexpected Internal State");
80 return Res;
81}
82
83/// GetValueInMiddleOfBlock - Construct SSA form, materializing a value that
84/// is live in the middle of the specified block.
85///
86/// GetValueInMiddleOfBlock is the same as GetValueAtEndOfBlock except in one
87/// important case: if there is a definition of the rewritten value after the
88/// 'use' in BB. Consider code like this:
89///
90/// X1 = ...
91/// SomeBB:
92/// use(X)
93/// X2 = ...
94/// br Cond, SomeBB, OutBB
95///
96/// In this case, there are two values (X1 and X2) added to the AvailableVals
97/// set by the client of the rewriter, and those values are both live out of
98/// their respective blocks. However, the use of X happens in the *middle* of
99/// a block. Because of this, we need to insert a new PHI node in SomeBB to
100/// merge the appropriate values, and this value isn't live out of the block.
101///
102Value *SSAUpdater::GetValueInMiddleOfBlock(BasicBlock *BB) {
103 // If there is no definition of the renamed variable in this block, just use
104 // GetValueAtEndOfBlock to do our work.
105 if (!getAvailableVals(AV).count(BB))
106 return GetValueAtEndOfBlock(BB);
107
108 // Otherwise, we have the hard case. Get the live-in values for each
109 // predecessor.
110 SmallVector<std::pair<BasicBlock*, Value*>, 8> PredValues;
111 Value *SingularValue = 0;
112
113 // We can get our predecessor info by walking the pred_iterator list, but it
114 // is relatively slow. If we already have PHI nodes in this block, walk one
115 // of them to get the predecessor list instead.
116 if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) {
117 for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) {
118 BasicBlock *PredBB = SomePhi->getIncomingBlock(i);
119 Value *PredVal = GetValueAtEndOfBlock(PredBB);
120 PredValues.push_back(std::make_pair(PredBB, PredVal));
121
122 // Compute SingularValue.
123 if (i == 0)
124 SingularValue = PredVal;
125 else if (PredVal != SingularValue)
126 SingularValue = 0;
127 }
128 } else {
129 bool isFirstPred = true;
130 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
131 BasicBlock *PredBB = *PI;
132 Value *PredVal = GetValueAtEndOfBlock(PredBB);
133 PredValues.push_back(std::make_pair(PredBB, PredVal));
134
135 // Compute SingularValue.
136 if (isFirstPred) {
137 SingularValue = PredVal;
138 isFirstPred = false;
139 } else if (PredVal != SingularValue)
140 SingularValue = 0;
141 }
142 }
143
144 // If there are no predecessors, just return undef.
145 if (PredValues.empty())
146 return UndefValue::get(PrototypeValue->getType());
147
148 // Otherwise, if all the merged values are the same, just use it.
149 if (SingularValue != 0)
150 return SingularValue;
151
152 // Otherwise, we do need a PHI: insert one now.
153 PHINode *InsertedPHI = PHINode::Create(PrototypeValue->getType(),
154 PrototypeValue->getName(),
155 &BB->front());
156 InsertedPHI->reserveOperandSpace(PredValues.size());
157
158 // Fill in all the predecessors of the PHI.
159 for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
160 InsertedPHI->addIncoming(PredValues[i].second, PredValues[i].first);
161
162 // See if the PHI node can be merged to a single value. This can happen in
163 // loop cases when we get a PHI of itself and one other value.
164 if (Value *ConstVal = InsertedPHI->hasConstantValue()) {
165 InsertedPHI->eraseFromParent();
166 return ConstVal;
167 }
168
169 // If the client wants to know about all new instructions, tell it.
170 if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);
171
172 DEBUG(errs() << " Inserted PHI: " << *InsertedPHI << "\n");
173 return InsertedPHI;
174}
175
176/// RewriteUse - Rewrite a use of the symbolic value. This handles PHI nodes,
177/// which use their value in the corresponding predecessor.
178void SSAUpdater::RewriteUse(Use &U) {
179 Instruction *User = cast<Instruction>(U.getUser());
180
181 Value *V;
182 if (PHINode *UserPN = dyn_cast<PHINode>(User))
183 V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U));
184 else
185 V = GetValueInMiddleOfBlock(User->getParent());
186
187 U.set(V);
188}
189
190
191/// GetValueAtEndOfBlockInternal - Check to see if AvailableVals has an entry
192/// for the specified BB and if so, return it. If not, construct SSA form by
193/// walking predecessors inserting PHI nodes as needed until we get to a block
194/// where the value is available.
195///
196Value *SSAUpdater::GetValueAtEndOfBlockInternal(BasicBlock *BB) {
197 AvailableValsTy &AvailableVals = getAvailableVals(AV);
198
199 // Query AvailableVals by doing an insertion of null.
200 std::pair<AvailableValsTy::iterator, bool> InsertRes =
201 AvailableVals.insert(std::make_pair(BB, WeakVH()));
202
203 // Handle the case when the insertion fails because we have already seen BB.
204 if (!InsertRes.second) {
205 // If the insertion failed, there are two cases. The first case is that the
206 // value is already available for the specified block. If we get this, just
207 // return the value.
208 if (InsertRes.first->second != 0)
209 return InsertRes.first->second;
210
211 // Otherwise, if the value we find is null, then this is the value is not
212 // known but it is being computed elsewhere in our recursion. This means
213 // that we have a cycle. Handle this by inserting a PHI node and returning
214 // it. When we get back to the first instance of the recursion we will fill
215 // in the PHI node.
216 return InsertRes.first->second =
217 PHINode::Create(PrototypeValue->getType(), PrototypeValue->getName(),
218 &BB->front());
219 }
220
221 // Okay, the value isn't in the map and we just inserted a null in the entry
222 // to indicate that we're processing the block. Since we have no idea what
223 // value is in this block, we have to recurse through our predecessors.
224 //
225 // While we're walking our predecessors, we keep track of them in a vector,
226 // then insert a PHI node in the end if we actually need one. We could use a
227 // smallvector here, but that would take a lot of stack space for every level
228 // of the recursion, just use IncomingPredInfo as an explicit stack.
229 IncomingPredInfoTy &IncomingPredInfo = getIncomingPredInfo(IPI);
230 unsigned FirstPredInfoEntry = IncomingPredInfo.size();
231
232 // As we're walking the predecessors, keep track of whether they are all
233 // producing the same value. If so, this value will capture it, if not, it
234 // will get reset to null. We distinguish the no-predecessor case explicitly
235 // below.
236 TrackingVH<Value> SingularValue;
237
238 // We can get our predecessor info by walking the pred_iterator list, but it
239 // is relatively slow. If we already have PHI nodes in this block, walk one
240 // of them to get the predecessor list instead.
241 if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) {
242 for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) {
243 BasicBlock *PredBB = SomePhi->getIncomingBlock(i);
244 Value *PredVal = GetValueAtEndOfBlockInternal(PredBB);
245 IncomingPredInfo.push_back(std::make_pair(PredBB, PredVal));
246
247 // Compute SingularValue.
248 if (i == 0)
249 SingularValue = PredVal;
250 else if (PredVal != SingularValue)
251 SingularValue = 0;
252 }
253 } else {
254 bool isFirstPred = true;
255 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
256 BasicBlock *PredBB = *PI;
257 Value *PredVal = GetValueAtEndOfBlockInternal(PredBB);
258 IncomingPredInfo.push_back(std::make_pair(PredBB, PredVal));
259
260 // Compute SingularValue.
261 if (isFirstPred) {
262 SingularValue = PredVal;
263 isFirstPred = false;
264 } else if (PredVal != SingularValue)
265 SingularValue = 0;
266 }
267 }
268
269 // If there are no predecessors, then we must have found an unreachable block
270 // just return 'undef'. Since there are no predecessors, InsertRes must not
271 // be invalidated.
272 if (IncomingPredInfo.size() == FirstPredInfoEntry)
273 return InsertRes.first->second = UndefValue::get(PrototypeValue->getType());
274
275 /// Look up BB's entry in AvailableVals. 'InsertRes' may be invalidated. If
276 /// this block is involved in a loop, a no-entry PHI node will have been
277 /// inserted as InsertedVal. Otherwise, we'll still have the null we inserted
278 /// above.
279 TrackingVH<Value> &InsertedVal = AvailableVals[BB];
280
281 // If all the predecessor values are the same then we don't need to insert a
282 // PHI. This is the simple and common case.
283 if (SingularValue) {
284 // If a PHI node got inserted, replace it with the singlar value and delete
285 // it.
286 if (InsertedVal) {
287 PHINode *OldVal = cast<PHINode>(InsertedVal);
288 // Be careful about dead loops. These RAUW's also update InsertedVal.
289 if (InsertedVal != SingularValue)
290 OldVal->replaceAllUsesWith(SingularValue);
291 else
292 OldVal->replaceAllUsesWith(UndefValue::get(InsertedVal->getType()));
293 OldVal->eraseFromParent();
294 } else {
295 InsertedVal = SingularValue;
296 }
297
| 1//===- SSAUpdater.cpp - Unstructured SSA Update Tool ----------------------===//
2//
3// The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This file implements the SSAUpdater class.
11//
12//===----------------------------------------------------------------------===//
13
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"
21using namespace llvm;
22
23typedef DenseMap<BasicBlock*, TrackingVH<Value> > AvailableValsTy;
24typedef std::vector<std::pair<BasicBlock*, TrackingVH<Value> > >
25 IncomingPredInfoTy;
26
27static AvailableValsTy &getAvailableVals(void *AV) {
28 return *static_cast<AvailableValsTy*>(AV);
29}
30
31static IncomingPredInfoTy &getIncomingPredInfo(void *IPI) {
32 return *static_cast<IncomingPredInfoTy*>(IPI);
33}
34
35
36SSAUpdater::SSAUpdater(SmallVectorImpl<PHINode*> *NewPHI)
37 : AV(0), PrototypeValue(0), IPI(0), InsertedPHIs(NewPHI) {}
38
39SSAUpdater::~SSAUpdater() {
40 delete &getAvailableVals(AV);
41 delete &getIncomingPredInfo(IPI);
42}
43
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.
46void SSAUpdater::Initialize(Value *ProtoValue) {
47 if (AV == 0)
48 AV = new AvailableValsTy();
49 else
50 getAvailableVals(AV).clear();
51
52 if (IPI == 0)
53 IPI = new IncomingPredInfoTy();
54 else
55 getIncomingPredInfo(IPI).clear();
56 PrototypeValue = ProtoValue;
57}
58
59/// HasValueForBlock - Return true if the SSAUpdater already has a value for
60/// the specified block.
61bool SSAUpdater::HasValueForBlock(BasicBlock *BB) const {
62 return getAvailableVals(AV).count(BB);
63}
64
65/// AddAvailableValue - Indicate that a rewritten value is available in the
66/// specified block with the specified value.
67void SSAUpdater::AddAvailableValue(BasicBlock *BB, Value *V) {
68 assert(PrototypeValue != 0 && "Need to initialize SSAUpdater");
69 assert(PrototypeValue->getType() == V->getType() &&
70 "All rewritten values must have the same type");
71 getAvailableVals(AV)[BB] = V;
72}
73
74/// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is
75/// live at the end of the specified block.
76Value *SSAUpdater::GetValueAtEndOfBlock(BasicBlock *BB) {
77 assert(getIncomingPredInfo(IPI).empty() && "Unexpected Internal State");
78 Value *Res = GetValueAtEndOfBlockInternal(BB);
79 assert(getIncomingPredInfo(IPI).empty() && "Unexpected Internal State");
80 return Res;
81}
82
83/// GetValueInMiddleOfBlock - Construct SSA form, materializing a value that
84/// is live in the middle of the specified block.
85///
86/// GetValueInMiddleOfBlock is the same as GetValueAtEndOfBlock except in one
87/// important case: if there is a definition of the rewritten value after the
88/// 'use' in BB. Consider code like this:
89///
90/// X1 = ...
91/// SomeBB:
92/// use(X)
93/// X2 = ...
94/// br Cond, SomeBB, OutBB
95///
96/// In this case, there are two values (X1 and X2) added to the AvailableVals
97/// set by the client of the rewriter, and those values are both live out of
98/// their respective blocks. However, the use of X happens in the *middle* of
99/// a block. Because of this, we need to insert a new PHI node in SomeBB to
100/// merge the appropriate values, and this value isn't live out of the block.
101///
102Value *SSAUpdater::GetValueInMiddleOfBlock(BasicBlock *BB) {
103 // If there is no definition of the renamed variable in this block, just use
104 // GetValueAtEndOfBlock to do our work.
105 if (!getAvailableVals(AV).count(BB))
106 return GetValueAtEndOfBlock(BB);
107
108 // Otherwise, we have the hard case. Get the live-in values for each
109 // predecessor.
110 SmallVector<std::pair<BasicBlock*, Value*>, 8> PredValues;
111 Value *SingularValue = 0;
112
113 // We can get our predecessor info by walking the pred_iterator list, but it
114 // is relatively slow. If we already have PHI nodes in this block, walk one
115 // of them to get the predecessor list instead.
116 if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) {
117 for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) {
118 BasicBlock *PredBB = SomePhi->getIncomingBlock(i);
119 Value *PredVal = GetValueAtEndOfBlock(PredBB);
120 PredValues.push_back(std::make_pair(PredBB, PredVal));
121
122 // Compute SingularValue.
123 if (i == 0)
124 SingularValue = PredVal;
125 else if (PredVal != SingularValue)
126 SingularValue = 0;
127 }
128 } else {
129 bool isFirstPred = true;
130 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
131 BasicBlock *PredBB = *PI;
132 Value *PredVal = GetValueAtEndOfBlock(PredBB);
133 PredValues.push_back(std::make_pair(PredBB, PredVal));
134
135 // Compute SingularValue.
136 if (isFirstPred) {
137 SingularValue = PredVal;
138 isFirstPred = false;
139 } else if (PredVal != SingularValue)
140 SingularValue = 0;
141 }
142 }
143
144 // If there are no predecessors, just return undef.
145 if (PredValues.empty())
146 return UndefValue::get(PrototypeValue->getType());
147
148 // Otherwise, if all the merged values are the same, just use it.
149 if (SingularValue != 0)
150 return SingularValue;
151
152 // Otherwise, we do need a PHI: insert one now.
153 PHINode *InsertedPHI = PHINode::Create(PrototypeValue->getType(),
154 PrototypeValue->getName(),
155 &BB->front());
156 InsertedPHI->reserveOperandSpace(PredValues.size());
157
158 // Fill in all the predecessors of the PHI.
159 for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
160 InsertedPHI->addIncoming(PredValues[i].second, PredValues[i].first);
161
162 // See if the PHI node can be merged to a single value. This can happen in
163 // loop cases when we get a PHI of itself and one other value.
164 if (Value *ConstVal = InsertedPHI->hasConstantValue()) {
165 InsertedPHI->eraseFromParent();
166 return ConstVal;
167 }
168
169 // If the client wants to know about all new instructions, tell it.
170 if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);
171
172 DEBUG(errs() << " Inserted PHI: " << *InsertedPHI << "\n");
173 return InsertedPHI;
174}
175
176/// RewriteUse - Rewrite a use of the symbolic value. This handles PHI nodes,
177/// which use their value in the corresponding predecessor.
178void SSAUpdater::RewriteUse(Use &U) {
179 Instruction *User = cast<Instruction>(U.getUser());
180
181 Value *V;
182 if (PHINode *UserPN = dyn_cast<PHINode>(User))
183 V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U));
184 else
185 V = GetValueInMiddleOfBlock(User->getParent());
186
187 U.set(V);
188}
189
190
191/// GetValueAtEndOfBlockInternal - Check to see if AvailableVals has an entry
192/// for the specified BB and if so, return it. If not, construct SSA form by
193/// walking predecessors inserting PHI nodes as needed until we get to a block
194/// where the value is available.
195///
196Value *SSAUpdater::GetValueAtEndOfBlockInternal(BasicBlock *BB) {
197 AvailableValsTy &AvailableVals = getAvailableVals(AV);
198
199 // Query AvailableVals by doing an insertion of null.
200 std::pair<AvailableValsTy::iterator, bool> InsertRes =
201 AvailableVals.insert(std::make_pair(BB, WeakVH()));
202
203 // Handle the case when the insertion fails because we have already seen BB.
204 if (!InsertRes.second) {
205 // If the insertion failed, there are two cases. The first case is that the
206 // value is already available for the specified block. If we get this, just
207 // return the value.
208 if (InsertRes.first->second != 0)
209 return InsertRes.first->second;
210
211 // Otherwise, if the value we find is null, then this is the value is not
212 // known but it is being computed elsewhere in our recursion. This means
213 // that we have a cycle. Handle this by inserting a PHI node and returning
214 // it. When we get back to the first instance of the recursion we will fill
215 // in the PHI node.
216 return InsertRes.first->second =
217 PHINode::Create(PrototypeValue->getType(), PrototypeValue->getName(),
218 &BB->front());
219 }
220
221 // Okay, the value isn't in the map and we just inserted a null in the entry
222 // to indicate that we're processing the block. Since we have no idea what
223 // value is in this block, we have to recurse through our predecessors.
224 //
225 // While we're walking our predecessors, we keep track of them in a vector,
226 // then insert a PHI node in the end if we actually need one. We could use a
227 // smallvector here, but that would take a lot of stack space for every level
228 // of the recursion, just use IncomingPredInfo as an explicit stack.
229 IncomingPredInfoTy &IncomingPredInfo = getIncomingPredInfo(IPI);
230 unsigned FirstPredInfoEntry = IncomingPredInfo.size();
231
232 // As we're walking the predecessors, keep track of whether they are all
233 // producing the same value. If so, this value will capture it, if not, it
234 // will get reset to null. We distinguish the no-predecessor case explicitly
235 // below.
236 TrackingVH<Value> SingularValue;
237
238 // We can get our predecessor info by walking the pred_iterator list, but it
239 // is relatively slow. If we already have PHI nodes in this block, walk one
240 // of them to get the predecessor list instead.
241 if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) {
242 for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) {
243 BasicBlock *PredBB = SomePhi->getIncomingBlock(i);
244 Value *PredVal = GetValueAtEndOfBlockInternal(PredBB);
245 IncomingPredInfo.push_back(std::make_pair(PredBB, PredVal));
246
247 // Compute SingularValue.
248 if (i == 0)
249 SingularValue = PredVal;
250 else if (PredVal != SingularValue)
251 SingularValue = 0;
252 }
253 } else {
254 bool isFirstPred = true;
255 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
256 BasicBlock *PredBB = *PI;
257 Value *PredVal = GetValueAtEndOfBlockInternal(PredBB);
258 IncomingPredInfo.push_back(std::make_pair(PredBB, PredVal));
259
260 // Compute SingularValue.
261 if (isFirstPred) {
262 SingularValue = PredVal;
263 isFirstPred = false;
264 } else if (PredVal != SingularValue)
265 SingularValue = 0;
266 }
267 }
268
269 // If there are no predecessors, then we must have found an unreachable block
270 // just return 'undef'. Since there are no predecessors, InsertRes must not
271 // be invalidated.
272 if (IncomingPredInfo.size() == FirstPredInfoEntry)
273 return InsertRes.first->second = UndefValue::get(PrototypeValue->getType());
274
275 /// Look up BB's entry in AvailableVals. 'InsertRes' may be invalidated. If
276 /// this block is involved in a loop, a no-entry PHI node will have been
277 /// inserted as InsertedVal. Otherwise, we'll still have the null we inserted
278 /// above.
279 TrackingVH<Value> &InsertedVal = AvailableVals[BB];
280
281 // If all the predecessor values are the same then we don't need to insert a
282 // PHI. This is the simple and common case.
283 if (SingularValue) {
284 // If a PHI node got inserted, replace it with the singlar value and delete
285 // it.
286 if (InsertedVal) {
287 PHINode *OldVal = cast<PHINode>(InsertedVal);
288 // Be careful about dead loops. These RAUW's also update InsertedVal.
289 if (InsertedVal != SingularValue)
290 OldVal->replaceAllUsesWith(SingularValue);
291 else
292 OldVal->replaceAllUsesWith(UndefValue::get(InsertedVal->getType()));
293 OldVal->eraseFromParent();
294 } else {
295 InsertedVal = SingularValue;
296 }
297
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