1 //
2 // Copyright (c) 2017 The ANGLE Project Authors. All rights reserved.
3 // Use of this source code is governed by a BSD-style license that can be
4 // found in the LICENSE file.
5 //
6 // RemoveUnreferencedVariables.cpp:
7 // Drop variables that are declared but never referenced in the AST. This avoids adding unnecessary
8 // initialization code for them. Also removes unreferenced struct types.
9 //
10
11 #include "compiler/translator/RemoveUnreferencedVariables.h"
12
13 #include "compiler/translator/IntermTraverse.h"
14 #include "compiler/translator/SymbolTable.h"
15
16 namespace sh
17 {
18
19 namespace
20 {
21
22 class CollectVariableRefCountsTraverser : public TIntermTraverser
23 {
24 public:
25 CollectVariableRefCountsTraverser();
26
27 using RefCountMap = std::unordered_map<int, unsigned int>;
getSymbolIdRefCounts()28 RefCountMap &getSymbolIdRefCounts() { return mSymbolIdRefCounts; }
getStructIdRefCounts()29 RefCountMap &getStructIdRefCounts() { return mStructIdRefCounts; }
30
31 void visitSymbol(TIntermSymbol *node) override;
32 bool visitAggregate(Visit visit, TIntermAggregate *node) override;
33 bool visitFunctionPrototype(Visit visit, TIntermFunctionPrototype *node) override;
34
35 private:
36 void incrementStructTypeRefCount(const TType &type);
37
38 RefCountMap mSymbolIdRefCounts;
39
40 // Structure reference counts are counted from symbols, constructors, function calls, function
41 // return values and from interface block and structure fields. We need to track both function
42 // calls and function return values since there's a compiler option not to prune unused
43 // functions. The type of a constant union may also be a struct, but statements that are just a
44 // constant union are always pruned, and if the constant union is used somehow it will get
45 // counted by something else.
46 RefCountMap mStructIdRefCounts;
47 };
48
CollectVariableRefCountsTraverser()49 CollectVariableRefCountsTraverser::CollectVariableRefCountsTraverser()
50 : TIntermTraverser(true, false, false)
51 {
52 }
53
incrementStructTypeRefCount(const TType & type)54 void CollectVariableRefCountsTraverser::incrementStructTypeRefCount(const TType &type)
55 {
56 if (type.isInterfaceBlock())
57 {
58 const auto *block = type.getInterfaceBlock();
59 ASSERT(block);
60
61 // We can end up incrementing ref counts of struct types referenced from an interface block
62 // multiple times for the same block. This doesn't matter, because interface blocks can't be
63 // pruned so we'll never do the reverse operation.
64 for (const auto &field : block->fields())
65 {
66 ASSERT(!field->type()->isInterfaceBlock());
67 incrementStructTypeRefCount(*field->type());
68 }
69 return;
70 }
71
72 const auto *structure = type.getStruct();
73 if (structure != nullptr)
74 {
75 auto structIter = mStructIdRefCounts.find(structure->uniqueId());
76 if (structIter == mStructIdRefCounts.end())
77 {
78 mStructIdRefCounts[structure->uniqueId()] = 1u;
79
80 for (const auto &field : structure->fields())
81 {
82 incrementStructTypeRefCount(*field->type());
83 }
84
85 return;
86 }
87 ++(structIter->second);
88 }
89 }
90
visitSymbol(TIntermSymbol * node)91 void CollectVariableRefCountsTraverser::visitSymbol(TIntermSymbol *node)
92 {
93 incrementStructTypeRefCount(node->getType());
94
95 auto iter = mSymbolIdRefCounts.find(node->getId());
96 if (iter == mSymbolIdRefCounts.end())
97 {
98 mSymbolIdRefCounts[node->getId()] = 1u;
99 return;
100 }
101 ++(iter->second);
102 }
103
visitAggregate(Visit visit,TIntermAggregate * node)104 bool CollectVariableRefCountsTraverser::visitAggregate(Visit visit, TIntermAggregate *node)
105 {
106 // This tracks struct references in both function calls and constructors.
107 incrementStructTypeRefCount(node->getType());
108 return true;
109 }
110
visitFunctionPrototype(Visit visit,TIntermFunctionPrototype * node)111 bool CollectVariableRefCountsTraverser::visitFunctionPrototype(Visit visit,
112 TIntermFunctionPrototype *node)
113 {
114 incrementStructTypeRefCount(node->getType());
115 return true;
116 }
117
118 // Traverser that removes all unreferenced variables on one traversal.
119 class RemoveUnreferencedVariablesTraverser : public TIntermTraverser
120 {
121 public:
122 RemoveUnreferencedVariablesTraverser(
123 CollectVariableRefCountsTraverser::RefCountMap *symbolIdRefCounts,
124 CollectVariableRefCountsTraverser::RefCountMap *structIdRefCounts,
125 TSymbolTable *symbolTable);
126
127 bool visitDeclaration(Visit visit, TIntermDeclaration *node) override;
128 void visitSymbol(TIntermSymbol *node) override;
129 bool visitAggregate(Visit visit, TIntermAggregate *node) override;
130
131 // Traverse loop and block nodes in reverse order. Note that this traverser does not track
132 // parent block positions, so insertStatementInParentBlock is unusable!
133 void traverseBlock(TIntermBlock *block) override;
134 void traverseLoop(TIntermLoop *loop) override;
135
136 private:
137 void removeVariableDeclaration(TIntermDeclaration *node, TIntermTyped *declarator);
138 void decrementStructTypeRefCount(const TType &type);
139
140 CollectVariableRefCountsTraverser::RefCountMap *mSymbolIdRefCounts;
141 CollectVariableRefCountsTraverser::RefCountMap *mStructIdRefCounts;
142 bool mRemoveReferences;
143 };
144
RemoveUnreferencedVariablesTraverser(CollectVariableRefCountsTraverser::RefCountMap * symbolIdRefCounts,CollectVariableRefCountsTraverser::RefCountMap * structIdRefCounts,TSymbolTable * symbolTable)145 RemoveUnreferencedVariablesTraverser::RemoveUnreferencedVariablesTraverser(
146 CollectVariableRefCountsTraverser::RefCountMap *symbolIdRefCounts,
147 CollectVariableRefCountsTraverser::RefCountMap *structIdRefCounts,
148 TSymbolTable *symbolTable)
149 : TIntermTraverser(true, false, true, symbolTable),
150 mSymbolIdRefCounts(symbolIdRefCounts),
151 mStructIdRefCounts(structIdRefCounts),
152 mRemoveReferences(false)
153 {
154 }
155
decrementStructTypeRefCount(const TType & type)156 void RemoveUnreferencedVariablesTraverser::decrementStructTypeRefCount(const TType &type)
157 {
158 auto *structure = type.getStruct();
159 if (structure != nullptr)
160 {
161 ASSERT(mStructIdRefCounts->find(structure->uniqueId()) != mStructIdRefCounts->end());
162 unsigned int structRefCount = --(*mStructIdRefCounts)[structure->uniqueId()];
163
164 if (structRefCount == 0)
165 {
166 for (const auto &field : structure->fields())
167 {
168 decrementStructTypeRefCount(*field->type());
169 }
170 }
171 }
172 }
173
removeVariableDeclaration(TIntermDeclaration * node,TIntermTyped * declarator)174 void RemoveUnreferencedVariablesTraverser::removeVariableDeclaration(TIntermDeclaration *node,
175 TIntermTyped *declarator)
176 {
177 if (declarator->getType().isStructSpecifier() && !declarator->getType().isNamelessStruct())
178 {
179 unsigned int structId = declarator->getType().getStruct()->uniqueId();
180 if ((*mStructIdRefCounts)[structId] > 1u)
181 {
182 // If this declaration declares a named struct type that is used elsewhere, we need to
183 // keep it. We can still change the declarator though so that it doesn't declare an
184 // unreferenced variable.
185
186 // Note that since we're not removing the entire declaration, the struct's reference
187 // count will end up being one less than the correct refcount. But since the struct
188 // declaration is kept, the incorrect refcount can't cause any other problems.
189
190 if (declarator->getAsSymbolNode() && declarator->getAsSymbolNode()->getSymbol().empty())
191 {
192 // Already an empty declaration - nothing to do.
193 return;
194 }
195 queueReplacementWithParent(node, declarator,
196 new TIntermSymbol(mSymbolTable->getEmptySymbolId(),
197 TString(""), declarator->getType()),
198 OriginalNode::IS_DROPPED);
199 return;
200 }
201 }
202
203 if (getParentNode()->getAsBlock())
204 {
205 TIntermSequence emptyReplacement;
206 mMultiReplacements.push_back(
207 NodeReplaceWithMultipleEntry(getParentNode()->getAsBlock(), node, emptyReplacement));
208 }
209 else
210 {
211 ASSERT(getParentNode()->getAsLoopNode());
212 queueReplacement(nullptr, OriginalNode::IS_DROPPED);
213 }
214 }
215
visitDeclaration(Visit visit,TIntermDeclaration * node)216 bool RemoveUnreferencedVariablesTraverser::visitDeclaration(Visit visit, TIntermDeclaration *node)
217 {
218 if (visit == PreVisit)
219 {
220 // SeparateDeclarations should have already been run.
221 ASSERT(node->getSequence()->size() == 1u);
222
223 TIntermTyped *declarator = node->getSequence()->back()->getAsTyped();
224 ASSERT(declarator);
225
226 // We can only remove variables that are not a part of the shader interface.
227 TQualifier qualifier = declarator->getQualifier();
228 if (qualifier != EvqTemporary && qualifier != EvqGlobal)
229 {
230 return true;
231 }
232
233 bool canRemoveVariable = false;
234 TIntermSymbol *symbolNode = declarator->getAsSymbolNode();
235 if (symbolNode != nullptr)
236 {
237 canRemoveVariable =
238 (*mSymbolIdRefCounts)[symbolNode->getId()] == 1u || symbolNode->getSymbol().empty();
239 }
240 TIntermBinary *initNode = declarator->getAsBinaryNode();
241 if (initNode != nullptr)
242 {
243 ASSERT(initNode->getLeft()->getAsSymbolNode());
244 int symbolId = initNode->getLeft()->getAsSymbolNode()->getId();
245 canRemoveVariable =
246 (*mSymbolIdRefCounts)[symbolId] == 1u && !initNode->getRight()->hasSideEffects();
247 }
248
249 if (canRemoveVariable)
250 {
251 removeVariableDeclaration(node, declarator);
252 mRemoveReferences = true;
253 }
254 return true;
255 }
256 ASSERT(visit == PostVisit);
257 mRemoveReferences = false;
258 return true;
259 }
260
visitSymbol(TIntermSymbol * node)261 void RemoveUnreferencedVariablesTraverser::visitSymbol(TIntermSymbol *node)
262 {
263 if (mRemoveReferences)
264 {
265 ASSERT(mSymbolIdRefCounts->find(node->getId()) != mSymbolIdRefCounts->end());
266 --(*mSymbolIdRefCounts)[node->getId()];
267
268 decrementStructTypeRefCount(node->getType());
269 }
270 }
271
visitAggregate(Visit visit,TIntermAggregate * node)272 bool RemoveUnreferencedVariablesTraverser::visitAggregate(Visit visit, TIntermAggregate *node)
273 {
274 if (mRemoveReferences)
275 {
276 decrementStructTypeRefCount(node->getType());
277 }
278 return true;
279 }
280
traverseBlock(TIntermBlock * node)281 void RemoveUnreferencedVariablesTraverser::traverseBlock(TIntermBlock *node)
282 {
283 // We traverse blocks in reverse order. This way reference counts can be decremented when
284 // removing initializers, and variables that become unused when initializers are removed can be
285 // removed on the same traversal.
286
287 ScopedNodeInTraversalPath addToPath(this, node);
288
289 bool visit = true;
290
291 TIntermSequence *sequence = node->getSequence();
292
293 if (preVisit)
294 visit = visitBlock(PreVisit, node);
295
296 if (visit)
297 {
298 for (auto iter = sequence->rbegin(); iter != sequence->rend(); ++iter)
299 {
300 (*iter)->traverse(this);
301 if (visit && inVisit)
302 {
303 if ((iter + 1) != sequence->rend())
304 visit = visitBlock(InVisit, node);
305 }
306 }
307 }
308
309 if (visit && postVisit)
310 visitBlock(PostVisit, node);
311 }
312
traverseLoop(TIntermLoop * node)313 void RemoveUnreferencedVariablesTraverser::traverseLoop(TIntermLoop *node)
314 {
315 // We traverse loops in reverse order as well. The loop body gets traversed before the init
316 // node.
317
318 ScopedNodeInTraversalPath addToPath(this, node);
319
320 bool visit = true;
321
322 if (preVisit)
323 visit = visitLoop(PreVisit, node);
324
325 if (visit)
326 {
327 // We don't need to traverse loop expressions or conditions since they can't be declarations
328 // in the AST (loops which have a declaration in their condition get transformed in the
329 // parsing stage).
330 ASSERT(node->getExpression() == nullptr ||
331 node->getExpression()->getAsDeclarationNode() == nullptr);
332 ASSERT(node->getCondition() == nullptr ||
333 node->getCondition()->getAsDeclarationNode() == nullptr);
334
335 if (node->getBody())
336 node->getBody()->traverse(this);
337
338 if (node->getInit())
339 node->getInit()->traverse(this);
340 }
341
342 if (visit && postVisit)
343 visitLoop(PostVisit, node);
344 }
345
346 } // namespace
347
RemoveUnreferencedVariables(TIntermBlock * root,TSymbolTable * symbolTable)348 void RemoveUnreferencedVariables(TIntermBlock *root, TSymbolTable *symbolTable)
349 {
350 CollectVariableRefCountsTraverser collector;
351 root->traverse(&collector);
352 RemoveUnreferencedVariablesTraverser traverser(&collector.getSymbolIdRefCounts(),
353 &collector.getStructIdRefCounts(), symbolTable);
354 root->traverse(&traverser);
355 traverser.updateTree();
356 }
357
358 } // namespace sh
359