1 //===- Dominators.cpp - Dominator Calculation -----------------------------===//
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 simple dominator construction algorithms for finding
11 // forward dominators. Postdominators are available in libanalysis, but are not
12 // included in libvmcore, because it's not needed. Forward dominators are
13 // needed to support the Verifier pass.
14 //
15 //===----------------------------------------------------------------------===//
16
17 #include "llvm/IR/Dominators.h"
18 #include "llvm/ADT/DepthFirstIterator.h"
19 #include "llvm/ADT/SmallPtrSet.h"
20 #include "llvm/ADT/SmallVector.h"
21 #include "llvm/IR/CFG.h"
22 #include "llvm/IR/Instructions.h"
23 #include "llvm/IR/PassManager.h"
24 #include "llvm/Support/CommandLine.h"
25 #include "llvm/Support/Compiler.h"
26 #include "llvm/Support/Debug.h"
27 #include "llvm/Support/GenericDomTreeConstruction.h"
28 #include "llvm/Support/raw_ostream.h"
29 #include <algorithm>
30 using namespace llvm;
31
32 // Always verify dominfo if expensive checking is enabled.
33 #ifdef XDEBUG
34 static bool VerifyDomInfo = true;
35 #else
36 static bool VerifyDomInfo = false;
37 #endif
38 static cl::opt<bool,true>
39 VerifyDomInfoX("verify-dom-info", cl::location(VerifyDomInfo),
40 cl::desc("Verify dominator info (time consuming)"));
41
isSingleEdge() const42 bool BasicBlockEdge::isSingleEdge() const {
43 const TerminatorInst *TI = Start->getTerminator();
44 unsigned NumEdgesToEnd = 0;
45 for (unsigned int i = 0, n = TI->getNumSuccessors(); i < n; ++i) {
46 if (TI->getSuccessor(i) == End)
47 ++NumEdgesToEnd;
48 if (NumEdgesToEnd >= 2)
49 return false;
50 }
51 assert(NumEdgesToEnd == 1);
52 return true;
53 }
54
55 //===----------------------------------------------------------------------===//
56 // DominatorTree Implementation
57 //===----------------------------------------------------------------------===//
58 //
59 // Provide public access to DominatorTree information. Implementation details
60 // can be found in Dominators.h, GenericDomTree.h, and
61 // GenericDomTreeConstruction.h.
62 //
63 //===----------------------------------------------------------------------===//
64
65 template class llvm::DomTreeNodeBase<BasicBlock>;
66 template class llvm::DominatorTreeBase<BasicBlock>;
67
68 template void llvm::Calculate<Function, BasicBlock *>(
69 DominatorTreeBase<GraphTraits<BasicBlock *>::NodeType> &DT, Function &F);
70 template void llvm::Calculate<Function, Inverse<BasicBlock *>>(
71 DominatorTreeBase<GraphTraits<Inverse<BasicBlock *>>::NodeType> &DT,
72 Function &F);
73
74 // dominates - Return true if Def dominates a use in User. This performs
75 // the special checks necessary if Def and User are in the same basic block.
76 // Note that Def doesn't dominate a use in Def itself!
dominates(const Instruction * Def,const Instruction * User) const77 bool DominatorTree::dominates(const Instruction *Def,
78 const Instruction *User) const {
79 const BasicBlock *UseBB = User->getParent();
80 const BasicBlock *DefBB = Def->getParent();
81
82 // Any unreachable use is dominated, even if Def == User.
83 if (!isReachableFromEntry(UseBB))
84 return true;
85
86 // Unreachable definitions don't dominate anything.
87 if (!isReachableFromEntry(DefBB))
88 return false;
89
90 // An instruction doesn't dominate a use in itself.
91 if (Def == User)
92 return false;
93
94 // The value defined by an invoke dominates an instruction only if it
95 // dominates every instruction in UseBB.
96 // A PHI is dominated only if the instruction dominates every possible use in
97 // the UseBB.
98 if (isa<InvokeInst>(Def) || isa<PHINode>(User))
99 return dominates(Def, UseBB);
100
101 if (DefBB != UseBB)
102 return dominates(DefBB, UseBB);
103
104 // Loop through the basic block until we find Def or User.
105 BasicBlock::const_iterator I = DefBB->begin();
106 for (; &*I != Def && &*I != User; ++I)
107 /*empty*/;
108
109 return &*I == Def;
110 }
111
112 // true if Def would dominate a use in any instruction in UseBB.
113 // note that dominates(Def, Def->getParent()) is false.
dominates(const Instruction * Def,const BasicBlock * UseBB) const114 bool DominatorTree::dominates(const Instruction *Def,
115 const BasicBlock *UseBB) const {
116 const BasicBlock *DefBB = Def->getParent();
117
118 // Any unreachable use is dominated, even if DefBB == UseBB.
119 if (!isReachableFromEntry(UseBB))
120 return true;
121
122 // Unreachable definitions don't dominate anything.
123 if (!isReachableFromEntry(DefBB))
124 return false;
125
126 if (DefBB == UseBB)
127 return false;
128
129 // Invoke results are only usable in the normal destination, not in the
130 // exceptional destination.
131 if (const auto *II = dyn_cast<InvokeInst>(Def)) {
132 BasicBlock *NormalDest = II->getNormalDest();
133 BasicBlockEdge E(DefBB, NormalDest);
134 return dominates(E, UseBB);
135 }
136
137 return dominates(DefBB, UseBB);
138 }
139
dominates(const BasicBlockEdge & BBE,const BasicBlock * UseBB) const140 bool DominatorTree::dominates(const BasicBlockEdge &BBE,
141 const BasicBlock *UseBB) const {
142 // Assert that we have a single edge. We could handle them by simply
143 // returning false, but since isSingleEdge is linear on the number of
144 // edges, the callers can normally handle them more efficiently.
145 assert(BBE.isSingleEdge() &&
146 "This function is not efficient in handling multiple edges");
147
148 // If the BB the edge ends in doesn't dominate the use BB, then the
149 // edge also doesn't.
150 const BasicBlock *Start = BBE.getStart();
151 const BasicBlock *End = BBE.getEnd();
152 if (!dominates(End, UseBB))
153 return false;
154
155 // Simple case: if the end BB has a single predecessor, the fact that it
156 // dominates the use block implies that the edge also does.
157 if (End->getSinglePredecessor())
158 return true;
159
160 // The normal edge from the invoke is critical. Conceptually, what we would
161 // like to do is split it and check if the new block dominates the use.
162 // With X being the new block, the graph would look like:
163 //
164 // DefBB
165 // /\ . .
166 // / \ . .
167 // / \ . .
168 // / \ | |
169 // A X B C
170 // | \ | /
171 // . \|/
172 // . NormalDest
173 // .
174 //
175 // Given the definition of dominance, NormalDest is dominated by X iff X
176 // dominates all of NormalDest's predecessors (X, B, C in the example). X
177 // trivially dominates itself, so we only have to find if it dominates the
178 // other predecessors. Since the only way out of X is via NormalDest, X can
179 // only properly dominate a node if NormalDest dominates that node too.
180 for (const_pred_iterator PI = pred_begin(End), E = pred_end(End);
181 PI != E; ++PI) {
182 const BasicBlock *BB = *PI;
183 if (BB == Start)
184 continue;
185
186 if (!dominates(End, BB))
187 return false;
188 }
189 return true;
190 }
191
dominates(const BasicBlockEdge & BBE,const Use & U) const192 bool DominatorTree::dominates(const BasicBlockEdge &BBE, const Use &U) const {
193 // Assert that we have a single edge. We could handle them by simply
194 // returning false, but since isSingleEdge is linear on the number of
195 // edges, the callers can normally handle them more efficiently.
196 assert(BBE.isSingleEdge() &&
197 "This function is not efficient in handling multiple edges");
198
199 Instruction *UserInst = cast<Instruction>(U.getUser());
200 // A PHI in the end of the edge is dominated by it.
201 PHINode *PN = dyn_cast<PHINode>(UserInst);
202 if (PN && PN->getParent() == BBE.getEnd() &&
203 PN->getIncomingBlock(U) == BBE.getStart())
204 return true;
205
206 // Otherwise use the edge-dominates-block query, which
207 // handles the crazy critical edge cases properly.
208 const BasicBlock *UseBB;
209 if (PN)
210 UseBB = PN->getIncomingBlock(U);
211 else
212 UseBB = UserInst->getParent();
213 return dominates(BBE, UseBB);
214 }
215
dominates(const Instruction * Def,const Use & U) const216 bool DominatorTree::dominates(const Instruction *Def, const Use &U) const {
217 Instruction *UserInst = cast<Instruction>(U.getUser());
218 const BasicBlock *DefBB = Def->getParent();
219
220 // Determine the block in which the use happens. PHI nodes use
221 // their operands on edges; simulate this by thinking of the use
222 // happening at the end of the predecessor block.
223 const BasicBlock *UseBB;
224 if (PHINode *PN = dyn_cast<PHINode>(UserInst))
225 UseBB = PN->getIncomingBlock(U);
226 else
227 UseBB = UserInst->getParent();
228
229 // Any unreachable use is dominated, even if Def == User.
230 if (!isReachableFromEntry(UseBB))
231 return true;
232
233 // Unreachable definitions don't dominate anything.
234 if (!isReachableFromEntry(DefBB))
235 return false;
236
237 // Invoke instructions define their return values on the edges to their normal
238 // successors, so we have to handle them specially.
239 // Among other things, this means they don't dominate anything in
240 // their own block, except possibly a phi, so we don't need to
241 // walk the block in any case.
242 if (const InvokeInst *II = dyn_cast<InvokeInst>(Def)) {
243 BasicBlock *NormalDest = II->getNormalDest();
244 BasicBlockEdge E(DefBB, NormalDest);
245 return dominates(E, U);
246 }
247
248 // If the def and use are in different blocks, do a simple CFG dominator
249 // tree query.
250 if (DefBB != UseBB)
251 return dominates(DefBB, UseBB);
252
253 // Ok, def and use are in the same block. If the def is an invoke, it
254 // doesn't dominate anything in the block. If it's a PHI, it dominates
255 // everything in the block.
256 if (isa<PHINode>(UserInst))
257 return true;
258
259 // Otherwise, just loop through the basic block until we find Def or User.
260 BasicBlock::const_iterator I = DefBB->begin();
261 for (; &*I != Def && &*I != UserInst; ++I)
262 /*empty*/;
263
264 return &*I != UserInst;
265 }
266
isReachableFromEntry(const Use & U) const267 bool DominatorTree::isReachableFromEntry(const Use &U) const {
268 Instruction *I = dyn_cast<Instruction>(U.getUser());
269
270 // ConstantExprs aren't really reachable from the entry block, but they
271 // don't need to be treated like unreachable code either.
272 if (!I) return true;
273
274 // PHI nodes use their operands on their incoming edges.
275 if (PHINode *PN = dyn_cast<PHINode>(I))
276 return isReachableFromEntry(PN->getIncomingBlock(U));
277
278 // Everything else uses their operands in their own block.
279 return isReachableFromEntry(I->getParent());
280 }
281
verifyDomTree() const282 void DominatorTree::verifyDomTree() const {
283 Function &F = *getRoot()->getParent();
284
285 DominatorTree OtherDT;
286 OtherDT.recalculate(F);
287 if (compare(OtherDT)) {
288 errs() << "DominatorTree is not up to date!\nComputed:\n";
289 print(errs());
290 errs() << "\nActual:\n";
291 OtherDT.print(errs());
292 abort();
293 }
294 }
295
296 //===----------------------------------------------------------------------===//
297 // DominatorTreeAnalysis and related pass implementations
298 //===----------------------------------------------------------------------===//
299 //
300 // This implements the DominatorTreeAnalysis which is used with the new pass
301 // manager. It also implements some methods from utility passes.
302 //
303 //===----------------------------------------------------------------------===//
304
run(Function & F)305 DominatorTree DominatorTreeAnalysis::run(Function &F) {
306 DominatorTree DT;
307 DT.recalculate(F);
308 return DT;
309 }
310
311 char DominatorTreeAnalysis::PassID;
312
DominatorTreePrinterPass(raw_ostream & OS)313 DominatorTreePrinterPass::DominatorTreePrinterPass(raw_ostream &OS) : OS(OS) {}
314
run(Function & F,FunctionAnalysisManager * AM)315 PreservedAnalyses DominatorTreePrinterPass::run(Function &F,
316 FunctionAnalysisManager *AM) {
317 OS << "DominatorTree for function: " << F.getName() << "\n";
318 AM->getResult<DominatorTreeAnalysis>(F).print(OS);
319
320 return PreservedAnalyses::all();
321 }
322
run(Function & F,FunctionAnalysisManager * AM)323 PreservedAnalyses DominatorTreeVerifierPass::run(Function &F,
324 FunctionAnalysisManager *AM) {
325 AM->getResult<DominatorTreeAnalysis>(F).verifyDomTree();
326
327 return PreservedAnalyses::all();
328 }
329
330 //===----------------------------------------------------------------------===//
331 // DominatorTreeWrapperPass Implementation
332 //===----------------------------------------------------------------------===//
333 //
334 // The implementation details of the wrapper pass that holds a DominatorTree
335 // suitable for use with the legacy pass manager.
336 //
337 //===----------------------------------------------------------------------===//
338
339 char DominatorTreeWrapperPass::ID = 0;
340 INITIALIZE_PASS(DominatorTreeWrapperPass, "domtree",
341 "Dominator Tree Construction", true, true)
342
runOnFunction(Function & F)343 bool DominatorTreeWrapperPass::runOnFunction(Function &F) {
344 DT.recalculate(F);
345 return false;
346 }
347
verifyAnalysis() const348 void DominatorTreeWrapperPass::verifyAnalysis() const {
349 if (VerifyDomInfo)
350 DT.verifyDomTree();
351 }
352
print(raw_ostream & OS,const Module *) const353 void DominatorTreeWrapperPass::print(raw_ostream &OS, const Module *) const {
354 DT.print(OS);
355 }
356
357