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/Analysis/Dominators.h"
18 #include "llvm/Support/CFG.h"
19 #include "llvm/Support/Compiler.h"
20 #include "llvm/Support/Debug.h"
21 #include "llvm/ADT/DepthFirstIterator.h"
22 #include "llvm/ADT/SmallPtrSet.h"
23 #include "llvm/ADT/SmallVector.h"
24 #include "llvm/Analysis/DominatorInternals.h"
25 #include "llvm/Assembly/Writer.h"
26 #include "llvm/Instructions.h"
27 #include "llvm/Support/raw_ostream.h"
28 #include "llvm/Support/CommandLine.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
42 //===----------------------------------------------------------------------===//
43 // DominatorTree Implementation
44 //===----------------------------------------------------------------------===//
45 //
46 // Provide public access to DominatorTree information. Implementation details
47 // can be found in DominatorInternals.h.
48 //
49 //===----------------------------------------------------------------------===//
50
51 TEMPLATE_INSTANTIATION(class llvm::DomTreeNodeBase<BasicBlock>);
52 TEMPLATE_INSTANTIATION(class llvm::DominatorTreeBase<BasicBlock>);
53
54 char DominatorTree::ID = 0;
55 INITIALIZE_PASS(DominatorTree, "domtree",
56 "Dominator Tree Construction", true, true)
57
runOnFunction(Function & F)58 bool DominatorTree::runOnFunction(Function &F) {
59 DT->recalculate(F);
60 return false;
61 }
62
verifyAnalysis() const63 void DominatorTree::verifyAnalysis() const {
64 if (!VerifyDomInfo) return;
65
66 Function &F = *getRoot()->getParent();
67
68 DominatorTree OtherDT;
69 OtherDT.getBase().recalculate(F);
70 if (compare(OtherDT)) {
71 errs() << "DominatorTree is not up to date!\nComputed:\n";
72 print(errs());
73 errs() << "\nActual:\n";
74 OtherDT.print(errs());
75 abort();
76 }
77 }
78
print(raw_ostream & OS,const Module *) const79 void DominatorTree::print(raw_ostream &OS, const Module *) const {
80 DT->print(OS);
81 }
82
83 // dominates - Return true if Def dominates a use in User. This performs
84 // the special checks necessary if Def and User are in the same basic block.
85 // Note that Def doesn't dominate a use in Def itself!
dominates(const Instruction * Def,const Instruction * User) const86 bool DominatorTree::dominates(const Instruction *Def,
87 const Instruction *User) const {
88 const BasicBlock *UseBB = User->getParent();
89 const BasicBlock *DefBB = Def->getParent();
90
91 // Any unreachable use is dominated, even if Def == User.
92 if (!isReachableFromEntry(UseBB))
93 return true;
94
95 // Unreachable definitions don't dominate anything.
96 if (!isReachableFromEntry(DefBB))
97 return false;
98
99 // An instruction doesn't dominate a use in itself.
100 if (Def == User)
101 return false;
102
103 // The value defined by an invoke dominates an instruction only if
104 // it dominates every instruction in UseBB.
105 // A PHI is dominated only if the instruction dominates every possible use
106 // in the UseBB.
107 if (isa<InvokeInst>(Def) || isa<PHINode>(User))
108 return dominates(Def, UseBB);
109
110 if (DefBB != UseBB)
111 return dominates(DefBB, UseBB);
112
113 // Loop through the basic block until we find Def or User.
114 BasicBlock::const_iterator I = DefBB->begin();
115 for (; &*I != Def && &*I != User; ++I)
116 /*empty*/;
117
118 return &*I == Def;
119 }
120
121 // true if Def would dominate a use in any instruction in UseBB.
122 // note that dominates(Def, Def->getParent()) is false.
dominates(const Instruction * Def,const BasicBlock * UseBB) const123 bool DominatorTree::dominates(const Instruction *Def,
124 const BasicBlock *UseBB) const {
125 const BasicBlock *DefBB = Def->getParent();
126
127 // Any unreachable use is dominated, even if DefBB == UseBB.
128 if (!isReachableFromEntry(UseBB))
129 return true;
130
131 // Unreachable definitions don't dominate anything.
132 if (!isReachableFromEntry(DefBB))
133 return false;
134
135 if (DefBB == UseBB)
136 return false;
137
138 const InvokeInst *II = dyn_cast<InvokeInst>(Def);
139 if (!II)
140 return dominates(DefBB, UseBB);
141
142 // Invoke results are only usable in the normal destination, not in the
143 // exceptional destination.
144 BasicBlock *NormalDest = II->getNormalDest();
145 if (!dominates(NormalDest, UseBB))
146 return false;
147
148 // Simple case: if the normal destination has a single predecessor, the
149 // fact that it dominates the use block implies that we also do.
150 if (NormalDest->getSinglePredecessor())
151 return true;
152
153 // The normal edge from the invoke is critical. Conceptually, what we would
154 // like to do is split it and check if the new block dominates the use.
155 // With X being the new block, the graph would look like:
156 //
157 // DefBB
158 // /\ . .
159 // / \ . .
160 // / \ . .
161 // / \ | |
162 // A X B C
163 // | \ | /
164 // . \|/
165 // . NormalDest
166 // .
167 //
168 // Given the definition of dominance, NormalDest is dominated by X iff X
169 // dominates all of NormalDest's predecessors (X, B, C in the example). X
170 // trivially dominates itself, so we only have to find if it dominates the
171 // other predecessors. Since the only way out of X is via NormalDest, X can
172 // only properly dominate a node if NormalDest dominates that node too.
173 for (pred_iterator PI = pred_begin(NormalDest),
174 E = pred_end(NormalDest); PI != E; ++PI) {
175 const BasicBlock *BB = *PI;
176 if (BB == DefBB)
177 continue;
178
179 if (!DT->isReachableFromEntry(BB))
180 continue;
181
182 if (!dominates(NormalDest, BB))
183 return false;
184 }
185 return true;
186 }
187
dominates(const Instruction * Def,const Use & U) const188 bool DominatorTree::dominates(const Instruction *Def,
189 const Use &U) const {
190 Instruction *UserInst = dyn_cast<Instruction>(U.getUser());
191
192 // Instructions do not dominate non-instructions.
193 if (!UserInst)
194 return false;
195
196 const BasicBlock *DefBB = Def->getParent();
197
198 // Determine the block in which the use happens. PHI nodes use
199 // their operands on edges; simulate this by thinking of the use
200 // happening at the end of the predecessor block.
201 const BasicBlock *UseBB;
202 if (PHINode *PN = dyn_cast<PHINode>(UserInst))
203 UseBB = PN->getIncomingBlock(U);
204 else
205 UseBB = UserInst->getParent();
206
207 // Any unreachable use is dominated, even if Def == User.
208 if (!isReachableFromEntry(UseBB))
209 return true;
210
211 // Unreachable definitions don't dominate anything.
212 if (!isReachableFromEntry(DefBB))
213 return false;
214
215 // Invoke instructions define their return values on the edges
216 // to their normal successors, so we have to handle them specially.
217 // Among other things, this means they don't dominate anything in
218 // their own block, except possibly a phi, so we don't need to
219 // walk the block in any case.
220 if (const InvokeInst *II = dyn_cast<InvokeInst>(Def)) {
221 // A PHI in the normal successor using the invoke's return value is
222 // dominated by the invoke's return value.
223 if (isa<PHINode>(UserInst) &&
224 UserInst->getParent() == II->getNormalDest() &&
225 cast<PHINode>(UserInst)->getIncomingBlock(U) == DefBB)
226 return true;
227
228 // Otherwise use the instruction-dominates-block query, which
229 // handles the crazy case of an invoke with a critical edge
230 // properly.
231 return dominates(Def, UseBB);
232 }
233
234 // If the def and use are in different blocks, do a simple CFG dominator
235 // tree query.
236 if (DefBB != UseBB)
237 return dominates(DefBB, UseBB);
238
239 // Ok, def and use are in the same block. If the def is an invoke, it
240 // doesn't dominate anything in the block. If it's a PHI, it dominates
241 // everything in the block.
242 if (isa<PHINode>(UserInst))
243 return true;
244
245 // Otherwise, just loop through the basic block until we find Def or User.
246 BasicBlock::const_iterator I = DefBB->begin();
247 for (; &*I != Def && &*I != UserInst; ++I)
248 /*empty*/;
249
250 return &*I != UserInst;
251 }
252
isReachableFromEntry(const Use & U) const253 bool DominatorTree::isReachableFromEntry(const Use &U) const {
254 Instruction *I = dyn_cast<Instruction>(U.getUser());
255
256 // ConstantExprs aren't really reachable from the entry block, but they
257 // don't need to be treated like unreachable code either.
258 if (!I) return true;
259
260 // PHI nodes use their operands on their incoming edges.
261 if (PHINode *PN = dyn_cast<PHINode>(I))
262 return isReachableFromEntry(PN->getIncomingBlock(U));
263
264 // Everything else uses their operands in their own block.
265 return isReachableFromEntry(I->getParent());
266 }
267