1 //===- Dominators.cpp - Dominator Calculation -----------------------------===//
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 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.
15 //===----------------------------------------------------------------------===//
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"
32 // Always verify dominfo if expensive checking is enabled.
34 static bool VerifyDomInfo = true;
36 static bool VerifyDomInfo = false;
38 static cl::opt<bool,true>
39 VerifyDomInfoX("verify-dom-info", cl::location(VerifyDomInfo),
40 cl::desc("Verify dominator info (time consuming)"));
42 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)
48 if (NumEdgesToEnd >= 2)
51 assert(NumEdgesToEnd == 1);
55 //===----------------------------------------------------------------------===//
56 // DominatorTree Implementation
57 //===----------------------------------------------------------------------===//
59 // Provide public access to DominatorTree information. Implementation details
60 // can be found in DominatorInternals.h.
62 //===----------------------------------------------------------------------===//
64 TEMPLATE_INSTANTIATION(class llvm::DomTreeNodeBase<BasicBlock>);
65 TEMPLATE_INSTANTIATION(class llvm::DominatorTreeBase<BasicBlock>);
67 char DominatorTree::ID = 0;
68 INITIALIZE_PASS(DominatorTree, "domtree",
69 "Dominator Tree Construction", true, true)
71 bool DominatorTree::runOnFunction(Function &F) {
76 void DominatorTree::verifyAnalysis() const {
77 if (!VerifyDomInfo) return;
79 Function &F = *getRoot()->getParent();
81 DominatorTree OtherDT;
82 OtherDT.getBase().recalculate(F);
83 if (compare(OtherDT)) {
84 errs() << "DominatorTree is not up to date!\nComputed:\n";
86 errs() << "\nActual:\n";
87 OtherDT.print(errs());
92 void DominatorTree::print(raw_ostream &OS, const Module *) const {
96 // dominates - Return true if Def dominates a use in User. This performs
97 // the special checks necessary if Def and User are in the same basic block.
98 // Note that Def doesn't dominate a use in Def itself!
99 bool DominatorTree::dominates(const Instruction *Def,
100 const Instruction *User) const {
101 const BasicBlock *UseBB = User->getParent();
102 const BasicBlock *DefBB = Def->getParent();
104 // Any unreachable use is dominated, even if Def == User.
105 if (!isReachableFromEntry(UseBB))
108 // Unreachable definitions don't dominate anything.
109 if (!isReachableFromEntry(DefBB))
112 // An instruction doesn't dominate a use in itself.
116 // The value defined by an invoke dominates an instruction only if
117 // it dominates every instruction in UseBB.
118 // A PHI is dominated only if the instruction dominates every possible use
120 if (isa<InvokeInst>(Def) || isa<PHINode>(User))
121 return dominates(Def, UseBB);
124 return dominates(DefBB, UseBB);
126 // Loop through the basic block until we find Def or User.
127 BasicBlock::const_iterator I = DefBB->begin();
128 for (; &*I != Def && &*I != User; ++I)
134 // true if Def would dominate a use in any instruction in UseBB.
135 // note that dominates(Def, Def->getParent()) is false.
136 bool DominatorTree::dominates(const Instruction *Def,
137 const BasicBlock *UseBB) const {
138 const BasicBlock *DefBB = Def->getParent();
140 // Any unreachable use is dominated, even if DefBB == UseBB.
141 if (!isReachableFromEntry(UseBB))
144 // Unreachable definitions don't dominate anything.
145 if (!isReachableFromEntry(DefBB))
151 const InvokeInst *II = dyn_cast<InvokeInst>(Def);
153 return dominates(DefBB, UseBB);
155 // Invoke results are only usable in the normal destination, not in the
156 // exceptional destination.
157 BasicBlock *NormalDest = II->getNormalDest();
158 BasicBlockEdge E(DefBB, NormalDest);
159 return dominates(E, UseBB);
162 bool DominatorTree::dominates(const BasicBlockEdge &BBE,
163 const BasicBlock *UseBB) const {
164 // Assert that we have a single edge. We could handle them by simply
165 // returning false, but since isSingleEdge is linear on the number of
166 // edges, the callers can normally handle them more efficiently.
167 assert(BBE.isSingleEdge());
169 // If the BB the edge ends in doesn't dominate the use BB, then the
170 // edge also doesn't.
171 const BasicBlock *Start = BBE.getStart();
172 const BasicBlock *End = BBE.getEnd();
173 if (!dominates(End, UseBB))
176 // Simple case: if the end BB has a single predecessor, the fact that it
177 // dominates the use block implies that the edge also does.
178 if (End->getSinglePredecessor())
181 // The normal edge from the invoke is critical. Conceptually, what we would
182 // like to do is split it and check if the new block dominates the use.
183 // With X being the new block, the graph would look like:
196 // Given the definition of dominance, NormalDest is dominated by X if X
197 // dominates all of NormalDest's predecessors (X, B, C in the example). X
198 // trivially dominates itself, so we only have to find if it dominates the
199 // other predecessors. Since the only way out of X is via NormalDest, X can
200 // only properly dominate a node if NormalDest dominates that node too.
201 for (const_pred_iterator PI = pred_begin(End), E = pred_end(End);
203 const BasicBlock *BB = *PI;
207 if (!dominates(End, BB))
213 bool DominatorTree::dominates(const BasicBlockEdge &BBE,
214 const Use &U) const {
215 // Assert that we have a single edge. We could handle them by simply
216 // returning false, but since isSingleEdge is linear on the number of
217 // edges, the callers can normally handle them more efficiently.
218 assert(BBE.isSingleEdge());
220 Instruction *UserInst = cast<Instruction>(U.getUser());
221 // A PHI in the end of the edge is dominated by it.
222 PHINode *PN = dyn_cast<PHINode>(UserInst);
223 if (PN && PN->getParent() == BBE.getEnd() &&
224 PN->getIncomingBlock(U) == BBE.getStart())
227 // Otherwise use the edge-dominates-block query, which
228 // handles the crazy critical edge cases properly.
229 const BasicBlock *UseBB;
231 UseBB = PN->getIncomingBlock(U);
233 UseBB = UserInst->getParent();
234 return dominates(BBE, UseBB);
237 bool DominatorTree::dominates(const Instruction *Def,
238 const Use &U) const {
239 Instruction *UserInst = cast<Instruction>(U.getUser());
240 const BasicBlock *DefBB = Def->getParent();
242 // Determine the block in which the use happens. PHI nodes use
243 // their operands on edges; simulate this by thinking of the use
244 // happening at the end of the predecessor block.
245 const BasicBlock *UseBB;
246 if (PHINode *PN = dyn_cast<PHINode>(UserInst))
247 UseBB = PN->getIncomingBlock(U);
249 UseBB = UserInst->getParent();
251 // Any unreachable use is dominated, even if Def == User.
252 if (!isReachableFromEntry(UseBB))
255 // Unreachable definitions don't dominate anything.
256 if (!isReachableFromEntry(DefBB))
259 // Invoke instructions define their return values on the edges
260 // to their normal successors, so we have to handle them specially.
261 // Among other things, this means they don't dominate anything in
262 // their own block, except possibly a phi, so we don't need to
263 // walk the block in any case.
264 if (const InvokeInst *II = dyn_cast<InvokeInst>(Def)) {
265 BasicBlock *NormalDest = II->getNormalDest();
266 BasicBlockEdge E(DefBB, NormalDest);
267 return dominates(E, U);
270 // If the def and use are in different blocks, do a simple CFG dominator
273 return dominates(DefBB, UseBB);
275 // Ok, def and use are in the same block. If the def is an invoke, it
276 // doesn't dominate anything in the block. If it's a PHI, it dominates
277 // everything in the block.
278 if (isa<PHINode>(UserInst))
281 // Otherwise, just loop through the basic block until we find Def or User.
282 BasicBlock::const_iterator I = DefBB->begin();
283 for (; &*I != Def && &*I != UserInst; ++I)
286 return &*I != UserInst;
289 bool DominatorTree::isReachableFromEntry(const Use &U) const {
290 Instruction *I = dyn_cast<Instruction>(U.getUser());
292 // ConstantExprs aren't really reachable from the entry block, but they
293 // don't need to be treated like unreachable code either.
296 // PHI nodes use their operands on their incoming edges.
297 if (PHINode *PN = dyn_cast<PHINode>(I))
298 return isReachableFromEntry(PN->getIncomingBlock(U));
300 // Everything else uses their operands in their own block.
301 return isReachableFromEntry(I->getParent());