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>
30using namespace llvm;
31
32// Always verify dominfo if expensive checking is enabled.
33#ifdef XDEBUG
34static bool VerifyDomInfo = true;
35#else
36static bool VerifyDomInfo = false;
37#endif
38static cl::opt<bool,true>
39VerifyDomInfoX("verify-dom-info", cl::location(VerifyDomInfo),
40               cl::desc("Verify dominator info (time consuming)"));
41
42bool 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
65template class llvm::DomTreeNodeBase<BasicBlock>;
66template class llvm::DominatorTreeBase<BasicBlock>;
67
68template void llvm::Calculate<Function, BasicBlock *>(
69    DominatorTreeBase<GraphTraits<BasicBlock *>::NodeType> &DT, Function &F);
70template 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!
77bool 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.
114bool 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
140bool 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
192bool 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
216bool 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
267bool 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
282void 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
305DominatorTree DominatorTreeAnalysis::run(Function &F) {
306  DominatorTree DT;
307  DT.recalculate(F);
308  return DT;
309}
310
311char DominatorTreeAnalysis::PassID;
312
313DominatorTreePrinterPass::DominatorTreePrinterPass(raw_ostream &OS) : OS(OS) {}
314
315PreservedAnalyses 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
323PreservedAnalyses 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
339char DominatorTreeWrapperPass::ID = 0;
340INITIALIZE_PASS(DominatorTreeWrapperPass, "domtree",
341                "Dominator Tree Construction", true, true)
342
343bool DominatorTreeWrapperPass::runOnFunction(Function &F) {
344  DT.recalculate(F);
345  return false;
346}
347
348void DominatorTreeWrapperPass::verifyAnalysis() const {
349    if (VerifyDomInfo)
350      DT.verifyDomTree();
351}
352
353void DominatorTreeWrapperPass::print(raw_ostream &OS, const Module *) const {
354  DT.print(OS);
355}
356
357