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/Support/CommandLine.h"
24#include "llvm/Support/Compiler.h"
25#include "llvm/Support/Debug.h"
26#include "llvm/Support/GenericDomTreeConstruction.h"
27#include "llvm/Support/raw_ostream.h"
28#include <algorithm>
29using namespace llvm;
30
31// Always verify dominfo if expensive checking is enabled.
32#ifdef XDEBUG
33static bool VerifyDomInfo = true;
34#else
35static bool VerifyDomInfo = false;
36#endif
37static cl::opt<bool,true>
38VerifyDomInfoX("verify-dom-info", cl::location(VerifyDomInfo),
39               cl::desc("Verify dominator info (time consuming)"));
40
41bool BasicBlockEdge::isSingleEdge() const {
42  const TerminatorInst *TI = Start->getTerminator();
43  unsigned NumEdgesToEnd = 0;
44  for (unsigned int i = 0, n = TI->getNumSuccessors(); i < n; ++i) {
45    if (TI->getSuccessor(i) == End)
46      ++NumEdgesToEnd;
47    if (NumEdgesToEnd >= 2)
48      return false;
49  }
50  assert(NumEdgesToEnd == 1);
51  return true;
52}
53
54//===----------------------------------------------------------------------===//
55//  DominatorTree Implementation
56//===----------------------------------------------------------------------===//
57//
58// Provide public access to DominatorTree information.  Implementation details
59// can be found in Dominators.h, GenericDomTree.h, and
60// GenericDomTreeConstruction.h.
61//
62//===----------------------------------------------------------------------===//
63
64TEMPLATE_INSTANTIATION(class llvm::DomTreeNodeBase<BasicBlock>);
65TEMPLATE_INSTANTIATION(class llvm::DominatorTreeBase<BasicBlock>);
66
67#define LLVM_COMMA ,
68TEMPLATE_INSTANTIATION(void llvm::Calculate<Function LLVM_COMMA BasicBlock *>(
69    DominatorTreeBase<GraphTraits<BasicBlock *>::NodeType> &DT LLVM_COMMA
70        Function &F));
71TEMPLATE_INSTANTIATION(
72    void llvm::Calculate<Function LLVM_COMMA Inverse<BasicBlock *> >(
73        DominatorTreeBase<GraphTraits<Inverse<BasicBlock *> >::NodeType> &DT
74            LLVM_COMMA Function &F));
75#undef LLVM_COMMA
76
77// dominates - Return true if Def dominates a use in User. This performs
78// the special checks necessary if Def and User are in the same basic block.
79// Note that Def doesn't dominate a use in Def itself!
80bool DominatorTree::dominates(const Instruction *Def,
81                              const Instruction *User) const {
82  const BasicBlock *UseBB = User->getParent();
83  const BasicBlock *DefBB = Def->getParent();
84
85  // Any unreachable use is dominated, even if Def == User.
86  if (!isReachableFromEntry(UseBB))
87    return true;
88
89  // Unreachable definitions don't dominate anything.
90  if (!isReachableFromEntry(DefBB))
91    return false;
92
93  // An instruction doesn't dominate a use in itself.
94  if (Def == User)
95    return false;
96
97  // The value defined by an invoke dominates an instruction only if
98  // it dominates every instruction in UseBB.
99  // A PHI is dominated only if the instruction dominates every possible use
100  // in the UseBB.
101  if (isa<InvokeInst>(Def) || isa<PHINode>(User))
102    return dominates(Def, UseBB);
103
104  if (DefBB != UseBB)
105    return dominates(DefBB, UseBB);
106
107  // Loop through the basic block until we find Def or User.
108  BasicBlock::const_iterator I = DefBB->begin();
109  for (; &*I != Def && &*I != User; ++I)
110    /*empty*/;
111
112  return &*I == Def;
113}
114
115// true if Def would dominate a use in any instruction in UseBB.
116// note that dominates(Def, Def->getParent()) is false.
117bool DominatorTree::dominates(const Instruction *Def,
118                              const BasicBlock *UseBB) const {
119  const BasicBlock *DefBB = Def->getParent();
120
121  // Any unreachable use is dominated, even if DefBB == UseBB.
122  if (!isReachableFromEntry(UseBB))
123    return true;
124
125  // Unreachable definitions don't dominate anything.
126  if (!isReachableFromEntry(DefBB))
127    return false;
128
129  if (DefBB == UseBB)
130    return false;
131
132  const InvokeInst *II = dyn_cast<InvokeInst>(Def);
133  if (!II)
134    return dominates(DefBB, UseBB);
135
136  // Invoke results are only usable in the normal destination, not in the
137  // exceptional destination.
138  BasicBlock *NormalDest = II->getNormalDest();
139  BasicBlockEdge E(DefBB, NormalDest);
140  return dominates(E, UseBB);
141}
142
143bool DominatorTree::dominates(const BasicBlockEdge &BBE,
144                              const BasicBlock *UseBB) const {
145  // Assert that we have a single edge. We could handle them by simply
146  // returning false, but since isSingleEdge is linear on the number of
147  // edges, the callers can normally handle them more efficiently.
148  assert(BBE.isSingleEdge());
149
150  // If the BB the edge ends in doesn't dominate the use BB, then the
151  // edge also doesn't.
152  const BasicBlock *Start = BBE.getStart();
153  const BasicBlock *End = BBE.getEnd();
154  if (!dominates(End, UseBB))
155    return false;
156
157  // Simple case: if the end BB has a single predecessor, the fact that it
158  // dominates the use block implies that the edge also does.
159  if (End->getSinglePredecessor())
160    return true;
161
162  // The normal edge from the invoke is critical. Conceptually, what we would
163  // like to do is split it and check if the new block dominates the use.
164  // With X being the new block, the graph would look like:
165  //
166  //        DefBB
167  //          /\      .  .
168  //         /  \     .  .
169  //        /    \    .  .
170  //       /      \   |  |
171  //      A        X  B  C
172  //      |         \ | /
173  //      .          \|/
174  //      .      NormalDest
175  //      .
176  //
177  // Given the definition of dominance, NormalDest is dominated by X iff X
178  // dominates all of NormalDest's predecessors (X, B, C in the example). X
179  // trivially dominates itself, so we only have to find if it dominates the
180  // other predecessors. Since the only way out of X is via NormalDest, X can
181  // only properly dominate a node if NormalDest dominates that node too.
182  for (const_pred_iterator PI = pred_begin(End), E = pred_end(End);
183       PI != E; ++PI) {
184    const BasicBlock *BB = *PI;
185    if (BB == Start)
186      continue;
187
188    if (!dominates(End, BB))
189      return false;
190  }
191  return true;
192}
193
194bool DominatorTree::dominates(const BasicBlockEdge &BBE, const Use &U) const {
195  // Assert that we have a single edge. We could handle them by simply
196  // returning false, but since isSingleEdge is linear on the number of
197  // edges, the callers can normally handle them more efficiently.
198  assert(BBE.isSingleEdge());
199
200  Instruction *UserInst = cast<Instruction>(U.getUser());
201  // A PHI in the end of the edge is dominated by it.
202  PHINode *PN = dyn_cast<PHINode>(UserInst);
203  if (PN && PN->getParent() == BBE.getEnd() &&
204      PN->getIncomingBlock(U) == BBE.getStart())
205    return true;
206
207  // Otherwise use the edge-dominates-block query, which
208  // handles the crazy critical edge cases properly.
209  const BasicBlock *UseBB;
210  if (PN)
211    UseBB = PN->getIncomingBlock(U);
212  else
213    UseBB = UserInst->getParent();
214  return dominates(BBE, UseBB);
215}
216
217bool DominatorTree::dominates(const Instruction *Def, const Use &U) const {
218  Instruction *UserInst = cast<Instruction>(U.getUser());
219  const BasicBlock *DefBB = Def->getParent();
220
221  // Determine the block in which the use happens. PHI nodes use
222  // their operands on edges; simulate this by thinking of the use
223  // happening at the end of the predecessor block.
224  const BasicBlock *UseBB;
225  if (PHINode *PN = dyn_cast<PHINode>(UserInst))
226    UseBB = PN->getIncomingBlock(U);
227  else
228    UseBB = UserInst->getParent();
229
230  // Any unreachable use is dominated, even if Def == User.
231  if (!isReachableFromEntry(UseBB))
232    return true;
233
234  // Unreachable definitions don't dominate anything.
235  if (!isReachableFromEntry(DefBB))
236    return false;
237
238  // Invoke instructions define their return values on the edges
239  // to their normal successors, so we have to handle them specially.
240  // Among other things, this means they don't dominate anything in
241  // their own block, except possibly a phi, so we don't need to
242  // walk the block in any case.
243  if (const InvokeInst *II = dyn_cast<InvokeInst>(Def)) {
244    BasicBlock *NormalDest = II->getNormalDest();
245    BasicBlockEdge E(DefBB, NormalDest);
246    return dominates(E, U);
247  }
248
249  // If the def and use are in different blocks, do a simple CFG dominator
250  // tree query.
251  if (DefBB != UseBB)
252    return dominates(DefBB, UseBB);
253
254  // Ok, def and use are in the same block. If the def is an invoke, it
255  // doesn't dominate anything in the block. If it's a PHI, it dominates
256  // everything in the block.
257  if (isa<PHINode>(UserInst))
258    return true;
259
260  // Otherwise, just loop through the basic block until we find Def or User.
261  BasicBlock::const_iterator I = DefBB->begin();
262  for (; &*I != Def && &*I != UserInst; ++I)
263    /*empty*/;
264
265  return &*I != UserInst;
266}
267
268bool DominatorTree::isReachableFromEntry(const Use &U) const {
269  Instruction *I = dyn_cast<Instruction>(U.getUser());
270
271  // ConstantExprs aren't really reachable from the entry block, but they
272  // don't need to be treated like unreachable code either.
273  if (!I) return true;
274
275  // PHI nodes use their operands on their incoming edges.
276  if (PHINode *PN = dyn_cast<PHINode>(I))
277    return isReachableFromEntry(PN->getIncomingBlock(U));
278
279  // Everything else uses their operands in their own block.
280  return isReachableFromEntry(I->getParent());
281}
282
283void DominatorTree::verifyDomTree() const {
284  if (!VerifyDomInfo)
285    return;
286
287  Function &F = *getRoot()->getParent();
288
289  DominatorTree OtherDT;
290  OtherDT.recalculate(F);
291  if (compare(OtherDT)) {
292    errs() << "DominatorTree is not up to date!\nComputed:\n";
293    print(errs());
294    errs() << "\nActual:\n";
295    OtherDT.print(errs());
296    abort();
297  }
298}
299
300//===----------------------------------------------------------------------===//
301//  DominatorTreeWrapperPass Implementation
302//===----------------------------------------------------------------------===//
303//
304// The implementation details of the wrapper pass that holds a DominatorTree.
305//
306//===----------------------------------------------------------------------===//
307
308char DominatorTreeWrapperPass::ID = 0;
309INITIALIZE_PASS(DominatorTreeWrapperPass, "domtree",
310                "Dominator Tree Construction", true, true)
311
312bool DominatorTreeWrapperPass::runOnFunction(Function &F) {
313  DT.recalculate(F);
314  return false;
315}
316
317void DominatorTreeWrapperPass::verifyAnalysis() const { DT.verifyDomTree(); }
318
319void DominatorTreeWrapperPass::print(raw_ostream &OS, const Module *) const {
320  DT.print(OS);
321}
322
323