ReachableCode.cpp revision 0f3b4ca1764cd6d457f511d340fba504f41763c3 
1//=- ReachableCodePathInsensitive.cpp ---------------------------*- C++ --*-==//
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 a flow-sensitive, path-insensitive analysis of
11// determining reachable blocks within a CFG.
12//
13//===----------------------------------------------------------------------===//
14
15#include "llvm/ADT/BitVector.h"
16#include "llvm/ADT/SmallVector.h"
17#include "clang/AST/Expr.h"
18#include "clang/AST/ExprCXX.h"
19#include "clang/AST/ExprObjC.h"
20#include "clang/AST/StmtCXX.h"
21#include "clang/Analysis/Analyses/ReachableCode.h"
22#include "clang/Analysis/CFG.h"
23#include "clang/Analysis/AnalysisContext.h"
24#include "clang/Basic/SourceManager.h"
25
26using namespace clang;
27
28namespace {
29class DeadCodeScan {
30  llvm::BitVector Visited;
31  llvm::BitVector &Reachable;
32  llvm::SmallVector<const CFGBlock *, 10> WorkList;
33
34  typedef llvm::SmallVector<std::pair<const CFGBlock *, const Stmt *>, 12>
35      DeferredLocsTy;
36
37  DeferredLocsTy DeferredLocs;
38
39public:
40  DeadCodeScan(llvm::BitVector &reachable)
41    : Visited(reachable.size()),
42      Reachable(reachable) {}
43
44  void enqueue(const CFGBlock *block);
45  unsigned scanBackwards(const CFGBlock *Start,
46                         clang::reachable_code::Callback &CB);
47
48  bool isDeadCodeRoot(const CFGBlock *Block);
49
50  const Stmt *findDeadCode(const CFGBlock *Block);
51
52  void reportDeadCode(const Stmt *S,
53                      clang::reachable_code::Callback &CB);
54};
55}
56
57void DeadCodeScan::enqueue(const CFGBlock *block) {
58  unsigned blockID = block->getBlockID();
59  if (Reachable[blockID] || Visited[blockID])
60    return;
61  Visited[blockID] = true;
62  WorkList.push_back(block);
63}
64
65bool DeadCodeScan::isDeadCodeRoot(const clang::CFGBlock *Block) {
66  bool isDeadRoot = true;
67
68  for (CFGBlock::const_pred_iterator I = Block->pred_begin(),
69        E = Block->pred_end(); I != E; ++I) {
70    if (const CFGBlock *PredBlock = *I) {
71      unsigned blockID = PredBlock->getBlockID();
72      if (Visited[blockID]) {
73        isDeadRoot = false;
74        continue;
75      }
76      if (!Reachable[blockID]) {
77        isDeadRoot = false;
78        Visited[blockID] = true;
79        WorkList.push_back(PredBlock);
80        continue;
81      }
82    }
83  }
84
85  return isDeadRoot;
86}
87
88static bool isValidDeadStmt(const Stmt *S) {
89  SourceLocation Loc = S->getLocStart();
90  if (!(Loc.isValid() && !Loc.isMacroID()))
91    return false;
92  if (const BinaryOperator *BO = dyn_cast<BinaryOperator>(S)) {
93    return BO->getOpcode() != BO_Comma;
94  }
95  return true;
96}
97
98const Stmt *DeadCodeScan::findDeadCode(const clang::CFGBlock *Block) {
99  for (CFGBlock::const_iterator I = Block->begin(), E = Block->end(); I!=E; ++I)
100    if (const CFGStmt *CS = I->getAs<CFGStmt>()) {
101      const Stmt *S = CS->getStmt();
102      if (isValidDeadStmt(S))
103        return S;
104    }
105
106  if (CFGTerminator T = Block->getTerminator()) {
107    const Stmt *S = T.getStmt();
108    if (isValidDeadStmt(S))
109      return S;
110  }
111
112  return 0;
113}
114
115static int SrcCmp(const void *p1, const void *p2) {
116  return
117    ((std::pair<const CFGBlock *, const Stmt *>*) p2)->second->getLocStart() <
118    ((std::pair<const CFGBlock *, const Stmt *>*) p1)->second->getLocStart();
119}
120
121unsigned DeadCodeScan::scanBackwards(const clang::CFGBlock *Start,
122                                     clang::reachable_code::Callback &CB) {
123
124  unsigned count = 0;
125  enqueue(Start);
126
127  while (!WorkList.empty()) {
128    const CFGBlock *Block = WorkList.pop_back_val();
129
130    // It is possible that this block has been marked reachable after
131    // it was enqueued.
132    if (Reachable[Block->getBlockID()])
133      continue;
134
135    // Look for any dead code within the block.
136    const Stmt *S = findDeadCode(Block);
137
138    if (!S) {
139      // No dead code.  Possibly an empty block.  Look at dead predecessors.
140      for (CFGBlock::const_pred_iterator I = Block->pred_begin(),
141           E = Block->pred_end(); I != E; ++I) {
142        if (const CFGBlock *predBlock = *I)
143          enqueue(predBlock);
144      }
145      continue;
146    }
147
148    if (isDeadCodeRoot(Block)) {
149      reportDeadCode(S, CB);
150      count += clang::reachable_code::ScanReachableFromBlock(Block, Reachable);
151    }
152    else {
153      // Record this statement as the possibly best location in a
154      // strongly-connected component of dead code for emitting a
155      // warning.
156      DeferredLocs.push_back(std::make_pair(Block, S));
157    }
158  }
159
160  // If we didn't find a dead root, then report the dead code with the
161  // earliest location.
162  if (!DeferredLocs.empty()) {
163    llvm::array_pod_sort(DeferredLocs.begin(), DeferredLocs.end(), SrcCmp);
164    for (DeferredLocsTy::iterator I = DeferredLocs.begin(),
165          E = DeferredLocs.end(); I != E; ++I) {
166      const CFGBlock *block = I->first;
167      if (Reachable[block->getBlockID()])
168        continue;
169      reportDeadCode(I->second, CB);
170      count += clang::reachable_code::ScanReachableFromBlock(block, Reachable);
171    }
172  }
173
174  return count;
175}
176
177static SourceLocation GetUnreachableLoc(const Stmt *S,
178                                        SourceRange &R1,
179                                        SourceRange &R2) {
180  R1 = R2 = SourceRange();
181
182  if (const Expr *Ex = dyn_cast<Expr>(S))
183    S = Ex->IgnoreParenImpCasts();
184
185  switch (S->getStmtClass()) {
186    case Expr::BinaryOperatorClass: {
187      const BinaryOperator *BO = cast<BinaryOperator>(S);
188      return BO->getOperatorLoc();
189    }
190    case Expr::UnaryOperatorClass: {
191      const UnaryOperator *UO = cast<UnaryOperator>(S);
192      R1 = UO->getSubExpr()->getSourceRange();
193      return UO->getOperatorLoc();
194    }
195    case Expr::CompoundAssignOperatorClass: {
196      const CompoundAssignOperator *CAO = cast<CompoundAssignOperator>(S);
197      R1 = CAO->getLHS()->getSourceRange();
198      R2 = CAO->getRHS()->getSourceRange();
199      return CAO->getOperatorLoc();
200    }
201    case Expr::BinaryConditionalOperatorClass:
202    case Expr::ConditionalOperatorClass: {
203      const AbstractConditionalOperator *CO =
204        cast<AbstractConditionalOperator>(S);
205      return CO->getQuestionLoc();
206    }
207    case Expr::MemberExprClass: {
208      const MemberExpr *ME = cast<MemberExpr>(S);
209      R1 = ME->getSourceRange();
210      return ME->getMemberLoc();
211    }
212    case Expr::ArraySubscriptExprClass: {
213      const ArraySubscriptExpr *ASE = cast<ArraySubscriptExpr>(S);
214      R1 = ASE->getLHS()->getSourceRange();
215      R2 = ASE->getRHS()->getSourceRange();
216      return ASE->getRBracketLoc();
217    }
218    case Expr::CStyleCastExprClass: {
219      const CStyleCastExpr *CSC = cast<CStyleCastExpr>(S);
220      R1 = CSC->getSubExpr()->getSourceRange();
221      return CSC->getLParenLoc();
222    }
223    case Expr::CXXFunctionalCastExprClass: {
224      const CXXFunctionalCastExpr *CE = cast <CXXFunctionalCastExpr>(S);
225      R1 = CE->getSubExpr()->getSourceRange();
226      return CE->getTypeBeginLoc();
227    }
228    case Stmt::CXXTryStmtClass: {
229      return cast<CXXTryStmt>(S)->getHandler(0)->getCatchLoc();
230    }
231    case Expr::ObjCBridgedCastExprClass: {
232      const ObjCBridgedCastExpr *CSC = cast<ObjCBridgedCastExpr>(S);
233      R1 = CSC->getSubExpr()->getSourceRange();
234      return CSC->getLParenLoc();
235    }
236    default: ;
237  }
238  R1 = S->getSourceRange();
239  return S->getLocStart();
240}
241
242void DeadCodeScan::reportDeadCode(const Stmt *S,
243                                  clang::reachable_code::Callback &CB) {
244  SourceRange R1, R2;
245  SourceLocation Loc = GetUnreachableLoc(S, R1, R2);
246  CB.HandleUnreachable(Loc, R1, R2);
247}
248
249namespace clang { namespace reachable_code {
250
251unsigned ScanReachableFromBlock(const CFGBlock *Start,
252                                llvm::BitVector &Reachable) {
253  unsigned count = 0;
254
255  // Prep work queue
256  SmallVector<const CFGBlock*, 32> WL;
257
258  // The entry block may have already been marked reachable
259  // by the caller.
260  if (!Reachable[Start->getBlockID()]) {
261    ++count;
262    Reachable[Start->getBlockID()] = true;
263  }
264
265  WL.push_back(Start);
266
267  // Find the reachable blocks from 'Start'.
268  while (!WL.empty()) {
269    const CFGBlock *item = WL.pop_back_val();
270
271    // Look at the successors and mark then reachable.
272    for (CFGBlock::const_succ_iterator I = item->succ_begin(),
273         E = item->succ_end(); I != E; ++I)
274      if (const CFGBlock *B = *I) {
275        unsigned blockID = B->getBlockID();
276        if (!Reachable[blockID]) {
277          Reachable.set(blockID);
278          WL.push_back(B);
279          ++count;
280        }
281      }
282  }
283  return count;
284}
285
286void FindUnreachableCode(AnalysisContext &AC, Callback &CB) {
287  CFG *cfg = AC.getCFG();
288  if (!cfg)
289    return;
290
291  // Scan for reachable blocks from the entrance of the CFG.
292  // If there are no unreachable blocks, we're done.
293  llvm::BitVector reachable(cfg->getNumBlockIDs());
294  unsigned numReachable = ScanReachableFromBlock(&cfg->getEntry(), reachable);
295  if (numReachable == cfg->getNumBlockIDs())
296    return;
297
298  // If there aren't explicit EH edges, we should include the 'try' dispatch
299  // blocks as roots.
300  if (!AC.getCFGBuildOptions().AddEHEdges) {
301    for (CFG::try_block_iterator I = cfg->try_blocks_begin(),
302         E = cfg->try_blocks_end() ; I != E; ++I) {
303      numReachable += ScanReachableFromBlock(*I, reachable);
304    }
305    if (numReachable == cfg->getNumBlockIDs())
306      return;
307  }
308
309  // There are some unreachable blocks.  We need to find the root blocks that
310  // contain code that should be considered unreachable.
311  for (CFG::iterator I = cfg->begin(), E = cfg->end(); I != E; ++I) {
312    const CFGBlock *block = *I;
313    // A block may have been marked reachable during this loop.
314    if (reachable[block->getBlockID()])
315      continue;
316
317    DeadCodeScan DS(reachable);
318    numReachable += DS.scanBackwards(block, CB);
319
320    if (numReachable == cfg->getNumBlockIDs())
321      return;
322  }
323}
324
325}} // end namespace clang::reachable_code
326