ReachableCode.cpp revision 767b3d2000a00c56e1a3c19372810e2b7d66b76c 
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 "clang/Analysis/Analyses/ReachableCode.h"
16#include "clang/AST/Expr.h"
17#include "clang/AST/ExprCXX.h"
18#include "clang/AST/ExprObjC.h"
19#include "clang/AST/StmtCXX.h"
20#include "clang/Analysis/AnalysisContext.h"
21#include "clang/Analysis/CFG.h"
22#include "clang/Basic/SourceManager.h"
23#include "llvm/ADT/BitVector.h"
24#include "llvm/ADT/SmallVector.h"
25
26using namespace clang;
27
28namespace {
29class DeadCodeScan {
30  llvm::BitVector Visited;
31  llvm::BitVector &Reachable;
32  SmallVector<const CFGBlock *, 10> WorkList;
33
34  typedef 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  if (S->getLocStart().isInvalid())
90    return false;
91  if (const BinaryOperator *BO = dyn_cast<BinaryOperator>(S))
92    return BO->getOpcode() != BO_Comma;
93  return true;
94}
95
96const Stmt *DeadCodeScan::findDeadCode(const clang::CFGBlock *Block) {
97  for (CFGBlock::const_iterator I = Block->begin(), E = Block->end(); I!=E; ++I)
98    if (Optional<CFGStmt> CS = I->getAs<CFGStmt>()) {
99      const Stmt *S = CS->getStmt();
100      if (isValidDeadStmt(S))
101        return S;
102    }
103
104  if (CFGTerminator T = Block->getTerminator()) {
105    const Stmt *S = T.getStmt();
106    if (isValidDeadStmt(S))
107      return S;
108  }
109
110  return 0;
111}
112
113static int SrcCmp(const std::pair<const CFGBlock *, const Stmt *> *p1,
114                  const std::pair<const CFGBlock *, const Stmt *> *p2) {
115  return p2->second->getLocStart() < p1->second->getLocStart();
116}
117
118unsigned DeadCodeScan::scanBackwards(const clang::CFGBlock *Start,
119                                     clang::reachable_code::Callback &CB) {
120
121  unsigned count = 0;
122  enqueue(Start);
123
124  while (!WorkList.empty()) {
125    const CFGBlock *Block = WorkList.pop_back_val();
126
127    // It is possible that this block has been marked reachable after
128    // it was enqueued.
129    if (Reachable[Block->getBlockID()])
130      continue;
131
132    // Look for any dead code within the block.
133    const Stmt *S = findDeadCode(Block);
134
135    if (!S) {
136      // No dead code.  Possibly an empty block.  Look at dead predecessors.
137      for (CFGBlock::const_pred_iterator I = Block->pred_begin(),
138           E = Block->pred_end(); I != E; ++I) {
139        if (const CFGBlock *predBlock = *I)
140          enqueue(predBlock);
141      }
142      continue;
143    }
144
145    // Specially handle macro-expanded code.
146    if (S->getLocStart().isMacroID()) {
147      count += clang::reachable_code::ScanReachableFromBlock(Block, Reachable);
148      continue;
149    }
150
151    if (isDeadCodeRoot(Block)) {
152      reportDeadCode(S, CB);
153      count += clang::reachable_code::ScanReachableFromBlock(Block, Reachable);
154    }
155    else {
156      // Record this statement as the possibly best location in a
157      // strongly-connected component of dead code for emitting a
158      // warning.
159      DeferredLocs.push_back(std::make_pair(Block, S));
160    }
161  }
162
163  // If we didn't find a dead root, then report the dead code with the
164  // earliest location.
165  if (!DeferredLocs.empty()) {
166    llvm::array_pod_sort(DeferredLocs.begin(), DeferredLocs.end(), SrcCmp);
167    for (DeferredLocsTy::iterator I = DeferredLocs.begin(),
168          E = DeferredLocs.end(); I != E; ++I) {
169      const CFGBlock *block = I->first;
170      if (Reachable[block->getBlockID()])
171        continue;
172      reportDeadCode(I->second, CB);
173      count += clang::reachable_code::ScanReachableFromBlock(block, Reachable);
174    }
175  }
176
177  return count;
178}
179
180static SourceLocation GetUnreachableLoc(const Stmt *S,
181                                        SourceRange &R1,
182                                        SourceRange &R2) {
183  R1 = R2 = SourceRange();
184
185  if (const Expr *Ex = dyn_cast<Expr>(S))
186    S = Ex->IgnoreParenImpCasts();
187
188  switch (S->getStmtClass()) {
189    case Expr::BinaryOperatorClass: {
190      const BinaryOperator *BO = cast<BinaryOperator>(S);
191      return BO->getOperatorLoc();
192    }
193    case Expr::UnaryOperatorClass: {
194      const UnaryOperator *UO = cast<UnaryOperator>(S);
195      R1 = UO->getSubExpr()->getSourceRange();
196      return UO->getOperatorLoc();
197    }
198    case Expr::CompoundAssignOperatorClass: {
199      const CompoundAssignOperator *CAO = cast<CompoundAssignOperator>(S);
200      R1 = CAO->getLHS()->getSourceRange();
201      R2 = CAO->getRHS()->getSourceRange();
202      return CAO->getOperatorLoc();
203    }
204    case Expr::BinaryConditionalOperatorClass:
205    case Expr::ConditionalOperatorClass: {
206      const AbstractConditionalOperator *CO =
207        cast<AbstractConditionalOperator>(S);
208      return CO->getQuestionLoc();
209    }
210    case Expr::MemberExprClass: {
211      const MemberExpr *ME = cast<MemberExpr>(S);
212      R1 = ME->getSourceRange();
213      return ME->getMemberLoc();
214    }
215    case Expr::ArraySubscriptExprClass: {
216      const ArraySubscriptExpr *ASE = cast<ArraySubscriptExpr>(S);
217      R1 = ASE->getLHS()->getSourceRange();
218      R2 = ASE->getRHS()->getSourceRange();
219      return ASE->getRBracketLoc();
220    }
221    case Expr::CStyleCastExprClass: {
222      const CStyleCastExpr *CSC = cast<CStyleCastExpr>(S);
223      R1 = CSC->getSubExpr()->getSourceRange();
224      return CSC->getLParenLoc();
225    }
226    case Expr::CXXFunctionalCastExprClass: {
227      const CXXFunctionalCastExpr *CE = cast <CXXFunctionalCastExpr>(S);
228      R1 = CE->getSubExpr()->getSourceRange();
229      return CE->getLocStart();
230    }
231    case Stmt::CXXTryStmtClass: {
232      return cast<CXXTryStmt>(S)->getHandler(0)->getCatchLoc();
233    }
234    case Expr::ObjCBridgedCastExprClass: {
235      const ObjCBridgedCastExpr *CSC = cast<ObjCBridgedCastExpr>(S);
236      R1 = CSC->getSubExpr()->getSourceRange();
237      return CSC->getLParenLoc();
238    }
239    default: ;
240  }
241  R1 = S->getSourceRange();
242  return S->getLocStart();
243}
244
245void DeadCodeScan::reportDeadCode(const Stmt *S,
246                                  clang::reachable_code::Callback &CB) {
247  SourceRange R1, R2;
248  SourceLocation Loc = GetUnreachableLoc(S, R1, R2);
249  CB.HandleUnreachable(Loc, R1, R2);
250}
251
252namespace clang { namespace reachable_code {
253
254void Callback::anchor() { }
255
256unsigned ScanReachableFromBlock(const CFGBlock *Start,
257                                llvm::BitVector &Reachable) {
258  unsigned count = 0;
259
260  // Prep work queue
261  SmallVector<const CFGBlock*, 32> WL;
262
263  // The entry block may have already been marked reachable
264  // by the caller.
265  if (!Reachable[Start->getBlockID()]) {
266    ++count;
267    Reachable[Start->getBlockID()] = true;
268  }
269
270  WL.push_back(Start);
271
272  // Find the reachable blocks from 'Start'.
273  while (!WL.empty()) {
274    const CFGBlock *item = WL.pop_back_val();
275
276    // Look at the successors and mark then reachable.
277    for (CFGBlock::const_succ_iterator I = item->succ_begin(),
278         E = item->succ_end(); I != E; ++I)
279      if (const CFGBlock *B = *I) {
280        unsigned blockID = B->getBlockID();
281        if (!Reachable[blockID]) {
282          Reachable.set(blockID);
283          WL.push_back(B);
284          ++count;
285        }
286      }
287  }
288  return count;
289}
290
291void FindUnreachableCode(AnalysisDeclContext &AC, Callback &CB) {
292  CFG *cfg = AC.getCFG();
293  if (!cfg)
294    return;
295
296  // Scan for reachable blocks from the entrance of the CFG.
297  // If there are no unreachable blocks, we're done.
298  llvm::BitVector reachable(cfg->getNumBlockIDs());
299  unsigned numReachable = ScanReachableFromBlock(&cfg->getEntry(), reachable);
300  if (numReachable == cfg->getNumBlockIDs())
301    return;
302
303  // If there aren't explicit EH edges, we should include the 'try' dispatch
304  // blocks as roots.
305  if (!AC.getCFGBuildOptions().AddEHEdges) {
306    for (CFG::try_block_iterator I = cfg->try_blocks_begin(),
307         E = cfg->try_blocks_end() ; I != E; ++I) {
308      numReachable += ScanReachableFromBlock(*I, reachable);
309    }
310    if (numReachable == cfg->getNumBlockIDs())
311      return;
312  }
313
314  // There are some unreachable blocks.  We need to find the root blocks that
315  // contain code that should be considered unreachable.
316  for (CFG::iterator I = cfg->begin(), E = cfg->end(); I != E; ++I) {
317    const CFGBlock *block = *I;
318    // A block may have been marked reachable during this loop.
319    if (reachable[block->getBlockID()])
320      continue;
321
322    DeadCodeScan DS(reachable);
323    numReachable += DS.scanBackwards(block, CB);
324
325    if (numReachable == cfg->getNumBlockIDs())
326      return;
327  }
328}
329
330}} // end namespace clang::reachable_code
331