ReachableCode.cpp revision 579ee4f9f5c7b8f939621c8008337a3c1c679957 
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  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 (const 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 void *p1, const void *p2) {
114  return
115    ((std::pair<const CFGBlock *, const Stmt *>*) p2)->second->getLocStart() <
116    ((std::pair<const CFGBlock *, const Stmt *>*) p1)->second->getLocStart();
117}
118
119unsigned DeadCodeScan::scanBackwards(const clang::CFGBlock *Start,
120                                     clang::reachable_code::Callback &CB) {
121
122  unsigned count = 0;
123  enqueue(Start);
124
125  while (!WorkList.empty()) {
126    const CFGBlock *Block = WorkList.pop_back_val();
127
128    // It is possible that this block has been marked reachable after
129    // it was enqueued.
130    if (Reachable[Block->getBlockID()])
131      continue;
132
133    // Look for any dead code within the block.
134    const Stmt *S = findDeadCode(Block);
135
136    if (!S) {
137      // No dead code.  Possibly an empty block.  Look at dead predecessors.
138      for (CFGBlock::const_pred_iterator I = Block->pred_begin(),
139           E = Block->pred_end(); I != E; ++I) {
140        if (const CFGBlock *predBlock = *I)
141          enqueue(predBlock);
142      }
143      continue;
144    }
145
146    // Specially handle macro-expanded code.
147    if (S->getLocStart().isMacroID()) {
148      count += clang::reachable_code::ScanReachableFromBlock(Block, Reachable);
149      continue;
150    }
151
152    if (isDeadCodeRoot(Block)) {
153      reportDeadCode(S, CB);
154      count += clang::reachable_code::ScanReachableFromBlock(Block, Reachable);
155    }
156    else {
157      // Record this statement as the possibly best location in a
158      // strongly-connected component of dead code for emitting a
159      // warning.
160      DeferredLocs.push_back(std::make_pair(Block, S));
161    }
162  }
163
164  // If we didn't find a dead root, then report the dead code with the
165  // earliest location.
166  if (!DeferredLocs.empty()) {
167    llvm::array_pod_sort(DeferredLocs.begin(), DeferredLocs.end(), SrcCmp);
168    for (DeferredLocsTy::iterator I = DeferredLocs.begin(),
169          E = DeferredLocs.end(); I != E; ++I) {
170      const CFGBlock *block = I->first;
171      if (Reachable[block->getBlockID()])
172        continue;
173      reportDeadCode(I->second, CB);
174      count += clang::reachable_code::ScanReachableFromBlock(block, Reachable);
175    }
176  }
177
178  return count;
179}
180
181static SourceLocation GetUnreachableLoc(const Stmt *S,
182                                        SourceRange &R1,
183                                        SourceRange &R2) {
184  R1 = R2 = SourceRange();
185
186  if (const Expr *Ex = dyn_cast<Expr>(S))
187    S = Ex->IgnoreParenImpCasts();
188
189  switch (S->getStmtClass()) {
190    case Expr::BinaryOperatorClass: {
191      const BinaryOperator *BO = cast<BinaryOperator>(S);
192      return BO->getOperatorLoc();
193    }
194    case Expr::UnaryOperatorClass: {
195      const UnaryOperator *UO = cast<UnaryOperator>(S);
196      R1 = UO->getSubExpr()->getSourceRange();
197      return UO->getOperatorLoc();
198    }
199    case Expr::CompoundAssignOperatorClass: {
200      const CompoundAssignOperator *CAO = cast<CompoundAssignOperator>(S);
201      R1 = CAO->getLHS()->getSourceRange();
202      R2 = CAO->getRHS()->getSourceRange();
203      return CAO->getOperatorLoc();
204    }
205    case Expr::BinaryConditionalOperatorClass:
206    case Expr::ConditionalOperatorClass: {
207      const AbstractConditionalOperator *CO =
208        cast<AbstractConditionalOperator>(S);
209      return CO->getQuestionLoc();
210    }
211    case Expr::MemberExprClass: {
212      const MemberExpr *ME = cast<MemberExpr>(S);
213      R1 = ME->getSourceRange();
214      return ME->getMemberLoc();
215    }
216    case Expr::ArraySubscriptExprClass: {
217      const ArraySubscriptExpr *ASE = cast<ArraySubscriptExpr>(S);
218      R1 = ASE->getLHS()->getSourceRange();
219      R2 = ASE->getRHS()->getSourceRange();
220      return ASE->getRBracketLoc();
221    }
222    case Expr::CStyleCastExprClass: {
223      const CStyleCastExpr *CSC = cast<CStyleCastExpr>(S);
224      R1 = CSC->getSubExpr()->getSourceRange();
225      return CSC->getLParenLoc();
226    }
227    case Expr::CXXFunctionalCastExprClass: {
228      const CXXFunctionalCastExpr *CE = cast <CXXFunctionalCastExpr>(S);
229      R1 = CE->getSubExpr()->getSourceRange();
230      return CE->getTypeBeginLoc();
231    }
232    case Stmt::CXXTryStmtClass: {
233      return cast<CXXTryStmt>(S)->getHandler(0)->getCatchLoc();
234    }
235    case Expr::ObjCBridgedCastExprClass: {
236      const ObjCBridgedCastExpr *CSC = cast<ObjCBridgedCastExpr>(S);
237      R1 = CSC->getSubExpr()->getSourceRange();
238      return CSC->getLParenLoc();
239    }
240    default: ;
241  }
242  R1 = S->getSourceRange();
243  return S->getLocStart();
244}
245
246void DeadCodeScan::reportDeadCode(const Stmt *S,
247                                  clang::reachable_code::Callback &CB) {
248  SourceRange R1, R2;
249  SourceLocation Loc = GetUnreachableLoc(S, R1, R2);
250  CB.HandleUnreachable(Loc, R1, R2);
251}
252
253namespace clang { namespace reachable_code {
254
255unsigned ScanReachableFromBlock(const CFGBlock *Start,
256                                llvm::BitVector &Reachable) {
257  unsigned count = 0;
258
259  // Prep work queue
260  SmallVector<const CFGBlock*, 32> WL;
261
262  // The entry block may have already been marked reachable
263  // by the caller.
264  if (!Reachable[Start->getBlockID()]) {
265    ++count;
266    Reachable[Start->getBlockID()] = true;
267  }
268
269  WL.push_back(Start);
270
271  // Find the reachable blocks from 'Start'.
272  while (!WL.empty()) {
273    const CFGBlock *item = WL.pop_back_val();
274
275    // Look at the successors and mark then reachable.
276    for (CFGBlock::const_succ_iterator I = item->succ_begin(),
277         E = item->succ_end(); I != E; ++I)
278      if (const CFGBlock *B = *I) {
279        unsigned blockID = B->getBlockID();
280        if (!Reachable[blockID]) {
281          Reachable.set(blockID);
282          WL.push_back(B);
283          ++count;
284        }
285      }
286  }
287  return count;
288}
289
290void FindUnreachableCode(AnalysisDeclContext &AC, Callback &CB) {
291  CFG *cfg = AC.getCFG();
292  if (!cfg)
293    return;
294
295  // Scan for reachable blocks from the entrance of the CFG.
296  // If there are no unreachable blocks, we're done.
297  llvm::BitVector reachable(cfg->getNumBlockIDs());
298  unsigned numReachable = ScanReachableFromBlock(&cfg->getEntry(), reachable);
299  if (numReachable == cfg->getNumBlockIDs())
300    return;
301
302  // If there aren't explicit EH edges, we should include the 'try' dispatch
303  // blocks as roots.
304  if (!AC.getCFGBuildOptions().AddEHEdges) {
305    for (CFG::try_block_iterator I = cfg->try_blocks_begin(),
306         E = cfg->try_blocks_end() ; I != E; ++I) {
307      numReachable += ScanReachableFromBlock(*I, reachable);
308    }
309    if (numReachable == cfg->getNumBlockIDs())
310      return;
311  }
312
313  // There are some unreachable blocks.  We need to find the root blocks that
314  // contain code that should be considered unreachable.
315  for (CFG::iterator I = cfg->begin(), E = cfg->end(); I != E; ++I) {
316    const CFGBlock *block = *I;
317    // A block may have been marked reachable during this loop.
318    if (reachable[block->getBlockID()])
319      continue;
320
321    DeadCodeScan DS(reachable);
322    numReachable += DS.scanBackwards(block, CB);
323
324    if (numReachable == cfg->getNumBlockIDs())
325      return;
326  }
327}
328
329}} // end namespace clang::reachable_code
330