summaryrefslogtreecommitdiff
path: root/clang/lib/StaticAnalyzer/Checkers/UnreachableCodeChecker.cpp
blob: 5a13ed0a2e17bfb7c1211ad5aa29d32c3abb5939 (about) (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
//==- UnreachableCodeChecker.cpp - Generalized dead code checker -*- C++ -*-==//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
// This file implements a generalized unreachable code checker using a
// path-sensitive analysis. We mark any path visited, and then walk the CFG as a
// post-analysis to determine what was never visited.
//
// A similar flow-sensitive only check exists in Analysis/ReachableCode.cpp
//===----------------------------------------------------------------------===//

#include "ClangSACheckers.h"
#include "clang/StaticAnalyzer/Core/Checker.h"
#include "clang/StaticAnalyzer/Core/CheckerManager.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/ExplodedGraph.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/SVals.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerHelpers.h"
#include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.h"
#include "clang/AST/ParentMap.h"
#include "clang/Basic/Builtins.h"
#include "clang/Basic/SourceManager.h"
#include "llvm/ADT/SmallSet.h"

// The number of CFGBlock pointers we want to reserve memory for. This is used
// once for each function we analyze.
#define DEFAULT_CFGBLOCKS 256

using namespace clang;
using namespace ento;

namespace {
class UnreachableCodeChecker : public Checker<check::EndAnalysis> {
public:
  void checkEndAnalysis(ExplodedGraph &G, BugReporter &B,
                        ExprEngine &Eng) const;
private:
  typedef llvm::SmallSet<unsigned, DEFAULT_CFGBLOCKS> CFGBlocksSet;

  static inline const Stmt *getUnreachableStmt(const CFGBlock *CB);
  static void FindUnreachableEntryPoints(const CFGBlock *CB,
                                         CFGBlocksSet &reachable,
                                         CFGBlocksSet &visited);
  static bool isInvalidPath(const CFGBlock *CB, const ParentMap &PM);
  static inline bool isEmptyCFGBlock(const CFGBlock *CB);
};
}

void UnreachableCodeChecker::checkEndAnalysis(ExplodedGraph &G,
                                              BugReporter &B,
                                              ExprEngine &Eng) const {
  CFGBlocksSet reachable, visited;
  
  if (Eng.hasWorkRemaining())
    return;

  const Decl *D = 0;
  CFG *C = 0;
  ParentMap *PM = 0;
  const LocationContext *LC = 0;
  // Iterate over ExplodedGraph
  for (ExplodedGraph::node_iterator I = G.nodes_begin(), E = G.nodes_end();
      I != E; ++I) {
    const ProgramPoint &P = I->getLocation();
    LC = P.getLocationContext();

    if (!D)
      D = LC->getAnalysisDeclContext()->getDecl();
    // Save the CFG if we don't have it already
    if (!C)
      C = LC->getAnalysisDeclContext()->getUnoptimizedCFG();
    if (!PM)
      PM = &LC->getParentMap();

    if (const BlockEntrance *BE = dyn_cast<BlockEntrance>(&P)) {
      const CFGBlock *CB = BE->getBlock();
      reachable.insert(CB->getBlockID());
    }
  }

  // Bail out if we didn't get the CFG or the ParentMap.
  if (!D || !C || !PM)
    return;
  
  // Don't do anything for template instantiations.  Proving that code
  // in a template instantiation is unreachable means proving that it is
  // unreachable in all instantiations.
  if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(D))
    if (FD->isTemplateInstantiation())
      return;

  // Find CFGBlocks that were not covered by any node
  for (CFG::const_iterator I = C->begin(), E = C->end(); I != E; ++I) {
    const CFGBlock *CB = *I;
    // Check if the block is unreachable
    if (reachable.count(CB->getBlockID()))
      continue;

    // Check if the block is empty (an artificial block)
    if (isEmptyCFGBlock(CB))
      continue;

    // Find the entry points for this block
    if (!visited.count(CB->getBlockID()))
      FindUnreachableEntryPoints(CB, reachable, visited);

    // This block may have been pruned; check if we still want to report it
    if (reachable.count(CB->getBlockID()))
      continue;

    // Check for false positives
    if (CB->size() > 0 && isInvalidPath(CB, *PM))
      continue;

    // It is good practice to always have a "default" label in a "switch", even
    // if we should never get there. It can be used to detect errors, for
    // instance. Unreachable code directly under a "default" label is therefore
    // likely to be a false positive.
    if (const Stmt *label = CB->getLabel())
      if (label->getStmtClass() == Stmt::DefaultStmtClass)
        continue;

    // Special case for __builtin_unreachable.
    // FIXME: This should be extended to include other unreachable markers,
    // such as llvm_unreachable.
    if (!CB->empty()) {
      bool foundUnreachable = false;
      for (CFGBlock::const_iterator ci = CB->begin(), ce = CB->end();
           ci != ce; ++ci) {
        if (const CFGStmt *S = (*ci).getAs<CFGStmt>())
          if (const CallExpr *CE = dyn_cast<CallExpr>(S->getStmt())) {
            if (CE->isBuiltinCall() == Builtin::BI__builtin_unreachable) {
              foundUnreachable = true;
              break;
            }
          }
      }
      if (foundUnreachable)
        continue;
    }

    // We found a block that wasn't covered - find the statement to report
    SourceRange SR;
    PathDiagnosticLocation DL;
    SourceLocation SL;
    if (const Stmt *S = getUnreachableStmt(CB)) {
      SR = S->getSourceRange();
      DL = PathDiagnosticLocation::createBegin(S, B.getSourceManager(), LC);
      SL = DL.asLocation();
      if (SR.isInvalid() || !SL.isValid())
        continue;
    }
    else
      continue;

    // Check if the SourceLocation is in a system header
    const SourceManager &SM = B.getSourceManager();
    if (SM.isInSystemHeader(SL) || SM.isInExternCSystemHeader(SL))
      continue;

    B.EmitBasicReport(D, "Unreachable code", "Dead code",
                      "This statement is never executed", DL, SR);
  }
}

// Recursively finds the entry point(s) for this dead CFGBlock.
void UnreachableCodeChecker::FindUnreachableEntryPoints(const CFGBlock *CB,
                                                        CFGBlocksSet &reachable,
                                                        CFGBlocksSet &visited) {
  visited.insert(CB->getBlockID());

  for (CFGBlock::const_pred_iterator I = CB->pred_begin(), E = CB->pred_end();
      I != E; ++I) {
    if (!reachable.count((*I)->getBlockID())) {
      // If we find an unreachable predecessor, mark this block as reachable so
      // we don't report this block
      reachable.insert(CB->getBlockID());
      if (!visited.count((*I)->getBlockID()))
        // If we haven't previously visited the unreachable predecessor, recurse
        FindUnreachableEntryPoints(*I, reachable, visited);
    }
  }
}

// Find the Stmt* in a CFGBlock for reporting a warning
const Stmt *UnreachableCodeChecker::getUnreachableStmt(const CFGBlock *CB) {
  for (CFGBlock::const_iterator I = CB->begin(), E = CB->end(); I != E; ++I) {
    if (const CFGStmt *S = I->getAs<CFGStmt>())
      return S->getStmt();
  }
  if (const Stmt *S = CB->getTerminator())
    return S;
  else
    return 0;
}

// Determines if the path to this CFGBlock contained an element that infers this
// block is a false positive. We assume that FindUnreachableEntryPoints has
// already marked only the entry points to any dead code, so we need only to
// find the condition that led to this block (the predecessor of this block.)
// There will never be more than one predecessor.
bool UnreachableCodeChecker::isInvalidPath(const CFGBlock *CB,
                                           const ParentMap &PM) {
  // We only expect a predecessor size of 0 or 1. If it is >1, then an external
  // condition has broken our assumption (for example, a sink being placed by
  // another check). In these cases, we choose not to report.
  if (CB->pred_size() > 1)
    return true;

  // If there are no predecessors, then this block is trivially unreachable
  if (CB->pred_size() == 0)
    return false;

  const CFGBlock *pred = *CB->pred_begin();

  // Get the predecessor block's terminator conditon
  const Stmt *cond = pred->getTerminatorCondition();

  //assert(cond && "CFGBlock's predecessor has a terminator condition");
  // The previous assertion is invalid in some cases (eg do/while). Leaving
  // reporting of these situations on at the moment to help triage these cases.
  if (!cond)
    return false;

  // Run each of the checks on the conditions
  if (containsMacro(cond) || containsEnum(cond)
      || containsStaticLocal(cond) || containsBuiltinOffsetOf(cond)
      || containsStmt<UnaryExprOrTypeTraitExpr>(cond))
    return true;

  return false;
}

// Returns true if the given CFGBlock is empty
bool UnreachableCodeChecker::isEmptyCFGBlock(const CFGBlock *CB) {
  return CB->getLabel() == 0       // No labels
      && CB->size() == 0           // No statements
      && CB->getTerminator() == 0; // No terminator
}

void ento::registerUnreachableCodeChecker(CheckerManager &mgr) {
  mgr.registerChecker<UnreachableCodeChecker>();
}