UnreachableCodeChecker.cpp revision 180f47959a066795cc0f409433023af448bb0328 
1//==- UnreachableCodeChecker.cpp - Generalized dead code checker -*- 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// This file implements a generalized unreachable code checker using a
10// path-sensitive analysis. We mark any path visited, and then walk the CFG as a
11// post-analysis to determine what was never visited.
12//
13// A similar flow-sensitive only check exists in Analysis/ReachableCode.cpp
14//===----------------------------------------------------------------------===//
15
16#include "ClangSACheckers.h"
17#include "clang/StaticAnalyzer/Core/Checker.h"
18#include "clang/StaticAnalyzer/Core/CheckerManager.h"
19#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
20#include "clang/StaticAnalyzer/Core/PathSensitive/ExplodedGraph.h"
21#include "clang/StaticAnalyzer/Core/PathSensitive/SVals.h"
22#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerHelpers.h"
23#include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.h"
24#include "clang/AST/ParentMap.h"
25#include "clang/Basic/Builtins.h"
26#include "clang/Basic/SourceManager.h"
27#include "llvm/ADT/SmallPtrSet.h"
28
29// The number of CFGBlock pointers we want to reserve memory for. This is used
30// once for each function we analyze.
31#define DEFAULT_CFGBLOCKS 256
32
33using namespace clang;
34using namespace ento;
35
36namespace {
37class UnreachableCodeChecker : public Checker<check::EndAnalysis> {
38public:
39  void checkEndAnalysis(ExplodedGraph &G, BugReporter &B,
40                        ExprEngine &Eng) const;
41private:
42  typedef llvm::SmallSet<unsigned, DEFAULT_CFGBLOCKS> CFGBlocksSet;
43
44  static inline const Stmt *getUnreachableStmt(const CFGBlock *CB);
45  static void FindUnreachableEntryPoints(const CFGBlock *CB,
46                                         CFGBlocksSet &reachable,
47                                         CFGBlocksSet &visited);
48  static bool isInvalidPath(const CFGBlock *CB, const ParentMap &PM);
49  static inline bool isEmptyCFGBlock(const CFGBlock *CB);
50};
51}
52
53void UnreachableCodeChecker::checkEndAnalysis(ExplodedGraph &G,
54                                              BugReporter &B,
55                                              ExprEngine &Eng) const {
56  CFGBlocksSet reachable, visited;
57
58  if (Eng.hasWorkRemaining())
59    return;
60
61  CFG *C = 0;
62  ParentMap *PM = 0;
63  const LocationContext *LC = 0;
64  // Iterate over ExplodedGraph
65  for (ExplodedGraph::node_iterator I = G.nodes_begin(), E = G.nodes_end();
66      I != E; ++I) {
67    const ProgramPoint &P = I->getLocation();
68    LC = P.getLocationContext();
69
70    // Save the CFG if we don't have it already
71    if (!C)
72      C = LC->getAnalysisDeclContext()->getUnoptimizedCFG();
73    if (!PM)
74      PM = &LC->getParentMap();
75
76    if (const BlockEntrance *BE = dyn_cast<BlockEntrance>(&P)) {
77      const CFGBlock *CB = BE->getBlock();
78      reachable.insert(CB->getBlockID());
79    }
80  }
81
82  // Bail out if we didn't get the CFG or the ParentMap.
83  if (!C || !PM)
84    return;
85
86  // Find CFGBlocks that were not covered by any node
87  for (CFG::const_iterator I = C->begin(), E = C->end(); I != E; ++I) {
88    const CFGBlock *CB = *I;
89    // Check if the block is unreachable
90    if (reachable.count(CB->getBlockID()))
91      continue;
92
93    // Check if the block is empty (an artificial block)
94    if (isEmptyCFGBlock(CB))
95      continue;
96
97    // Find the entry points for this block
98    if (!visited.count(CB->getBlockID()))
99      FindUnreachableEntryPoints(CB, reachable, visited);
100
101    // This block may have been pruned; check if we still want to report it
102    if (reachable.count(CB->getBlockID()))
103      continue;
104
105    // Check for false positives
106    if (CB->size() > 0 && isInvalidPath(CB, *PM))
107      continue;
108
109    // Special case for __builtin_unreachable.
110    // FIXME: This should be extended to include other unreachable markers,
111    // such as llvm_unreachable.
112    if (!CB->empty()) {
113      bool foundUnreachable = false;
114      for (CFGBlock::const_iterator ci = CB->begin(), ce = CB->end();
115           ci != ce; ++ci) {
116        if (const CFGStmt *S = (*ci).getAs<CFGStmt>())
117          if (const CallExpr *CE = dyn_cast<CallExpr>(S->getStmt())) {
118            if (CE->isBuiltinCall() == Builtin::BI__builtin_unreachable) {
119              foundUnreachable = true;
120              break;
121            }
122          }
123      }
124      if (foundUnreachable)
125        continue;
126    }
127
128    // We found a block that wasn't covered - find the statement to report
129    SourceRange SR;
130    PathDiagnosticLocation DL;
131    SourceLocation SL;
132    if (const Stmt *S = getUnreachableStmt(CB)) {
133      SR = S->getSourceRange();
134      DL = PathDiagnosticLocation::createBegin(S, B.getSourceManager(), LC);
135      SL = DL.asLocation();
136      if (SR.isInvalid() || !SL.isValid())
137        continue;
138    }
139    else
140      continue;
141
142    // Check if the SourceLocation is in a system header
143    const SourceManager &SM = B.getSourceManager();
144    if (SM.isInSystemHeader(SL) || SM.isInExternCSystemHeader(SL))
145      continue;
146
147    B.EmitBasicReport("Unreachable code", "Dead code", "This statement is never"
148        " executed", DL, SR);
149  }
150}
151
152// Recursively finds the entry point(s) for this dead CFGBlock.
153void UnreachableCodeChecker::FindUnreachableEntryPoints(const CFGBlock *CB,
154                                                        CFGBlocksSet &reachable,
155                                                        CFGBlocksSet &visited) {
156  visited.insert(CB->getBlockID());
157
158  for (CFGBlock::const_pred_iterator I = CB->pred_begin(), E = CB->pred_end();
159      I != E; ++I) {
160    if (!reachable.count((*I)->getBlockID())) {
161      // If we find an unreachable predecessor, mark this block as reachable so
162      // we don't report this block
163      reachable.insert(CB->getBlockID());
164      if (!visited.count((*I)->getBlockID()))
165        // If we haven't previously visited the unreachable predecessor, recurse
166        FindUnreachableEntryPoints(*I, reachable, visited);
167    }
168  }
169}
170
171// Find the Stmt* in a CFGBlock for reporting a warning
172const Stmt *UnreachableCodeChecker::getUnreachableStmt(const CFGBlock *CB) {
173  for (CFGBlock::const_iterator I = CB->begin(), E = CB->end(); I != E; ++I) {
174    if (const CFGStmt *S = I->getAs<CFGStmt>())
175      return S->getStmt();
176  }
177  if (const Stmt *S = CB->getTerminator())
178    return S;
179  else
180    return 0;
181}
182
183// Determines if the path to this CFGBlock contained an element that infers this
184// block is a false positive. We assume that FindUnreachableEntryPoints has
185// already marked only the entry points to any dead code, so we need only to
186// find the condition that led to this block (the predecessor of this block.)
187// There will never be more than one predecessor.
188bool UnreachableCodeChecker::isInvalidPath(const CFGBlock *CB,
189                                           const ParentMap &PM) {
190  // We only expect a predecessor size of 0 or 1. If it is >1, then an external
191  // condition has broken our assumption (for example, a sink being placed by
192  // another check). In these cases, we choose not to report.
193  if (CB->pred_size() > 1)
194    return true;
195
196  // If there are no predecessors, then this block is trivially unreachable
197  if (CB->pred_size() == 0)
198    return false;
199
200  const CFGBlock *pred = *CB->pred_begin();
201
202  // Get the predecessor block's terminator conditon
203  const Stmt *cond = pred->getTerminatorCondition();
204
205  //assert(cond && "CFGBlock's predecessor has a terminator condition");
206  // The previous assertion is invalid in some cases (eg do/while). Leaving
207  // reporting of these situations on at the moment to help triage these cases.
208  if (!cond)
209    return false;
210
211  // Run each of the checks on the conditions
212  if (containsMacro(cond) || containsEnum(cond)
213      || containsStaticLocal(cond) || containsBuiltinOffsetOf(cond)
214      || containsStmt<UnaryExprOrTypeTraitExpr>(cond))
215    return true;
216
217  return false;
218}
219
220// Returns true if the given CFGBlock is empty
221bool UnreachableCodeChecker::isEmptyCFGBlock(const CFGBlock *CB) {
222  return CB->getLabel() == 0       // No labels
223      && CB->size() == 0           // No statements
224      && CB->getTerminator() == 0; // No terminator
225}
226
227void ento::registerUnreachableCodeChecker(CheckerManager &mgr) {
228  mgr.registerChecker<UnreachableCodeChecker>();
229}
230