src/llvm/tools/clang/lib/StaticAnalyzer/Checkers/UnreachableCodeChecker.cpp

   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/SmallSet.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
  33 using namespace clang;
  34 using namespace ento;
  35
  36 namespace {
  37 class UnreachableCodeChecker : public Checker<check::EndAnalysis> {
  38 public:
  39   void checkEndAnalysis(ExplodedGraph &G, BugReporter &B,
  40                         ExprEngine &Eng) const;
  41 private:
  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
  53 void UnreachableCodeChecker::checkEndAnalysis(ExplodedGraph &G,
  54                                               BugReporter &B,
  55                                               ExprEngine &Eng) const {
  56   CFGBlocksSet reachable, visited;
  57
  58   if (Eng.hasWorkRemaining())
  59     return;
  60
  61   const Decl *D = 0;
  62   CFG *C = 0;
  63   ParentMap *PM = 0;
  64   const LocationContext *LC = 0;
  65   // Iterate over ExplodedGraph
  66   for (ExplodedGraph::node_iterator I = G.nodes_begin(), E = G.nodes_end();
  67       I != E; ++I) {
  68     const ProgramPoint &P = I->getLocation();
  69     LC = P.getLocationContext();
  70
  71     if (!D)
  72       D = LC->getAnalysisDeclContext()->getDecl();
  73     // Save the CFG if we don't have it already
  74     if (!C)
  75       C = LC->getAnalysisDeclContext()->getUnoptimizedCFG();
  76     if (!PM)
  77       PM = &LC->getParentMap();
  78
  79     if (const BlockEntrance *BE = dyn_cast<BlockEntrance>(&P)) {
  80       const CFGBlock *CB = BE->getBlock();
  81       reachable.insert(CB->getBlockID());
  82     }
  83   }
  84
  85   // Bail out if we didn't get the CFG or the ParentMap.
  86   if (!D || !C || !PM)
  87     return;
  88
  89   // Don't do anything for template instantiations.  Proving that code
  90   // in a template instantiation is unreachable means proving that it is
  91   // unreachable in all instantiations.
  92   if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(D))
  93     if (FD->isTemplateInstantiation())
  94       return;
  95
  96   // Find CFGBlocks that were not covered by any node
  97   for (CFG::const_iterator I = C->begin(), E = C->end(); I != E; ++I) {
  98     const CFGBlock *CB = *I;
  99     // Check if the block is unreachable
 100     if (reachable.count(CB->getBlockID()))
 101       continue;
 102
 103     // Check if the block is empty (an artificial block)
 104     if (isEmptyCFGBlock(CB))
 105       continue;
 106
 107     // Find the entry points for this block
 108     if (!visited.count(CB->getBlockID()))
 109       FindUnreachableEntryPoints(CB, reachable, visited);
 110
 111     // This block may have been pruned; check if we still want to report it
 112     if (reachable.count(CB->getBlockID()))
 113       continue;
 114
 115     // Check for false positives
 116     if (CB->size() > 0 && isInvalidPath(CB, *PM))
 117       continue;
 118
 119     // It is good practice to always have a "default" label in a "switch", even
 120     // if we should never get there. It can be used to detect errors, for
 121     // instance. Unreachable code directly under a "default" label is therefore
 122     // likely to be a false positive.
 123     if (const Stmt *label = CB->getLabel())
 124       if (label->getStmtClass() == Stmt::DefaultStmtClass)
 125         continue;
 126
 127     // Special case for __builtin_unreachable.
 128     // FIXME: This should be extended to include other unreachable markers,
 129     // such as llvm_unreachable.
 130     if (!CB->empty()) {
 131       bool foundUnreachable = false;
 132       for (CFGBlock::const_iterator ci = CB->begin(), ce = CB->end();
 133            ci != ce; ++ci) {
 134         if (const CFGStmt *S = (*ci).getAs<CFGStmt>())
 135           if (const CallExpr *CE = dyn_cast<CallExpr>(S->getStmt())) {
 136             if (CE->isBuiltinCall() == Builtin::BI__builtin_unreachable) {
 137               foundUnreachable = true;
 138               break;
 139             }
 140           }
 141       }
 142       if (foundUnreachable)
 143         continue;
 144     }
 145
 146     // We found a block that wasn't covered - find the statement to report
 147     SourceRange SR;
 148     PathDiagnosticLocation DL;
 149     SourceLocation SL;
 150     if (const Stmt *S = getUnreachableStmt(CB)) {
 151       SR = S->getSourceRange();
 152       DL = PathDiagnosticLocation::createBegin(S, B.getSourceManager(), LC);
 153       SL = DL.asLocation();
 154       if (SR.isInvalid() || !SL.isValid())
 155         continue;
 156     }
 157     else
 158       continue;
 159
 160     // Check if the SourceLocation is in a system header
 161     const SourceManager &SM = B.getSourceManager();
 162     if (SM.isInSystemHeader(SL) || SM.isInExternCSystemHeader(SL))
 163       continue;
 164
 165     B.EmitBasicReport(D, "Unreachable code", "Dead code",
 166                       "This statement is never executed", DL, SR);
 167   }
 168 }
 169
 170 // Recursively finds the entry point(s) for this dead CFGBlock.
 171 void UnreachableCodeChecker::FindUnreachableEntryPoints(const CFGBlock *CB,
 172                                                         CFGBlocksSet &reachable,
 173                                                         CFGBlocksSet &visited) {
 174   visited.insert(CB->getBlockID());
 175
 176   for (CFGBlock::const_pred_iterator I = CB->pred_begin(), E = CB->pred_end();
 177       I != E; ++I) {
 178     if (!reachable.count((*I)->getBlockID())) {
 179       // If we find an unreachable predecessor, mark this block as reachable so
 180       // we don't report this block
 181       reachable.insert(CB->getBlockID());
 182       if (!visited.count((*I)->getBlockID()))
 183         // If we haven't previously visited the unreachable predecessor, recurse
 184         FindUnreachableEntryPoints(*I, reachable, visited);
 185     }
 186   }
 187 }
 188
 189 // Find the Stmt* in a CFGBlock for reporting a warning
 190 const Stmt *UnreachableCodeChecker::getUnreachableStmt(const CFGBlock *CB) {
 191   for (CFGBlock::const_iterator I = CB->begin(), E = CB->end(); I != E; ++I) {
 192     if (const CFGStmt *S = I->getAs<CFGStmt>())
 193       return S->getStmt();
 194   }
 195   if (const Stmt *S = CB->getTerminator())
 196     return S;
 197   else
 198     return 0;
 199 }
 200
 201 // Determines if the path to this CFGBlock contained an element that infers this
 202 // block is a false positive. We assume that FindUnreachableEntryPoints has
 203 // already marked only the entry points to any dead code, so we need only to
 204 // find the condition that led to this block (the predecessor of this block.)
 205 // There will never be more than one predecessor.
 206 bool UnreachableCodeChecker::isInvalidPath(const CFGBlock *CB,
 207                                            const ParentMap &PM) {
 208   // We only expect a predecessor size of 0 or 1. If it is >1, then an external
 209   // condition has broken our assumption (for example, a sink being placed by
 210   // another check). In these cases, we choose not to report.
 211   if (CB->pred_size() > 1)
 212     return true;
 213
 214   // If there are no predecessors, then this block is trivially unreachable
 215   if (CB->pred_size() == 0)
 216     return false;
 217
 218   const CFGBlock *pred = *CB->pred_begin();
 219
 220   // Get the predecessor block's terminator conditon
 221   const Stmt *cond = pred->getTerminatorCondition();
 222
 223   //assert(cond && "CFGBlock's predecessor has a terminator condition");
 224   // The previous assertion is invalid in some cases (eg do/while). Leaving
 225   // reporting of these situations on at the moment to help triage these cases.
 226   if (!cond)
 227     return false;
 228
 229   // Run each of the checks on the conditions
 230   if (containsMacro(cond) || containsEnum(cond)
 231       || containsStaticLocal(cond) || containsBuiltinOffsetOf(cond)
 232       || containsStmt<UnaryExprOrTypeTraitExpr>(cond))
 233     return true;
 234
 235   return false;
 236 }
 237
 238 // Returns true if the given CFGBlock is empty
 239 bool UnreachableCodeChecker::isEmptyCFGBlock(const CFGBlock *CB) {
 240   return CB->getLabel() == 0       // No labels
 241       && CB->size() == 0           // No statements
 242       && CB->getTerminator() == 0; // No terminator
 243 }
 244
 245 void ento::registerUnreachableCodeChecker(CheckerManager &mgr) {
 246   mgr.registerChecker<UnreachableCodeChecker>();
 247 }