UnreachableCodeChecker.cpp revision 1d26f48dc2eea1c07431ca1519d7034a21b9bcff
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 ASTContext &Ctx = B.getContext(); 87 88 // Find CFGBlocks that were not covered by any node 89 for (CFG::const_iterator I = C->begin(), E = C->end(); I != E; ++I) { 90 const CFGBlock *CB = *I; 91 // Check if the block is unreachable 92 if (reachable.count(CB->getBlockID())) 93 continue; 94 95 // Check if the block is empty (an artificial block) 96 if (isEmptyCFGBlock(CB)) 97 continue; 98 99 // Find the entry points for this block 100 if (!visited.count(CB->getBlockID())) 101 FindUnreachableEntryPoints(CB, reachable, visited); 102 103 // This block may have been pruned; check if we still want to report it 104 if (reachable.count(CB->getBlockID())) 105 continue; 106 107 // Check for false positives 108 if (CB->size() > 0 && isInvalidPath(CB, *PM)) 109 continue; 110 111 // Special case for __builtin_unreachable. 112 // FIXME: This should be extended to include other unreachable markers, 113 // such as llvm_unreachable. 114 if (!CB->empty()) { 115 bool foundUnreachable = false; 116 for (CFGBlock::const_iterator ci = CB->begin(), ce = CB->end(); 117 ci != ce; ++ci) { 118 if (const CFGStmt *S = (*ci).getAs<CFGStmt>()) 119 if (const CallExpr *CE = dyn_cast<CallExpr>(S->getStmt())) { 120 if (CE->isBuiltinCall(Ctx) == Builtin::BI__builtin_unreachable) { 121 foundUnreachable = true; 122 break; 123 } 124 } 125 } 126 if (foundUnreachable) 127 continue; 128 } 129 130 // We found a block that wasn't covered - find the statement to report 131 SourceRange SR; 132 PathDiagnosticLocation DL; 133 SourceLocation SL; 134 if (const Stmt *S = getUnreachableStmt(CB)) { 135 SR = S->getSourceRange(); 136 DL = PathDiagnosticLocation::createBegin(S, B.getSourceManager(), LC); 137 SL = DL.asLocation(); 138 if (SR.isInvalid() || !SL.isValid()) 139 continue; 140 } 141 else 142 continue; 143 144 // Check if the SourceLocation is in a system header 145 const SourceManager &SM = B.getSourceManager(); 146 if (SM.isInSystemHeader(SL) || SM.isInExternCSystemHeader(SL)) 147 continue; 148 149 B.EmitBasicReport("Unreachable code", "Dead code", "This statement is never" 150 " executed", DL, SR); 151 } 152} 153 154// Recursively finds the entry point(s) for this dead CFGBlock. 155void UnreachableCodeChecker::FindUnreachableEntryPoints(const CFGBlock *CB, 156 CFGBlocksSet &reachable, 157 CFGBlocksSet &visited) { 158 visited.insert(CB->getBlockID()); 159 160 for (CFGBlock::const_pred_iterator I = CB->pred_begin(), E = CB->pred_end(); 161 I != E; ++I) { 162 if (!reachable.count((*I)->getBlockID())) { 163 // If we find an unreachable predecessor, mark this block as reachable so 164 // we don't report this block 165 reachable.insert(CB->getBlockID()); 166 if (!visited.count((*I)->getBlockID())) 167 // If we haven't previously visited the unreachable predecessor, recurse 168 FindUnreachableEntryPoints(*I, reachable, visited); 169 } 170 } 171} 172 173// Find the Stmt* in a CFGBlock for reporting a warning 174const Stmt *UnreachableCodeChecker::getUnreachableStmt(const CFGBlock *CB) { 175 for (CFGBlock::const_iterator I = CB->begin(), E = CB->end(); I != E; ++I) { 176 if (const CFGStmt *S = I->getAs<CFGStmt>()) 177 return S->getStmt(); 178 } 179 if (const Stmt *S = CB->getTerminator()) 180 return S; 181 else 182 return 0; 183} 184 185// Determines if the path to this CFGBlock contained an element that infers this 186// block is a false positive. We assume that FindUnreachableEntryPoints has 187// already marked only the entry points to any dead code, so we need only to 188// find the condition that led to this block (the predecessor of this block.) 189// There will never be more than one predecessor. 190bool UnreachableCodeChecker::isInvalidPath(const CFGBlock *CB, 191 const ParentMap &PM) { 192 // We only expect a predecessor size of 0 or 1. If it is >1, then an external 193 // condition has broken our assumption (for example, a sink being placed by 194 // another check). In these cases, we choose not to report. 195 if (CB->pred_size() > 1) 196 return true; 197 198 // If there are no predecessors, then this block is trivially unreachable 199 if (CB->pred_size() == 0) 200 return false; 201 202 const CFGBlock *pred = *CB->pred_begin(); 203 204 // Get the predecessor block's terminator conditon 205 const Stmt *cond = pred->getTerminatorCondition(); 206 207 //assert(cond && "CFGBlock's predecessor has a terminator condition"); 208 // The previous assertion is invalid in some cases (eg do/while). Leaving 209 // reporting of these situations on at the moment to help triage these cases. 210 if (!cond) 211 return false; 212 213 // Run each of the checks on the conditions 214 if (containsMacro(cond) || containsEnum(cond) 215 || containsStaticLocal(cond) || containsBuiltinOffsetOf(cond) 216 || containsStmt<UnaryExprOrTypeTraitExpr>(cond)) 217 return true; 218 219 return false; 220} 221 222// Returns true if the given CFGBlock is empty 223bool UnreachableCodeChecker::isEmptyCFGBlock(const CFGBlock *CB) { 224 return CB->getLabel() == 0 // No labels 225 && CB->size() == 0 // No statements 226 && CB->getTerminator() == 0; // No terminator 227} 228 229void ento::registerUnreachableCodeChecker(CheckerManager &mgr) { 230 mgr.registerChecker<UnreachableCodeChecker>(); 231} 232