UnreachableCodeChecker.cpp revision 28cd22d7c2d2458575ce9cc19dfe63c6321010ce
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 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("Unreachable code", "Dead code", "This statement is never" 166 " executed", DL, SR); 167 } 168} 169 170// Recursively finds the entry point(s) for this dead CFGBlock. 171void 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 190const 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. 206bool 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 239bool 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 245void ento::registerUnreachableCodeChecker(CheckerManager &mgr) { 246 mgr.registerChecker<UnreachableCodeChecker>(); 247} 248