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/AST/ParentMap.h" 18#include "clang/Basic/Builtins.h" 19#include "clang/Basic/SourceManager.h" 20#include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.h" 21#include "clang/StaticAnalyzer/Core/Checker.h" 22#include "clang/StaticAnalyzer/Core/CheckerManager.h" 23#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h" 24#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerHelpers.h" 25#include "clang/StaticAnalyzer/Core/PathSensitive/ExplodedGraph.h" 26#include "clang/StaticAnalyzer/Core/PathSensitive/SVals.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 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 = nullptr; 62 CFG *C = nullptr; 63 ParentMap *PM = nullptr; 64 const LocationContext *LC = nullptr; 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 if (!LC->inTopFrame()) 71 continue; 72 73 if (!D) 74 D = LC->getAnalysisDeclContext()->getDecl(); 75 76 // Save the CFG if we don't have it already 77 if (!C) 78 C = LC->getAnalysisDeclContext()->getUnoptimizedCFG(); 79 if (!PM) 80 PM = &LC->getParentMap(); 81 82 if (Optional<BlockEntrance> BE = P.getAs<BlockEntrance>()) { 83 const CFGBlock *CB = BE->getBlock(); 84 reachable.insert(CB->getBlockID()); 85 } 86 } 87 88 // Bail out if we didn't get the CFG or the ParentMap. 89 if (!D || !C || !PM) 90 return; 91 92 // Don't do anything for template instantiations. Proving that code 93 // in a template instantiation is unreachable means proving that it is 94 // unreachable in all instantiations. 95 if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(D)) 96 if (FD->isTemplateInstantiation()) 97 return; 98 99 // Find CFGBlocks that were not covered by any node 100 for (CFG::const_iterator I = C->begin(), E = C->end(); I != E; ++I) { 101 const CFGBlock *CB = *I; 102 // Check if the block is unreachable 103 if (reachable.count(CB->getBlockID())) 104 continue; 105 106 // Check if the block is empty (an artificial block) 107 if (isEmptyCFGBlock(CB)) 108 continue; 109 110 // Find the entry points for this block 111 if (!visited.count(CB->getBlockID())) 112 FindUnreachableEntryPoints(CB, reachable, visited); 113 114 // This block may have been pruned; check if we still want to report it 115 if (reachable.count(CB->getBlockID())) 116 continue; 117 118 // Check for false positives 119 if (CB->size() > 0 && isInvalidPath(CB, *PM)) 120 continue; 121 122 // It is good practice to always have a "default" label in a "switch", even 123 // if we should never get there. It can be used to detect errors, for 124 // instance. Unreachable code directly under a "default" label is therefore 125 // likely to be a false positive. 126 if (const Stmt *label = CB->getLabel()) 127 if (label->getStmtClass() == Stmt::DefaultStmtClass) 128 continue; 129 130 // Special case for __builtin_unreachable. 131 // FIXME: This should be extended to include other unreachable markers, 132 // such as llvm_unreachable. 133 if (!CB->empty()) { 134 bool foundUnreachable = false; 135 for (CFGBlock::const_iterator ci = CB->begin(), ce = CB->end(); 136 ci != ce; ++ci) { 137 if (Optional<CFGStmt> S = (*ci).getAs<CFGStmt>()) 138 if (const CallExpr *CE = dyn_cast<CallExpr>(S->getStmt())) { 139 if (CE->getBuiltinCallee() == Builtin::BI__builtin_unreachable) { 140 foundUnreachable = true; 141 break; 142 } 143 } 144 } 145 if (foundUnreachable) 146 continue; 147 } 148 149 // We found a block that wasn't covered - find the statement to report 150 SourceRange SR; 151 PathDiagnosticLocation DL; 152 SourceLocation SL; 153 if (const Stmt *S = getUnreachableStmt(CB)) { 154 SR = S->getSourceRange(); 155 DL = PathDiagnosticLocation::createBegin(S, B.getSourceManager(), LC); 156 SL = DL.asLocation(); 157 if (SR.isInvalid() || !SL.isValid()) 158 continue; 159 } 160 else 161 continue; 162 163 // Check if the SourceLocation is in a system header 164 const SourceManager &SM = B.getSourceManager(); 165 if (SM.isInSystemHeader(SL) || SM.isInExternCSystemHeader(SL)) 166 continue; 167 168 B.EmitBasicReport(D, this, "Unreachable code", "Dead code", 169 "This statement is never executed", DL, SR); 170 } 171} 172 173// Recursively finds the entry point(s) for this dead CFGBlock. 174void UnreachableCodeChecker::FindUnreachableEntryPoints(const CFGBlock *CB, 175 CFGBlocksSet &reachable, 176 CFGBlocksSet &visited) { 177 visited.insert(CB->getBlockID()); 178 179 for (CFGBlock::const_pred_iterator I = CB->pred_begin(), E = CB->pred_end(); 180 I != E; ++I) { 181 if (!*I) 182 continue; 183 184 if (!reachable.count((*I)->getBlockID())) { 185 // If we find an unreachable predecessor, mark this block as reachable so 186 // we don't report this block 187 reachable.insert(CB->getBlockID()); 188 if (!visited.count((*I)->getBlockID())) 189 // If we haven't previously visited the unreachable predecessor, recurse 190 FindUnreachableEntryPoints(*I, reachable, visited); 191 } 192 } 193} 194 195// Find the Stmt* in a CFGBlock for reporting a warning 196const Stmt *UnreachableCodeChecker::getUnreachableStmt(const CFGBlock *CB) { 197 for (CFGBlock::const_iterator I = CB->begin(), E = CB->end(); I != E; ++I) { 198 if (Optional<CFGStmt> S = I->getAs<CFGStmt>()) 199 return S->getStmt(); 200 } 201 if (const Stmt *S = CB->getTerminator()) 202 return S; 203 else 204 return nullptr; 205} 206 207// Determines if the path to this CFGBlock contained an element that infers this 208// block is a false positive. We assume that FindUnreachableEntryPoints has 209// already marked only the entry points to any dead code, so we need only to 210// find the condition that led to this block (the predecessor of this block.) 211// There will never be more than one predecessor. 212bool UnreachableCodeChecker::isInvalidPath(const CFGBlock *CB, 213 const ParentMap &PM) { 214 // We only expect a predecessor size of 0 or 1. If it is >1, then an external 215 // condition has broken our assumption (for example, a sink being placed by 216 // another check). In these cases, we choose not to report. 217 if (CB->pred_size() > 1) 218 return true; 219 220 // If there are no predecessors, then this block is trivially unreachable 221 if (CB->pred_size() == 0) 222 return false; 223 224 const CFGBlock *pred = *CB->pred_begin(); 225 if (!pred) 226 return false; 227 228 // Get the predecessor block's terminator conditon 229 const Stmt *cond = pred->getTerminatorCondition(); 230 231 //assert(cond && "CFGBlock's predecessor has a terminator condition"); 232 // The previous assertion is invalid in some cases (eg do/while). Leaving 233 // reporting of these situations on at the moment to help triage these cases. 234 if (!cond) 235 return false; 236 237 // Run each of the checks on the conditions 238 if (containsMacro(cond) || containsEnum(cond) 239 || containsStaticLocal(cond) || containsBuiltinOffsetOf(cond) 240 || containsStmt<UnaryExprOrTypeTraitExpr>(cond)) 241 return true; 242 243 return false; 244} 245 246// Returns true if the given CFGBlock is empty 247bool UnreachableCodeChecker::isEmptyCFGBlock(const CFGBlock *CB) { 248 return CB->getLabel() == nullptr // No labels 249 && CB->size() == 0 // No statements 250 && !CB->getTerminator(); // No terminator 251} 252 253void ento::registerUnreachableCodeChecker(CheckerManager &mgr) { 254 mgr.registerChecker<UnreachableCodeChecker>(); 255} 256