1 //==- UnreachableCodeChecker.cpp - Generalized dead code checker -*- C++ -*-==//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
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.
13 // A similar flow-sensitive only check exists in Analysis/ReachableCode.cpp
14 //===----------------------------------------------------------------------===//
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"
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
33 using namespace clang;
37 class UnreachableCodeChecker : public Checker<check::EndAnalysis> {
39 void checkEndAnalysis(ExplodedGraph &G, BugReporter &B,
40 ExprEngine &Eng) const;
42 typedef llvm::SmallSet<unsigned, DEFAULT_CFGBLOCKS> CFGBlocksSet;
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);
53 void UnreachableCodeChecker::checkEndAnalysis(ExplodedGraph &G,
55 ExprEngine &Eng) const {
56 CFGBlocksSet reachable, visited;
58 if (Eng.hasWorkRemaining())
64 const LocationContext *LC = 0;
65 // Iterate over ExplodedGraph
66 for (ExplodedGraph::node_iterator I = G.nodes_begin(), E = G.nodes_end();
68 const ProgramPoint &P = I->getLocation();
69 LC = P.getLocationContext();
70 if (!LC->inTopFrame())
74 D = LC->getAnalysisDeclContext()->getDecl();
76 // Save the CFG if we don't have it already
78 C = LC->getAnalysisDeclContext()->getUnoptimizedCFG();
80 PM = &LC->getParentMap();
82 if (Optional<BlockEntrance> BE = P.getAs<BlockEntrance>()) {
83 const CFGBlock *CB = BE->getBlock();
84 reachable.insert(CB->getBlockID());
88 // Bail out if we didn't get the CFG or the ParentMap.
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())
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()))
106 // Check if the block is empty (an artificial block)
107 if (isEmptyCFGBlock(CB))
110 // Find the entry points for this block
111 if (!visited.count(CB->getBlockID()))
112 FindUnreachableEntryPoints(CB, reachable, visited);
114 // This block may have been pruned; check if we still want to report it
115 if (reachable.count(CB->getBlockID()))
118 // Check for false positives
119 if (CB->size() > 0 && isInvalidPath(CB, *PM))
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)
130 // Special case for __builtin_unreachable.
131 // FIXME: This should be extended to include other unreachable markers,
132 // such as llvm_unreachable.
134 bool foundUnreachable = false;
135 for (CFGBlock::const_iterator ci = CB->begin(), ce = CB->end();
137 if (Optional<CFGStmt> S = (*ci).getAs<CFGStmt>())
138 if (const CallExpr *CE = dyn_cast<CallExpr>(S->getStmt())) {
139 if (CE->isBuiltinCall() == Builtin::BI__builtin_unreachable) {
140 foundUnreachable = true;
145 if (foundUnreachable)
149 // We found a block that wasn't covered - find the statement to report
151 PathDiagnosticLocation DL;
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())
163 // Check if the SourceLocation is in a system header
164 const SourceManager &SM = B.getSourceManager();
165 if (SM.isInSystemHeader(SL) || SM.isInExternCSystemHeader(SL))
168 B.EmitBasicReport(D, "Unreachable code", "Dead code",
169 "This statement is never executed", DL, SR);
173 // Recursively finds the entry point(s) for this dead CFGBlock.
174 void UnreachableCodeChecker::FindUnreachableEntryPoints(const CFGBlock *CB,
175 CFGBlocksSet &reachable,
176 CFGBlocksSet &visited) {
177 visited.insert(CB->getBlockID());
179 for (CFGBlock::const_pred_iterator I = CB->pred_begin(), E = CB->pred_end();
181 if (!reachable.count((*I)->getBlockID())) {
182 // If we find an unreachable predecessor, mark this block as reachable so
183 // we don't report this block
184 reachable.insert(CB->getBlockID());
185 if (!visited.count((*I)->getBlockID()))
186 // If we haven't previously visited the unreachable predecessor, recurse
187 FindUnreachableEntryPoints(*I, reachable, visited);
192 // Find the Stmt* in a CFGBlock for reporting a warning
193 const Stmt *UnreachableCodeChecker::getUnreachableStmt(const CFGBlock *CB) {
194 for (CFGBlock::const_iterator I = CB->begin(), E = CB->end(); I != E; ++I) {
195 if (Optional<CFGStmt> S = I->getAs<CFGStmt>())
198 if (const Stmt *S = CB->getTerminator())
204 // Determines if the path to this CFGBlock contained an element that infers this
205 // block is a false positive. We assume that FindUnreachableEntryPoints has
206 // already marked only the entry points to any dead code, so we need only to
207 // find the condition that led to this block (the predecessor of this block.)
208 // There will never be more than one predecessor.
209 bool UnreachableCodeChecker::isInvalidPath(const CFGBlock *CB,
210 const ParentMap &PM) {
211 // We only expect a predecessor size of 0 or 1. If it is >1, then an external
212 // condition has broken our assumption (for example, a sink being placed by
213 // another check). In these cases, we choose not to report.
214 if (CB->pred_size() > 1)
217 // If there are no predecessors, then this block is trivially unreachable
218 if (CB->pred_size() == 0)
221 const CFGBlock *pred = *CB->pred_begin();
223 // Get the predecessor block's terminator conditon
224 const Stmt *cond = pred->getTerminatorCondition();
226 //assert(cond && "CFGBlock's predecessor has a terminator condition");
227 // The previous assertion is invalid in some cases (eg do/while). Leaving
228 // reporting of these situations on at the moment to help triage these cases.
232 // Run each of the checks on the conditions
233 if (containsMacro(cond) || containsEnum(cond)
234 || containsStaticLocal(cond) || containsBuiltinOffsetOf(cond)
235 || containsStmt<UnaryExprOrTypeTraitExpr>(cond))
241 // Returns true if the given CFGBlock is empty
242 bool UnreachableCodeChecker::isEmptyCFGBlock(const CFGBlock *CB) {
243 return CB->getLabel() == 0 // No labels
244 && CB->size() == 0 // No statements
245 && CB->getTerminator() == 0; // No terminator
248 void ento::registerUnreachableCodeChecker(CheckerManager &mgr) {
249 mgr.registerChecker<UnreachableCodeChecker>();