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/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"
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())
63 // Iterate over ExplodedGraph
64 for (ExplodedGraph::node_iterator I = G.nodes_begin(), E = G.nodes_end();
66 const ProgramPoint &P = I->getLocation();
67 const LocationContext *LC = P.getLocationContext();
69 // Save the CFG if we don't have it already
71 C = LC->getAnalysisContext()->getUnoptimizedCFG();
73 PM = &LC->getParentMap();
75 if (const BlockEntrance *BE = dyn_cast<BlockEntrance>(&P)) {
76 const CFGBlock *CB = BE->getBlock();
77 reachable.insert(CB->getBlockID());
81 // Bail out if we didn't get the CFG or the ParentMap.
85 ASTContext &Ctx = B.getContext();
87 // Find CFGBlocks that were not covered by any node
88 for (CFG::const_iterator I = C->begin(), E = C->end(); I != E; ++I) {
89 const CFGBlock *CB = *I;
90 // Check if the block is unreachable
91 if (reachable.count(CB->getBlockID()))
94 // Check if the block is empty (an artificial block)
95 if (isEmptyCFGBlock(CB))
98 // Find the entry points for this block
99 if (!visited.count(CB->getBlockID()))
100 FindUnreachableEntryPoints(CB, reachable, visited);
102 // This block may have been pruned; check if we still want to report it
103 if (reachable.count(CB->getBlockID()))
106 // Check for false positives
107 if (CB->size() > 0 && isInvalidPath(CB, *PM))
110 // Special case for __builtin_unreachable.
111 // FIXME: This should be extended to include other unreachable markers,
112 // such as llvm_unreachable.
114 CFGElement First = CB->front();
115 if (const CFGStmt *S = First.getAs<CFGStmt>()) {
116 if (const CallExpr *CE = dyn_cast<CallExpr>(S->getStmt())) {
117 if (CE->isBuiltinCall(Ctx) == Builtin::BI__builtin_unreachable)
123 // We found a block that wasn't covered - find the statement to report
126 if (const Stmt *S = getUnreachableStmt(CB)) {
127 SR = S->getSourceRange();
128 SL = S->getLocStart();
129 if (SR.isInvalid() || SL.isInvalid())
135 // Check if the SourceLocation is in a system header
136 const SourceManager &SM = B.getSourceManager();
137 if (SM.isInSystemHeader(SL) || SM.isInExternCSystemHeader(SL))
140 B.EmitBasicReport("Unreachable code", "Dead code", "This statement is never"
141 " executed", SL, SR);
145 // Recursively finds the entry point(s) for this dead CFGBlock.
146 void UnreachableCodeChecker::FindUnreachableEntryPoints(const CFGBlock *CB,
147 CFGBlocksSet &reachable,
148 CFGBlocksSet &visited) {
149 visited.insert(CB->getBlockID());
151 for (CFGBlock::const_pred_iterator I = CB->pred_begin(), E = CB->pred_end();
153 if (!reachable.count((*I)->getBlockID())) {
154 // If we find an unreachable predecessor, mark this block as reachable so
155 // we don't report this block
156 reachable.insert(CB->getBlockID());
157 if (!visited.count((*I)->getBlockID()))
158 // If we haven't previously visited the unreachable predecessor, recurse
159 FindUnreachableEntryPoints(*I, reachable, visited);
164 // Find the Stmt* in a CFGBlock for reporting a warning
165 const Stmt *UnreachableCodeChecker::getUnreachableStmt(const CFGBlock *CB) {
166 for (CFGBlock::const_iterator I = CB->begin(), E = CB->end(); I != E; ++I) {
167 if (const CFGStmt *S = I->getAs<CFGStmt>())
170 if (const Stmt *S = CB->getTerminator())
176 // Determines if the path to this CFGBlock contained an element that infers this
177 // block is a false positive. We assume that FindUnreachableEntryPoints has
178 // already marked only the entry points to any dead code, so we need only to
179 // find the condition that led to this block (the predecessor of this block.)
180 // There will never be more than one predecessor.
181 bool UnreachableCodeChecker::isInvalidPath(const CFGBlock *CB,
182 const ParentMap &PM) {
183 // We only expect a predecessor size of 0 or 1. If it is >1, then an external
184 // condition has broken our assumption (for example, a sink being placed by
185 // another check). In these cases, we choose not to report.
186 if (CB->pred_size() > 1)
189 // If there are no predecessors, then this block is trivially unreachable
190 if (CB->pred_size() == 0)
193 const CFGBlock *pred = *CB->pred_begin();
195 // Get the predecessor block's terminator conditon
196 const Stmt *cond = pred->getTerminatorCondition();
198 //assert(cond && "CFGBlock's predecessor has a terminator condition");
199 // The previous assertion is invalid in some cases (eg do/while). Leaving
200 // reporting of these situations on at the moment to help triage these cases.
204 // Run each of the checks on the conditions
205 if (containsMacro(cond) || containsEnum(cond)
206 || containsStaticLocal(cond) || containsBuiltinOffsetOf(cond)
207 || containsStmt<UnaryExprOrTypeTraitExpr>(cond))
213 // Returns true if the given CFGBlock is empty
214 bool UnreachableCodeChecker::isEmptyCFGBlock(const CFGBlock *CB) {
215 return CB->getLabel() == 0 // No labels
216 && CB->size() == 0 // No statements
217 && CB->getTerminator() == 0; // No terminator
220 void ento::registerUnreachableCodeChecker(CheckerManager &mgr) {
221 mgr.registerChecker<UnreachableCodeChecker>();