1//==- UnreachableCodeChecker.cpp - Generalized dead code checker -*- C++ -*-==//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8// This file implements a generalized unreachable code checker using a
9// path-sensitive analysis. We mark any path visited, and then walk the CFG as a
10// post-analysis to determine what was never visited.
11//
12// A similar flow-sensitive only check exists in Analysis/ReachableCode.cpp
13//===----------------------------------------------------------------------===//
14
15#include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
16#include "clang/AST/ParentMap.h"
17#include "clang/Basic/Builtins.h"
18#include "clang/Basic/SourceManager.h"
19#include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.h"
20#include "clang/StaticAnalyzer/Core/Checker.h"
21#include "clang/StaticAnalyzer/Core/CheckerManager.h"
22#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
23#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerHelpers.h"
24#include "clang/StaticAnalyzer/Core/PathSensitive/ExplodedGraph.h"
25#include "clang/StaticAnalyzer/Core/PathSensitive/SVals.h"
26#include "llvm/ADT/SmallSet.h"
27#include <optional>
28
29using namespace clang;
30using namespace ento;
31
32namespace {
33class UnreachableCodeChecker : public Checker<check::EndAnalysis> {
34public:
35 void checkEndAnalysis(ExplodedGraph &G, BugReporter &B,
36 ExprEngine &Eng) const;
37private:
38 typedef llvm::SmallSet<unsigned, 32> CFGBlocksSet;
39
40 static inline const Stmt *getUnreachableStmt(const CFGBlock *CB);
41 static void FindUnreachableEntryPoints(const CFGBlock *CB,
42 CFGBlocksSet &reachable,
43 CFGBlocksSet &visited);
44 static bool isInvalidPath(const CFGBlock *CB, const ParentMap &PM);
45 static inline bool isEmptyCFGBlock(const CFGBlock *CB);
46};
47}
48
49void UnreachableCodeChecker::checkEndAnalysis(ExplodedGraph &G,
50 BugReporter &B,
51 ExprEngine &Eng) const {
52 CFGBlocksSet reachable, visited;
53
54 if (Eng.hasWorkRemaining())
55 return;
56
57 const Decl *D = nullptr;
58 CFG *C = nullptr;
59 const ParentMap *PM = nullptr;
60 const LocationContext *LC = nullptr;
61 // Iterate over ExplodedGraph
62 for (const ExplodedNode &N : G.nodes()) {
63 const ProgramPoint &P = N.getLocation();
64 LC = P.getLocationContext();
65 if (!LC->inTopFrame())
66 continue;
67
68 if (!D)
69 D = LC->getAnalysisDeclContext()->getDecl();
70
71 // Save the CFG if we don't have it already
72 if (!C)
73 C = LC->getAnalysisDeclContext()->getUnoptimizedCFG();
74 if (!PM)
75 PM = &LC->getParentMap();
76
77 if (std::optional<BlockEntrance> BE = P.getAs<BlockEntrance>()) {
78 const CFGBlock *CB = BE->getBlock();
79 reachable.insert(V: CB->getBlockID());
80 }
81 }
82
83 // Bail out if we didn't get the CFG or the ParentMap.
84 if (!D || !C || !PM)
85 return;
86
87 // Don't do anything for template instantiations. Proving that code
88 // in a template instantiation is unreachable means proving that it is
89 // unreachable in all instantiations.
90 if (const FunctionDecl *FD = dyn_cast<FunctionDecl>(Val: D))
91 if (FD->isTemplateInstantiation())
92 return;
93
94 // Find CFGBlocks that were not covered by any node
95 for (const CFGBlock *CB : *C) {
96 // Check if the block is unreachable
97 if (reachable.count(V: CB->getBlockID()))
98 continue;
99
100 // Check if the block is empty (an artificial block)
101 if (isEmptyCFGBlock(CB))
102 continue;
103
104 // Find the entry points for this block
105 if (!visited.count(V: CB->getBlockID()))
106 FindUnreachableEntryPoints(CB, reachable, visited);
107
108 // This block may have been pruned; check if we still want to report it
109 if (reachable.count(V: CB->getBlockID()))
110 continue;
111
112 // Check for false positives
113 if (isInvalidPath(CB, PM: *PM))
114 continue;
115
116 // It is good practice to always have a "default" label in a "switch", even
117 // if we should never get there. It can be used to detect errors, for
118 // instance. Unreachable code directly under a "default" label is therefore
119 // likely to be a false positive.
120 if (const Stmt *label = CB->getLabel())
121 if (label->getStmtClass() == Stmt::DefaultStmtClass)
122 continue;
123
124 // Special case for __builtin_unreachable.
125 // FIXME: This should be extended to include other unreachable markers,
126 // such as llvm_unreachable.
127 if (!CB->empty()) {
128 bool foundUnreachable = false;
129 for (CFGBlock::const_iterator ci = CB->begin(), ce = CB->end();
130 ci != ce; ++ci) {
131 if (std::optional<CFGStmt> S = (*ci).getAs<CFGStmt>())
132 if (const CallExpr *CE = dyn_cast<CallExpr>(Val: S->getStmt())) {
133 if (CE->getBuiltinCallee() == Builtin::BI__builtin_unreachable ||
134 CE->isBuiltinAssumeFalse(Ctx: Eng.getContext())) {
135 foundUnreachable = true;
136 break;
137 }
138 }
139 }
140 if (foundUnreachable)
141 continue;
142 }
143
144 // We found a block that wasn't covered - find the statement to report
145 SourceRange SR;
146 PathDiagnosticLocation DL;
147 SourceLocation SL;
148 if (const Stmt *S = getUnreachableStmt(CB)) {
149 // In macros, 'do {...} while (0)' is often used. Don't warn about the
150 // condition 0 when it is unreachable.
151 if (S->getBeginLoc().isMacroID())
152 if (const auto *I = dyn_cast<IntegerLiteral>(Val: S))
153 if (I->getValue() == 0ULL)
154 if (const Stmt *Parent = PM->getParent(S))
155 if (isa<DoStmt>(Val: Parent))
156 continue;
157 SR = S->getSourceRange();
158 DL = PathDiagnosticLocation::createBegin(S, SM: B.getSourceManager(), LAC: LC);
159 SL = DL.asLocation();
160 if (SR.isInvalid() || !SL.isValid())
161 continue;
162 if (isa<CXXTryStmt>(Val: S))
163 continue;
164 }
165 else
166 continue;
167
168 // Check if the SourceLocation is in a system header
169 const SourceManager &SM = B.getSourceManager();
170 if (SM.isInSystemHeader(Loc: SL) || SM.isInExternCSystemHeader(Loc: SL))
171 continue;
172
173 B.EmitBasicReport(DeclWithIssue: D, Checker: this, BugName: "Unreachable code", BugCategory: categories::UnusedCode,
174 BugStr: "This statement is never executed", Loc: DL, Ranges: SR);
175 }
176}
177
178// Recursively finds the entry point(s) for this dead CFGBlock.
179void UnreachableCodeChecker::FindUnreachableEntryPoints(const CFGBlock *CB,
180 CFGBlocksSet &reachable,
181 CFGBlocksSet &visited) {
182 visited.insert(V: CB->getBlockID());
183
184 for (const CFGBlock *PredBlock : CB->preds()) {
185 if (!PredBlock)
186 continue;
187
188 if (!reachable.count(V: PredBlock->getBlockID())) {
189 // If we find an unreachable predecessor, mark this block as reachable so
190 // we don't report this block
191 reachable.insert(V: CB->getBlockID());
192 if (!visited.count(V: PredBlock->getBlockID()))
193 // If we haven't previously visited the unreachable predecessor, recurse
194 FindUnreachableEntryPoints(CB: PredBlock, reachable, visited);
195 }
196 }
197}
198
199// Find the Stmt* in a CFGBlock for reporting a warning
200const Stmt *UnreachableCodeChecker::getUnreachableStmt(const CFGBlock *CB) {
201 for (const CFGElement &Elem : *CB) {
202 if (std::optional<CFGStmt> S = Elem.getAs<CFGStmt>()) {
203 if (!isa<DeclStmt>(Val: S->getStmt()))
204 return S->getStmt();
205 }
206 }
207 return CB->getTerminatorStmt();
208}
209
210// Determines if the path to this CFGBlock contained an element that infers this
211// block is a false positive. We assume that FindUnreachableEntryPoints has
212// already marked only the entry points to any dead code, so we need only to
213// find the condition that led to this block (the predecessor of this block.)
214// There will never be more than one predecessor.
215bool UnreachableCodeChecker::isInvalidPath(const CFGBlock *CB,
216 const ParentMap &PM) {
217 // We only expect a predecessor size of 0 or 1. If it is >1, then an external
218 // condition has broken our assumption (for example, a sink being placed by
219 // another check). In these cases, we choose not to report.
220 if (CB->pred_size() > 1)
221 return true;
222
223 // If there are no predecessors, then this block is trivially unreachable
224 if (CB->pred_size() == 0)
225 return false;
226
227 const CFGBlock *pred = *CB->pred_begin();
228 if (!pred)
229 return false;
230
231 // Get the predecessor block's terminator condition
232 const Stmt *cond = pred->getTerminatorCondition();
233
234 //assert(cond && "CFGBlock's predecessor has a terminator condition");
235 // The previous assertion is invalid in some cases (eg do/while). Leaving
236 // reporting of these situations on at the moment to help triage these cases.
237 if (!cond)
238 return false;
239
240 // Run each of the checks on the conditions
241 return containsMacro(S: cond) || containsEnum(S: cond) ||
242 containsStaticLocal(S: cond) || containsBuiltinOffsetOf(S: cond) ||
243 containsStmt<UnaryExprOrTypeTraitExpr>(S: cond);
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->getTerminatorStmt(); // No terminator
251}
252
253void ento::registerUnreachableCodeChecker(CheckerManager &mgr) {
254 mgr.registerChecker<UnreachableCodeChecker>();
255}
256
257bool ento::shouldRegisterUnreachableCodeChecker(const CheckerManager &mgr) {
258 return true;
259}
260