1//===--- NoRecursionCheck.cpp - clang-tidy --------------------------------===//
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
9#include "NoRecursionCheck.h"
10#include "clang/AST/ASTContext.h"
11#include "clang/ASTMatchers/ASTMatchFinder.h"
12#include "clang/Analysis/CallGraph.h"
13#include "llvm/ADT/SCCIterator.h"
14
15using namespace clang::ast_matchers;
16
17namespace clang::tidy::misc {
18
19namespace {
20
21/// Much like SmallSet, with two differences:
22/// 1. It can *only* be constructed from an ArrayRef<>. If the element count
23/// is small, there is no copy and said storage *must* outlive us.
24/// 2. it is immutable, the way it was constructed it will stay.
25template <typename T, unsigned SmallSize> class ImmutableSmallSet {
26 ArrayRef<T> Vector;
27 llvm::DenseSet<T> Set;
28
29 static_assert(SmallSize <= 32, "N should be small");
30
31 bool isSmall() const { return Set.empty(); }
32
33public:
34 using size_type = size_t;
35
36 ImmutableSmallSet() = delete;
37 ImmutableSmallSet(const ImmutableSmallSet &) = delete;
38 ImmutableSmallSet(ImmutableSmallSet &&) = delete;
39 T &operator=(const ImmutableSmallSet &) = delete;
40 T &operator=(ImmutableSmallSet &&) = delete;
41
42 // WARNING: Storage *must* outlive us if we decide that the size is small.
43 ImmutableSmallSet(ArrayRef<T> Storage) {
44 // Is size small-enough to just keep using the existing storage?
45 if (Storage.size() <= SmallSize) {
46 Vector = Storage;
47 return;
48 }
49
50 // We've decided that it isn't performant to keep using vector.
51 // Let's migrate the data into Set.
52 Set.reserve(Storage.size());
53 Set.insert_range(Storage);
54 }
55
56 /// count - Return 1 if the element is in the set, 0 otherwise.
57 size_type count(const T &V) const {
58 if (isSmall()) {
59 // Since the collection is small, just do a linear search.
60 return llvm::is_contained(Vector, V) ? 1 : 0;
61 }
62
63 return Set.count(V);
64 }
65};
66
67/// Much like SmallSetVector, but with one difference:
68/// when the size is \p SmallSize or less, when checking whether an element is
69/// already in the set or not, we perform linear search over the vector,
70/// but if the size is larger than \p SmallSize, we look in set.
71/// FIXME: upstream this into SetVector/SmallSetVector itself.
72template <typename T, unsigned SmallSize> class SmartSmallSetVector {
73public:
74 using size_type = size_t;
75
76private:
77 SmallVector<T, SmallSize> Vector;
78 llvm::DenseSet<T> Set;
79
80 static_assert(SmallSize <= 32, "N should be small");
81
82 // Are we still using Vector for uniqness tracking?
83 bool isSmall() const { return Set.empty(); }
84
85 // Will one more entry cause Vector to switch away from small-size storage?
86 bool entiretyOfVectorSmallSizeIsOccupied() const {
87 assert(isSmall() && Vector.size() <= SmallSize &&
88 "Shouldn't ask if we have already [should have] migrated into Set.");
89 return Vector.size() == SmallSize;
90 }
91
92 void populateSet() {
93 assert(Set.empty() && "Should not have already utilized the Set.");
94 // Magical growth factor prediction - to how many elements do we expect to
95 // sanely grow after switching away from small-size storage?
96 const size_t NewMaxElts = 4 * Vector.size();
97 Vector.reserve(NewMaxElts);
98 Set.reserve(NewMaxElts);
99 Set.insert_range(Vector);
100 }
101
102 /// count - Return 1 if the element is in the set, 0 otherwise.
103 size_type count(const T &V) const {
104 if (isSmall()) {
105 // Since the collection is small, just do a linear search.
106 return llvm::is_contained(Vector, V) ? 1 : 0;
107 }
108 // Look-up in the Set.
109 return Set.count(V);
110 }
111
112 bool setInsert(const T &V) {
113 if (count(V) != 0)
114 return false; // Already exists.
115 // Does not exist, Can/need to record it.
116 if (isSmall()) { // Are we still using Vector for uniqness tracking?
117 // Will one more entry fit within small-sized Vector?
118 if (!entiretyOfVectorSmallSizeIsOccupied())
119 return true; // We'll insert into vector right afterwards anyway.
120 // Time to switch to Set.
121 populateSet();
122 }
123 // Set time!
124 // Note that this must be after `populateSet()` might have been called.
125 bool SetInsertionSucceeded = Set.insert(V).second;
126 (void)SetInsertionSucceeded;
127 assert(SetInsertionSucceeded && "We did check that no such value existed");
128 return true;
129 }
130
131public:
132 /// Insert a new element into the SmartSmallSetVector.
133 /// \returns true if the element was inserted into the SmartSmallSetVector.
134 bool insert(const T &X) {
135 bool Result = setInsert(X);
136 if (Result)
137 Vector.push_back(X);
138 return Result;
139 }
140
141 /// Clear the SmartSmallSetVector and return the underlying vector.
142 decltype(Vector) takeVector() {
143 Set.clear();
144 return std::move(Vector);
145 }
146};
147
148constexpr unsigned SmallCallStackSize = 16;
149constexpr unsigned SmallSCCSize = 32;
150
151using CallStackTy =
152 llvm::SmallVector<CallGraphNode::CallRecord, SmallCallStackSize>;
153
154// In given SCC, find *some* call stack that will be cyclic.
155// This will only find *one* such stack, it might not be the smallest one,
156// and there may be other loops.
157CallStackTy pathfindSomeCycle(ArrayRef<CallGraphNode *> SCC) {
158 // We'll need to be able to performantly look up whether some CallGraphNode
159 // is in SCC or not, so cache all the SCC elements in a set.
160 const ImmutableSmallSet<CallGraphNode *, SmallSCCSize> SCCElts(SCC);
161
162 // Is node N part if the current SCC?
163 auto NodeIsPartOfSCC = [&SCCElts](CallGraphNode *N) {
164 return SCCElts.count(V: N) != 0;
165 };
166
167 // Track the call stack that will cause a cycle.
168 SmartSmallSetVector<CallGraphNode::CallRecord, SmallCallStackSize>
169 CallStackSet;
170
171 // Arbitrarily take the first element of SCC as entry point.
172 CallGraphNode::CallRecord EntryNode(SCC.front(), /*CallExpr=*/nullptr);
173 // Continue recursing into subsequent callees that are part of this SCC,
174 // and are thus known to be part of the call graph loop, until loop forms.
175 CallGraphNode::CallRecord *Node = &EntryNode;
176 while (true) {
177 // Did we see this node before?
178 if (!CallStackSet.insert(X: *Node))
179 break; // Cycle completed! Note that didn't insert the node into stack!
180 // Else, perform depth-first traversal: out of all callees, pick first one
181 // that is part of this SCC. This is not guaranteed to yield shortest cycle.
182 Node = llvm::find_if(Range: Node->Callee->callees(), P: NodeIsPartOfSCC);
183 }
184
185 // Note that we failed to insert the last node, that completes the cycle.
186 // But we really want to have it. So insert it manually into stack only.
187 CallStackTy CallStack = CallStackSet.takeVector();
188 CallStack.emplace_back(Args&: *Node);
189
190 return CallStack;
191}
192
193} // namespace
194
195void NoRecursionCheck::registerMatchers(MatchFinder *Finder) {
196 Finder->addMatcher(NodeMatch: translationUnitDecl().bind(ID: "TUDecl"), Action: this);
197}
198
199void NoRecursionCheck::handleSCC(ArrayRef<CallGraphNode *> SCC) {
200 assert(!SCC.empty() && "Empty SCC does not make sense.");
201
202 // First of all, call out every strongly connected function.
203 for (CallGraphNode *N : SCC) {
204 FunctionDecl *D = N->getDefinition();
205 diag(D->getLocation(), "function %0 is within a recursive call chain") << D;
206 }
207
208 // Now, SCC only tells us about strongly connected function declarations in
209 // the call graph. It doesn't *really* tell us about the cycles they form.
210 // And there may be more than one cycle in SCC.
211 // So let's form a call stack that eventually exposes *some* cycle.
212 const CallStackTy EventuallyCyclicCallStack = pathfindSomeCycle(SCC);
213 assert(!EventuallyCyclicCallStack.empty() && "We should've found the cycle");
214
215 // While last node of the call stack does cause a loop, due to the way we
216 // pathfind the cycle, the loop does not necessarily begin at the first node
217 // of the call stack, so drop front nodes of the call stack until it does.
218 const auto CyclicCallStack =
219 ArrayRef<CallGraphNode::CallRecord>(EventuallyCyclicCallStack)
220 .drop_until(Pred: [LastNode = EventuallyCyclicCallStack.back()](
221 CallGraphNode::CallRecord FrontNode) {
222 return FrontNode == LastNode;
223 });
224 assert(CyclicCallStack.size() >= 2 && "Cycle requires at least 2 frames");
225
226 // Which function we decided to be the entry point that lead to the recursion?
227 FunctionDecl *CycleEntryFn = CyclicCallStack.front().Callee->getDefinition();
228 // And now, for ease of understanding, let's print the call sequence that
229 // forms the cycle in question.
230 diag(CycleEntryFn->getLocation(),
231 "example recursive call chain, starting from function %0",
232 DiagnosticIDs::Note)
233 << CycleEntryFn;
234 for (int CurFrame = 1, NumFrames = CyclicCallStack.size();
235 CurFrame != NumFrames; ++CurFrame) {
236 CallGraphNode::CallRecord PrevNode = CyclicCallStack[CurFrame - 1];
237 CallGraphNode::CallRecord CurrNode = CyclicCallStack[CurFrame];
238
239 Decl *PrevDecl = PrevNode.Callee->getDecl();
240 Decl *CurrDecl = CurrNode.Callee->getDecl();
241
242 diag(CurrNode.CallExpr->getBeginLoc(),
243 "Frame #%0: function %1 calls function %2 here:", DiagnosticIDs::Note)
244 << CurFrame << cast<NamedDecl>(Val: PrevDecl) << cast<NamedDecl>(Val: CurrDecl);
245 }
246
247 diag(CyclicCallStack.back().CallExpr->getBeginLoc(),
248 "... which was the starting point of the recursive call chain; there "
249 "may be other cycles",
250 DiagnosticIDs::Note);
251}
252
253void NoRecursionCheck::check(const MatchFinder::MatchResult &Result) {
254 // Build call graph for the entire translation unit.
255 const auto *TU = Result.Nodes.getNodeAs<TranslationUnitDecl>(ID: "TUDecl");
256 CallGraph CG;
257 CG.addToCallGraph(const_cast<TranslationUnitDecl *>(TU));
258
259 // Look for cycles in call graph,
260 // by looking for Strongly Connected Components (SCC's)
261 for (llvm::scc_iterator<CallGraph *> SCCI = llvm::scc_begin(G: &CG),
262 SCCE = llvm::scc_end(G: &CG);
263 SCCI != SCCE; ++SCCI) {
264 if (!SCCI.hasCycle()) // We only care about cycles, not standalone nodes.
265 continue;
266 handleSCC(SCC: *SCCI);
267 }
268}
269
270} // namespace clang::tidy::misc
271

source code of clang-tools-extra/clang-tidy/misc/NoRecursionCheck.cpp