Constexpr recurse

clang-tools-extra/clang-tidy/bsl/BslTidyModule.cpp

@@ -41,6 +41,7 @@
 #include "LiteralsUserDefinedCheck.h"
 #include "NamespaceGlobalCheck.h"
 #include "NodiscardCheck.h"
+#include "NoRecursionCheck.h"
 #include "NonPodClassdefCheck.h"
 #include "NonPodStaticCheck.h"
 #include "OpBitwiseOperandsCheck.h"
@@ -131,6 +132,8 @@ class BslModule : public ClangTidyModule {
         "bsl-namespace-global");
     CheckFactories.registerCheck<NodiscardCheck>(
         "bsl-nodiscard");
+    CheckFactories.registerCheck<NoRecursionCheck>(
+        "bsl-no-recursion");
     CheckFactories.registerCheck<NonPodClassdefCheck>(
         "bsl-non-pod-classdef");
     CheckFactories.registerCheck<NonPodStaticCheck>(

clang-tools-extra/clang-tidy/bsl/CMakeLists.txt

@@ -35,6 +35,7 @@ add_clang_library(clangTidyBslModule
   LiteralsUserDefinedCheck.cpp
   NamespaceGlobalCheck.cpp
   NodiscardCheck.cpp
+  NoRecursionCheck.cpp
   NonPodClassdefCheck.cpp
   NonPodStaticCheck.cpp
   OpBitwiseOperandsCheck.cpp

clang-tools-extra/clang-tidy/bsl/NoRecursionCheck.cpp

@@ -0,0 +1,281 @@
+//===--- NoRecursionCheck.cpp - clang-tidy --------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "NoRecursionCheck.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/Analysis/CallGraph.h"
+#include "llvm/ADT/DenseMapInfo.h"
+#include "llvm/ADT/SCCIterator.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang {
+namespace tidy {
+namespace bsl {
+
+namespace {
+
+/// Much like SmallSet, with two differences:
+/// 1. It can *only* be constructed from an ArrayRef<>. If the element count
+///    is small, there is no copy and said storage *must* outlive us.
+/// 2. it is immutable, the way it was constructed it will stay.
+template <typename T, unsigned SmallSize> class ImmutableSmallSet {
+  ArrayRef<T> Vector;
+  llvm::DenseSet<T> Set;
+
+  static_assert(SmallSize <= 32, "N should be small");
+
+  bool isSmall() const { return Set.empty(); }
+
+public:
+  using size_type = size_t;
+
+  ImmutableSmallSet() = delete;
+  ImmutableSmallSet(const ImmutableSmallSet &) = delete;
+  ImmutableSmallSet(ImmutableSmallSet &&) = delete;
+  T &operator=(const ImmutableSmallSet &) = delete;
+  T &operator=(ImmutableSmallSet &&) = delete;
+
+  // WARNING: Storage *must* outlive us if we decide that the size is small.
+  ImmutableSmallSet(ArrayRef<T> Storage) {
+    // Is size small-enough to just keep using the existing storage?
+    if (Storage.size() <= SmallSize) {
+      Vector = Storage;
+      return;
+    }
+
+    // We've decided that it isn't performant to keep using vector.
+    // Let's migrate the data into Set.
+    Set.reserve(Storage.size());
+    Set.insert(Storage.begin(), Storage.end());
+  }
+
+  /// count - Return 1 if the element is in the set, 0 otherwise.
+  size_type count(const T &V) const {
+    if (isSmall()) {
+      // Since the collection is small, just do a linear search.
+      return llvm::find(Vector, V) == Vector.end() ? 0 : 1;
+    }
+
+    return Set.count(V);
+  }
+};
+
+/// Much like SmallSetVector, but with one difference:
+/// when the size is \p SmallSize or less, when checking whether an element is
+/// already in the set or not, we perform linear search over the vector,
+/// but if the size is larger than \p SmallSize, we look in set.
+/// FIXME: upstream this into SetVector/SmallSetVector itself.
+template <typename T, unsigned SmallSize> class SmartSmallSetVector {
+public:
+  using size_type = size_t;
+
+private:
+  SmallVector<T, SmallSize> Vector;
+  llvm::DenseSet<T> Set;
+
+  static_assert(SmallSize <= 32, "N should be small");
+
+  // Are we still using Vector for uniqness tracking?
+  bool isSmall() const { return Set.empty(); }
+
+  // Will one more entry cause Vector to switch away from small-size storage?
+  bool entiretyOfVectorSmallSizeIsOccupied() const {
+    assert(isSmall() && Vector.size() <= SmallSize &&
+           "Shouldn't ask if we have already [should have] migrated into Set.");
+    return Vector.size() == SmallSize;
+  }
+
+  void populateSet() {
+    assert(Set.empty() && "Should not have already utilized the Set.");
+    // Magical growth factor prediction - to how many elements do we expect to
+    // sanely grow after switching away from small-size storage?
+    const size_t NewMaxElts = 4 * Vector.size();
+    Vector.reserve(NewMaxElts);
+    Set.reserve(NewMaxElts);
+    Set.insert(Vector.begin(), Vector.end());
+  }
+
+  /// count - Return 1 if the element is in the set, 0 otherwise.
+  size_type count(const T &V) const {
+    if (isSmall()) {
+      // Since the collection is small, just do a linear search.
+      return llvm::find(Vector, V) == Vector.end() ? 0 : 1;
+    }
+    // Look-up in the Set.
+    return Set.count(V);
+  }
+
+  bool setInsert(const T &V) {
+    if (count(V) != 0)
+      return false; // Already exists.
+    // Does not exist, Can/need to record it.
+    if (isSmall()) { // Are we still using Vector for uniqness tracking?
+      // Will one more entry fit within small-sized Vector?
+      if (!entiretyOfVectorSmallSizeIsOccupied())
+        return true; // We'll insert into vector right afterwards anyway.
+      // Time to switch to Set.
+      populateSet();
+    }
+    // Set time!
+    // Note that this must be after `populateSet()` might have been called.
+    bool SetInsertionSucceeded = Set.insert(V).second;
+    (void)SetInsertionSucceeded;
+    assert(SetInsertionSucceeded && "We did check that no such value existed");
+    return true;
+  }
+
+public:
+  /// Insert a new element into the SmartSmallSetVector.
+  /// \returns true if the element was inserted into the SmartSmallSetVector.
+  bool insert(const T &X) {
+    bool result = setInsert(X);
+    if (result)
+      Vector.push_back(X);
+    return result;
+  }
+
+  /// Clear the SmartSmallSetVector and return the underlying vector.
+  decltype(Vector) takeVector() {
+    Set.clear();
+    return std::move(Vector);
+  }
+};
+
+constexpr unsigned SmallCallStackSize = 16;
+constexpr unsigned SmallSCCSize = 32;
+
+using CallStackTy =
+    llvm::SmallVector<CallGraphNode::CallRecord, SmallCallStackSize>;
+
+// In given SCC, find *some* call stack that will be cyclic.
+// This will only find *one* such stack, it might not be the smallest one,
+// and there may be other loops.
+CallStackTy PathfindSomeCycle(ArrayRef<CallGraphNode *> SCC) {
+  // We'll need to be able to performantly look up whether some CallGraphNode
+  // is in SCC or not, so cache all the SCC elements in a set.
+  const ImmutableSmallSet<CallGraphNode *, SmallSCCSize> SCCElts(SCC);
+
+  // Is node N part if the current SCC?
+  auto NodeIsPartOfSCC = [&SCCElts](CallGraphNode *N) {
+    return SCCElts.count(N) != 0;
+  };
+
+  // Track the call stack that will cause a cycle.
+  SmartSmallSetVector<CallGraphNode::CallRecord, SmallCallStackSize>
+      CallStackSet;
+
+  // Arbitrairly take the first element of SCC as entry point.
+  CallGraphNode::CallRecord EntryNode(SCC.front(), /*CallExpr=*/nullptr);
+  // Continue recursing into subsequent callees that are part of this SCC,
+  // and are thus known to be part of the call graph loop, until loop forms.
+  CallGraphNode::CallRecord *Node = &EntryNode;
+  while (true) {
+    // Did we see this node before?
+    if (!CallStackSet.insert(*Node))
+      break; // Cycle completed! Note that didn't insert the node into stack!
+    // Else, perform depth-first traversal: out of all callees, pick first one
+    // that is part of this SCC. This is not guaranteed to yield shortest cycle.
+    Node = llvm::find_if(Node->Callee->callees(), NodeIsPartOfSCC);
+  }
+
+  // Note that we failed to insert the last node, that completes the cycle.
+  // But we really want to have it. So insert it manually into stack only.
+  CallStackTy CallStack = CallStackSet.takeVector();
+  CallStack.emplace_back(*Node);
+
+  return CallStack;
+}
+
+} // namespace
+
+void NoRecursionCheck::registerMatchers(MatchFinder *Finder) {
+  Finder->addMatcher(translationUnitDecl().bind("TUDecl"), this);
+}
+
+void NoRecursionCheck::handleSCC(ArrayRef<CallGraphNode *> SCC) {
+  assert(!SCC.empty() && "Empty SCC does not make sense.");
+
+  // First of all, call out every stongly connected function.
+  for (CallGraphNode *N : SCC) {
+    FunctionDecl *D = N->getDefinition();
+    // getOuterLexicalRecordContext
+    if (!D->isConstexprSpecified())
+      diag(D->getLocation(), "function %0 is within a recursive call chain")
+          << D;
+  }
+
+  // Now, SCC only tells us about strongly connected function declarations in
+  // the call graph. It doesn't *really* tell us about the cycles they form.
+  // And there may be more than one cycle in SCC.
+  // So let's form a call stack that eventually exposes *some* cycle.
+  const CallStackTy EventuallyCyclicCallStack = PathfindSomeCycle(SCC);
+  assert(!EventuallyCyclicCallStack.empty() && "We should've found the cycle");
+
+  // While last node of the call stack does cause a loop, due to the way we
+  // pathfind the cycle, the loop does not nessesairly begin at the first node
+  // of the call stack, so drop front nodes of the call stack until it does.
+  const auto CyclicCallStack =
+      ArrayRef<CallGraphNode::CallRecord>(EventuallyCyclicCallStack)
+          .drop_until([LastNode = EventuallyCyclicCallStack.back()](
+                          CallGraphNode::CallRecord FrontNode) {
+            return FrontNode == LastNode;
+          });
+  assert(CyclicCallStack.size() >= 2 && "Cycle requires at least 2 frames");
+
+  // Which function we decided to be the entry point that lead to the recursion?
+  FunctionDecl *CycleEntryFn = CyclicCallStack.front().Callee->getDefinition();
+  // And now, for ease of understanding, let's print the call sequence that
+  // forms the cycle in question.
+  if (!CycleEntryFn->isConstexprSpecified()) {
+    diag(CycleEntryFn->getLocation(),
+         "example recursive call chain, starting from function %0",
+         DiagnosticIDs::Note)
+        << CycleEntryFn;
+    for (int CurFrame = 1, NumFrames = CyclicCallStack.size();
+         CurFrame != NumFrames; ++CurFrame) {
+      CallGraphNode::CallRecord PrevNode = CyclicCallStack[CurFrame - 1];
+      CallGraphNode::CallRecord CurrNode = CyclicCallStack[CurFrame];
+
+      Decl *PrevDecl = PrevNode.Callee->getDecl();
+      Decl *CurrDecl = CurrNode.Callee->getDecl();
+
+      diag(
+          CurrNode.CallExpr->getBeginLoc(),
+          "Frame #%0: function %1 calls function %2 here:", DiagnosticIDs::Note)
+          << CurFrame << cast<NamedDecl>(PrevDecl) << cast<NamedDecl>(CurrDecl);
+    }
+
+    diag(CyclicCallStack.back().CallExpr->getBeginLoc(),
+         "... which was the starting point of the recursive call chain; there "
+         "may be other cycles",
+         DiagnosticIDs::Note);
+  }
+}
+
+void NoRecursionCheck::check(const MatchFinder::MatchResult &Result) {
+  // Build call graph for the entire translation unit.
+  const auto *TU = Result.Nodes.getNodeAs<TranslationUnitDecl>("TUDecl");
+  CallGraph CG;
+  CG.addToCallGraph(const_cast<TranslationUnitDecl *>(TU));
+
+  // Look for cycles in call graph,
+  // by looking for Strongly Connected Comonents (SCC's)
+  for (llvm::scc_iterator<CallGraph *> SCCI = llvm::scc_begin(&CG),
+                                       SCCE = llvm::scc_end(&CG);
+       SCCI != SCCE; ++SCCI) {
+    if (!SCCI.hasCycle()) // We only care about cycles, not standalone nodes.
+      continue;
+    handleSCC(*SCCI);
+  }
+}
+
+} // namespace bsl
+} // namespace tidy
+} // namespace clang

clang-tools-extra/clang-tidy/bsl/NoRecursionCheck.h

@@ -0,0 +1,44 @@
+//===--- NoRecursionCheck.h - clang-tidy ------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_BSL_NORECURSIONCHECK_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_BSL_NORECURSIONCHECK_H
+
+#include "../ClangTidyCheck.h"
+
+namespace clang {
+
+class CallGraphNode;
+
+namespace tidy {
+namespace bsl {
+
+/// Finds strongly connected functions (by analyzing call graph for SCC's
+/// that are loops), diagnoses each function in the cycle,
+/// and displays one example of possible call graph loop (recursion).
+/// Makes exception for constexpr functions, including lambdas
+/// implicitly marked constexpr in C++17 or later.
+///
+/// For the user-facing documentation see:
+/// http://clang.llvm.org/extra/clang-tidy/checks/bsl-no-recursion.html
+class NoRecursionCheck : public ClangTidyCheck {
+public:
+  NoRecursionCheck(StringRef Name, ClangTidyContext *Context)
+      : ClangTidyCheck(Name, Context) {}
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+
+private:
+  void handleSCC(ArrayRef<CallGraphNode *> SCC);
+};
+
+} // namespace bsl
+} // namespace tidy
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_BSL_NORECURSIONCHECK_H

clang-tools-extra/docs/ReleaseNotes.rst

@@ -219,6 +219,11 @@ New checks
 
   Warns if [[nodiscard]] or [[maybe_unused]] are missing
 
+- New :doc:`bsl-no-recursion
+  <clang-tidy/checks/bsl-no-recursion>` check.
+
+  Finds recursive functions and diagnoses them unless constexpr.
+
 - New :doc:`bsl-non-pod-classdef
   <clang-tidy/checks/bsl-non-pod-classdef>` check.