[analyzer] Introduce the check::BlockEntrance checker callback by balazs-benics-sonarsource · Pull Request #140924 · llvm/llvm-project (original) (raw)

@llvm/pr-subscribers-clang

@llvm/pr-subscribers-clang-static-analyzer-1

Author: Balázs Benics (balazs-benics-sonarsource)

Changes

Tranersing the CFG blocks of a function is a fundamental operation. Many C++ constructs can create splits in the control-flow, such as if, for, and similar control structures or ternary expressions, gnu conditionals, gotos, switches and possibly more.

Checkers should be able to get notifications about entering or leaving a CFG block of interest.

Note that in the ExplodedGraph there is always a BlockEntrance ProgramPoint right after the BlockEdge ProgramPoint. I considered naming this callback check::BlockEdge, but then that may leave the observer of the graph puzzled to see BlockEdge points followed more BlockEdge nodes describing the same CFG transition. This confusion could also apply to Bug Report Visitors too.

Because of this, I decided to hook BlockEntrance ProgramPoints instead. The same confusion applies here, but I find this still a better place TBH. There would only appear only one BlockEntrance ProgramPoint in the graph if no checkers modify the state or emit a bug report. Otherwise they modify some GDM (aka. State) thus create a new ExplodedNode with the same BlockEntrance ProgramPoint in the graph.

CPP-6484

Patch is 23.05 KiB, truncated to 20.00 KiB below, full version: https://github.com/llvm/llvm-project/pull/140924.diff

10 Files Affected:

• (modified) clang/include/clang/StaticAnalyzer/Core/Checker.h (+20)
• (modified) clang/include/clang/StaticAnalyzer/Core/CheckerManager.h (+13)
• (modified) clang/include/clang/StaticAnalyzer/Core/PathSensitive/ExprEngine.h (+4)
• (modified) clang/lib/StaticAnalyzer/Checkers/CheckerDocumentation.cpp (+14-1)
• (modified) clang/lib/StaticAnalyzer/Core/CheckerManager.cpp (+45-5)
• (modified) clang/lib/StaticAnalyzer/Core/CoreEngine.cpp (+18-7)
• (modified) clang/lib/StaticAnalyzer/Core/ExprEngine.cpp (+13)
• (modified) clang/lib/StaticAnalyzer/Core/ExprEngineC.cpp (+3-3)
• (added) clang/unittests/StaticAnalyzer/BlockEntranceCallbackTest.cpp (+283)
• (modified) clang/unittests/StaticAnalyzer/CMakeLists.txt (+1)

diff --git a/clang/include/clang/StaticAnalyzer/Core/Checker.h b/clang/include/clang/StaticAnalyzer/Core/Checker.h index a54c5bee612f6..1b348dcce5ea7 100644 --- a/clang/include/clang/StaticAnalyzer/Core/Checker.h +++ b/clang/include/clang/StaticAnalyzer/Core/Checker.h @@ -221,6 +221,22 @@ class Bind { } };

+class BlockEntrance {

• template
• static void _checkBlockEntrance(void *Checker,

•                              const clang::BlockEntrance &Entrance,

•                              CheckerContext &C) {

• ((const CHECKER *)Checker)->checkBlockEntrance(Entrance, C);
• }
• +public:
• template
• static void _register(CHECKER *checker, CheckerManager &mgr) {
• mgr._registerForBlockEntrance(CheckerManager::CheckBlockEntranceFunc(

•    checker, _checkBlockEntrance<CHECKER>));

• } +};
• class EndAnalysis { template static void _checkEndAnalysis(void *checker, ExplodedGraph &G, @@ -548,6 +564,8 @@ class CheckerProgramPointTag : public SimpleProgramPointTag { template <typename CHECK1, typename... CHECKs> class Checker : public CHECK1, public CHECKs..., public CheckerBase { public:
• using BlockEntrance = clang::BlockEntrance;
• template static void _register(CHECKER *checker, CheckerManager &mgr) { CHECK1::_register(checker, mgr);

@@ -558,6 +576,8 @@ class Checker : public CHECK1, public CHECKs..., public CheckerBase { template class Checker : public CHECK1, public CheckerBase { public:

• using BlockEntrance = clang::BlockEntrance;
• template static void _register(CHECKER *checker, CheckerManager &mgr) { CHECK1::_register(checker, mgr);

diff --git a/clang/include/clang/StaticAnalyzer/Core/CheckerManager.h b/clang/include/clang/StaticAnalyzer/Core/CheckerManager.h index 03ffadd346d0b..b5fefdb75401d 100644 --- a/clang/include/clang/StaticAnalyzer/Core/CheckerManager.h +++ b/clang/include/clang/StaticAnalyzer/Core/CheckerManager.h @@ -376,6 +376,12 @@ class CheckerManager { const Stmt *S, ExprEngine &Eng, const ProgramPoint &PP);

• /// Run checkers after taking a control flow edge.
• void runCheckersForBlockEntrance(ExplodedNodeSet &Dst,

•                               const ExplodedNodeSet &Src,

•                               const BlockEntrance &Entrance,

•                               ExprEngine &Eng) const;

• /// Run checkers for end of analysis. void runCheckersForEndAnalysis(ExplodedGraph &G, BugReporter &BR, ExprEngine &Eng);

@@ -528,6 +534,9 @@ class CheckerManager { using CheckBindFunc = CheckerFn<void(SVal location, SVal val, const Stmt *S, CheckerContext &)>;

• using CheckBlockEntranceFunc =

•  CheckerFn<void(const BlockEntrance &, CheckerContext &)>;

• using CheckEndAnalysisFunc = CheckerFn<void (ExplodedGraph &, BugReporter &, ExprEngine &)>;

@@ -589,6 +598,8 @@ class CheckerManager {

void _registerForBind(CheckBindFunc checkfn);

• void _registerForBlockEntrance(CheckBlockEntranceFunc checkfn);

• void _registerForEndAnalysis(CheckEndAnalysisFunc checkfn);

  void _registerForBeginFunction(CheckBeginFunctionFunc checkfn);

@@ -695,6 +706,8 @@ class CheckerManager {

std::vector BindCheckers;

• std::vector BlockEntranceCheckers;

• std::vector EndAnalysisCheckers;

  std::vector BeginFunctionCheckers;

diff --git a/clang/include/clang/StaticAnalyzer/Core/PathSensitive/ExprEngine.h b/clang/include/clang/StaticAnalyzer/Core/PathSensitive/ExprEngine.h index b8a4dcbc727a6..6370586e218ef 100644 --- a/clang/include/clang/StaticAnalyzer/Core/PathSensitive/ExprEngine.h +++ b/clang/include/clang/StaticAnalyzer/Core/PathSensitive/ExprEngine.h @@ -321,6 +321,10 @@ class ExprEngine { NodeBuilderWithSinks &nodeBuilder, ExplodedNode *Pred);

• void runCheckersForBlockEntrance(const NodeBuilderContext &BldCtx,

•                               const BlockEntrance &Entrance,

•                               ExplodedNode *Pred, ExplodedNodeSet &Dst);

• /// ProcessBranch - Called by CoreEngine. Used to generate successor nodes by /// processing the 'effects' of a branch condition. If the branch condition /// is a loop condition, IterationsCompletedInLoop is the number of completed diff --git a/clang/lib/StaticAnalyzer/Checkers/CheckerDocumentation.cpp b/clang/lib/StaticAnalyzer/Checkers/CheckerDocumentation.cpp index 153a1b1acbfa1..8bfe677693362 100644 --- a/clang/lib/StaticAnalyzer/Checkers/CheckerDocumentation.cpp +++ b/clang/lib/StaticAnalyzer/Checkers/CheckerDocumentation.cpp @@ -16,7 +16,6 @@ #include "clang/StaticAnalyzer/Core/Checker.h" #include "clang/StaticAnalyzer/Core/CheckerManager.h" #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"

-#include "clang/StaticAnalyzer/Core/PathSensitive/ProgramStateTrait.h"

using namespace clang; using namespace ento; @@ -46,6 +45,7 @@ class CheckerDocumentation check::EndAnalysis, check::EndFunction, check::EndOfTranslationUnit,

•      check::BlockEntrance,
       check::Event<ImplicitNullDerefEvent>,
       check::LiveSymbols,
       check::Location,

@@ -166,6 +166,19 @@ class CheckerDocumentation /// check::Bind void checkBind(SVal Loc, SVal Val, const Stmt *S, CheckerContext &) const {}

• /// Called after a CFG edge is taken within a function.
• ///
• /// This callback can be used to obtain information about potential branching
• /// points or any other constructs that involve traversing a CFG edge.
• /// Note that when inlining a call, there is no CFG edge between the caller
• /// and the callee. One will only see the edge between the entry block and
• /// the body of the function once inlined.
• ///
• /// \param E The ProgramPoint that describes the transition.
• ///
• /// check::BlockEntrance
• void checkBlockEntrance(const BlockEntrance &E, CheckerContext &) const {}
• /// Called whenever a symbol becomes dead. /// /// This callback should be used by the checkers to aggressively clean diff --git a/clang/lib/StaticAnalyzer/Core/CheckerManager.cpp b/clang/lib/StaticAnalyzer/Core/CheckerManager.cpp index 7ae86f133904b..b77ead60af258 100644 --- a/clang/lib/StaticAnalyzer/Core/CheckerManager.cpp +++ b/clang/lib/StaticAnalyzer/Core/CheckerManager.cpp @@ -42,11 +42,11 @@ bool CheckerManager::hasPathSensitiveCheckers() const { return IfAnyAreNonEmpty( StmtCheckers, PreObjCMessageCheckers, ObjCMessageNilCheckers, PostObjCMessageCheckers, PreCallCheckers, PostCallCheckers,

•  LocationCheckers, BindCheckers, EndAnalysisCheckers,

•  BeginFunctionCheckers, EndFunctionCheckers, BranchConditionCheckers,

•  NewAllocatorCheckers, LiveSymbolsCheckers, DeadSymbolsCheckers,

•  RegionChangesCheckers, PointerEscapeCheckers, EvalAssumeCheckers,

•  EvalCallCheckers, EndOfTranslationUnitCheckers);

•  LocationCheckers, BindCheckers, BlockEntranceCheckers,

•  EndAnalysisCheckers, BeginFunctionCheckers, EndFunctionCheckers,

•  BranchConditionCheckers, NewAllocatorCheckers, LiveSymbolsCheckers,

•  DeadSymbolsCheckers, RegionChangesCheckers, PointerEscapeCheckers,

•  EvalAssumeCheckers, EvalCallCheckers, EndOfTranslationUnitCheckers);

}

void CheckerManager::reportInvalidCheckerOptionValue( @@ -420,6 +420,42 @@ void CheckerManager::runCheckersForBind(ExplodedNodeSet &Dst, expandGraphWithCheckers(C, Dst, Src); }

+namespace { +struct CheckBlockEntranceContext {

• using CheckBlockEntranceFunc = CheckerManager::CheckBlockEntranceFunc;
• using CheckersTy = std::vector;
• const CheckersTy &Checkers;
• const BlockEntrance &Entrance;
• ExprEngine &Eng;
• CheckBlockEntranceContext(const CheckersTy &Checkers,

•                        const BlockEntrance &Entrance, ExprEngine &Eng)

•  : Checkers(Checkers), Entrance(Entrance), Eng(Eng) {}

• auto checkers_begin() const { return Checkers.begin(); }
• auto checkers_end() const { return Checkers.end(); }
• void runChecker(CheckBlockEntranceFunc CheckFn, NodeBuilder &Bldr,

•              ExplodedNode *Pred) {

• llvm::TimeTraceScope TimeScope(

•    checkerScopeName("BlockEntrance", CheckFn.Checker));

• CheckerContext C(Bldr, Eng, Pred, Entrance.withTag(CheckFn.Checker));
• CheckFn(Entrance, C);
• } +};
• +} // namespace
• +void CheckerManager::runCheckersForBlockEntrance(ExplodedNodeSet &Dst,

•                                             const ExplodedNodeSet &Src,

•                                             const BlockEntrance &Entrance,

•                                             ExprEngine &Eng) const {

• CheckBlockEntranceContext C(BlockEntranceCheckers, Entrance, Eng);
• llvm::TimeTraceScope TimeScope{"CheckerManager::runCheckersForBlockEntrance"};
• expandGraphWithCheckers(C, Dst, Src); +}
• void CheckerManager::runCheckersForEndAnalysis(ExplodedGraph &G, BugReporter &BR, ExprEngine &Eng) {

@@ -877,6 +913,10 @@ void CheckerManager::_registerForBind(CheckBindFunc checkfn) { BindCheckers.push_back(checkfn); }

+void CheckerManager::_registerForBlockEntrance(CheckBlockEntranceFunc checkfn) {

• BlockEntranceCheckers.push_back(checkfn); +}

• void CheckerManager::_registerForEndAnalysis(CheckEndAnalysisFunc checkfn) { EndAnalysisCheckers.push_back(checkfn); } diff --git a/clang/lib/StaticAnalyzer/Core/CoreEngine.cpp b/clang/lib/StaticAnalyzer/Core/CoreEngine.cpp index bedb11f8b94a5..bdc2704cc424e 100644 --- a/clang/lib/StaticAnalyzer/Core/CoreEngine.cpp +++ b/clang/lib/StaticAnalyzer/Core/CoreEngine.cpp @@ -306,26 +306,37 @@ void CoreEngine::HandleBlockEdge(const BlockEdge &L, ExplodedNode *Pred) { } }

• ExplodedNodeSet CheckerNodes;

• BlockEntrance BE(L.getSrc(), L.getDst(), Pred->getLocationContext());

• ExprEng.runCheckersForBlockEntrance(BuilderCtx, BE, Pred, CheckerNodes);

• // Process the final state transition.

• ExprEng.processEndOfFunction(BuilderCtx, Pred, RS);

• for (ExplodedNode *P : CheckerNodes) {

•  ExprEng.processEndOfFunction(BuilderCtx, P, RS);

• }

  // This path is done. Don't enqueue any more nodes. return;

}

// Call into the ExprEngine to process entering the CFGBlock.

• ExplodedNodeSet dstNodes; BlockEntrance BE(L.getSrc(), L.getDst(), Pred->getLocationContext());
• NodeBuilderWithSinks nodeBuilder(Pred, dstNodes, BuilderCtx, BE);
• ExprEng.processCFGBlockEntrance(L, nodeBuilder, Pred);

• ExplodedNodeSet DstNodes;

• NodeBuilderWithSinks NodeBuilder(Pred, DstNodes, BuilderCtx, BE);

• ExprEng.processCFGBlockEntrance(L, NodeBuilder, Pred);

  // Auto-generate a node.

• if (!nodeBuilder.hasGeneratedNodes()) {
• nodeBuilder.generateNode(Pred->State, Pred);

• if (!NodeBuilder.hasGeneratedNodes()) {
• NodeBuilder.generateNode(Pred->State, Pred);
• }
• ExplodedNodeSet CheckerNodes;
• for (auto *N : DstNodes) {
• ExprEng.runCheckersForBlockEntrance(BuilderCtx, BE, N, CheckerNodes);

}

// Enqueue nodes onto the worklist.

• enqueue(dstNodes);

• enqueue(CheckerNodes); }

void CoreEngine::HandleBlockEntrance(const BlockEntrance &L, diff --git a/clang/lib/StaticAnalyzer/Core/ExprEngine.cpp b/clang/lib/StaticAnalyzer/Core/ExprEngine.cpp index 1afd4b52eb354..992e451d33081 100644 --- a/clang/lib/StaticAnalyzer/Core/ExprEngine.cpp +++ b/clang/lib/StaticAnalyzer/Core/ExprEngine.cpp @@ -2619,6 +2619,19 @@ void ExprEngine::processCFGBlockEntrance(const BlockEdge &L, } }

+void ExprEngine::runCheckersForBlockEntrance(const NodeBuilderContext &BldCtx,

•                                         const BlockEntrance &Entrance,

•                                         ExplodedNode *Pred,

•                                         ExplodedNodeSet &Dst) {

• llvm::PrettyStackTraceFormat CrashInfo(

•  "Processing block entrance B%d -> B%d",

•  Entrance.getPreviousBlock()->getBlockID(),

•  Entrance.getBlock()->getBlockID());

• currBldrCtx = &BldCtx;

• getCheckerManager().runCheckersForBlockEntrance(Dst, Pred, Entrance, *this);

• currBldrCtx = nullptr; +}

• //===----------------------------------------------------------------------===// // Branch processing. //===----------------------------------------------------------------------===// diff --git a/clang/lib/StaticAnalyzer/Core/ExprEngineC.cpp b/clang/lib/StaticAnalyzer/Core/ExprEngineC.cpp index b91954eda19b8..fa8e669b6bb2f 100644 --- a/clang/lib/StaticAnalyzer/Core/ExprEngineC.cpp +++ b/clang/lib/StaticAnalyzer/Core/ExprEngineC.cpp @@ -711,9 +711,10 @@ void ExprEngine::VisitLogicalExpr(const BinaryOperator* B, ExplodedNode *Pred, }

  ExplodedNode *N = Pred;

• while (!N->getLocation().getAs()) {

• while (!N->getLocation().getAs()) { ProgramPoint P = N->getLocation();

• assert(P.getAs()|| P.getAs());

• assert(P.getAs() || P.getAs() ||

•       P.getAs<BlockEntrance>());

  (void) P; if (N->pred_size() != 1) { // We failed to track back where we came from.

@@ -729,7 +730,6 @@ void ExprEngine::VisitLogicalExpr(const BinaryOperator* B, ExplodedNode *Pred, return; }

• N = *N->pred_begin(); BlockEdge BE = N->getLocation().castAs(); SVal X;

diff --git a/clang/unittests/StaticAnalyzer/BlockEntranceCallbackTest.cpp b/clang/unittests/StaticAnalyzer/BlockEntranceCallbackTest.cpp new file mode 100644 index 0000000000000..729c6d6ebf3ba --- /dev/null +++ b/clang/unittests/StaticAnalyzer/BlockEntranceCallbackTest.cpp @@ -0,0 +1,283 @@ +//===- unittests/StaticAnalyzer/BlockEntranceCallbackTest.cpp -------------===// +// +// 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 "CheckerRegistration.h" +#include "clang/Analysis/AnalysisDeclContext.h" +#include "clang/Analysis/ProgramPoint.h" +#include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.h" +#include "clang/StaticAnalyzer/Core/BugReporter/BugType.h" +#include "clang/StaticAnalyzer/Core/Checker.h" +#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h" +#include "clang/StaticAnalyzer/Core/PathSensitive/ProgramState_Fwd.h" +#include "clang/StaticAnalyzer/Frontend/AnalysisConsumer.h" +#include "clang/StaticAnalyzer/Frontend/CheckerRegistry.h" +#include "llvm/ADT/STLExtras.h" +#include "llvm/ADT/StringExtras.h" +#include "llvm/ADT/StringRef.h" +#include "llvm/Support/FormatVariadic.h" +#include "llvm/Support/raw_ostream.h" +#include "gtest/gtest.h" + +#include + +using namespace clang; +using namespace ento; + +namespace { + +class BlockEntranceCallbackTester : public Checkercheck::BlockEntrance {

• const BugType Bug{this, "BlockEntranceTester"};
• +public:
• void checkBlockEntrance(const BlockEntrance &Entrance,

•                      CheckerContext &C) const {

• ExplodedNode *Node = C.generateNonFatalErrorNode(C.getState());
• if (!Node)

•  return;

• const auto *FD =

•    cast<FunctionDecl>(C.getLocationContext()->getStackFrame()->getDecl());

• std::string Description = llvm::formatv(

•    "Within '{0}' B{1} -> B{2}", FD->getIdentifier()->getName(),

•    Entrance.getPreviousBlock()->getBlockID(),

•    Entrance.getBlock()->getBlockID());

• auto Report =

•    std::make_unique<PathSensitiveBugReport>(Bug, Description, Node);

• C.emitReport(std::move(Report));
• } +};
• +void registerBlockEntranceTester(CheckerManager &Mgr) {
• Mgr.registerChecker(); +}
• +bool shouldRegisterBlockEntranceTester(const CheckerManager &) { return true; }
• +void addBlockEntranceTester(AnalysisASTConsumer &AnalysisConsumer,

•                        AnalyzerOptions &AnOpts) {

• AnOpts.CheckersAndPackages = {{"test.BlockEntranceTester", true}};
• AnalysisConsumer.AddCheckerRegistrationFn([](CheckerRegistry &Registry) {
• Registry.addChecker(

•    &registerBlockEntranceTester, &shouldRegisterBlockEntranceTester,

•    "test.BlockEntranceTester", "EmptyDescription", "EmptyDocsUri",

•    /*IsHidden=*/false);

• }); +}
• +llvm::SmallVector parseEachDiag(StringRef Diags) {
• llvm::SmallVector Fragments;
• llvm::SplitString(Diags, Fragments, "\n");
• llvm::for_each(Fragments, [](StringRef &Fragment) {
• Fragment.consume_front("test.BlockEntranceTester: ");
• });
• llvm::sort(Fragments);
• return Fragments; +}
• +bool runChecker(const std::string &Code, std::string &Diags) {
• std::string RawDiags;
• bool Res =

•  runCheckerOnCode<addBlockEntranceTester>(Code, RawDiags,

•                                           /*OnlyEmitWarnings=*/true);

• llvm::raw_string_ostream OS(Diags);
• llvm::interleave(parseEachDiag(RawDiags), OS, "\n");
• return Res; +}
• +std::string expected(SmallVector Diags) {
• llvm::sort(Diags);
• std::string Result;
• llvm::raw_string_ostream OS(Result);
• llvm::interleave(Diags, OS, "\n");
• return Result; +}
• +TEST(BlockEntranceTester, FromEntryToExit) {
• constexpr auto Code = R"cpp(
• void top() {
• // empty
• })cpp";
• std::string Diags;
• EXPECT_TRUE(runChecker(Code, Diags));
• EXPECT_EQ(expected({"Within 'top' B1 -> B0"}), Diags); +}
• +TEST(BlockEntranceTester, SingleOpaqueIfCondition) {
• constexpr auto Code = R"cpp(
• bool coin();
• int glob;
• void top() {
• if (coin()) {

•  glob = 1;

• } else {

•  glob = 2;

• }
• glob = 3;
• })cpp";
• std::string Diags;
• EXPECT_TRUE(runChecker(Code, Diags));
• EXPECT_EQ(expected({

•            "Within 'top' B1 -> B0",

•            "Within 'top' B2 -> B1",

•            "Within 'top' B3 -> B1",

•            "Within 'top' B4 -> B2",

•            "Within 'top' B4 -> B3",

•            "Within 'top' B5 -> B4",

•        }),

•        Diags);

• // entry true exit
• // B5 -------> B4 --> B2 --> B1 --> B0
• // | ^
• // | false |
• // v |
• // B3 -----------------------+ +}
• +TEST(BlockEntranceTester, TrivialIfCondition) {
• constexpr auto Code = R"cpp(
• bool coin();
• int glob;
• void top() {
• int cond = true;
• if (cond) {

•  glob = 1;

• } else {

•  glob = 2;

• }
• glob = 3;
• })cpp";
• std::string Diags;
• EXPECT_TRUE(runChecker(Code, Diags));
• EXPECT_EQ(expected({

•            "Within 'top' B1 -> B0",

•            "Within 'top' B3 -> B1",

•            "Within 'top' B4 -> B3",

•            "Within 'top' B5 -> B4",

•        }),

•        Diags);

• // entry true exit
• // B5 ----------> B4 --> B3 --> B1 --> B0 +}
• +TEST(BlockEntranceTester, AcrossFunctions) {
• constexpr auto Code = R"cpp(
• bool coin();
• int glob;
• void nested() { glob = 1; }
• void top() {
• glob = 0;
• nested();
• glob = 2;
• })cpp";
• std::string Diags;
• EXPECT_TRUE(runChecker(Code, Diags));
• EXPECT_EQ(

•  expected({

•      // Going from the "top" entry artificial node to the "top" body.

•      // Ideally, we shouldn't observe this edge because it's artificial.

•      "Within 'top' B2 -> B1",

•      // We encounter the call to "nested()" in the "top" body, thus we have

•      // a "CallEnter" node, but importantly, we also elide the transition

•      // to the "entry" node of "nested()".

•      // We only see the edge from the "nested()" entry to the "nested()"

•      // body:

•      "Within 'nested' B2 -> B1",

•      // Once we return from "nested()", we transition to the "exit" node of

•      // "nested()":

•      "Within 'nested' B1 -> B0",

•      // We will eventually return to the "top" body, thus we transition to

•      // its "exit" node:

... [truncated]