source: src/ControlStruct/MultiLevelExit.cpp @ d0296db6

//
// Cforall Version 1.0.0 Copyright (C) 2015 University of Waterloo
//
// The contents of this file are covered under the licence agreement in the
// file "LICENCE" distributed with Cforall.
//
// MultiLevelExit.cpp -- Replaces CFA's local control flow with C's versions.
//
// Author           : Andrew Beach
// Created On       : Mon Nov  1 13:48:00 2021
// Last Modified By : Peter A. Buhr
// Last Modified On : Thu Dec 14 17:34:12 2023
// Update Count     : 39
//

#include "MultiLevelExit.hpp"

#include <set>

#include "AST/Pass.hpp"
#include "AST/Stmt.hpp"
#include "LabelGenerator.hpp"

using namespace std;
using namespace ast;

namespace ControlStruct {

namespace {

/// The return context is used to remember if returns are allowed and if
/// not, why not. It is the nearest local control flow blocking construct.
enum ReturnContext {
        MayReturn,
        InTryWithHandler,
        InResumeHandler,
        InFinally,
};

class Entry {
  public:
        const Stmt * stmt;
  private:
        // Organized like a manual ADT. Avoids creating a bunch of dead data.
        struct Target {
                Label label;
                bool used = false;
                Target( const Label & label ) : label( label ) {}
                Target() : label( CodeLocation(), "" ) {}
        };
        Target firstTarget;
        Target secondTarget;

        enum Kind {
                ForStmtK, WhileDoStmtK, CompoundStmtK, IfStmtK, CaseClauseK, SwitchStmtK, TryStmtK
        } kind;

        bool fallDefaultValid = true;

        static Label & useTarget( Target & target ) {
                target.used = true;
                return target.label;
        }
  public:
        Entry( const ForStmt * stmt, Label breakExit, Label contExit ) :
                stmt( stmt ), firstTarget( breakExit ), secondTarget( contExit ), kind( ForStmtK ) {}
        Entry( const WhileDoStmt * stmt, Label breakExit, Label contExit ) :
                stmt( stmt ), firstTarget( breakExit ), secondTarget( contExit ), kind( WhileDoStmtK ) {}
        Entry( const CompoundStmt *stmt, Label breakExit ) :
                stmt( stmt ), firstTarget( breakExit ), secondTarget(), kind( CompoundStmtK ) {}
        Entry( const IfStmt *stmt, Label breakExit ) :
                stmt( stmt ), firstTarget( breakExit ), secondTarget(), kind( IfStmtK ) {}
        Entry( const CaseClause *, const CompoundStmt *stmt, Label fallExit ) :
                stmt( stmt ), firstTarget( fallExit ), secondTarget(), kind( CaseClauseK ) {}
        Entry( const SwitchStmt *stmt, Label breakExit, Label fallDefaultExit ) :
                stmt( stmt ), firstTarget( breakExit ), secondTarget( fallDefaultExit ), kind( SwitchStmtK ) {}
        Entry( const TryStmt *stmt, Label breakExit ) :
                stmt( stmt ), firstTarget( breakExit ), secondTarget(), kind( TryStmtK ) {}

        // Check if this entry can be the target of the given type of control flow.
        bool isContTarget() const { return kind <= WhileDoStmtK; }
        bool isBreakTarget() const { return kind != CaseClauseK; }
        bool isFallTarget() const { return kind == CaseClauseK; }
        bool isFallDefaultTarget() const { return kind == SwitchStmtK; }

        // Check if this entry can be the target of an unlabelled break.
        bool isUnlabelledBreakTarget() const { return kind <= WhileDoStmtK || kind == SwitchStmtK; }

        // These routines set a target as being "used" by a BranchStmt
        Label useContExit() { assert( kind <= WhileDoStmtK ); return useTarget(secondTarget); }
        Label useBreakExit() { assert( kind != CaseClauseK ); return useTarget(firstTarget); }
        Label useFallExit() { assert( kind == CaseClauseK );  return useTarget(firstTarget); }
        Label useFallDefaultExit() { assert( kind == SwitchStmtK ); return useTarget(secondTarget); }

        // These routines check if a specific label for a statement is used by a BranchStmt
        bool isContUsed() const { assert( kind <= WhileDoStmtK ); return secondTarget.used; }
        bool isBreakUsed() const { assert( kind != CaseClauseK ); return firstTarget.used; }
        bool isFallUsed() const { assert( kind == CaseClauseK ); return firstTarget.used; }
        bool isFallDefaultUsed() const { assert( kind == SwitchStmtK ); return secondTarget.used; }
        void seenDefault() { fallDefaultValid = false; }
        bool isFallDefaultValid() const { return fallDefaultValid; }
};

// Helper predicates used in find_if calls (it doesn't take methods):
bool isBreakTarget( const Entry & entry ) {
        return entry.isBreakTarget();
}

bool isContinueTarget( const Entry & entry ) {
        return entry.isContTarget();
}

bool isFallthroughTarget( const Entry & entry ) {
        return entry.isFallTarget();
}

bool isFallthroughDefaultTarget( const Entry & entry ) {
        return entry.isFallDefaultTarget();
}

bool isUnlabelledBreakTarget( const Entry & entry ) {
        return entry.isUnlabelledBreakTarget();
}

struct MultiLevelExitCore final :
        public WithVisitorRef<MultiLevelExitCore>,
        public WithShortCircuiting, public WithGuards {
        MultiLevelExitCore( const LabelToStmt & lt );

        void previsit( const FunctionDecl * );

        const CompoundStmt * previsit( const CompoundStmt * );
        const BranchStmt * postvisit( const BranchStmt * );
        void previsit( const WhileDoStmt * );
        const WhileDoStmt * postvisit( const WhileDoStmt * );
        void previsit( const ForStmt * );
        const ForStmt * postvisit( const ForStmt * );
        const CaseClause * previsit( const CaseClause * );
        void previsit( const IfStmt * );
        const IfStmt * postvisit( const IfStmt * );
        void previsit( const SwitchStmt * );
        const SwitchStmt * postvisit( const SwitchStmt * );
        void previsit( const ReturnStmt * );
        void previsit( const TryStmt * );
        void postvisit( const TryStmt * );
        void previsit( const CatchClause * );
        void previsit( const FinallyClause * );

        const Stmt * mutateLoop( const Stmt * body, Entry& );

        const LabelToStmt & target_table;
        set<Label> fallthrough_labels;
        vector<Entry> enclosing_control_structures;
        Label break_label;
        ReturnContext ret_context;

        template<typename LoopNode>
        void prehandleLoopStmt( const LoopNode * loopStmt );
        template<typename LoopNode>
        const LoopNode * posthandleLoopStmt( const LoopNode * loopStmt );

        list<ptr<Stmt>> fixBlock(
                const list<ptr<Stmt>> & kids, bool caseClause );

        void enterSealedContext( ReturnContext );

        template<typename UnaryPredicate>
        auto findEnclosingControlStructure( UnaryPredicate pred ) {
                return find_if( enclosing_control_structures.rbegin(),
                                                enclosing_control_structures.rend(), pred );
        }
};

NullStmt * labelledNullStmt( const CodeLocation & cl, const Label & label ) {
        return new NullStmt( cl, vector<Label>{ label } );
}

MultiLevelExitCore::MultiLevelExitCore( const LabelToStmt & lt ) :
        target_table( lt ), break_label( CodeLocation(), "" ),
        ret_context( ReturnContext::MayReturn )
{}

void MultiLevelExitCore::previsit( const FunctionDecl * ) {
        visit_children = false;
}

const CompoundStmt * MultiLevelExitCore::previsit(
                const CompoundStmt * stmt ) {
        visit_children = false;

        // if the stmt is labelled then generate a label to check in postvisit if the label is used
        bool isLabeled = ! stmt->labels.empty();
        if ( isLabeled ) {
                Label breakLabel = newLabel( "blockBreak", stmt );
                enclosing_control_structures.emplace_back( stmt, breakLabel );
                GuardAction( [this]() { enclosing_control_structures.pop_back(); } );
        }

        auto mutStmt = mutate( stmt );
        // A child statement may set the break label.
        mutStmt->kids = fixBlock( stmt->kids, false );

        if ( isLabeled ) {
                assert( ! enclosing_control_structures.empty() );
                Entry & entry = enclosing_control_structures.back();
                if ( ! entry.useBreakExit().empty() ) {
                        break_label = entry.useBreakExit();
                }
        }
        return mutStmt;
}

size_t getUnusedIndex( const Stmt * stmt, const Label & originalTarget ) {
        const size_t size = stmt->labels.size();

        // If the label is empty, do not add unused attribute.
        if ( originalTarget.empty() ) return size;

        // Search for a label that matches the originalTarget.
        for ( size_t i = 0 ; i < size ; ++i ) {
                const Label & label = stmt->labels[i];
                if ( label == originalTarget ) {
                        for ( const Attribute * attr : label.attributes ) {
                                if ( attr->name == "unused" ) return size;
                        }
                        return i;
                }
        }
        assertf( false, "CFA internal error: could not find label '%s' on statement %s",
                         originalTarget.name.c_str(), toString( stmt ).c_str() );
}

const Stmt * addUnused( const Stmt * stmt, const Label & originalTarget ) {
        size_t i = getUnusedIndex( stmt, originalTarget );
        if ( i == stmt->labels.size() ) {
                return stmt;
        }
        Stmt * mutStmt = mutate( stmt );
        mutStmt->labels[i].attributes.push_back( new Attribute( "unused" ) );
        return mutStmt;
}

// This routine updates targets on enclosing control structures to indicate which
//     label is used by the BranchStmt that is passed
const BranchStmt * MultiLevelExitCore::postvisit( const BranchStmt * stmt ) {
        vector<Entry>::reverse_iterator targetEntry =
                enclosing_control_structures.rend();

        // Labels on different stmts require different approaches to access
        switch ( stmt->kind ) {
        case BranchStmt::Goto:
                return stmt;
        case BranchStmt::Continue:
        case BranchStmt::Break: {
                bool isContinue = stmt->kind == BranchStmt::Continue;
                // Handle unlabeled continue.
                if ( isContinue && stmt->target.empty() ) {
                        targetEntry = findEnclosingControlStructure( isContinueTarget );
                        if ( targetEntry == enclosing_control_structures.rend() ) {
                                SemanticError( stmt->location,
                                               "\"continue\" outside a loop" );
                        }
                // Handle unlabeled break.
                } else if ( stmt->target.empty() ) {
                        targetEntry = findEnclosingControlStructure( isUnlabelledBreakTarget );
                        if ( targetEntry == enclosing_control_structures.rend() ) {
                                SemanticError( stmt->location,
                                               "\"break\" outside a loop or \"switch\"" );
                        }
                // Handle labeled break and continue.
                } else {
                        // Lookup label in table to find attached control structure.
                        targetEntry = findEnclosingControlStructure(
                                [ targetStmt = target_table.at(stmt->target) ](auto entry){
                                          return entry.stmt == targetStmt;
                                } );
                        // Ensure that selected target is valid.
                        if ( targetEntry == enclosing_control_structures.rend()
                                        || ( isContinue ? !isContinueTarget( *targetEntry ) : !isBreakTarget( *targetEntry ) ) ) {
                                SemanticError( stmt->location, toString( (isContinue ? "\"continue\"" : "\"break\""),
                                               " target must be an enclosing ", (isContinue ? "loop: " : "control structure: "),
                                               stmt->originalTarget ) );
                        }
                }
                break;
        }
        // handle fallthrough in case/switch stmts
        case BranchStmt::FallThrough: {
                targetEntry = findEnclosingControlStructure( isFallthroughTarget );
                // Check that target is valid.
                if ( targetEntry == enclosing_control_structures.rend() ) {
                        SemanticError( stmt->location, "\"fallthrough\" must be enclosed in a \"switch\" or \"choose\"" );
                }
                if ( ! stmt->target.empty() ) {
                        // Labelled fallthrough: target must be a valid fallthough label.
                        if ( ! fallthrough_labels.count( stmt->target ) ) {
                                SemanticError( stmt->location, toString( "\"fallthrough\" target must be a later case statement: ",
                                                                                                                   stmt->originalTarget ) );
                        }
                        return new BranchStmt( stmt->location, BranchStmt::Goto, stmt->originalTarget );
                }
                break;
        }
        case BranchStmt::FallThroughDefault: {
                targetEntry = findEnclosingControlStructure( isFallthroughDefaultTarget );

                // Check if in switch or choose statement.
                if ( targetEntry == enclosing_control_structures.rend() ) {
                        SemanticError( stmt->location, "\"fallthrough\" must be enclosed in a \"switch\" or \"choose\"" );
                }

                // Check if switch or choose has default clause.
                auto switchStmt = strict_dynamic_cast< const SwitchStmt * >( targetEntry->stmt );
                bool foundDefault = false;
                for ( auto caseStmt : switchStmt->cases ) {
                        if ( caseStmt->isDefault() ) {
                                foundDefault = true;
                                break;
                        }
                }
                if ( ! foundDefault ) {
                        SemanticError( stmt->location, "\"fallthrough default\" must be enclosed in a \"switch\" or \"choose\""
                                                   "control structure with a \"default\" clause" );
                }
                break;
        }
        default:
                assert( false );
        }

        // Branch error checks: get the appropriate label name, which is always replaced.
        Label exitLabel( CodeLocation(), "" );
        switch ( stmt->kind ) {
        case BranchStmt::Break:
                assert( ! targetEntry->useBreakExit().empty() );
                exitLabel = targetEntry->useBreakExit();
                break;
        case BranchStmt::Continue:
                assert( ! targetEntry->useContExit().empty() );
                exitLabel = targetEntry->useContExit();
                break;
        case BranchStmt::FallThrough:
                assert( ! targetEntry->useFallExit().empty() );
                exitLabel = targetEntry->useFallExit();
                break;
        case BranchStmt::FallThroughDefault:
                assert( ! targetEntry->useFallDefaultExit().empty() );
                exitLabel = targetEntry->useFallDefaultExit();
                // Check that fallthrough default comes before the default clause.
                if ( ! targetEntry->isFallDefaultValid() ) {
                        SemanticError( stmt->location, "\"fallthrough default\" must precede the \"default\" clause" );
                }
                break;
        default:
                assert(0);
        }
        assert( !exitLabel.empty() );

        // Add unused attribute to silence warnings.
        targetEntry->stmt = addUnused( targetEntry->stmt, stmt->originalTarget );

        // Replace with goto to make later passes more uniform.
        return new BranchStmt( stmt->location, BranchStmt::Goto, exitLabel );
}

void MultiLevelExitCore::previsit( const WhileDoStmt * stmt ) {
        return prehandleLoopStmt( stmt );
}

const WhileDoStmt * MultiLevelExitCore::postvisit( const WhileDoStmt * stmt ) {
        return posthandleLoopStmt( stmt );
}

void MultiLevelExitCore::previsit( const ForStmt * stmt ) {
        return prehandleLoopStmt( stmt );
}

const ForStmt * MultiLevelExitCore::postvisit( const ForStmt * stmt ) {
        return posthandleLoopStmt( stmt );
}

// Mimic what the built-in push_front would do anyways. It is O(n).
void push_front( vector<ptr<Stmt>> & vec, const Stmt * element ) {
        vec.emplace_back( nullptr );
        for ( size_t i = vec.size() - 1 ; 0 < i ; --i ) {
                vec[ i ] = std::move( vec[ i - 1 ] );
        }
        vec[ 0 ] = element;
}

const CaseClause * MultiLevelExitCore::previsit( const CaseClause * stmt ) {
        visit_children = false;

        // If default, mark seen.
        if ( stmt->isDefault() ) {
                assert( ! enclosing_control_structures.empty() );
                enclosing_control_structures.back().seenDefault();
        }

        // The cond may not exist, but if it does update it now.
        visitor->maybe_accept( stmt, &CaseClause::cond );

        // Just save the mutated node for simplicity.
        CaseClause * mutStmt = mutate( stmt );

        Label fallLabel = newLabel( "fallThrough", stmt->location );
        if ( ! mutStmt->stmts.empty() ) {
                // These should already be in a block.
                auto first = mutStmt->stmts.front().get_and_mutate();
                auto block = strict_dynamic_cast<CompoundStmt *>( first );

                // Ensure that the stack isn't corrupted by exceptions in fixBlock.
                auto guard = makeFuncGuard(
                        [&](){ enclosing_control_structures.emplace_back( mutStmt, block, fallLabel ); },
                        [this](){ enclosing_control_structures.pop_back(); }
                        );

                block->kids = fixBlock( block->kids, true );

                // Add fallthrough label if necessary.
                assert( ! enclosing_control_structures.empty() );
                Entry & entry = enclosing_control_structures.back();
                if ( entry.isFallUsed() ) {
                        mutStmt->stmts.push_back( labelledNullStmt( block->location, entry.useFallExit() ) );
                }
        }
        assert( ! enclosing_control_structures.empty() );
        Entry & entry = enclosing_control_structures.back();
        assertf( dynamic_cast< const SwitchStmt * >( entry.stmt ),
                         "CFA internal error: control structure enclosing a case clause must be a switch, but is: %s",
                         toString( entry.stmt ).c_str() );
        if ( mutStmt->isDefault() ) {
                if ( entry.isFallDefaultUsed() ) {
                        // Add fallthrough default label if necessary.
                        push_front( mutStmt->stmts, labelledNullStmt( stmt->location, entry.useFallDefaultExit() ) );
                }
        }
        return mutStmt;
}

void MultiLevelExitCore::previsit( const IfStmt * stmt ) {
        bool labeledBlock = ! stmt->labels.empty();
        if ( labeledBlock ) {
                Label breakLabel = newLabel( "blockBreak", stmt );
                enclosing_control_structures.emplace_back( stmt, breakLabel );
                GuardAction( [this](){ enclosing_control_structures.pop_back(); } );
        }
}

const IfStmt * MultiLevelExitCore::postvisit( const IfStmt * stmt ) {
        bool labeledBlock = ! stmt->labels.empty();
        if ( labeledBlock ) {
                auto this_label = enclosing_control_structures.back().useBreakExit();
                if ( ! this_label.empty() ) {
                        break_label = this_label;
                }
        }
        return stmt;
}

static bool isDefaultCase( const ptr<CaseClause> & caseClause ) {
        return caseClause->isDefault();
}

void MultiLevelExitCore::previsit( const SwitchStmt * stmt ) {
        Label label = newLabel( "switchBreak", stmt );
        auto it = find_if( stmt->cases.rbegin(), stmt->cases.rend(), isDefaultCase );

        const CaseClause * defaultCase = it != stmt->cases.rend() ? (*it) : nullptr;
        Label defaultLabel = defaultCase ? newLabel( "fallThroughDefault", defaultCase->location ) : Label( stmt->location, "" );
        enclosing_control_structures.emplace_back( stmt, label, defaultLabel );
        GuardAction( [this]() { enclosing_control_structures.pop_back(); } );

        // Collect valid labels for fallthrough. It starts with all labels at this level, then remove as each is seen during
        // traversal.
        for ( const CaseClause * caseStmt : stmt->cases ) {
                if ( caseStmt->stmts.empty() ) continue;
                auto block = caseStmt->stmts.front().strict_as<CompoundStmt>();
                for ( const Stmt * stmt : block->kids ) {
                        for ( const Label & l : stmt->labels ) {
                                fallthrough_labels.insert( l );
                        }
                }
        }
}

const SwitchStmt * MultiLevelExitCore::postvisit( const SwitchStmt * stmt ) {
        assert( ! enclosing_control_structures.empty() );
        Entry & entry = enclosing_control_structures.back();
        assert( entry.stmt == stmt );

        // Only run to generate the break label.
        if ( entry.isBreakUsed() ) {
                // To keep the switch statements uniform (all direct children of a SwitchStmt should be CastStmts), append the
                // exit label and break to the last case, create a default case if no cases.
                SwitchStmt * mutStmt = mutate( stmt );
                if ( mutStmt->cases.empty() ) {
                        mutStmt->cases.push_back( new CaseClause( mutStmt->location, nullptr, {} ) );
                }

                auto mutCase = mutStmt->cases.back().get_and_mutate();

                Label label( mutCase->location, "breakLabel" );
                auto branch = new BranchStmt( mutCase->location, BranchStmt::Break, label );
                branch->labels.push_back( entry.useBreakExit() );
                mutCase->stmts.push_back( branch );

                return mutStmt;
        }
        return stmt;
}

void MultiLevelExitCore::previsit( const ReturnStmt * stmt ) {
        char const * context;
        switch ( ret_context ) {
        case ReturnContext::MayReturn:
                return;
        case ReturnContext::InTryWithHandler:
                context = "try statement with a catch clause";
                break;
        case ReturnContext::InResumeHandler:
                context = "catchResume clause";
                break;
        case ReturnContext::InFinally:
                context = "finally clause";
                break;
        default:
                assert(0);
        }
        SemanticError( stmt->location, "\"return\" may not appear in a %s", context );
}

bool hasTerminate( const TryStmt * stmt ) {
        for ( auto clause : stmt->handlers ) {
                if ( ast::Terminate == clause->kind ) return true;
        }
        return false;
}

void MultiLevelExitCore::previsit( const TryStmt * stmt ) {
        visit_children = false;

        bool isLabeled = ! stmt->labels.empty();
        if ( isLabeled ) {
                Label breakLabel = newLabel( "blockBreak", stmt );
                enclosing_control_structures.emplace_back( stmt, breakLabel );
                GuardAction([this](){ enclosing_control_structures.pop_back(); } );
        }

        // Try statements/try blocks are only sealed with a termination handler.
        if ( hasTerminate( stmt ) ) {
                // This is just enterSealedContext except scoped to the block.
                // And that is because the state must change for a single field.
                ValueGuard< ReturnContext > guard0( ret_context );
                ret_context = ReturnContext::InTryWithHandler;
                auto guard = makeFuncGuard( [](){}, [this, old = std::move(enclosing_control_structures)](){ enclosing_control_structures = std::move(old); });
                enclosing_control_structures = vector<Entry>();
                visitor->maybe_accept( stmt, &TryStmt::body );
        } else {
                visitor->maybe_accept( stmt, &TryStmt::body );
        }

        visitor->maybe_accept( stmt, &TryStmt::handlers );
        visitor->maybe_accept( stmt, &TryStmt::finally );
}

void MultiLevelExitCore::postvisit( const TryStmt * stmt ) {
        bool isLabeled = ! stmt->labels.empty();
        if ( isLabeled ) {
                auto this_label = enclosing_control_structures.back().useBreakExit();
                if ( ! this_label.empty() ) {
                        break_label = this_label;
                }
        }
}

void MultiLevelExitCore::previsit( const CatchClause * clause ) {
        if ( ast::Resume == clause->kind ) {
                enterSealedContext( ReturnContext::InResumeHandler );
        }
}

void MultiLevelExitCore::previsit( const FinallyClause * ) {
        enterSealedContext( ReturnContext::InFinally );
}

const Stmt * MultiLevelExitCore::mutateLoop(
        const Stmt * body, Entry & entry ) {
        if ( entry.isBreakUsed() ) {
                break_label = entry.useBreakExit();
        }

        // if continue is used insert a continue label into the back of the body of the loop
        if ( entry.isContUsed() ) {
                // {
                //  body
                //  ContinueLabel: ;
                // }
                return new CompoundStmt( body->location, {
                        body,
                        labelledNullStmt( body->location, entry.useContExit() ),
                } );
        }

        return body;
}

template<typename LoopNode>
void MultiLevelExitCore::prehandleLoopStmt( const LoopNode * loopStmt ) {
        // Create temporary labels and mark the enclosing loop before traversal.
        // The labels will be folded in if they are used.
        Label breakLabel = newLabel( "loopBreak", loopStmt );
        Label contLabel = newLabel( "loopContinue", loopStmt );
        enclosing_control_structures.emplace_back( loopStmt, breakLabel, contLabel );

        GuardAction( [this](){ enclosing_control_structures.pop_back(); } );

        // Because of fixBlock, this should be empty now (and must be).
        assert( nullptr == loopStmt->else_ );
}

template<typename LoopNode>
const LoopNode * MultiLevelExitCore::posthandleLoopStmt( const LoopNode * loopStmt ) {
        assert( ! enclosing_control_structures.empty() );
        Entry & entry = enclosing_control_structures.back();
        assert( entry.stmt == loopStmt );

        // Now check if the labels are used and add them if so.
        return mutate_field( loopStmt, &LoopNode::body, mutateLoop( loopStmt->body, entry ) );
}

list<ptr<Stmt>> MultiLevelExitCore::fixBlock(
        const list<ptr<Stmt>> & kids, bool is_case_clause ) {
        // Unfortunately cannot use automatic error collection.
        SemanticErrorException errors;

        list<ptr<Stmt>> ret;

        // Manually visit each child.
        for ( const ptr<Stmt> & kid : kids ) {
                if ( is_case_clause ) {
                        // Once a label is seen, it's no longer a valid for fallthrough.
                        for ( const Label & l : kid->labels ) {
                                fallthrough_labels.erase( l );
                        }
                }

                ptr<Stmt> else_stmt = nullptr;
                const Stmt * to_visit;
                // check if loop node and if so add else clause if it exists
                if ( auto ptr = kid.as<WhileDoStmt>() ; ptr && ptr->else_ ) {
                        else_stmt = ptr->else_;
                        to_visit = mutate_field( ptr, &WhileDoStmt::else_, nullptr );
                } else if ( auto ptr = kid.as<ForStmt>() ; ptr && ptr->else_ ) {
                        else_stmt = ptr->else_;
                        to_visit = mutate_field( ptr, &ForStmt::else_, nullptr );
                } else {
                        to_visit = kid.get();
                }

                // This is the main (safe) visit of the child node.
                try {
                        ret.push_back( to_visit->accept( *visitor ) );
                } catch ( SemanticErrorException & e ) {
                        errors.append( e );
                }

                // The following sections handle visiting loop else clause and makes
                // sure breaking from a loop body does not execute that clause.
                Label local_break_label = std::move( break_label );
                break_label = Label( CodeLocation(), "" );

                if ( else_stmt ) try {
                        ret.push_back( else_stmt->accept( *visitor ) );
                } catch ( SemanticErrorException & e ) {
                        errors.append( e );
                }

                if ( !break_label.empty() ) {
                        ret.push_back( labelledNullStmt( ret.back()->location, break_label ) );
                        break_label = Label( CodeLocation(), "" );
                }

                // This handles a break from the body or non-loop statement.
                if ( !local_break_label.empty() ) {
                        ret.push_back( labelledNullStmt( ret.back()->location, local_break_label ) );
                }
        }

        errors.throwIfNonEmpty();
        return ret;
}

void MultiLevelExitCore::enterSealedContext( ReturnContext enter_context ) {
        GuardAction([this, old = std::move(enclosing_control_structures)](){ enclosing_control_structures = std::move(old); });
        enclosing_control_structures = vector<Entry>();
        GuardValue( ret_context ) = enter_context;
}

} // namespace

const CompoundStmt * multiLevelExitUpdate(
                const CompoundStmt * stmt, const LabelToStmt & labelTable ) {
        // Must start in the body, so FunctionDecls can be a stopping point.
        Pass<MultiLevelExitCore> visitor( labelTable );
        return stmt->accept( visitor );
}

} // namespace ControlStruct

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// mode: c++ //
// compile-command: "make install" //
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