source: src/GenPoly/InstantiateGeneric.cc @ 8a97248

//
// Cforall Version 1.0.0 Copyright (C) 2016 University of Waterloo
//
// The contents of this file are covered under the licence agreement in the
// file "LICENCE" distributed with Cforall.
//
// InstantiateGeneric.cc --
//
// Author           : Aaron B. Moss
// Created On       : Thu Aug 04 18:33:00 2016
// Last Modified By : Andrew Beach
// Last Modified On : Wed Jul 16 10:17:00 2020
// Update Count     : 2
//
#include "InstantiateGeneric.h"

#include <cassert>                     // for assertf, assert
#include <iterator>                    // for back_inserter, inserter
#include <list>                        // for list, _List_const_iterator
#include <utility>                     // for move, pair
#include <vector>                      // for vector

#include "CodeGen/OperatorTable.h"
#include "Common/PassVisitor.h"        // for PassVisitor, WithDeclsToAdd
#include "Common/ScopedMap.h"          // for ScopedMap
#include "Common/UniqueName.h"         // for UniqueName
#include "Common/utility.h"            // for deleteAll, cloneAll
#include "GenPoly.h"                   // for isPolyType, typesPolyCompatible
#include "InitTweak/InitTweak.h"
#include "ResolvExpr/AdjustExprType.hpp"  // for adjustExprType
#include "ResolvExpr/Unify.h"          // for typesCompatible
#include "ScopedSet.h"                 // for ScopedSet, ScopedSet<>::iterator
#include "ScrubTyVars.h"               // for ScrubTyVars
#include "SynTree/Declaration.h"       // for StructDecl, UnionDecl, TypeDecl
#include "SynTree/Expression.h"        // for TypeExpr, Expression
#include "SynTree/Mutator.h"           // for mutateAll
#include "SynTree/Type.h"              // for StructInstType, UnionInstType
#include "SynTree/TypeSubstitution.h"  // for TypeSubstitution
#include "SynTree/Visitor.h"           // for acceptAll


namespace GenPoly {

        /// Abstracts type equality for a list of parameter types
        struct TypeList {
                TypeList() : params() {}
                TypeList( const std::list< Type* > &_params ) : params() { cloneAll(_params, params); }
                TypeList( std::list< Type* > &&_params ) : params( _params ) {}

                TypeList( const TypeList &that ) : params() { cloneAll(that.params, params); }
                TypeList( TypeList &&that ) : params( std::move( that.params ) ) {}

                /// Extracts types from a list of TypeExpr*
                TypeList( const std::list< TypeExpr* >& _params ) : params() {
                        for ( std::list< TypeExpr* >::const_iterator param = _params.begin(); param != _params.end(); ++param ) {
                                params.push_back( (*param)->get_type()->clone() );
                        }
                }

                TypeList& operator= ( const TypeList &that ) {
                        deleteAll( params );

                        params.clear();
                        cloneAll( that.params, params );

                        return *this;
                }

                TypeList& operator= ( TypeList &&that ) {
                        deleteAll( params );

                        params = std::move( that.params );

                        return *this;
                }

                ~TypeList() { deleteAll( params ); }

                bool operator== ( const TypeList& that ) const {
                        if ( params.size() != that.params.size() ) return false;

                        for ( std::list< Type* >::const_iterator it = params.begin(), jt = that.params.begin(); it != params.end(); ++it, ++jt ) {
                                if ( ! typesPolyCompatible( *it, *jt ) ) return false;
                        }
                        return true;
                }

                std::list< Type* > params;  ///< Instantiation parameters
        };

        /// Maps a key and a TypeList to the some value, accounting for scope
        template< typename Key, typename Value >
        class InstantiationMap {
                /// Wraps value for a specific (Key, TypeList) combination
                typedef std::pair< TypeList, Value* > Instantiation;
                /// List of TypeLists paired with their appropriate values
                typedef std::vector< Instantiation > ValueList;
                /// Underlying map type; maps keys to a linear list of corresponding TypeLists and values
                typedef ScopedMap< Key*, ValueList > InnerMap;

                InnerMap instantiations;  ///< instantiations

        public:
                /// Starts a new scope
                void beginScope() { instantiations.beginScope(); }

                /// Ends a scope
                void endScope() { instantiations.endScope(); }

                /// Gets the value for the (key, typeList) pair, returns NULL on none such.
                Value *lookup( Key *key, const std::list< TypeExpr* >& params ) const {
                        TypeList typeList( params );

                        // scan scopes for matches to the key
                        for ( typename InnerMap::const_iterator insts = instantiations.find( key ); insts != instantiations.end(); insts = instantiations.findNext( insts, key ) ) {
                                for ( typename ValueList::const_reverse_iterator inst = insts->second.rbegin(); inst != insts->second.rend(); ++inst ) {
                                        if ( inst->first == typeList ) return inst->second;
                                }
                        }
                        // no matching instantiations found
                        return 0;
                }

                /// Adds a value for a (key, typeList) pair to the current scope
                void insert( Key *key, const std::list< TypeExpr* > &params, Value *value ) {
                        auto it = instantiations.findAt( instantiations.currentScope(), key );
                        if ( it == instantiations.end() ) {
                                instantiations.insert( key, ValueList{ Instantiation{ TypeList( params ), value } } );
                        } else {
                                it->second.push_back( Instantiation{ TypeList( params ), value } );
                        }
                }
        };

        /// Possible options for a given specialization of a generic type
        enum class genericType {
                dtypeStatic,  ///< Concrete instantiation based solely on {d,f}type-to-void conversions
                concrete,     ///< Concrete instantiation requiring at least one parameter type
                dynamic       ///< No concrete instantiation
        };

        genericType& operator |= ( genericType& gt, const genericType& ht ) {
                switch ( gt ) {
                case genericType::dtypeStatic:
                        gt = ht;
                        break;
                case genericType::concrete:
                        if ( ht == genericType::dynamic ) { gt = genericType::dynamic; }
                        break;
                case genericType::dynamic:
                        // nothing possible
                        break;
                }
                return gt;
        }

        /// Add cast to dtype-static member expressions so that type information is not lost in GenericInstantiator
        struct FixDtypeStatic final : public WithGuards, public WithVisitorRef<FixDtypeStatic>, public WithShortCircuiting, public WithStmtsToAdd {
                Expression * postmutate( MemberExpr * memberExpr );

                void premutate( ApplicationExpr * appExpr );
                void premutate( AddressExpr * addrExpr );

                template<typename AggrInst>
                Expression * fixMemberExpr( AggrInst * inst, MemberExpr * memberExpr );

                bool isLvalueArg = false;
        };

        /// Mutator pass that replaces concrete instantiations of generic types with actual struct declarations, scoped appropriately
        struct GenericInstantiator final : public WithConstTypeSubstitution, public WithDeclsToAdd, public WithVisitorRef<GenericInstantiator>, public WithGuards {
                /// Map of (generic type, parameter list) pairs to concrete type instantiations
                InstantiationMap< AggregateDecl, AggregateDecl > instantiations;
                /// Set of types which are dtype-only generic (and therefore have static layout)
                std::set<AggregateDecl *> dtypeStatics;
                /// Namer for concrete types
                UniqueName typeNamer;
                /// Should not make use of type environment to replace types of function parameter and return values.
                bool inFunctionType = false;
                /// Index of current member, used to recreate MemberExprs with the member from an instantiation
                int memberIndex = -1;
                GenericInstantiator() : instantiations(), dtypeStatics(), typeNamer("_conc_") {}

                Type* postmutate( StructInstType *inst );
                Type* postmutate( UnionInstType *inst );

                // fix MemberExprs to use the member from the instantiation
                void premutate( MemberExpr * memberExpr );
                Expression * postmutate( MemberExpr * memberExpr );

                void premutate( FunctionType * ) {
                        GuardValue( inFunctionType );
                        inFunctionType = true;
                }

                void beginScope();
                void endScope();
        private:
                /// Wrap instantiation lookup for structs
                StructDecl* lookup( StructInstType *inst, const std::list< TypeExpr* > &typeSubs ) { return (StructDecl*)instantiations.lookup( inst->get_baseStruct(), typeSubs ); }
                /// Wrap instantiation lookup for unions
                UnionDecl* lookup( UnionInstType *inst, const std::list< TypeExpr* > &typeSubs ) { return (UnionDecl*)instantiations.lookup( inst->get_baseUnion(), typeSubs ); }
                /// Wrap instantiation insertion for structs
                void insert( StructInstType *inst, const std::list< TypeExpr* > &typeSubs, StructDecl *decl ) { instantiations.insert( inst->get_baseStruct(), typeSubs, decl ); }
                /// Wrap instantiation insertion for unions
                void insert( UnionInstType *inst, const std::list< TypeExpr* > &typeSubs, UnionDecl *decl ) { instantiations.insert( inst->get_baseUnion(), typeSubs, decl ); }

                void replaceParametersWithConcrete( std::list< Expression* >& params );
                Type *replaceWithConcrete( Type *type, bool doClone );

                /// Strips a dtype-static aggregate decl of its type parameters, marks it as stripped
                void stripDtypeParams( AggregateDecl *base, std::list< TypeDecl* >& baseParams, const std::list< TypeExpr* >& typeSubs );
        };

        void instantiateGeneric( std::list< Declaration* > &translationUnit ) {
                PassVisitor<FixDtypeStatic> fixer;
                PassVisitor<GenericInstantiator> instantiator;

                mutateAll( translationUnit, fixer );
                mutateAll( translationUnit, instantiator );
        }

        bool isDtypeStatic( const std::list< TypeDecl* >& baseParams ) {
                return std::all_of( baseParams.begin(), baseParams.end(), []( TypeDecl * td ) { return ! td->isComplete(); } );
        }

        /// Makes substitutions of params into baseParams; returns dtypeStatic if there is a concrete instantiation based only on {d,f}type-to-void conversions,
        /// concrete if there is a concrete instantiation requiring at least one parameter type, and dynamic if there is no concrete instantiation
        genericType makeSubstitutions( const std::list< TypeDecl* >& baseParams, const std::list< Expression* >& params, std::list< TypeExpr* >& out ) {
                genericType gt = genericType::dtypeStatic;

                // substitute concrete types for given parameters, and incomplete types for placeholders
                std::list< TypeDecl* >::const_iterator baseParam = baseParams.begin();
                std::list< Expression* >::const_iterator param = params.begin();
                for ( ; baseParam != baseParams.end() && param != params.end(); ++baseParam, ++param ) {
                        TypeExpr *paramType = dynamic_cast< TypeExpr* >( *param );
                        assert(paramType && "Aggregate parameters should be type expressions");

                        if ( (*baseParam)->isComplete() ) {
                                // substitute parameter for complete (otype or sized dtype) type
                                if ( isPolyType( paramType->get_type() ) ) {
                                        // substitute polymorphic parameter type in to generic type
                                        out.push_back( paramType->clone() );
                                        gt = genericType::dynamic;
                                } else {
                                        // normalize possibly dtype-static parameter type
                                        out.push_back( new TypeExpr{
                                                ScrubTyVars::scrubAll( paramType->get_type()->clone() ) } );
                                        gt |= genericType::concrete;
                                }
                        } else switch ( (*baseParam)->get_kind() ) {
                                case TypeDecl::Dtype:
                                        // can pretend that any incomplete dtype is `void`
                                        out.push_back( new TypeExpr( new VoidType( Type::Qualifiers() ) ) );
                                        break;
                                case TypeDecl::Ftype:
                                        // can pretend that any ftype is `void (*)(void)`
                                        out.push_back( new TypeExpr( new FunctionType( Type::Qualifiers(), false ) ) );
                                        break;
                                case TypeDecl::Ttype:
                                        assertf( false, "Ttype parameters are not currently allowed as parameters to generic types." );
                                        break;
                                default:
                                        assertf( false, "Unhandled type parameter kind" );
                                        break;
                        }
                }

                assertf( baseParam == baseParams.end() && param == params.end(), "Type parameters should match type variables" );
                return gt;
        }

        /// Substitutes types of members of in according to baseParams => typeSubs, appending the result to out
        void substituteMembers( const std::list< Declaration* >& in, const std::list< TypeDecl* >& baseParams, const std::list< TypeExpr* >& typeSubs,
                                                        std::list< Declaration* >& out ) {
                // substitute types into new members
                TypeSubstitution subs( baseParams.begin(), baseParams.end(), typeSubs.begin() );
                for ( std::list< Declaration* >::const_iterator member = in.begin(); member != in.end(); ++member ) {
                        Declaration *newMember = (*member)->clone();
                        subs.apply(newMember);
                        out.push_back( newMember );
                }
        }

        /// Substitutes types of members according to baseParams => typeSubs, working in-place
        void substituteMembers( std::list< Declaration* >& members, const std::list< TypeDecl* >& baseParams, const std::list< TypeExpr* >& typeSubs ) {
                // substitute types into new members
                TypeSubstitution subs( baseParams.begin(), baseParams.end(), typeSubs.begin() );
                for ( std::list< Declaration* >::iterator member = members.begin(); member != members.end(); ++member ) {
                        subs.apply(*member);
                }
        }

        /// Strips the instances's type parameters
        void stripInstParams( ReferenceToType *inst ) {
                deleteAll( inst->get_parameters() );
                inst->get_parameters().clear();
        }

        template< typename AggrInst >
        static AggrInst * asForward( AggrInst * decl ) {
                if ( !decl->body ) {
                        return nullptr;
                }
                decl = decl->clone();
                decl->body = false;
                deleteAll( decl->members );
                decl->members.clear();
                return decl;
        }

        void GenericInstantiator::stripDtypeParams( AggregateDecl *base, std::list< TypeDecl* >& baseParams, const std::list< TypeExpr* >& typeSubs ) {
                substituteMembers( base->get_members(), baseParams, typeSubs );

                // xxx - can't delete type parameters because they may have assertions that are used
                // deleteAll( baseParams );
                baseParams.clear();

                dtypeStatics.insert( base );
        }

        /// xxx - more or less copied from box -- these should be merged with those somehow...
        void GenericInstantiator::replaceParametersWithConcrete( std::list< Expression* >& params ) {
                for ( std::list< Expression* >::iterator param = params.begin(); param != params.end(); ++param ) {
                        TypeExpr *paramType = dynamic_cast< TypeExpr* >( *param );
                        assertf(paramType, "Aggregate parameters should be type expressions");
                        paramType->set_type( replaceWithConcrete( paramType->get_type(), false ) );
                }
        }

        Type *GenericInstantiator::replaceWithConcrete( Type *type, bool doClone ) {
                if ( TypeInstType *typeInst = dynamic_cast< TypeInstType * >( type ) ) {
                        if ( env && ! inFunctionType ) {
                                Type *concrete = env->lookup( typeInst->get_name() );
                                if ( concrete ) {
                                        return concrete->clone();
                                }
                                else return typeInst->clone();
                        }
                } else if ( StructInstType *structType = dynamic_cast< StructInstType* >( type ) ) {
                        if ( doClone ) {
                                structType = structType->clone();
                        }
                        replaceParametersWithConcrete( structType->get_parameters() );
                        return structType;
                } else if ( UnionInstType *unionType = dynamic_cast< UnionInstType* >( type ) ) {
                        if ( doClone ) {
                                unionType = unionType->clone();
                        }
                        replaceParametersWithConcrete( unionType->get_parameters() );
                        return unionType;
                }
                return type;
        }


        Type* GenericInstantiator::postmutate( StructInstType *inst ) {
                // exit early if no need for further mutation
                if ( inst->get_parameters().empty() ) return inst;

                // need to replace type variables to ensure that generic types are instantiated for the return values of polymorphic functions (in particular, for thunks, because they are not [currently] copy constructed).
                replaceWithConcrete( inst, false );

                // check for an already-instantiatiated dtype-static type
                if ( dtypeStatics.find( inst->get_baseStruct() ) != dtypeStatics.end() ) {
                        stripInstParams( inst );
                        return inst;
                }

                // check if type can be concretely instantiated; put substitutions into typeSubs
                assertf( inst->get_baseParameters(), "Base struct has parameters" );
                std::list< TypeExpr* > typeSubs;
                genericType gt = makeSubstitutions( *inst->get_baseParameters(), inst->get_parameters(), typeSubs );
                switch ( gt ) {
                case genericType::dtypeStatic:
                        stripDtypeParams( inst->get_baseStruct(), *inst->get_baseParameters(), typeSubs );
                        stripInstParams( inst );
                        break;

                case genericType::concrete: {
                        // make concrete instantiation of generic type
                        StructDecl *concDecl = lookup( inst, typeSubs );
                        if ( ! concDecl ) {
                                // set concDecl to new type, insert type declaration into statements to add
                                concDecl = new StructDecl( typeNamer.newName( inst->get_name() ) );
                                concDecl->set_body( inst->get_baseStruct()->has_body() );
                                substituteMembers( inst->get_baseStruct()->get_members(), *inst->get_baseParameters(), typeSubs, concDecl->get_members() );
                                // Forward declare before recursion. (TODO: Only when needed, #199.)
                                insert( inst, typeSubs, concDecl );
                                if ( StructDecl *forwardDecl = asForward( concDecl ) ) {
                                        declsToAddBefore.push_back( forwardDecl );
                                }
                                concDecl->acceptMutator( *visitor ); // recursively instantiate members
                                declsToAddBefore.push_back( concDecl ); // must occur before declaration is added so that member instantiations appear first
                        }
                        StructInstType *newInst = new StructInstType( inst->get_qualifiers(), concDecl->get_name() );
                        newInst->set_baseStruct( concDecl );

                        delete inst;
                        inst = newInst;
                        break;
                }

                case genericType::dynamic:
                        // do nothing
                        break;
                }

                deleteAll( typeSubs );
                return inst;
        }

        Type* GenericInstantiator::postmutate( UnionInstType *inst ) {
                // exit early if no need for further mutation
                if ( inst->get_parameters().empty() ) return inst;

                // check for an already-instantiatiated dtype-static type
                if ( dtypeStatics.find( inst->get_baseUnion() ) != dtypeStatics.end() ) {
                        stripInstParams( inst );
                        return inst;
                }

                // check if type can be concretely instantiated; put substitutions into typeSubs
                assert( inst->get_baseParameters() && "Base union has parameters" );
                std::list< TypeExpr* > typeSubs;
                genericType gt = makeSubstitutions( *inst->get_baseParameters(), inst->get_parameters(), typeSubs );
                switch ( gt ) {
                case genericType::dtypeStatic:
                        stripDtypeParams( inst->get_baseUnion(), *inst->get_baseParameters(), typeSubs );
                        stripInstParams( inst );
                        break;

                case genericType::concrete:
                {
                        // make concrete instantiation of generic type
                        UnionDecl *concDecl = lookup( inst, typeSubs );
                        if ( ! concDecl ) {
                                // set concDecl to new type, insert type declaration into statements to add
                                concDecl = new UnionDecl( typeNamer.newName( inst->get_name() ) );
                                concDecl->set_body( inst->get_baseUnion()->has_body() );
                                substituteMembers( inst->get_baseUnion()->get_members(), *inst->get_baseParameters(), typeSubs, concDecl->get_members() );
                                // Forward declare before recursion. (TODO: Only when needed, #199.)
                                insert( inst, typeSubs, concDecl );
                                if ( UnionDecl *forwardDecl = asForward( concDecl ) ) {
                                        declsToAddBefore.push_back( forwardDecl );
                                }
                                concDecl->acceptMutator( *visitor ); // recursively instantiate members
                                declsToAddBefore.push_back( concDecl ); // must occur before declaration is added so that member instantiations appear first
                        }
                        UnionInstType *newInst = new UnionInstType( inst->get_qualifiers(), concDecl->get_name() );
                        newInst->set_baseUnion( concDecl );

                        delete inst;
                        inst = newInst;
                        break;
                }
                case genericType::dynamic:
                        // do nothing
                        break;
                }

                deleteAll( typeSubs );
                return inst;
        }

        namespace {
                bool isGenericType( Type * t ) {
                        if ( StructInstType * inst = dynamic_cast< StructInstType * >( t ) ) {
                                return ! inst->parameters.empty();
                        } else if ( UnionInstType * inst = dynamic_cast< UnionInstType * >( t ) ) {
                                return ! inst->parameters.empty();
                        }
                        return false;
                }
        }

        void GenericInstantiator::premutate( MemberExpr * memberExpr ) {
                GuardValue( memberIndex );
                memberIndex = -1;
                if ( isGenericType( memberExpr->aggregate->result ) ) {
                        // find the location of the member
                        AggregateDecl * aggr = memberExpr->aggregate->result->getAggr();
                        std::list< Declaration * > & members = aggr->members;
                        memberIndex = std::distance( members.begin(), std::find( members.begin(), members.end(), memberExpr->member ) );
                        assertf( memberIndex < (int)members.size(), "Could not find member %s in generic type %s", toString( memberExpr->member ).c_str(), toString( memberExpr->aggregate ).c_str() );
                }
        }

        Expression * GenericInstantiator::postmutate( MemberExpr * memberExpr ) {
                if ( memberIndex != -1 ) {
                        // using the location from the generic type, find the member in the instantiation and rebuild the member expression
                        AggregateDecl * aggr = memberExpr->aggregate->result->getAggr();
                        assertf( memberIndex < (int)aggr->members.size(), "Instantiation somehow has fewer members than the generic type." );
                        Declaration * member = *std::next( aggr->members.begin(), memberIndex );
                        assertf( member->name == memberExpr->member->name, "Instantiation has different member order than the generic type. %s / %s", toString( member ).c_str(), toString( memberExpr->member ).c_str() );
                        DeclarationWithType * field = strict_dynamic_cast< DeclarationWithType * >( member );
                        MemberExpr * ret = new MemberExpr( field, memberExpr->aggregate->clone() );
                        ResolvExpr::adjustExprType( ret->result ); // pointer decay
                        std::swap( ret->env, memberExpr->env );
                        delete memberExpr;
                        return ret;
                }
                return memberExpr;
        }

        void GenericInstantiator::beginScope() {
                instantiations.beginScope();
        }

        void GenericInstantiator::endScope() {
                instantiations.endScope();
        }

        template< typename AggrInst >
        Expression * FixDtypeStatic::fixMemberExpr( AggrInst * inst, MemberExpr * memberExpr ) {
                // need to cast dtype-static member expressions to their actual type before that type is erased.
                // NOTE: the casts here have the third argument (isGenerated) set to false so that these casts persist until Box, where they are needed.
                auto & baseParams = *inst->get_baseParameters();
                if ( isDtypeStatic( baseParams ) ) {
                        if ( ! ResolvExpr::typesCompatible( memberExpr->result, memberExpr->member->get_type(), SymTab::Indexer() ) ) {
                                // type of member and type of expression differ
                                Type * concType = memberExpr->result->clone();
                                if ( isLvalueArg ) {
                                        // result must be C lvalue, so make a new reference variable with the correct actual type to replace the member expression
                                        //   forall(dtype T)
                                        //   struct Ptr {
                                        //     T * x
                                        //   };
                                        //   Ptr(int) p;
                                        //   int i;
                                        //   p.x = &i;
                                        // becomes
                                        //   int *& _dtype_static_member_0 = (int **)&p.x;
                                        //   _dtype_static_member_0 = &i;
                                        // Note: this currently creates more temporaries than is strictly necessary, since it does not check for duplicate uses of the same member expression.
                                        static UniqueName tmpNamer( "_dtype_static_member_" );
                                        Expression * init = new CastExpr( new AddressExpr( memberExpr ), new PointerType( Type::Qualifiers(), concType->clone() ), false );
                                        ObjectDecl * tmp = ObjectDecl::newObject( tmpNamer.newName(), new ReferenceType( Type::Qualifiers(), concType ), new SingleInit( init ) );
                                        stmtsToAddBefore.push_back( new DeclStmt( tmp ) );
                                        return new VariableExpr( tmp );
                                } else {
                                        // can simply add a cast to actual type
                                        return new CastExpr( memberExpr, concType, false );
                                }
                        }
                }
                return memberExpr;
        }

        Expression * FixDtypeStatic::postmutate( MemberExpr * memberExpr ) {
                Type * aggrType = memberExpr->aggregate->result;
                if ( isGenericType( aggrType ) ) {
                        if ( StructInstType * inst = dynamic_cast< StructInstType * >( aggrType ) ) {
                                return fixMemberExpr( inst, memberExpr );
                        } else if ( UnionInstType * inst = dynamic_cast< UnionInstType * >( aggrType ) ) {
                                return fixMemberExpr( inst, memberExpr );
                        }
                }
                return memberExpr;
        }

        void FixDtypeStatic::premutate( ApplicationExpr * appExpr ) {
                GuardValue( isLvalueArg );
                isLvalueArg = false;
                DeclarationWithType * function = InitTweak::getFunction( appExpr );
                if ( function->linkage == LinkageSpec::Intrinsic && CodeGen::isAssignment( function->name ) ) {
                        // explicitly visit children because only the first argument must be a C lvalue.
                        visit_children = false;
                        appExpr->env = maybeMutate( appExpr->env, *visitor );
                        appExpr->result = maybeMutate( appExpr->result, *visitor );
                        appExpr->function = maybeMutate( appExpr->function, *visitor );
                        isLvalueArg = true;
                        for ( Expression * arg : appExpr->args ) {
                                arg = maybeMutate( arg, *visitor );
                                isLvalueArg = false;
                        }
                }
        }

        void FixDtypeStatic::premutate( AddressExpr * ) {
                // argument of & must be C lvalue
                GuardValue( isLvalueArg );
                isLvalueArg = true;
        }
} // namespace GenPoly

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