source: src/InitTweak/FixInitNew.cpp @ 33b7d49

#include "FixInit.h"

#include <stddef.h>                    // for NULL
#include <algorithm>                   // for set_difference, copy_if
#include <cassert>                     // for assert, strict_dynamic_cast
#include <iostream>                    // for operator<<, ostream, basic_ost...
#include <iterator>                    // for insert_iterator, back_inserter
#include <list>                        // for _List_iterator, list, list<>::...
#include <map>                         // for _Rb_tree_iterator, _Rb_tree_co...
#include <memory>                      // for allocator_traits<>::value_type
#include <set>                         // for set, set<>::value_type
#include <unordered_map>               // for unordered_map, unordered_map<>...
#include <unordered_set>               // for unordered_set
#include <utility>                     // for pair

#include "CodeGen/GenType.h"           // for genPrettyType
#include "CodeGen/OperatorTable.h"
#include "Common/CodeLocationTools.hpp"
#include "Common/PassVisitor.h"        // for PassVisitor, WithStmtsToAdd
#include "Common/SemanticError.h"      // for SemanticError
#include "Common/UniqueName.h"         // for UniqueName
#include "Common/utility.h"            // for CodeLocation, ValueGuard, toSt...
#include "FixGlobalInit.h"             // for fixGlobalInit
#include "GenInit.h"                   // for genCtorDtor
#include "GenPoly/GenPoly.h"           // for getFunctionType
#include "InitTweak.h"                 // for getFunctionName, getCallArg
#include "ResolvExpr/Resolver.h"       // for findVoidExpression
#include "ResolvExpr/typeops.h"        // for typesCompatible
#include "SymTab/Autogen.h"            // for genImplicitCall
#include "SymTab/Indexer.h"            // for Indexer
#include "SymTab/Mangler.h"            // for Mangler
#include "SynTree/LinkageSpec.h"       // for C, Spec, Cforall, isBuiltin
#include "SynTree/Attribute.h"         // for Attribute
#include "SynTree/Constant.h"          // for Constant
#include "SynTree/Declaration.h"       // for ObjectDecl, FunctionDecl, Decl...
#include "SynTree/Expression.h"        // for UniqueExpr, VariableExpr, Unty...
#include "SynTree/Initializer.h"       // for ConstructorInit, SingleInit
#include "SynTree/Label.h"             // for Label, operator<
#include "SynTree/Mutator.h"           // for mutateAll, Mutator, maybeMutate
#include "SynTree/Statement.h"         // for ExprStmt, CompoundStmt, Branch...
#include "SynTree/Type.h"              // for Type, Type::StorageClasses
#include "SynTree/TypeSubstitution.h"  // for TypeSubstitution, operator<<
#include "SynTree/DeclReplacer.h"      // for DeclReplacer
#include "SynTree/Visitor.h"           // for acceptAll, maybeAccept
#include "Validate/FindSpecialDecls.h" // for dtorStmt, dtorStructDestroy

#include "AST/Expr.hpp"
#include "AST/Node.hpp"
#include "AST/Pass.hpp"
#include "AST/Print.hpp"
#include "AST/SymbolTable.hpp"
#include "AST/Type.hpp"
#include "AST/DeclReplacer.hpp"

extern bool ctordtorp; // print all debug
extern bool ctorp; // print ctor debug
extern bool cpctorp; // print copy ctor debug
extern bool dtorp; // print dtor debug
#define PRINT( text ) if ( ctordtorp ) { text }
#define CP_CTOR_PRINT( text ) if ( ctordtorp || cpctorp ) { text }
#define DTOR_PRINT( text ) if ( ctordtorp || dtorp ) { text }

namespace InitTweak {
namespace {
        struct SelfAssignChecker {
                void previsit( const ast::ApplicationExpr * appExpr );
        };

        struct StmtExprResult {
                const ast::StmtExpr * previsit( const ast::StmtExpr * stmtExpr );
        };

        /// wrap function application expressions as ImplicitCopyCtorExpr nodes so that it is easy to identify which
        /// function calls need their parameters to be copy constructed
        struct InsertImplicitCalls : public ast::WithConstTypeSubstitution, public ast::WithShortCircuiting {
                const ast::Expr * postvisit( const ast::ApplicationExpr * appExpr );

                // only handles each UniqueExpr once
                // if order of visit does not change, this should be safe
                void previsit (const ast::UniqueExpr *);

                std::unordered_set<decltype(ast::UniqueExpr::id)> visitedIds;
        };

        /// generate temporary ObjectDecls for each argument and return value of each ImplicitCopyCtorExpr,
        /// generate/resolve copy construction expressions for each, and generate/resolve destructors for both
        /// arguments and return value temporaries
        struct ResolveCopyCtors final : public ast::WithGuards, public ast::WithStmtsToAdd<>, public ast::WithSymbolTable, public ast::WithShortCircuiting, public ast::WithVisitorRef<ResolveCopyCtors> {
                const ast::Expr * postvisit( const ast::ImplicitCopyCtorExpr * impCpCtorExpr );
                const ast::StmtExpr * previsit( const ast::StmtExpr * stmtExpr );
                const ast::UniqueExpr * previsit( const ast::UniqueExpr * unqExpr );

                /// handles distant mutations of environment manually.
                /// WithConstTypeSubstitution cannot remember where the environment is from

                /// MUST be called at start of overload previsit
                void previsit( const ast::Expr * expr);
                /// MUST be called at return of overload postvisit
                const ast::Expr * postvisit(const ast::Expr * expr);

                /// create and resolve ctor/dtor expression: fname(var, [cpArg])
                const ast::Expr * makeCtorDtor( const std::string & fname, const ast::ObjectDecl * var, const ast::Expr * cpArg = nullptr );
                /// true if type does not need to be copy constructed to ensure correctness
                bool skipCopyConstruct( const ast::Type * type );
                ast::ptr< ast::Expr > copyConstructArg( const ast::Expr * arg, const ast::ImplicitCopyCtorExpr * impCpCtorExpr, const ast::Type * formal );
                ast::Expr * destructRet( const ast::ObjectDecl * ret, const ast::Expr * arg );
        private:
                /// hack to implement WithTypeSubstitution while conforming to mutation safety.
                ast::TypeSubstitution * env;
                bool                    envModified;
        };

        /// collects constructed object decls - used as a base class
        struct ObjDeclCollector : public ast::WithGuards, public ast::WithShortCircuiting {
                // use ordered data structure to maintain ordering for set_difference and for consistent error messages
                typedef std::list< const ast::ObjectDecl * > ObjectSet;
                void previsit( const ast::CompoundStmt *compoundStmt );
                void previsit( const ast::DeclStmt *stmt );

                // don't go into other functions
                void previsit( const ast::FunctionDecl * ) { visit_children = false; }

          protected:
                ObjectSet curVars;
        };

        // debug
        template<typename ObjectSet>
        struct PrintSet {
                PrintSet( const ObjectSet & objs ) : objs( objs ) {}
                const ObjectSet & objs;
        };
        template<typename ObjectSet>
        PrintSet<ObjectSet> printSet( const ObjectSet & objs ) { return PrintSet<ObjectSet>( objs ); }
        template<typename ObjectSet>
        std::ostream & operator<<( std::ostream & out, const PrintSet<ObjectSet> & set) {
                out << "{ ";
                for ( auto & obj : set.objs ) {
                        out << obj->name << ", " ;
                } // for
                out << " }";
                return out;
        }

        struct LabelFinder final : public ObjDeclCollector {
                typedef std::map< std::string, ObjectSet > LabelMap;
                // map of Label -> live variables at that label
                LabelMap vars;

                typedef ObjDeclCollector Parent;
                using Parent::previsit;
                void previsit( const ast::Stmt * stmt );

                void previsit( const ast::CompoundStmt *compoundStmt );
                void previsit( const ast::DeclStmt *stmt );
        };

        /// insert destructor calls at the appropriate places.  must happen before CtorInit nodes are removed
        /// (currently by FixInit)
        struct InsertDtors final : public ObjDeclCollector, public ast::WithStmtsToAdd<> {
                typedef std::list< ObjectDecl * > OrderedDecls;
                typedef std::list< OrderedDecls > OrderedDeclsStack;

                InsertDtors( ast::Pass<LabelFinder> & finder ) : finder( finder ), labelVars( finder.core.vars ) {}

                typedef ObjDeclCollector Parent;
                using Parent::previsit;

                void previsit( const ast::FunctionDecl * funcDecl );

                void previsit( const ast::BranchStmt * stmt );
        private:
                void handleGoto( const ast::BranchStmt * stmt );

                ast::Pass<LabelFinder> & finder;
                LabelFinder::LabelMap & labelVars;
                OrderedDeclsStack reverseDeclOrder;
        };

        /// expand each object declaration to use its constructor after it is declared.
        struct FixInit : public ast::WithStmtsToAdd<> {
                static void fixInitializers( ast::TranslationUnit &translationUnit );

                const ast::DeclWithType * postvisit( const ast::ObjectDecl *objDecl );

                std::list< ast::ptr< ast::Decl > > staticDtorDecls;
        };

        /// generate default/copy ctor and dtor calls for user-defined struct ctor/dtors
        /// for any member that is missing a corresponding ctor/dtor call.
        /// error if a member is used before constructed
        struct GenStructMemberCalls final : public ast::WithGuards, public ast::WithShortCircuiting, public ast::WithSymbolTable, public ast::WithVisitorRef<GenStructMemberCalls> {
                void previsit( const ast::FunctionDecl * funcDecl );
                const ast::DeclWithType * postvisit( const ast::FunctionDecl * funcDecl );

                void previsit( const ast::MemberExpr * memberExpr );
                void previsit( const ast::ApplicationExpr * appExpr );

                /// Note: this post mutate used to be in a separate visitor. If this pass breaks, one place to examine is whether it is
                /// okay for this part of the recursion to occur alongside the rest.
                const ast::Expr * postvisit( const ast::UntypedExpr * expr );

                SemanticErrorException errors;
          private:
                template< typename... Params >
                void emit( CodeLocation, const Params &... params );

                ast::FunctionDecl * function = nullptr;
                std::set< const ast::DeclWithType * > unhandled;
                std::map< const ast::DeclWithType *, CodeLocation > usedUninit;
                const ast::ObjectDecl * thisParam = nullptr;
                bool isCtor = false; // true if current function is a constructor
                const ast::StructDecl * structDecl = nullptr;
        };

        /// expands ConstructorExpr nodes into comma expressions, using a temporary for the first argument
        struct FixCtorExprs final : public ast::WithDeclsToAdd<>, public ast::WithSymbolTable, public ast::WithShortCircuiting {
                const ast::Expr * postvisit( const ast::ConstructorExpr * ctorExpr );
        };

        /// add CompoundStmts around top-level expressions so that temporaries are destroyed in the correct places.
        struct SplitExpressions : public ast::WithShortCircuiting {
                ast::Stmt * postvisit( const ast::ExprStmt * stmt );
                void previsit( const ast::TupleAssignExpr * expr );
        };
} // namespace

void fix( ast::TranslationUnit & translationUnit, bool inLibrary ) {
        ast::Pass<SelfAssignChecker>::run( translationUnit );

        // fixes StmtExpr to properly link to their resulting expression
        ast::Pass<StmtExprResult>::run( translationUnit );

        // fixes ConstructorInit for global variables. should happen before fixInitializers.
        InitTweak::fixGlobalInit( translationUnit, inLibrary );

        // must happen before ResolveCopyCtors because temporaries have to be inserted into the correct scope
        ast::Pass<SplitExpressions>::run( translationUnit );

        ast::Pass<InsertImplicitCalls>::run( translationUnit );

        // Needs to happen before ResolveCopyCtors, because argument/return temporaries should not be considered in
        // error checking branch statements
        {
                ast::Pass<LabelFinder> finder;
                ast::Pass<InsertDtors>::run( translationUnit, finder );
        }

        ast::Pass<ResolveCopyCtors>::run( translationUnit );
        FixInit::fixInitializers( translationUnit );
        ast::Pass<GenStructMemberCalls>::run( translationUnit );

        // Needs to happen after GenStructMemberCalls, since otherwise member constructors exprs
        // don't have the correct form, and a member can be constructed more than once.
        ast::Pass<FixCtorExprs>::run( translationUnit );
}

namespace {
        /// find and return the destructor used in `input`. If `input` is not a simple destructor call, generate a thunk
        /// that wraps the destructor, insert it into `stmtsToAdd` and return the new function declaration
        const ast::DeclWithType * getDtorFunc( const ast::ObjectDecl * objDecl, const ast::Stmt * input, std::list< ast::ptr<ast::Stmt> > & stmtsToAdd ) {
                const CodeLocation loc = input->location;
                // unwrap implicit statement wrapper
                // Statement * dtor = input;
                assert( input );
                // std::list< const ast::Expr * > matches;
                auto matches = collectCtorDtorCalls( input );

                if ( dynamic_cast< const ast::ExprStmt * >( input ) ) {
                        // only one destructor call in the expression
                        if ( matches.size() == 1 ) {
                                auto func = getFunction( matches.front() );
                                assertf( func, "getFunction failed to find function in %s", toString( matches.front() ).c_str() );

                                // cleanup argument must be a function, not an object (including function pointer)
                                if ( auto dtorFunc = dynamic_cast< const ast::FunctionDecl * > ( func ) ) {
                                        if ( dtorFunc->type->forall.empty() ) {
                                                // simple case where the destructor is a monomorphic function call - can simply
                                                // use that function as the cleanup function.
                                                return func;
                                        }
                                }
                        }
                }

                // otherwise the cleanup is more complicated - need to build a single argument cleanup function that
                // wraps the more complicated code.
                static UniqueName dtorNamer( "__cleanup_dtor" );
                std::string name = dtorNamer.newName();
                ast::FunctionDecl * dtorFunc = SymTab::genDefaultFunc( loc, name, objDecl->type->stripReferences(), false );
                stmtsToAdd.push_back( new ast::DeclStmt(loc, dtorFunc ) );

                // the original code contains uses of objDecl - replace them with the newly generated 'this' parameter.
                const ast::ObjectDecl * thisParam = getParamThis( dtorFunc );
                const ast::Expr * replacement = new ast::VariableExpr( loc, thisParam );

                auto base = replacement->result->stripReferences();
                if ( dynamic_cast< const ast::ArrayType * >( base ) || dynamic_cast< const ast::TupleType * > ( base ) ) {
                        // need to cast away reference for array types, since the destructor is generated without the reference type,
                        // and for tuple types since tuple indexing does not work directly on a reference
                        replacement = new ast::CastExpr( replacement, base );
                }
                auto dtor = ast::DeclReplacer::replace( input, ast::DeclReplacer::ExprMap{ std::make_pair( objDecl, replacement ) } );
                auto mutStmts = dtorFunc->stmts.get_and_mutate();
                mutStmts->push_back(strict_dynamic_cast<const ast::Stmt *>( dtor ));
                dtorFunc->stmts = mutStmts;

                return dtorFunc;
        }

        void FixInit::fixInitializers( ast::TranslationUnit & translationUnit ) {
                ast::Pass<FixInit> fixer;

                // can't use mutateAll, because need to insert declarations at top-level
                // can't use DeclMutator, because sometimes need to insert IfStmt, etc.
                SemanticErrorException errors;
                for ( auto i = translationUnit.decls.begin(); i != translationUnit.decls.end(); ++i ) {
                        try {
                                // maybeAccept( *i, fixer ); translationUnit should never contain null
                                *i = (*i)->accept(fixer);
                                translationUnit.decls.splice( i, fixer.core.staticDtorDecls );
                        } catch( SemanticErrorException &e ) {
                                errors.append( e );
                        } // try
                } // for
                if ( ! errors.isEmpty() ) {
                        throw errors;
                } // if
        }

        const ast::StmtExpr * StmtExprResult::previsit( const ast::StmtExpr * stmtExpr ) {
                // we might loose the result expression here so add a pointer to trace back
                assert( stmtExpr->result );
                const ast::Type * result = stmtExpr->result;
                if ( ! result->isVoid() ) {
                        auto mutExpr = mutate(stmtExpr);
                        const ast::CompoundStmt * body = mutExpr->stmts;
                        assert( ! body->kids.empty() );
                        mutExpr->resultExpr = body->kids.back().strict_as<ast::ExprStmt>();
                        return mutExpr;
                }
                return stmtExpr;
        }

        ast::Stmt * SplitExpressions::postvisit( const ast::ExprStmt * stmt ) {
                // wrap each top-level ExprStmt in a block so that destructors for argument and return temporaries are destroyed
                // in the correct places
                ast::CompoundStmt * ret = new ast::CompoundStmt( stmt->location, { stmt } );
                return ret;
        }

        void SplitExpressions::previsit( const ast::TupleAssignExpr * ) {
                // don't do this within TupleAssignExpr, since it is already broken up into multiple expressions
                visit_children = false;
        }

        // Relatively simple structural comparison for expressions, needed to determine
        // if two expressions are "the same" (used to determine if self assignment occurs)
        struct StructuralChecker {
                // Strip all casts and then dynamic_cast.
                template<typename T>
                static const T * cast( const ast::Expr * expr ) {
                        // this might be too permissive. It's possible that only particular casts are relevant.
                        while ( auto cast = dynamic_cast< const ast::CastExpr * >( expr ) ) {
                                expr = cast->arg;
                        }
                        return dynamic_cast< const T * >( expr );
                }

                void previsit( const ast::Expr * ) {
                        // anything else does not qualify
                        result = false;
                }

                // ignore casts
                void previsit( const ast::CastExpr * ) {}

                void previsit( const ast::MemberExpr * memExpr ) {
                        if ( auto otherMember = cast< ast::MemberExpr >( other ) ) {
                                if ( otherMember->member == memExpr->member ) {
                                        other = otherMember->aggregate;
                                        return;
                                }
                        }
                        result = false;
                }

                void previsit( const ast::VariableExpr * varExpr ) {
                        if ( auto otherVar = cast< ast::VariableExpr >( other ) ) {
                                if ( otherVar->var == varExpr->var ) {
                                        return;
                                }
                        }
                        result = false;
                }

                void previsit( const ast::AddressExpr * ) {
                        if ( auto addrExpr = cast< ast::AddressExpr >( other ) ) {
                                other = addrExpr->arg;
                                return;
                        }
                        result = false;
                }

                const ast::Expr * other;
                bool result = true;
                StructuralChecker( const ast::Expr * other ) : other(other) {}
        };

        bool structurallySimilar( const ast::Expr * e1, const ast::Expr * e2 ) {
                return ast::Pass<StructuralChecker>::read( e1, e2 );
        }

        void SelfAssignChecker::previsit( const ast::ApplicationExpr * appExpr ) {
                auto function = getFunction( appExpr );
                // Doesn't use isAssignment, because ?+=?, etc. should not count as self-assignment.
                if ( function->name == "?=?" && appExpr->args.size() == 2
                                // Check for structural similarity (same variable use, ignore casts, etc.
                                // (but does not look too deeply, anything looking like a function is off limits).
                                && structurallySimilar( appExpr->args.front(), appExpr->args.back() ) ) {
                        SemanticWarning( appExpr->location, Warning::SelfAssignment, toCString( appExpr->args.front() ) );
                }
        }

        const ast::Expr * InsertImplicitCalls::postvisit( const ast::ApplicationExpr * appExpr ) {
                if ( auto function = appExpr->func.as<ast::VariableExpr>() ) {
                        if ( function->var->linkage.is_builtin ) {
                                // optimization: don't need to copy construct in order to call intrinsic functions
                                return appExpr;
                        } else if ( auto funcDecl = function->var.as<ast::DeclWithType>() ) {
                                auto ftype = dynamic_cast< const ast::FunctionType * >( GenPoly::getFunctionType( funcDecl->get_type() ) );
                                assertf( ftype, "Function call without function type: %s", toString( funcDecl ).c_str() );
                                if ( CodeGen::isConstructor( funcDecl->name ) && ftype->params.size() == 2 ) {
                                        auto t1 = getPointerBase( ftype->params.front() );
                                        auto t2 = ftype->params.back();
                                        assert( t1 );

                                        if ( ResolvExpr::typesCompatible( t1, t2 ) ) {
                                                // optimization: don't need to copy construct in order to call a copy constructor
                                                return appExpr;
                                        } // if
                                } else if ( CodeGen::isDestructor( funcDecl->name ) ) {
                                        // correctness: never copy construct arguments to a destructor
                                        return appExpr;
                                } // if
                        } // if
                } // if
                CP_CTOR_PRINT( std::cerr << "InsertImplicitCalls: adding a wrapper " << appExpr << std::endl; )

                // wrap each function call so that it is easy to identify nodes that have to be copy constructed
                ast::ptr<ast::TypeSubstitution> tmp = appExpr->env;
                auto mutExpr = mutate(appExpr);
                mutExpr->env = nullptr;

                auto expr = new ast::ImplicitCopyCtorExpr( appExpr->location, mutExpr );
                // Move the type substitution to the new top-level, if it is attached to the appExpr.
                // Ensure it is not deleted with the ImplicitCopyCtorExpr by removing it before deletion.
                // The substitution is needed to obtain the type of temporary variables so that copy constructor
                // calls can be resolved.
                assert( typeSubs );
                // assert (mutExpr->env);
                expr->env = tmp;
                // mutExpr->env = nullptr;
                //std::swap( expr->env, appExpr->env );
                return expr;
        }

        void ResolveCopyCtors::previsit(const ast::Expr * expr) {
                if (expr->env) {
                        GuardValue(env);
                        GuardValue(envModified);
                        env = expr->env->clone();
                        envModified = false;
                }
        }

        const ast::Expr * ResolveCopyCtors::postvisit(const ast::Expr * expr) {
                if (expr->env) {
                        if (envModified) {
                                auto mutExpr = mutate(expr);
                                mutExpr->env = env;
                                return mutExpr;
                        }
                        else {
                                // env was not mutated, skip and delete the shallow copy
                                delete env;
                                return expr;
                        }
                }
                else {
                        return expr;
                }
        }

        bool ResolveCopyCtors::skipCopyConstruct( const ast::Type * type ) { return ! isConstructable( type ); }

        const ast::Expr * ResolveCopyCtors::makeCtorDtor( const std::string & fname, const ast::ObjectDecl * var, const ast::Expr * cpArg ) {
                assert( var );
                assert (var->isManaged());
                assert (!cpArg || cpArg->isManaged());
                // arrays are not copy constructed, so this should always be an ExprStmt
                ast::ptr< ast::Stmt > stmt = genCtorDtor(var->location, fname, var, cpArg );
                assertf( stmt, "ResolveCopyCtors: genCtorDtor returned nullptr: %s / %s / %s", fname.c_str(), toString( var ).c_str(), toString( cpArg ).c_str() );
                auto exprStmt = stmt.strict_as<ast::ImplicitCtorDtorStmt>()->callStmt.strict_as<ast::ExprStmt>();
                ast::ptr<ast::Expr> untyped = exprStmt->expr; // take ownership of expr
                // exprStmt->expr = nullptr;

                // resolve copy constructor
                // should only be one alternative for copy ctor and dtor expressions, since all arguments are fixed
                // (VariableExpr and already resolved expression)
                CP_CTOR_PRINT( std::cerr << "ResolvingCtorDtor " << untyped << std::endl; )
                ast::ptr<ast::Expr> resolved = ResolvExpr::findVoidExpression(untyped, symtab);
                assert( resolved );
                if ( resolved->env ) {
                        // Extract useful information and discard new environments. Keeping them causes problems in PolyMutator passes.
                        env->add( *resolved->env );
                        envModified = true;
                        // delete resolved->env;
                        auto mut = mutate(resolved.get());
                        assertf(mut == resolved.get(), "newly resolved expression must be unique");
                        mut->env = nullptr;
                } // if
                // delete stmt;
                if ( auto assign = resolved.as<ast::TupleAssignExpr>() ) {
                        // fix newly generated StmtExpr
                        previsit( assign->stmtExpr );
                }
                return resolved.release();
        }

        ast::ptr<ast::Expr> ResolveCopyCtors::copyConstructArg(
                const ast::Expr * arg, const ast::ImplicitCopyCtorExpr * impCpCtorExpr, const ast::Type * formal )
        {
                static UniqueName tempNamer("_tmp_cp");
                assert( env );
                const CodeLocation loc = impCpCtorExpr->location;
                // CP_CTOR_PRINT( std::cerr << "Type Substitution: " << *env << std::endl; )
                assert( arg->result );
                ast::ptr<ast::Type> result = arg->result;
                if ( skipCopyConstruct( result ) ) return arg; // skip certain non-copyable types

                // type may involve type variables, so apply type substitution to get temporary variable's actual type,
                // since result type may not be substituted (e.g., if the type does not appear in the parameter list)
                // Use applyFree so that types bound in function pointers are not substituted, e.g. in forall(dtype T) void (*)(T).

                // xxx - this originally mutates arg->result in place. is it correct?
                result = env->applyFree( result.get() ).node;
                auto mutResult = result.get_and_mutate();
                mutResult->set_const(false);

                auto mutArg = mutate(arg);
                mutArg->result = mutResult;

                ast::ptr<ast::Expr> guard = mutArg;

                ast::ptr<ast::ObjectDecl> tmp = new ast::ObjectDecl(loc, "__tmp", mutResult, nullptr );

                // create and resolve copy constructor
                CP_CTOR_PRINT( std::cerr << "makeCtorDtor for an argument" << std::endl; )
                auto cpCtor = makeCtorDtor( "?{}", tmp, mutArg );

                if ( auto appExpr = dynamic_cast< const ast::ApplicationExpr * >( cpCtor ) ) {
                        // if the chosen constructor is intrinsic, the copy is unnecessary, so
                        // don't create the temporary and don't call the copy constructor
                        auto function = appExpr->func.strict_as<ast::VariableExpr>();
                        if ( function->var->linkage == ast::Linkage::Intrinsic ) {
                                // arguments that need to be boxed need a temporary regardless of whether the copy constructor is intrinsic,
                                // so that the object isn't changed inside of the polymorphic function
                                if ( ! GenPoly::needsBoxing( formal, result, impCpCtorExpr->callExpr, env ) ) {
                                        // xxx - should arg->result be mutated? see comment above.
                                        return guard;
                                }
                        }
                }

                // set a unique name for the temporary once it's certain the call is necessary
                auto mut = tmp.get_and_mutate();
                assertf (mut == tmp, "newly created ObjectDecl must be unique");
                mut->name = tempNamer.newName();

                // replace argument to function call with temporary
                stmtsToAddBefore.push_back( new ast::DeclStmt(loc, tmp ) );
                arg = cpCtor;
                return destructRet( tmp, arg );

                // impCpCtorExpr->dtors.push_front( makeCtorDtor( "^?{}", tmp ) );
        }

        ast::Expr * ResolveCopyCtors::destructRet( const ast::ObjectDecl * ret, const ast::Expr * arg ) {
                // TODO: refactor code for generating cleanup attribute, since it's common and reused in ~3-4 places
                // check for existing cleanup attribute before adding another(?)
                // need to add __Destructor for _tmp_cp variables as well

                assertf( ast::dtorStruct, "Destructor generation requires __Destructor definition." );
                assertf( ast::dtorStruct->members.size() == 2, "__Destructor definition does not have expected fields." );
                assertf( ast::dtorStructDestroy, "Destructor generation requires __destroy_Destructor." );

                const CodeLocation loc = ret->location;

                // generate a __Destructor for ret that calls the destructor
                auto res = makeCtorDtor( "^?{}", ret );
                auto dtor = mutate(res);

                // if the chosen destructor is intrinsic, elide the generated dtor handler
                if ( arg && isIntrinsicCallExpr( dtor ) ) {
                        return new ast::CommaExpr(loc, arg, new ast::VariableExpr(loc, ret ) );
                        // return;
                }

                if ( ! dtor->env ) dtor->env = maybeClone( env );
                auto dtorFunc = getDtorFunc( ret, new ast::ExprStmt(loc, dtor ), stmtsToAddBefore );

                auto dtorStructType = new ast::StructInstType(ast::dtorStruct);

                // what does this do???
                dtorStructType->params.push_back( new ast::TypeExpr(loc, new ast::VoidType() ) );

                // cast destructor pointer to void (*)(void *), to silence GCC incompatible pointer warnings
                auto dtorFtype = new ast::FunctionType();
                dtorFtype->params.push_back( new ast::PointerType(new ast::VoidType( ) ) );
                auto dtorType = new ast::PointerType( dtorFtype );

                static UniqueName namer( "_ret_dtor" );
                auto retDtor = new ast::ObjectDecl(loc, namer.newName(), dtorStructType, new ast::ListInit(loc, { new ast::SingleInit(loc, ast::ConstantExpr::null(loc) ), new ast::SingleInit(loc, new ast::CastExpr( new ast::VariableExpr(loc, dtorFunc ), dtorType ) ) } ) );
                retDtor->attributes.push_back( new ast::Attribute( "cleanup", { new ast::VariableExpr(loc, ast::dtorStructDestroy ) } ) );
                stmtsToAddBefore.push_back( new ast::DeclStmt(loc, retDtor ) );

                if ( arg ) {
                        auto member = new ast::MemberExpr(loc, ast::dtorStruct->members.front().strict_as<ast::DeclWithType>(), new ast::VariableExpr(loc, retDtor ) );
                        auto object = new ast::CastExpr( new ast::AddressExpr( new ast::VariableExpr(loc, ret ) ), new ast::PointerType(new ast::VoidType() ) );
                        ast::Expr * assign = createBitwiseAssignment( member, object );
                        return new ast::CommaExpr(loc, new ast::CommaExpr(loc, arg, assign ), new ast::VariableExpr(loc, ret ) );
                }
                return nullptr;
                // impCpCtorExpr->get_dtors().push_front( makeCtorDtor( "^?{}", ret ) );
        }

        const ast::Expr * ResolveCopyCtors::postvisit( const ast::ImplicitCopyCtorExpr *impCpCtorExpr ) {
                CP_CTOR_PRINT( std::cerr << "ResolveCopyCtors: " << impCpCtorExpr << std::endl; )

                ast::ApplicationExpr * appExpr = mutate(impCpCtorExpr->callExpr.get());
                const ast::ObjectDecl * returnDecl = nullptr;
                const CodeLocation loc = appExpr->location;

                // take each argument and attempt to copy construct it.
                auto ftype = GenPoly::getFunctionType( appExpr->func->result );
                assert( ftype );
                auto & params = ftype->params;
                auto iter = params.begin();
                for ( auto & arg : appExpr->args ) {
                        const ast::Type * formal = nullptr;
                        if ( iter != params.end() ) { // does not copy construct C-style variadic arguments
                                // DeclarationWithType * param = *iter++;
                                formal = *iter++;
                        }

                        arg = copyConstructArg( arg, impCpCtorExpr, formal );
                } // for

                // each return value from the call needs to be connected with an ObjectDecl at the call site, which is
                // initialized with the return value and is destructed later
                // xxx - handle named return values?
                const ast::Type * result = appExpr->result;
                if ( ! result->isVoid() ) {
                        static UniqueName retNamer("_tmp_cp_ret");
                        // result = result->clone();
                        auto subResult = env->apply( result ).node;
                        auto ret = new ast::ObjectDecl(loc, retNamer.newName(), subResult, nullptr );
                        auto mutType = mutate(ret->type.get());
                        mutType->set_const( false );
                        ret->type = mutType;
                        returnDecl = ret;
                        stmtsToAddBefore.push_back( new ast::DeclStmt(loc, ret ) );
                        CP_CTOR_PRINT( std::cerr << "makeCtorDtor for a return" << std::endl; )
                } // for
                CP_CTOR_PRINT( std::cerr << "after Resolving: " << impCpCtorExpr << std::endl; )
                // ------------------------------------------------------

                CP_CTOR_PRINT( std::cerr << "Coming out the back..." << impCpCtorExpr << std::endl; )

                // detach fields from wrapper node so that it can be deleted without deleting too much

                // xxx - actual env might be somewhere else, need to keep invariant

                // deletion of wrapper should be handled by pass template now

                // impCpCtorExpr->callExpr = nullptr;
                assert (appExpr->env == nullptr);
                appExpr->env = impCpCtorExpr->env;
                // std::swap( impCpCtorExpr->env, appExpr->env );
                // assert( impCpCtorExpr->env == nullptr );
                // delete impCpCtorExpr;

                if ( returnDecl ) {
                        ast::Expr * assign = createBitwiseAssignment( new ast::VariableExpr(loc, returnDecl ), appExpr );
                        if ( ! dynamic_cast< const ast::ReferenceType * >( result ) ) {
                                // destructing reference returns is bad because it can cause multiple destructor calls to the same object - the returned object is not a temporary
                                assign = destructRet( returnDecl, assign );
                                assert(assign);
                        } else {
                                assign = new ast::CommaExpr(loc, assign, new ast::VariableExpr(loc, returnDecl ) );
                        }
                        // move env from appExpr to retExpr
                        // std::swap( assign->env, appExpr->env );
                        assign->env = appExpr->env;
                        // actual env is handled by common routine that replaces WithTypeSubstitution
                        return postvisit((const ast::Expr *)assign);
                } else {
                        return postvisit((const ast::Expr *)appExpr);
                } // if
        }

        const ast::StmtExpr * ResolveCopyCtors::previsit( const ast::StmtExpr * _stmtExpr ) {
                // function call temporaries should be placed at statement-level, rather than nested inside of a new statement expression,
                // since temporaries can be shared across sub-expressions, e.g.
                //   [A, A] f();       // decl
                //   g([A] x, [A] y);  // decl
                //   g(f());           // call
                // f is executed once, so the return temporary is shared across the tuple constructors for x and y.
                // Explicitly mutating children instead of mutating the inner compound statement forces the temporaries to be added
                // to the outer context, rather than inside of the statement expression.

                // call the common routine that replaces WithTypeSubstitution
                previsit((const ast::Expr *) _stmtExpr);

                visit_children = false;
                const CodeLocation loc = _stmtExpr->location;

                assert( env );

                symtab.enterScope();
                // visit all statements
                auto stmtExpr = mutate(_stmtExpr);
                auto mutStmts = mutate(stmtExpr->stmts.get());

                auto & stmts = mutStmts->kids;
                for ( auto & stmt : stmts ) {
                        stmt = stmt->accept( *visitor );
                } // for
                stmtExpr->stmts = mutStmts;
                symtab.leaveScope();

                assert( stmtExpr->result );
                // const ast::Type * result = stmtExpr->result;
                if ( ! stmtExpr->result->isVoid() ) {
                        static UniqueName retNamer("_tmp_stmtexpr_ret");

                        // result = result->clone();
                        auto result = env->apply( stmtExpr->result.get() ).node;
                        if ( ! InitTweak::isConstructable( result ) ) {
                                // delete result;
                                return stmtExpr;
                        }
                        auto mutResult = result.get_and_mutate();
                        mutResult->set_const(false);

                        // create variable that will hold the result of the stmt expr
                        auto ret = new ast::ObjectDecl(loc, retNamer.newName(), mutResult, nullptr );
                        stmtsToAddBefore.push_back( new ast::DeclStmt(loc, ret ) );

                        assertf(
                                stmtExpr->resultExpr,
                                "Statement-Expression should have a resulting expression at %s:%d",
                                stmtExpr->location.filename.c_str(),
                                stmtExpr->location.first_line
                        );

                        const ast::ExprStmt * last = stmtExpr->resultExpr;
                        // xxx - if this is non-unique, need to copy while making resultExpr ref
                        assertf(last->unique(), "attempt to modify weakly shared statement");
                        auto mutLast = mutate(last);
                        // above assertion means in-place mutation is OK
                        try {
                                mutLast->expr = makeCtorDtor( "?{}", ret, mutLast->expr );
                        } catch(...) {
                                std::cerr << "*CFA internal error: ";
                                std::cerr << "can't resolve implicit constructor";
                                std::cerr << " at " << stmtExpr->location.filename;
                                std::cerr << ":" << stmtExpr->location.first_line << std::endl;

                                abort();
                        }

                        // add destructors after current statement
                        stmtsToAddAfter.push_back( new ast::ExprStmt(loc, makeCtorDtor( "^?{}", ret ) ) );

                        // must have a non-empty body, otherwise it wouldn't have a result
                        assert( ! stmts.empty() );

                        // if there is a return decl, add a use as the last statement; will not have return decl on non-constructable returns
                        stmts.push_back( new ast::ExprStmt(loc, new ast::VariableExpr(loc, ret ) ) );
                } // if

                assert( stmtExpr->returnDecls.empty() );
                assert( stmtExpr->dtors.empty() );

                return stmtExpr;
        }

        // to prevent warnings ('_unq0' may be used uninitialized in this function),
        // insert an appropriate zero initializer for UniqueExpr temporaries.
        ast::Init * makeInit( const ast::Type * t, CodeLocation const & loc ) {
                if ( auto inst = dynamic_cast< const ast::StructInstType * >( t ) ) {
                        // initizer for empty struct must be empty
                        if ( inst->base->members.empty() ) {
                                return new ast::ListInit( loc, {} );
                        }
                } else if ( auto inst = dynamic_cast< const ast::UnionInstType * >( t ) ) {
                        // initizer for empty union must be empty
                        if ( inst->base->members.empty() ) {
                                return new ast::ListInit( loc, {} );
                        }
                }

                return new ast::ListInit( loc, {
                        new ast::SingleInit( loc, ast::ConstantExpr::from_int( loc, 0 ) )
                } );
        }

        const ast::UniqueExpr * ResolveCopyCtors::previsit( const ast::UniqueExpr * unqExpr ) {
                visit_children = false;
                // xxx - hack to prevent double-handling of unique exprs, otherwise too many temporary variables and destructors are generated
                static std::unordered_map< int, const ast::UniqueExpr * > unqMap;
                auto mutExpr = mutate(unqExpr);
                if ( ! unqMap.count( unqExpr->id ) ) {
                        // resolve expr and find its

                        auto impCpCtorExpr = mutExpr->expr.as<ast::ImplicitCopyCtorExpr>();
                        // PassVisitor<ResolveCopyCtors> fixer;

                        mutExpr->expr = mutExpr->expr->accept( *visitor );
                        // it should never be necessary to wrap a void-returning expression in a UniqueExpr - if this assumption changes, this needs to be rethought
                        assert( unqExpr->result );
                        if ( impCpCtorExpr ) {
                                auto comma = unqExpr->expr.strict_as<ast::CommaExpr>();
                                auto var = comma->arg2.strict_as<ast::VariableExpr>();
                                // note the variable used as the result from the call
                                mutExpr->var = var;
                        } else {
                                // expr isn't a call expr, so create a new temporary variable to use to hold the value of the unique expression
                                mutExpr->object = new ast::ObjectDecl( mutExpr->location, toString("_unq", mutExpr->id), mutExpr->result, makeInit( mutExpr->result, mutExpr->location ) );
                                mutExpr->var = new ast::VariableExpr( mutExpr->location, mutExpr->object );
                        }

                        // stmtsToAddBefore.splice( stmtsToAddBefore.end(), fixer.pass.stmtsToAddBefore );
                        // stmtsToAddAfter.splice( stmtsToAddAfter.end(), fixer.pass.stmtsToAddAfter );
                        unqMap[mutExpr->id] = mutExpr;
                } else {
                        // take data from other UniqueExpr to ensure consistency
                        // delete unqExpr->get_expr();
                        mutExpr->expr = unqMap[mutExpr->id]->expr;
                        // delete unqExpr->result;
                        mutExpr->result = mutExpr->expr->result;
                }
                return mutExpr;
        }

        const ast::DeclWithType * FixInit::postvisit( const ast::ObjectDecl *_objDecl ) {
                const CodeLocation loc = _objDecl->location;

                // since this removes the init field from objDecl, it must occur after children are mutated (i.e. postvisit)
                if ( ast::ptr<ast::ConstructorInit> ctorInit = _objDecl->init.as<ast::ConstructorInit>() ) {
                        auto objDecl = mutate(_objDecl);

                        // could this be non-unique?
                        if (objDecl != _objDecl) {
                                std::cerr << "FixInit: non-unique object decl " << objDecl->location << objDecl->name << std::endl;
                        }
                        // a decision should have been made by the resolver, so ctor and init are not both non-NULL
                        assert( ! ctorInit->ctor || ! ctorInit->init );
                        if ( const ast::Stmt * ctor = ctorInit->ctor ) {
                                if ( objDecl->storage.is_static ) {
                                        addDataSectionAttribute(objDecl);
                                        // originally wanted to take advantage of gcc nested functions, but
                                        // we get memory errors with this approach. To remedy this, the static
                                        // variable is hoisted when the destructor needs to be called.
                                        //
                                        // generate:
                                        // static T __objName_static_varN;
                                        // void __objName_dtor_atexitN() {
                                        //   __dtor__...;
                                        // }
                                        // int f(...) {
                                        //   ...
                                        //   static bool __objName_uninitialized = true;
                                        //   if (__objName_uninitialized) {
                                        //     __ctor(__objName);
                                        //     __objName_uninitialized = false;
                                        //     atexit(__objName_dtor_atexitN);
                                        //   }
                                        //   ...
                                        // }

                                        static UniqueName dtorCallerNamer( "_dtor_atexit" );

                                        // static bool __objName_uninitialized = true
                                        auto boolType = new ast::BasicType( ast::BasicType::Kind::Bool );
                                        auto boolInitExpr = new ast::SingleInit(loc, ast::ConstantExpr::from_int(loc, 1 ) );
                                        auto isUninitializedVar = new ast::ObjectDecl(loc, objDecl->mangleName + "_uninitialized", boolType, boolInitExpr, ast::Storage::Static, ast::Linkage::Cforall);
                                        isUninitializedVar->fixUniqueId();

                                        // __objName_uninitialized = false;
                                        auto setTrue = new ast::UntypedExpr(loc, new ast::NameExpr(loc, "?=?" ) );
                                        setTrue->args.push_back( new ast::VariableExpr(loc, isUninitializedVar ) );
                                        setTrue->args.push_back( ast::ConstantExpr::from_int(loc, 0 ) );

                                        // generate body of if
                                        auto initStmts = new ast::CompoundStmt(loc);
                                        auto & body = initStmts->kids;
                                        body.push_back( ctor );
                                        body.push_back( new ast::ExprStmt(loc, setTrue ) );

                                        // put it all together
                                        auto ifStmt = new ast::IfStmt(loc, new ast::VariableExpr(loc, isUninitializedVar ), initStmts, 0 );
                                        stmtsToAddAfter.push_back( new ast::DeclStmt(loc, isUninitializedVar ) );
                                        stmtsToAddAfter.push_back( ifStmt );

                                        const ast::Stmt * dtor = ctorInit->dtor;

                                        // these should be automatically managed once reassigned
                                        // objDecl->set_init( nullptr );
                                        // ctorInit->set_ctor( nullptr );
                                        // ctorInit->set_dtor( nullptr );
                                        if ( dtor ) {
                                                // if the object has a non-trivial destructor, have to
                                                // hoist it and the object into the global space and
                                                // call the destructor function with atexit.

                                                // Statement * dtorStmt = dtor->clone();

                                                // void __objName_dtor_atexitN(...) {...}
                                                ast::FunctionDecl * dtorCaller = new ast::FunctionDecl(loc, objDecl->mangleName + dtorCallerNamer.newName(), {}, {}, {}, new ast::CompoundStmt(loc, {dtor}), ast::Storage::Static, ast::Linkage::C );
                                                dtorCaller->fixUniqueId();
                                                // dtorCaller->stmts->push_back( dtor );

                                                // atexit(dtor_atexit);
                                                auto callAtexit = new ast::UntypedExpr(loc, new ast::NameExpr(loc, "atexit" ) );
                                                callAtexit->args.push_back( new ast::VariableExpr(loc, dtorCaller ) );

                                                body.push_back( new ast::ExprStmt(loc, callAtexit ) );

                                                // hoist variable and dtor caller decls to list of decls that will be added into global scope
                                                staticDtorDecls.push_back( objDecl );
                                                staticDtorDecls.push_back( dtorCaller );

                                                // need to rename object uniquely since it now appears
                                                // at global scope and there could be multiple function-scoped
                                                // static variables with the same name in different functions.
                                                // Note: it isn't sufficient to modify only the mangleName, because
                                                // then subsequent Indexer passes can choke on seeing the object's name
                                                // if another object has the same name and type. An unfortunate side-effect
                                                // of renaming the object is that subsequent NameExprs may fail to resolve,
                                                // but there shouldn't be any remaining past this point.
                                                static UniqueName staticNamer( "_static_var" );
                                                objDecl->name = objDecl->name + staticNamer.newName();
                                                objDecl->mangleName = Mangle::mangle( objDecl );
                                                objDecl->init = nullptr;

                                                // xxx - temporary hack: need to return a declaration, but want to hoist the current object out of this scope
                                                // create a new object which is never used
                                                static UniqueName dummyNamer( "_dummy" );
                                                auto dummy = new ast::ObjectDecl(loc, dummyNamer.newName(), new ast::PointerType(new ast::VoidType()), nullptr, ast::Storage::Static, ast::Linkage::Cforall, 0, { new ast::Attribute("unused") } );
                                                // delete ctorInit;
                                                return dummy;
                                        } else {
                                                objDecl->init = nullptr;
                                                return objDecl;
                                        }
                                } else {
                                        auto implicit = strict_dynamic_cast< const ast::ImplicitCtorDtorStmt * > ( ctor );
                                        auto ctorStmt = implicit->callStmt.as<ast::ExprStmt>();
                                        const ast::ApplicationExpr * ctorCall = nullptr;
                                        if ( ctorStmt && (ctorCall = isIntrinsicCallExpr( ctorStmt->expr )) && ctorCall->args.size() == 2 ) {
                                                // clean up intrinsic copy constructor calls by making them into SingleInits
                                                const ast::Expr * ctorArg = ctorCall->args.back();
                                                // ctorCall should be gone afterwards
                                                auto mutArg = mutate(ctorArg);
                                                mutArg->env = ctorCall->env;
                                                // std::swap( ctorArg->env, ctorCall->env );
                                                objDecl->init = new ast::SingleInit(loc, mutArg );

                                                // ctorCall->args.pop_back();
                                        } else {
                                                stmtsToAddAfter.push_back( ctor );
                                                objDecl->init = nullptr;
                                                // ctorInit->ctor = nullptr;
                                        }

                                        const ast::Stmt * dtor = ctorInit->dtor;
                                        if ( dtor ) {
                                                auto implicit = strict_dynamic_cast< const ast::ImplicitCtorDtorStmt * >( dtor );
                                                const ast::Stmt * dtorStmt = implicit->callStmt;

                                                // don't need to call intrinsic dtor, because it does nothing, but
                                                // non-intrinsic dtors must be called
                                                if ( ! isIntrinsicSingleArgCallStmt( dtorStmt ) ) {
                                                        // set dtor location to the object's location for error messages
                                                        auto dtorFunc = getDtorFunc( objDecl, dtorStmt, stmtsToAddBefore );
                                                        objDecl->attributes.push_back( new ast::Attribute( "cleanup", { new ast::VariableExpr(loc, dtorFunc ) } ) );
                                                        // ctorInit->dtor = nullptr;
                                                } // if
                                        }
                                } // if
                        } else if ( const ast::Init * init = ctorInit->init ) {
                                objDecl->init = init;
                                // ctorInit->init = nullptr;
                        } else {
                                // no constructor and no initializer, which is okay
                                objDecl->init = nullptr;
                        } // if
                        // delete ctorInit;
                        return objDecl;
                } // if
                return _objDecl;
        }

        void ObjDeclCollector::previsit( const ast::CompoundStmt * ) {
                GuardValue( curVars );
        }

        void ObjDeclCollector::previsit( const ast::DeclStmt * stmt ) {
                // keep track of all variables currently in scope
                if ( auto objDecl = stmt->decl.as<ast::ObjectDecl>() ) {
                        curVars.push_back( objDecl );
                } // if
        }

        void LabelFinder::previsit( const ast::Stmt * stmt ) {
                // for each label, remember the variables in scope at that label.
                for ( auto l : stmt->labels ) {
                        vars[l] = curVars;
                } // for
        }

        void LabelFinder::previsit( const ast::CompoundStmt * stmt ) {
                previsit( (const ast::Stmt *) stmt );
                Parent::previsit( stmt );
        }

        void LabelFinder::previsit( const ast::DeclStmt * stmt ) {
                previsit( (const ast::Stmt *)stmt );
                Parent::previsit( stmt );
        }


        void InsertDtors::previsit( const ast::FunctionDecl * funcDecl ) {
                // each function needs to have its own set of labels
                GuardValue( labelVars );
                labelVars.clear();
                // LabelFinder does not recurse into FunctionDecl, so need to visit
                // its children manually.
                if (funcDecl->type) funcDecl->type->accept(finder);
                // maybeAccept( funcDecl->type, finder );
                if (funcDecl->stmts) funcDecl->stmts->accept(finder) ;

                // all labels for this function have been collected, insert destructors as appropriate via implicit recursion.
        }

        // Handle break/continue/goto in the same manner as C++.  Basic idea: any objects that are in scope at the
        // BranchStmt but not at the labelled (target) statement must be destructed.  If there are any objects in scope
        // at the target location but not at the BranchStmt then those objects would be uninitialized so notify the user
        // of the error.  See C++ Reference 6.6 Jump Statements for details.
        void InsertDtors::handleGoto( const ast::BranchStmt * stmt ) {
                // can't do anything for computed goto
                if ( stmt->computedTarget ) return;

                assertf( stmt->target.name != "", "BranchStmt missing a label: %s", toString( stmt ).c_str() );
                // S_L = lvars = set of objects in scope at label definition
                // S_G = curVars = set of objects in scope at goto statement
                ObjectSet & lvars = labelVars[ stmt->target ];

                DTOR_PRINT(
                        std::cerr << "at goto label: " << stmt->target.name << std::endl;
                        std::cerr << "S_G = " << printSet( curVars ) << std::endl;
                        std::cerr << "S_L = " << printSet( lvars ) << std::endl;
                )


                // std::set_difference requires that the inputs be sorted.
                lvars.sort();
                curVars.sort();

                ObjectSet diff;
                // S_L-S_G results in set of objects whose construction is skipped - it's an error if this set is non-empty
                std::set_difference( lvars.begin(), lvars.end(), curVars.begin(), curVars.end(), std::inserter( diff, diff.begin() ) );
                DTOR_PRINT(
                        std::cerr << "S_L-S_G = " << printSet( diff ) << std::endl;
                )
                if ( ! diff.empty() ) {
                        SemanticError( stmt, std::string("jump to label '") + stmt->target.name + "' crosses initialization of " + (*diff.begin())->name + " " );
                } // if
        }

        void InsertDtors::previsit( const ast::BranchStmt * stmt ) {
                switch( stmt->kind ) {
                  case ast::BranchStmt::Continue:
                  case ast::BranchStmt::Break:
                        // could optimize the break/continue case, because the S_L-S_G check is unnecessary (this set should
                        // always be empty), but it serves as a small sanity check.
                  case ast::BranchStmt::Goto:
                        handleGoto( stmt );
                        break;
                  default:
                        assert( false );
                } // switch
        }

        bool checkWarnings( const ast::FunctionDecl * funcDecl ) {
                // only check for warnings if the current function is a user-defined
                // constructor or destructor
                if ( ! funcDecl ) return false;
                if ( ! funcDecl->stmts ) return false;
                return CodeGen::isCtorDtor( funcDecl->name ) && ! funcDecl->linkage.is_overrideable;
        }

        void GenStructMemberCalls::previsit( const ast::FunctionDecl * funcDecl ) {
                GuardValue( function );
                GuardValue( unhandled );
                GuardValue( usedUninit );
                GuardValue( thisParam );
                GuardValue( isCtor );
                GuardValue( structDecl );
                errors = SemanticErrorException();  // clear previous errors

                // need to start with fresh sets
                unhandled.clear();
                usedUninit.clear();

                function = mutate(funcDecl);
                // could this be non-unique?
                if (function != funcDecl) {
                        std::cerr << "GenStructMemberCalls: non-unique FunctionDecl " << funcDecl->location << funcDecl->name << std::endl;
                }

                isCtor = CodeGen::isConstructor( function->name );
                if ( checkWarnings( function ) ) {
                        // const ast::FunctionType * type = function->type;
                        // assert( ! type->params.empty() );
                        thisParam = function->params.front().strict_as<ast::ObjectDecl>();
                        auto thisType = getPointerBase( thisParam->get_type() );
                        auto structType = dynamic_cast< const ast::StructInstType * >( thisType );
                        if ( structType ) {
                                structDecl = structType->base;
                                for ( auto & member : structDecl->members ) {
                                        if ( auto field = member.as<ast::ObjectDecl>() ) {
                                                // record all of the struct type's members that need to be constructed or
                                                // destructed by the end of the function
                                                unhandled.insert( field );
                                        }
                                }
                        }
                }
        }

        const ast::DeclWithType * GenStructMemberCalls::postvisit( const ast::FunctionDecl * funcDecl ) {
                // remove the unhandled objects from usedUninit, because a call is inserted
                // to handle them - only objects that are later constructed are used uninitialized.
                std::map< const ast::DeclWithType *, CodeLocation > diff;
                // need the comparator since usedUninit and unhandled have different types
                struct comp_t {
                        typedef decltype(usedUninit)::value_type usedUninit_t;
                        typedef decltype(unhandled)::value_type unhandled_t;
                        bool operator()(usedUninit_t x, unhandled_t y) { return x.first < y; }
                        bool operator()(unhandled_t x, usedUninit_t y) { return x < y.first; }
                } comp;
                std::set_difference( usedUninit.begin(), usedUninit.end(), unhandled.begin(), unhandled.end(), std::inserter( diff, diff.begin() ), comp );
                for ( auto p : diff ) {
                        auto member = p.first;
                        auto loc = p.second;
                        // xxx - make error message better by also tracking the location that the object is constructed at?
                        emit( loc, "in ", function->name, ", field ", member->name, " used before being constructed" );
                }

                const CodeLocation loc = funcDecl->location;

                if ( ! unhandled.empty() ) {
                        auto mutStmts = function->stmts.get_and_mutate();
                        // need to explicitly re-add function parameters to the indexer in order to resolve copy constructors
                        auto guard = makeFuncGuard( [this]() { symtab.enterScope(); }, [this]() { symtab.leaveScope(); } );
                        symtab.addFunction( function );

                        // need to iterate through members in reverse in order for
                        // ctor/dtor statements to come out in the right order
                        for ( auto & member : reverseIterate( structDecl->members ) ) {
                                auto field = member.as<ast::ObjectDecl>();
                                // skip non-DWT members
                                if ( ! field ) continue;
                                // skip non-constructable members
                                if ( ! tryConstruct( field ) ) continue;
                                // skip handled members
                                if ( ! unhandled.count( field ) ) continue;

                                // insert and resolve default/copy constructor call for each field that's unhandled
                                // std::list< const ast::Stmt * > stmt;
                                ast::Expr * arg2 = nullptr;
                                if ( function->name == "?{}" && isCopyFunction( function ) ) {
                                        // if copy ctor, need to pass second-param-of-this-function.field
                                        // std::list< DeclarationWithType * > & params = function->get_functionType()->get_parameters();
                                        assert( function->params.size() == 2 );
                                        arg2 = new ast::MemberExpr(funcDecl->location, field, new ast::VariableExpr(funcDecl->location, function->params.back() ) );
                                }
                                InitExpander_new srcParam( arg2 );
                                // cast away reference type and construct field.
                                ast::Expr * thisExpr = new ast::CastExpr(funcDecl->location, new ast::VariableExpr(funcDecl->location, thisParam ), thisParam->get_type()->stripReferences());
                                ast::Expr * memberDest = new ast::MemberExpr(funcDecl->location, field, thisExpr );
                                ast::ptr<ast::Stmt> callStmt = SymTab::genImplicitCall( srcParam, memberDest, loc, function->name, field, static_cast<SymTab::LoopDirection>(isCtor) );

                                if ( callStmt ) {
                                        // auto & callStmt = stmt.front();

                                        try {
                                                callStmt = callStmt->accept( *visitor );
                                                if ( isCtor ) {
                                                        mutStmts->push_front( callStmt );
                                                } else { // TODO: don't generate destructor function/object for intrinsic calls
                                                        // destructor statements should be added at the end
                                                        // function->get_statements()->push_back( callStmt );

                                                        // Optimization: do not need to call intrinsic destructors on members
                                                        if ( isIntrinsicSingleArgCallStmt( callStmt ) ) continue;

                                                        // __Destructor _dtor0 = { (void *)&b.a1, (void (*)(void *)_destroy_A };
                                                        std::list< ast::ptr<ast::Stmt> > stmtsToAdd;

                                                        static UniqueName memberDtorNamer = { "__memberDtor" };
                                                        assertf( ast::dtorStruct, "builtin __Destructor not found." );
                                                        assertf( ast::dtorStructDestroy, "builtin __destroy_Destructor not found." );

                                                        ast::Expr * thisExpr = new ast::CastExpr( new ast::AddressExpr( new ast::VariableExpr(loc, thisParam ) ), new ast::PointerType( new ast::VoidType(), ast::CV::Qualifiers() ) );
                                                        ast::Expr * dtorExpr = new ast::VariableExpr(loc, getDtorFunc( thisParam, callStmt, stmtsToAdd ) );

                                                        // cast destructor pointer to void (*)(void *), to silence GCC incompatible pointer warnings
                                                        auto dtorFtype = new ast::FunctionType();
                                                        dtorFtype->params.emplace_back( new ast::PointerType( new ast::VoidType() ) );
                                                        auto dtorType = new ast::PointerType( dtorFtype );

                                                        auto destructor = new ast::ObjectDecl(loc, memberDtorNamer.newName(), new ast::StructInstType( ast::dtorStruct ), new ast::ListInit(loc, { new ast::SingleInit(loc, thisExpr ), new ast::SingleInit(loc, new ast::CastExpr( dtorExpr, dtorType ) ) } ) );
                                                        destructor->attributes.push_back( new ast::Attribute( "cleanup", { new ast::VariableExpr( loc, ast::dtorStructDestroy ) } ) );
                                                        mutStmts->push_front( new ast::DeclStmt(loc, destructor ) );
                                                        mutStmts->kids.splice( mutStmts->kids.begin(), stmtsToAdd );
                                                }
                                        } catch ( SemanticErrorException & error ) {
                                                emit( funcDecl->location, "in ", function->name , ", field ", field->name, " not explicitly ", isCtor ? "constructed" : "destructed",  " and no ", isCtor ? "default constructor" : "destructor", " found" );
                                        }
                                }
                        }
                        function->stmts = mutStmts;
                }
                if (! errors.isEmpty()) {
                        throw errors;
                }
                // return funcDecl;
                return function;
        }

        /// true if expr is effectively just the 'this' parameter
        bool isThisExpression( const ast::Expr * expr, const ast::DeclWithType * thisParam ) {
                // TODO: there are more complicated ways to pass 'this' to a constructor, e.g. &*, *&, etc.
                if ( auto varExpr = dynamic_cast< const ast::VariableExpr * >( expr ) ) {
                        return varExpr->var == thisParam;
                } else if ( auto castExpr = dynamic_cast< const ast::CastExpr * > ( expr ) ) {
                        return isThisExpression( castExpr->arg, thisParam );
                }
                return false;
        }

        /// returns a MemberExpr if expr is effectively just member access on the 'this' parameter, else nullptr
        const ast::MemberExpr * isThisMemberExpr( const ast::Expr * expr, const ast::DeclWithType * thisParam ) {
                if ( auto memberExpr = dynamic_cast< const ast::MemberExpr * >( expr ) ) {
                        if ( isThisExpression( memberExpr->aggregate, thisParam ) ) {
                                return memberExpr;
                        }
                } else if ( auto castExpr = dynamic_cast< const ast::CastExpr * >( expr ) ) {
                        return isThisMemberExpr( castExpr->arg, thisParam );
                }
                return nullptr;
        }

        void GenStructMemberCalls::previsit( const ast::ApplicationExpr * appExpr ) {
                if ( ! checkWarnings( function ) ) {
                        visit_children = false;
                        return;
                }

                std::string fname = getFunctionName( appExpr );
                if ( fname == function->name ) {
                        // call to same kind of function
                        const ast::Expr * firstParam = appExpr->args.front();

                        if ( isThisExpression( firstParam, thisParam ) ) {
                                // if calling another constructor on thisParam, assume that function handles
                                // all members - if it doesn't a warning will appear in that function.
                                unhandled.clear();
                        } else if ( auto memberExpr = isThisMemberExpr( firstParam, thisParam ) ) {
                                // if first parameter is a member expression on the this parameter,
                                // then remove the member from unhandled set.
                                if ( isThisExpression( memberExpr->aggregate, thisParam ) ) {
                                        unhandled.erase( memberExpr->member );
                                }
                        }
                }
        }

        void GenStructMemberCalls::previsit( const ast::MemberExpr * memberExpr ) {
                if ( ! checkWarnings( function ) || ! isCtor ) {
                        visit_children = false;
                        return;
                }

                if ( isThisExpression( memberExpr->aggregate, thisParam ) ) {
                        if ( unhandled.count( memberExpr->member ) ) {
                                // emit a warning because a member was used before it was constructed
                                usedUninit.insert( { memberExpr->member, memberExpr->location } );
                        }
                }
        }

        template< typename Visitor, typename... Params >
        void error( Visitor & v, CodeLocation loc, const Params &... params ) {
                SemanticErrorException err( loc, toString( params... ) );
                v.errors.append( err );
        }

        template< typename... Params >
        void GenStructMemberCalls::emit( CodeLocation loc, const Params &... params ) {
                // toggle warnings vs. errors here.
                // warn( params... );
                error( *this, loc, params... );
        }

        const ast::Expr * GenStructMemberCalls::postvisit( const ast::UntypedExpr * untypedExpr ) {
                // xxx - functions returning ast::ptr seems wrong...
                auto res = ResolvExpr::findVoidExpression( untypedExpr, symtab );
                // Fix CodeLocation (at least until resolver is fixed).
                auto fix = localFillCodeLocations( untypedExpr->location, res.release() );
                return strict_dynamic_cast<const ast::Expr *>( fix );
        }

        void InsertImplicitCalls::previsit(const ast::UniqueExpr * unqExpr) {
                if (visitedIds.count(unqExpr->id)) visit_children = false;
                else visitedIds.insert(unqExpr->id);
        }

        const ast::Expr * FixCtorExprs::postvisit( const ast::ConstructorExpr * ctorExpr ) {
                const CodeLocation loc = ctorExpr->location;
                static UniqueName tempNamer( "_tmp_ctor_expr" );
                // xxx - is the size check necessary?
                assert( ctorExpr->result && ctorExpr->result->size() == 1 );

                // xxx - this can be TupleAssignExpr now. Need to properly handle this case.
                // take possession of expr and env
                ast::ptr<ast::ApplicationExpr> callExpr = ctorExpr->callExpr.strict_as<ast::ApplicationExpr>();
                ast::ptr<ast::TypeSubstitution> env = ctorExpr->env;
                // ctorExpr->set_callExpr( nullptr );
                // ctorExpr->set_env( nullptr );

                // xxx - ideally we would reuse the temporary generated from the copy constructor passes from within firstArg if it exists and not generate a temporary if it's unnecessary.
                auto tmp = new ast::ObjectDecl(loc, tempNamer.newName(), callExpr->args.front()->result );
                declsToAddBefore.push_back( tmp );
                // delete ctorExpr;

                // build assignment and replace constructor's first argument with new temporary
                auto mutCallExpr = callExpr.get_and_mutate();
                const ast::Expr * firstArg = callExpr->args.front();
                ast::Expr * assign = new ast::UntypedExpr(loc, new ast::NameExpr(loc, "?=?" ), { new ast::AddressExpr(loc, new ast::VariableExpr(loc, tmp ) ), new ast::AddressExpr( firstArg ) } );
                firstArg = new ast::VariableExpr(loc, tmp );
                mutCallExpr->args.front() = firstArg;

                // resolve assignment and dispose of new env
                auto resolved = ResolvExpr::findVoidExpression( assign, symtab );
                auto mut = resolved.get_and_mutate();
                assertf(resolved.get() == mut, "newly resolved expression must be unique");
                mut->env = nullptr;

                // for constructor expr:
                //   T x;
                //   x{};
                // results in:
                //   T x;
                //   T & tmp;
                //   &tmp = &x, ?{}(tmp), tmp
                ast::CommaExpr * commaExpr = new ast::CommaExpr(loc, resolved, new ast::CommaExpr(loc, mutCallExpr, new ast::VariableExpr(loc, tmp ) ) );
                commaExpr->env = env;
                return commaExpr;
        }
} // namespace
} // namespace InitTweak

// Local Variables: //
// tab-width: 4 //
// mode: c++ //
// compile-command: "make install" //
// End: //