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Timestamp:
May 1, 2017, 1:39:55 PM (7 years ago)
Author:
Rob Schluntz <rschlunt@…>
Branches:
ADT, aaron-thesis, arm-eh, ast-experimental, cleanup-dtors, deferred_resn, demangler, enum, forall-pointer-decay, jacob/cs343-translation, jenkins-sandbox, master, new-ast, new-ast-unique-expr, new-env, no_list, persistent-indexer, pthread-emulation, qualifiedEnum, resolv-new, with_gc
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2055098
Parents:
b3d70eba
Message:

final revisions to thesis

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  • doc/rob_thesis/ctordtor.tex

    rb3d70eba r12d3187  
    66% doesn't seem possible to do this without allowing ttype on generic structs?
    77
    8 Since \CFA is a true systems language, it does not provide a garbage collector.
    9 As well, \CFA is not an object-oriented programming language, \ie, structures cannot have routine members.
     8Since \CFA is a true systems language, it does not require a garbage collector.
     9As well, \CFA is not an object-oriented programming language, \ie, structures cannot have methods.
     10While structures can have function pointer members, this is different from methods, since methods have implicit access to structure members and methods cannot be reassigned.
    1011Nevertheless, one important goal is to reduce programming complexity and increase safety.
    1112To that end, \CFA provides support for implicit pre/post-execution of routines for objects, via constructors and destructors.
     
    3233The key difference between assignment and initialization being that assignment occurs on a live object (\ie, an object that contains data).
    3334It is important to note that this means @x@ could have been used uninitialized prior to being assigned, while @y@ could not be used uninitialized.
    34 Use of uninitialized variables yields undefined behaviour, which is a common source of errors in C programs.
     35Use of uninitialized variables yields undefined behaviour \cite[p.~558]{C11}, which is a common source of errors in C programs.
    3536
    3637Initialization of a declaration is strictly optional, permitting uninitialized variables to exist.
     
    7071int x2 = opaque_get(x, 2);
    7172\end{cfacode}
    72 This pattern is cumbersome to use since every access becomes a function call.
     73This pattern is cumbersome to use since every access becomes a function call, requiring awkward syntax and a performance cost.
    7374While useful in some situations, this compromise is too restrictive.
    7475Furthermore, even with this idiom it is easy to make mistakes, such as forgetting to destroy an object or destroying it multiple times.
    7576
    7677A constructor provides a way of ensuring that the necessary aspects of object initialization is performed, from setting up invariants to providing compile- and run-time checks for appropriate initialization parameters.
    77 This goal is achieved through a guarantee that a constructor is called implicitly after every object is allocated from a type with associated constructors, as part of an object's definition.
    78 Since a constructor is called on every object of a managed type, it is impossible to forget to initialize such objects, as long as all constructors perform some sensible form of initialization.
     78This goal is achieved through a \emph{guarantee} that a constructor is called \emph{implicitly} after every object is allocated from a type with associated constructors, as part of an object's \emph{definition}.
     79Since a constructor is called on every object of a managed type, it is \emph{impossible} to forget to initialize such objects, as long as all constructors perform some sensible form of initialization.
    7980
    8081In \CFA, a constructor is a function with the name @?{}@.
     
    114115In other words, a default constructor is a constructor that takes a single argument: the @this@ parameter.
    115116
    116 In \CFA, a destructor is a function much like a constructor, except that its name is \lstinline!^?{}! and it takes only one argument.
     117In \CFA, a destructor is a function much like a constructor, except that its name is \lstinline!^?{}! \footnote{Originally, the name @~?{}@ was chosen for destructors, to provide familiarity to \CC programmers. Unforunately, this name causes parsing conflicts with the bitwise-not operator when used with operator syntax (see section \ref{sub:syntax}.)} and it takes only one argument.
    117118A destructor for the @Array@ type can be defined as:
    118119\begin{cfacode}
     
    135136On line 2, @z@ is initialized with the value of @x@, while on line 3, @y@ is assigned the value of @x@.
    136137The key distinction between initialization and assignment is that a value to be initialized does not hold any meaningful values, whereas an object to be assigned might.
    137 In particular, these cases cannot be handled the same way because in the former case @z@ does not currently own an array, while @y@ does.
     138In particular, these cases cannot be handled the same way because in the former case @z@ has no array, while @y@ does.
     139A \emph{copy constructor} is used to perform initialization using another object of the same type.
    138140
    139141\begin{cfacode}[emph={other}, emphstyle=\color{red}]
     
    151153}
    152154\end{cfacode}
    153 The two functions above handle these cases.
    154 The first function is called a \emph{copy constructor}, because it constructs its argument by copying the values from another object of the same type.
     155The two functions above handle the cases of initialization and assignment.
     156The first function is called a copy constructor, because it constructs its argument by copying the values from another object of the same type.
    155157The second function is the standard copy-assignment operator.
     158\CFA does not currently have the concept of reference types, so the most appropriate type for the source object in copy constructors and assignment operators is a value type.
     159Appropriate care is taken in the implementation to avoid recursive calls to the copy constructor.
    156160The four functions (default constructor, destructor, copy constructor, and assignment operator) are special in that they safely control the state of most objects.
    157161
     
    216220A * y = malloc();  // copy construct: ?{}(&y, malloc())
    217221
     222^?{}(&x);   // explicit destroy x, in different order
    218223?{}(&x);    // explicit construct x, second construction
     224^?{}(y);    // explicit destroy y
    219225?{}(y, x);  // explit construct y from x, second construction
    220 ^?{}(&x);   // explicit destroy x, in different order
    221 ^?{}(y);    // explicit destroy y
    222226
    223227// implicit ^?{}(&y);
     
    279283\end{cfacode}
    280284In this example, @malloc@ dynamically allocates storage and initializes it using a constructor, all before assigning it into the variable @x@.
     285Intuitively, the expression-resolver determines that @malloc@ returns some type @T *@, as does the constructor expression since it returns the type of its argument.
     286This type flows outwards to the declaration site where the expected type is known to be @X *@, thus the first argument to the constructor must be @X *@, narrowing the search space.
     287
    281288If this extension is not present, constructing dynamically allocated objects is much more cumbersome, requiring separate initialization of the pointer and initialization of the pointed-to memory.
    282289\begin{cfacode}
     
    300307It should be noted that this technique is not exclusive to @malloc@, and allows a user to write a custom allocator that can be idiomatically used in much the same way as a constructed @malloc@ call.
    301308
    302 It should be noted that while it is possible to use operator syntax with destructors, destructors invalidate their argument, thus operator syntax with destructors is a statement and does not produce a value.
     309While it is possible to use operator syntax with destructors, destructors invalidate their argument, thus operator syntax with destructors is void-typed expression.
    303310
    304311\subsection{Function Generation}
     
    308315If the translator can expect these functions to exist, then it can unconditionally attempt to resolve them.
    309316Moreover, the existence of a standard interface allows polymorphic code to interoperate with new types seamlessly.
     317While automatic generation of assignment functions is present in previous versions of \CFA, the the implementation has been largely rewritten to accomodate constructors and destructors.
    310318
    311319To mimic the behaviour of standard C, the default constructor and destructor for all of the basic types and for all pointer types are defined to do nothing, while the copy constructor and assignment operator perform a bitwise copy of the source parameter (as in \CC).
     320This default is intended to maintain backwards compatibility and performance, by not imposing unexpected operations for a C programmer, as a zero-default behaviour would.
     321However, it is possible for a user to define such constructors so that variables are safely zeroed by default, if desired.
     322%%%%%%%%%%%%%%%%%%%%%%%%%% line width %%%%%%%%%%%%%%%%%%%%%%%%%%
     323\begin{cfacode}
     324void ?{}(int * i) { *i = 0; }
     325forall(dtype T) void ?{}(T ** p) { *p = 0; }  // any pointer type
     326void f() {
     327  int x;    // initialized to 0
     328  int * p;  // initialized to 0
     329}
     330\end{cfacode}
     331%%%%%%%%%%%%%%%%%%%%%%%%%% line width %%%%%%%%%%%%%%%%%%%%%%%%%%
    312332
    313333There are several options for user-defined types: structures, unions, and enumerations.
    314334To aid in ease of use, the standard set of four functions is automatically generated for a user-defined type after its definition is completed.
    315335By auto-generating these functions, it is ensured that legacy C code continues to work correctly in every context where \CFA expects these functions to exist, since they are generated for every complete type.
     336As well, these functions are always generated, since they may be needed by polymorphic functions.
     337With that said, the generated functions are not called implicitly unless they are non-trivial, and are never exported, making it simple for the optimizer to strip them away when they are not used.
    316338
    317339The generated functions for enumerations are the simplest.
     
    338360}
    339361\end{cfacode}
    340 In the future, \CFA will introduce strongly-typed enumerations, like those in \CC.
     362In the future, \CFA will introduce strongly-typed enumerations, like those in \CC, wherein enumerations create a new type distinct from @int@ so that integral values require an explicit cast to be stored in an enumeration variable.
    341363The existing generated routines are sufficient to express this restriction, since they are currently set up to take in values of that enumeration type.
    342364Changes related to this feature only need to affect the expression resolution phase, where more strict rules will be applied to prevent implicit conversions from integral types to enumeration types, but should continue to permit conversions from enumeration types to @int@.
     
    492514In addition to freedom, \ateq provides a simple path for migrating legacy C code to \CFA, in that objects can be moved from C-style initialization to \CFA gradually and individually.
    493515It is worth noting that the use of unmanaged objects can be tricky to get right, since there is no guarantee that the proper invariants are established on an unmanaged object.
    494 It is recommended that most objects be managed by sensible constructors and destructors, except where absolutely necessary.
     516It is recommended that most objects be managed by sensible constructors and destructors, except where absolutely necessary, such as memory-mapped devices, trigger devices, I/O controllers, etc.
    495517
    496518When a user declares any constructor or destructor, the corresponding intrinsic/generated function and all field constructors for that type are hidden, so that they are not found during expression resolution until the user-defined function goes out of scope.
     
    545567\end{cfacode}
    546568However, if the translator sees a sub-object used within the body of a constructor, but does not see a constructor call that uses the sub-object as the target of a constructor, then the translator assumes the object is to be implicitly constructed (copy constructed in a copy constructor and default constructed in any other constructor).
     569To override this rule, \ateq can be used to force the translator to trust the programmer's discretion.
     570This form of \ateq is not yet implemented.
    547571\begin{cfacode}
    548572void ?{}(A * a) {
     
    556580}
    557581
     582void ?{}(A * a, int x) {
     583  // object forwarded to another constructor,
     584  // does not implicitly construct any members
     585  (&a){};
     586}
     587
    558588void ^?{}(A * a) {
    559589  ^(&a->x){}; // explicit destructor call
    560590} // z, y, w implicitly destructed, in this order
    561591\end{cfacode}
    562 If at any point, the @this@ parameter is passed directly as the target of another constructor, then it is assumed that constructor handles the initialization of all of the object's members and no implicit constructor calls are added.
    563 To override this rule, \ateq can be used to force the translator to trust the programmer's discretion.
    564 This form of \ateq is not yet implemented.
     592If at any point, the @this@ parameter is passed directly as the target of another constructor, then it is assumed the other constructor handles the initialization of all of the object's members and no implicit constructor calls are added to the current constructor.
    565593
    566594Despite great effort, some forms of C syntax do not work well with constructors in \CFA.
     
    619647The body of @A@ has been omitted, since only the constructor interfaces are important.
    620648
    621 It should be noted that unmanaged objects can still make use of designations and nested initializers in \CFA.
     649It should be noted that unmanaged objects, i.e. objects that have only trivial constructors, can still make use of designations and nested initializers in \CFA.
    622650It is simple to overcome this limitation for managed objects by making use of compound literals, so that the arguments to the constructor call are explicitly typed.
     651%%%%%%%%%%%%%%%%%%%%%%%%%% line width %%%%%%%%%%%%%%%%%%%%%%%%%%
     652\begin{cfacode}
     653struct B { int x; };
     654struct C { int y; };
     655struct A { B b; C c; };
     656void ?{}(A *, B);
     657void ?{}(A *, C);
     658
     659A a = {
     660  (C){ 10 } // disambiguate with compound literal
     661};
     662\end{cfacode}
     663%%%%%%%%%%%%%%%%%%%%%%%%%% line width %%%%%%%%%%%%%%%%%%%%%%%%%%
    623664
    624665\subsection{Implicit Destructors}
     
    744785\end{cfacode}
    745786
     787While \CFA supports the GCC computed-goto extension, the behaviour of managed objects in combination with computed-goto is undefined.
     788\begin{cfacode}
     789void f(int val) {
     790  void * l = val == 0 ? &&L1 : &&L2;
     791  {
     792      A x;
     793    L1: ;
     794      goto *l;  // branches differently depending on argument
     795  }
     796  L2: ;
     797}
     798\end{cfacode}
     799Likewise, destructors are not executed at scope-exit due to a computed-goto in \CC, as of g++ version 6.2.
     800
    746801\subsection{Implicit Copy Construction}
    747802\label{s:implicit_copy_construction}
     
    750805Exempt from these rules are intrinsic and built-in functions.
    751806It should be noted that unmanaged objects are subject to copy constructor calls when passed as arguments to a function or when returned from a function, since they are not the \emph{target} of the copy constructor call.
    752 That is, since the parameter is not marked as an unmanaged object using \ateq, it is be copy constructed if it is returned by value or passed as an argument to another function, so to guarantee consistent behaviour, unmanaged objects must be copy constructed when passed as arguments.
     807That is, since the parameter is not marked as an unmanaged object using \ateq, it is copy constructed if it is returned by value or passed as an argument to another function, so to guarantee consistent behaviour, unmanaged objects must be copy constructed when passed as arguments.
    753808These semantics are important to bear in mind when using unmanaged objects, and could produce unexpected results when mixed with objects that are explicitly constructed.
    754809\begin{cfacode}
    755 struct A;
     810struct A { ... };
    756811void ?{}(A *);
    757812void ?{}(A *, A);
     
    810865It should be noted that reference types will allow specifying that a value does not need to be copied, however reference types do not provide a means of preventing implicit copy construction from uses of the reference, so the problem is still present when passing or returning the reference by value.
    811866
     867Adding implicit copy construction imposes the additional runtime cost of the copy constructor for every argument and return value in a function call.
     868This cost is necessary to maintain appropriate value semantics when calling a function.
     869In the future, return-value-optimization (RVO) can be implemented for \CFA to elide unnecessary copy construction and destruction of temporary objects.
     870This cost is not present for types with trivial copy constructors and destructors.
     871
    812872A known issue with this implementation is that the argument and return value temporaries are not guaranteed to have the same address for their entire lifetimes.
    813873In the previous example, since @_retval_f@ is allocated and constructed in @f@, then returned by value, the internal data is bitwise copied into the caller's stack frame.
     
    908968\subsection{Array Initialization}
    909969Arrays are a special case in the C type-system.
    910 C arrays do not carry around their size, making it impossible to write a standalone \CFA function that constructs or destructs an array while maintaining the standard interface for constructors and destructors.
     970Type checking largely ignores size information for C arrays, making it impossible to write a standalone \CFA function that constructs or destructs an array, while maintaining the standard interface for constructors and destructors.
    911971Instead, \CFA defines the initialization and destruction of an array recursively.
    912972That is, when an array is defined, each of its elements is constructed in order from element 0 up to element $n-1$.
     
    11471207\end{cfacode}
    11481208
     1209This implementation comes at the runtime cost of an additional branch for every @static@ local variable, each time the function is called.
     1210Since initializers are not required to be compile-time constant expressions, they can involve global variables, function arguments, function calls, etc.
     1211As a direct consequence, @static@ local variables cannot be initialized with an attribute-constructor routines like global variables can.
     1212However, in the case where the variable is unmanaged and has a compile-time constant initializer, a C-compliant initializer is generated and the additional cost is not present.
     1213\CC shares the same semantics for its @static@ local variables.
     1214
    11491215\subsection{Polymorphism}
    11501216As mentioned in section \ref{sub:polymorphism}, \CFA currently has 3 type-classes that are used to designate polymorphic data types: @otype@, @dtype@, and @ftype@.
     
    11721238These additions allow @f@'s body to create and destroy objects of type @T@, and pass objects of type @T@ as arguments to other functions, following the normal \CFA rules.
    11731239A point of note here is that objects can be missing default constructors (and eventually other functions through deleted functions), so it is important for \CFA programmers to think carefully about the operations needed by their function, as to not over-constrain the acceptable parameter types and prevent potential reuse.
     1240
     1241These additional assertion parameters impose a runtime cost on all managed temporary objects created in polymorphic code, even those with trivial constructors and destructors.
     1242This cost is necessary because polymorphic code does not know the actual type at compile-time, due to separate compilation.
     1243Since trivial constructors and destructors either do not perform operations or are simply bit-wise copy operations, the imposed cost is essentially the cost of the function calls.
     1244
     1245\section{Summary}
     1246
     1247When creating a new object of a managed type, it is guaranteed that a constructor is be called to initialize the object at its definition point, and is destructed when the object's lifetime ends.
     1248Destructors are called in the reverse order of construction.
     1249
     1250Every argument passed to a function is copy constructed into a temporary object that is passed by value to the functions and destructed at the end of the statement.
     1251Function return values are copy constructed inside the function at the return statement, passed by value to the call-site, and destructed at the call-site at the end of the statement.
     1252
     1253Every complete object type has a default constructor, copy constructor, assignment operator, and destructor.
     1254To accomplish this, these functions are generated as appropriate for new types.
     1255User-defined functions shadow built-in and automatically generated functions, so it is possible to specialize the behaviour of a type.
     1256Furthermore, default constructors and aggregate field constructors are hidden when \emph{any} constructor is defined.
     1257
     1258Objects dynamically allocated with @malloc@, \ateq objects, and objects with only trivial constructors and destructors are unmanaged.
     1259Unmanaged objects are never the target of an implicit constructor or destructor call.
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