# Changeset 0eb18557 for doc/rob_thesis/ctordtor.tex

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Timestamp:
Apr 12, 2017, 3:54:28 PM (5 years ago)
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aaron-thesis, arm-eh, 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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e869e434
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eaa2f3a1
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thesis updates based on Peter's feedback

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 reaa2f3a1 Since \CFA is a true systems language, it does not provide a garbage collector. As well, \CFA is not an object-oriented programming language, i.e., structures cannot have routine members. As well, \CFA is not an object-oriented programming language, \ie, structures cannot have routine members. Nevertheless, one important goal is to reduce programming complexity and increase safety. To that end, \CFA provides support for implicit pre/post-execution of routines for objects, via constructors and destructors. Next, @x@ is assigned the value of @y@. In the last line, @z@ is implicitly initialized to 0 since it is marked @static@. The key difference between assignment and initialization being that assignment occurs on a live object (i.e., an object that contains data). The key difference between assignment and initialization being that assignment occurs on a live object (\ie, an object that contains data). It is important to note that this means @x@ could have been used uninitialized prior to being assigned, while @y@ could not be used uninitialized. Use of uninitialized variables yields undefined behaviour, which is a common source of errors in C programs. In \CFA, a constructor is a function with the name @?{}@. Like other operators in \CFA, the name represents the syntax used to call the constructor, e.g., @struct S = { ... };@. Like other operators in \CFA, the name represents the syntax used to call the constructor, \eg, @struct S = { ... };@. Every constructor must have a return type of @void@ and at least one parameter, the first of which is colloquially referred to as the \emph{this} parameter, as in many object-oriented programming-languages (however, a programmer can give it an arbitrary name). The @this@ parameter must have a pointer type, whose base type is the type of object that the function constructs. In other words, a default constructor is a constructor that takes a single argument: the @this@ parameter. In \CFA, a destructor is a function much like a constructor, except that its name is \lstinline!^?{}! and it take only one argument. A destructor for the @Array@ type can be defined as such. In \CFA, a destructor is a function much like a constructor, except that its name is \lstinline!^?{}! and it takes only one argument. A destructor for the @Array@ type can be defined as: \begin{cfacode} void ^?{}(Array * arr) { } \end{cfacode} In \CFA, constructors are called implicitly in initialization contexts. \begin{cfacode} Array x, y = { 20, 0xdeadbeef }, z = y; \end{cfacode} In \CFA, constructor calls look just like C initializers, which allows them to be inserted into legacy C code with minimal code changes, and also provides a very simple syntax that veteran C programmers are familiar with. One downside of reusing C initialization syntax is that it isn't possible to determine whether an object is constructed just by looking at its declaration, since that requires knowledge of whether the type is managed at that point. Constructor calls look just like C initializers, which allows them to be inserted into legacy C code with minimal code changes, and also provides a very simple syntax that veteran C programmers are familiar with. One downside of reusing C initialization syntax is that it is not possible to determine whether an object is constructed just by looking at its declaration, since that requires knowledge of whether the type is managed at that point in the program. This example generates the following code \end{cfacode} Finally, constructors and destructors support \emph{operator syntax}. Like other operators in \CFA, the function name mirrors the use-case, in that the first $N$ arguments fill in the place of the question mark. Like other operators in \CFA, the function name mirrors the use-case, in that the question marks are placeholders for the first $N$ arguments. This syntactic form is similar to the new initialization syntax in \CCeleven, except that it is used in expression contexts, rather than declaration contexts. \begin{cfacode} Like other operators, the function name @?{}@ matches its operator syntax. For example, @(&x){}@ calls the default constructor on the variable @x@, and produces @&x@ as a result. A key example for this capability is the use of constructor expressions to initialize the result of a call to standard C routine @malloc@. A key example for this capability is the use of constructor expressions to initialize the result of a call to @malloc@. \begin{cfacode} struct X { ... }; void ?{}(X *, double); X * x = malloc(sizeof(X)){ 1.5 }; X * x = malloc(){ 1.5 }; \end{cfacode} In this example, @malloc@ dynamically allocates storage and initializes it using a constructor, all before assigning it into the variable @x@. If 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. \begin{cfacode} X * x = malloc(sizeof(X)); X * x = malloc(); x{ 1.5 }; \end{cfacode} struct X *_tmp_ctor; struct X *x = ?{}(  // construct result of malloc _tmp_ctor=malloc(sizeof(struct X)), // store result of malloc _tmp_ctor=malloc_T(sizeof(struct X), _Alignof(struct X)), // store result of malloc 1.5 ), _tmp_ctor; // produce constructed result of malloc It 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. It is also possible to use operator syntax with destructors. Unlike constructors, operator syntax with destructors is a statement and thus does not produce a value, since the destructed object is invalidated by the use of a destructor. For example, \lstinline!^(&x){}! calls the destructor on the variable @x@. 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. \subsection{Function Generation} The field constructors are constructors that consume a prefix of the structure's member-list. That is, $N$ constructors are built of the form @void ?{}(S *, T$_{\text{M}_0}$)@, @void ?{}(S *, T$_{\text{M}_0}$, T$_{\text{M}_1}$)@, ..., @void ?{}(S *, T$_{\text{M}_0}$, T$_{\text{M}_1}$, ..., T$_{\text{M}_{N-1}}$)@, where members are copy constructed if they have a corresponding positional argument and are default constructed otherwise. The addition of field constructors allows structures in \CFA to be used naturally in the same ways as used in C (i.e., to initialize any prefix of the structure), e.g., @A a0 = { b }, a1 = { b, c }@. The addition of field constructors allows structures in \CFA to be used naturally in the same ways as used in C (\ie, to initialize any prefix of the structure), \eg, @A a0 = { b }, a1 = { b, c }@. Extending the previous example, the following constructors are implicitly generated for @A@. \begin{cfacode} \subsection{Using Constructors and Destructors} Implicitly generated constructor and destructor calls ignore the outermost type qualifiers, e.g. @const@ and @volatile@, on a type by way of a cast on the first argument to the function. Implicitly generated constructor and destructor calls ignore the outermost type qualifiers, \eg @const@ and @volatile@, on a type by way of a cast on the first argument to the function. For example, \begin{cfacode} Here, @&s@ and @&s2@ are cast to unqualified pointer types. This mechanism allows the same constructors and destructors to be used for qualified objects as for unqualified objects. This applies only to implicitly generated constructor calls. This rule applies only to implicitly generated constructor calls. Hence, explicitly re-initializing qualified objects with a constructor requires an explicit cast. Instead, @a2->x@ is initialized to @0@ as if it were a C object, because of the explicit initializer. In addition to freedom, \ateq provides a simple path to migrating legacy C code to \CFA, in that objects can be moved from C-style initialization to \CFA gradually and individually. In 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. It 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. It is recommended that most objects be managed by sensible constructors and destructors, except where absolutely necessary. { void ?{}(S * s, int i) { s->x = i*2; } // locally hide autogen constructors S s4;  // error S s5 = { 3 };  // okay S s6 = { 4, 5 };  // error S s4;  // error, no default constructor S s5 = { 3 };  // okay, local constructor S s6 = { 4, 5 };  // error, no field constructor S s7 = s5; // okay } In this example, the inner scope declares a constructor from @int@ to @S@, which hides the default constructor and field constructors until the end of the scope. When defining a constructor or destructor for a struct @S@, any members that are not explicitly constructed or destructed are implicitly constructed or destructed automatically. When defining a constructor or destructor for a structure @S@, any members that are not explicitly constructed or destructed are implicitly constructed or destructed automatically. If an explicit call is present, then that call is taken in preference to any implicitly generated call. A consequence of this rule is that it is possible, unlike \CC, to precisely control the order of construction and destruction of sub-objects on a per-constructor basis, whereas in \CC sub-object initialization and destruction is always performed based on the declaration order. In practice, however, there could be many objects that can be constructed from a given @int@ (or, indeed, any arbitrary parameter list), and thus a complete solution to this problem would require fully exploring all possibilities. More precisely, constructor calls cannot have a nesting depth greater than the number of array components in the type of the initialized object, plus one. More precisely, constructor calls cannot have a nesting depth greater than the number of array dimensions in the type of the initialized object, plus one. For example, \begin{cfacode} { {14 }, { 15 } }   // a2[1] }; A a3[4] = { { { 11 }, { 12 } },  // error A a3[4] = { // 1 dimension => max depth 2 { { 11 }, { 12 } },  // error, three levels deep { 80 }, { 90 }, { 100 } } \label{sub:implicit_dtor} Destructors are automatically called at the end of the block in which the object is declared. In addition to this, destructors are automatically called when statements manipulate control flow to leave a block in which the object is declared, e.g., with return, break, continue, and goto statements. In addition to this, destructors are automatically called when statements manipulate control flow to leave a block in which the object is declared, \eg, with return, break, continue, and goto statements. The example below demonstrates a simple routine with multiple return statements. \begin{cfacode} Exempt from these rules are intrinsic and built-in functions. It 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. That is, since the parameter is not marked as an unmanaged object using \ateq, it will 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. This is an important detail to bear in mind when using unmanaged objects, and could produce unexpected results when mixed with objects that are explicitly constructed. 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. These semantics are important to bear in mind when using unmanaged objects, and could produce unexpected results when mixed with objects that are explicitly constructed. \begin{cfacode} struct A; identity(z);  // copy construct z into x \end{cfacode} Note that @z@ is copy constructed into a temporary variable to be passed as an argument, which is also destructed after the call. Note that unmanaged argument @z@ is logically copy constructed into managed parameter @x@; however, the translator must copy construct into a temporary variable to be passed as an argument, which is also destructed after the call. A compiler could by-pass the argument temporaries since it is in control of the calling conventions and knows exactly where the called-function's parameters live. This generates the following This transformation provides @f@ with the address of the return variable so that it can be constructed into directly. It is worth pointing out that this kind of signature rewriting already occurs in polymorphic functions that return by value, as discussed in \cite{Bilson03}. A key difference in this case is that every function would need to be rewritten like this, since types can switch between managed and unmanaged at different scope levels, e.g. A key difference in this case is that every function would need to be rewritten like this, since types can switch between managed and unmanaged at different scope levels, \eg \begin{cfacode} struct A { int v; }; Furthermore, it is not possible to overload C functions, so using @extern "C"@ to declare functions is of limited use. It would be possible to regain some control by adding an attribute to structs that specifies whether they can be managed or not (perhaps \emph{manageable} or \emph{unmanageable}), and to emit an error in the case that a constructor or destructor is declared for an unmanageable type. Ideally, structs should be manageable by default, since otherwise the default case becomes more verbose. It would be possible to regain some control by adding an attribute to structures that specifies whether they can be managed or not (perhaps \emph{manageable} or \emph{unmanageable}), and to emit an error in the case that a constructor or destructor is declared for an unmanageable type. Ideally, structures should be manageable by default, since otherwise the default case becomes more verbose. This means that in general, function signatures would have to be rewritten, and in a select few cases the signatures would not be rewritten. \begin{cfacode} C h();  // rewritten void h(C *); \end{cfacode} An alternative is to instead make the attribute \emph{identifiable}, which states that objects of this type use the @this@ parameter as an identity. An alternative is to make the attribute \emph{identifiable}, which states that objects of this type use the @this@ parameter as an identity. This strikes more closely to the visible problem, in that only types marked as identifiable would need to have the return value moved into the parameter list, and every other type could remain the same. Furthermore, no restrictions would need to be placed on whether objects can be constructed. \subsection{Global Initialization} In standard C, global variables can only be initialized to compile-time constant expressions. This places strict limitations on the programmer's ability to control the default values of objects. In standard C, global variables can only be initialized to compile-time constant expressions, which places strict limitations on the programmer's ability to control the default values of objects. In \CFA, constructors and destructors are guaranteed to be run on global objects, allowing arbitrary code to be run before and after the execution of the main routine. By default, objects within a translation unit are constructed in declaration order, and destructed in the reverse order. The default order of construction of objects amongst translation units is unspecified. It is, however, guaranteed that any global objects in the standard library are initialized prior to the initialization of any object in the user program. It is, however, guaranteed that any global objects in the standard library are initialized prior to the initialization of any object in a user program. This feature is implemented in the \CFA translator by grouping every global constructor call into a function with the GCC attribute \emph{constructor}, which performs most of the heavy lifting \cite[6.31.1]{GCCExtensions}. %   https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Attributes.html#C_002b_002b-Attributes % suggestion: implement this in CFA by picking objects with a specified priority and pulling them into their own init functions (could even group them by priority level -> map>) and pull init_priority forward into constructor and destructor attributes with the same priority level GCC provides an attribute @init_priority@, which allows specifying the relative priority for initialization of global objects on a per-object basis in \CC. GCC provides an attribute @init_priority@ in \CC, which allows specifying the relative priority for initialization of global objects on a per-object basis. A similar attribute can be implemented in \CFA by pulling marked objects into global constructor/destructor-attribute functions with the specified priority. For example, In standard C, it is possible to mark variables that are local to a function with the @static@ storage class. Unlike normal local variables, a @static@ local variable is defined to live for the entire duration of the program, so that each call to the function has access to the same variable with the same address and value as it had in the previous call to the function. Much like global variables, in C @static@ variables can only be initialized to a \emph{compile-time constant value} so that a compiler is able to create storage for the variable and initialize it at compile-time. Much like global variables, @static@ variables can only be initialized to a \emph{compile-time constant value} so that a compiler is able to create storage for the variable and initialize it at compile-time. Yet again, this rule is too restrictive for a language with constructors and destructors. Instead, \CFA modifies the definition of a @static@ local variable so that objects are guaranteed to be live from the time control flow reaches their declaration, until the end of the program, since the initializer expression is not necessarily a compile-time constant, but can depend on the current execution state of the function. Since standard C does not allow access to a @static@ local variable before the first time control flow reaches the declaration, this restriction does not preclude any valid C code. Since the initializer expression is not necessarily a compile-time constant and can depend on the current execution state of the function, \CFA modifies the definition of a @static@ local variable so that objects are guaranteed to be live from the time control flow reaches their declaration, until the end of the program. Since standard C does not allow access to a @static@ local variable before the first time control flow reaches the declaration, this change does not preclude any valid C code. Local objects with @static@ storage class are only implicitly constructed and destructed once for the duration of the program. The object is constructed when its declaration is reached for the first time. Since the parameter to @atexit@ is a parameter-less function, some additional tweaking is required. First, the @static@ variable must be hoisted up to global scope and uniquely renamed to prevent name clashes with other global objects. Second, a function is built which calls the destructor for the newly hoisted variable. If necessary, a local structure may need to be hoisted, as well. Second, a function is built that calls the destructor for the newly hoisted variable. Finally, the newly generated function is registered with @atexit@, instead of registering the destructor directly. Since @atexit@ calls functions in the reverse order in which they are registered, @static@ local variables are guaranteed to be destructed in the reverse order that they are constructed, which may differ between multiple executions of the same program. void f(T); \end{cfacode} This allows easily specifying constraints that are common to all complete object types very simply. Now that \CFA has constructors and destructors, more of a complete object's behaviour can be specified by than was previously possible. This allows easily specifying constraints that are common to all complete object-types very simply. Now that \CFA has constructors and destructors, more of a complete object's behaviour can be specified than was previously possible. As such, @otype@ has been augmented to include assertions for a default constructor, copy constructor, and destructor. That is, the previous example is now equivalent to \begin{cfacode} forall(dtype T | sized(T) | { T ?=?(T *, T); void ?{}(T *); void ?{}(T *, T); void ^?{}(T *); }) forall(dtype T | sized(T) | { T ?=?(T *, T); void ?{}(T *); void ?{}(T *, T); void ^?{}(T *); }) void f(T); \end{cfacode} This allows @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. A 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. These 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. A 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.