\chapter{Implementation} % Goes over how all the features are implemented. \section{Virtual System} % Virtual table rules. Virtual tables, the pointer to them and the cast. The \CFA virtual system only has one public facing feature: virtual casts. However there is a lot of structure to support that and provide some other features for the standard library. All of this is accessed through a field inserted at the beginning of every virtual type. Currently it is called @virtual_table@ but it is not ment to be accessed by the user. This field is a pointer to the type's virtual table instance. It is assigned once during the object's construction and left alone after that. \subsection{Virtual Table Construction} For each virtual type a virtual table is constructed. This is both a new type and an instance of that type. Other instances of the type could be created but the system doesn't use them. So this section will go over the creation of the type and the instance. Creating the single instance is actually very important. The address of the table acts as the unique identifier for the virtual type. Similarly the first field in every virtual table is the parent's id; a pointer to the parent virtual table instance. The remaining fields contain the type's virtual members. First come the ones present on the parent type, in the same order as they were the parent, and then any that this type introduces. The types of the ones inherited from the parent may have a slightly modified type, in that references to the dispatched type are replaced with the current virtual type. These are always taken by pointer or reference. The structure itself is created where the virtual type is created. The name of the type is created by mangling the name of the base type. The name of the instance is also generated by name mangling. The fields are initialized automatically. The parent field is initialized by getting the type of the parent field and using that to calculate the mangled name of the parent's virtual table type. There are two special fields that are included like normal fields but have special initialization rules: the @size@ field is the type's size and is initialized with a sizeof expression, the @align@ field is the type's alignment and uses an alignof expression. The remaining fields are resolved to a name matching the field's name and type using the normal visibility and overload resolution rules of the type system. These operations are split up into several groups depending on where they take place which can vary for monomorphic and polymorphic types. The first devision is between the declarations and the definitions. Declarations, such as a function signature or a structure's name, must always be visible but may be repeated so they go in headers. Definitions, such as function bodies and a structure's layout, don't have to be visible on use but must occur exactly once and go into source files. The declarations include the virtual type definition and forward declarations of the virtual table instance, constructor, message function and @get_exception_vtable@. The definition includes the storage and initialization of the virtual table instance and the bodies of the three functions. Monomorphic instances put all of these two groups in one place each. Polymorphic instances also split out the core declarations and definitions from the per-instance information. The virtual table type and most of the functions are polymorphic so they are all part of the core. The virtual table instance and the @get_exception_vtable@ function. Coroutines and threads need instances of @CoroutineCancelled@ and @ThreadCancelled@ respectively to use all of their functionality. When a new data type is declared with @coroutine@ or @thread@ the forward declaration for the instance is created as well. The definition of the virtual table is created at the definition of the main function. \subsection{Virtual Cast} Virtual casts are implemented as a function call that does the check and a old C-style cast to do the type conversion. The C-cast is just to make sure the generated code is correct so the rest of the section is about that function. The function is @__cfa__virtual_cast@ and it is implemented in the standard library. It takes a pointer to the target type's virtual table and the object pointer being cast. The function is very simple, getting the object's virtual table pointer and then checking to see if it or any of its ancestors, by using the parent pointers, are the same as the target type virtual table pointer. It does this in a simple loop. For the generated code a forward decaration of the virtual works as follows. There is a forward declaration of @__cfa__virtual_cast@ in every cfa file so it can just be used. The object argument is the expression being cast so that is just placed in the argument list. To build the target type parameter the compiler will create a mapping from concrete type-name -- so for polymorphic types the parameters are filled in -- to virtual table address. Every virtual table declaraction is added to the this table; repeats are ignored unless they have conflicting definitions. This does mean the declaractions have to be in scope, but they should usually be introduced as part of the type definition. \section{Exceptions} % Anything about exception construction. \section{Unwinding} % Adapt the unwind chapter, just describe the sections of libunwind used. % Mention that termination and cancellation use it. Maybe go into why % resumption doesn't as well. Many modern languages work with an interal stack that function push and pop their local data to. Stack unwinding removes large sections of the stack, often across functions. At a very basic level this can be done with @setjmp@ \& @longjmp@ which simply move the top of the stack, discarding everything on the stack above a certain point. However this ignores all the clean-up code that should be run when certain sections of the stack are removed (for \CFA these are from destructors and finally clauses) and also requires that the point to which the stack is being unwound is known ahead of time. libunwind is used to address both of these problems. Libunwind, provided in @unwind.h@ on most platorms, is a C library that provides \CPP style stack unwinding. Its operation is divided into two phases. The search phase -- phase 1 -- is used to scan the stack and decide where the unwinding will stop, this allows for a dynamic target. The clean-up phase -- phase 2 -- does the actual unwinding and also runs any clean-up code as it goes. To use the libunwind each function must have a personality function and an LSDA (Language Specific Data Area). Libunwind actually does very little, it simply moves down the stack from function to function. Most of the actions are implemented by the personality function which libunwind calls on every function. Since this is shared across many functions or even every function in a language it will need a bit more information. This is provided by the LSDA which has the unique information for each function. Theoretically the LSDA can contain anything but conventionally it is a table with entries reperenting areas of the function and what has to be done there during unwinding. These areas are described in terms of where the instruction pointer is. If the current value of the instruction pointer is between two values reperenting the beginning and end of a region then execution is currently being executed. These are used to mark out try blocks and the scopes of objects with destructors to run. GCC will generate an LSDA and attach its personality function with the @-fexceptions@ flag. However this only handles the cleanup attribute. This attribute is used on a variable and specifies a function that should be run when the variable goes out of scope. The function is passed a pointer to the object as well so it can be used to mimic destructors. It however cannot be used to mimic try statements. \subsection{Implementing Personality Functions} Personality functions have a complex interface specified by libunwind. This section will cover some of the important parts of that interface. \begin{lstlisting} typedef _Unwind_Reason_Code (*_Unwind_Personality_Fn)( int version, _Unwind_Action action, _Unwind_Exception_Class exception_class, _Unwind_Exception * exception, struct _Unwind_Context * context); \end{lstlisting} The return value, the reason code, is an enumeration of possible messages that can be passed several places in libunwind. It includes a number of messages for special cases (some of which should never be used by the personality function) and error codes but unless otherwise noted the personality function should always return @_URC_CONTINUE_UNWIND@. The @version@ argument is the verson of the implementation that is calling the personality function. At this point it appears to always be 1 and it will likely stay that way until a new version of the API is updated. The @action@ argument is set of flags that tell the personality function when it is being called and what it must do on this invocation. The flags are as follows: \begin{itemize} \item@_UA_SEARCH_PHASE@: This flag is set whenever the personality function is called during the search phase. The personality function should decide if unwinding will stop in this function or not. If it does then the personality function should return @_URC_HANDLER_FOUND@. \item@_UA_CLEANUP_PHASE@: This flag is set whenever the personality function is called during the cleanup phase. If no other flags are set this means the entire frame will be unwound and all cleanup code should be run. \item@_UA_HANDLER_FRAME@: This flag is set during the cleanup phase on the function frame that found the handler. The personality function must prepare to return to normal code execution and return @_URC_INSTALL_CONTEXT@. \item@_UA_FORCE_UNWIND@: This flag is set if the personality function is called through a forced unwind call. Forced unwind only performs the cleanup phase and uses a different means to decide when to stop. See its section below. \end{itemize} The @exception_class@ argument is a copy of the @exception@'s @exception_class@ field. The @exception@ argument is a pointer to the user provided storage object. It has two public fields, the exception class which is actually just a number that identifies the exception handling mechanism that created it and the other is the clean-up function. The clean-up function is called if the exception needs to The @context@ argument is a pointer to an opaque type. This is passed to the many helper functions that can be called inside the personality function. \subsection{Raise Exception} This could be considered the central function of libunwind. It preforms the two staged unwinding the library is built around and most of the rest of the interface of libunwind is here to support it. It's signature is as follows: \begin{lstlisting} _Unwind_Reason_Code _Unwind_RaiseException(_Unwind_Exception *); \end{lstlisting} When called the function begins the search phase, calling the personality function of the most recent stack frame. It will continue to call personality functions traversing the stack new-to-old until a function finds a handler or the end of the stack is reached. In the latter case raise exception will return with @_URC_END_OF_STACK@. Once a handler has been found raise exception continues onto the the cleanup phase. Once again it will call the personality functins of each stack frame from newest to oldest. This pass will stop at the stack frame that found the handler last time, if that personality function does not install the handler it is an error. If an error is encountered raise exception will return either @_URC_FATAL_PHASE1_ERROR@ or @_URC_FATAL_PHASE2_ERROR@ depending on when the error occured. \subsection{Forced Unwind} This is the second big function in libunwind. It also unwinds a stack but it does not use the search phase. Instead another function, the stop function, is used to decide when to stop. \begin{lstlisting} _Unwind_Reason_Code _Unwind_ForcedUnwind( _Unwind_Exception *, _Unwind_Stop_Fn, void *); \end{lstlisting} The exception is the same as the one passed to raise exception. The extra arguments are the stop function and the stop parameter. The stop function has a similar interface as a personality function, except it is also passed the stop parameter. \begin{lstlisting} typedef _Unwind_Reason_Code (*_Unwind_Stop_Fn)( int version, _Unwind_Action action, _Unwind_Exception_Class exception_class, _Unwind_Exception * exception, struct _Unwind_Context * context, void * stop_parameter); \end{lstlisting} The stop function is called at every stack frame before the personality function is called and then once more once after all frames of the stack have been unwound. Each time it is called the stop function should return @_URC_NO_REASON@ or transfer control directly to other code outside of libunwind. The framework does not provide any assistance here. Its arguments are the same as the paired personality function. The actions @_UA_CLEANUP_PHASE@ and @_UA_FORCE_UNWIND@ are always set when it is called. By the official standard that is all but both GCC and Clang add an extra action on the last call at the end of the stack: @_UA_END_OF_STACK@. \section{Exception Context} % Should I have another independent section? % There are only two things in it, top_resume and current_exception. How it is % stored changes depending on wheither or not the thread-library is linked. The exception context is a piece of global storage used to maintain data across different exception operations and to communicate between different components. Each stack has its own exception context. In a purely sequental program, using only core Cforall, there is only one stack and the context is global. However if the library @libcfathread@ is linked then there can be multiple stacks so they will each need their own. To handle this code always gets the exception context from the function @this_exception_context@. The main exception handling code is in @libcfa@ and that library also defines the function as a weak symbol so it acts as a default. Meanwhile in @libcfathread@ the function is defined as a strong symbol that replaces it when the libraries are linked together. The version of the function defined in @libcfa@ is very simple. It returns a pointer to a global static variable. With only one stack this global instance is associated with the only stack. The version of the function defined in @libcfathread@ has to handle more as there are multiple stacks. The exception context is included as part of the per-stack data stored as part of coroutines. In the cold data section, stored at the base of each stack, is the exception context for that stack. The @this_exception_context@ uses the concurrency library to get the current coroutine and through it the cold data section and the exception context. \section{Termination} % Memory management & extra information, the custom function used to implement % catches. Talk about GCC nested functions. Termination exceptions use libunwind quite heavily because it matches the intended use from \CPP exceptions very closely. The main complication is that since the \CFA compiler works by translating to C code it cannot generate the assembly to form the LSDA for try blocks or destructors. \subsection{Memory Management} The first step of termination is to copy the exception into memory managed by the exception system. Currently the system just uses malloc, without reserved memory or and ``small allocation" optimizations. The exception handling mechanism manages memory for the exception as well as memory for libunwind and the system's own per-exception storage. Exceptions are stored in variable sized block. The first component is a fixed sized data structure that contains the information for libunwind and the exception system. The second component is a blob of memory that is big enough to store the exception. Macros with pointer arthritic and type cast are used to move between the components or go from the embedded @_Unwind_Exception@ to the entire node. All of these nodes are strung together in a linked list. One linked list per stack, with the head stored in the exception context. Within each linked list the most recently thrown exception is at the head and the older exceptions are further down the list. This list format allows exceptions to be thrown while a different exception is being handled. Only the exception at the head of the list is currently being handled, the other will wait for the exceptions before them to be removed. The virtual members in the exception's virtual table. The size of the exception, the copy function and the free function are all in the virtual table so they are decided per-exception type. The size and copy function are used right away when the exception is copied in to managed memory. After the exception is handled the free function is used to clean up the exception and then the entire node is passed to free. \subsection{Try Statements \& Catch Clauses} The try statements with termination handlers have a pretty complex conversion to compensate for the lack of assembly generation. Libunwind requires an LSDA (Language Specific Data Area) and personality function for a function to unwind across it. The LSDA in particular is hard to generate at the level of C which is what the \CFA compiler outputs so a work-around is used. This work around is a function called @__cfaehm_try_terminate@ in the standard library. The contents of a try block and the termination handlers are converted into functions. These are then passed to the try terminate function and it calls them. This puts the try statements in their own functions so that no function has to deal with both termination handlers and destructors. This function has some custom embedded assembly that defines its personality function and LSDA. This is hand coded in C which is why there is only one version of it, the compiler has no capability to generate it. The personality function is structured so that it may be expanded, but really it only handles this one function. Notably it does not handle any destructors so the function is constructed so that it does need to run it. The three functions passed to try terminate are: \begin{itemize} \item The try function: This function is the try block, all the code inside the try block is placed inside the try function. It takes no parameters and has no return value. This function is called during regular execution to run the try block. \item The match function: This function decides if this try statement should handle any given termination exception. It takes a pointer to the exception and returns 0 if the exception is not handled here. Otherwise the return value is the id of the handler that should handle the exception. It is called during the search phase. It is constructed from the conditional part of each handler. It runs each check in turn, first checking to see if the object \item The catch function: This function handles the exception. It takes a pointer to the exception and the handler's id and returns nothing. It is called after the clean-up phase. It is constructed by stitching together the bodies of each handler \end{itemize} All three are created with GCC nested functions. GCC nested functions can be used to create closures, functions that can refer to the state of other functions on the stack. This allows the functions to refer to the main function and all the variables in scope. These nested functions and all other functions besides @__cfaehm_try_terminate@ in \CFA use the GCC personality function and the @-fexceptions@ flag to generate the LSDA. This allows destructors to be implemented with the cleanup attribute. \section{Resumption} % The stack-local data, the linked list of nodes. Resumption uses a list of nodes for its stack traversal. The head of the list is stored in the exception context. The nodes in the list just have a pointer to the next node and a pointer to the handler function. The on a resumption throw the this list is traversed. At each node the handler function is called and is passed the exception by pointer. It returns true if the exception was handled and false otherwise. The handler function does both the matching and catching. It tries each the condition of @catchResume@ in order, top-to-bottom and until it finds a handler that matches. If no handler matches then the function returns false. Otherwise the matching handler is run, if it completes successfully the function returns true. Rethrows, through the @throwResume;@ statement, cause the function to return true. % Recursive Resumption Stuff: Blocking out part of the stack is accomplished by updating the front of the list as the search continues. Before the handler at a node is called the head of the list is updated to the next node of the current node. After the search is complete, successful or not, the head of the list is reset. This means the current handler and every handler that has already been checked are not on the list while a handler is run. If a resumption is thrown during the handling of another resumption the active handlers and all the other handler checked up to this point will not be checked again. This structure also supports new handler added while the resumption is being handled. These are added to the front of the list, pointing back along the stack -- the first one will point over all the checked handlers -- and the ordering is maintained. \subsection{Libunwind Compatibility} Resumption does not use libunwind for two simple reasons. The first is that it does not have to unwind anything so would never need to use the clean-up phase. Still the search phase could be used to make it free to enter or exit a try statement with resumption handlers in the same way termination handlers are for the same trade off in the cost of the throw. This is where the second reason comes in, there is no way to return from a search without installing a handler or raising an error. Although work arounds could be created none seemed to be worth it for the prototype. This implementation has no difference in behaviour and is much simpler. % Seriously, just compare the size of the two chapters and then consider % that unwind is required knowledge for that chapter. \section{Finally} % Uses destructors and GCC nested functions. Finally clauses are a simple decomposition to some of the existing features. The code in the block is placed into a GCC nested function with a unique name, no arguments or return values. This nested function is then set as the clean-up function of an empty object that is declared at the beginning of a block placed around the contexts of the try statement. The rest is handled by GCC. The try block and all handlers are inside the block. When they are complete control exits the block and the empty object is cleaned up, which runs the function that contains the finally code. \section{Cancellation} % Stack selections, the three internal unwind functions. Cancellation also uses libunwind to do its stack traversal and unwinding, however it uses a different primary function @_Unwind_ForcedUnwind@. Details of its interface can be found in the unwind section. The first step of cancellation is to find the stack was cancelled and which type of stack it is. Luckily the threads library stores the main thread pointer and the current thread pointer and every thread stores a pointer to its main coroutine and the coroutine it is currently executing. So if the the current thread's main and current coroutine do not match, it is a coroutine cancellation. Otherwise if the main and current thread do not match, it is a thread cancellation. Otherwise it is a main thread cancellation. However if the threading library is not linked then execution must be on the main stack as that is the only one that exists. So the entire check is skipped using the linker and weak symbols. Instead the main thread cancellation is unconditionally preformed. Regardless of how they are choosen afterwords the stop function and the stop parameter are passed to the forced unwind functon. The general pattern of all three stop functions is the same, they continue unwinding until the end of stack when they do there primary work. Main stack cancellation it is very simple. The ``transfer" is just an abort, the program stops executing. The coroutine cancellation stores the exception on the coroutine and then does a coroutine context switch. The rest is handled inside resume. Every time control returns from a resumed thread there is a check to see if it is cancelled. If it is the exception is retrieved and the CoroutineCancelled exception is constructed and loaded. It is then thrown as a regular exception with the default handler coming from the context of the resumption call. The thread cancellation stores the exception on the thread's main stack and then returns to the scheduler. The rest is handled by the joiner. The wait for the joined thread to finish works the same but after that it checks to see if there was a cancellation. If there was the exception is retrieved and the ThreadCancelled exception is constructed. The default handler is passed in as a function pointer. If it is null (as it is for the auto-generated joins on destructor call) it a default is used that simply calls abort; which gives the required handling on implicate join.