Index: docs/SupportLibrary.rst =================================================================== --- docs/SupportLibrary.rst +++ docs/SupportLibrary.rst @@ -0,0 +1,246 @@ +=============== +Support Library +=============== + +Abstract +======== + +This document provides some details on LLVM's Support Library, located in the +source at ``lib/Support`` and ``include/llvm/Support``. The library's purpose +is to shield LLVM from the differences between operating systems for the few +services LLVM needs from the operating system. Much of LLVM is written using +portability features of standard C++. However, in a few areas, system dependent +facilities are needed and the Support Library is the wrapper around those +system calls. + +By centralizing LLVM's use of operating system interfaces, we make it possible +for the LLVM tool chain and runtime libraries to be more easily ported to new +platforms since (theoretically) only ``lib/Support`` needs to be ported. This +library also unclutters the rest of LLVM from #ifdef use and special cases for +specific operating systems. Such uses are replaced with simple calls to the +interfaces provided in ``include/llvm/Support``. + +Note that the Support Library is not intended to be a complete operating system +wrapper (such as the Adaptive Communications Environment (ACE) or Apache +Portable Runtime (APR)), but only provides the functionality necessary to +support LLVM. + +The Support Library was originally referred to as the System Library, written +by Reid Spencer who formulated the design based on similar work originating +from the eXtensible Programming System (XPS). Several people helped with the +effort; especially, Jeff Cohen and Henrik Bach on the Win32 port. + +Keeping LLVM Portable +===================== + +In order to keep LLVM portable, LLVM developers should adhere to a set of +portability rules associated with the Support Library. Adherence to these rules +should help the Support Library achieve its goal of shielding LLVM from the +variations in operating system interfaces and doing so efficiently. The +following sections define the rules needed to fulfill this objective. + +Don't Include System Headers +---------------------------- + +Except in ``lib/Support``, no LLVM source code should directly ``#include`` a +system header. Care has been taken to remove all such ``#includes`` from LLVM +while ``lib/Support`` was being developed. Specifically this means that header +files like "``unistd.h``", "``windows.h``", "``stdio.h``", and "``string.h``" +are forbidden to be included by LLVM source code outside the implementation of +``lib/Support``. + +To obtain system-dependent functionality, existing interfaces to the system +found in ``include/llvm/Support`` should be used. If an appropriate interface is +not available, it should be added to ``include/llvm/Support`` and implemented in +``lib/Support`` for all supported platforms. + +Don't Expose System Headers +--------------------------- + +The Support Library must shield LLVM from **all** system headers. To obtain +system level functionality, LLVM source must ``#include "llvm/System/Thing.h"`` +and nothing else. This means that ``Thing.h`` cannot expose any system header +files. This protects LLVM from accidentally using system specific functionality +and only allows it via the ``lib/Support`` interface. + +Use Standard C Headers +---------------------- + +The **standard** C headers (the ones beginning with "c") are allowed to be +exposed through the ``lib/Support`` interface. These headers and the things they +declare are considered to be platform agnostic. LLVM source files may include +them directly or obtain their inclusion through ``lib/Support`` interfaces. + +Use Standard C++ Headers +------------------------ + +The **standard** C++ headers from the standard C++ library and standard +template library may be exposed through the ``lib/Support`` interface. These +headers and the things they declare are considered to be platform agnostic. +LLVM source files may include them or obtain their inclusion through +``lib/Support`` interfaces. + +High Level Interface +-------------------- + +The entry points specified in the interface of ``lib/Support`` must be aimed at +completing some reasonably high level task needed by LLVM. We do not want to +simply wrap each operating system call. It would be preferable to wrap several +operating system calls that are always used in conjunction with one another by +LLVM. + +For example, consider what is needed to execute a program, wait for it to +complete, and return its result code. On Unix, this involves the following +operating system calls: ``getenv``, ``fork``, ``execve``, and ``wait``. The +correct thing for ``lib/Support`` to provide is a function, say +``ExecuteProgramAndWait``, that implements the functionality completely. what +we don't want is wrappers for the operating system calls involved. + +There must **not** be a one-to-one relationship between operating system +calls and the Support library's interface. Any such interface function will be +suspicious. + +No Unused Functionality +----------------------- + +There must be no functionality specified in the interface of ``lib/Support`` +that isn't actually used by LLVM. We're not writing a general purpose operating +system wrapper here, just enough to satisfy LLVM's needs. And, LLVM doesn't +need much. This design goal aims to keep the ``lib/Support`` interface small and +understandable which should foster its actual use and adoption. + +No Duplicate Implementations +---------------------------- + +The implementation of a function for a given platform must be written exactly +once. This implies that it must be possible to apply a function's +implementation to multiple operating systems if those operating systems can +share the same implementation. This rule applies to the set of operating +systems supported for a given class of operating system (e.g. Unix, Win32). + +No Virtual Methods +------------------ + +The Support Library interfaces can be called quite frequently by LLVM. In order +to make those calls as efficient as possible, we discourage the use of virtual +methods. There is no need to use inheritance for implementation differences, it +just adds complexity. The ``#include`` mechanism works just fine. + +No Exposed Functions +-------------------- + +Any functions defined by system libraries (i.e. not defined by ``lib/Support``) +must not be exposed through the ``lib/Support`` interface, even if the header +file for that function is not exposed. This prevents inadvertent use of system +specific functionality. + +For example, the ``stat`` system call is notorious for having variations in the +data it provides. ``lib/Support`` must not declare ``stat`` nor allow it to be +declared. Instead it should provide its own interface to discovering +information about files and directories. Those interfaces may be implemented in +terms of ``stat`` but that is strictly an implementation detail. The interface +provided by the Support Library must be implemented on all platforms (even +those without ``stat``). + +No Exposed Data +--------------- + +Any data defined by system libraries (i.e. not defined by ``lib/Support``) must +not be exposed through the ``lib/Support`` interface, even if the header file +for that function is not exposed. As with functions, this prevents inadvertent +use of data that might not exist on all platforms. + +Minimize Soft Errors +-------------------- + +Operating system interfaces will generally provide error results for every +little thing that could go wrong. In almost all cases, you can divide these +error results into two groups: normal/good/soft and abnormal/bad/hard. That is, +some of the errors are simply information like "file not found", "insufficient +privileges", etc. while other errors are much harder like "out of space", "bad +disk sector", or "system call interrupted". We'll call the first group "*soft*" +errors and the second group "*hard*" errors. + +``lib/Support`` must always attempt to minimize soft errors. This is a design +requirement because the minimization of soft errors can affect the granularity +and the nature of the interface. In general, if you find that you're wanting to +throw soft errors, you must review the granularity of the interface because it +is likely you're trying to implement something that is too low level. The rule +of thumb is to provide interface functions that **can't** fail, except when +faced with hard errors. + +For a trivial example, suppose we wanted to add an "``OpenFileForWriting``" +function. For many operating systems, if the file doesn't exist, attempting to +open the file will produce an error. However, ``lib/Support`` should not simply +throw that error if it occurs because its a soft error. The problem is that the +interface function, ``OpenFileForWriting`` is too low level. It should be +``OpenOrCreateFileForWriting``. In the case of the soft "doesn't exist" error, +this function would just create it and then open it for writing. + +This design principle needs to be maintained in ``lib/Support`` because it +avoids the propagation of soft error handling throughout the rest of LLVM. +Hard errors will generally just cause a termination for an LLVM tool so don't +be bashful about throwing them. + +Rules of thumb: + +#. Don't throw soft errors, only hard errors. + +#. If you're tempted to throw a soft error, re-think the interface. + +#. Handle internally the most common normal/good/soft error conditions + so the rest of LLVM doesn't have to. + +No throw Specifications +----------------------- + +None of the ``lib/Support`` interface functions may be declared with C++ +``throw()`` specifications on them. This requirement makes sure that the +compiler does not insert additional exception handling code into the interface +functions. This is a performance consideration: ``lib/Support`` functions are +at the bottom of many call chains and as such can be frequently called. We +need them to be as efficient as possible. However, no routines in the system +library should actually throw exceptions. + +Code Organization +----------------- + +Implementations of the Support Library interface are separated by their general +class of operating system. Currently only Unix and Win32 classes are defined +but more could be added for other operating system classifications. To +distinguish which implementation to compile, the code in ``lib/Support`` uses +the ``LLVM_ON_UNIX`` and ``_WIN32`` ``#defines``. Each source file in +``lib/Support``, after implementing the generic (operating system independent) +functionality needs to include the correct implementation using a set of +``#if defined(LLVM_ON_XYZ)`` directives. For example, if we had +``lib/Support/File.cpp``, we'd expect to see in that file: + +.. code-block:: c++ + + #if defined(LLVM_ON_UNIX) + #include "Unix/File.inc" + #endif + #if defined(_WIN32) + #include "Windows/File.inc" + #endif + +The implementation in ``lib/Support/Unix/File.cpp`` should handle all Unix +variants. The implementation in ``lib/Support/Windows/File.cpp`` should handle +all Windows variants. What this does is quickly differentiate the basic class +of operating system that will provide the implementation. The specific details +for a given platform must still be determined through the use of ``#ifdef``. + +Consistent Semantics +-------------------- + +The implementation of a ``lib/Support`` interface can vary drastically between +platforms. That's okay as long as the end result of the interface function is +the same. For example, a function to create a directory is pretty straight +forward on all operating system. System V IPC on the other hand isn't even +supported on all platforms. Instead of "supporting" System V IPC, +``lib/Support`` should provide an interface to the basic concept of +inter-process communications. The implementations might use System V IPC if +that was available or named pipes, or whatever gets the job done effectively +for a given operating system. In all cases, the interface and the +implementation must be semantically consistent. +