[Docs] [Support] System Library to Support Library transition along with minor corrections to reflect it.

System Library has been a long deprecated term along with the path lib/System, having been superseded/renamed
to the Support Library a long time ago. These patches reflect those changes in documentation as well as
update some outdated examples and provide context to the origin of the Support Library.

Differential Revision: https://reviews.llvm.org/D52107

llvm-svn: 342500

1 files changed, 4 insertions, 241 deletions

diff --git a/llvm/docs/SystemLibrary.rst b/llvm/docs/SystemLibrary.rst
index dba446b476d..4409532153a 100644
--- a/llvm/docs/SystemLibrary.rst
+++ b/llvm/docs/SystemLibrary.rst
@@ -2,245 +2,8 @@
 System Library
 ==============
 
-Abstract
-========
-
-This document provides some details on LLVM's System Library, located in the
-source at ``lib/System`` and ``include/llvm/System``. 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 System 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/System`` 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/System``.
-
-Note that the System 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 System Library was 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 System Library. Adherence to these rules
-should help the System 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/System``, no LLVM source code should directly ``#include`` a
-system header. Care has been taken to remove all such ``#includes`` from LLVM
-while ``lib/System`` 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/System``.
-
-To obtain system-dependent functionality, existing interfaces to the system
-found in ``include/llvm/System`` should be used. If an appropriate interface is
-not available, it should be added to ``include/llvm/System`` and implemented in
-``lib/System`` for all supported platforms.
-
-Don't Expose System Headers
----------------------------
-
-The System 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/System`` interface.
-
-Use Standard C Headers
-----------------------
-
-The **standard** C headers (the ones beginning with "c") are allowed to be
-exposed through the ``lib/System`` 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/System`` interfaces.
-
-Use Standard C++ Headers
-------------------------
-
-The **standard** C++ headers from the standard C++ library and standard
-template library may be exposed through the ``lib/System`` 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/System`` interfaces.
-
-High Level Interface
---------------------
-
-The entry points specified in the interface of ``lib/System`` 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/System`` 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 System library's interface. Any such interface function will be
-suspicious.
-
-No Unused Functionality
------------------------
-
-There must be no functionality specified in the interface of ``lib/System``
-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/System`` 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 System 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/System``)
-must not be exposed through the ``lib/System`` 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/System`` 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 System 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/System``) must
-not be exposed through the ``lib/System`` 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/System`` 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/System`` 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/System`` 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/System`` 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/System`` 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 System 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/System`` uses
-the ``LLVM_ON_UNIX`` and ``_WIN32`` ``#defines``.  Each source file in
-``lib/System``, 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/System/File.cpp``, we'd expect to see in that file:
-
-.. code-block:: c++
-
-  #if defined(LLVM_ON_UNIX)
-  #include "Unix/File.cpp"
-  #endif
-  #if defined(_WIN32)
-  #include "Win32/File.cpp"
-  #endif
-
-The implementation in ``lib/System/Unix/File.cpp`` should handle all Unix
-variants. The implementation in ``lib/System/Win32/File.cpp`` should handle all
-Win32 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/System`` 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/System`` 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.
+Moved
+=====
 
+The System Library has been renamed to Support Library with documentation
+available at :doc:`SupportLibrary`. Please, change your links to that page.