[LLVMdev] The system library is gone for a long time.
罗勇刚(Yonggang Luo)
luoyonggang at gmail.com
Sun May 26 09:09:54 PDT 2013
>From 1d658dd52ca3973109e370103a7dd3485a4ee11f Mon Sep 17 00:00:00 2001
From: Yonggang Luo <luoyonggang at gmail.com>
Date: Mon, 27 May 2013 00:07:16 +0800
Subject: [PATCH] The System library is merged into Support library.
---
docs/SystemLibrary.rst | 104 ++++++++++++++++++++++++-------------------------
docs/index.rst | 4 +-
2 files changed, 54 insertions(+), 54 deletions(-)
diff --git a/docs/SystemLibrary.rst b/docs/SystemLibrary.rst
index 0d0f4fa..4c6226c 100644
--- a/docs/SystemLibrary.rst
+++ b/docs/SystemLibrary.rst
@@ -1,31 +1,31 @@
==============
-System Library
+Support 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
+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 System Library is the wrapper around those system
+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/System`` needs to be ported. This
+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/System``.
+interfaces provided in ``include/llvm/Support``.
-Note that the System Library is not intended to be a complete operating system
+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 System Library was written by Reid Spencer who formulated the design based
+The Support 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.
@@ -34,56 +34,56 @@ 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
+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
+Don't Include Support Headers
----------------------------
-Except in ``lib/System``, no LLVM source code should directly ``#include`` a
+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/System`` was being developed. Specifically this means that header
+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/System``.
+``lib/Support``.
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.
+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
+Don't Expose Support Headers
---------------------------
-The System Library must shield LLVM from **all** system headers. To obtain
-system level functionality, LLVM source must ``#include "llvm/System/Thing.h"``
+The Support Library must shield LLVM from **all** system headers. To obtain
+system level functionality, LLVM source must ``#include
"llvm/Support/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.
+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/System`` interface. These headers and the things they
+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/System`` interfaces.
+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/System`` interface. These
+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/System`` interfaces.
+``lib/Support`` interfaces.
High Level Interface
--------------------
-The entry points specified in the interface of ``lib/System`` must be aimed at
+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
@@ -92,21 +92,21 @@ 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
+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 System library's interface. Any such interface function will be
+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/System``
+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/System`` interface small and
+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
@@ -121,7 +121,7 @@ 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
+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.
@@ -129,24 +129,24 @@ 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
+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/System`` must not declare ``stat`` nor allow it to be
+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 System Library must be implemented on all platforms (even those
+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/System``) must
-not be exposed through the ``lib/System`` interface, even if the header file
+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.
@@ -161,7 +161,7 @@ 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
+``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
@@ -171,13 +171,13 @@ 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
+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/System`` because it
+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.
@@ -194,10 +194,10 @@ Rules of thumb:
No throw Specifications
-----------------------
-None of the ``lib/System`` interface functions may be declared with C++
+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/System`` functions are at
+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.
@@ -205,28 +205,28 @@ library should actually throw exceptions.
Code Organization
-----------------
-Implementations of the System Library interface are separated by their general
+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/System`` uses
+distinguish which implementation to compile, the code in ``lib/Support`` uses
the ``LLVM_ON_UNIX`` and ``LLVM_ON_WIN32`` ``#defines`` provided via configure
-through the ``llvm/Config/config.h`` file. Each source file in ``lib/System``,
+through the ``llvm/Config/config.h`` file. 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/System/File.cpp``, we'd expect to see in that file:
+``lib/Support/Path.cpp``, we'd expect to see in that file:
.. code-block:: c++
#if defined(LLVM_ON_UNIX)
- #include "Unix/File.cpp"
+ #include "Unix/Path.inc"
#endif
#if defined(LLVM_ON_WIN32)
- #include "Win32/File.cpp"
+ #include "Windows/Path.inc"
#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
+The implementation in ``lib/Support/Unix/Path.inc`` should handle all Unix
+variants. The implementation in ``lib/Support/Windows/Path.inc``
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``.
@@ -234,12 +234,12 @@ 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
+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/System`` should provide an interface to the basic concept of
+``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
diff --git a/docs/index.rst b/docs/index.rst
index 6b182da..28107d7 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -271,8 +271,8 @@ For API clients and LLVM developers.
:doc:`BitCodeFormat`
This describes the file format and encoding used for LLVM "bc" files.
-:doc:`System Library <SystemLibrary>`
- This document describes the LLVM System Library (``lib/System``) and
+:doc:`Support Library <SystemLibrary>`
+ This document describes the LLVM Support Library (``lib/Support``) and
how to keep LLVM source code portable
:doc:`LinkTimeOptimization`
--
1.8.1.msysgit.1
2013/5/26 Rafael Espíndola <rafael.espindola at gmail.com>:
> On 25 May 2013 15:30, Sean Silva <silvas at purdue.edu> wrote:
>> This will break existing URLs. Until we have a way to set up redirects the
>> file name should stay the same.
>
> Would a SystemLibrary.rst saying it was replaced with the support library be ok?
>
>> -- Sean Silva
>
> Cheers,
> Rafael
--
此致
礼
罗勇刚
Yours
sincerely,
Yonggang Luo
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