mirror of
https://github.com/RPCS3/llvm.git
synced 2024-12-21 03:28:31 +00:00
2ba4bd97d1
against the developer policy to include this sort of thing as SVN blame already captures this in a far more fine-grained way. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172109 91177308-0d34-0410-b5e6-96231b3b80d8
248 lines
12 KiB
ReStructuredText
248 lines
12 KiB
ReStructuredText
==============
|
|
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 ``LLVM_ON_WIN32`` ``#defines`` provided via configure
|
|
through the ``llvm/Config/config.h`` file. 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(LLVM_ON_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.
|
|
|