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LLVM 3.4
author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Thu, 12 Dec 2013 13:56:28 +0900
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1 ==============
2 System Library
3 ==============
4
5 Abstract
6 ========
7
8 This document provides some details on LLVM's System Library, located in the
9 source at ``lib/System`` and ``include/llvm/System``. The library's purpose is
10 to shield LLVM from the differences between operating systems for the few
11 services LLVM needs from the operating system. Much of LLVM is written using
12 portability features of standard C++. However, in a few areas, system dependent
13 facilities are needed and the System Library is the wrapper around those system
14 calls.
15
16 By centralizing LLVM's use of operating system interfaces, we make it possible
17 for the LLVM tool chain and runtime libraries to be more easily ported to new
18 platforms since (theoretically) only ``lib/System`` needs to be ported. This
19 library also unclutters the rest of LLVM from #ifdef use and special cases for
20 specific operating systems. Such uses are replaced with simple calls to the
21 interfaces provided in ``include/llvm/System``.
22
23 Note that the System Library is not intended to be a complete operating system
24 wrapper (such as the Adaptive Communications Environment (ACE) or Apache
25 Portable Runtime (APR)), but only provides the functionality necessary to
26 support LLVM.
27
28 The System Library was written by Reid Spencer who formulated the design based
29 on similar work originating from the eXtensible Programming System (XPS).
30 Several people helped with the effort; especially, Jeff Cohen and Henrik Bach
31 on the Win32 port.
32
33 Keeping LLVM Portable
34 =====================
35
36 In order to keep LLVM portable, LLVM developers should adhere to a set of
37 portability rules associated with the System Library. Adherence to these rules
38 should help the System Library achieve its goal of shielding LLVM from the
39 variations in operating system interfaces and doing so efficiently. The
40 following sections define the rules needed to fulfill this objective.
41
42 Don't Include System Headers
43 ----------------------------
44
45 Except in ``lib/System``, no LLVM source code should directly ``#include`` a
46 system header. Care has been taken to remove all such ``#includes`` from LLVM
47 while ``lib/System`` was being developed. Specifically this means that header
48 files like "``unistd.h``", "``windows.h``", "``stdio.h``", and "``string.h``"
49 are forbidden to be included by LLVM source code outside the implementation of
50 ``lib/System``.
51
52 To obtain system-dependent functionality, existing interfaces to the system
53 found in ``include/llvm/System`` should be used. If an appropriate interface is
54 not available, it should be added to ``include/llvm/System`` and implemented in
55 ``lib/System`` for all supported platforms.
56
57 Don't Expose System Headers
58 ---------------------------
59
60 The System Library must shield LLVM from **all** system headers. To obtain
61 system level functionality, LLVM source must ``#include "llvm/System/Thing.h"``
62 and nothing else. This means that ``Thing.h`` cannot expose any system header
63 files. This protects LLVM from accidentally using system specific functionality
64 and only allows it via the ``lib/System`` interface.
65
66 Use Standard C Headers
67 ----------------------
68
69 The **standard** C headers (the ones beginning with "c") are allowed to be
70 exposed through the ``lib/System`` interface. These headers and the things they
71 declare are considered to be platform agnostic. LLVM source files may include
72 them directly or obtain their inclusion through ``lib/System`` interfaces.
73
74 Use Standard C++ Headers
75 ------------------------
76
77 The **standard** C++ headers from the standard C++ library and standard
78 template library may be exposed through the ``lib/System`` interface. These
79 headers and the things they declare are considered to be platform agnostic.
80 LLVM source files may include them or obtain their inclusion through
81 ``lib/System`` interfaces.
82
83 High Level Interface
84 --------------------
85
86 The entry points specified in the interface of ``lib/System`` must be aimed at
87 completing some reasonably high level task needed by LLVM. We do not want to
88 simply wrap each operating system call. It would be preferable to wrap several
89 operating system calls that are always used in conjunction with one another by
90 LLVM.
91
92 For example, consider what is needed to execute a program, wait for it to
93 complete, and return its result code. On Unix, this involves the following
94 operating system calls: ``getenv``, ``fork``, ``execve``, and ``wait``. The
95 correct thing for ``lib/System`` to provide is a function, say
96 ``ExecuteProgramAndWait``, that implements the functionality completely. what
97 we don't want is wrappers for the operating system calls involved.
98
99 There must **not** be a one-to-one relationship between operating system
100 calls and the System library's interface. Any such interface function will be
101 suspicious.
102
103 No Unused Functionality
104 -----------------------
105
106 There must be no functionality specified in the interface of ``lib/System``
107 that isn't actually used by LLVM. We're not writing a general purpose operating
108 system wrapper here, just enough to satisfy LLVM's needs. And, LLVM doesn't
109 need much. This design goal aims to keep the ``lib/System`` interface small and
110 understandable which should foster its actual use and adoption.
111
112 No Duplicate Implementations
113 ----------------------------
114
115 The implementation of a function for a given platform must be written exactly
116 once. This implies that it must be possible to apply a function's
117 implementation to multiple operating systems if those operating systems can
118 share the same implementation. This rule applies to the set of operating
119 systems supported for a given class of operating system (e.g. Unix, Win32).
120
121 No Virtual Methods
122 ------------------
123
124 The System Library interfaces can be called quite frequently by LLVM. In order
125 to make those calls as efficient as possible, we discourage the use of virtual
126 methods. There is no need to use inheritance for implementation differences, it
127 just adds complexity. The ``#include`` mechanism works just fine.
128
129 No Exposed Functions
130 --------------------
131
132 Any functions defined by system libraries (i.e. not defined by ``lib/System``)
133 must not be exposed through the ``lib/System`` interface, even if the header
134 file for that function is not exposed. This prevents inadvertent use of system
135 specific functionality.
136
137 For example, the ``stat`` system call is notorious for having variations in the
138 data it provides. ``lib/System`` must not declare ``stat`` nor allow it to be
139 declared. Instead it should provide its own interface to discovering
140 information about files and directories. Those interfaces may be implemented in
141 terms of ``stat`` but that is strictly an implementation detail. The interface
142 provided by the System Library must be implemented on all platforms (even those
143 without ``stat``).
144
145 No Exposed Data
146 ---------------
147
148 Any data defined by system libraries (i.e. not defined by ``lib/System``) must
149 not be exposed through the ``lib/System`` interface, even if the header file
150 for that function is not exposed. As with functions, this prevents inadvertent
151 use of data that might not exist on all platforms.
152
153 Minimize Soft Errors
154 --------------------
155
156 Operating system interfaces will generally provide error results for every
157 little thing that could go wrong. In almost all cases, you can divide these
158 error results into two groups: normal/good/soft and abnormal/bad/hard. That is,
159 some of the errors are simply information like "file not found", "insufficient
160 privileges", etc. while other errors are much harder like "out of space", "bad
161 disk sector", or "system call interrupted". We'll call the first group "*soft*"
162 errors and the second group "*hard*" errors.
163
164 ``lib/System`` must always attempt to minimize soft errors. This is a design
165 requirement because the minimization of soft errors can affect the granularity
166 and the nature of the interface. In general, if you find that you're wanting to
167 throw soft errors, you must review the granularity of the interface because it
168 is likely you're trying to implement something that is too low level. The rule
169 of thumb is to provide interface functions that **can't** fail, except when
170 faced with hard errors.
171
172 For a trivial example, suppose we wanted to add an "``OpenFileForWriting``"
173 function. For many operating systems, if the file doesn't exist, attempting to
174 open the file will produce an error. However, ``lib/System`` should not simply
175 throw that error if it occurs because its a soft error. The problem is that the
176 interface function, ``OpenFileForWriting`` is too low level. It should be
177 ``OpenOrCreateFileForWriting``. In the case of the soft "doesn't exist" error,
178 this function would just create it and then open it for writing.
179
180 This design principle needs to be maintained in ``lib/System`` because it
181 avoids the propagation of soft error handling throughout the rest of LLVM.
182 Hard errors will generally just cause a termination for an LLVM tool so don't
183 be bashful about throwing them.
184
185 Rules of thumb:
186
187 #. Don't throw soft errors, only hard errors.
188
189 #. If you're tempted to throw a soft error, re-think the interface.
190
191 #. Handle internally the most common normal/good/soft error conditions
192 so the rest of LLVM doesn't have to.
193
194 No throw Specifications
195 -----------------------
196
197 None of the ``lib/System`` interface functions may be declared with C++
198 ``throw()`` specifications on them. This requirement makes sure that the
199 compiler does not insert additional exception handling code into the interface
200 functions. This is a performance consideration: ``lib/System`` functions are at
201 the bottom of many call chains and as such can be frequently called. We need
202 them to be as efficient as possible. However, no routines in the system
203 library should actually throw exceptions.
204
205 Code Organization
206 -----------------
207
208 Implementations of the System Library interface are separated by their general
209 class of operating system. Currently only Unix and Win32 classes are defined
210 but more could be added for other operating system classifications. To
211 distinguish which implementation to compile, the code in ``lib/System`` uses
212 the ``LLVM_ON_UNIX`` and ``LLVM_ON_WIN32`` ``#defines`` provided via configure
213 through the ``llvm/Config/config.h`` file. Each source file in ``lib/System``,
214 after implementing the generic (operating system independent) functionality
215 needs to include the correct implementation using a set of
216 ``#if defined(LLVM_ON_XYZ)`` directives. For example, if we had
217 ``lib/System/File.cpp``, we'd expect to see in that file:
218
219 .. code-block:: c++
220
221 #if defined(LLVM_ON_UNIX)
222 #include "Unix/File.cpp"
223 #endif
224 #if defined(LLVM_ON_WIN32)
225 #include "Win32/File.cpp"
226 #endif
227
228 The implementation in ``lib/System/Unix/File.cpp`` should handle all Unix
229 variants. The implementation in ``lib/System/Win32/File.cpp`` should handle all
230 Win32 variants. What this does is quickly differentiate the basic class of
231 operating system that will provide the implementation. The specific details for
232 a given platform must still be determined through the use of ``#ifdef``.
233
234 Consistent Semantics
235 --------------------
236
237 The implementation of a ``lib/System`` interface can vary drastically between
238 platforms. That's okay as long as the end result of the interface function is
239 the same. For example, a function to create a directory is pretty straight
240 forward on all operating system. System V IPC on the other hand isn't even
241 supported on all platforms. Instead of "supporting" System V IPC,
242 ``lib/System`` should provide an interface to the basic concept of
243 inter-process communications. The implementations might use System V IPC if
244 that was available or named pipes, or whatever gets the job done effectively
245 for a given operating system. In all cases, the interface and the
246 implementation must be semantically consistent.
247