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author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Sun, 31 Jan 2016 17:34:49 +0900
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1 ==============
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2 System Library
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3 ==============
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4
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5 Abstract
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6 ========
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7
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8 This document provides some details on LLVM's System Library, located in the
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9 source at ``lib/System`` and ``include/llvm/System``. The library's purpose is
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10 to shield LLVM from the differences between operating systems for the few
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11 services LLVM needs from the operating system. Much of LLVM is written using
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12 portability features of standard C++. However, in a few areas, system dependent
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13 facilities are needed and the System Library is the wrapper around those system
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14 calls.
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15
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16 By centralizing LLVM's use of operating system interfaces, we make it possible
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17 for the LLVM tool chain and runtime libraries to be more easily ported to new
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18 platforms since (theoretically) only ``lib/System`` needs to be ported. This
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19 library also unclutters the rest of LLVM from #ifdef use and special cases for
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20 specific operating systems. Such uses are replaced with simple calls to the
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21 interfaces provided in ``include/llvm/System``.
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22
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23 Note that the System Library is not intended to be a complete operating system
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24 wrapper (such as the Adaptive Communications Environment (ACE) or Apache
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25 Portable Runtime (APR)), but only provides the functionality necessary to
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26 support LLVM.
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27
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28 The System Library was written by Reid Spencer who formulated the design based
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29 on similar work originating from the eXtensible Programming System (XPS).
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30 Several people helped with the effort; especially, Jeff Cohen and Henrik Bach
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31 on the Win32 port.
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32
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33 Keeping LLVM Portable
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34 =====================
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35
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36 In order to keep LLVM portable, LLVM developers should adhere to a set of
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37 portability rules associated with the System Library. Adherence to these rules
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38 should help the System Library achieve its goal of shielding LLVM from the
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39 variations in operating system interfaces and doing so efficiently. The
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40 following sections define the rules needed to fulfill this objective.
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41
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42 Don't Include System Headers
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43 ----------------------------
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44
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45 Except in ``lib/System``, no LLVM source code should directly ``#include`` a
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46 system header. Care has been taken to remove all such ``#includes`` from LLVM
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47 while ``lib/System`` was being developed. Specifically this means that header
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48 files like "``unistd.h``", "``windows.h``", "``stdio.h``", and "``string.h``"
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49 are forbidden to be included by LLVM source code outside the implementation of
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50 ``lib/System``.
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51
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52 To obtain system-dependent functionality, existing interfaces to the system
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53 found in ``include/llvm/System`` should be used. If an appropriate interface is
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54 not available, it should be added to ``include/llvm/System`` and implemented in
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55 ``lib/System`` for all supported platforms.
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56
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57 Don't Expose System Headers
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58 ---------------------------
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59
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60 The System Library must shield LLVM from **all** system headers. To obtain
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61 system level functionality, LLVM source must ``#include "llvm/System/Thing.h"``
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62 and nothing else. This means that ``Thing.h`` cannot expose any system header
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63 files. This protects LLVM from accidentally using system specific functionality
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64 and only allows it via the ``lib/System`` interface.
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65
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66 Use Standard C Headers
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67 ----------------------
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68
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69 The **standard** C headers (the ones beginning with "c") are allowed to be
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70 exposed through the ``lib/System`` interface. These headers and the things they
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71 declare are considered to be platform agnostic. LLVM source files may include
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72 them directly or obtain their inclusion through ``lib/System`` interfaces.
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73
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74 Use Standard C++ Headers
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75 ------------------------
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76
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77 The **standard** C++ headers from the standard C++ library and standard
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78 template library may be exposed through the ``lib/System`` interface. These
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79 headers and the things they declare are considered to be platform agnostic.
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80 LLVM source files may include them or obtain their inclusion through
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81 ``lib/System`` interfaces.
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82
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83 High Level Interface
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84 --------------------
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85
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86 The entry points specified in the interface of ``lib/System`` must be aimed at
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87 completing some reasonably high level task needed by LLVM. We do not want to
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88 simply wrap each operating system call. It would be preferable to wrap several
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89 operating system calls that are always used in conjunction with one another by
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90 LLVM.
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91
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92 For example, consider what is needed to execute a program, wait for it to
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93 complete, and return its result code. On Unix, this involves the following
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94 operating system calls: ``getenv``, ``fork``, ``execve``, and ``wait``. The
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95 correct thing for ``lib/System`` to provide is a function, say
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96 ``ExecuteProgramAndWait``, that implements the functionality completely. what
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97 we don't want is wrappers for the operating system calls involved.
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98
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99 There must **not** be a one-to-one relationship between operating system
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100 calls and the System library's interface. Any such interface function will be
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101 suspicious.
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102
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103 No Unused Functionality
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104 -----------------------
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105
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106 There must be no functionality specified in the interface of ``lib/System``
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107 that isn't actually used by LLVM. We're not writing a general purpose operating
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108 system wrapper here, just enough to satisfy LLVM's needs. And, LLVM doesn't
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109 need much. This design goal aims to keep the ``lib/System`` interface small and
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110 understandable which should foster its actual use and adoption.
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111
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112 No Duplicate Implementations
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113 ----------------------------
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114
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115 The implementation of a function for a given platform must be written exactly
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116 once. This implies that it must be possible to apply a function's
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117 implementation to multiple operating systems if those operating systems can
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118 share the same implementation. This rule applies to the set of operating
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119 systems supported for a given class of operating system (e.g. Unix, Win32).
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120
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121 No Virtual Methods
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122 ------------------
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123
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124 The System Library interfaces can be called quite frequently by LLVM. In order
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125 to make those calls as efficient as possible, we discourage the use of virtual
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126 methods. There is no need to use inheritance for implementation differences, it
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127 just adds complexity. The ``#include`` mechanism works just fine.
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128
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129 No Exposed Functions
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
130 --------------------
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
131
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
132 Any functions defined by system libraries (i.e. not defined by ``lib/System``)
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
133 must not be exposed through the ``lib/System`` interface, even if the header
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
134 file for that function is not exposed. This prevents inadvertent use of system
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
135 specific functionality.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
136
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
137 For example, the ``stat`` system call is notorious for having variations in the
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
138 data it provides. ``lib/System`` must not declare ``stat`` nor allow it to be
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
139 declared. Instead it should provide its own interface to discovering
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
140 information about files and directories. Those interfaces may be implemented in
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
141 terms of ``stat`` but that is strictly an implementation detail. The interface
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
142 provided by the System Library must be implemented on all platforms (even those
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
143 without ``stat``).
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
144
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
145 No Exposed Data
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
146 ---------------
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
147
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
148 Any data defined by system libraries (i.e. not defined by ``lib/System``) must
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
149 not be exposed through the ``lib/System`` interface, even if the header file
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
150 for that function is not exposed. As with functions, this prevents inadvertent
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
151 use of data that might not exist on all platforms.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
152
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
153 Minimize Soft Errors
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
154 --------------------
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
155
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
156 Operating system interfaces will generally provide error results for every
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
157 little thing that could go wrong. In almost all cases, you can divide these
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
158 error results into two groups: normal/good/soft and abnormal/bad/hard. That is,
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
159 some of the errors are simply information like "file not found", "insufficient
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
160 privileges", etc. while other errors are much harder like "out of space", "bad
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
161 disk sector", or "system call interrupted". We'll call the first group "*soft*"
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
162 errors and the second group "*hard*" errors.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
163
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
164 ``lib/System`` must always attempt to minimize soft errors. This is a design
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
165 requirement because the minimization of soft errors can affect the granularity
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
166 and the nature of the interface. In general, if you find that you're wanting to
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
167 throw soft errors, you must review the granularity of the interface because it
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
168 is likely you're trying to implement something that is too low level. The rule
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
169 of thumb is to provide interface functions that **can't** fail, except when
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
170 faced with hard errors.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
171
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
172 For a trivial example, suppose we wanted to add an "``OpenFileForWriting``"
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
173 function. For many operating systems, if the file doesn't exist, attempting to
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
174 open the file will produce an error. However, ``lib/System`` should not simply
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
175 throw that error if it occurs because its a soft error. The problem is that the
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
176 interface function, ``OpenFileForWriting`` is too low level. It should be
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
177 ``OpenOrCreateFileForWriting``. In the case of the soft "doesn't exist" error,
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
178 this function would just create it and then open it for writing.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
179
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
180 This design principle needs to be maintained in ``lib/System`` because it
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
181 avoids the propagation of soft error handling throughout the rest of LLVM.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
182 Hard errors will generally just cause a termination for an LLVM tool so don't
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
183 be bashful about throwing them.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
184
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
185 Rules of thumb:
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
186
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
187 #. Don't throw soft errors, only hard errors.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
188
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
189 #. If you're tempted to throw a soft error, re-think the interface.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
190
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
191 #. Handle internally the most common normal/good/soft error conditions
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
192 so the rest of LLVM doesn't have to.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
193
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
194 No throw Specifications
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
195 -----------------------
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
196
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
197 None of the ``lib/System`` interface functions may be declared with C++
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
198 ``throw()`` specifications on them. This requirement makes sure that the
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
199 compiler does not insert additional exception handling code into the interface
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
200 functions. This is a performance consideration: ``lib/System`` functions are at
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
201 the bottom of many call chains and as such can be frequently called. We need
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
202 them to be as efficient as possible. However, no routines in the system
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
203 library should actually throw exceptions.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
204
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
205 Code Organization
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
206 -----------------
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
207
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
208 Implementations of the System Library interface are separated by their general
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
209 class of operating system. Currently only Unix and Win32 classes are defined
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
210 but more could be added for other operating system classifications. To
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
211 distinguish which implementation to compile, the code in ``lib/System`` uses
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
212 the ``LLVM_ON_UNIX`` and ``LLVM_ON_WIN32`` ``#defines`` provided via configure
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
213 through the ``llvm/Config/config.h`` file. Each source file in ``lib/System``,
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
214 after implementing the generic (operating system independent) functionality
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
215 needs to include the correct implementation using a set of
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
216 ``#if defined(LLVM_ON_XYZ)`` directives. For example, if we had
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
217 ``lib/System/File.cpp``, we'd expect to see in that file:
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
218
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
219 .. code-block:: c++
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
220
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
221 #if defined(LLVM_ON_UNIX)
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
222 #include "Unix/File.cpp"
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
223 #endif
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
224 #if defined(LLVM_ON_WIN32)
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
225 #include "Win32/File.cpp"
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
226 #endif
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
227
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
228 The implementation in ``lib/System/Unix/File.cpp`` should handle all Unix
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
229 variants. The implementation in ``lib/System/Win32/File.cpp`` should handle all
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
230 Win32 variants. What this does is quickly differentiate the basic class of
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
231 operating system that will provide the implementation. The specific details for
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
232 a given platform must still be determined through the use of ``#ifdef``.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
233
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
234 Consistent Semantics
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
235 --------------------
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
236
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
237 The implementation of a ``lib/System`` interface can vary drastically between
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
238 platforms. That's okay as long as the end result of the interface function is
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
239 the same. For example, a function to create a directory is pretty straight
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
240 forward on all operating system. System V IPC on the other hand isn't even
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
241 supported on all platforms. Instead of "supporting" System V IPC,
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
242 ``lib/System`` should provide an interface to the basic concept of
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
243 inter-process communications. The implementations might use System V IPC if
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
244 that was available or named pipes, or whatever gets the job done effectively
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
245 for a given operating system. In all cases, the interface and the
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
246 implementation must be semantically consistent.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
247