1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<HTML>
<HEAD>
<TITLE>AARM95 - Shared Variable Control</TITLE>
<META NAME="Author" CONTENT="JTC1/SC22/WG9/ARG, by Randall Brukardt, ARG Editor">
<META NAME="GENERATOR" CONTENT="Arm_Form.Exe, Ada Reference Manual generator">
<STYLE type="text/css">
DIV.paranum {position: absolute; font-family: Arial, Helvetica, sans-serif; left: 0.5 em; top: auto}
TT {font-family: "Courier New", monospace}
DT {display: compact}
DIV.Normal {font-family: "Times New Roman", Times, serif; margin-bottom: 0.6em}
DIV.Wide {font-family: "Times New Roman", Times, serif; margin-top: 0.6em; margin-bottom: 0.6em}
DIV.Annotations {font-family: "Times New Roman", Times, serif; margin-left: 4.0em; margin-bottom: 0.6em}
DIV.WideAnnotations {font-family: "Times New Roman", Times, serif; margin-left: 4.0em; margin-top: 0.6em; margin-bottom: 0.6em}
DIV.Index {font-family: "Times New Roman", Times, serif}
DIV.SyntaxSummary {font-family: "Times New Roman", Times, serif; margin-left: 2.0em; margin-bottom: 0.4em}
DIV.Notes {font-family: "Times New Roman", Times, serif; margin-left: 2.0em; margin-bottom: 0.6em}
DIV.NotesHeader {font-family: "Times New Roman", Times, serif; margin-left: 2.0em}
DIV.SyntaxIndented {font-family: "Times New Roman", Times, serif; margin-left: 2.0em; margin-bottom: 0.4em}
DIV.Indented {font-family: "Times New Roman", Times, serif; margin-left: 6.0em; margin-bottom: 0.6em}
DIV.CodeIndented {font-family: "Times New Roman", Times, serif; margin-left: 4.0em; margin-bottom: 0.6em}
DIV.SmallIndented {font-family: "Times New Roman", Times, serif; margin-left: 10.0em; margin-bottom: 0.6em}
DIV.SmallCodeIndented {font-family: "Times New Roman", Times, serif; margin-left: 8.0em; margin-bottom: 0.6em}
DIV.Examples {font-family: "Courier New", monospace; margin-left: 2.0em; margin-bottom: 0.6em}
DIV.SmallExamples {font-family: "Courier New", monospace; font-size: 80%; margin-left: 7.5em; margin-bottom: 0.6em}
DIV.IndentedExamples {font-family: "Courier New", monospace; margin-left: 8.0em; margin-bottom: 0.6em}
DIV.SmallIndentedExamples {font-family: "Courier New", monospace; font-size: 80%; margin-left: 15.0em; margin-bottom: 0.6em}
UL.Bulleted {font-family: "Times New Roman", Times, serif; margin-left: 2.0em; margin-right: 2.0em; margin-top: 0em; margin-bottom: 0.5em}
UL.SmallBulleted {font-family: "Times New Roman", Times, serif; margin-left: 6.0em; margin-right: 6.0em; margin-top: 0em; margin-bottom: 0.5em}
UL.NestedBulleted {font-family: "Times New Roman", Times, serif; margin-left: 4.0em; margin-right: 4.0em; margin-top: 0em; margin-bottom: 0.5em}
UL.SmallNestedBulleted {font-family: "Times New Roman", Times, serif; margin-left: 8.0em; margin-right: 8.0em; margin-top: 0em; margin-bottom: 0.5em}
UL.IndentedBulleted {font-family: "Times New Roman", Times, serif; margin-left: 8.0em; margin-right: 8.0em; margin-top: 0em; margin-bottom: 0.5em}
UL.CodeIndentedBulleted {font-family: "Times New Roman", Times, serif; margin-left: 6.0em; margin-right: 6.0em; margin-top: 0em; margin-bottom: 0.5em}
UL.CodeIndentedNestedBulleted {font-family: "Times New Roman", Times, serif; margin-left: 8.0em; margin-right: 8.0em; margin-top: 0em; margin-bottom: 0.5em}
UL.SyntaxIndentedBulleted {font-family: "Times New Roman", Times, serif; margin-left: 4.0em; margin-right: 4.0em; margin-top: 0em; margin-bottom: 0.5em}
UL.NotesBulleted {font-family: "Times New Roman", Times, serif; margin-left: 4.0em; margin-right: 4.0em; margin-top: 0em; margin-bottom: 0.5em}
UL.NotesNestedBulleted {font-family: "Times New Roman", Times, serif; margin-left: 6.0em; margin-right: 6.0em; margin-top: 0em; margin-bottom: 0.5em}
DL.Hanging {font-family: "Times New Roman", Times, serif; margin-top: 0em; margin-bottom: 0.6em}
DD.Hanging {margin-left: 6.0em}
DL.IndentedHanging {font-family: "Times New Roman", Times, serif; margin-left: 4.0em; margin-top: 0em; margin-bottom: 0.6em}
DD.IndentedHanging {margin-left: 2.0em}
DL.HangingInBulleted {font-family: "Times New Roman", Times, serif; margin-left: 2.0em; margin-right: 2.0em; margin-top: 0em; margin-bottom: 0.5em}
DD.HangingInBulleted {margin-left: 4.0em}
DL.SmallHanging {font-family: "Times New Roman", Times, serif; margin-left: 4.0em; margin-top: 0em; margin-bottom: 0.6em}
DD.SmallHanging {margin-left: 7.5em}
DL.SmallIndentedHanging {font-family: "Times New Roman", Times, serif; margin-left: 8.0em; margin-top: 0em; margin-bottom: 0.6em}
DD.SmallIndentedHanging {margin-left: 2.0em}
DL.SmallHangingInBulleted {font-family: "Times New Roman", Times, serif; margin-left: 6.0em; margin-right: 6.0em; margin-top: 0em; margin-bottom: 0.5em}
DD.SmallHangingInBulleted {margin-left: 5.0em}
DL.Enumerated {font-family: "Times New Roman", Times, serif; margin-right: 0.0em; margin-top: 0em; margin-bottom: 0.5em}
DD.Enumerated {margin-left: 2.0em}
DL.SmallEnumerated {font-family: "Times New Roman", Times, serif; margin-left: 4.0em; margin-right: 4.0em; margin-top: 0em; margin-bottom: 0.5em}
DD.SmallEnumerated {margin-left: 2.5em}
DL.NestedEnumerated {font-family: "Times New Roman", Times, serif; margin-left: 2.0em; margin-right: 2.0em; margin-top: 0em; margin-bottom: 0.5em}
DL.SmallNestedEnumerated {font-family: "Times New Roman", Times, serif; margin-left: 6.0em; margin-right: 6.0em; margin-top: 0em; margin-bottom: 0.5em}
</STYLE>
</HEAD>
<BODY TEXT="#000000" BGCOLOR="#FFFFF0" LINK="#0000FF" VLINK="#800080" ALINK="#FF0000">
<P><A HREF="AA-TOC.html">Contents</A> <A HREF="AA-0-29.html">Index</A> <A HREF="AA-C-5.html">Previous</A> <A HREF="AA-C-7.html">Next</A></P>
<HR>
<H1> C.6 Shared Variable Control</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="Normal"> [This clause specifies representation pragmas
that control the use of shared variables.] </DIV>
<H4 ALIGN=CENTER>Syntax</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="SyntaxIndented" Style="margin-bottom: 0.2em">The form for
pragmas Atomic, Volatile, Atomic_Components, and Volatile_Components
is as follows: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="SyntaxIndented"> <B>pragma</B> <A NAME="I6692"></A>Atomic(<A NAME="I6693"></A><FONT FACE="Arial, Helvetica">local_name</FONT>);</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="SyntaxIndented"> <B>pragma</B> <A NAME="I6694"></A>Volatile(<A NAME="I6695"></A><FONT FACE="Arial, Helvetica">local_name</FONT>);</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="SyntaxIndented"> <B>pragma</B> <A NAME="I6696"></A>Atomic_Components(<I>array_</I><A NAME="I6697"></A><FONT FACE="Arial, Helvetica">local_name</FONT>);</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="SyntaxIndented"> <B>pragma</B> <A NAME="I6698"></A>Volatile_Components(<I>array_</I><A NAME="I6699"></A><FONT FACE="Arial, Helvetica">local_name</FONT>);</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6700"></A>An <I>atomic</I> type is one
to which a pragma Atomic applies. An <I>atomic</I> object (including
a component) is one to which a pragma Atomic applies, or a component
of an array to which a pragma Atomic_Components applies, or any object
of an atomic type.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6701"></A>A <I>volatile</I> type is
one to which a pragma Volatile applies. A <I>volatile</I> object (including
a component) is one to which a pragma Volatile applies, or a component
of an array to which a pragma Volatile_Components applies, or any object
of a volatile type. In addition, every atomic type or object is also
defined to be volatile. Finally, if an object is volatile, then so are
all of its subcomponents [(the same does not apply to atomic)]. </DIV>
<H4 ALIGN=CENTER>Name Resolution Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<DIV Class="Normal"> The <FONT FACE="Arial, Helvetica">local_name</FONT>
in an Atomic or Volatile pragma shall resolve to denote either an <FONT FACE="Arial, Helvetica">object_declaration</FONT>,
a non-inherited <FONT FACE="Arial, Helvetica">component_declaration</FONT>,
or a <FONT FACE="Arial, Helvetica">full_type_declaration</FONT>. The
<I>array_</I><FONT FACE="Arial, Helvetica">local_name</FONT> in an Atomic_Components
or Volatile_Components pragma shall resolve to denote the declaration
of an array type or an array object of an anonymous type.</DIV>
<H4 ALIGN=CENTER>Legality Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6702"></A>It is illegal to apply either
an Atomic or Atomic_Components pragma to an object or type if the implementation
cannot support the indivisible reads and updates required by the pragma
(see below).</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<DIV Class="Normal"> It is illegal to specify the Size attribute of
an atomic object, the Component_Size attribute for an array type with
atomic components, or the layout attributes of an atomic component, in
a way that prevents the implementation from performing the required indivisible
reads and updates.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<DIV Class="Normal"> If an atomic object is passed as a parameter,
then the type of the formal parameter shall either be atomic or allow
pass by copy [(that is, not be a nonatomic by-reference type)]. If an
atomic object is used as an actual for a generic formal object of mode
<B>in out</B>, then the type of the generic formal object shall be atomic.
If the <FONT FACE="Arial, Helvetica">prefix</FONT> of an <FONT FACE="Arial, Helvetica">attribute_reference</FONT>
for an Access attribute denotes an atomic object [(including a component)],
then the designated type of the resulting access type shall be atomic.
If an atomic type is used as an actual for a generic formal derived type,
then the ancestor of the formal type shall be atomic or allow pass by
copy. Corresponding rules apply to volatile objects and types.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>13</FONT></DIV>
<DIV Class="Normal"> If a pragma Volatile, Volatile_Components, Atomic,
or Atomic_Components applies to a stand-alone constant object, then a
pragma Import shall also apply to it. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>13.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>Hence, no
initialization expression is allowed for such a constant. Note that a
constant that is atomic or volatile because of its type is allowed. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>13.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>Stand-alone constants
that are explicitly specified as Atomic or Volatile only make sense if
they are being manipulated outside the Ada program. From the Ada perspective
the object is read-only. Nevertheless, if imported and atomic or volatile,
the implementation should presume it might be altered externally. For
an imported stand-alone constant that is not atomic or volatile, the
implementation can assume that it will not be altered. </FONT></DIV>
<H4 ALIGN=CENTER>Static Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>14</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6703"></A><A NAME="I6704"></A><A NAME="I6705"></A><A NAME="I6706"></A><A NAME="I6707"></A><A NAME="I6708"></A><A NAME="I6709"></A><A NAME="I6710"></A>These
<FONT FACE="Arial, Helvetica">pragma</FONT>s are representation pragmas
(see <A HREF="AA-13-1.html">13.1</A>).</DIV>
<H4 ALIGN=CENTER>Dynamic Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>15</FONT></DIV>
<DIV Class="Normal"> For an atomic object (including an atomic component)
all reads and updates of the object as a whole are indivisible.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>16</FONT></DIV>
<DIV Class="Normal"> For a volatile object all reads and updates of
the object as a whole are performed directly to memory. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>16.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Implementation Note: </B>This
precludes any use of register temporaries, caches, and other similar
optimizations for that object. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>17</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6711"></A>Two actions are sequential
(see <A HREF="AA-9-10.html">9.10</A>) if each is the read or update of
the same atomic object.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>18</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6712"></A>If a type is atomic or volatile
and it is not a by-copy type, then the type is defined to be a by-reference
type. If any subcomponent of a type is atomic or volatile, then the type
is defined to be a by-reference type.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>19</FONT></DIV>
<DIV Class="Normal"> If an actual parameter is atomic or volatile,
and the corresponding formal parameter is not, then the parameter is
passed by copy. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>19.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Implementation Note: </B>Note
that in the case where such a parameter is normally passed by reference,
a copy of the actual will have to be produced at the call-site, and a
pointer to the copy passed to the formal parameter. If the actual is
atomic, any copying has to use indivisible read on the way in, and indivisible
write on the way out. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>19.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>It has to be known
at compile time whether an atomic or a volatile parameter is to be passed
by copy or by reference. For some types, it is unspecified whether parameters
are passed by copy or by reference. The above rules further specify the
parameter passing rules involving atomic and volatile types and objects.
</FONT></DIV>
<H4 ALIGN=CENTER>Implementation Requirements</H4>
<DIV Class="Paranum"><FONT SIZE=-2>20</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6713"></A>The external effect of a
program (see <A HREF="AA-1-1-3.html">1.1.3</A>) is defined to include
each read and update of a volatile or atomic object. The implementation
shall not generate any memory reads or updates of atomic or volatile
objects other than those specified by the program. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Discussion: </B>The presumption
is that volatile or atomic objects might reside in an ``active'' part
of the address space where each read has a potential side-effect, and
at the very least might deliver a different value.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20.b</FONT></DIV>
<DIV Class="Annotations" Style="margin-bottom: 0.4em"><FONT SIZE=-1>The
rule above and the definition of external effect are intended to prevent
(at least) the following incorrect optimizations, where V is a volatile
variable: </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20.c</FONT></DIV>
<UL Class="SmallBulleted"><FONT SIZE=-1><LI TYPE=DISC>X:= V; Y:=V; cannot be allowed to be translated as Y:=V;
X:=V;</LI></FONT></UL>
<DIV Class="Paranum"><FONT SIZE=-2>20.d</FONT></DIV>
<UL Class="SmallBulleted"><FONT SIZE=-1><LI TYPE=DISC>Deleting redundant loads: X:= V; X:= V; shall read the
value of V from memory twice.</LI></FONT></UL>
<DIV Class="Paranum"><FONT SIZE=-2>20.e</FONT></DIV>
<UL Class="SmallBulleted"><FONT SIZE=-1><LI TYPE=DISC>Deleting redundant stores: V:= X; V:= X; shall write into
V twice.</LI></FONT></UL>
<DIV Class="Paranum"><FONT SIZE=-2>20.f</FONT></DIV>
<UL Class="SmallBulleted"><FONT SIZE=-1><LI TYPE=DISC>Extra stores: V:= X+Y; should not translate to something
like V:= X; V:= V+Y;</LI></FONT></UL>
<DIV Class="Paranum"><FONT SIZE=-2>20.g</FONT></DIV>
<UL Class="SmallBulleted"><FONT SIZE=-1><LI TYPE=DISC>Extra loads: X:= V; Y:= X+Z; X:=X+B; should not translate
to something like Y:= V+Z; X:= V+B;</LI></FONT></UL>
<DIV Class="Paranum"><FONT SIZE=-2>20.h</FONT></DIV>
<UL Class="SmallBulleted"><FONT SIZE=-1><LI TYPE=DISC>Reordering of loads from volatile variables: X:= V1; Y:=
V2; (whether or not V1 = V2) should not translate to Y:= V2; X:= V1;</LI></FONT></UL>
<DIV Class="Paranum"><FONT SIZE=-2>20.i</FONT></DIV>
<UL Class="SmallBulleted"><FONT SIZE=-1><LI TYPE=DISC>Reordering of stores to volatile variables: V1:= X; V2:=
X; should not translate to V2:=X; V1:= X; </LI></FONT></UL>
<DIV Class="Paranum"><FONT SIZE=-2>21</FONT></DIV>
<DIV Class="Normal"> If a pragma Pack applies to a type any of whose
subcomponents are atomic, the implementation shall not pack the atomic
subcomponents more tightly than that for which it can support indivisible
reads and updates. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>21.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Implementation Note: </B>A
warning might be appropriate if no packing whatsoever can be achieved.
</FONT></DIV>
<DIV Class="NotesHeader"><FONT SIZE=-1>NOTES</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>22</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>9 An imported volatile or
atomic constant behaves as a constant (i.e. read-only) with respect to
other parts of the Ada program, but can still be modified by an ``external
source.''</FONT></DIV>
<H4 ALIGN=CENTER>Incompatibilities With Ada 83</H4>
<DIV Class="Paranum"><FONT SIZE=-2>22.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><A NAME="I6714"></A>Pragma Atomic
replaces Ada 83's pragma Shared. The name ``Shared'' was confusing, because
the pragma was not used to mark variables as shared. </FONT></DIV>
<HR>
<P><A HREF="AA-TOC.html">Contents</A> <A HREF="AA-0-29.html">Index</A> <A HREF="AA-C-5.html">Previous</A> <A HREF="AA-C-7.html">Next</A> <A HREF="AA-TTL.html">Legal</A></P>
</BODY>
</HTML>
|
