13.5.1 Record Representation Clauses
1
[A
record_representation_clause
specifies the storage representation of records and record extensions,
that is, the order, position, and size of components (including discriminants,
if any).
{bit field: See record_representation_clause}
]
Language Design Principles
1.a/2
{
AI95-00114-01}
It should be feasible for an implementation to use negative offsets in
the representation of composite types. However, no implementation should
be forced to support negative offsets. Therefore
,
in the interest of uniformity, negative offsets should be disallowed
in
record_representation_clauses.
Syntax
2
record_representation_clause ::=
for first_subtype_local_name use
record [
mod_clause]
{
component_clause}
end record;
3
component_clause ::=
component_local_name at position range first_bit ..
last_bit;
4
position ::= static_expression
5
first_bit ::= static_simple_expression
6
last_bit ::= static_simple_expression
6.a
Reason: First_bit
and
last_bit need to be
simple_expression
instead of
expression for the same reason
as in
range (see
3.5,
“
Scalar Types”).
Name Resolution Rules
7
{expected type (component_clause
expressions) [partial]} {expected
type (position) [partial]} {expected
type (first_bit) [partial]} {expected
type (last_bit) [partial]} Each
position,
first_bit, and
last_bit
is expected to be of any integer type.
7.a
Ramification: These need not have the
same integer type.
Legality Rules
8/2
{
AI95-00436-01}
The
first_subtype_local_name of a
record_representation_clause
shall denote a specific
nonlimited record
or record extension subtype.
8.a
Ramification: As for all type-related
representation items, the local_name is required
to denote a first subtype.
9
If the component_local_name
is a direct_name, the local_name
shall denote a component of the type. For a record extension, the component
shall not be inherited, and shall not be a discriminant that corresponds
to a discriminant of the parent type. If the component_local_name
has an attribute_designator, the direct_name
of the local_name shall denote either the
declaration of the type or a component of the type, and the attribute_designator
shall denote an implementation-defined implicit component of the type.
10
The position, first_bit,
and last_bit shall be static expressions.
The value of position and first_bit
shall be nonnegative. The value of last_bit
shall be no less than first_bit – 1.
10.a
Ramification: A component_clause
such as “X at 4 range 0..–1;” is allowed
if X can fit in zero bits.
10.1/2
{
AI95-00133-01}
If the nondefault bit ordering applies to the type,
then either: 10.2/2
- the value of
last_bit shall be less than the size of the
largest machine scalar; or
10.3/2
- the value of
first_bit shall be zero and the value of last_bit
+ 1 shall be a multiple of System.Storage_Unit.
11
At most one component_clause
is allowed for each component of the type, including for each discriminant
(component_clauses may be given for some,
all, or none of the components). Storage places within a component_list
shall not overlap, unless they are for components in distinct variants
of the same variant_part.
12
A name that denotes a component of a type is not
allowed within a record_representation_clause
for the type, except as the component_local_name
of a component_clause.
12.a
Reason: It
might seem strange to make the record_representation_clause
part of the declarative region, and then disallow mentions of the components
within almost all of the record_representation_clause.
The alternative would be to treat the component_local_name
like a formal parameter name in a subprogram call (in terms of visibility).
However, this rule would imply slightly different semantics, because
(given the actual rule) the components can hide other declarations. This
was the rule in Ada 83, and we see no reason to change it. The following,
for example, was and is illegal:
12.b
type T is
record
X : Integer;
end record;
X : constant := 31; -- Same defining name as the component.
for T use
record
X at 0 range 0..X; -- Illegal!
end record;
12.c
The component X hides the named number X throughout
the record_representation_clause.
Static Semantics
13/2
{
AI95-00133-01}
A
record_representation_clause (without the
mod_clause) specifies the layout.
The storage place attributes (see 13.5.2)
are taken from the values of the position,
first_bit, and last_bit
expressions after normalizing those values so that first_bit
is less than Storage_Unit.13.1/2
{
AI95-00133-01}
If the default bit ordering applies to the type,
the position, first_bit,
and last_bit of each component_clause
directly specify the position and size of the corresponding component.13.2/2
{
AI95-00133-01}
If the nondefault bit ordering applies to the type
then the layout is determined as follows: 13.3/2
- the component_clauses
for which the value of last_bit is greater
than or equal to the size of the largest machine scalar directly specify
the position and size of the corresponding component;
13.4/2
- for other component_clauses,
all of the components having the same value of position
are considered to be part of a single machine scalar, located at that
position; this machine scalar has a size which
is the smallest machine scalar size larger than the largest last_bit
for all component_clauses at that position;
the first_bit and last_bit
of each component_clause are then interpreted
as bit offsets in this machine scalar.
13.a/2
This paragraph
was deleted.Ramification: {
AI95-00133-01}
For example, if Storage_Unit is 8, then “C
at 0 range 24..31;” defines C'Position = 3, C'First_Bit
= 0, and C'Last_Bit = 7. This is true of machines with either bit ordering.
13.b
A component_clause
also determines the value of the Size attribute of the component, since
this attribute is related to First_Bit and Last_Bit.
14
[A record_representation_clause
for a record extension does not override the layout of the parent part;]
if the layout was specified for the parent type, it is inherited by the
record extension.
Implementation Permissions
15
An implementation may generate implementation-defined
components (for example, one containing the offset of another component).
An implementation may generate names that denote such implementation-defined
components; such names shall be implementation-defined attribute_references.
An implementation may allow such implementation-defined names to be used
in record_representation_clauses. An implementation
can restrict such component_clauses in any
manner it sees fit.
15.a
Implementation defined: Implementation-defined
components.
15.b
Ramification: Of course, since the semantics
of implementation-defined attributes is implementation defined, the implementation
need not support these names in all situations. They might be purely
for the purpose of component_clauses, for
example. The visibility rules for such names are up to the implementation.
15.c
We do not allow such component names to be normal
identifiers — that would constitute blanket permission to do all
kinds of evil things.
15.d
Discussion: {
dope}
Such
implementation-defined components are known in the vernacular as “dope.”
Their main purpose is for storing offsets of components that depend on
discriminants.
16
If a record_representation_clause
is given for an untagged derived type, the storage place attributes for
all of the components of the derived type may differ from those of the
corresponding components of the parent type, even for components whose
storage place is not specified explicitly in the record_representation_clause.
16.a
Reason: This is clearly necessary, since
the whole record may need to be laid out differently.
Implementation Advice
17
{recommended
level of support (record_representation_clause) [partial]}
The recommended level of support for
record_representation_clauses
is:
17.1/2
- {AI95-00133-01}
An implementation should support machine scalars
that correspond to all of the integer, floating point, and address formats
supported by the machine.
18
- An implementation should support storage
places that can be extracted with a load, mask, shift sequence of machine
code, and set with a load, shift, mask, store sequence, given the available
machine instructions and run-time model.
19
- A storage place should be supported
if its size is equal to the Size of the component subtype, and it starts
and ends on a boundary that obeys the Alignment of the component subtype.
20/2
- {AI95-00133-01}
For If the default
bit ordering applies to the declaration of a given type, then for
a component with a subtype whose subtype's
Size is less than the word size, any storage place that does not
cross an aligned word boundary should be supported.
20.a
Reason: The above recommendations are
sufficient to define interfaces to most interesting hardware. This causes
less implementation burden than the definition in ACID, which requires
arbitrary bit alignments of arbitrarily large components. Since the ACID
definition is neither enforced by the ACVC, nor supported by all implementations,
it seems OK for us to weaken it.
21
- An implementation may reserve a storage
place for the tag field of a tagged type, and disallow other components
from overlapping that place.
21.a
Ramification: Similar permission for
other dope is not granted.
22
- An implementation need not support
a component_clause for a component of an extension
part if the storage place is not after the storage places of all components
of the parent type, whether or not those storage places had been specified.
22.a
Reason: These restrictions are probably
necessary if block equality operations are to be feasible for class-wide
types. For block comparison to work, the implementation typically has
to fill in any gaps with zero (or one) bits. If a “gap” in
the parent type is filled in with a component in a type extension, then
this won't work when a class-wide object is passed by reference, as is
required.
22.b/2
Implementation Advice:
The recommended level of support for
record_representation_clauses should be followed.
23
12 If no component_clause
is given for a component, then the choice of the storage place for the
component is left to the implementation. If component_clauses
are given for all components, the record_representation_clause
completely specifies the representation of the type and will be obeyed
exactly by the implementation.
23.a
Ramification: The visibility rules prevent
the name of a component of the type from appearing in a record_representation_clause
at any place except for the component_local_name
of a component_clause. However, since the
record_representation_clause is part of the
declarative region of the type declaration, the component names hide
outer homographs throughout.
23.b/1
{
8652/0009}
{
AI95-00137-01}
A
record_representation_clause cannot be given
for a protected type, even though protected types, like record types,
have components. The primary reason for this rule is that there is likely
to be too much dope in a protected type — entry queues, bit maps
for barrier values, etc. In order to control the representation of the
user-defined components, simply declare a record type, give it a
record_representation_clause representation_clause,
and give the protected type one component whose type is the record type.
Alternatively, if the protected object is protecting something like a
device register, it makes more sense to keep the thing being protected
outside the protected object (possibly with a pointer to it in the protected
object), in order to keep implementation-defined components out of the
way.
Examples
24
Example of specifying
the layout of a record type:
25
Word : constant := 4; -- storage element is byte, 4 bytes per word
26
type State is (A,M,W,P);
type Mode is (Fix, Dec, Exp, Signif);
27
type Byte_Mask is array (0..7) of Boolean;
type State_Mask is array (State) of Boolean;
type Mode_Mask is array (Mode) of Boolean;
28
type Program_Status_Word is
record
System_Mask : Byte_Mask;
Protection_Key : Integer range 0 .. 3;
Machine_State : State_Mask;
Interrupt_Cause : Interruption_Code;
Ilc : Integer range 0 .. 3;
Cc : Integer range 0 .. 3;
Program_Mask : Mode_Mask;
Inst_Address : Address;
end record;
29
for Program_Status_Word use
record
System_Mask at 0*Word range 0 .. 7;
Protection_Key at 0*Word range 10 .. 11; -- bits 8,9 unused
Machine_State at 0*Word range 12 .. 15;
Interrupt_Cause at 0*Word range 16 .. 31;
Ilc at 1*Word range 0 .. 1; -- second word
Cc at 1*Word range 2 .. 3;
Program_Mask at 1*Word range 4 .. 7;
Inst_Address at 1*Word range 8 .. 31;
end record;
30
for Program_Status_Word'Size use 8*System.Storage_Unit;
for Program_Status_Word'Alignment use 8;
31
13 Note on the example: The record_representation_clause
defines the record layout. The Size clause guarantees that (at least)
eight storage elements are used for objects of the type. The Alignment
clause guarantees that aliased, imported, or exported objects of the
type will have addresses divisible by eight.
Wording Changes from Ada 83
31.a
31.b
We have clarified that implementation-defined
component names have to be in the form of an attribute_reference
of a component or of the first subtype itself; surely Ada 83 did not
intend to allow arbitrary identifiers.
31.c
The RM83-13.4(7) wording incorrectly allows
components in non-variant records to overlap. We have corrected that
oversight.
Incompatibilities With Ada 95
31.d/2
{
AI95-00133-01}
{incompatibilities with Ada 95} Amendment
Correction: The meaning of a record_representation_clause
for the nondefault bit order is now clearly defined. Thus, such clauses
can be portably written. In order to do that though, the equivalence
of bit 1 in word 1 to bit 9 in word 0 (for a machine with Storage_Unit
= 8) had to be dropped for the nondefault bit order. Any record_representation_clauses
which depends on that equivalence will break (although such code would
imply a non-contiguous representation for a component, and it seems unlikely
that compilers were supporting that anyway). Extensions to Ada 95
31.e/2
{
AI95-00436-01}
{extensions to Ada 95} Amendment
Correction: The undocumented (and likely unintentional) incompatibility
with Ada 83 caused by not allowing record_representation_clauses
on limited record types is removed.
Sponsored by Ada-Europe