C.4 Preelaboration Requirements
[This clause specifies additional implementation
and documentation requirements for the Preelaborate pragma (see
10.2.1).]
Implementation Requirements
The implementation shall not incur any run-time overhead
for the elaboration checks of subprograms and protected_bodies
declared in preelaborated library units.
The implementation shall not execute any memory write
operations after load time for the elaboration of constant objects declared
immediately within the declarative region of a preelaborated library
package, so long as the subtype and initial expression (or default initial
expressions if initialized by default) of the
object_declaration
satisfy the following restrictions.
{load
time} The meaning of
load time
is implementation defined.
Discussion: On systems where the image
of the partition is initially copied from disk to RAM, or from ROM to
RAM, prior to starting execution of the partition, the intention is that
“load time” consist of this initial copying step. On other
systems, load time and run time might actually be interspersed.
Any
subtype_mark
denotes a statically constrained subtype, with statically constrained
subcomponents, if any;
{
AI95-00161-01}
no
subtype_mark
denotes a controlled type, a private type, a private extension, a generic
formal private type, a generic formal derived type, or a descendant of
such a type;
Reason: For an implementation that uses
the registration method of finalization, a controlled object will require
some code executed to register the object at the appropriate point. The
other types are those that might have a controlled component.
None of these types were allowed in preelaborated units in Ada 95. These
types are covered by the Implementation Advice, of course, so they should
still execute as little code as possible.
any
allocator
is for an access-to-constant type;
any uses of predefined operators appear only within
static expressions;
any
primaries that are
names, other
than
attribute_references
for the Access or Address attributes, appear only within static expressions;
Ramification: This cuts out
attribute_references
that are not static, except for Access and Address.
any
name
that is not part of a static expression is an expanded name or
direct_name
that statically denotes some entity;
Ramification: This cuts out
function_calls
and
type_conversions
that are not static, including calls on attribute functions like 'Image
and 'Value.
no language-defined check associated with the elaboration
of the
object_declaration
can fail.
Reason: {
AI95-00114-01}
The intent is that aggregates all of whose scalar subcomponents are static
and all of whose access subcomponents are
null, allocators for
access-to-constant types, or X'Access, will be supported with no run-time
code generated.
Documentation Requirements
The implementation shall document any circumstances
under which the elaboration of a preelaborated package causes code to
be executed at run time.
Documentation Requirement: Any circumstances
when the elaboration of a preelaborated package causes code to be executed.
The implementation shall document whether the method
used for initialization of preelaborated variables allows a partition
to be restarted without reloading.
Documentation Requirement: Whether a
partition can be restarted without reloading.
This paragraph
was deleted.
Discussion: {
AI95-00114-01}
This covers the issue of the run-time system itself being restartable,
so that need not be a separate Documentation Requirement.
Implementation Advice
It is recommended that preelaborated packages be
implemented in such a way that there should be little or no code executed
at run time for the elaboration of entities not already covered by the
Implementation Requirements.
Implementation Advice: Preelaborated
packages should be implemented such that little or no code is executed
at run time for the elaboration of entities.
Wording Changes from Ada 95
{
AI95-00161-01}
Added wording to exclude the additional kinds of types allowed in preelaborated
units from the Implementation Requirements.
Sponsored by Ada-Europe