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C.5 Pragma Discard_Names

1
   [A pragma Discard_Names may be used to request a reduction in storage used for the names of certain entities.]

Syntax

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The form of a pragma Discard_Names is as follows:
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  pragma Discard_Names[([On => ] local_name)];
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A pragma Discard_Names is allowed only immediately within a declarative_part, immediately within a package_specification, or as a configuration pragma.

Legality Rules

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   The local_name (if present) shall denote a non-derived enumeration [first] subtype, a tagged [first] subtype, or an exception. The pragma applies to the type or exception. Without a local_name, the pragma applies to all such entities declared after the pragma, within the same declarative region. Alternatively, the pragma can be used as a configuration pragma. If the pragma applies to a type, then it applies also to all descendants of the type.

Static Semantics

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   {representation pragma (Discard_Names) [partial]} {pragma, representation (Discard_Names) [partial]} If a local_name is given, then a pragma Discard_Names is a representation pragma.
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   If the pragma applies to an enumeration type, then the semantics of the Wide_Image and Wide_Value attributes are implementation defined for that type; [the semantics of Image and Value are still defined in terms of Wide_Image and Wide_Value.] In addition, the semantics of Text_IO.Enumeration_IO are implementation defined. If the pragma applies to a tagged type, then the semantics of the Tags.Expanded_Name function are implementation defined for that type. If the pragma applies to an exception, then the semantics of the Exceptions.Exception_Name function are implementation defined for that exception.
7.a
Implementation defined: The semantics of pragma Discard_Names.
7.b
Ramification: The Width attribute is still defined in terms of Image.
7.c
The semantics of S'Wide_Image and S'Wide_Value are implementation defined for any subtype of an enumeration type to which the pragma applies. (The pragma actually names the first subtype, of course.)

Implementation Advice

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   If the pragma applies to an entity, then the implementation should reduce the amount of storage used for storing names associated with that entity.
8.a
Reason: A typical implementation of the Image attribute for enumeration types is to store a table containing the names of all the enumeration literals. Pragma Discard_Names allows the implementation to avoid storing such a table without having to prove that the Image attribute is never used (which can be difficult in the presence of separate compilation).
8.b
We did not specify the semantics of the Image attribute in the presence of this pragma because different semantics might be desirable in different situations. In some cases, it might make sense to use the Image attribute to print out a useful value that can be used to identify the entity given information in compiler-generated listings. In other cases, it might make sense to get an error at compile time or at run time. In cases where memory is plentiful, the simplest implementation makes sense: ignore the pragma. Implementations that are capable of avoiding the extra storage in cases where the Image attribute is never used might also wish to ignore the pragma.
8.c
The same applies to the Tags.Expanded_Name and Exceptions.Exception_Name functions.

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