G.2 Numeric Performance Requirements
Implementation Requirements
1
{accuracy}
{strict mode}
Implementations shall provide a user-selectable mode
in which the accuracy and other numeric performance requirements detailed
in the following subclauses are observed. This mode, referred to as the
strict mode, may or may not be the default mode; it directly affects
the results of the predefined arithmetic operations of real types and
the results of the subprograms in children of the Numerics package, and
indirectly affects the operations in other language defined packages.
{relaxed mode} Implementations
shall also provide the opposing mode, which is known as the
relaxed
mode.
1.a
Reason: On the assumption that the users
of an implementation that does not support the Numerics Annex have no
particular need for numerical performance, such an implementation has
no obligation to meet any particular requirements in this area. On the
other hand, users of an implementation that does support the Numerics
Annex are provided with a way of ensuring that their programs achieve
a known level of numerical performance and that the performance is portable
to other such implementations. The relaxed mode is provided to allow
implementers to offer an efficient but not fully accurate alternative
in the case that the strict mode entails a time overhead that some users
may find excessive. In some of its areas of impact, the relaxed mode
may be fully equivalent to the strict mode.
1.b
Implementation Note: The relaxed mode
may, for example, be used to exploit the implementation of (some of)
the elementary functions in hardware, when available. Such implementations
often do not meet the accuracy requirements of the strict mode, or do
not meet them over the specified range of parameter values, but compensate
in other ways that may be important to the user, such as their extreme
speed.
1.c
Ramification: For implementations supporting
the Numerics Annex, the choice of mode has no effect on the selection
of a representation for a real type or on the values of attributes of
a real type.
Implementation Permissions
2
Either mode may be the default mode.
2.a
Implementation defined: Whether the strict
mode or the relaxed mode is the default.
3
The two modes need not actually be different.
Extensions to Ada 83
3.a
{
extensions to Ada 83}
The
choice between strict and relaxed numeric performance was not available
in Ada 83.
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