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11.6 Exceptions and Optimization
1
This
clause gives permission to the implementation to perform certain ``optimizations''
that do not necessarily preserve the canonical semantics.
Dynamic Semantics
2
The rest of this International
Standard (outside this clause) defines the
canonical semantics
of the language. The canonical semantics of a given (legal) program determines
a set of possible external effects that can result from the execution
of the program with given inputs.
3
As explained in
1.1.3,
``
Conformity of an Implementation with the Standard'',
the external effect of a program is defined in terms of its interactions
with its external environment. Hence, the implementation can perform
any internal actions whatsoever, in any order or in parallel, so long
as the external effect of the execution of the program is one that is
allowed by the canonical semantics, or by the rules of this clause.
Implementation Permissions
4
The following additional
permissions are granted to the implementation:
5
- An implementation
need not always raise an exception when a language-defined check fails.
Instead, the operation that failed the check can simply yield an undefined
result. The exception need be raised by the implementation only if,
in the absence of raising it, the value of this undefined result would
have some effect on the external interactions of the program. In determining
this, the implementation shall not presume that an undefined result has
a value that belongs to its subtype, nor even to the base range of its
type, if scalar. Having removed the raise of the exception, the canonical
semantics will in general allow the implementation to omit the code for
the check, and some or all of the operation itself.
6
- If an exception is raised due to the
failure of a language-defined check, then upon reaching the corresponding
exception_handler (or the termination
of the task, if none), the external interactions that have occurred need
reflect only that the exception was raised somewhere within the execution
of the sequence_of_statements with
the handler (or the task_body),
possibly earlier (or later if the interactions are independent of the
result of the checked operation) than that defined by the canonical semantics,
but not within the execution of some abort-deferred operation or independent
subprogram that does not dynamically enclose the execution of the construct
whose check failed. An independent subprogram is
one that is defined outside the library unit containing the construct
whose check failed, and has no Inline pragma
applied to it. Any
assignment that occurred outside of such abort-deferred operations or
independent subprograms can be disrupted by the raising of the exception,
causing the object or its parts to become abnormal, and certain subsequent
uses of the object to be erroneous, as explained in 13.9.1.
7
3 The permissions granted
by this clause can have an effect on the semantics of a program only
if the program fails a language-defined check.
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