Ada '83 Language Reference Manual

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11.6. Exceptions and Optimization

The purpose of this section is to specify the conditions under which an implementation is allowed to perform certain actions either earlier or later than specified by other rules of the language.

In general, when the language rules specify an order for certain actions (the canonical order), an implementation may only use an alternative order if it can guarantee that the effect of the program is not changed by the reordering. In particular, no exception should arise for the execution of the reordered program if none arises for the execution of the program in the canonical order. When, on the other hand, the order of certain actions is not defined by the language, any order can be used by the implementation. (For example, the arguments of a predefined operator can be evaluated in any order since the rules given in section 4.5 do not require a specific order of evaluation.)

Additional freedom is left to an implementation for reordering actions involving predefined operations that are either predefined operators or basic operations other than assignments. This freedom is left, as defined below, even in the case where the execution of these predefined operations may propagate a (predefined) exception:

1. For the purpose of establishing whether the same effect is obtained by the execution of certain actions in the canonical and in an alternative order, it can be assumed that none of the predefined operations invoked by these actions propagates a (predefined) exception, provided that the two following requirements are met by the alternative order: first, an operation must not be invoked in the alternative order if it is not invoked in the canonical order; second, for each operation, the innermost enclosing frame or accept statement must be the same in the alternative order as in the canonical order, and the same exception handlers must apply.
   
   
2. Within an expression, the association of operators with operands is specified by the syntax. However, for a sequence of predefined operators of the same precedence level (and in the absence of parentheses imposing a specific association), any association of operators with operands is allowed if it satisfies the following requirement: an integer result must be equal to that given by the canonical left-to-right order; a real result must belong to the result model interval defined for the canonical left-to-right order (see 4.5.7). Such a reordering is allowed even if it may remove an exception, or introduce a further predefined exception.

A preedefined operation need not be invoked at all, if its only possible effect is to propagate a predefined exception. Similarly, a predefined operation need not be invoked if the removal of subsequent operations by the above rule renders this invocation ineffective.

Notes:

Rule (b) applies to predefined operators but not to the short-circuit control forms.

The expression SPEED < 300_000.0 can be replaced by TRUE if the value 300_000.0 lies outside the base type of SPEED, even though the implicit conversion of the numeric literal would raise the exception NUMERIC_ERROR.

Example:

    declare
       N : INTEGER;
    begin
       N := 0;               --  (1)
       for J in 1 .. 10 loop
          N := N + J**A(K);  --  A and K are global variables
       end loop;
       PUT(N);
    exception
       when others => PUT("Some error arose"); PUT(N);
    end; 

The evaluation of A(K) may be performed before the loop, and possibly immediately before the assignment statement (1) even if this evaluation can raise an exception. Consequently, within the exception handler, the value of N is either the undefined initial value or a value later assigned. On the other hand, the evaluation of A(K) cannot be moved before begin since an exception would then be handled by a different handler. For this reason, the initialization of N in the declaration itself would exclude the possibility of having an undefined initial value of N in the handler.

References: accept statement, accuracy of real operations, assignment, base type, basic operation, conversion, error situation, exception, exception handler, frame, numeric_error exception, predefined operator, predefined subprogram, propagation of an exception, real type, undefined value.

Rationale references: 14.5.4 Implementation of Exception Handling

Address any questions or comments to adainfo@sw-eng.falls-church.va.us.