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6.4.2 Management of Overflows in GNAT

To deal with the portability issue, and with the problem of mathematical versus run-time interpretation of the expressions in assertions, GNAT provides comprehensive control over the handling of intermediate overflow. GNAT can operate in three modes, and furthermore, permits separate selection of operating modes for the expressions within assertions (here the term ‘assertions’ is used in the technical sense, which includes preconditions and so forth) and for expressions appearing outside assertions.

The three modes are:

Note that these modes apply only to the evaluation of predefined arithmetic, membership, and comparison operators for signed integer arithmetic.

For fixed-point arithmetic, checks can be suppressed. But if checks are enabled then fixed-point values are always checked for overflow against the base type for intermediate expressions (that is such checks always operate in the equivalent of STRICT mode).

For floating-point, on nearly all architectures, Machine_Overflows is False, and IEEE infinities are generated, so overflow exceptions are never raised. If you want to avoid infinities, and check that final results of expressions are in range, then you can declare a constrained floating-point type, and range checks will be carried out in the normal manner (with infinite values always failing all range checks).


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