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3.2.5 Run-Time Checks

If you compile with the default options, GNAT will insert many run-time checks into the compiled code, including code that performs range checking against constraints, but not arithmetic overflow checking for integer operations (including division by zero) or checks for access before elaboration on subprogram calls. All other run-time checks, as required by the Ada 95 Reference Manual, are generated by default. The following gcc switches refine this default behavior:

Suppress all run-time checks as though pragma Suppress (all_checks) had been present in the source. Validity checks are also suppressed (in other words -gnatp also implies -gnatVn. Use this switch to improve the performance of the code at the expense of safety in the presence of invalid data or program bugs.
Enables overflow checking for integer operations. This causes GNAT to generate slower and larger executable programs by adding code to check for overflow (resulting in raising Constraint_Error as required by standard Ada semantics). These overflow checks correspond to situations in which the true value of the result of an operation may be outside the base range of the result type. The following example shows the distinction:
          X1 : Integer := Integer'Last;
          X2 : Integer range 1 .. 5 := 5;
          X1 := X1 + 1;   -- -gnato required to catch the Constraint_Error
          X2 := X2 + 1;   -- range check, -gnato has no effect here

Here the first addition results in a value that is outside the base range of Integer, and hence requires an overflow check for detection of the constraint error. The second increment operation results in a violation of the explicit range constraint, and such range checks are always performed. Basically the compiler can assume that in the absence of the -gnato switch that any value of type xxx is in range of the base type of xxx.

Note that the -gnato switch does not affect the code generated for any floating-point operations; it applies only to integer semantics). For floating-point, GNAT has the Machine_Overflows attribute set to False and the normal mode of operation is to generate IEEE NaN and infinite values on overflow or invalid operations (such as dividing 0.0 by 0.0).

The reason that we distinguish overflow checking from other kinds of range constraint checking is that a failure of an overflow check can generate an incorrect value, but cannot cause erroneous behavior. This is unlike the situation with a constraint check on an array subscript, where failure to perform the check can result in random memory description, or the range check on a case statement, where failure to perform the check can cause a wild jump.

Note again that -gnato is off by default, so overflow checking is not performed in default mode. This means that out of the box, with the default settings, GNAT does not do all the checks expected from the language description in the Ada Reference Manual. If you want all constraint checks to be performed, as described in this Manual, then you must explicitly use the -gnato switch either on the gnatmake or gcc command.

Enables dynamic checks for access-before-elaboration on subprogram calls and generic instantiations. For full details of the effect and use of this switch, See Compiling Using gcc.

The setting of these switches only controls the default setting of the checks. You may modify them using either Suppress (to remove checks) or Unsuppress (to add back suppressed checks) pragmas in the program source.