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3.2.6 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), checks for access before elaboration on subprogram calls, or stack overflow checking. 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;
          X3 : Integer := Integer'Last;
          X4 : Integer range 1 .. 5 := 5;
          F  : Float := 2.0E+20;
          X1 := X1 + 1;
          X2 := X2 + 1;
          X3 := Integer (F);
          X4 := Integer (F);

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. Thus the first assignment to X1 raises a Constraint_Error exception only if -gnato is set.

The second increment operation results in a violation of the explicit range constraint, and such range checks are always performed (unless specifically suppressed with a pragma suppress or the use of -gnatp).

The two conversions of F both result in values that are outside the base range of type Integer and thus will raise Constraint_Error exceptions only if -gnato is used. The fact that the result of the second conversion is assigned to variable X4 with a restricted range is irrelevant, since the problem is in the conversion, not the assignment.

Basically the rule is that in the default mode (-gnato not used), the generated code assures that all integer variables stay within their declared ranges, or within the base range if there is no declared range. This prevents any serious problems like indexes out of range for array operations.

What is not checked in default mode is an overflow that results in an in-range, but incorrect value. In the above example, the assignments to X1, X2, X3 all give results that are within the range of the target variable, but the result is wrong in the sense that it is too large to be represented correctly. Typically the assignment to X1 will result in wrap around to the largest negative number. The conversions of F will result in some Integer value and if that integer value is out of the X4 range then the subsequent assignment would generate an exception.

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.
Activates stack overflow checking. For full details of the effect and use of this switch see Stack Overflow Checking.

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.