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3.2.6 Run-Time Checks

By default, the following checks are suppressed: integer overflow checks, stack overflow checks, and checks for access before elaboration on subprogram calls. All other checks, including range checks and array bounds checks, are turned on by default. The following gcc switches refine this default behavior.

-gnatp
This switch causes the unit to be compiled as though pragma Suppress (All_checks) had been present in the source. Validity checks are also eliminated (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.

Note that when checks are suppressed, the compiler is allowed, but not required, to omit the checking code. If the run-time cost of the checking code is zero or near-zero, the compiler will generate it even if checks are suppressed. In particular, if the compiler can prove that a certain check will necessarily fail, it will generate code to do an unconditional “raise”, even if checks are suppressed. The compiler warns in this case. Another case in which checks may not be eliminated is when they are embedded in certain run time routines such as math library routines.

Of course, run-time checks are omitted whenever the compiler can prove that they will not fail, whether or not checks are suppressed.

Note that if you suppress a check that would have failed, program execution is erroneous, which means the behavior is totally unpredictable. The program might crash, or print wrong answers, or do anything else. It might even do exactly what you wanted it to do (and then it might start failing mysteriously next week or next year). The compiler will generate code based on the assumption that the condition being checked is true, which can result in erroneous execution if that assumption is wrong.

The checks subject to suppression include all the checks defined by the Ada standard, the additional implementation defined checks Alignment_Check, Duplicated_Tag_Check, Predicate_Check, and Validity_Check, as well as any checks introduced using pragma Check_Name. Note that Atomic_Synchronization is not automatically suppressed by use of this option.

If the code depends on certain checks being active, you can use pragma Unsuppress either as a configuration pragma or as a local pragma to make sure that a specified check is performed even if gnatp is specified.

The -gnatp switch has no effect if a subsequent -gnat-p switch appears.

-gnat-p
This switch cancels the effect of a previous gnatp switch.
-gnato??
This switch controls the mode used for computing intermediate arithmetic integer operations, and also enables overflow checking. For a full description of overflow mode and checking control, see the “Overflow Check Handling in GNAT” appendix in this User's Guide.

Overflow checks are always enabled by this switch. The argument controls the mode, using the codes

If two digits are present after -gnato then the first digit sets the mode for expressions outside assertions, and the second digit sets the mode for expressions within assertions. Here assertions is used in the technical sense (which includes for example precondition and postcondition expressions).

If one digit is present, the corresponding mode is applicable to both expressions within and outside assertion expressions.

If no digits are present, the default is to enable overflow checks and set STRICT mode for both kinds of expressions. This is compatible with the use of -gnato in previous versions of GNAT.

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, unlike for example the failure of a range check, can result in an incorrect value, but cannot cause random memory destruction (like an out of range subscript), or a wild jump (from an out of range case value). Overflow checking is also quite expensive in time and space, since in general it requires the use of double length arithmetic.

Note again that the default is -gnato00, 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.

-gnatE
Enables dynamic checks for access-before-elaboration on subprogram calls and generic instantiations. Note that -gnatE is not necessary for safety, because in the default mode, GNAT ensures statically that the checks would not fail. For full details of the effect and use of this switch, See Compiling with gcc.
-fstack-check
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.