By default, GNAT generates all run-time checks, except stack overflow checks and checks for access before elaboration on subprogram calls. The latter are not required in default mode because all necessary checking is done at compile time.
The GNAT switch, -gnatp allows you to modify this default; see
Run-Time Checks.
Our experience is that the default is suitable for most development purposes.
Elaboration checks are off by default and also not needed by default since GNAT uses a static elaboration analysis approach that avoids the need for run-time checking. This manual contains a full chapter discussing the issue of elaboration checks and you should read this chapter if the default is not satisfactory for your use,
For validity checks, the minimal checks required by the Ada Reference
Manual (for case statements and assignments to array elements) are enabled
by default. You can suppress these by using the -gnatVn switch.
Note that in Ada 83, there were no validity checks, so if the Ada 83 mode
is acceptable (or when comparing GNAT performance with an Ada 83 compiler),
it may be reasonable to routinely use -gnatVn. Validity checks
are also suppressed entirely if you use -gnatp.
Note that the setting of the switches controls the default setting of
the checks. You may modify them using either pragma Suppress (to
remove checks) or pragma Unsuppress (to add back suppressed
checks) in your program source.