AARM95 - Suppressing Checks
Author: JTC1/SC22/WG9/ARG, by Randall Brukardt, ARG Editor

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# 11.5 Suppressing Checks

1
A pragma Suppress gives permission to an implementation to omit certain language-defined checks.
2
A language-defined check (or simply, a ``check'') is one of the situations defined by this International Standard that requires a check to be made at run time to determine whether some condition is true. A check fails when the condition being checked is false, causing an exception to be raised.
2.a
Discussion: All such checks are defined under ``Dynamic Semantics'' in clauses and subclauses throughout the standard.

#### Syntax

3
The form of a pragma Suppress is as follows:
4
pragma Suppress(identifier [, [On =>] name]);
5
A pragma Suppress is allowed only immediately within a declarative_part, immediately within a package_specification, or as a configuration pragma.

#### Legality Rules

6
The identifier shall be the name of a check. The name (if present) shall statically denote some entity.
7
For a pragma Suppress that is immediately within a package_specification and includes a name, the name shall denote an entity (or several overloaded subprograms) declared immediately within the package_specification.

#### Static Semantics

8
A pragma Suppress gives permission to an implementation to omit the named check from the place of the pragma to the end of the innermost enclosing declarative region, or, if the pragma is given in a package_specification and includes a name, to the end of the scope of the named entity. If the pragma includes a name, the permission applies only to checks performed on the named entity, or, for a subtype, on objects and values of its type. Otherwise, the permission applies to all entities. If permission has been given to suppress a given check, the check is said to be suppressed.
8.a
Ramification: A check is suppressed even if the implementation chooses not to actually generate better code. This allows the implementation to raise Program_Error, for example, if the erroneousness is detected.
9
The following are the language-defined checks:
10

- [The following checks correspond to situations in which the exception Constraint_Error is raised upon failure.]

11/1

{8652/0036} Access_Check
  [When evaluating a dereference (explicit or implicit), check that the value of the name is not null. When passing an actual parameter to a formal access parameter, check that the value of the actual parameter is not null. When evaluating a discriminant_association for an access discriminant, check that the value of the discriminant is not null.]

12

Discriminant_Check
  [Check that the discriminants of a composite value have the values imposed by a discriminant constraint. Also, when accessing a record component, check that it exists for the current discriminant values.]

13

Division_Check
  [Check that the second operand is not zero for the operations /, rem and mod.]

14

Index_Check
  [Check that the bounds of an array value are equal to the corresponding bounds of an index constraint. Also, when accessing a component of an array object, check for each dimension that the given index value belongs to the range defined by the bounds of the array object. Also, when accessing a slice of an array object, check that the given discrete range is compatible with the range defined by the bounds of the array object.]

15

Length_Check
  [Check that two arrays have matching components, in the case of array subtype conversions, and logical operators for arrays of boolean components.]

16

Overflow_Check
  [Check that a scalar value is within the base range of its type, in cases where the implementation chooses to raise an exception instead of returning the correct mathematical result.]

17

Range_Check
  [Check that a scalar value satisfies a range constraint. Also, for the elaboration of a subtype_indication, check that the constraint (if present) is compatible with the subtype denoted by the subtype_mark. Also, for an aggregate, check that an index or discriminant value belongs to the corresponding subtype. Also, check that when the result of an operation yields an array, the value of each component belongs to the component subtype.]

18

Tag_Check
  [Check that operand tags in a dispatching call are all equal. Check for the correct tag on tagged type conversions, for an assignment_statement, and when returning a tagged limited object from a function.]

19

- [The following checks correspond to situations in which the exception Program_Error is raised upon failure.]

20

Elaboration_Check
  [When a subprogram or protected entry is called, a task activation is accomplished, or a generic instantiation is elaborated, check that the body of the corresponding unit has already been elaborated.]

21

Accessibility_Check
  [Check the accessibility level of an entity or view.]

22

- [The following check corresponds to situations in which the exception Storage_Error is raised upon failure.]

23

Storage_Check
  [Check that evaluation of an allocator does not require more space than is available for a storage pool. Check that the space available for a task or subprogram has not been exceeded.]

23.a
Reason: We considered splitting this out into three categories: Pool_Check (for allocators), Stack_Check (for stack usage), and Heap_Check (for implicit use of the heap -- use of the heap other than through an allocator). Storage_Check would then represent the union of these three. However, there seems to be no compelling reason to do this, given that it is not feasible to split Storage_Error.
24

- [The following check corresponds to all situations in which any predefined exception is raised.]

25

All_Checks
  Represents the union of all checks; [suppressing All_Checks suppresses all checks.]

25.a
Ramification: All_Checks includes both language-defined and implementation-defined checks.

#### Erroneous Execution

26
If a given check has been suppressed, and the corresponding error situation occurs, the execution of the program is erroneous.

#### Implementation Permissions

27
An implementation is allowed to place restrictions on Suppress pragmas. An implementation is allowed to add additional check names, with implementation-defined semantics. When Overflow_Check has been suppressed, an implementation may also suppress an unspecified subset of the Range_Checks.
27.a
Reason: The permission to restrict is given so the implementation can give an error message when the requested suppression is nonsense, such as suppressing a Range_Check on a task type. It would be verbose and pointless to list all the cases of nonsensical language-defined checks in the standard, and since the list of checks is open-ended, we can't list the restrictions for implementation-defined checks anyway.
27.b
Implementation defined: Implementation-defined check names.
27.c
Discussion: For Overflow_Check, the intention is that the implementation will suppress any Range_Checks that are implemented in the same manner as Overflow_Checks (unless they are free).

#### Implementation Advice

28
The implementation should minimize the code executed for checks that have been suppressed.
28.a
Implementation Note: However, if a given check comes for free (for example, the hardware automatically performs the check in parallel with doing useful work) or nearly free (for example, the check is a tiny portion of an expensive run-time system call), the implementation should not bother to suppress the check. Similarly, if the implementation detects the failure at compile time and provides a warning message, there is no need to actually suppress the check.
NOTES
29
2 There is no guarantee that a suppressed check is actually removed; hence a pragma Suppress should be used only for efficiency reasons.

#### Examples

30
Examples of suppressing checks:
31
`pragma Suppress(Range_Check);
pragma Suppress(Index_Check, On => Table);`

#### Extensions to Ada 83

31.a
A pragma Suppress is allowed as a configuration pragma. A pragma Suppress without a name is allowed in a package_specification.
31.b
Additional check names are added. We allow implementations to define their own checks.

#### Wording Changes from Ada 83

31.c
We define the checks in a distributed manner. Therefore, the long list of what checks apply to what is merely a NOTE.
31.d
We have removed the detailed rules about what is allowed in a pragma Suppress, and allow implementations to invent their own. The RM83 rules weren't quite right, and such a change is necessary anyway in the presence of implementation-defined checks.
31.e
We make it clear that the difference between a Range_Check and an Overflow_Check is fuzzy. This was true in Ada 83, given RM83-11.6, but it was not clear. We considered removing Overflow_Check from the language or making it obsolescent, just as we did for Numeric_Error. However, we kept it for upward compatibility, and because it may be useful on machines where range checking costs more than overflow checking, but overflow checking still costs something. Different compilers will suppress different checks when asked to suppress Overflow_Check -- the non-uniformity in this case is not harmful, and removing it would have a serious impact on optimizers.
31.f
Under Access_Check, dereferences cover the cases of selected_component, indexed_component, slice, and attribute that are listed in RM83, as well as the new explicit_dereference, which was included in selected_component in RM83.

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