GNAT Reference Manual: Implementation Advice

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# 5. Implementation Advice

The main text of the Ada Reference Manual describes the required behavior of all Ada compilers, and the GNAT compiler conforms to these requirements.

In addition, there are sections throughout the Ada Reference Manual headed by the phrase "Implementation advice". These sections are not normative, i.e., they do not specify requirements that all compilers must follow. Rather they provide advice on generally desirable behavior. You may wonder why they are not requirements. The most typical answer is that they describe behavior that seems generally desirable, but cannot be provided on all systems, or which may be undesirable on some systems.

As far as practical, GNAT follows the implementation advice sections in the Ada Reference Manual. This chapter contains a table giving the reference manual section number, paragraph number and several keywords for each advice. Each entry consists of the text of the advice followed by the GNAT interpretation of this advice. Most often, this simply says "followed", which means that GNAT follows the advice. However, in a number of cases, GNAT deliberately deviates from this advice, in which case the text describes what GNAT does and why.

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## 1.1.3(20): Error Detection

If an implementation detects the use of an unsupported Specialized Needs Annex feature at run time, it should raise `Program_Error` if feasible. Not relevant. All specialized needs annex features are either supported, or diagnosed at compile time.

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## 1.1.3(31): Child Units

If an implementation wishes to provide implementation-defined extensions to the functionality of a language-defined library unit, it should normally do so by adding children to the library unit. Followed.

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## 1.1.5(12): Bounded Errors

If an implementation detects a bounded error or erroneous execution, it should raise `Program_Error`. Followed in all cases in which the implementation detects a bounded error or erroneous execution. Not all such situations are detected at runtime.

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## 2.8(16): Pragmas

Normally, implementation-defined pragmas should have no semantic effect for error-free programs; that is, if the implementation-defined pragmas are removed from a working program, the program should still be legal, and should still have the same semantics. The following implementation defined pragmas are exceptions to this rule:

`Abort_Defer`
  Affects semantics
`Ada_83`
  Affects legality
`Assert`
  Affects semantics
`CPP_Class`
  Affects semantics
`CPP_Constructor`
  Affects semantics
`Debug`
  Affects semantics
`Interface_Name`
  Affects semantics
`Machine_Attribute`
  Affects semantics
`Unimplemented_Unit`
  Affects legality
`Unchecked_Union`
  Affects semantics

In each of the above cases, it is essential to the purpose of the pragma that this advice not be followed. For details see the separate section on implementation defined pragmas.

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## 2.8(17-19): Pragmas

Normally, an implementation should not define pragmas that can make an illegal program legal, except as follows: A pragma used to complete a declaration, such as a pragma `Import`; A pragma used to configure the environment by adding, removing, or replacing `library_items`. See response to paragraph 16 of this same section.

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## 3.5.2(5): Alternative Character Sets

If an implementation supports a mode with alternative interpretations for `Character` and `Wide_Character`, the set of graphic characters of `Character` should nevertheless remain a proper subset of the set of graphic characters of `Wide_Character`. Any character set "localizations" should be reflected in the results of the subprograms defined in the language-defined package `Characters.Handling` (see A.3) available in such a mode. In a mode with an alternative interpretation of `Character`, the implementation should also support a corresponding change in what is a legal `identifier_letter`. Not all wide character modes follow this advice, in particular the JIS and IEC modes reflect standard usage in Japan, and in these encoding, the upper half of the Latin-1 set is not part of the wide-character subset, since the most significant bit is used for wide character encoding. However, this only applies to the external forms. Internally there is no such restriction.

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## 3.5.4(28): Integer Types

An implementation should support `Long_Integer` in addition to `Integer` if the target machine supports 32-bit (or longer) arithmetic. No other named integer subtypes are recommended for package `Standard`. Instead, appropriate named integer subtypes should be provided in the library package `Interfaces` (see B.2). `Long_Integer` is supported. Other standard integer types are supported so this advice is not fully followed. These types are supported for convenient interface to C, and so that all hardware types of the machine are easily available.

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## 3.5.4(29): Integer Types

An implementation for a two's complement machine should support modular types with a binary modulus up to `System.Max_Int*2+2`. An implementation should support a non-binary modules up to `Integer'Last`. Followed.

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## 3.5.5(8): Enumeration Values

For the evaluation of a call on `S'Pos` for an enumeration subtype, if the value of the operand does not correspond to the internal code for any enumeration literal of its type (perhaps due to an un-initialized variable), then the implementation should raise `Program_Error`. This is particularly important for enumeration types with noncontiguous internal codes specified by an enumeration_representation_clause. Followed.

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## 3.5.7(17): Float Types

An implementation should support `Long_Float` in addition to `Float` if the target machine supports 11 or more digits of precision. No other named floating point subtypes are recommended for package `Standard`. Instead, appropriate named floating point subtypes should be provided in the library package `Interfaces` (see B.2). `Short_Float` and `Long_Long_Float` are also provided. The former provides improved compatibility with other implementations supporting this type. The latter corresponds to the highest precision floating-point type supported by the hardware. On most machines, this will be the same as `Long_Float`, but on some machines, it will correspond to the IEEE extended form. The notable case is all ia32 (x86) implementations, where `Long_Long_Float` corresponds to the 80-bit extended precision format supported in hardware on this processor. Note that the 128-bit format on SPARC is not supported, since this is a software rather than a hardware format.

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## 3.6.2(11): Multidimensional Arrays

An implementation should normally represent multidimensional arrays in row-major order, consistent with the notation used for multidimensional array aggregates (see 4.3.3). However, if a pragma `Convention` (`Fortran`, ...) applies to a multidimensional array type, then column-major order should be used instead (see B.5, "Interfacing with Fortran"). Followed.

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## 9.6(30-31): Duration'Small

Whenever possible in an implementation, the value of `Duration'Small` should be no greater than 100 microseconds. Followed. (`Duration'Small` = 10**(-9)).

The time base for `delay_relative_statements` should be monotonic; it need not be the same time base as used for `Calendar.Clock`. Followed.

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## 10.2.1(12): Consistent Representation

In an implementation, a type declared in a pre-elaborated package should have the same representation in every elaboration of a given version of the package, whether the elaborations occur in distinct executions of the same program, or in executions of distinct programs or partitions that include the given version. Followed, except in the case of tagged types. Tagged types involve implicit pointers to a local copy of a dispatch table, and these pointers have representations which thus depend on a particular elaboration of the package. It is not easy to see how it would be possible to follow this advice without severely impacting efficiency of execution.

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## 11.4.1(19): Exception Information

`Exception_Message` by default and `Exception_Information` should produce information useful for debugging. `Exception_Message` should be short, about one line. `Exception_Information` can be long. `Exception_Message` should not include the `Exception_Name`. `Exception_Information` should include both the `Exception_Name` and the `Exception_Message`. Followed. For each exception that doesn't have a specified `Exception_Message`, the compiler generates one containing the location of the raise statement. This location has the form "file:line", where file is the short file name (without path information) and line is the line number in the file. Note that in the case of the Zero Cost Exception mechanism, these messages become redundant with the Exception_Information that contains a full backtrace of the calling sequence, so they are disabled. To disable explicitly the generation of the source location message, use the Pragma `Discard_Names`.

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## 11.5(28): Suppression of Checks

The implementation should minimize the code executed for checks that have been suppressed. Followed.

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## 13.1 (21-24): Representation Clauses

The recommended level of support for all representation items is qualified as follows: An implementation need not support representation items containing non-static expressions, except that an implementation should support a representation item for a given entity if each non-static expression in the representation item is a name that statically denotes a constant declared before the entity. Followed. In fact, GNAT goes beyond the recommended level of support by allowing nonstatic expressions in some representation clauses even without the need to declare constants initialized with the values of such expressions. For example:

      X : Integer;
      Y : Float;
      for Y'Address use X'Address;>>

An implementation need not support a specification for the `Size` for a given composite subtype, nor the size or storage place for an object (including a component) of a given composite subtype, unless the constraints on the subtype and its composite subcomponents (if any) are all static constraints. Followed. Size Clauses are not permitted on non-static components, as described above.

An aliased component, or a component whose type is by-reference, should always be allocated at an addressable location. Followed.

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## 13.2(6-8): Packed Types

If a type is packed, then the implementation should try to minimize storage allocated to objects of the type, possibly at the expense of speed of accessing components, subject to reasonable complexity in addressing calculations. The recommended level of support pragma `Pack` is:

For a packed record type, the components should be packed as tightly as possible subject to the Sizes of the component subtypes, and subject to any `record_representation_clause` that applies to the type; the implementation may, but need not, reorder components or cross aligned word boundaries to improve the packing. A component whose `Size` is greater than the word size may be allocated an integral number of words. Followed. Tight packing of arrays is supported for all component sizes up to 64-bits. If the array component size is 1 (that is to say, if the component is a boolean type or an enumeration type with two values) then values of the type are implicitly initialized to zero. This happens both for objects of the packed type, and for objects that have a subcomponent of the packed type.

An implementation should support Address clauses for imported subprograms. Followed.

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## 13.3(14-19): Address Clauses

For an array X, `X'Address` should point at the first component of the array, and not at the array bounds. Followed.

The recommended level of support for the `Address` attribute is:

`X'Address` should produce a useful result if X is an object that is aliased or of a by-reference type, or is an entity whose `Address` has been specified. Followed. A valid address will be produced even if none of those conditions have been met. If necessary, the object is forced into memory to ensure the address is valid.

An implementation should support `Address` clauses for imported subprograms. Followed.

Objects (including subcomponents) that are aliased or of a by-reference type should be allocated on storage element boundaries. Followed.

If the `Address` of an object is specified, or it is imported or exported, then the implementation should not perform optimizations based on assumptions of no aliases. Followed.

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## 13.3(29-35): Alignment Clauses

The recommended level of support for the `Alignment` attribute for subtypes is:

An implementation should support specified Alignments that are factors and multiples of the number of storage elements per word, subject to the following: Followed.

An implementation need not support specified `Alignment`s for combinations of `Size`s and `Alignment`s that cannot be easily loaded and stored by available machine instructions. Followed.

An implementation need not support specified `Alignment`s that are greater than the maximum `Alignment` the implementation ever returns by default. Followed.

The recommended level of support for the `Alignment` attribute for objects is:

Same as above, for subtypes, but in addition: Followed.

For stand-alone library-level objects of statically constrained subtypes, the implementation should support all `Alignment`s supported by the target linker. For example, page alignment is likely to be supported for such objects, but not for subtypes. Followed.

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## 13.3(42-43): Size Clauses

The recommended level of support for the `Size` attribute of objects is:

A `Size` clause should be supported for an object if the specified `Size` is at least as large as its subtype's `Size`, and corresponds to a size in storage elements that is a multiple of the object's `Alignment` (if the `Alignment` is nonzero). Followed.

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## 13.3(50-56): Size Clauses

If the `Size` of a subtype is specified, and allows for efficient independent addressability (see 9.10) on the target architecture, then the `Size` of the following objects of the subtype should equal the `Size` of the subtype:

Aliased objects (including components). Followed.

`Size` clause on a composite subtype should not affect the internal layout of components. Followed. But note that this can be overridden by use of the implementation pragma Implicit_Packing in the case of packed arrays.

The recommended level of support for the `Size` attribute of subtypes is: The `Size` (if not specified) of a static discrete or fixed point subtype should be the number of bits needed to represent each value belonging to the subtype using an unbiased representation, leaving space for a sign bit only if the subtype contains negative values. If such a subtype is a first subtype, then an implementation should support a specified `Size` for it that reflects this representation. Followed.

For a subtype implemented with levels of indirection, the `Size` should include the size of the pointers, but not the size of what they point at. Followed.

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## 13.3(71-73): Component Size Clauses

The recommended level of support for the `Component_Size` attribute is: An implementation need not support specified `Component_Sizes` that are less than the `Size` of the component subtype. Followed.

An implementation should support specified `Component_Size`s that are factors and multiples of the word size. For such `Component_Size`s, the array should contain no gaps between components. For other `Component_Size`s (if supported), the array should contain no gaps between components when packing is also specified; the implementation should forbid this combination in cases where it cannot support a no-gaps representation. Followed.

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## 13.4(9-10): Enumeration Representation Clauses

The recommended level of support for enumeration representation clauses is:

An implementation need not support enumeration representation clauses for boolean types, but should at minimum support the internal codes in the range `System.Min_Int.System.Max_Int`. Followed.

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## 13.5.1(17-22): Record Representation Clauses

The recommended level of support for
`record_representation_clauses` is:

An implementation should support storage places that can be extracted with a load, mask, shift sequence of machine code, and set with a load, shift, mask, store sequence, given the available machine instructions and run-time model. Followed.

A storage place should be supported if its size is equal to the `Size` of the component subtype, and it starts and ends on a boundary that obeys the `Alignment` of the component subtype. Followed.

If the default bit ordering applies to the declaration of a given type, then for a component whose subtype's `Size` is less than the word size, any storage place that does not cross an aligned word boundary should be supported. Followed.

An implementation may reserve a storage place for the tag field of a tagged type, and disallow other components from overlapping that place. Followed. The storage place for the tag field is the beginning of the tagged record, and its size is Address'Size. GNAT will reject an explicit component clause for the tag field.

An implementation need not support a `component_clause` for a component of an extension part if the storage place is not after the storage places of all components of the parent type, whether or not those storage places had been specified. Followed. The above advice on record representation clauses is followed, and all mentioned features are implemented.

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## 13.5.2(5): Storage Place Attributes

If a component is represented using some form of pointer (such as an offset) to the actual data of the component, and this data is contiguous with the rest of the object, then the storage place attributes should reflect the place of the actual data, not the pointer. If a component is allocated discontinuously from the rest of the object, then a warning should be generated upon reference to one of its storage place attributes. Followed. There are no such components in GNAT.

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## 13.5.3(7-8): Bit Ordering

The recommended level of support for the non-default bit ordering is: If `Word_Size` = `Storage_Unit`, then the implementation should support the non-default bit ordering in addition to the default bit ordering. Followed. Word size does not equal storage size in this implementation. Thus non-default bit ordering is not supported.

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## 13.7(37): Address as Private

`Address` should be of a private type. Followed.

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## 13.7.1(16): Address Operations

Operations in `System` and its children should reflect the target environment semantics as closely as is reasonable. For example, on most machines, it makes sense for address arithmetic to "wrap around". Operations that do not make sense should raise `Program_Error`. Followed. Address arithmetic is modular arithmetic that wraps around. No operation raises `Program_Error`, since all operations make sense.

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## 13.9(14-17): Unchecked Conversion

The `Size` of an array object should not include its bounds; hence, the bounds should not be part of the converted data. Followed.

The implementation should not generate unnecessary run-time checks to ensure that the representation of S is a representation of the target type. It should take advantage of the permission to return by reference when possible. Restrictions on unchecked conversions should be avoided unless required by the target environment. Followed. There are no restrictions on unchecked conversion. A warning is generated if the source and target types do not have the same size since the semantics in this case may be target dependent.

The recommended level of support for unchecked conversions is: Unchecked conversions should be supported and should be reversible in the cases where this clause defines the result. To enable meaningful use of unchecked conversion, a contiguous representation should be used for elementary subtypes, for statically constrained array subtypes whose component subtype is one of the subtypes described in this paragraph, and for record subtypes without discriminants whose component subtypes are described in this paragraph. Followed.

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## 13.11(23-25): Implicit Heap Usage

An implementation should document any cases in which it dynamically allocates heap storage for a purpose other than the evaluation of an allocator. Followed, the only other points at which heap storage is dynamically allocated are as follows:

- At initial elaboration time, to allocate dynamically sized global objects.

- To allocate space for a task when a task is created.

- To extend the secondary stack dynamically when needed. The secondary stack is used for returning variable length results.

A default (implementation-provided) storage pool for an access-to-constant type should not have overhead to support deallocation of individual objects. Followed.

A storage pool for an anonymous access type should be created at the point of an allocator for the type, and be reclaimed when the designated object becomes inaccessible. Followed.

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## 13.11.2(17): Unchecked De-allocation

For a standard storage pool, `Free` should actually reclaim the storage. Followed.

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## 13.13.2(17): Stream Oriented Attributes

If a stream element is the same size as a storage element, then the normal in-memory representation should be used by `Read` and `Write` for scalar objects. Otherwise, `Read` and `Write` should use the smallest number of stream elements needed to represent all values in the base range of the scalar type.

Followed. By default, GNAT uses the interpretation suggested by AI-195, which specifies using the size of the first subtype. However, such an implementation is based on direct binary representations and is therefore target- and endianness-dependent. To address this issue, GNAT also supplies an alternate implementation of the stream attributes `Read` and `Write`, which uses the target-independent XDR standard representation for scalar types. The XDR implementation is provided as an alternative body of the `System.Stream_Attributes` package, in the file ``s-stratt-xdr.adb'` in the GNAT library. There is no ``s-stratt-xdr.ads'` file. In order to install the XDR implementation, do the following:

1. Replace the default implementation of the `System.Stream_Attributes` package with the XDR implementation. For example on a Unix platform issue the commands:

       $ mv s-stratt.adb s-stratt-default.adb
       $ mv s-stratt-xdr.adb s-stratt.adb

2. Rebuild the GNAT run-time library as documented in section `GNAT and Libraries' in GNAT User's Guide.

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## A.1(52): Names of Predefined Numeric Types

If an implementation provides additional named predefined integer types, then the names should end with ``Integer'` as in ``Long_Integer'`. If an implementation provides additional named predefined floating point types, then the names should end with ``Float'` as in ``Long_Float'`. Followed.

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## A.3.2(49): `Ada.Characters.Handling`

If an implementation provides a localized definition of `Character` or `Wide_Character`, then the effects of the subprograms in `Characters.Handling` should reflect the localizations. See also 3.5.2. Followed. GNAT provides no such localized definitions.

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## A.4.4(106): Bounded-Length String Handling

Bounded string objects should not be implemented by implicit pointers and dynamic allocation. Followed. No implicit pointers or dynamic allocation are used.

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## A.5.2(46-47): Random Number Generation

Any storage associated with an object of type `Generator` should be reclaimed on exit from the scope of the object. Followed.

If the generator period is sufficiently long in relation to the number of distinct initiator values, then each possible value of `Initiator` passed to `Reset` should initiate a sequence of random numbers that does not, in a practical sense, overlap the sequence initiated by any other value. If this is not possible, then the mapping between initiator values and generator states should be a rapidly varying function of the initiator value. Followed. The generator period is sufficiently long for the first condition here to hold true.

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## A.10.7(23): `Get_Immediate`

The `Get_Immediate` procedures should be implemented with unbuffered input. For a device such as a keyboard, input should be available if a key has already been typed, whereas for a disk file, input should always be available except at end of file. For a file associated with a keyboard-like device, any line-editing features of the underlying operating system should be disabled during the execution of `Get_Immediate`. Followed on all targets except VxWorks. For VxWorks, there is no way to provide this functionality that does not result in the input buffer being flushed before the `Get_Immediate` call. A special unit `Interfaces.Vxworks.IO` is provided that contains routines to enable this functionality.

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## B.1(39-41): Pragma `Export`

If an implementation supports pragma `Export` to a given language, then it should also allow the main subprogram to be written in that language. It should support some mechanism for invoking the elaboration of the Ada library units included in the system, and for invoking the finalization of the environment task. On typical systems, the recommended mechanism is to provide two subprograms whose link names are `adainit` and `adafinal`. `adainit` should contain the elaboration code for library units. `adafinal` should contain the finalization code. These subprograms should have no effect the second and subsequent time they are called. Followed.

Automatic elaboration of pre-elaborated packages should be provided when pragma `Export` is supported. Followed when the main program is in Ada. If the main program is in a foreign language, then `adainit` must be called to elaborate pre-elaborated packages.

For each supported convention L other than `Intrinsic`, an implementation should support `Import` and `Export` pragmas for objects of L-compatible types and for subprograms, and pragma `Convention` for L-eligible types and for subprograms, presuming the other language has corresponding features. Pragma `Convention` need not be supported for scalar types. Followed.

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## B.2(12-13): Package `Interfaces`

For each implementation-defined convention identifier, there should be a child package of package Interfaces with the corresponding name. This package should contain any declarations that would be useful for interfacing to the language (implementation) represented by the convention. Any declarations useful for interfacing to any language on the given hardware architecture should be provided directly in `Interfaces`. Followed. An additional package not defined in the Ada Reference Manual is `Interfaces.CPP`, used for interfacing to C++.

An implementation supporting an interface to C, COBOL, or Fortran should provide the corresponding package or packages described in the following clauses. Followed. GNAT provides all the packages described in this section.

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## B.3(63-71): Interfacing with C

An implementation should support the following interface correspondences between Ada and C. Followed.

An Ada procedure corresponds to a void-returning C function. Followed.

An Ada function corresponds to a non-void C function. Followed.

An Ada `in` scalar parameter is passed as a scalar argument to a C function. Followed.

An Ada `in` parameter of an access-to-object type with designated type T is passed as a `t*` argument to a C function, where t is the C type corresponding to the Ada type T. Followed.

An Ada access T parameter, or an Ada `out` or `in out` parameter of an elementary type T, is passed as a `t*` argument to a C function, where t is the C type corresponding to the Ada type T. In the case of an elementary `out` or `in out` parameter, a pointer to a temporary copy is used to preserve by-copy semantics. Followed.

An Ada parameter of a record type T, of any mode, is passed as a `t*` argument to a C function, where t is the C structure corresponding to the Ada type T. Followed. This convention may be overridden by the use of the C_Pass_By_Copy pragma, or Convention, or by explicitly specifying the mechanism for a given call using an extended import or export pragma.

An Ada parameter of an array type with component type T, of any mode, is passed as a `t*` argument to a C function, where t is the C type corresponding to the Ada type T. Followed.

An Ada parameter of an access-to-subprogram type is passed as a pointer to a C function whose prototype corresponds to the designated subprogram's specification. Followed.

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## B.4(95-98): Interfacing with COBOL

An Ada implementation should support the following interface correspondences between Ada and COBOL. Followed.

An Ada access T parameter is passed as a ``BY REFERENCE'` data item of the COBOL type corresponding to T. Followed.

An Ada in scalar parameter is passed as a ``BY CONTENT'` data item of the corresponding COBOL type. Followed.

Any other Ada parameter is passed as a ``BY REFERENCE'` data item of the COBOL type corresponding to the Ada parameter type; for scalars, a local copy is used if necessary to ensure by-copy semantics. Followed.

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## B.5(22-26): Interfacing with Fortran

An Ada implementation should support the following interface correspondences between Ada and Fortran: Followed.

An Ada procedure corresponds to a Fortran subroutine. Followed.

An Ada function corresponds to a Fortran function. Followed.

An Ada parameter of an elementary, array, or record type T is passed as a T argument to a Fortran procedure, where T is the Fortran type corresponding to the Ada type T, and where the INTENT attribute of the corresponding dummy argument matches the Ada formal parameter mode; the Fortran implementation's parameter passing conventions are used. For elementary types, a local copy is used if necessary to ensure by-copy semantics. Followed.

An Ada parameter of an access-to-subprogram type is passed as a reference to a Fortran procedure whose interface corresponds to the designated subprogram's specification. Followed.

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## C.1(3-5): Access to Machine Operations

The machine code or intrinsic support should allow access to all operations normally available to assembly language programmers for the target environment, including privileged instructions, if any. Followed.

The interfacing pragmas (see Annex B) should support interface to assembler; the default assembler should be associated with the convention identifier `Assembler`. Followed.

If an entity is exported to assembly language, then the implementation should allocate it at an addressable location, and should ensure that it is retained by the linking process, even if not otherwise referenced from the Ada code. The implementation should assume that any call to a machine code or assembler subprogram is allowed to read or update every object that is specified as exported. Followed.

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## C.1(10-16): Access to Machine Operations

The implementation should ensure that little or no overhead is associated with calling intrinsic and machine-code subprograms. Followed for both intrinsics and machine-code subprograms.

It is recommended that intrinsic subprograms be provided for convenient access to any machine operations that provide special capabilities or efficiency and that are not otherwise available through the language constructs. Followed. A full set of machine operation intrinsic subprograms is provided.

Atomic read-modify-write operations--e.g., test and set, compare and swap, decrement and test, enqueue/dequeue. Followed on any target supporting such operations.

Standard numeric functions--e.g., sin, log. Followed on any target supporting such operations.

String manipulation operations--e.g., translate and test. Followed on any target supporting such operations.

Vector operations--e.g., compare vector against thresholds. Followed on any target supporting such operations.

Direct operations on I/O ports. Followed on any target supporting such operations.

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## C.3(28): Interrupt Support

If the `Ceiling_Locking` policy is not in effect, the implementation should provide means for the application to specify which interrupts are to be blocked during protected actions, if the underlying system allows for a finer-grain control of interrupt blocking. Followed. The underlying system does not allow for finer-grain control of interrupt blocking.

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## C.3.1(20-21): Protected Procedure Handlers

Whenever possible, the implementation should allow interrupt handlers to be called directly by the hardware. Followed on any target where the underlying operating system permits such direct calls.

Whenever practical, violations of any implementation-defined restrictions should be detected before run time. Followed. Compile time warnings are given when possible.

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## C.3.2(25): Package `Interrupts`

If implementation-defined forms of interrupt handler procedures are supported, such as protected procedures with parameters, then for each such form of a handler, a type analogous to `Parameterless_Handler` should be specified in a child package of `Interrupts`, with the same operations as in the predefined package Interrupts. Followed.

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## C.4(14): Pre-elaboration Requirements

It is recommended that pre-elaborated packages be implemented in such a way that there should be little or no code executed at run time for the elaboration of entities not already covered by the Implementation Requirements. Followed. Executable code is generated in some cases, e.g. loops to initialize large arrays.

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## C.5(8): Pragma `Discard_Names`

If the pragma applies to an entity, then the implementation should reduce the amount of storage used for storing names associated with that entity. Followed.

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## C.7.2(30): The Package Task_Attributes

Some implementations are targeted to domains in which memory use at run time must be completely deterministic. For such implementations, it is recommended that the storage for task attributes will be pre-allocated statically and not from the heap. This can be accomplished by either placing restrictions on the number and the size of the task's attributes, or by using the pre-allocated storage for the first N attribute objects, and the heap for the others. In the latter case, N should be documented. Not followed. This implementation is not targeted to such a domain.

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## D.3(17): Locking Policies

The implementation should use names that end with ``_Locking'` for locking policies defined by the implementation. Followed. Two implementation-defined locking policies are defined, whose names (`Inheritance_Locking` and `Concurrent_Readers_Locking`) follow this suggestion.

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## D.4(16): Entry Queuing Policies

Names that end with ``_Queuing'` should be used for all implementation-defined queuing policies. Followed. No such implementation-defined queuing policies exist.

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## D.6(9-10): Preemptive Abort

Even though the `abort_statement` is included in the list of potentially blocking operations (see 9.5.1), it is recommended that this statement be implemented in a way that never requires the task executing the `abort_statement` to block. Followed.

On a multi-processor, the delay associated with aborting a task on another processor should be bounded; the implementation should use periodic polling, if necessary, to achieve this. Followed.

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## D.7(21): Tasking Restrictions

When feasible, the implementation should take advantage of the specified restrictions to produce a more efficient implementation. GNAT currently takes advantage of these restrictions by providing an optimized run time when the Ravenscar profile and the GNAT restricted run time set of restrictions are specified. See pragma `Profile (Ravenscar)` and pragma `Profile (Restricted)` for more details.

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## D.8(47-49): Monotonic Time

When appropriate, implementations should provide configuration mechanisms to change the value of `Tick`. Such configuration mechanisms are not appropriate to this implementation and are thus not supported.

It is recommended that `Calendar.Clock` and `Real_Time.Clock` be implemented as transformations of the same time base. Followed.

It is recommended that the best time base which exists in the underlying system be available to the application through `Clock`. Best may mean highest accuracy or largest range. Followed.

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## E.5(28-29): Partition Communication Subsystem

Whenever possible, the PCS on the called partition should allow for multiple tasks to call the RPC-receiver with different messages and should allow them to block until the corresponding subprogram body returns. Followed by GLADE, a separately supplied PCS that can be used with GNAT.

The `Write` operation on a stream of type `Params_Stream_Type` should raise `Storage_Error` if it runs out of space trying to write the `Item` into the stream. Followed by GLADE, a separately supplied PCS that can be used with GNAT.

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## F(7): COBOL Support

If COBOL (respectively, C) is widely supported in the target environment, implementations supporting the Information Systems Annex should provide the child package `Interfaces.COBOL` (respectively, `Interfaces.C`) specified in Annex B and should support a `convention_identifier` of COBOL (respectively, C) in the interfacing pragmas (see Annex B), thus allowing Ada programs to interface with programs written in that language. Followed.

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## F.1(2): Decimal Radix Support

Packed decimal should be used as the internal representation for objects of subtype S when S'Machine_Radix = 10. Not followed. GNAT ignores S'Machine_Radix and always uses binary representations.

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## G: Numerics

If Fortran (respectively, C) is widely supported in the target environment, implementations supporting the Numerics Annex should provide the child package `Interfaces.Fortran` (respectively, `Interfaces.C`) specified in Annex B and should support a `convention_identifier` of Fortran (respectively, C) in the interfacing pragmas (see Annex B), thus allowing Ada programs to interface with programs written in that language. Followed.

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## G.1.1(56-58): Complex Types

Because the usual mathematical meaning of multiplication of a complex operand and a real operand is that of the scaling of both components of the former by the latter, an implementation should not perform this operation by first promoting the real operand to complex type and then performing a full complex multiplication. In systems that, in the future, support an Ada binding to IEC 559:1989, the latter technique will not generate the required result when one of the components of the complex operand is infinite. (Explicit multiplication of the infinite component by the zero component obtained during promotion yields a NaN that propagates into the final result.) Analogous advice applies in the case of multiplication of a complex operand and a pure-imaginary operand, and in the case of division of a complex operand by a real or pure-imaginary operand. Not followed.

Similarly, because the usual mathematical meaning of addition of a complex operand and a real operand is that the imaginary operand remains unchanged, an implementation should not perform this operation by first promoting the real operand to complex type and then performing a full complex addition. In implementations in which the `Signed_Zeros` attribute of the component type is `True` (and which therefore conform to IEC 559:1989 in regard to the handling of the sign of zero in predefined arithmetic operations), the latter technique will not generate the required result when the imaginary component of the complex operand is a negatively signed zero. (Explicit addition of the negative zero to the zero obtained during promotion yields a positive zero.) Analogous advice applies in the case of addition of a complex operand and a pure-imaginary operand, and in the case of subtraction of a complex operand and a real or pure-imaginary operand. Not followed.

Implementations in which `Real'Signed_Zeros` is `True` should attempt to provide a rational treatment of the signs of zero results and result components. As one example, the result of the `Argument` function should have the sign of the imaginary component of the parameter `X` when the point represented by that parameter lies on the positive real axis; as another, the sign of the imaginary component of the `Compose_From_Polar` function should be the same as (respectively, the opposite of) that of the `Argument` parameter when that parameter has a value of zero and the `Modulus` parameter has a nonnegative (respectively, negative) value. Followed.

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## G.1.2(49): Complex Elementary Functions

Implementations in which `Complex_Types.Real'Signed_Zeros` is `True` should attempt to provide a rational treatment of the signs of zero results and result components. For example, many of the complex elementary functions have components that are odd functions of one of the parameter components; in these cases, the result component should have the sign of the parameter component at the origin. Other complex elementary functions have zero components whose sign is opposite that of a parameter component at the origin, or is always positive or always negative. Followed.

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## G.2.4(19): Accuracy Requirements

The versions of the forward trigonometric functions without a `Cycle` parameter should not be implemented by calling the corresponding version with a `Cycle` parameter of `2.0*Numerics.Pi`, since this will not provide the required accuracy in some portions of the domain. For the same reason, the version of `Log` without a `Base` parameter should not be implemented by calling the corresponding version with a `Base` parameter of `Numerics.e`. Followed.

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## G.2.6(15): Complex Arithmetic Accuracy

The version of the `Compose_From_Polar` function without a `Cycle` parameter should not be implemented by calling the corresponding version with a `Cycle` parameter of `2.0*Numerics.Pi`, since this will not provide the required accuracy in some portions of the domain. Followed.

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## H.6(15/2): Pragma Partition_Elaboration_Policy

If the partition elaboration policy is `Sequential` and the Environment task becomes permanently blocked during elaboration then the partition is deadlocked and it is recommended that the partition be immediately terminated. Not followed.

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