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6.16 Determining the Representations chosen by GNAT

Although the descriptions in this section are intended to be complete, it is often easier to simply experiment to see what GNAT accepts and what the effect is on the layout of types and objects.

As required by the Ada RM, if a representation clause is not accepted, then it must be rejected as illegal by the compiler. However, when a representation clause or pragma is accepted, there can still be questions of what the compiler actually does. For example, if a partial record representation clause specifies the location of some components and not others, then where are the non-specified components placed? Or if pragma Pack is used on a record, then exactly where are the resulting fields placed? The section on pragma Pack in this chapter can be used to answer the second question, but it is often easier to just see what the compiler does.

For this purpose, GNAT provides the option -gnatR. If you compile with this option, then the compiler will output information on the actual representations chosen, in a format similar to source representation clauses. For example, if we compile the package:

     package q is
        type r (x : boolean) is tagged record
           case x is
              when True => S : String (1 .. 100);
              when False => null;
           end case;
        end record;
        type r2 is new r (false) with record
           y2 : integer;
        end record;
        for r2 use record
           y2 at 16 range 0 .. 31;
        end record;
        type x is record
           y : character;
        end record;
        type x1 is array (1 .. 10) of x;
        for x1'component_size use 11;
        type ia is access integer;
        type Rb1 is array (1 .. 13) of Boolean;
        pragma Pack (rb1);
        type Rb2 is array (1 .. 65) of Boolean;
        pragma Pack (rb2);
        type x2 is record
           l1 : Boolean;
           l2 : Duration;
           l3 : Float;
           l4 : Boolean;
           l5 : Rb1;
           l6 : Rb2;
        end record;
        pragma Pack (x2);
     end q;

using the switch -gnatR we obtain the following output:

     Representation information for unit q
     for r'Size use ??;
     for r'Alignment use 4;
     for r use record
        x    at 4 range  0 .. 7;
        _tag at 0 range  0 .. 31;
        s    at 5 range  0 .. 799;
     end record;
     for r2'Size use 160;
     for r2'Alignment use 4;
     for r2 use record
        x       at  4 range  0 .. 7;
        _tag    at  0 range  0 .. 31;
        _parent at  0 range  0 .. 63;
        y2      at 16 range  0 .. 31;
     end record;
     for x'Size use 8;
     for x'Alignment use 1;
     for x use record
        y at 0 range  0 .. 7;
     end record;
     for x1'Size use 112;
     for x1'Alignment use 1;
     for x1'Component_Size use 11;
     for rb1'Size use 13;
     for rb1'Alignment use 2;
     for rb1'Component_Size use 1;
     for rb2'Size use 72;
     for rb2'Alignment use 1;
     for rb2'Component_Size use 1;
     for x2'Size use 224;
     for x2'Alignment use 4;
     for x2 use record
        l1 at  0 range  0 .. 0;
        l2 at  0 range  1 .. 64;
        l3 at 12 range  0 .. 31;
        l4 at 16 range  0 .. 0;
        l5 at 16 range  1 .. 13;
        l6 at 18 range  0 .. 71;
     end record;

The Size values are actually the Object_Size, i.e. the default size that will be allocated for objects of the type. The ?? size for type r indicates that we have a variant record, and the actual size of objects will depend on the discriminant value.

The Alignment values show the actual alignment chosen by the compiler for each record or array type.

The record representation clause for type r shows where all fields are placed, including the compiler generated tag field (whose location cannot be controlled by the programmer).

The record representation clause for the type extension r2 shows all the fields present, including the parent field, which is a copy of the fields of the parent type of r2, i.e. r1.

The component size and size clauses for types rb1 and rb2 show the exact effect of pragma Pack on these arrays, and the record representation clause for type x2 shows how pragma Pack affects this record type.

In some cases, it may be useful to cut and paste the representation clauses generated by the compiler into the original source to fix and guarantee the actual representation to be used.