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6.1 Alignment Clauses

GNAT requires that all alignment clauses specify a power of 2, and all default alignments are always a power of 2. The default alignment values are as follows:

An alignment clause may always specify a larger alignment than the default value, up to some maximum value dependent on the target (obtainable by using the attribute reference Standard'Maximum_Alignment). The only case where it is permissible to specify a smaller alignment than the default value is for a record with a record representation clause. In this case, packable fields for which a component clause is given still result in a default alignment corresponding to the original type, but this may be overridden, since these components in fact only require an alignment of one byte. For example, given

       type V is record
          A : Integer;
       end record;
     
       for V use record
          A at 0  range 0 .. 31;
       end record;
     
       for V'alignment use 1;

The default alignment for the type V is 4, as a result of the Integer field in the record, but since this field is placed with a component clause, it is permissible, as shown, to override the default alignment of the record with a smaller value.