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2.116 Pragma Optimize_Alignment


pragma Optimize_Alignment (TIME | SPACE | OFF);

This is a configuration pragma which affects the choice of default alignments for types and objects where no alignment is explicitly specified. There is a time/space trade-off in the selection of these values. Large alignments result in more efficient code, at the expense of larger data space, since sizes have to be increased to match these alignments. Smaller alignments save space, but the access code is slower. The normal choice of default alignments for types and individual alignment promotions for objects (which is what you get if you do not use this pragma, or if you use an argument of OFF), tries to balance these two requirements.

Specifying SPACE causes smaller default alignments to be chosen in two cases. First any packed record is given an alignment of 1. Second, if a size is given for the type, then the alignment is chosen to avoid increasing this size. For example, consider:

type R is record
   X : Integer;
   Y : Character;
end record;

for R'Size use 5*8;

In the default mode, this type gets an alignment of 4, so that access to the Integer field X are efficient. But this means that objects of the type end up with a size of 8 bytes. This is a valid choice, since sizes of objects are allowed to be bigger than the size of the type, but it can waste space if for example fields of type R appear in an enclosing record. If the above type is compiled in Optimize_Alignment (Space) mode, the alignment is set to 1.

However, there is one case in which SPACE is ignored. If a variable length record (that is a discriminated record with a component which is an array whose length depends on a discriminant), has a pragma Pack, then it is not in general possible to set the alignment of such a record to one, so the pragma is ignored in this case (with a warning).

Specifying SPACE also disables alignment promotions for standalone objects, which occur when the compiler increases the alignment of a specific object without changing the alignment of its type.

Specifying TIME causes larger default alignments to be chosen in the case of small types with sizes that are not a power of 2. For example, consider:

type R is record
   A : Character;
   B : Character;
   C : Boolean;
end record;

pragma Pack (R);
for R'Size use 17;

The default alignment for this record is normally 1, but if this type is compiled in Optimize_Alignment (Time) mode, then the alignment is set to 4, which wastes space for objects of the type, since they are now 4 bytes long, but results in more efficient access when the whole record is referenced.

As noted above, this is a configuration pragma, and there is a requirement that all units in a partition be compiled with a consistent setting of the optimization setting. This would normally be achieved by use of a configuration pragma file containing the appropriate setting. The exception to this rule is that units with an explicit configuration pragma in the same file as the source unit are excluded from the consistency check, as are all predefined units. The latter are compiled by default in pragma Optimize_Alignment (Off) mode if no pragma appears at the start of the file.

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