5.11.8 Specialized Arrays

       

Common Lisp supports specialized array element types through the :element-type argument to make-array. When an array has a specialized element type, only elements of that type can be stored in the array. From this restriction comes two major efficiency advantages:

• A specialized array can save space by packing multiple elements into a single word. For example, a base-char array can have 4 elements per word, and a bit array can have 32. This space-efficient representation is possible because it is not necessary to separately indicate the type of each element.
• The elements in a specialized array can be given the same non-descriptor representation as the one used in registers and on the stack, eliminating the need for representation conversions when reading and writing array elements. For objects with pointer descriptor representations (such as floats and word integers) there is also a substantial consing reduction because it is not necessary to allocate a new object every time an array element is modified.

These are the specialized element types currently supported:

bit
(unsigned-byte 2)
(unsigned-byte 4)
(unsigned-byte 8)
(unsigned-byte 16)
(unsigned-byte 32)
base-character
single-float
double-float

Some versions of CMU Common Lisp also support the following specialized element types:

(signed-byte 8)
(signed-byte 16)
(signed-byte 30)
(signed-byte 32)

Although a simple-vector can hold any type of object, t should still be considered a specialized array type, since arrays with element type t are specialized to hold descriptors.

When using non-descriptor representations, it is particularly important to make sure that array accesses are open-coded, since in addition to the generic operation overhead, efficiency is lost when the array element is converted to a descriptor so that it can be passed to (or from) the generic access routine. You can detect inefficient array accesses by enabling efficiency notes, see section 5.13. See section 5.10.3.