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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:

These are the specialized element types currently supported:

(unsigned-byte 2)
(unsigned-byte 4)
(unsigned-byte 8)
(unsigned-byte 16)
(unsigned-byte 32)
(signed-byte 8)
(signed-byte 16)
(signed-byte 30)
(signed-byte 32)
(complex single-float)
(complex double-float)
(complex ext:double-double-float)

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 efficiency-notes. See array-types.

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