3.4 Using optimized constructs

3.4.4 CASE macro optimization

You can improve the run-time performance of a call to the Common Lisp case macro by using only fixnum key values within a certain range. the Compiler can then expand the macro into code that jumps directly to a memory location instead of using anif statement for every case.

The Compiler can optimize an application of the case macro that satisfies the following qualifications:

All keys must be of typefixnum except the last key, which can be the symbolt, the symbolotherwise, or a fixnum integer.
The range of the keys must satisfy the following property:

: (highest-key - lowest-key) < (10 * number-of-keys)

The lowest key must be less than 68, and the highest key must be greater than -60.

For example, the following code can be fully optimized:

(case x (1 'one)
        (2 'two)
        (4 'four)
        (t 'other-num))

The Advanced User's Guide - 9 SEP 1996

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