5.13 Efficiency Notes

Efficiency notes are messages that warn the user that the compiler has chosen a relatively inefficient implementation for some operation. Usually an efficiency note reflects the compiler’s desire for more type information. If the type of the values concerned is known to the programmer, then additional declarations can be used to get a more efficient implementation.

Efficiency notes are controlled by the extensions:inhibit-warnings (see optimize-declaration) optimization quality. When speed is greater than extensions:inhibit-warnings, efficiency notes are enabled. Note that this implicitly enables efficiency notes whenever speed is increased from its default of 1.

Consider this program with an obscure missing declaration:

(defun eff-note (x y z)
  (declare (fixnum x y z))
  (the fixnum (+ x y z)))

If compiled with (speed 3) (safety 0), this note is given:

In: DEFUN EFF-NOTE
  (+ X Y Z)
==>
  (+ (+ X Y) Z)
Note: Forced to do inline (signed-byte 32) arithmetic (cost 3).
      Unable to do inline fixnum arithmetic (cost 2) because:
      The first argument is a (INTEGER -1073741824 1073741822),
      not a FIXNUM.

This efficiency note tells us that the result of the intermediate computation (+ x y) is not known to be a fixnum, so the addition of the intermediate sum to z must be done less efficiently. This can be fixed by changing the definition of eff-note:

(defun eff-note (x y z)
  (declare (fixnum x y z))
  (the fixnum (+ (the fixnum (+ x y)) z)))

• Type Uncertainty
• Efficiency Notes and Type Checking
• Representation Efficiency Notes
• Verbosity Control