Sophie: sbcl-1.0.31-1mdv2010.0 i586

sbcl-1.0.31-1mdv2010.0.i586.rpm

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<h3 class="section">6.1 Slot access</h3>

<p><a name="index-Slot-access-136"></a>

<h4 class="subsection">6.1.1 Structure object slot access</h4>

<p>Structure slot accessors are efficient only if the compiler is able to
open code them: compiling a call to a structure slot accessor before
the structure is defined, declaring one <code>notinline</code>, or passing
it as a functional argument to another function causes severe
perfomance degradation.

<h4 class="subsection">6.1.2 Standard object slot access</h4>

<p>The most efficient way to access a slot of a <code>standard-object</code> is
by using <code>slot-value</code> with a constant slot name argument inside a
<code>defmethod</code> body, where the variable holding the instance is a
specializer parameter of the method and is never assigned to. The cost
is roughly 1.6 times that of an open coded structure slot accessor.

   <p>Second most efficient way is to use a CLOS slot accessor, or
<code>slot-value</code> with a constant slot name argument, but in
circumstances other than specified above. This may be up to 3 times as
slow as the method described above.

   <p>Example:

<pre class="lisp">     (defclass foo () ((bar)))
     
     ;; Fast: specializer and never assigned to
     (defmethod quux ((foo foo) new)
       (let ((old (slot-value foo 'bar)))
         (setf (slot-value foo 'bar) new)
         old))
     
     ;; Slow: not a specializer
     (defmethod quux ((foo foo) new)
       (let* ((temp foo)
              (old (slot-value temp 'bar)))
         (setf (slot-value temp 'bar) new)
         old))
     
     ;; Slow: assignment to FOO
     (defmethod quux ((foo foo) new)
       (let ((old (slot-value foo 'bar)))
         (setf (slot-value foo 'bar) new)
         (setf foo new)
         old))
</pre>
   <p>Note that when profiling code such as this, the first few calls to the
generic function are not representative, as the dispatch mechanism is
lazily set up during those calls.

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