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123 lines
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<title>Compiler Transformations - SBCL Internals</title>
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<a name="Compiler-Transformations"></a>
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Next: <a rel="next" accesskey="n" href="MOP-Optimizations.html#MOP-Optimizations">MOP Optimizations</a>,
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Previous: <a rel="previous" accesskey="p" href="Basic-Implementation.html#Basic-Implementation">Basic Implementation</a>,
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Up: <a rel="up" accesskey="u" href="Slot_002dValue.html#Slot_002dValue">Slot-Value</a>
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<h3 class="section">7.2 Compiler Transformations</h3>
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<p>The compiler can assist in optimizing calls to <code>slot-value</code>: in
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particular, and despite the highly-dynamic nature of CLOS, compile-time
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knowledge of the name of the slot being accessed permits precomputation
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of much of the access (along with a branch to the slow path in case the
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parameters of the access change between compile-time and run-time).
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<h4 class="subsection">7.2.1 Within Methods</h4>
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<p><a name="index-permutation-vector-12"></a>
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If the object being accessed is a required parameter to the method,
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where the parameter variable is unmodified in the method body, and the
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slot name is a compile-time constant, then fast slot access can be
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supported through <dfn>permutation vectors</dfn>.
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<p>(FIXME: what about the metaclasses of the object? Does it have to be
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standard-class, or can it be funcallable-standard-class? Surely
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structure-class objects could be completely optimized if the class
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definition and slot name are both known at compile-time.)
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<p>Permutation vectors are built up and maintained to associate a
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compile-time index associated with a slot name with an index into the
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slot vector for a class of objects. The permutation vector applicable
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to a given method call (FIXME: or effective method? set of classes?
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something else?) is passed to the method body, and slots are accessed by
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looking up the index to the slot vector in the permutation vector, then
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looking up the value from the slot vector. (FIXME: a diagram would
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help, if I understood this bit well enough to draw a diagram).
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<p>Subsequent redefinitions of classes or of methods on
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<code>slot-value-using-class</code> cause an invalid index to be written into
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the permutation vector, and the call falls back to a full call to
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<code>slot-value</code>.
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<p>If the conditions for (structure or) permutation vector slot access
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optimization are not met, optimization of <code>slot-value</code> within
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methods falls back to the same as for calls to <code>slot-value</code> outside
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of methods, below.
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<h4 class="subsection">7.2.2 Outside of Methods</h4>
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<p><a name="index-load_002dtime_002dvalue-13"></a>
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A call to <code>slot-value</code> with a compile-time constant slot
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<var>name</var> argument is compiled into a call to a generic function
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named <code>(sb-pcl::slot-accessor :global </code><var>name</var><code> sb-pcl::reader)</code>,
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together with code providing load-time assurance (via
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<code>load-time-value</code>) that the generic function is bound and has a
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suitable accessor method. This generic function then benefits from the
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same optimizations as ordinary accessors, described in
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<a href="Accessor-Discriminating-Functions.html#Accessor-Discriminating-Functions">Accessor Discriminating Functions</a>.
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<p>(FIXME: how does this get invalidated if we later add methods on
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<code>slot-value-using-class</code>? Hm, maybe it isn't. I think this is
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probably a bug, and that adding methods to <code>slot-value-using-class</code>
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needs to invalidate accessor caches. Bah, humbug. Test code in
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<a href="ex_003abuggycache.html#ex_003abuggycache">ex:buggycache</a>, and note that I think that the analogous case
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involving adding or removing methods from
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<code>compute-applicable-methods</code> is handled correctly by
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<code>update-all-c-a-m-gf-info</code>.)
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<div class="float">
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<a name="ex_003abuggycache"></a>
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<pre class="example"> (defclass foo () ((a :initform 0)))
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(defun foo (x) (slot-value x 'a))
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(foo (make-instance 'foo)) ; => 0
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(defmethod slot-value-using-class :after
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((class std-class) (object foo)
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(slotd standard-effective-slot-definition))
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(print "hi"))
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(foo (make-instance 'foo)) ; => 0, no print
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(defclass bar (foo) ((a :initform 1)))
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(foo (make-instance 'bar)) ; => 1 and prints "hi"
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(foo (make-instance 'foo)) ; => 0, no print
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</pre>
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<p><strong class="float-caption">Example 7.3</strong></p></div>
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</body></html>
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