70 lines
3.3 KiB
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70 lines
3.3 KiB
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<title>Implementation of Funcallable Instances - SBCL Internals</title>
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<a name="Implementation-of-Funcallable-Instances"></a>
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Previous: <a rel="previous" accesskey="p" href="Overview-of-Funcallable-Instances.html#Overview-of-Funcallable-Instances">Overview of Funcallable Instances</a>,
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Up: <a rel="up" accesskey="u" href="Funcallable-Instances.html#Funcallable-Instances">Funcallable Instances</a>
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<h3 class="section">5.2 Implementation of Funcallable Instances</h3>
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<p>The first word after the header of a funcallable instance points to a
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dedicated trampoline function (known as
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<code>funcallable_instance_tramp</code> in SBCL 0.9.17) which is responsible
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for calling the funcallable instance function, kept in the second word
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after the header. The remaining words of a funcallable instance are
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firstly the <code>layout</code>, and then the slots.
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<p>The implementation of funcallable instances inherited from CMUCL
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differed in that there were two slots for the function: one for the
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underlying <code>simple-fun</code>, and one for the function itself (which is
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distinct from the <code>simple-fun</code> in the case of a closure. This,
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coupled with an instruction in the prologue of a closure's function to
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fetch the function from the latter slot, allowed a trampolineless
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calling sequence for funcallable instances; however, drawbacks included
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the loss of object identity for the funcallable instance function (if a
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funcallable instance was set as the function of another, updates to the
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first would not be reflected in calls to the second) and, more
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importantly, a race condition in calling funcallable instances from one
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thread while setting its funcallable instance function in another. The
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current implementation, described in the paragraph above, does not
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suffer from these problems (the function of a funcallable instance can
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be set atomically and retains its identity) at the cost of an additional
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layer of indirection.
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</body></html>
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