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82 lines
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<title>Calling Convention - SBCL Internals</title>
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<a name="Calling-Convention"></a>
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Next: <a rel="next" accesskey="n" href="Discriminating-Functions.html#Discriminating-Functions">Discriminating Functions</a>,
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<h2 class="chapter">2 Calling Convention</h2>
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<ul class="menu">
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<li><a accesskey="1" href="Assembly-Routines.html#Assembly-Routines">Assembly Routines</a>
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<li><a accesskey="2" href="Local-Calls.html#Local-Calls">Local Calls</a>
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<li><a accesskey="3" href="Full-Calls.html#Full-Calls">Full Calls</a>
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<li><a accesskey="4" href="Unknown_002dValues-Returns.html#Unknown_002dValues-Returns">Unknown-Values Returns</a>
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<li><a accesskey="5" href="IR2-Conversion.html#IR2-Conversion">IR2 Conversion</a>
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<li><a accesskey="6" href="Additional-Notes.html#Additional-Notes">Additional Notes</a>
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</ul>
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<p>The calling convention used within Lisp code on SBCL/x86 was,
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for the longest time, really bad. If it weren't for the fact
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that it predates modern x86 CPUs, one might almost believe it to
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have been designed explicitly to defeat the branch-prediction
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hardware therein. This chapter is somewhat of a brain-dump of
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information that might be useful when attempting to improve the
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situation further, mostly written immediately after having made
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a dent in the problem.
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<p>Assumptions about the calling convention are embedded throughout
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the system. The runtime knows how to call in to Lisp and receive
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a value from Lisp, the assembly-routines have intimate knowledge
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of what registers are involved in a call situation,
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<samp><span class="file">src/compiler/target/call.lisp</span></samp> contains the VOPs involved in
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implementing function call/return, and <samp><span class="file">src/compiler/ir2tran.lisp</span></samp> has
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assumptions about frame allocation and argument/return-value
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passing locations.
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<p>The current round of changes has been limited to VOPs, assembly-routines,
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related support functions, and the required support in the runtime.
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<p>Note that most of this documentation also applies to other CPUs,
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modulo the actual registers involved, the displacement used
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in the single-value return convention, and the fact that they
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use the “old” convention anywhere it is mentioned.
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