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61 lines
1.5 KiB
HTML
61 lines
1.5 KiB
HTML
<!--
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vim: ts=4 sw=4 expandtab ft=html
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-->
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<html>
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<head>
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<title>Information on LLDS accurate garbage collection and C code</title>
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</head>
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<body>
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<h1>Information on LLDS accurate garbage collection and C code</h1>
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When handwritten code is called from Mercury,
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the garbage collection scheduler doesn't know anything about the code,
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so it cannot replace the succip on the stack (if there is one)
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with the collector's address.
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<p>
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If the handwritten code calls no other code, then this is fine,
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the scheduler knows it can replace the succip variable
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and when a proceed() occurs,
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execution will return to mercury code which it knows about.
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<p>
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If handwritten code calls other handwritten code, we have a problem,
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as succip will be saved on the stack
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and we don't know where on the stack it is stored.
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So we use a global variable 'saved_succip' which is succip is saved into.
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Care must be taken to save saved_succip on the stack
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so it doesn't get clobbered.
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So
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<pre>
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detstackvar(1) = (int) succip;
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</pre>
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becomes
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<pre>
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detstackvar(1) = (int) saved_succip;
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saved_succip = (int) succip;
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</pre>
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and, when restoring,
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<pre>
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succip = (int) detstackvar(1);
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</pre>
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becomes
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<pre>
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succip = saved_succip;
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saved_succip = detstackvar(1);
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</pre>
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<p>
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(With appropriate LVALUE_CASTs).
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<p>
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In this way, garbage collection
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always knows where the succip is stored in handwritten code.
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<p>
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The garbage collection code must check that
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the current execution is not still in a handwritten predicate --
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if it is, it must re-schedule (essentially just the same as before).
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</body>
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</html>
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