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We had been maintaining two copies of these notes, this change attempts to
merge them into one set. The cononical location for these notes is here in
compiler/notes/, they will be removed from the www repository.
I've also updated a number of URLs.
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compiler/notes/coding_standards.html:
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compiler/notes/gc_and_c_code.html:
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Merge in differences with the versions of these files in the www
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75 lines
1.6 KiB
HTML
75 lines
1.6 KiB
HTML
<html>
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<head>
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<title>
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Information On LLDS Accurate Garbage Collection And C Code
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</title>
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</head>
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<body
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bgcolor="#ffffff"
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text="#000000"
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>
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<hr>
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<!-------------------------->
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When handwritten code is called from Mercury, the garbage collection
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scheduler doesn't know anything about the code, so it cannot replace
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the succip on the stack (if there is one) 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, the
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scheduler knows it can replace the succip variable and when a
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proceed() occurs execution will return to mercury code which it
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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 and we don't know where on
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the stack it is stored. So we use a global variable 'saved_succip' which
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is succip is saved into. Care must be taken to save saved_succip on the
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stack so it doesn't get clobbered. <br>
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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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(With appropriate LVALUE_CASTs).
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<p>
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In this way, garbage collection always knows where the succip is stored
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in handwritten code.
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<p>
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The garbage collection code must check that the current execution is not
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still in a handwritten predicate - if it is, it must re-schedule (essentially
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just the same as before).
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<p>
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<hr>
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<!-------------------------->
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</body>
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</html>
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