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166 lines
6.4 KiB
HTML
166 lines
6.4 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>User GC - GNU Compiler Collection (GCC) Internals</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="GNU Compiler Collection (GCC) Internals">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Type-Information.html#Type-Information" title="Type Information">
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<link rel="prev" href="Inheritance-and-GTY.html#Inheritance-and-GTY" title="Inheritance and GTY">
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<link rel="next" href="GGC-Roots.html#GGC-Roots" title="GGC Roots">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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``GNU Free Documentation License''.
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development.-->
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<style type="text/css"><!--
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--></style>
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</head>
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<body>
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<div class="node">
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<a name="User-GC"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="GGC-Roots.html#GGC-Roots">GGC Roots</a>,
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Previous: <a rel="previous" accesskey="p" href="Inheritance-and-GTY.html#Inheritance-and-GTY">Inheritance and GTY</a>,
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Up: <a rel="up" accesskey="u" href="Type-Information.html#Type-Information">Type Information</a>
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<hr>
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</div>
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<h3 class="section">22.3 Support for user-provided GC marking routines</h3>
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<p><a name="index-user-gc-5094"></a>The garbage collector supports types for which no automatic marking
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code is generated. For these types, the user is required to provide
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three functions: one to act as a marker for garbage collection, and
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two functions to act as marker and pointer walker for pre-compiled
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headers.
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<p>Given a structure <code>struct GTY((user)) my_struct</code>, the following functions
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should be defined to mark <code>my_struct</code>:
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<pre class="smallexample"> void gt_ggc_mx (my_struct *p)
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{
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/* This marks field 'fld'. */
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gt_ggc_mx (p->fld);
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}
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void gt_pch_nx (my_struct *p)
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{
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/* This marks field 'fld'. */
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gt_pch_nx (tp->fld);
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}
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void gt_pch_nx (my_struct *p, gt_pointer_operator op, void *cookie)
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{
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/* For every field 'fld', call the given pointer operator. */
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op (&(tp->fld), cookie);
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}
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</pre>
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<p>In general, each marker <code>M</code> should call <code>M</code> for every
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pointer field in the structure. Fields that are not allocated in GC
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or are not pointers must be ignored.
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<p>For embedded lists (e.g., structures with a <code>next</code> or <code>prev</code>
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pointer), the marker must follow the chain and mark every element in
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it.
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<p>Note that the rules for the pointer walker <code>gt_pch_nx (my_struct
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*, gt_pointer_operator, void *)</code> are slightly different. In this
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case, the operation <code>op</code> must be applied to the <em>address</em> of
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every pointer field.
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<h4 class="subsection">22.3.1 User-provided marking routines for template types</h4>
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<p>When a template type <code>TP</code> is marked with <code>GTY</code>, all
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instances of that type are considered user-provided types. This means
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that the individual instances of <code>TP</code> do not need to be marked
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with <code>GTY</code>. The user needs to provide template functions to mark
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all the fields of the type.
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<p>The following code snippets represent all the functions that need to
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be provided. Note that type <code>TP</code> may reference to more than one
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type. In these snippets, there is only one type <code>T</code>, but there
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could be more.
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<pre class="smallexample"> template<typename T>
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void gt_ggc_mx (TP<T> *tp)
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{
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extern void gt_ggc_mx (T&);
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/* This marks field 'fld' of type 'T'. */
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gt_ggc_mx (tp->fld);
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}
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template<typename T>
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void gt_pch_nx (TP<T> *tp)
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{
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extern void gt_pch_nx (T&);
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/* This marks field 'fld' of type 'T'. */
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gt_pch_nx (tp->fld);
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}
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template<typename T>
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void gt_pch_nx (TP<T *> *tp, gt_pointer_operator op, void *cookie)
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{
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/* For every field 'fld' of 'tp' with type 'T *', call the given
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pointer operator. */
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op (&(tp->fld), cookie);
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}
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template<typename T>
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void gt_pch_nx (TP<T> *tp, gt_pointer_operator, void *cookie)
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{
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extern void gt_pch_nx (T *, gt_pointer_operator, void *);
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/* For every field 'fld' of 'tp' with type 'T', call the pointer
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walker for all the fields of T. */
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gt_pch_nx (&(tp->fld), op, cookie);
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}
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</pre>
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<p>Support for user-defined types is currently limited. The following
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restrictions apply:
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<ol type=1 start=1>
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<li>Type <code>TP</code> and all the argument types <code>T</code> must be
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marked with <code>GTY</code>.
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<li>Type <code>TP</code> can only have type names in its argument list.
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<li>The pointer walker functions are different for <code>TP<T></code> and
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<code>TP<T *></code>. In the case of <code>TP<T></code>, references to
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<code>T</code> must be handled by calling <code>gt_pch_nx</code> (which
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will, in turn, walk all the pointers inside fields of <code>T</code>).
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In the case of <code>TP<T *></code>, references to <code>T *</code> must be
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handled by calling the <code>op</code> function on the address of the
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pointer (see the code snippets above).
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</ol>
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</body></html>
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