145 lines
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145 lines
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<head>
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<title>GNU Compiler Collection (GCC) Internals: Per-Function Data</title>
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Target-Macros.html#Target-Macros" rel="up" title="Target Macros">
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<link href="Storage-Layout.html#Storage-Layout" rel="next" title="Storage Layout">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Per_002dFunction-Data"></a>
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<div class="header">
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<p>
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Next: <a href="Storage-Layout.html#Storage-Layout" accesskey="n" rel="next">Storage Layout</a>, Previous: <a href="Run_002dtime-Target.html#Run_002dtime-Target" accesskey="p" rel="prev">Run-time Target</a>, Up: <a href="Target-Macros.html#Target-Macros" accesskey="u" rel="up">Target Macros</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Defining-data-structures-for-per_002dfunction-information_002e"></a>
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<h3 class="section">17.4 Defining data structures for per-function information.</h3>
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<a name="index-per_002dfunction-data"></a>
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<a name="index-data-structures"></a>
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<p>If the target needs to store information on a per-function basis, GCC
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provides a macro and a couple of variables to allow this. Note, just
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using statics to store the information is a bad idea, since GCC supports
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nested functions, so you can be halfway through encoding one function
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when another one comes along.
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</p>
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<p>GCC defines a data structure called <code>struct function</code> which
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contains all of the data specific to an individual function. This
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structure contains a field called <code>machine</code> whose type is
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<code>struct machine_function *</code>, which can be used by targets to point
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to their own specific data.
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</p>
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<p>If a target needs per-function specific data it should define the type
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<code>struct machine_function</code> and also the macro <code>INIT_EXPANDERS</code>.
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This macro should be used to initialize the function pointer
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<code>init_machine_status</code>. This pointer is explained below.
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</p>
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<p>One typical use of per-function, target specific data is to create an
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RTX to hold the register containing the function’s return address. This
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RTX can then be used to implement the <code>__builtin_return_address</code>
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function, for level 0.
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</p>
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<p>Note—earlier implementations of GCC used a single data area to hold
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all of the per-function information. Thus when processing of a nested
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function began the old per-function data had to be pushed onto a
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stack, and when the processing was finished, it had to be popped off the
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stack. GCC used to provide function pointers called
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<code>save_machine_status</code> and <code>restore_machine_status</code> to handle
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the saving and restoring of the target specific information. Since the
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single data area approach is no longer used, these pointers are no
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longer supported.
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</p>
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<dl>
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<dt><a name="index-INIT_005fEXPANDERS"></a>Macro: <strong>INIT_EXPANDERS</strong></dt>
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<dd><p>Macro called to initialize any target specific information. This macro
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is called once per function, before generation of any RTL has begun.
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The intention of this macro is to allow the initialization of the
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function pointer <code>init_machine_status</code>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-init_005fmachine_005fstatus"></a>Variable: <em>void (*)(struct function *)</em> <strong>init_machine_status</strong></dt>
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<dd><p>If this function pointer is non-<code>NULL</code> it will be called once per
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function, before function compilation starts, in order to allow the
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target to perform any target specific initialization of the
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<code>struct function</code> structure. It is intended that this would be
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used to initialize the <code>machine</code> of that structure.
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</p>
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<p><code>struct machine_function</code> structures are expected to be freed by GC.
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Generally, any memory that they reference must be allocated by using
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GC allocation, including the structure itself.
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</p></dd></dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Storage-Layout.html#Storage-Layout" accesskey="n" rel="next">Storage Layout</a>, Previous: <a href="Run_002dtime-Target.html#Run_002dtime-Target" accesskey="p" rel="prev">Run-time Target</a>, Up: <a href="Target-Macros.html#Target-Macros" accesskey="u" rel="up">Target Macros</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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