300 lines
16 KiB
HTML
300 lines
16 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2016 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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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>GNU Compiler Collection (GCC) Internals: Varargs</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals: Varargs">
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<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Varargs">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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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="Trampolines.html#Trampolines" rel="next" title="Trampolines">
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<link href="Miscellaneous-Register-Hooks.html#Miscellaneous-Register-Hooks" rel="prev" title="Miscellaneous Register Hooks">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Varargs"></a>
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<div class="header">
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<p>
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Next: <a href="Trampolines.html#Trampolines" accesskey="n" rel="next">Trampolines</a>, Previous: <a href="Stack-and-Calling.html#Stack-and-Calling" accesskey="p" rel="prev">Stack and Calling</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="Implementing-the-Varargs-Macros"></a>
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<h3 class="section">17.10 Implementing the Varargs Macros</h3>
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<a name="index-varargs-implementation"></a>
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<p>GCC comes with an implementation of <code><varargs.h></code> and
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<code><stdarg.h></code> that work without change on machines that pass arguments
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on the stack. Other machines require their own implementations of
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varargs, and the two machine independent header files must have
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conditionals to include it.
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</p>
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<p>ISO <code><stdarg.h></code> differs from traditional <code><varargs.h></code> mainly in
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the calling convention for <code>va_start</code>. The traditional
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implementation takes just one argument, which is the variable in which
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to store the argument pointer. The ISO implementation of
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<code>va_start</code> takes an additional second argument. The user is
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supposed to write the last named argument of the function here.
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</p>
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<p>However, <code>va_start</code> should not use this argument. The way to find
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the end of the named arguments is with the built-in functions described
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below.
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</p>
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<dl>
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<dt><a name="index-_005f_005fbuiltin_005fsaveregs"></a>Macro: <strong>__builtin_saveregs</strong> <em>()</em></dt>
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<dd><p>Use this built-in function to save the argument registers in memory so
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that the varargs mechanism can access them. Both ISO and traditional
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versions of <code>va_start</code> must use <code>__builtin_saveregs</code>, unless
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you use <code>TARGET_SETUP_INCOMING_VARARGS</code> (see below) instead.
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</p>
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<p>On some machines, <code>__builtin_saveregs</code> is open-coded under the
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control of the target hook <code>TARGET_EXPAND_BUILTIN_SAVEREGS</code>. On
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other machines, it calls a routine written in assembler language,
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found in <samp>libgcc2.c</samp>.
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</p>
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<p>Code generated for the call to <code>__builtin_saveregs</code> appears at the
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beginning of the function, as opposed to where the call to
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<code>__builtin_saveregs</code> is written, regardless of what the code is.
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This is because the registers must be saved before the function starts
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to use them for its own purposes.
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</p></dd></dl>
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<dl>
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<dt><a name="index-_005f_005fbuiltin_005fnext_005farg"></a>Macro: <strong>__builtin_next_arg</strong> <em>(<var>lastarg</var>)</em></dt>
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<dd><p>This builtin returns the address of the first anonymous stack
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argument, as type <code>void *</code>. If <code>ARGS_GROW_DOWNWARD</code>, it
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returns the address of the location above the first anonymous stack
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argument. Use it in <code>va_start</code> to initialize the pointer for
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fetching arguments from the stack. Also use it in <code>va_start</code> to
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verify that the second parameter <var>lastarg</var> is the last named argument
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of the current function.
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</p></dd></dl>
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<dl>
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<dt><a name="index-_005f_005fbuiltin_005fclassify_005ftype"></a>Macro: <strong>__builtin_classify_type</strong> <em>(<var>object</var>)</em></dt>
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<dd><p>Since each machine has its own conventions for which data types are
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passed in which kind of register, your implementation of <code>va_arg</code>
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has to embody these conventions. The easiest way to categorize the
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specified data type is to use <code>__builtin_classify_type</code> together
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with <code>sizeof</code> and <code>__alignof__</code>.
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</p>
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<p><code>__builtin_classify_type</code> ignores the value of <var>object</var>,
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considering only its data type. It returns an integer describing what
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kind of type that is—integer, floating, pointer, structure, and so on.
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</p>
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<p>The file <samp>typeclass.h</samp> defines an enumeration that you can use to
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interpret the values of <code>__builtin_classify_type</code>.
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</p></dd></dl>
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<p>These machine description macros help implement varargs:
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</p>
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<dl>
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<dt><a name="index-TARGET_005fEXPAND_005fBUILTIN_005fSAVEREGS"></a>Target Hook: <em>rtx</em> <strong>TARGET_EXPAND_BUILTIN_SAVEREGS</strong> <em>(void)</em></dt>
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<dd><p>If defined, this hook produces the machine-specific code for a call to
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<code>__builtin_saveregs</code>. This code will be moved to the very
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beginning of the function, before any parameter access are made. The
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return value of this function should be an RTX that contains the value
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to use as the return of <code>__builtin_saveregs</code>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fSETUP_005fINCOMING_005fVARARGS"></a>Target Hook: <em>void</em> <strong>TARGET_SETUP_INCOMING_VARARGS</strong> <em>(cumulative_args_t <var>args_so_far</var>, machine_mode <var>mode</var>, tree <var>type</var>, int *<var>pretend_args_size</var>, int <var>second_time</var>)</em></dt>
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<dd><p>This target hook offers an alternative to using
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<code>__builtin_saveregs</code> and defining the hook
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<code>TARGET_EXPAND_BUILTIN_SAVEREGS</code>. Use it to store the anonymous
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register arguments into the stack so that all the arguments appear to
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have been passed consecutively on the stack. Once this is done, you can
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use the standard implementation of varargs that works for machines that
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pass all their arguments on the stack.
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</p>
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<p>The argument <var>args_so_far</var> points to the <code>CUMULATIVE_ARGS</code> data
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structure, containing the values that are obtained after processing the
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named arguments. The arguments <var>mode</var> and <var>type</var> describe the
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last named argument—its machine mode and its data type as a tree node.
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</p>
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<p>The target hook should do two things: first, push onto the stack all the
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argument registers <em>not</em> used for the named arguments, and second,
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store the size of the data thus pushed into the <code>int</code>-valued
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variable pointed to by <var>pretend_args_size</var>. The value that you
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store here will serve as additional offset for setting up the stack
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frame.
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</p>
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<p>Because you must generate code to push the anonymous arguments at
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compile time without knowing their data types,
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<code>TARGET_SETUP_INCOMING_VARARGS</code> is only useful on machines that
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have just a single category of argument register and use it uniformly
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for all data types.
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</p>
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<p>If the argument <var>second_time</var> is nonzero, it means that the
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arguments of the function are being analyzed for the second time. This
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happens for an inline function, which is not actually compiled until the
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end of the source file. The hook <code>TARGET_SETUP_INCOMING_VARARGS</code> should
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not generate any instructions in this case.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fSTRICT_005fARGUMENT_005fNAMING"></a>Target Hook: <em>bool</em> <strong>TARGET_STRICT_ARGUMENT_NAMING</strong> <em>(cumulative_args_t <var>ca</var>)</em></dt>
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<dd><p>Define this hook to return <code>true</code> if the location where a function
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argument is passed depends on whether or not it is a named argument.
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</p>
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<p>This hook controls how the <var>named</var> argument to <code>TARGET_FUNCTION_ARG</code>
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is set for varargs and stdarg functions. If this hook returns
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<code>true</code>, the <var>named</var> argument is always true for named
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arguments, and false for unnamed arguments. If it returns <code>false</code>,
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but <code>TARGET_PRETEND_OUTGOING_VARARGS_NAMED</code> returns <code>true</code>,
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then all arguments are treated as named. Otherwise, all named arguments
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except the last are treated as named.
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</p>
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<p>You need not define this hook if it always returns <code>false</code>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fCALL_005fARGS"></a>Target Hook: <em>void</em> <strong>TARGET_CALL_ARGS</strong> <em>(rtx, <var>tree</var>)</em></dt>
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<dd><p>While generating RTL for a function call, this target hook is invoked once
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for each argument passed to the function, either a register returned by
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<code>TARGET_FUNCTION_ARG</code> or a memory location. It is called just
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before the point where argument registers are stored. The type of the
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function to be called is also passed as the second argument; it is
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<code>NULL_TREE</code> for libcalls. The <code>TARGET_END_CALL_ARGS</code> hook is
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invoked just after the code to copy the return reg has been emitted.
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This functionality can be used to perform special setup of call argument
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registers if a target needs it.
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For functions without arguments, the hook is called once with <code>pc_rtx</code>
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passed instead of an argument register.
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Most ports do not need to implement anything for this hook.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fEND_005fCALL_005fARGS"></a>Target Hook: <em>void</em> <strong>TARGET_END_CALL_ARGS</strong> <em>(void)</em></dt>
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<dd><p>This target hook is invoked while generating RTL for a function call,
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just after the point where the return reg is copied into a pseudo. It
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signals that all the call argument and return registers for the just
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emitted call are now no longer in use.
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Most ports do not need to implement anything for this hook.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fPRETEND_005fOUTGOING_005fVARARGS_005fNAMED"></a>Target Hook: <em>bool</em> <strong>TARGET_PRETEND_OUTGOING_VARARGS_NAMED</strong> <em>(cumulative_args_t <var>ca</var>)</em></dt>
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<dd><p>If you need to conditionally change ABIs so that one works with
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<code>TARGET_SETUP_INCOMING_VARARGS</code>, but the other works like neither
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<code>TARGET_SETUP_INCOMING_VARARGS</code> nor <code>TARGET_STRICT_ARGUMENT_NAMING</code> was
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defined, then define this hook to return <code>true</code> if
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<code>TARGET_SETUP_INCOMING_VARARGS</code> is used, <code>false</code> otherwise.
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Otherwise, you should not define this hook.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fLOAD_005fBOUNDS_005fFOR_005fARG"></a>Target Hook: <em>rtx</em> <strong>TARGET_LOAD_BOUNDS_FOR_ARG</strong> <em>(rtx <var>slot</var>, rtx <var>arg</var>, rtx <var>slot_no</var>)</em></dt>
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<dd><p>This hook is used by expand pass to emit insn to load bounds of
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<var>arg</var> passed in <var>slot</var>. Expand pass uses this hook in case
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bounds of <var>arg</var> are not passed in register. If <var>slot</var> is a
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memory, then bounds are loaded as for regular pointer loaded from
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memory. If <var>slot</var> is not a memory then <var>slot_no</var> is an integer
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constant holding number of the target dependent special slot which
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should be used to obtain bounds. Hook returns RTX holding loaded bounds.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fSTORE_005fBOUNDS_005fFOR_005fARG"></a>Target Hook: <em>void</em> <strong>TARGET_STORE_BOUNDS_FOR_ARG</strong> <em>(rtx <var>arg</var>, rtx <var>slot</var>, rtx <var>bounds</var>, rtx <var>slot_no</var>)</em></dt>
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<dd><p>This hook is used by expand pass to emit insns to store <var>bounds</var> of
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<var>arg</var> passed in <var>slot</var>. Expand pass uses this hook in case
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<var>bounds</var> of <var>arg</var> are not passed in register. If <var>slot</var> is a
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memory, then <var>bounds</var> are stored as for regular pointer stored in
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memory. If <var>slot</var> is not a memory then <var>slot_no</var> is an integer
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constant holding number of the target dependent special slot which
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should be used to store <var>bounds</var>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fLOAD_005fRETURNED_005fBOUNDS"></a>Target Hook: <em>rtx</em> <strong>TARGET_LOAD_RETURNED_BOUNDS</strong> <em>(rtx <var>slot</var>)</em></dt>
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<dd><p>This hook is used by expand pass to emit insn to load bounds
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returned by function call in <var>slot</var>. Hook returns RTX holding
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loaded bounds.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fSTORE_005fRETURNED_005fBOUNDS"></a>Target Hook: <em>void</em> <strong>TARGET_STORE_RETURNED_BOUNDS</strong> <em>(rtx <var>slot</var>, rtx <var>bounds</var>)</em></dt>
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<dd><p>This hook is used by expand pass to emit insn to store <var>bounds</var>
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returned by function call into <var>slot</var>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fCHKP_005fFUNCTION_005fVALUE_005fBOUNDS"></a>Target Hook: <em>rtx</em> <strong>TARGET_CHKP_FUNCTION_VALUE_BOUNDS</strong> <em>(const_tree <var>ret_type</var>, const_tree <var>fn_decl_or_type</var>, bool <var>outgoing</var>)</em></dt>
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<dd><p>Define this to return an RTX representing the place where a function
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returns bounds for returned pointers. Arguments meaning is similar to
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<code>TARGET_FUNCTION_VALUE</code>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-TARGET_005fSETUP_005fINCOMING_005fVARARG_005fBOUNDS"></a>Target Hook: <em>void</em> <strong>TARGET_SETUP_INCOMING_VARARG_BOUNDS</strong> <em>(cumulative_args_t <var>args_so_far</var>, enum machine_mode <var>mode</var>, tree <var>type</var>, int *<var>pretend_args_size</var>, int <var>second_time</var>)</em></dt>
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<dd><p>Use it to store bounds for anonymous register arguments stored
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into the stack. Arguments meaning is similar to
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<code>TARGET_SETUP_INCOMING_VARARGS</code>.
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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="Trampolines.html#Trampolines" accesskey="n" rel="next">Trampolines</a>, Previous: <a href="Stack-and-Calling.html#Stack-and-Calling" accesskey="p" rel="prev">Stack and Calling</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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