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117 lines
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117 lines
5.9 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Interface - 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="prev" href="Portability.html#Portability" title="Portability">
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<link rel="next" href="Libgcc.html#Libgcc" title="Libgcc">
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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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</head>
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<body>
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<div class="node">
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<a name="Interface"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Libgcc.html#Libgcc">Libgcc</a>,
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Previous: <a rel="previous" accesskey="p" href="Portability.html#Portability">Portability</a>,
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Up: <a rel="up" accesskey="u" href="index.html#Top">Top</a>
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<hr>
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</div>
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<h2 class="chapter">3 Interfacing to GCC Output</h2>
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<p><a name="index-interfacing-to-GCC-output-7"></a><a name="index-run_002dtime-conventions-8"></a><a name="index-function-call-conventions-9"></a><a name="index-conventions_002c-run_002dtime-10"></a>
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GCC is normally configured to use the same function calling convention
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normally in use on the target system. This is done with the
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machine-description macros described (see <a href="Target-Macros.html#Target-Macros">Target Macros</a>).
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<p><a name="index-unions_002c-returning-11"></a><a name="index-structures_002c-returning-12"></a><a name="index-returning-structures-and-unions-13"></a>However, returning of structure and union values is done differently on
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some target machines. As a result, functions compiled with PCC
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returning such types cannot be called from code compiled with GCC,
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and vice versa. This does not cause trouble often because few Unix
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library routines return structures or unions.
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<p>GCC code returns structures and unions that are 1, 2, 4 or 8 bytes
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long in the same registers used for <code>int</code> or <code>double</code> return
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values. (GCC typically allocates variables of such types in
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registers also.) Structures and unions of other sizes are returned by
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storing them into an address passed by the caller (usually in a
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register). The target hook <code>TARGET_STRUCT_VALUE_RTX</code>
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tells GCC where to pass this address.
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<p>By contrast, PCC on most target machines returns structures and unions
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of any size by copying the data into an area of static storage, and then
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returning the address of that storage as if it were a pointer value.
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The caller must copy the data from that memory area to the place where
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the value is wanted. This is slower than the method used by GCC, and
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fails to be reentrant.
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<p>On some target machines, such as RISC machines and the 80386, the
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standard system convention is to pass to the subroutine the address of
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where to return the value. On these machines, GCC has been
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configured to be compatible with the standard compiler, when this method
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is used. It may not be compatible for structures of 1, 2, 4 or 8 bytes.
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<p><a name="index-argument-passing-14"></a><a name="index-passing-arguments-15"></a>GCC uses the system's standard convention for passing arguments. On
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some machines, the first few arguments are passed in registers; in
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others, all are passed on the stack. It would be possible to use
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registers for argument passing on any machine, and this would probably
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result in a significant speedup. But the result would be complete
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incompatibility with code that follows the standard convention. So this
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change is practical only if you are switching to GCC as the sole C
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compiler for the system. We may implement register argument passing on
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certain machines once we have a complete GNU system so that we can
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compile the libraries with GCC.
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<p>On some machines (particularly the SPARC), certain types of arguments
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are passed “by invisible reference”. This means that the value is
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stored in memory, and the address of the memory location is passed to
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the subroutine.
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<p><a name="index-g_t_0040code_007blongjmp_007d-and-automatic-variables-16"></a>If you use <code>longjmp</code>, beware of automatic variables. ISO C says that
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automatic variables that are not declared <code>volatile</code> have undefined
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values after a <code>longjmp</code>. And this is all GCC promises to do,
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because it is very difficult to restore register variables correctly, and
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one of GCC's features is that it can put variables in registers without
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your asking it to.
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<!-- Copyright (C) 2003-2015 Free Software Foundation, Inc. -->
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<!-- This is part of the GCC manual. -->
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<!-- For copying conditions, see the file gcc.texi. -->
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<!-- Contributed by Aldy Hernandez <aldy@quesejoda.com> -->
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</body></html>
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