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192 lines
10 KiB
HTML
192 lines
10 KiB
HTML
4 years ago
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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) 1988-2018 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 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Uninitialized Data (GNU Compiler Collection (GCC) Internals)</title>
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<meta name="description" content="Uninitialized Data (GNU Compiler Collection (GCC) Internals)">
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<meta name="keywords" content="Uninitialized Data (GNU Compiler Collection (GCC) Internals)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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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="Assembler-Format.html#Assembler-Format" rel="up" title="Assembler Format">
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<link href="Label-Output.html#Label-Output" rel="next" title="Label Output">
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<link href="Data-Output.html#Data-Output" rel="prev" title="Data Output">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Uninitialized-Data"></a>
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<div class="header">
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<p>
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Next: <a href="Label-Output.html#Label-Output" accesskey="n" rel="next">Label Output</a>, Previous: <a href="Data-Output.html#Data-Output" accesskey="p" rel="prev">Data Output</a>, Up: <a href="Assembler-Format.html#Assembler-Format" accesskey="u" rel="up">Assembler Format</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="Output-of-Uninitialized-Variables"></a>
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<h4 class="subsection">18.20.3 Output of Uninitialized Variables</h4>
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<p>Each of the macros in this section is used to do the whole job of
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outputting a single uninitialized variable.
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</p>
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<dl>
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<dt><a name="index-ASM_005fOUTPUT_005fCOMMON"></a>Macro: <strong>ASM_OUTPUT_COMMON</strong> <em>(<var>stream</var>, <var>name</var>, <var>size</var>, <var>rounded</var>)</em></dt>
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<dd><p>A C statement (sans semicolon) to output to the stdio stream
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<var>stream</var> the assembler definition of a common-label named
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<var>name</var> whose size is <var>size</var> bytes. The variable <var>rounded</var>
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is the size rounded up to whatever alignment the caller wants. It is
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possible that <var>size</var> may be zero, for instance if a struct with no
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other member than a zero-length array is defined. In this case, the
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backend must output a symbol definition that allocates at least one
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byte, both so that the address of the resulting object does not compare
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equal to any other, and because some object formats cannot even express
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the concept of a zero-sized common symbol, as that is how they represent
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an ordinary undefined external.
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</p>
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<p>Use the expression <code>assemble_name (<var>stream</var>, <var>name</var>)</code> to
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output the name itself; before and after that, output the additional
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assembler syntax for defining the name, and a newline.
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</p>
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<p>This macro controls how the assembler definitions of uninitialized
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common global variables are output.
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</p></dd></dl>
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<dl>
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<dt><a name="index-ASM_005fOUTPUT_005fALIGNED_005fCOMMON"></a>Macro: <strong>ASM_OUTPUT_ALIGNED_COMMON</strong> <em>(<var>stream</var>, <var>name</var>, <var>size</var>, <var>alignment</var>)</em></dt>
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<dd><p>Like <code>ASM_OUTPUT_COMMON</code> except takes the required alignment as a
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separate, explicit argument. If you define this macro, it is used in
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place of <code>ASM_OUTPUT_COMMON</code>, and gives you more flexibility in
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handling the required alignment of the variable. The alignment is specified
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as the number of bits.
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</p></dd></dl>
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<dl>
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<dt><a name="index-ASM_005fOUTPUT_005fALIGNED_005fDECL_005fCOMMON"></a>Macro: <strong>ASM_OUTPUT_ALIGNED_DECL_COMMON</strong> <em>(<var>stream</var>, <var>decl</var>, <var>name</var>, <var>size</var>, <var>alignment</var>)</em></dt>
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<dd><p>Like <code>ASM_OUTPUT_ALIGNED_COMMON</code> except that <var>decl</var> of the
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variable to be output, if there is one, or <code>NULL_TREE</code> if there
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is no corresponding variable. If you define this macro, GCC will use it
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in place of both <code>ASM_OUTPUT_COMMON</code> and
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<code>ASM_OUTPUT_ALIGNED_COMMON</code>. Define this macro when you need to see
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the variable’s decl in order to chose what to output.
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</p></dd></dl>
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<dl>
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<dt><a name="index-ASM_005fOUTPUT_005fALIGNED_005fBSS"></a>Macro: <strong>ASM_OUTPUT_ALIGNED_BSS</strong> <em>(<var>stream</var>, <var>decl</var>, <var>name</var>, <var>size</var>, <var>alignment</var>)</em></dt>
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<dd><p>A C statement (sans semicolon) to output to the stdio stream
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<var>stream</var> the assembler definition of uninitialized global <var>decl</var> named
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<var>name</var> whose size is <var>size</var> bytes. The variable <var>alignment</var>
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is the alignment specified as the number of bits.
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</p>
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<p>Try to use function <code>asm_output_aligned_bss</code> defined in file
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<samp>varasm.c</samp> when defining this macro. If unable, use the expression
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<code>assemble_name (<var>stream</var>, <var>name</var>)</code> to output the name itself;
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before and after that, output the additional assembler syntax for defining
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the name, and a newline.
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</p>
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<p>There are two ways of handling global BSS. One is to define this macro.
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The other is to have <code>TARGET_ASM_SELECT_SECTION</code> return a
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switchable BSS section (see <a href="File-Framework.html#TARGET_005fHAVE_005fSWITCHABLE_005fBSS_005fSECTIONS">TARGET_HAVE_SWITCHABLE_BSS_SECTIONS</a>).
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You do not need to do both.
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</p>
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<p>Some languages do not have <code>common</code> data, and require a
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non-common form of global BSS in order to handle uninitialized globals
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efficiently. C++ is one example of this. However, if the target does
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not support global BSS, the front end may choose to make globals
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common in order to save space in the object file.
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</p></dd></dl>
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<dl>
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<dt><a name="index-ASM_005fOUTPUT_005fLOCAL"></a>Macro: <strong>ASM_OUTPUT_LOCAL</strong> <em>(<var>stream</var>, <var>name</var>, <var>size</var>, <var>rounded</var>)</em></dt>
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<dd><p>A C statement (sans semicolon) to output to the stdio stream
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<var>stream</var> the assembler definition of a local-common-label named
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<var>name</var> whose size is <var>size</var> bytes. The variable <var>rounded</var>
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is the size rounded up to whatever alignment the caller wants.
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</p>
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<p>Use the expression <code>assemble_name (<var>stream</var>, <var>name</var>)</code> to
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output the name itself; before and after that, output the additional
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assembler syntax for defining the name, and a newline.
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</p>
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<p>This macro controls how the assembler definitions of uninitialized
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static variables are output.
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</p></dd></dl>
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<dl>
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<dt><a name="index-ASM_005fOUTPUT_005fALIGNED_005fLOCAL"></a>Macro: <strong>ASM_OUTPUT_ALIGNED_LOCAL</strong> <em>(<var>stream</var>, <var>name</var>, <var>size</var>, <var>alignment</var>)</em></dt>
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<dd><p>Like <code>ASM_OUTPUT_LOCAL</code> except takes the required alignment as a
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separate, explicit argument. If you define this macro, it is used in
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place of <code>ASM_OUTPUT_LOCAL</code>, and gives you more flexibility in
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handling the required alignment of the variable. The alignment is specified
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as the number of bits.
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</p></dd></dl>
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<dl>
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<dt><a name="index-ASM_005fOUTPUT_005fALIGNED_005fDECL_005fLOCAL"></a>Macro: <strong>ASM_OUTPUT_ALIGNED_DECL_LOCAL</strong> <em>(<var>stream</var>, <var>decl</var>, <var>name</var>, <var>size</var>, <var>alignment</var>)</em></dt>
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<dd><p>Like <code>ASM_OUTPUT_ALIGNED_DECL</code> except that <var>decl</var> of the
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variable to be output, if there is one, or <code>NULL_TREE</code> if there
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is no corresponding variable. If you define this macro, GCC will use it
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in place of both <code>ASM_OUTPUT_DECL</code> and
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<code>ASM_OUTPUT_ALIGNED_DECL</code>. Define this macro when you need to see
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the variable’s decl in order to chose what to output.
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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="Label-Output.html#Label-Output" accesskey="n" rel="next">Label Output</a>, Previous: <a href="Data-Output.html#Data-Output" accesskey="p" rel="prev">Data Output</a>, Up: <a href="Assembler-Format.html#Assembler-Format" accesskey="u" rel="up">Assembler Format</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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</body>
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</html>
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