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162 lines
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162 lines
8.6 KiB
HTML
4 years ago
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<html lang="en">
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<title>Uninitialized Data - GNU Compiler Collection (GCC) Internals</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Assembler-Format.html#Assembler-Format" title="Assembler Format">
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<link rel="prev" href="Data-Output.html#Data-Output" title="Data Output">
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<link rel="next" href="Label-Output.html#Label-Output" title="Label Output">
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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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(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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<body>
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<div class="node">
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<a name="Uninitialized-Data"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Label-Output.html#Label-Output">Label Output</a>,
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Previous: <a rel="previous" accesskey="p" href="Data-Output.html#Data-Output">Data Output</a>,
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Up: <a rel="up" accesskey="u" href="Assembler-Format.html#Assembler-Format">Assembler Format</a>
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<hr>
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</div>
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<h4 class="subsection">17.20.3 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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<div class="defun">
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— Macro: <b>ASM_OUTPUT_COMMON</b> (<var>stream, name, size, rounded</var>)<var><a name="index-ASM_005fOUTPUT_005fCOMMON-4608"></a></var><br>
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<blockquote><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>Use the expression <code>assemble_name (</code><var>stream</var><code>, </code><var>name</var><code>)</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>This macro controls how the assembler definitions of uninitialized
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common global variables are output.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>ASM_OUTPUT_ALIGNED_COMMON</b> (<var>stream, name, size, alignment</var>)<var><a name="index-ASM_005fOUTPUT_005fALIGNED_005fCOMMON-4609"></a></var><br>
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<blockquote><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></blockquote></div>
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<div class="defun">
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— Macro: <b>ASM_OUTPUT_ALIGNED_DECL_COMMON</b> (<var>stream, decl, name, size, alignment</var>)<var><a name="index-ASM_005fOUTPUT_005fALIGNED_005fDECL_005fCOMMON-4610"></a></var><br>
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<blockquote><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></blockquote></div>
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<div class="defun">
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— Macro: <b>ASM_OUTPUT_ALIGNED_BSS</b> (<var>stream, decl, name, size, alignment</var>)<var><a name="index-ASM_005fOUTPUT_005fALIGNED_005fBSS-4611"></a></var><br>
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<blockquote><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>Try to use function <code>asm_output_aligned_bss</code> defined in file
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<samp><span class="file">varasm.c</span></samp> when defining this macro. If unable, use the expression
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<code>assemble_name (</code><var>stream</var><code>, </code><var>name</var><code>)</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>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="TARGET_005fHAVE_005fSWITCHABLE_005fBSS_005fSECTIONS.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>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></blockquote></div>
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<div class="defun">
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— Macro: <b>ASM_OUTPUT_LOCAL</b> (<var>stream, name, size, rounded</var>)<var><a name="index-ASM_005fOUTPUT_005fLOCAL-4612"></a></var><br>
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<blockquote><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>Use the expression <code>assemble_name (</code><var>stream</var><code>, </code><var>name</var><code>)</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>This macro controls how the assembler definitions of uninitialized
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static variables are output.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>ASM_OUTPUT_ALIGNED_LOCAL</b> (<var>stream, name, size, alignment</var>)<var><a name="index-ASM_005fOUTPUT_005fALIGNED_005fLOCAL-4613"></a></var><br>
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<blockquote><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></blockquote></div>
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<div class="defun">
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— Macro: <b>ASM_OUTPUT_ALIGNED_DECL_LOCAL</b> (<var>stream, decl, name, size, alignment</var>)<var><a name="index-ASM_005fOUTPUT_005fALIGNED_005fDECL_005fLOCAL-4614"></a></var><br>
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<blockquote><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></blockquote></div>
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</body></html>
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