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111 lines
4.5 KiB
HTML
111 lines
4.5 KiB
HTML
<!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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<!-- This file documents the GNU linker LD
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(SiFive Binutils 2.32.0-2020.04.1)
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version 2.32.
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Copyright (C) 1991-2019 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
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or any later version published by the Free Software Foundation;
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with no Invariant Sections, with no Front-Cover Texts, and with no
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Back-Cover Texts. A copy of the license is included in the
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section entitled "GNU Free Documentation License". -->
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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>Evaluation (LD)</title>
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<meta name="description" content="Evaluation (LD)">
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<meta name="keywords" content="Evaluation (LD)">
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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="LD-Index.html#LD-Index" rel="index" title="LD Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Expressions.html#Expressions" rel="up" title="Expressions">
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<link href="Expression-Section.html#Expression-Section" rel="next" title="Expression Section">
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<link href="Operators.html#Operators" rel="prev" title="Operators">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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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="Evaluation"></a>
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<div class="header">
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<p>
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Next: <a href="Expression-Section.html#Expression-Section" accesskey="n" rel="next">Expression Section</a>, Previous: <a href="Operators.html#Operators" accesskey="p" rel="prev">Operators</a>, Up: <a href="Expressions.html#Expressions" accesskey="u" rel="up">Expressions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Evaluation-1"></a>
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<h4 class="subsection">3.10.7 Evaluation</h4>
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<a name="index-lazy-evaluation"></a>
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<a name="index-expression-evaluation-order"></a>
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<p>The linker evaluates expressions lazily. It only computes the value of
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an expression when absolutely necessary.
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</p>
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<p>The linker needs some information, such as the value of the start
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address of the first section, and the origins and lengths of memory
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regions, in order to do any linking at all. These values are computed
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as soon as possible when the linker reads in the linker script.
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</p>
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<p>However, other values (such as symbol values) are not known or needed
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until after storage allocation. Such values are evaluated later, when
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other information (such as the sizes of output sections) is available
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for use in the symbol assignment expression.
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</p>
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<p>The sizes of sections cannot be known until after allocation, so
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assignments dependent upon these are not performed until after
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allocation.
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</p>
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<p>Some expressions, such as those depending upon the location counter
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‘<samp>.</samp>’, must be evaluated during section allocation.
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</p>
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<p>If the result of an expression is required, but the value is not
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available, then an error results. For example, a script like the
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following
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</p><div class="smallexample">
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<pre class="smallexample">SECTIONS
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{
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.text 9+this_isnt_constant :
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{ *(.text) }
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}
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</pre></div>
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<p>will cause the error message ‘<samp>non constant expression for initial
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address</samp>’.
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</p>
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</body>
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</html>
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