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149 lines
6.6 KiB
HTML
149 lines
6.6 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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<!-- 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>Xtensa (LD)</title>
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<meta name="description" content="Xtensa (LD)">
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<meta name="keywords" content="Xtensa (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="Machine-Dependent.html#Machine-Dependent" rel="up" title="Machine Dependent">
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<link href="BFD.html#BFD" rel="next" title="BFD">
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<link href="WIN32.html#WIN32" rel="prev" title="WIN32">
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<style type="text/css">
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<!--
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a.summary-letter {text-decoration: none}
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blockquote.indentedblock {margin-right: 0em}
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blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
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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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div.lisp {margin-left: 3.2em}
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div.smalldisplay {margin-left: 3.2em}
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div.smallexample {margin-left: 3.2em}
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div.smalllisp {margin-left: 3.2em}
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kbd {font-style: oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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pre.menu-preformatted {font-family: serif}
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pre.smalldisplay {font-family: inherit; font-size: smaller}
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pre.smallexample {font-size: smaller}
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pre.smallformat {font-family: inherit; font-size: smaller}
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pre.smalllisp {font-size: smaller}
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span.nolinebreak {white-space: nowrap}
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span.roman {font-family: initial; font-weight: normal}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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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="Xtensa"></a>
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<div class="header">
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<p>
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Previous: <a href="WIN32.html#WIN32" accesskey="p" rel="prev">WIN32</a>, Up: <a href="Machine-Dependent.html#Machine-Dependent" accesskey="u" rel="up">Machine Dependent</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="ld-and-Xtensa-Processors"></a>
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<h3 class="section">4.17 <code>ld</code> and Xtensa Processors</h3>
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<a name="index-Xtensa-processors"></a>
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<p>The default <code>ld</code> behavior for Xtensa processors is to interpret
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<code>SECTIONS</code> commands so that lists of explicitly named sections in a
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specification with a wildcard file will be interleaved when necessary to
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keep literal pools within the range of PC-relative load offsets. For
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example, with the command:
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</p>
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<div class="smallexample">
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<pre class="smallexample">SECTIONS
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{
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.text : {
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*(.literal .text)
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}
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}
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</pre></div>
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<p><code>ld</code> may interleave some of the <code>.literal</code>
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and <code>.text</code> sections from different object files to ensure that the
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literal pools are within the range of PC-relative load offsets. A valid
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interleaving might place the <code>.literal</code> sections from an initial
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group of files followed by the <code>.text</code> sections of that group of
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files. Then, the <code>.literal</code> sections from the rest of the files
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and the <code>.text</code> sections from the rest of the files would follow.
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</p>
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<a name="index-_002d_002drelax-on-Xtensa"></a>
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<a name="index-relaxing-on-Xtensa"></a>
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<p>Relaxation is enabled by default for the Xtensa version of <code>ld</code> and
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provides two important link-time optimizations. The first optimization
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is to combine identical literal values to reduce code size. A redundant
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literal will be removed and all the <code>L32R</code> instructions that use it
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will be changed to reference an identical literal, as long as the
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location of the replacement literal is within the offset range of all
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the <code>L32R</code> instructions. The second optimization is to remove
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unnecessary overhead from assembler-generated “longcall” sequences of
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<code>L32R</code>/<code>CALLX<var>n</var></code> when the target functions are within
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range of direct <code>CALL<var>n</var></code> instructions.
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</p>
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<p>For each of these cases where an indirect call sequence can be optimized
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to a direct call, the linker will change the <code>CALLX<var>n</var></code>
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instruction to a <code>CALL<var>n</var></code> instruction, remove the <code>L32R</code>
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instruction, and remove the literal referenced by the <code>L32R</code>
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instruction if it is not used for anything else. Removing the
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<code>L32R</code> instruction always reduces code size but can potentially
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hurt performance by changing the alignment of subsequent branch targets.
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By default, the linker will always preserve alignments, either by
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switching some instructions between 24-bit encodings and the equivalent
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density instructions or by inserting a no-op in place of the <code>L32R</code>
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instruction that was removed. If code size is more important than
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performance, the <samp>--size-opt</samp> option can be used to prevent the
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linker from widening density instructions or inserting no-ops, except in
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a few cases where no-ops are required for correctness.
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</p>
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<p>The following Xtensa-specific command-line options can be used to
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control the linker:
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</p>
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<a name="index-Xtensa-options"></a>
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<dl compact="compact">
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<dt><samp>--size-opt</samp></dt>
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<dd><p>When optimizing indirect calls to direct calls, optimize for code size
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more than performance. With this option, the linker will not insert
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no-ops or widen density instructions to preserve branch target
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alignment. There may still be some cases where no-ops are required to
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preserve the correctness of the code.
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</p></dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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Previous: <a href="WIN32.html#WIN32" accesskey="p" rel="prev">WIN32</a>, Up: <a href="Machine-Dependent.html#Machine-Dependent" accesskey="u" rel="up">Machine Dependent</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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</body>
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</html>
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