You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
183 lines
9.2 KiB
HTML
183 lines
9.2 KiB
HTML
4 years ago
|
<html lang="en">
|
||
|
|
<head>
|
||
|
|
<title>Alpha-Relocs - Using as</title>
|
||
|
|
<meta http-equiv="Content-Type" content="text/html">
|
||
|
|
<meta name="description" content="Using as">
|
||
|
|
<meta name="generator" content="makeinfo 4.13">
|
||
|
|
<link title="Top" rel="start" href="index.html#Top">
|
||
|
|
<link rel="up" href="Alpha-Syntax.html#Alpha-Syntax" title="Alpha Syntax">
|
||
|
|
<link rel="prev" href="Alpha_002dRegs.html#Alpha_002dRegs" title="Alpha-Regs">
|
||
|
|
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
|
||
|
|
<!--
|
||
|
|
This file documents the GNU Assembler "as".
|
||
|
|
|
||
|
|
Copyright (C) 1991-2015 Free Software Foundation, Inc.
|
||
|
|
|
||
|
|
Permission is granted to copy, distribute and/or modify this document
|
||
|
|
under the terms of the GNU Free Documentation License, Version 1.3
|
||
|
|
or any later version published by the Free Software Foundation;
|
||
|
|
with no Invariant Sections, with no Front-Cover Texts, and with no
|
||
|
|
Back-Cover Texts. A copy of the license is included in the
|
||
|
|
section entitled ``GNU Free Documentation License''.
|
||
|
|
|
||
|
|
-->
|
||
|
|
<meta http-equiv="Content-Style-Type" content="text/css">
|
||
|
|
<style type="text/css"><!--
|
||
|
|
pre.display { font-family:inherit }
|
||
|
|
pre.format { font-family:inherit }
|
||
|
|
pre.smalldisplay { font-family:inherit; font-size:smaller }
|
||
|
|
pre.smallformat { font-family:inherit; font-size:smaller }
|
||
|
|
pre.smallexample { font-size:smaller }
|
||
|
|
pre.smalllisp { font-size:smaller }
|
||
|
|
span.sc { font-variant:small-caps }
|
||
|
|
span.roman { font-family:serif; font-weight:normal; }
|
||
|
|
span.sansserif { font-family:sans-serif; font-weight:normal; }
|
||
|
|
--></style>
|
||
|
|
</head>
|
||
|
|
<body>
|
||
|
|
<div class="node">
|
||
|
|
<a name="Alpha-Relocs"></a>
|
||
|
|
<a name="Alpha_002dRelocs"></a>
|
||
|
|
<p>
|
||
|
|
Previous: <a rel="previous" accesskey="p" href="Alpha_002dRegs.html#Alpha_002dRegs">Alpha-Regs</a>,
|
||
|
|
Up: <a rel="up" accesskey="u" href="Alpha-Syntax.html#Alpha-Syntax">Alpha Syntax</a>
|
||
|
|
<hr>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
<h5 class="subsubsection">9.2.3.3 Relocations</h5>
|
||
|
|
|
||
|
|
<p><a name="index-Alpha-relocations-585"></a><a name="index-relocations_002c-Alpha-586"></a>
|
||
|
|
Some of these relocations are available for ECOFF, but mostly
|
||
|
|
only for ELF. They are modeled after the relocation format
|
||
|
|
introduced in Digital Unix 4.0, but there are additions.
|
||
|
|
|
||
|
|
<p>The format is ‘<samp><span class="samp">!</span><var>tag</var></samp>’ or ‘<samp><span class="samp">!</span><var>tag</var><span class="samp">!</span><var>number</var></samp>’
|
||
|
|
where <var>tag</var> is the name of the relocation. In some cases
|
||
|
|
<var>number</var> is used to relate specific instructions.
|
||
|
|
|
||
|
|
<p>The relocation is placed at the end of the instruction like so:
|
||
|
|
|
||
|
|
<pre class="example"> ldah $0,a($29) !gprelhigh
|
||
|
|
lda $0,a($0) !gprellow
|
||
|
|
ldq $1,b($29) !literal!100
|
||
|
|
ldl $2,0($1) !lituse_base!100
|
||
|
|
</pre>
|
||
|
|
<dl>
|
||
|
|
<dt><code>!literal</code><dt><code>!literal!</code><var>N</var><dd>Used with an <code>ldq</code> instruction to load the address of a symbol
|
||
|
|
from the GOT.
|
||
|
|
|
||
|
|
<p>A sequence number <var>N</var> is optional, and if present is used to pair
|
||
|
|
<code>lituse</code> relocations with this <code>literal</code> relocation. The
|
||
|
|
<code>lituse</code> relocations are used by the linker to optimize the code
|
||
|
|
based on the final location of the symbol.
|
||
|
|
|
||
|
|
<p>Note that these optimizations are dependent on the data flow of the
|
||
|
|
program. Therefore, if <em>any</em> <code>lituse</code> is paired with a
|
||
|
|
<code>literal</code> relocation, then <em>all</em> uses of the register set by
|
||
|
|
the <code>literal</code> instruction must also be marked with <code>lituse</code>
|
||
|
|
relocations. This is because the original <code>literal</code> instruction
|
||
|
|
may be deleted or transformed into another instruction.
|
||
|
|
|
||
|
|
<p>Also note that there may be a one-to-many relationship between
|
||
|
|
<code>literal</code> and <code>lituse</code>, but not a many-to-one. That is, if
|
||
|
|
there are two code paths that load up the same address and feed the
|
||
|
|
value to a single use, then the use may not use a <code>lituse</code>
|
||
|
|
relocation.
|
||
|
|
|
||
|
|
<br><dt><code>!lituse_base!</code><var>N</var><dd>Used with any memory format instruction (e.g. <code>ldl</code>) to indicate
|
||
|
|
that the literal is used for an address load. The offset field of the
|
||
|
|
instruction must be zero. During relaxation, the code may be altered
|
||
|
|
to use a gp-relative load.
|
||
|
|
|
||
|
|
<br><dt><code>!lituse_jsr!</code><var>N</var><dd>Used with a register branch format instruction (e.g. <code>jsr</code>) to
|
||
|
|
indicate that the literal is used for a call. During relaxation, the
|
||
|
|
code may be altered to use a direct branch (e.g. <code>bsr</code>).
|
||
|
|
|
||
|
|
<br><dt><code>!lituse_jsrdirect!</code><var>N</var><dd>Similar to <code>lituse_jsr</code>, but also that this call cannot be vectored
|
||
|
|
through a PLT entry. This is useful for functions with special calling
|
||
|
|
conventions which do not allow the normal call-clobbered registers to be
|
||
|
|
clobbered.
|
||
|
|
|
||
|
|
<br><dt><code>!lituse_bytoff!</code><var>N</var><dd>Used with a byte mask instruction (e.g. <code>extbl</code>) to indicate
|
||
|
|
that only the low 3 bits of the address are relevant. During relaxation,
|
||
|
|
the code may be altered to use an immediate instead of a register shift.
|
||
|
|
|
||
|
|
<br><dt><code>!lituse_addr!</code><var>N</var><dd>Used with any other instruction to indicate that the original address
|
||
|
|
is in fact used, and the original <code>ldq</code> instruction may not be
|
||
|
|
altered or deleted. This is useful in conjunction with <code>lituse_jsr</code>
|
||
|
|
to test whether a weak symbol is defined.
|
||
|
|
|
||
|
|
<pre class="example"> ldq $27,foo($29) !literal!1
|
||
|
|
beq $27,is_undef !lituse_addr!1
|
||
|
|
jsr $26,($27),foo !lituse_jsr!1
|
||
|
|
</pre>
|
||
|
|
<br><dt><code>!lituse_tlsgd!</code><var>N</var><dd>Used with a register branch format instruction to indicate that the
|
||
|
|
literal is the call to <code>__tls_get_addr</code> used to compute the
|
||
|
|
address of the thread-local storage variable whose descriptor was
|
||
|
|
loaded with <code>!tlsgd!</code><var>N</var>.
|
||
|
|
|
||
|
|
<br><dt><code>!lituse_tlsldm!</code><var>N</var><dd>Used with a register branch format instruction to indicate that the
|
||
|
|
literal is the call to <code>__tls_get_addr</code> used to compute the
|
||
|
|
address of the base of the thread-local storage block for the current
|
||
|
|
module. The descriptor for the module must have been loaded with
|
||
|
|
<code>!tlsldm!</code><var>N</var>.
|
||
|
|
|
||
|
|
<br><dt><code>!gpdisp!</code><var>N</var><dd>Used with <code>ldah</code> and <code>lda</code> to load the GP from the current
|
||
|
|
address, a-la the <code>ldgp</code> macro. The source register for the
|
||
|
|
<code>ldah</code> instruction must contain the address of the <code>ldah</code>
|
||
|
|
instruction. There must be exactly one <code>lda</code> instruction paired
|
||
|
|
with the <code>ldah</code> instruction, though it may appear anywhere in
|
||
|
|
the instruction stream. The immediate operands must be zero.
|
||
|
|
|
||
|
|
<pre class="example"> bsr $26,foo
|
||
|
|
ldah $29,0($26) !gpdisp!1
|
||
|
|
lda $29,0($29) !gpdisp!1
|
||
|
|
</pre>
|
||
|
|
<br><dt><code>!gprelhigh</code><dd>Used with an <code>ldah</code> instruction to add the high 16 bits of a
|
||
|
|
32-bit displacement from the GP.
|
||
|
|
|
||
|
|
<br><dt><code>!gprellow</code><dd>Used with any memory format instruction to add the low 16 bits of a
|
||
|
|
32-bit displacement from the GP.
|
||
|
|
|
||
|
|
<br><dt><code>!gprel</code><dd>Used with any memory format instruction to add a 16-bit displacement
|
||
|
|
from the GP.
|
||
|
|
|
||
|
|
<br><dt><code>!samegp</code><dd>Used with any branch format instruction to skip the GP load at the
|
||
|
|
target address. The referenced symbol must have the same GP as the
|
||
|
|
source object file, and it must be declared to either not use <code>$27</code>
|
||
|
|
or perform a standard GP load in the first two instructions via the
|
||
|
|
<code>.prologue</code> directive.
|
||
|
|
|
||
|
|
<br><dt><code>!tlsgd</code><dt><code>!tlsgd!</code><var>N</var><dd>Used with an <code>lda</code> instruction to load the address of a TLS
|
||
|
|
descriptor for a symbol in the GOT.
|
||
|
|
|
||
|
|
<p>The sequence number <var>N</var> is optional, and if present it used to
|
||
|
|
pair the descriptor load with both the <code>literal</code> loading the
|
||
|
|
address of the <code>__tls_get_addr</code> function and the <code>lituse_tlsgd</code>
|
||
|
|
marking the call to that function.
|
||
|
|
|
||
|
|
<p>For proper relaxation, both the <code>tlsgd</code>, <code>literal</code> and
|
||
|
|
<code>lituse</code> relocations must be in the same extended basic block.
|
||
|
|
That is, the relocation with the lowest address must be executed
|
||
|
|
first at runtime.
|
||
|
|
|
||
|
|
<br><dt><code>!tlsldm</code><dt><code>!tlsldm!</code><var>N</var><dd>Used with an <code>lda</code> instruction to load the address of a TLS
|
||
|
|
descriptor for the current module in the GOT.
|
||
|
|
|
||
|
|
<p>Similar in other respects to <code>tlsgd</code>.
|
||
|
|
|
||
|
|
<br><dt><code>!gotdtprel</code><dd>Used with an <code>ldq</code> instruction to load the offset of the TLS
|
||
|
|
symbol within its module's thread-local storage block. Also known
|
||
|
|
as the dynamic thread pointer offset or dtp-relative offset.
|
||
|
|
|
||
|
|
<br><dt><code>!dtprelhi</code><dt><code>!dtprello</code><dt><code>!dtprel</code><dd>Like <code>gprel</code> relocations except they compute dtp-relative offsets.
|
||
|
|
|
||
|
|
<br><dt><code>!gottprel</code><dd>Used with an <code>ldq</code> instruction to load the offset of the TLS
|
||
|
|
symbol from the thread pointer. Also known as the tp-relative offset.
|
||
|
|
|
||
|
|
<br><dt><code>!tprelhi</code><dt><code>!tprello</code><dt><code>!tprel</code><dd>Like <code>gprel</code> relocations except they compute tp-relative offsets.
|
||
|
|
</dl>
|
||
|
|
|
||
|
|
</body></html>
|
||
|
|
|