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209 lines
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209 lines
8.6 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 Assembler "as".
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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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-->
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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>RISC-V-Directives (Using as)</title>
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<meta name="description" content="RISC-V-Directives (Using as)">
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<meta name="keywords" content="RISC-V-Directives (Using as)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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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="AS-Index.html#AS-Index" rel="index" title="AS Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="RISC_002dV_002dDependent.html#RISC_002dV_002dDependent" rel="up" title="RISC-V-Dependent">
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<link href="RISC_002dV_002dFormats.html#RISC_002dV_002dFormats" rel="next" title="RISC-V-Formats">
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<link href="RISC_002dV_002dOptions.html#RISC_002dV_002dOptions" rel="prev" title="RISC-V-Options">
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</style>
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</head>
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<body lang="en">
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<a name="RISC_002dV_002dDirectives"></a>
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<div class="header">
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<p>
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Next: <a href="RISC_002dV_002dFormats.html#RISC_002dV_002dFormats" accesskey="n" rel="next">RISC-V-Formats</a>, Previous: <a href="RISC_002dV_002dOptions.html#RISC_002dV_002dOptions" accesskey="p" rel="prev">RISC-V-Options</a>, Up: <a href="RISC_002dV_002dDependent.html#RISC_002dV_002dDependent" accesskey="u" rel="up">RISC-V-Dependent</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="RISC_002dV-Directives"></a>
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<h4 class="subsection">9.37.2 RISC-V Directives</h4>
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<a name="index-machine-directives_002c-RISC_002dV"></a>
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<a name="index-RISC_002dV-machine-directives"></a>
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<p>The following table lists all available RISC-V specific directives.
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</p>
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<dl compact="compact">
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<dd>
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<a name="index-align-directive-1"></a>
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</dd>
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<dt><code>.align <var>size-log-2</var></code></dt>
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<dd><p>Align to the given boundary, with the size given as log2 the number of bytes to
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align to.
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</p>
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<a name="index-Data-directives"></a>
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</dd>
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<dt><code>.half <var>value</var></code></dt>
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<dt><code>.word <var>value</var></code></dt>
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<dt><code>.dword <var>value</var></code></dt>
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<dd><p>Emits a half-word, word, or double-word value at the current position.
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</p>
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<a name="index-DTP_002drelative-data-directives"></a>
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</dd>
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<dt><code>.dtprelword <var>value</var></code></dt>
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<dt><code>.dtpreldword <var>value</var></code></dt>
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<dd><p>Emits a DTP-relative word (or double-word) at the current position. This is
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meant to be used by the compiler in shared libraries for DWARF debug info for
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thread local variables.
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</p>
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<a name="index-BSS-directive"></a>
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</dd>
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<dt><code>.bss</code></dt>
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<dd><p>Sets the current section to the BSS section.
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</p>
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<a name="index-LEB128-directives"></a>
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</dd>
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<dt><code>.uleb128 <var>value</var></code></dt>
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<dt><code>.sleb128 <var>value</var></code></dt>
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<dd><p>Emits a signed or unsigned LEB128 value at the current position. This only
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accepts constant expressions, because symbol addresses can change with
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relaxation, and we don’t support relocations to modify LEB128 values at link
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time.
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</p>
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<a name="index-Option-directive"></a>
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<a name="index-option-directive"></a>
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</dd>
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<dt><code>.option <var>argument</var></code></dt>
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<dd><p>Modifies RISC-V specific assembler options inline with the assembly code.
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This is used when particular instruction sequences must be assembled with a
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specific set of options. For example, since we relax addressing sequences to
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shorter GP-relative sequences when possible the initial load of GP must not be
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relaxed and should be emitted as something like
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</p>
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<div class="smallexample">
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<pre class="smallexample"> .option push
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.option norelax
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la gp, __global_pointer$
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.option pop
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</pre></div>
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<p>in order to produce after linker relaxation the expected
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</p>
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<div class="smallexample">
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<pre class="smallexample"> auipc gp, %pcrel_hi(__global_pointer$)
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addi gp, gp, %pcrel_lo(__global_pointer$)
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</pre></div>
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<p>instead of just
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</p>
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<div class="smallexample">
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<pre class="smallexample"> addi gp, gp, 0
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</pre></div>
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<p>It’s not expected that options are changed in this manner during regular use,
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but there are a handful of esoteric cases like the one above where users need
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to disable particular features of the assembler for particular code sequences.
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The complete list of option arguments is shown below:
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</p>
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<dl compact="compact">
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<dt><code>push</code></dt>
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<dt><code>pop</code></dt>
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<dd><p>Pushes or pops the current option stack. These should be used whenever
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changing an option in line with assembly code in order to ensure the user’s
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command-line options are respected for the bulk of the file being assembled.
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</p>
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</dd>
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<dt><code>rvc</code></dt>
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<dt><code>norvc</code></dt>
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<dd><p>Enables or disables the generation of compressed instructions. Instructions
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are opportunistically compressed by the RISC-V assembler when possible, but
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sometimes this behavior is not desirable.
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</p>
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</dd>
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<dt><code>pic</code></dt>
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<dt><code>nopic</code></dt>
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<dd><p>Enables or disables position-independent code generation. Unless you really
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know what you’re doing, this should only be at the top of a file.
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</p>
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</dd>
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<dt><code>relax</code></dt>
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<dt><code>norelax</code></dt>
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<dd><p>Enables or disables relaxation. The RISC-V assembler and linker
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opportunistically relax some code sequences, but sometimes this behavior is not
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desirable.
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</p></dd>
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</dl>
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<a name="index-INSN-directives"></a>
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</dd>
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<dt><code>.insn <var>value</var></code></dt>
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<dt><code>.insn <var>value</var></code></dt>
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<dd><p>This directive permits the numeric representation of an instructions
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and makes the assembler insert the operands according to one of the
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instruction formats for ‘<samp>.insn</samp>’ (<a href="RISC_002dV_002dFormats.html#RISC_002dV_002dFormats">RISC-V-Formats</a>).
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For example, the instruction ‘<samp>add a0, a1, a2</samp>’ could be written as
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‘<samp>.insn r 0x33, 0, 0, a0, a1, a2</samp>’.
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</p>
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<a name="index-_002eattribute-directive_002c-RISC_002dV"></a>
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</dd>
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<dt><code>.attribute <var>tag</var>, <var>value</var></code></dt>
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<dd><p>Set the object attribute <var>tag</var> to <var>value</var>.
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</p>
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<p>The <var>tag</var> is either an attribute number, or one of the following:
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<code>Tag_RISCV_arch</code>, <code>Tag_RISCV_stack_align</code>,
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<code>Tag_RISCV_unaligned_access</code>, <code>Tag_RISCV_priv_spec</code>,
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<code>Tag_RISCV_priv_spec_minor</code>, <code>Tag_RISCV_priv_spec_revision</code>.
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</p>
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</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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Next: <a href="RISC_002dV_002dFormats.html#RISC_002dV_002dFormats" accesskey="n" rel="next">RISC-V-Formats</a>, Previous: <a href="RISC_002dV_002dOptions.html#RISC_002dV_002dOptions" accesskey="p" rel="prev">RISC-V-Options</a>, Up: <a href="RISC_002dV_002dDependent.html#RISC_002dV_002dDependent" accesskey="u" rel="up">RISC-V-Dependent</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-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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