131 lines
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131 lines
7.0 KiB
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<title>RISC-V-Directives - Using as</title>
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<meta name="description" content="Using as">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="RISC_002dV_002dDependent.html#RISC_002dV_002dDependent" title="RISC-V-Dependent">
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<link rel="prev" href="RISC_002dV_002dOptions.html#RISC_002dV_002dOptions" title="RISC-V-Options">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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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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</head>
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<body>
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<div class="node">
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<a name="RISC-V-Directives"></a>
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<a name="RISC_002dV_002dDirectives"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="RISC_002dV_002dFormats.html#RISC_002dV_002dFormats">RISC-V-Formats</a>,
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Previous: <a rel="previous" accesskey="p" href="RISC_002dV_002dOptions.html#RISC_002dV_002dOptions">RISC-V-Options</a>,
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Up: <a rel="up" accesskey="u" href="RISC_002dV_002dDependent.html#RISC_002dV_002dDependent">RISC-V-Dependent</a>
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<hr>
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</div>
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<h4 class="subsection">9.38.2 RISC-V Directives</h4>
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<p><a name="index-machine-directives_002c-RISC_002dV-1988"></a><a name="index-RISC_002dV-machine-directives-1989"></a>
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The following table lists all available RISC-V specific directives.
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<a name="index-g_t_0040code_007balign_007d-directive-1990"></a>
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<dl><dt><code>.align </code><var>size-log-2</var><dd>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><a name="index-Data-directives-1991"></a><br><dt><code>.half </code><var>value</var><dt><code>.word </code><var>value</var><dt><code>.dword </code><var>value</var><dd>Emits a half-word, word, or double-word value at the current position.
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<p><a name="index-DTP_002drelative-data-directives-1992"></a><br><dt><code>.dtprelword </code><var>value</var><dt><code>.dtpreldword </code><var>value</var><dd>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><a name="index-BSS-directive-1993"></a><br><dt><code>.bss</code><dd>Sets the current section to the BSS section.
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<p><a name="index-LEB128-directives-1994"></a><br><dt><code>.uleb128 </code><var>value</var><dt><code>.sleb128 </code><var>value</var><dd>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><a name="index-Option-directive-1995"></a><a name="index-g_t_0040code_007boption_007d-directive-1996"></a><br><dt><code>.option </code><var>argument</var><dd>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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<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>
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<p>in order to produce after linker relaxation the expected
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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>
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<p>instead of just
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<pre class="smallexample"> addi gp, gp, 0
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</pre>
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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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<dl>
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<dt><code>push</code><dt><code>pop</code><dd>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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<br><dt><code>rvc</code><dt><code>norvc</code><dd>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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<br><dt><code>pic</code><dt><code>nopic</code><dd>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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<br><dt><code>relax</code><dt><code>norelax</code><dd>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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</dl>
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<p><a name="index-INSN-directives-1997"></a><br><dt><code>.insn </code><var>value</var><dt><code>.insn </code><var>value</var><dd>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><span class="samp">.insn</span></samp>’ (<a href="RISC_002dV_002dFormats.html#RISC_002dV_002dFormats">RISC-V-Formats</a>).
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For example, the instruction ‘<samp><span class="samp">add a0, a1, a2</span></samp>’ could be written as
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‘<samp><span class="samp">.insn r 0x33, 0, 0, a0, a1, a2</span></samp>’.
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<p><a name="index-g_t_0040code_007b_002eattribute_007d-directive_002c-RISC_002dV-1998"></a><br><dt><code>.attribute </code><var>tag</var><code>, </code><var>value</var><dd>Set the object attribute <var>tag</var> to <var>value</var>.
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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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</dl>
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</body></html>
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