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153 lines
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153 lines
7.3 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Basic Asm - Using the GNU Compiler Collection (GCC)</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC)">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C" title="Using Assembly Language with C">
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<link rel="next" href="Extended-Asm.html#Extended-Asm" title="Extended Asm">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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Copyright (C) 1988-2015 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 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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``GNU Free Documentation License''.
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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</head>
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<body>
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<div class="node">
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<a name="Basic-Asm"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Extended-Asm.html#Extended-Asm">Extended Asm</a>,
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Up: <a rel="up" accesskey="u" href="Using-Assembly-Language-with-C.html#Using-Assembly-Language-with-C">Using Assembly Language with C</a>
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<hr>
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</div>
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<h4 class="subsection">6.43.1 Basic Asm — Assembler Instructions Without Operands</h4>
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<p><a name="index-basic-_0040code_007basm_007d-3421"></a><a name="index-assembly-language-in-C_002c-basic-3422"></a>
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A basic <code>asm</code> statement has the following syntax:
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<pre class="example"> asm <span class="roman">[</span> volatile <span class="roman">]</span> ( <var>AssemblerInstructions</var> )
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</pre>
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<p>The <code>asm</code> keyword is a GNU extension.
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When writing code that can be compiled with <samp><span class="option">-ansi</span></samp> and the
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various <samp><span class="option">-std</span></samp> options, use <code>__asm__</code> instead of
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<code>asm</code> (see <a href="Alternate-Keywords.html#Alternate-Keywords">Alternate Keywords</a>).
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<h5 class="subsubheading">Qualifiers</h5>
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<dl>
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<dt><code>volatile</code><dd>The optional <code>volatile</code> qualifier has no effect.
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All basic <code>asm</code> blocks are implicitly volatile.
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</dl>
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<h5 class="subsubheading">Parameters</h5>
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<dl>
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<dt><var>AssemblerInstructions</var><dd>This is a literal string that specifies the assembler code. The string can
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contain any instructions recognized by the assembler, including directives.
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GCC does not parse the assembler instructions themselves and
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does not know what they mean or even whether they are valid assembler input.
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<p>You may place multiple assembler instructions together in a single <code>asm</code>
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string, separated by the characters normally used in assembly code for the
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system. A combination that works in most places is a newline to break the
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line, plus a tab character (written as ‘<samp><span class="samp">\n\t</span></samp>’).
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Some assemblers allow semicolons as a line separator. However,
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note that some assembler dialects use semicolons to start a comment.
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</dl>
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<h5 class="subsubheading">Remarks</h5>
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<p>Using extended <code>asm</code> typically produces smaller, safer, and more
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efficient code, and in most cases it is a better solution than basic
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<code>asm</code>. However, there are two situations where only basic <code>asm</code>
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can be used:
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<ul>
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<li>Extended <code>asm</code> statements have to be inside a C
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function, so to write inline assembly language at file scope (“top-level”),
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outside of C functions, you must use basic <code>asm</code>.
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You can use this technique to emit assembler directives,
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define assembly language macros that can be invoked elsewhere in the file,
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or write entire functions in assembly language.
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<li>Functions declared
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with the <code>naked</code> attribute also require basic <code>asm</code>
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(see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>).
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</ul>
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<p>Safely accessing C data and calling functions from basic <code>asm</code> is more
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complex than it may appear. To access C data, it is better to use extended
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<code>asm</code>.
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<p>Do not expect a sequence of <code>asm</code> statements to remain perfectly
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consecutive after compilation. If certain instructions need to remain
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consecutive in the output, put them in a single multi-instruction <code>asm</code>
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statement. Note that GCC's optimizers can move <code>asm</code> statements
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relative to other code, including across jumps.
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<p><code>asm</code> statements may not perform jumps into other <code>asm</code> statements.
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GCC does not know about these jumps, and therefore cannot take
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account of them when deciding how to optimize. Jumps from <code>asm</code> to C
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labels are only supported in extended <code>asm</code>.
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<p>Under certain circumstances, GCC may duplicate (or remove duplicates of) your
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assembly code when optimizing. This can lead to unexpected duplicate
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symbol errors during compilation if your assembly code defines symbols or
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labels.
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<p>Since GCC does not parse the <var>AssemblerInstructions</var>, it has no
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visibility of any symbols it references. This may result in GCC discarding
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those symbols as unreferenced.
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<p>The compiler copies the assembler instructions in a basic <code>asm</code>
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verbatim to the assembly language output file, without
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processing dialects or any of the ‘<samp><span class="samp">%</span></samp>’ operators that are available with
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extended <code>asm</code>. This results in minor differences between basic
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<code>asm</code> strings and extended <code>asm</code> templates. For example, to refer to
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registers you might use ‘<samp><span class="samp">%eax</span></samp>’ in basic <code>asm</code> and
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‘<samp><span class="samp">%%eax</span></samp>’ in extended <code>asm</code>.
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<p>On targets such as x86 that support multiple assembler dialects,
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all basic <code>asm</code> blocks use the assembler dialect specified by the
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<samp><span class="option">-masm</span></samp> command-line option (see <a href="x86-Options.html#x86-Options">x86 Options</a>).
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Basic <code>asm</code> provides no
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mechanism to provide different assembler strings for different dialects.
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<p>Here is an example of basic <code>asm</code> for i386:
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<pre class="example"> /* Note that this code will not compile with -masm=intel */
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#define DebugBreak() asm("int $3")
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</pre>
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</body></html>
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