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235 lines
10 KiB
HTML
235 lines
10 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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<!-- Copyright (C) 1988-2018 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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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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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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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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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>Conversions (GNU Compiler Collection (GCC) Internals)</title>
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<meta name="description" content="Conversions (GNU Compiler Collection (GCC) Internals)">
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<meta name="keywords" content="Conversions (GNU Compiler Collection (GCC) Internals)">
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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="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="RTL.html#RTL" rel="up" title="RTL">
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<link href="RTL-Declarations.html#RTL-Declarations" rel="next" title="RTL Declarations">
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<link href="Vector-Operations.html#Vector-Operations" rel="prev" title="Vector Operations">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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blockquote.indentedblock {margin-right: 0em}
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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="Conversions"></a>
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<div class="header">
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<p>
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Next: <a href="RTL-Declarations.html#RTL-Declarations" accesskey="n" rel="next">RTL Declarations</a>, Previous: <a href="Vector-Operations.html#Vector-Operations" accesskey="p" rel="prev">Vector Operations</a>, Up: <a href="RTL.html#RTL" accesskey="u" rel="up">RTL</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Conversions-1"></a>
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<h3 class="section">14.13 Conversions</h3>
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<a name="index-conversions"></a>
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<a name="index-machine-mode-conversions"></a>
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<p>All conversions between machine modes must be represented by
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explicit conversion operations. For example, an expression
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which is the sum of a byte and a full word cannot be written as
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<code>(plus:SI (reg:QI 34) (reg:SI 80))</code> because the <code>plus</code>
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operation requires two operands of the same machine mode.
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Therefore, the byte-sized operand is enclosed in a conversion
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operation, as in
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</p>
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<div class="smallexample">
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<pre class="smallexample">(plus:SI (sign_extend:SI (reg:QI 34)) (reg:SI 80))
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</pre></div>
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<p>The conversion operation is not a mere placeholder, because there
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may be more than one way of converting from a given starting mode
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to the desired final mode. The conversion operation code says how
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to do it.
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</p>
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<p>For all conversion operations, <var>x</var> must not be <code>VOIDmode</code>
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because the mode in which to do the conversion would not be known.
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The conversion must either be done at compile-time or <var>x</var>
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must be placed into a register.
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</p>
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<dl compact="compact">
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<dd><a name="index-sign_005fextend"></a>
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</dd>
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<dt><code>(sign_extend:<var>m</var> <var>x</var>)</code></dt>
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<dd><p>Represents the result of sign-extending the value <var>x</var>
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to machine mode <var>m</var>. <var>m</var> must be a fixed-point mode
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and <var>x</var> a fixed-point value of a mode narrower than <var>m</var>.
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</p>
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<a name="index-zero_005fextend"></a>
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</dd>
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<dt><code>(zero_extend:<var>m</var> <var>x</var>)</code></dt>
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<dd><p>Represents the result of zero-extending the value <var>x</var>
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to machine mode <var>m</var>. <var>m</var> must be a fixed-point mode
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and <var>x</var> a fixed-point value of a mode narrower than <var>m</var>.
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</p>
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<a name="index-float_005fextend"></a>
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</dd>
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<dt><code>(float_extend:<var>m</var> <var>x</var>)</code></dt>
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<dd><p>Represents the result of extending the value <var>x</var>
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to machine mode <var>m</var>. <var>m</var> must be a floating point mode
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and <var>x</var> a floating point value of a mode narrower than <var>m</var>.
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</p>
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<a name="index-truncate"></a>
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</dd>
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<dt><code>(truncate:<var>m</var> <var>x</var>)</code></dt>
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<dd><p>Represents the result of truncating the value <var>x</var>
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to machine mode <var>m</var>. <var>m</var> must be a fixed-point mode
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and <var>x</var> a fixed-point value of a mode wider than <var>m</var>.
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</p>
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<a name="index-ss_005ftruncate"></a>
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</dd>
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<dt><code>(ss_truncate:<var>m</var> <var>x</var>)</code></dt>
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<dd><p>Represents the result of truncating the value <var>x</var>
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to machine mode <var>m</var>, using signed saturation in the case of
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overflow. Both <var>m</var> and the mode of <var>x</var> must be fixed-point
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modes.
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</p>
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<a name="index-us_005ftruncate"></a>
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</dd>
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<dt><code>(us_truncate:<var>m</var> <var>x</var>)</code></dt>
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<dd><p>Represents the result of truncating the value <var>x</var>
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to machine mode <var>m</var>, using unsigned saturation in the case of
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overflow. Both <var>m</var> and the mode of <var>x</var> must be fixed-point
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modes.
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</p>
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<a name="index-float_005ftruncate"></a>
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</dd>
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<dt><code>(float_truncate:<var>m</var> <var>x</var>)</code></dt>
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<dd><p>Represents the result of truncating the value <var>x</var>
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to machine mode <var>m</var>. <var>m</var> must be a floating point mode
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and <var>x</var> a floating point value of a mode wider than <var>m</var>.
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</p>
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<a name="index-float"></a>
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</dd>
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<dt><code>(float:<var>m</var> <var>x</var>)</code></dt>
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<dd><p>Represents the result of converting fixed point value <var>x</var>,
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regarded as signed, to floating point mode <var>m</var>.
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</p>
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<a name="index-unsigned_005ffloat"></a>
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</dd>
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<dt><code>(unsigned_float:<var>m</var> <var>x</var>)</code></dt>
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<dd><p>Represents the result of converting fixed point value <var>x</var>,
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regarded as unsigned, to floating point mode <var>m</var>.
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</p>
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<a name="index-fix"></a>
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</dd>
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<dt><code>(fix:<var>m</var> <var>x</var>)</code></dt>
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<dd><p>When <var>m</var> is a floating-point mode, represents the result of
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converting floating point value <var>x</var> (valid for mode <var>m</var>) to an
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integer, still represented in floating point mode <var>m</var>, by rounding
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towards zero.
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</p>
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<p>When <var>m</var> is a fixed-point mode, represents the result of
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converting floating point value <var>x</var> to mode <var>m</var>, regarded as
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signed. How rounding is done is not specified, so this operation may
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be used validly in compiling C code only for integer-valued operands.
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</p>
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<a name="index-unsigned_005ffix"></a>
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</dd>
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<dt><code>(unsigned_fix:<var>m</var> <var>x</var>)</code></dt>
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<dd><p>Represents the result of converting floating point value <var>x</var> to
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fixed point mode <var>m</var>, regarded as unsigned. How rounding is done
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is not specified.
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</p>
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<a name="index-fract_005fconvert"></a>
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</dd>
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<dt><code>(fract_convert:<var>m</var> <var>x</var>)</code></dt>
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<dd><p>Represents the result of converting fixed-point value <var>x</var> to
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fixed-point mode <var>m</var>, signed integer value <var>x</var> to
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fixed-point mode <var>m</var>, floating-point value <var>x</var> to
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fixed-point mode <var>m</var>, fixed-point value <var>x</var> to integer mode <var>m</var>
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regarded as signed, or fixed-point value <var>x</var> to floating-point mode <var>m</var>.
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When overflows or underflows happen, the results are undefined.
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</p>
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<a name="index-sat_005ffract"></a>
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</dd>
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<dt><code>(sat_fract:<var>m</var> <var>x</var>)</code></dt>
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<dd><p>Represents the result of converting fixed-point value <var>x</var> to
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fixed-point mode <var>m</var>, signed integer value <var>x</var> to
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fixed-point mode <var>m</var>, or floating-point value <var>x</var> to
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fixed-point mode <var>m</var>.
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When overflows or underflows happen, the results are saturated to the
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maximum or the minimum.
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</p>
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<a name="index-unsigned_005ffract_005fconvert"></a>
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</dd>
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<dt><code>(unsigned_fract_convert:<var>m</var> <var>x</var>)</code></dt>
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<dd><p>Represents the result of converting fixed-point value <var>x</var> to
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integer mode <var>m</var> regarded as unsigned, or unsigned integer value <var>x</var> to
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fixed-point mode <var>m</var>.
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When overflows or underflows happen, the results are undefined.
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</p>
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<a name="index-unsigned_005fsat_005ffract"></a>
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</dd>
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<dt><code>(unsigned_sat_fract:<var>m</var> <var>x</var>)</code></dt>
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<dd><p>Represents the result of converting unsigned integer value <var>x</var> to
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fixed-point mode <var>m</var>.
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When overflows or underflows happen, the results are saturated to the
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maximum or the minimum.
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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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Next: <a href="RTL-Declarations.html#RTL-Declarations" accesskey="n" rel="next">RTL Declarations</a>, Previous: <a href="Vector-Operations.html#Vector-Operations" accesskey="p" rel="prev">Vector Operations</a>, Up: <a href="RTL.html#RTL" accesskey="u" rel="up">RTL</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-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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