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157 lines
8.8 KiB
HTML
4 years ago
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<html lang="en">
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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="GNU Compiler Collection (GCC) Internals">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="RTL.html#RTL" title="RTL">
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<link rel="prev" href="Vector-Operations.html#Vector-Operations" title="Vector Operations">
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<link rel="next" href="RTL-Declarations.html#RTL-Declarations" title="RTL Declarations">
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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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(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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</head>
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<body>
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<div class="node">
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<a name="Conversions"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="RTL-Declarations.html#RTL-Declarations">RTL Declarations</a>,
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Previous: <a rel="previous" accesskey="p" href="Vector-Operations.html#Vector-Operations">Vector Operations</a>,
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Up: <a rel="up" accesskey="u" href="RTL.html#RTL">RTL</a>
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<hr>
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</div>
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<h3 class="section">13.13 Conversions</h3>
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<p><a name="index-conversions-3010"></a><a name="index-machine-mode-conversions-3011"></a>
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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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<pre class="smallexample"> (plus:SI (sign_extend:SI (reg:QI 34)) (reg:SI 80))
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</pre>
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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>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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<a name="index-sign_005fextend-3012"></a>
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<dl><dt><code>(sign_extend:</code><var>m</var> <var>x</var><code>)</code><dd>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><a name="index-zero_005fextend-3013"></a><br><dt><code>(zero_extend:</code><var>m</var> <var>x</var><code>)</code><dd>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><a name="index-float_005fextend-3014"></a><br><dt><code>(float_extend:</code><var>m</var> <var>x</var><code>)</code><dd>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><a name="index-truncate-3015"></a><br><dt><code>(truncate:</code><var>m</var> <var>x</var><code>)</code><dd>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><a name="index-ss_005ftruncate-3016"></a><br><dt><code>(ss_truncate:</code><var>m</var> <var>x</var><code>)</code><dd>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><a name="index-us_005ftruncate-3017"></a><br><dt><code>(us_truncate:</code><var>m</var> <var>x</var><code>)</code><dd>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><a name="index-float_005ftruncate-3018"></a><br><dt><code>(float_truncate:</code><var>m</var> <var>x</var><code>)</code><dd>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><a name="index-float-3019"></a><br><dt><code>(float:</code><var>m</var> <var>x</var><code>)</code><dd>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><a name="index-unsigned_005ffloat-3020"></a><br><dt><code>(unsigned_float:</code><var>m</var> <var>x</var><code>)</code><dd>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><a name="index-fix-3021"></a><br><dt><code>(fix:</code><var>m</var> <var>x</var><code>)</code><dd>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>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><a name="index-unsigned_005ffix-3022"></a><br><dt><code>(unsigned_fix:</code><var>m</var> <var>x</var><code>)</code><dd>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><a name="index-fract_005fconvert-3023"></a><br><dt><code>(fract_convert:</code><var>m</var> <var>x</var><code>)</code><dd>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><a name="index-sat_005ffract-3024"></a><br><dt><code>(sat_fract:</code><var>m</var> <var>x</var><code>)</code><dd>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><a name="index-unsigned_005ffract_005fconvert-3025"></a><br><dt><code>(unsigned_fract_convert:</code><var>m</var> <var>x</var><code>)</code><dd>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><a name="index-unsigned_005fsat_005ffract-3026"></a><br><dt><code>(unsigned_sat_fract:</code><var>m</var> <var>x</var><code>)</code><dd>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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</dl>
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</body></html>
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