137 lines
6.4 KiB
HTML
137 lines
6.4 KiB
HTML
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<head>
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<title>Using the GNU Compiler Collection (GCC): Half-Precision</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC): Half-Precision">
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<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Half-Precision">
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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="C-Extensions.html#C-Extensions" rel="up" title="C Extensions">
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<link href="Decimal-Float.html#Decimal-Float" rel="next" title="Decimal Float">
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<link href="Floating-Types.html#Floating-Types" rel="prev" title="Floating Types">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Half_002dPrecision"></a>
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<div class="header">
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<p>
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Next: <a href="Decimal-Float.html#Decimal-Float" accesskey="n" rel="next">Decimal Float</a>, Previous: <a href="Floating-Types.html#Floating-Types" accesskey="p" rel="prev">Floating Types</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</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="Half_002dPrecision-Floating-Point"></a>
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<h3 class="section">6.12 Half-Precision Floating Point</h3>
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<a name="index-half_002dprecision-floating-point"></a>
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<a name="index-_005f_005ffp16-data-type"></a>
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<p>On ARM targets, GCC supports half-precision (16-bit) floating point via
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the <code>__fp16</code> type. You must enable this type explicitly
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with the <samp>-mfp16-format</samp> command-line option in order to use it.
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</p>
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<p>ARM supports two incompatible representations for half-precision
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floating-point values. You must choose one of the representations and
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use it consistently in your program.
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</p>
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<p>Specifying <samp>-mfp16-format=ieee</samp> selects the IEEE 754-2008 format.
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This format can represent normalized values in the range of <em>2^{-14}</em> to 65504.
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There are 11 bits of significand precision, approximately 3
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decimal digits.
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</p>
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<p>Specifying <samp>-mfp16-format=alternative</samp> selects the ARM
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alternative format. This representation is similar to the IEEE
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format, but does not support infinities or NaNs. Instead, the range
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of exponents is extended, so that this format can represent normalized
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values in the range of <em>2^{-14}</em> to 131008.
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</p>
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<p>The <code>__fp16</code> type is a storage format only. For purposes
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of arithmetic and other operations, <code>__fp16</code> values in C or C++
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expressions are automatically promoted to <code>float</code>. In addition,
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you cannot declare a function with a return value or parameters
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of type <code>__fp16</code>.
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</p>
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<p>Note that conversions from <code>double</code> to <code>__fp16</code>
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involve an intermediate conversion to <code>float</code>. Because
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of rounding, this can sometimes produce a different result than a
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direct conversion.
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</p>
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<p>ARM provides hardware support for conversions between
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<code>__fp16</code> and <code>float</code> values
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as an extension to VFP and NEON (Advanced SIMD). GCC generates
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code using these hardware instructions if you compile with
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options to select an FPU that provides them;
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for example, <samp>-mfpu=neon-fp16 -mfloat-abi=softfp</samp>,
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in addition to the <samp>-mfp16-format</samp> option to select
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a half-precision format.
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</p>
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<p>Language-level support for the <code>__fp16</code> data type is
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independent of whether GCC generates code using hardware floating-point
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instructions. In cases where hardware support is not specified, GCC
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implements conversions between <code>__fp16</code> and <code>float</code> values
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as library calls.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Decimal-Float.html#Decimal-Float" accesskey="n" rel="next">Decimal Float</a>, Previous: <a href="Floating-Types.html#Floating-Types" accesskey="p" rel="prev">Floating Types</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</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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