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106 lines
4.9 KiB
HTML
106 lines
4.9 KiB
HTML
4 years ago
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<title>Half-Precision - 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 rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions">
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<link rel="prev" href="Floating-Types.html#Floating-Types" title="Floating Types">
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<link rel="next" href="Decimal-Float.html#Decimal-Float" title="Decimal Float">
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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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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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</head>
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<body>
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<div class="node">
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<a name="Half-Precision"></a>
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<a name="Half_002dPrecision"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Decimal-Float.html#Decimal-Float">Decimal Float</a>,
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Previous: <a rel="previous" accesskey="p" href="Floating-Types.html#Floating-Types">Floating Types</a>,
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Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a>
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<hr>
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</div>
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<h3 class="section">6.12 Half-Precision Floating Point</h3>
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<p><a name="index-half_002dprecision-floating-point-2923"></a><a name="index-g_t_0040code_007b_005f_005ffp16_007d-data-type-2924"></a>
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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><span class="option">-mfp16-format</span></samp> command-line option in order to use it.
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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>Specifying <samp><span class="option">-mfp16-format=ieee</span></samp> selects the IEEE 754-2008 format.
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This format can represent normalized values in the range of 2^-14 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>Specifying <samp><span class="option">-mfp16-format=alternative</span></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 2^-14 to 131008.
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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>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>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><span class="option">-mfpu=neon-fp16 -mfloat-abi=softfp</span></samp>,
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in addition to the <samp><span class="option">-mfp16-format</span></samp> option to select
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a half-precision format.
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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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</body></html>
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