<html lang="en"> <head> <title>Half-Precision - Using the GNU Compiler Collection (GCC)</title> <meta http-equiv="Content-Type" content="text/html"> <meta name="description" content="Using the GNU Compiler Collection (GCC)"> <meta name="generator" content="makeinfo 4.13"> <link title="Top" rel="start" href="index.html#Top"> <link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions"> <link rel="prev" href="Floating-Types.html#Floating-Types" title="Floating Types"> <link rel="next" href="Decimal-Float.html#Decimal-Float" title="Decimal Float"> <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> <!-- Copyright (C) 1988-2015 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being ``Funding Free Software'', the Front-Cover Texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the section entitled ``GNU Free Documentation License''. (a) The FSF's Front-Cover Text is: A GNU Manual (b) The FSF's Back-Cover Text is: You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.--> <meta http-equiv="Content-Style-Type" content="text/css"> <style type="text/css"><!-- pre.display { font-family:inherit } pre.format { font-family:inherit } pre.smalldisplay { font-family:inherit; font-size:smaller } pre.smallformat { font-family:inherit; font-size:smaller } pre.smallexample { font-size:smaller } pre.smalllisp { font-size:smaller } span.sc { font-variant:small-caps } span.roman { font-family:serif; font-weight:normal; } span.sansserif { font-family:sans-serif; font-weight:normal; } --></style> </head> <body> <div class="node"> <a name="Half-Precision"></a> <a name="Half_002dPrecision"></a> <p> Next: <a rel="next" accesskey="n" href="Decimal-Float.html#Decimal-Float">Decimal Float</a>, Previous: <a rel="previous" accesskey="p" href="Floating-Types.html#Floating-Types">Floating Types</a>, Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a> <hr> </div> <h3 class="section">6.12 Half-Precision Floating Point</h3> <p><a name="index-half_002dprecision-floating-point-2923"></a><a name="index-g_t_0040code_007b_005f_005ffp16_007d-data-type-2924"></a> On ARM targets, GCC supports half-precision (16-bit) floating point via the <code>__fp16</code> type. You must enable this type explicitly with the <samp><span class="option">-mfp16-format</span></samp> command-line option in order to use it. <p>ARM supports two incompatible representations for half-precision floating-point values. You must choose one of the representations and use it consistently in your program. <p>Specifying <samp><span class="option">-mfp16-format=ieee</span></samp> selects the IEEE 754-2008 format. This format can represent normalized values in the range of 2^-14 to 65504. There are 11 bits of significand precision, approximately 3 decimal digits. <p>Specifying <samp><span class="option">-mfp16-format=alternative</span></samp> selects the ARM alternative format. This representation is similar to the IEEE format, but does not support infinities or NaNs. Instead, the range of exponents is extended, so that this format can represent normalized values in the range of 2^-14 to 131008. <p>The <code>__fp16</code> type is a storage format only. For purposes of arithmetic and other operations, <code>__fp16</code> values in C or C++ expressions are automatically promoted to <code>float</code>. In addition, you cannot declare a function with a return value or parameters of type <code>__fp16</code>. <p>Note that conversions from <code>double</code> to <code>__fp16</code> involve an intermediate conversion to <code>float</code>. Because of rounding, this can sometimes produce a different result than a direct conversion. <p>ARM provides hardware support for conversions between <code>__fp16</code> and <code>float</code> values as an extension to VFP and NEON (Advanced SIMD). GCC generates code using these hardware instructions if you compile with options to select an FPU that provides them; for example, <samp><span class="option">-mfpu=neon-fp16 -mfloat-abi=softfp</span></samp>, in addition to the <samp><span class="option">-mfp16-format</span></samp> option to select a half-precision format. <p>Language-level support for the <code>__fp16</code> data type is independent of whether GCC generates code using hardware floating-point instructions. In cases where hardware support is not specified, GCC implements conversions between <code>__fp16</code> and <code>float</code> values as library calls. </body></html>