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| <h3 class="section">6.11 Half-Precision Floating Point</h3> |
| |
| <p><a name="index-half_002dprecision-floating-point-2234"></a><a name="index-g_t_0040code_007b_005f_005ffp16_007d-data-type-2235"></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. |
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