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6.12 Half-Precision Floating Point

On ARM targets, GCC supports half-precision (16-bit) floating point via the __fp16 type. You must enable this type explicitly with the ‘-mfp16-format’ command-line option in order to use it.

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.

Specifying ‘-mfp16-format=ieee’ 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.

Specifying ‘-mfp16-format=alternative’ 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.

The __fp16 type is a storage format only. For purposes of arithmetic and other operations, __fp16 values in C or C++ expressions are automatically promoted to float. In addition, you cannot declare a function with a return value or parameters of type __fp16.

Note that conversions from double to __fp16 involve an intermediate conversion to float. Because of rounding, this can sometimes produce a different result than a direct conversion.

ARM provides hardware support for conversions between __fp16 and float 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, ‘-mfpu=neon-fp16 -mfloat-abi=softfp’, in addition to the ‘-mfp16-format’ option to select a half-precision format.

Language-level support for the __fp16 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 __fp16 and float values as library calls.


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