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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.