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.