Half-precision floating-point number format
ARM® Compiler supports the half-precision floating-point
Half-precision is a floating-point format that occupies 16 bits. Architectures that support half-precision floating-point numbers include:
- The ARMv8 architecture.
- The ARMv7 FPv5 architecture.
- The ARMv7 VFPv4 architecture.
- The ARMv7 VFPv3 architecture (as an optional extension).
If the target hardware does not support half-precision floating-point
numbers, the compiler uses the floating-point library
fplib to provide software support for half-precision.
__fp16type is a storage format only. For purposes of arithmetic and other operations,
__fp16values in C or C++ expressions are automatically promoted to
Half-precision floating-point format
ARM Compiler uses the half-precision binary floating-point format defined by IEEE 754r, a revision to the IEEE 754 standard:
Figure 6-1 IEEE half-precision floating-point format
S (bit): Sign bit E (bits[14:10]): Biased exponent T (bits[9:0]): Mantissa.
The meanings of these fields are as follows:
IF E==31: IF T==0: Value = Signed infinity IF T!=0: Value = Nan T determines Quiet or Signalling: 0: Quiet NaN 1: Signalling NaN IF 0<E<31: Value = (-1)^S x 2^(E-15) x (1 + (2^(-10) x T)) IF E==0: IF T==0: Value = Signed zero IF T!=0: Value = (-1)^S x 2^(-14) x (0 + (2^(-10) x T))