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8.5. Declaring hls_float Data Types
The Intel® HLS Compiler Pro Edition includes the hls_float.h header file for arbitrary-precision floating-point number support. The floating-point representation for hls_float data types adopts the same IEEE 754 standard as native C++ float and double types.
The hls_float.h header file does not work with native compilers (g++ or MSVC).
An hls_float variable carries an explicit sign bit and an arbitrary number of bits for the exponent and mantissa.Due to the differences in the internal math implementations and rounding errors, the results from hls_float operations might not always be bit-accurate to those produced by C++ native floating-point types with the same exponent and mantissa bit widths.
- Include the hls_float.h header file in your component in the following manner:
- After you include the header file, declare your hls_float variables as follows:
The hls_float.h header file also provides aliases in the ihc namespace so that you can declare ihc::bfloat16 and ihc::bfloat19 data types directly.
hls_float<exponent_width, mantissa_width[,rounding_mode]>Where the template attributes are defined as follows:
- exponent_width, mantissa_width
The bit-width of the exponent and mantissa of the floating-point variable.
The hls_float data type supports the following exponent_width, mantissa_width combinations:
Table 20. Exponent- and Mantissa-Width Combinations Supported by the hls_float Data Type 5, 10 8, 7 8 , 10 8 , 17 8 , 23 8, 26 10, 35 11, 44 11, 52 15, 63
Optional parameter to specify the IEEE 754 rounding mode used when converting between data types.
Set the rounding mode with one of the following values:
Round to nearest, tie to even
This rounding mode is more accurate (0.5 ULP), but requires more FPGA area.
Round towards zero
This rounding mode is less accurate (1 ULP) and requires less FPGA area.
The hls_float data type supports a limited set of math operations. For details, see Operators and Return Types Supported by the hls_float Data Type.
80-bit extended precision has one explicit bit of fraction that is dropped when converting it to hls_float<15,63>.
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