Floating-point Arithmetic Precision for IA-32 Systems

-prec_div Option

The Intel® Fortran Compiler can change floating-point division computations into multiplication by the reciprocal of the denominator. Use -prec_div to disable floating point division-to-multiplication optimization resulting in more accurate division results. May have speed impact.

-pc{32|64|80} Option

Use the -pc{32|64|80} option to enable floating-point significand precision control. Some floating-point algorithms, created for specific IA-32 and Itanium®-based systems, are sensitive to the accuracy of the significand or fractional part of the floating-point value. Use appropriate version of the option to round the significand to the number of bits as follows:

-pc32: 24 bits (single precision)

-pc64: 53 bits (double precision)

-pc80: 64 bits (extended precision)

The default version is -pc80 for full floating-point precision.

This option enables full optimization. Using this option does not have the negative performance impact of using the -mp option because only the fractional part of the floating-point value is affected. The range of the exponent is not affected.

Note

This option only has effect  when the module being compiled contains the main program.

Caution

A change of the default precision control or rounding mode (for example, by using the -pc32 option or by user intervention) may affect the results returned by some of the mathematical functions.

Rounding Control, -rcd, -fp_port

The Intel Fortran Compiler uses the -rcd option to disable changing of rounding mode for floating-point-to-integer conversions.

The system default floating-point rounding mode is round-to-nearest. This means that values are rounded during floating-point calculations. However, the Fortran language requires floating-point values to be truncated when a conversion to an integer is involved. To do this, the compiler must change the rounding mode to truncation before each floating-point conversion and change it back afterwards.

The -rcd option disables the change to truncation of the rounding mode for all floating-point calculations, including floating-point-to-integer conversions. Turning on this option can improve performance, but floating-point conversions to integer will not conform to Fortran semantics.

You can also use the -fp_port option to round floating-point results at assignments and casts. May cause some speed impact, but also makes sure that rounding to the user-declared precision at assignments is always done. The -mp1 option implies -fp_port.