6.2.5 Nonstandard Arithmetic

One aspect of standard IEEE arithmetic, called gradual underflow, can be manually disabled. When disabled, the program is considered to be running with nonstandard arithmetic.

The IEEE standard for arithmetic specifies a way of handling underflowed results gradually by dynamically adjusting the radix point of the significand. In IEEE floating-point format, the radix point occurs before the significand, and there is an implicit leading bit of 1. Gradual underflow allows the implicit leading bit to be cleared to 0 and shifts the radix point into the significand when the result of a floating-point computation would otherwise underflow. With a SPARC processor this result is not accomplished in hardware but in software. If your program generates many underflows (perhaps a sign of a problem with your algorithm), you may experience a performance loss.

Gradual underflow can be disabled either by compiling with the -fns option or by calling the library routine nonstandard_arithmetic() from within the program to turn it off. Call standard_arithmetic() to turn gradual underflow back on.

To be effective, the application’s main program must be compiled with -fns. See the Fortran User’s Guide.

For legacy applications, take note that:

• The standard_arithmetic() subroutine replaces an earlier routine named gradual_underflow().

• The nonstandard_arithmetic() subroutine replaces an earlier routine named abrupt_underflow().

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  Note –

  The -fns option and the nonstandard_arithmetic() library routine are effective only on some SPARC systems.

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