As discussed in Chapter 2, IEEE Arithmetic, IEEE arithmetic handles underflowed results using gradual underflow. On some SPARC-based systems, gradual underflow is often implemented partly with software emulation of the arithmetic. If many calculations underflow, this can cause performance degradation.
To obtain some information about whether this is a case in a specific program, you can use ieee_retrospective or ieee_flags to determine if underflow exceptions occur, and check the amount of system time used by the program. If a program spends an unusually large amount of time in the operating system, and raises underflow exceptions, gradual underflow might be the cause. In this case, using non-IEEE arithmetic might speed up program execution.
The function nonstandard_arithmetic enables non-IEEE arithmetic modes on processors that support them. On SPARC systems, this function sets the NS (nonstandard arithmetic) bit in the floating-point status register. On x86 systems supporting the SSE instructions, this function sets the FTZ (flush to zero) bit in the MXCSR register; it also sets the DAZ (denormals are zero) bit in the MXCSR register on those processors that support this bit. Note that the effects of nonstandard modes vary from one processor to another and can cause otherwise robust software to malfunction. Nonstandard mode is not recommended for normal use.
The function standard_arithmetic resets the hardware to use the default IEEE arithmetic. Both functions have no effect on processors that provide only the default IEEE 754 style of arithmetic. SPARC T4 is one such processor.