Reducing the Impact of Denormal Exceptions

Denormalized floating-point values are those that are too small to be represented in the normal manner; that is, the mantissa cannot be left-justified. Denormal values require hardware or operating system interventions to handle the computation, so floating-point computations that result in denormal values may have an adverse impact on performance.

There are several ways to handle denormals to increase the performance of your application:

• Scale the values into the normalized range

• Use a higher precision data type with a larger range

• Flush denormals to zero

For example, you can translate them to normalized numbers by multiplying them using a large scalar number, doing the remaining computations in the normal space, then scaling back down to the denormal range. Consider using this method when the small denormal values benefit the program design.

If you change the type declaration of a variable, you might also need to change associated library calls, unless these are generic; . Another strategy that might result in increased performance is to increase the amount of precision of intermediate values using the -fp-model [double|extended] option. However, this strategy might not eliminate all denormal exceptions, so you must experiment with the performance of your application. You should verify that the gain in performance from eliminating denormals is greater than the overhead of using a data type with higher precision and greater dynamic range.

In many cases, denormal numbers can be treated safely as zero without adverse effects on program results. Depending on the target architecture, use flush-to-zero (FTZ) options.