User Guide

Contents

FP Assists

Metric Description

Certain floating point operations cannot be handled natively by the execution pipeline and must be performed by microcode (small programs injected into the execution stream). For example, when working with very small floating point values (so-called denormals), the floating-point units are not set up to perform these operations natively. Instead, a sequence of instructions to perform the computation on the denormal is injected into the pipeline. Since these microcode sequences might be hundreds of instructions long, these microcode assists are extremely deleterious to performance.

Possible Issues

A significant portion of execution time is spent in floating point assists.

Tips

Consider enabling the DAZ (Denormals Are Zero) and/or FTZ (Flush To Zero) options in your compiler to flush denormals to zero. This option may improve performance if the denormal values are not critical in your application. Also note that the DAZ and FTZ modes are not compatible with the IEEE Standard 754.

Product and Performance Information

1

Intel's compilers may or may not optimize to the same degree for non-Intel microprocessors for optimizations that are not unique to Intel microprocessors. These optimizations include SSE2, SSE3, and SSSE3 instruction sets and other optimizations. Intel does not guarantee the availability, functionality, or effectiveness of any optimization on microprocessors not manufactured by Intel. Microprocessor-dependent optimizations in this product are intended for use with Intel microprocessors. Certain optimizations not specific to Intel microarchitecture are reserved for Intel microprocessors. Please refer to the applicable product User and Reference Guides for more information regarding the specific instruction sets covered by this notice.

Notice revision #20110804