Intel® VTune™ Amplifier
Certain corner-case 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, microcode assists can be extremely deleterious to performance.
A significant portion of execution time is spent in microcode assists. Examine the FP_ASSIST and OTHER_ASSISTS events to determine the specific cause.