Many functions in the library do not map to single instruction. This makes hard to evaluate the performance impact of the functions. To combat this issue, instruction counts are provided. An instruction count is simply the number of instructions that are used to implement specific function at the specific vector width. The instructions are counted as follows:

• Any register-register moves and copies that do not cross the processor execution domains are ignored.
• General purpose domain instructions are ignored except when they move data to or from memory or one of the SIMD domains.
• If the implementation of a function is dependent on template arguments (for example, element selector), then the instruction count is defined as a range with both lower and upper bounds.
• If the function loads or computes static data, then the instruction count is defined as a range. The lower count calculated as if the loads from memory or computation didn't happen (for example, if the function was used in a small loop and there were enough registers to cache the data). The upper count is calculated as if the data needed to be recomputed each time.

If instruction count is not listed for specific architecture, then the function directly maps to one instruction. This rule does not apply to the following architectures:

X86_FMA3, X86_FMA4 and X86_XOP.

For these, if instruction count is not listed, the instruction counts should be interpreted as if the architecture is not supported.

Note, that instruction count is very, very imprecise way to measure performance. It is provided just as a quick way to estimate how well specific functionality maps to target architecture.