Auto CPU Dispatch (IA-32 only)

The -ax{M|i|K|W} options direct the compiler to find opportunities to generate separate versions of functions that use instructions supported on the specified processors (see table below). If the compiler finds such an opportunity, it first checks whether generating a processor-specific version of a function results in a performance gain. If this is the case, the compiler generates both a processor-specific version of a function and a generic version of the function. The generic version will run on any IA-32 processor.

At run time, one of the two versions is chosen to execute, depending on the processor the program is currently running on. In this way, the program can benefit from performance gains on more advanced processors, while still working properly on older processors.

The disadvantages of using -ax{M|i|K|W} are:

The size of the compiled binary increases because it contains both a processor-specific version and a generic version of the code.
Performance is affected by the run-time checks to determine which code to run.

Note
Programs that you compile with this option will execute on any IA-32 processor. Such compilations are, however, subject to any exclusive specialized code restrictions you impose during compilation with the -x option.

Option	Optimizes for...
-axM	Intel® Pentium® processors with MMX(TM) technology
-axi	Intel Pentium Pro and Intel Pentium II processors
-axK	Intel Pentium III processors. Implies i and M instructions.
-axW	Intel Pentium 4 processors, Intel® Xeon(TM) processors, and Intel® Pentium® M processors. Implies M, i, and K instructions.

Example

The compilation below will generate a single executable that includes:

A generic version for use on any x86-compatible processor.
A version optimized for Intel Pentium III, as long as there is a performance benefit.
A version optimized for Intel Pentium 4 processors, Intel Xeon processors, and Intel Pentium M processors, as long as there is a performance benefit.

prompt>icc -axKW prog.c