ax, Qax

IDE Equivalent

Windows: Code Generation > Add Processor-Optimized Code Path

Linux: Code Generation > Add Processor-Optimized Code Path

Mac OS X: Code Generation > Add Processor-Optimized Code Path

Architectures

IA-32, Intel® 64 architectures

Syntax

Arguments

Default

Description

This option tells the compiler to generate multiple, processor-specific auto-dispatch code paths for Intel processors if there is a performance benefit. It also generates a baseline code path. The Intel processor-specific auto-dispatch path is usually more optimized than the baseline path. Other options, such as O3, control how much optimization is performed on the baseline path.

The baseline code path is determined by the architecture specified by options -m or -x (Linux and Mac OS X) or options /arch or /Qx (Windows). While there are defaults for the -x or /Qx option that depend on the operating system being used, you can specify an architecture and optimization level for the baseline code that is higher or lower than the default. The specified architecture becomes the effective minimum architecture for the baseline code path.

If you specify both the -ax and -x options (Linux and Mac OS X) or the /Qax and /Qx options (Windows), the baseline code will only execute on Intel® processors compatible with the processor type specified by the -x or /Qx option.

If you specify both the -ax and -m options (Linux) or the /Qax and /arch options (Windows), the baseline code will execute on non-Intel processors compatible with the processor type specified by the -m or /arch option.

The -ax and /Qax options tell the compiler to find opportunities to generate separate versions of functions that take advantage of features of the specified Intel® processor.

If the compiler finds such an opportunity, it first checks whether generating a processor-specific version of a function is likely to result in a performance gain. If this is the case, the compiler generates both a processor-specific version of a function and a baseline version of the function. At run time, one of the versions is chosen to execute, depending on the Intel processor in use. In this way, the program can benefit from performance gains on more advanced Intel processors, while still working properly on older processors and non-Intel processors. A non-Intel processor always executes the baseline code path.

You can use more than one of the processor values by combining them. For example, you can specify -axSSE4.1,SSSE3 (Linux and Mac OS X) or /QaxSSE4.1,SSSE3 (Windows). You cannot combine the old style, deprecated options and the new options. For example, you cannot specify -axSSE4.1,T (Linux and Mac OS X) or /QaxSSE4.1,T (Windows).

Previous values W and K are deprecated. The details on replacements are as follows:

• Mac OS X systems: On these systems, there is no exact replacement for W or K. You can upgrade to the default option -msse3 (IA-32 architecture) or option -mssse3 (Intel® 64 architecture).

• Windows and Linux systems: The replacement for W is -msse2 (Linux) or /arch:SSE2 (Windows). There is no exact replacement for K. However, on Windows systems, /QaxK is interpreted as /arch:IA32; on Linux systems, -axK is interpreted as -mia32. You can also do one of the following:

  • Upgrade to option -msse2 (Linux) or option /arch:SSE2 (Windows). This will produce one code path that is specialized for Intel® SSE2. It will not run on earlier processors

  • Specify the two option combination -mia32 -axSSE2 (Linux) or /arch:IA32 /QaxSSE2 (Windows). This combination will produce an executable that runs on any processor with IA-32 architecture but with an additional specialized Intel® SSE2 code path.

Alternate Options

None