ax, Qax

Architecture Restrictions

Not available on Intel® 64 architecture targeting the Intel® Xeon Phi™ coprocessor x100 product family (formerly code name Knights Corner)

Syntax

Linux and OS X:

-axcode

Windows:

/Qaxcode

Arguments

code

Indicates to the compiler a feature set that it may target, including which instruction sets it may generate. The following descriptions refer to Intel® Streaming SIMD Extensions (Intel® SSE) and Supplemental Streaming SIMD Extensions (SSSE). Possible values are: COMMON-AVX512 May generate Intel® Advanced Vector Extensions 512 (Intel® AVX-512) Foundation instructions, Intel® AVX-512 Conflict Detection instructions, as well as the instructions enabled with CORE-AVX2. MIC-AVX512 May generate Intel® Advanced Vector Extensions 512 (Intel® AVX-512) Foundation instructions, Intel® AVX-512 Conflict Detection instructions, Intel® AVX-512 Exponential and Reciprocal instructions, Intel® AVX-512 Prefetch instructions for Intel® processors, and the instructions enabled with CORE-AVX2. CORE-AVX512 May generate Intel® Advanced Vector Extensions 512 (Intel® AVX-512) Foundation instructions, Intel® AVX-512 Conflict Detection instructions, Intel® AVX-512 Doubleword and Quadword instructions, Intel® AVX-512 Byte and Word instructions and Intel® AVX-512 Vector Length extensions, as well as the instructions enabled with CORE-AVX2. CORE-AVX2 May generate Intel® Advanced Vector Extensions 2 (Intel® AVX2), Intel® AVX, SSE4.2, SSE4.1, SSE3, SSE2, SSE, and SSSE3 instructions for Intel® processors. CORE-AVX-I May generate the RDRND instruction, Intel® Advanced Vector Extensions (Intel® AVX), Intel® SSE4.2, SSE4.1, SSE3, SSE2, SSE, and SSSE3 instructions for Intel® processors. AVX May generate Intel® Advanced Vector Extensions (Intel® AVX), Intel® SSE4.2, SSE4.1, SSE3, SSE2, SSE, and SSSE3 instructions for Intel® processors. SSE4.2 May generate Intel® SSE4.2, SSE4.1, SSE3, SSE2, SSE, and SSSE3 instructions for Intel processors. SSE4.1 May generate Intel® SSE4.1, SSE3, SSE2, SSE, and SSSE3 instructions for Intel® processors. SSSE3 May generate SSSE3 instructions and Intel® SSE3, SSE2, and SSE instructions for Intel® processors. For OS X* systems, this value is only supported on Intel® 64 architecture. This replaces value T, which is deprecated. SSE3 May generate Intel® SSE3, SSE2, and SSE instructions for Intel® processors. For OS X* systems, this value is only supported on IA-32 architecture. SSE2 May generate Intel® SSE2 and SSE instructions for Intel® processors. This value is not available on OS X* systems.

Default

OFF	No auto-dispatch code is generated. Feature-specific code is generated and is controlled by the setting of the following compiler options: Linux* OS: -m and -x Windows* OS: /arch and /Qx OS X*: -x

Description

This option tells the compiler to generate multiple, feature-specific auto-dispatch code paths for Intel® processors if there is a performance benefit. It also generates a baseline code path. The Intel feature-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* OS and OS X*) or options /arch or /Qx (Windows* OS). While there are defaults for the [Q]x 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 [Q]ax and [Q]x options, the baseline code will only execute on Intel® processors compatible with the setting specified for the [Q]x.

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

If you specify both the -ax and -march options (Linux OS and OS X), or the /Qax and /arch options (Windows OS), the compiler will not generate Intel-specific instructions.

The [Q]ax option tells the compiler to find opportunities to generate separate versions of functions that take advantage of features of the specified instruction features.

If the compiler finds such an opportunity, it first checks whether generating a feature-specific version of a function is likely to result in a performance gain. If this is the case, the compiler generates both a feature-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 feature values by combining them. For example, you can specify -axSSE4.1,SSSE3 (Linux OS and OS X) or /QaxSSE4.1,SSSE3 (Windows OS). You cannot combine the old style, deprecated options and the new options. For example, you cannot specify -axSSE4.1,T (Linux OS and OS X) or /QaxSSE4.1,T (Windows OS).

IDE Equivalent

Visual Studio: Code Generation > Add Processor-Optimized Code Path

Eclipse: None

Xcode: Optimization > (multiple values)

Alternate Options

None