Specifying Code Branches

Intel MKL provides a conditional numerical reproducibility (CNR) functionality that enables you to obtain reproducible results from MKL routines. When enabling CNR, you choose a specific code branch of Intel MKL that corresponds to the instruction set architecture (ISA) that you target. You can specify the code branch and other CNR options using the MKL_CBWR environment variable.

  • MKL_CBWR="<branch>[,STRICT]" or

  • MKL_CBWR="BRANCH=<branch>[,STRICT]"

Use the STRICT flag to enable strict CNR mode. For more information, see Reproducibility Conditions.

The <branch> placeholder specifies the CNR branch with one of the following values:




CNR mode uses the standard ISA-based dispatching model while ensuring fixed cache sizes, deterministic reductions, and static scheduling


Intel® Streaming SIMD Extensions 2 (Intel® SSE2) without rcpps/rsqrtps instructions


Intel SSE2


DEPRECATED. Intel® Streaming SIMD Extensions 3 (Intel® SSE3). This setting is kept for backward compatibility and is equivalent to SSE2.


Supplemental Streaming SIMD Extensions 3 (SSSE3)


Intel® Streaming SIMD Extensions 4.2 (Intel® SSE4.2)


Intel® Advanced Vector Extensions (Intel® AVX)


Intel® Advanced Vector Extensions 2 (Intel® AVX2)


Intel AVX-512 on Intel® Xeon® processors


Intel® Advanced Vector Extensions 512 (Intel® AVX-512) with support for Vector Neural Network Instructions


Intel® Advanced Vector Extensions 512 (Intel® AVX-512) on Intel® Xeon Phi™ processors


Intel® Advanced Vector Extensions 512 (Intel® AVX-512) with support for Vector Neural Network Instructions on Intel® Xeon Phi™ processors

When specifying the CNR branch, be aware of the following:

  • Reproducible results are provided under Reproducibility Conditions.

  • Settings other than AUTO or COMPATIBLE are available only for Intel processors.

  • To get the CNR branch optimized for the processor where your program is currently running, choose the value of AUTO or call the mkl_cbwr_get_auto_branch function.

  • Strict CNR mode is only supported for AVX2, AVX512, AVX512_E1, AVX512_MIC, and AVX512_MIC_E1 branches. You can also use strict CNR mode with the AUTO branch when running on Intel processors that support one of these instruction set architectures (ISAs).

Setting the MKL_CBWR environment variable or a call to an equivalent mkl_cbwr_set function fixes the code branch and sets the reproducibility mode.


  • If the value of the branch is incorrect or your processor or operating system does not support the specified ISA, CNR ignores this value and uses the AUTO branch without providing any warning messages.

  • Calls to functions that define the behavior of CNR must precede any of the math library functions that they control.

  • Settings specified by the functions take precedence over the settings specified by the environment variable.

See the Intel MKL Developer Reference for how to specify the branches using functions.

Optimization Notice

Intel's compilers may or may not optimize to the same degree for non-Intel microprocessors for optimizations that are not unique to Intel microprocessors. These optimizations include SSE2, SSE3, and SSSE3 instruction sets and other optimizations. Intel does not guarantee the availability, functionality, or effectiveness of any optimization on microprocessors not manufactured by Intel. Microprocessor-dependent optimizations in this product are intended for use with Intel microprocessors. Certain optimizations not specific to Intel microarchitecture are reserved for Intel microprocessors. Please refer to the applicable product User and Reference Guides for more information regarding the specific instruction sets covered by this notice.

Notice revision #20110804

For more complete information about compiler optimizations, see our Optimization Notice.
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