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I'm unsure about library support, but gcc and clang support function multi-versioning (FMV), which resolves the function based on CPUID the first time the function is called.

This LWN article has some additional information: https://lwn.net/Articles/691932/

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TIL! I guess it makes sense that popular numeric libraries like BLAS, Eigen, and so-on would take advantage of this, but I wonder how widely used it is overall.

For BLAS, it has enough legacy that the vendors just make their own libraries, so it would probably be best to use Intel MKL (which has it's own internal dispatching, but I don't think it uses FMV) if you are targeting Intel, and maybe AMD's branch of BLIS if you are targeting AMD. I believe Eigen comes with it's own version of BLAS built in, but it can be made to link to MKL at least, probably BLIS if you are really stubborn.

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