clang 14 supports target_clones

See https://releases.llvm.org/14.0.0/tools/clang/docs/AttributeReference.html#target-clones

Examples :

• clang 14 : https://godbolt.org/z/GPnd9jYvK
• clang 13 : https://godbolt.org/z/1r4c39q88
• gcc11 : https://godbolt.org/z/KWPqT5vqe
• gcc 8: https://godbolt.org/z/TK6Pd5r5n

Asking explicitly @ssnyder on this.

It seem that clang prefers this on declarations (above doc), while GCC prefers this on definitions. So to use it in both we need to apply to a declaration that is also a definitions.

The inline/header methods in bitscan are fine. For the other 2 cases I apply it to an Impl method in an anonymous namespace in the relevant .cxx where I places the bodies we want to be vectorized.