Comment by loeg

GCC has offered Function Multiversioning for about a decade now (GCC ~4.8 or 4.9). GCC 6's resolver apparently uses CPUID to resolve the ifunc once at program start: https://lwn.net/Articles/691932/ .

Clang added it in 7.0.0: https://releases.llvm.org/7.0.0/tools/clang/docs/AttributeRe...

A nice presentation on it: https://llvm.org/devmtg/2014-10/Slides/Christopher-Function%...