[libcxx-commits] [libcxx] [libcxx] Use generic builtins for popcount, clz and ctz (PR #86563)

[Nick Desaulniers via libcxx-commits]

nickdesaulniers wrote:

Right, so these builtins are nascent. Only unreleased versions of clang and gcc support them (top of tree).  So you'll need to use `__has_builtin` to detect support for them. `__has_builtin` also might not exist in older compiler versions, so you may need to check that.  Try to find what compiler versions libcxx currently supports; your solution will need to consider those older compilers which may lack the builtins.

https://github.com/llvm/llvm-project/pull/86563