[clang] [Clang] [Lexer] Detect SSE4.2 availability at runtime in fastParseASCIIIdentifier (PR #171914)

[Thibault Monnier via cfe-commits]

Thibault-Monnier wrote:

When compiling this with `SSE4.2` enabled, does the compiler remove the runtime check entirely, or does it have to do it anyway? I'm worried of a regression in this case.

https://github.com/llvm/llvm-project/pull/171914