Thibault-Monnier wrote:

> But I still feel it will open a Pandora’s box. I really don’t want to see a 
> situation decades from now still trapped in the original x86-64 without 
> extensions.

Sure, but does that really justify intentionally reducing performance for 
default builds? Maybe in other cases it does, but in this case the performance 
difference between runtime check and -march is negligeable (and even invisible 
because of the noise). Adding a runtime dispatch won't help distros switch to 
-march builds, but it is for the valid reason that -march makes no difference 
in this case.
The compromise seems to me more between improving performance (in the 
short-term at least) at the cost of 50 lines of code, and doing nothing. The 
nothing route is justifiable by the fact this PR may become somewhat obsolete 
in a few years from now, but at the same time the runtime dispatch doesn't seem 
to have any real negative aspect to it.

https://github.com/llvm/llvm-project/pull/180631
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