[Bug target/85100] __builtin_cpu_supports avx does not verify OS supports it

["njs at pobox dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85100

Nathaniel J. Smith <njs at pobox dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |njs at pobox dot com

--- Comment #1 from Nathaniel J. Smith <njs at pobox dot com> ---
For context here: NumPy currently uses __builtin_cpu_supports("avx") to decide
whether it can use AVX-accelerated numerical kernels. We've been getting
regular bug reports from users where this __builtin_cpu_supports("avx")
returned true, but then NumPy crashes with a SIGILL when it tries to use AVX.
(It seems to be related to some kind of relatively common virtualization
configurations.)

Examples:

https://github.com/numpy/numpy/issues/10787
https://github.com/numpy/numpy/issues/9532
https://github.com/numpy/numpy/issues/10330
https://github.com/numpy/numpy/issues/9534

Now that Julian finally figured it out, I guess we'll work around it by calling
xgetbv ourselves:

https://github.com/numpy/numpy/pull/10814

but it really seems like it would be better if __builtin_cpu_supports would
check this itself.