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

[jtaylor.debian at googlemail dot com]

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

            Bug ID: 85100
           Summary: __builtin_cpu_supports avx does not verify OS supports
                    it
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jtaylor.debian at googlemail dot com
  Target Milestone: ---

__builtin_cpu_supports("avx") checks that the cpu supports avx instructions,
but without OS support the instructions cannot be used.

To reproduce launch linux with the noxsave boot option. The function will
return nonzero as the feature is there but programs using avx will crash with
SIGILL.

This makes this function significantly less useful as you need to add
additional checks (using xgetbv) to verify operating system support.

At least this behavior it should be more clearly documented. Not everybody is
aware the operating system may not support or that you can disable it in linux
with a boot option.