[PATCH v2] x86: Require full ISA support for x86-64 level marker [BZ #27318]

[Florian Weimer]

* H. J. Lu via Libc-alpha:

> Since -march=sandybridge enables ISAs in x86-64 ISA level v3, the v3
> marker is set on libc.so.  We couldn't set the needed ISA marker to v2
> since this libc won't run on all v2 machines.  Technically, the v3 marker
> is correct.  But the resulting libc.so won't run on Sandy Brigde, which
> is a v2 machine, even when libc is compiled with -march=sandybridge:

The v3 marker is no better than the v2 marker because it still does not
cover all Sandy Bridge features implied by -march=sandybridge.  Pretty
much all model-based -march= options have this effect.

What's the intended effect of this change?  Only require x86-64-v3 if
all x86-64-v3 features are implied by the -march= setting?

Thanks,
Florian
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