[Bug gas/25550] Review .arch directives

[jbeulich at suse dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=25550

--- Comment #3 from Jan Beulich <jbeulich at suse dot com> ---
(In reply to H.J. Lu from comment #2)
> (In reply to Jan Beulich from comment #1)
> > However, as pointed out in the mail discussion already, consistent behavior
> > should result (which currently isn't the case):
> > 
> > 1. Since .arch generally enables prereqs for the specified ISA extension
> > (like AVX for AVX512F), disabling an extension also should disable
> > everything that it is considered a prereq for.
> 
> Is SSE2 a prerequisite for AVX?

This can be viewed either way, I guess. The history of the ISA extensions
suggests it is. But functionally nothing except the XMM registers have any
overlap, I think.

> > 2. Any ISA extension enabled should be enabled completely. The main anomaly
> > looks to be SSE/SSE2 insns accessing MMX registers, where strictly speaking
> > the MMX ISA extension isn't a prereq to the SSE* ones (it's only the
> > registers that are used), which gas also handles this way.
> 
> For view point of assembly codes, enable MMX register without MMX ISA
> makes no sense. For example, emms is needed for proper MMX usage.

Not necessarily, one can certainly get away without (using 4 FFREEP in a row).

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