[Bug gas/25550] Review .arch directives

[hjl.tools at gmail dot com]

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

--- Comment #2 from H.J. Lu <hjl.tools at gmail dot com> ---
(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?

> 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.

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