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[Bug gas/25550] Review .arch directives


From: hjl.tools at gmail dot com
Subject: [Bug gas/25550] Review .arch directives
Date: Fri, 14 Feb 2020 13:02:57 +0000

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

--- Comment #4 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to Jan Beulich from comment #3)
> (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.

Why is AVX a prerequisite for AVX512?

> > > 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).

What purpose does it serve?

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