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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 13:22:07 +0000

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

--- Comment #6 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to Jan Beulich from comment #5)
> (In reply to H.J. Lu from comment #4)
> > (In reply to Jan Beulich from comment #3)
> > > (In reply to H.J. Lu from comment #2)
> > > > 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?
> 
> The correlation is largely the same as the one between SSE* and AVX.

I agree with you on both counts.  I think treating MMX, SSE, AVX and AVX512
as separate ISA families gives assembler the most flexibility.  Of course,
we need to document whatever behavior we decide.

> > > > 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?
> 
> Well, I don't expect anyone would actively want to use it this way.

I think it is better to allow MMX register only if MMX ISA is enabled.

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