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List:       fedora-devel-list
Subject:    Re: Fedora 32 System-Wide Change proposal: x86-64 micro-architecture update
From:       Kevin Kofler <kevin.kofler () chello ! at>
Date:       2019-07-23 23:53:55
Message-ID: qh86mk$p38$1 () blaine ! gmane ! org
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Andrew Lutomirski wrote:
> Features like SSE2: enabling SSE2 as the basic floating point mechanism
> changes the ABI drastically.  But x86_64 already requires SSE2, so this is
> irrelevant.

For what it's worth, only the x86_64 ABI actually makes use of this. For 
i686 (32-bit), even when Fedora moved to requiring SSE2, the ABI was not 
changed (because that would have meant bootstrapping a whole new 
architecture, breaking compatibility with all existing binaries in the 
process). So i686 is stuck with the x87 ABI, copying floating-point data 
back and forth between x87 and SSE registers.

> Things like SSSE3, SSE3, SSE4, AVX1, AVX2, FMA, etc: for the most part,
> these accelerate existing algorithms.  I'm sure that someone somewhere has
> written an algorithm that requires FMA for enhanced precision, but
> otherwise pretty much any code that benefits from any of these features
> would work just fine, if slower, without the features.

FMA can also be emulated in pure software (while still using hardware 
floating-point instructions!), it is even implemented in glibc. So this also 
falls under "would work just fine, if slower, without the features".

> but FSGSBASE is not a credible requirement for Fedora since IIRC it's not
> supported on Sandy Bridge.  Even Intel seems to consider Sandy Bridge to
> be an important CPU to support.

AVX2 is also not supported on Sandy Bridge.

> I think that, for vector extensions, Fedora shouldn't require anything
> beyond SSE2 for basic functionality.  Instead, Fedora should figure out
> where there are material benefits to using them and find ways to make it
> easier for packagers to make them available.  Ideally this would all be
> figured out at runtime,

I agree.

> but install-time choices could make sense, too.

I think we should just do it right and figure it out at runtime. Otherwise, 
the user ends up having to manually install things such as those infamous 
atlas-sse* subpackages, and most users will not bother doing that, or even 
not even know that they exist or at least which one to pick.

        Kevin Kofler
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