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Subj : Re: AMD has cmpxchg16b now - sort of
To   : comp.programming.threads
From : nathanw
Date : Tue Mar 08 2005 09:22 am


[this seems more like a comp.arch topic than a c.p.t topic, but here
it is...]

"Joe Seigh" <jseigh_01@xemaps.com> writes:

> Although they introduced the new features midstream, making it an option
> probably wasn't the best way to go.

There has to be some way to tell whether the feature exists on a
processor when code is running on it. Your choices are an option bit
or a lookup table. The lookup table is going to be unmaintainable, so
an option bit is clearly the way to go

> That conbined with no external documentation on what processor
> models have what features makes it impossible to determine if the
> processor has the feature before buying it.

Is that a serious problem? For a large-scale buyer (say, building a
compute cluster where this feature will be critical), you do an
evaluation where you can test it. For an individual who wants to mess
around with atomic operations, take advantage of return policies,
third-party reviews, and the like. 

> They probably should have gone with a architectural version numbering
> scheme that let you know at what point features become committed to the
> architecture.

Option bits are a pretty well entrenched mechanism for this on several
architectures (not, notably, PowerPC, where you have to have a lookup
table based on the CPU ID to find out if you have an FPU, AltiVec, or
whatnot).

        - Nathan

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