The main question is do we care if the downgrade to sync on power3
hurts
performances (and does it ?) and what do we do for 32 bits as
currently,
no 32 bits implementation has lwsync afaik (though that might not be
true for long).
Some time ago, I benchmarked (*) a loop of "stw;sync" vs. "stw;eieio"
on a 750 (yeah, great micro-benchmark, heh); and the sync version
was a factor ~100 slower.
Also, we don't, I think, have verified that they all properly ignore
the
added bit and behave as sync rather than program checking..
The architecture books don't have a programming note about this
(like they do in similar cases), so either a) it works fine on
every implementation; or b) someone forgot to add such a note.
We shouldn't use lwsync on 32-bit (it's an awful performance hit
on "classic" cpus, and who knows what it does on embedded cpus
(bookE *shudder*)). For POWER3 and *star, i.e., all 64-bit? Perhaps
it'll be okay there, but if as you suggest some 32-bit CPUs will add
proper lwsync support soonish, maybe a feature fixup or some such is
in order.
Segher
(*) Not on purpose.
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