Re: [PATCH 1/3] X86: Optimise fls(), ffs() and fls64()

[Linus Torvalds]

On Fri, 26 Mar 2010, Scott Lurndal wrote:
> 
> I wonder if Intel's EM64 stuff makes this more deterministic, perhaps
> David's implementation would work for x86_64 only?

Limiting it to x86-64 would certainly remove all the worries about all the 
historical x86 clones.

I'd still worry about it for future Intel chips, though. I absolutely 
_detest_ relying on undocumented features - it pretty much always ends up 
biting you eventually. And conditional writeback is actually pretty nasty 
from a microarchitectural standpoint.

			Linus
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