Messages in this thread |  | | Date | Mon, 10 Mar 2008 07:29:45 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH] x86: Change x86 to use generic find_next_bit |
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* Alexander van Heukelum <heukelum@fastmail.fm> wrote:
> > ok, that's rather convincing. > > > > the generic version in lib/find_next_bit.c is open-coded C which gcc > > can optimize pretty nicely. > > > > the hand-coded assembly versions in arch/x86/lib/bitops_32.c mostly > > use the special x86 'bit search forward' (BSF) instruction - which i > > know from the days when the scheduler relied on it has some > > non-trivial setup costs. So especially when there's _small_ bitmasks > > involved, it's more expensive. > > Hi, > > BSF is fine, it doesn't need any special setup. [...]
under "setup costs" i mean cycles spent by the CPU itself - the instruction itself is simple (of course) and needs no setup. If you look at BSF performance you'll see that it has nontrivial overhead.
Ingo
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