Re: [PATCH 4/7] mac80211: make ieee80211_get_hdrlen_from_skb return unsigned

Harvey Harrison
2008-06-07T02:24:21Z

On Fri, 2008-06-06 at 16:15 -0400, Pavel Roskin wrote:
> On Fri, 2008-06-06 at 11:47 -0700, Harvey Harrison wrote:
>=20
> > unlikely() isn't some magic make-me-faster function, it just moves =
code
> > to the end of the function to get it out of the icache and jumps to=
it
> > in the unlikely case it is taken. When all there is is a return, I
> > don't think it even makes any difference. In both these cases a co=
mment
> > is probably more appropriate.
>=20
> If we don't more return to the end, we get something like:
>=20
> compare skb->len to 10
> if more or equal goto 1
> return
> 1: <the likely case>
>=20
> =EF=BB=BFI don't know much about modern processor design, but I remem=
ber reading
> that even a very short jump would flush the instruction pipeline and
> cause some delay.
>=20
> I would keep unlikely() and let the compiler think what to do with it=
=2E
>=20

On X86-32 at least the generated code is exactly the same with or
without the unlikely()s. I'll put them back in a follow-on if people
are really attached to them.

Harvey

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