Date: Wed, 22 Aug 2012 23:52:44 +0200 From: Luigi Rizzo <rizzo@iet.unipi.it> To: John Baldwin <jhb@freebsd.org> Cc: freebsd-hackers@freebsd.org, Adrian Chadd <adrian@freebsd.org>, Mitya <mitya@cabletv.dp.ua>, Wojciech Puchar <wojtek@wojtek.tensor.gdynia.pl>, freebsd-net@freebsd.org Subject: Re: Replace bcopy() to update ether_addr Message-ID: <20120822215244.GC67796@onelab2.iet.unipi.it> In-Reply-To: <201208221521.06954.jhb@freebsd.org> References: <50324DB4.6080905@cabletv.dp.ua> <201208220802.14588.jhb@freebsd.org> <CAJ-Vmo=1cbJn3pkSvoCq7y-kEGig-h1Vxo6M5V0=b9=MkfuMRA@mail.gmail.com> <201208221521.06954.jhb@freebsd.org>
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On Wed, Aug 22, 2012 at 03:21:06PM -0400, John Baldwin wrote: > On Wednesday, August 22, 2012 2:54:07 pm Adrian Chadd wrote: > > On 22 August 2012 05:02, John Baldwin <jhb@freebsd.org> wrote: > > > On Tuesday, August 21, 2012 12:34:42 pm Adrian Chadd wrote: > > >> Hi, > > >> > > >> What about just creating an ETHER_ADDR_COPY(dst, src) and putting that > > >> in a relevant include file, then hide the ugliness there? > > >> > > >> The same benefits will likely appear when copying wifi MAC addresses > > >> to/from headers. > > >> > > >> Thanks, I'm glad someone noticed this. > > > > > > I doubt we even _need_ the ugliness. We should just use *dst = *src > > > unless there is a compelling reason not to. > > > > Because it's not very clear? :-) I'd much prefer my array-of-things > > copies to be explicit. > > Eh? 'struct foo *src, *dst; *dst = *src' is pretty bog-standard C. That > isn't really all that obtuse. the thread has probably forked causing people to miss the explanation that Bruce gave: quite often the function is called by casting arbitrary pointers into 'struct foo *', so the compiler's expectations about alignment do not necessarily match the user's lies. Unfortunately we are building kernels with many compiler checks disabled, so there is a fair chance that the compiler will not detect such invalid casts. Probably addresses are aligned to 2-byte boundaries, but certainly not on a 4-byte, which means that a safe copy might require 3 instructions, even though a compiler could otherwise decide to align all non-packed 'struct foo' to a 4- or 8-byte boundary and possibly do the copy with 2 or even 1 instruction. I would also suggest to try the code i posted in response to bruce so you can check how good or bad are the various solutions on different architectures or CPUs, and see if there is a reasonable compromise. cheers luigi > > Also, the optimisation and compiler silliness may not be THAT obvious > > on intel (except when you're luigi and using netmap) but I can't help > > but wonder whether the same does hold for MIPS/ARM. Getting it wrong > > there will lead to some very very poor performing code. > > Don't you think there's a really good chance the compiler knows how to copy a > structure appropriately for each architecture already? > > -- > John Baldwin > _______________________________________________ > freebsd-hackers@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-hackers > To unsubscribe, send any mail to "freebsd-hackers-unsubscribe@freebsd.org"
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