netdev - RE: [PATCH] net: Remove branch in csum

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Message-ID: <476aa649389345db92f86e9103a848be@AcuMS.aculab.com>
Date:   Sun, 13 Feb 2022 17:47:52 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Segher Boessenkool' <segher@...nel.crashing.org>
CC:     'Christophe Leroy' <christophe.leroy@...roup.eu>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] net: Remove branch in csum_shift()

From: Segher Boessenkool 
> Sent: 13 February 2022 09:16
....
> 
> > What happens on x86-64?
> >
> > Trying to do the same in the x86 ipcsum code tended to make the code worse.
> > (Although that test is for an odd length fragment and can just be removed.)
> 
> In an ideal world the compiler could choose the optimal code sequences
> everywhere.  But that won't ever happen, the search space is way too
> big.  So compilers just use heuristics, not exhaustive search like
> superopt does.  There is a middle way of course, something with directed
> searches, and maybe in a few decades systems will be fast enough.  Until
> then we will very often see code that is 10% slower and 30% bigger than
> necessary.  A single insn more than needed isn't so bad :-)

But it can be a lot more than that.

> Making things branch-free is very much worth it here though!

I tried to find out where 'here' is.

I can't get godbolt to generate anything like that object code
for a call to csum_shift().

I can't actually get it to issue a rotate (x86 of ppc).

I think it is only a single instruction because the compiler
has saved 'offset & 1' much earlier instead of doing testing
'offset & 1' just prior to the conditional.
It certainly has a nasty habit of doing that pessimisation.

So while it helps a specific call site it may be much
worse in general.

I also suspect that the addc/addze pair could be removed
by passing the old checksum into csum_partial.

	David

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