From: Segher Boessenkool <segher@kernel.crashing.org>
To: Scott Wood <scottwood@freescale.com>
Cc: David Laight <David.Laight@ACULAB.COM>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Paul Mackerras <paulus@samba.org>,
"linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH v3 2/2] powerpc: add support for csum_add()
Date: Fri, 22 May 2015 16:39:56 -0500 [thread overview]
Message-ID: <20150522213956.GC7305@gate.crashing.org> (raw)
In-Reply-To: <1432323162.27761.274.camel@freescale.com>
On Fri, May 22, 2015 at 02:32:42PM -0500, Scott Wood wrote:
> > I'd also have thought that the 64bit C version above would be generally 'good'.
>
> It doesn't generate the addc/addze sequence. At least with GCC 4.8.2,
> it does something like:
>
> mr tmp0, csum
> li tmp1, 0
> li tmp2, 0
> addc tmp3, addend, tmp0
> adde csum, tmp2, tmp1
> add csum, csum, tmp3
Right. Don't expect older compilers to do sane things here.
All this begs a question... If it is worth spending so much time
micro-optimising this, why not pick the low-hanging fruit first?
Having a 32-bit accumulator for ones' complement sums, on a 64-bit
system, is not such a great idea.
Segher
next prev parent reply other threads:[~2015-05-22 22:08 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-05-19 15:18 [PATCH v3 0/2] Optimise some IP checksum functions Christophe Leroy
2015-05-19 15:18 ` [PATCH v3 1/2] powerpc: put csum_tcpudp_magic inline Christophe Leroy
2015-05-19 15:18 ` [PATCH v3 2/2] powerpc: add support for csum_add() Christophe Leroy
2015-05-22 15:57 ` David Laight
2015-05-22 19:32 ` Scott Wood
2015-05-22 21:39 ` Segher Boessenkool [this message]
2015-05-22 21:54 ` Scott Wood
2015-05-26 13:57 ` David Laight
2015-05-26 19:42 ` Scott Wood
2015-05-27 8:41 ` David Laight
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