linux-kernel - Re: [PATCH v3 2/2] powerpc: add support for csum

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <1432331679.27761.278.camel@freescale.com>
Date:	Fri, 22 May 2015 16:54:39 -0500
From:	Scott Wood <scottwood@...escale.com>
To:	Segher Boessenkool <segher@...nel.crashing.org>
CC:	David Laight <David.Laight@...LAB.COM>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"Christophe Leroy" <christophe.leroy@....fr>
Subject: Re: [PATCH v3 2/2] powerpc: add support for csum_add()

On Fri, 2015-05-22 at 16:39 -0500, Segher Boessenkool wrote:
> 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.

That would be a more intrusive change -- not (comparatively) low-hanging
fruit.  Plus, the person submitting these patches is focused on 32-bit.

-Scott


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/