From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:56392) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QMd3r-0003mU-8K for qemu-devel@nongnu.org; Wed, 18 May 2011 05:29:00 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1QMd3q-0001nw-6c for qemu-devel@nongnu.org; Wed, 18 May 2011 05:28:59 -0400 Received: from mx1.redhat.com ([209.132.183.28]:58801) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QMd3p-0001nr-TX for qemu-devel@nongnu.org; Wed, 18 May 2011 05:28:58 -0400 Message-ID: <4DD391FC.6070101@redhat.com> Date: Wed, 18 May 2011 11:31:40 +0200 From: Kevin Wolf MIME-Version: 1.0 References: <4DD38F03.7020209@gmail.com> In-Reply-To: <4DD38F03.7020209@gmail.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH] [qemu-img] CPU consuming optimization List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Dmitry Konishchev Cc: Stefan Hajnoczi , qemu-devel@nongnu.org, stanislav.ievlev@gmail.com Am 18.05.2011 11:18, schrieb Dmitry Konishchev: > On 18.05.2011 11:57, Stefan Hajnoczi wrote: >> Yes, optimizing is_not_zero() is good. The only additional thing I >> suggest is adding a comment before the function to document the length >> constraint. > > OK, fixed. > > > On 18.05.2011 12:05, Kevin Wolf wrote: >> A future bdrv_is_allocated() patch must make sure that the conversion >> falls back to a simple is_not_zero() when a backing file is used. > > Thanks, I'll take this into account. > > > Signed-off-by: Dmitry Konishchev > --- > qemu-img.c | 30 +++++++++++++++++++++++++++--- > 1 files changed, 27 insertions(+), 3 deletions(-) > > diff --git a/qemu-img.c b/qemu-img.c > index e825123..7665c2f 100644 > --- a/qemu-img.c > +++ b/qemu-img.c > @@ -496,14 +496,38 @@ static int img_commit(int argc, char **argv) > return 0; > } > > +/* > + * Checks whether the sector is not a zero sector. > + * > + * Attention! The len must be a multiple of 4 * sizeof(long) due to > + * restriction of optimizations in this function. > + */ > static int is_not_zero(const uint8_t *sector, int len) > { > + /* > + * Use long as the biggest available internal data type that fits > into the > + * CPU register and unroll the loop to smooth out the effect of memory > + * latency. > + */ > + > int i; > - len >>= 2; > - for(i = 0;i < len; i++) { > - if (((uint32_t *)sector)[i] != 0) > + len /= sizeof(long); > + > + long d0; > + long d1; > + long d2; > + long d3; Please move the declarations to the start of the function. I also would use a single line like "long d0, d1, d2, d3;", but that's up to you. > + > + for(i = 0; i < len; i += 4) { > + d0 = ((const long*) sector)[i + 0]; > + d1 = ((const long*) sector)[i + 1]; > + d2 = ((const long*) sector)[i + 2]; > + d3 = ((const long*) sector)[i + 3]; I would suggest to declare a const long* variable so that you don't have to cast each time you use, but that's probably a matter of taste. > + > + if (d0 || d1 || d2 || d3) > return 1; Coding style requires braces here. > } > + > return 0; > } Please make sure that your patch isn't line-wrapped when you send it for inclusion. git send-email will do the right thing. Kevin