From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:59299) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QMdyB-0006P0-Uc for qemu-devel@nongnu.org; Wed, 18 May 2011 06:27:12 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1QMdyA-0003Nd-Ns for qemu-devel@nongnu.org; Wed, 18 May 2011 06:27:11 -0400 Received: from mail-bw0-f45.google.com ([209.85.214.45]:43836) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QMdyA-0003NQ-H9 for qemu-devel@nongnu.org; Wed, 18 May 2011 06:27:10 -0400 Received: by bwz16 with SMTP id 16so1387972bwz.4 for ; Wed, 18 May 2011 03:27:09 -0700 (PDT) Message-ID: <4DD39EFB.8050400@gmail.com> Date: Wed, 18 May 2011 14:27:07 +0400 From: Dmitry Konishchev MIME-Version: 1.0 References: <4DD38F03.7020209@gmail.com> <4DD391FC.6070101@redhat.com> In-Reply-To: <4DD391FC.6070101@redhat.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed 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: Kevin Wolf Cc: Stefan Hajnoczi , qemu-devel@nongnu.org, stanislav.ievlev@gmail.com On 18.05.2011 13:31, Kevin Wolf wrote: > 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; >> } OK, fixed. Signed-off-by: Dmitry Konishchev --- qemu-img.c | 29 ++++++++++++++++++++++++++--- 1 files changed, 26 insertions(+), 3 deletions(-) diff --git a/qemu-img.c b/qemu-img.c index e825123..c849c6f 100644 --- a/qemu-img.c +++ b/qemu-img.c @@ -496,14 +496,37 @@ 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) + long d0, d1, d2, d3; + const long * const data = (const long *) sector; + + len /= sizeof(long); + + for(i = 0; i < len; i += 4) { + d0 = data[i + 0]; + d1 = data[i + 1]; + d2 = data[i + 2]; + d3 = data[i + 3]; + + if (d0 || d1 || d2 || d3) { return 1; + } } + return 0; } -- 1.7.4.1