From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:37068) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QMeZO-0006gq-E5 for qemu-devel@nongnu.org; Wed, 18 May 2011 07:05:39 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1QMeZM-0001P1-N4 for qemu-devel@nongnu.org; Wed, 18 May 2011 07:05:38 -0400 Received: from mail-bw0-f45.google.com ([209.85.214.45]:50918) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QMeZM-0001OE-FV for qemu-devel@nongnu.org; Wed, 18 May 2011 07:05:36 -0400 Received: by bwz16 with SMTP id 16so1413830bwz.4 for ; Wed, 18 May 2011 04:05:35 -0700 (PDT) From: Dmitry Konishchev Date: Wed, 18 May 2011 15:03:59 +0400 Message-Id: <1305716639-27846-1-git-send-email-konishchev@gmail.com> In-Reply-To: <4DD391FC.6070101@redhat.com> References: <4DD391FC.6070101@redhat.com> Subject: [Qemu-devel] [PATCH] is_not_zero() optimization in qemu-img List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Kevin Wolf Cc: Stefan Hajnoczi , Dmitry Konishchev , qemu-devel@nongnu.org, Stanislav Ievlev Please sorry - I've sent the previous email via Thunderbird which promised that won't wrap the lines but it did. :( Sending this via git send-email. 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