From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:60055) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fk342-00023F-7x for qemu-devel@nongnu.org; Mon, 30 Jul 2018 03:57:59 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fk33z-0006Vf-7k for qemu-devel@nongnu.org; Mon, 30 Jul 2018 03:57:58 -0400 Received: from mga04.intel.com ([192.55.52.120]:27685) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1fk33y-0006VB-S6 for qemu-devel@nongnu.org; Mon, 30 Jul 2018 03:57:55 -0400 Message-ID: <5B5EC604.4020301@intel.com> Date: Mon, 30 Jul 2018 16:02:12 +0800 From: Wei Wang MIME-Version: 1.0 References: <1532934837-5966-1-git-send-email-wei.w.wang@intel.com> <1532934837-5966-2-git-send-email-wei.w.wang@intel.com> In-Reply-To: <1532934837-5966-2-git-send-email-wei.w.wang@intel.com> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH 1/2] bitmap.h: add comments to BITMAP_LAST_WORD_MASK List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org, peterx@redhat.com, dgilbert@redhat.com, quintela@redhat.com Cc: mst@redhat.com On 07/30/2018 03:13 PM, Wei Wang wrote: > This macro was ported from Linux and we've reached an aggreement there > that the corner case "nbits = 0" is not applicable to this macro, because > when "nbits = 0", which means no bits to mask, this macro is expected to > return 0, instead of 0xffffffff. This patch simply adds a comment above > the macro as a note to users about the corner case. > > Signed-off-by: Wei Wang > CC: Dr. David Alan Gilbert > CC: Juan Quintela > CC: Peter Xu > --- > include/qemu/bitmap.h | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/include/qemu/bitmap.h b/include/qemu/bitmap.h > index 509eedd..f53c640 100644 > --- a/include/qemu/bitmap.h > +++ b/include/qemu/bitmap.h > @@ -60,6 +60,7 @@ > */ > > #define BITMAP_FIRST_WORD_MASK(start) (~0UL << ((start) & (BITS_PER_LONG - 1))) > +/* "nbits = 0" is not applicable to this macro. Callers should avoid that. */ > #define BITMAP_LAST_WORD_MASK(nbits) (~0UL >> (-(nbits) & (BITS_PER_LONG - 1))) > > #define DECLARE_BITMAP(name,bits) \ A better fix would be to directly change the macro to: nbits ? (~0UL >> (-(nbits) & (BITS_PER_LONG - 1))) : 0, so that we don't need to fix other callers like bitmap_full, bitmap_intersects. So just post this out for a discussion whether it's preferred to just adding note comments as we did for linux or fixing the macro directly. Best, Wei