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Date:   Mon, 2 Nov 2020 16:08:51 +0100
From:   David Sterba <dsterba@suse.cz>
To:     Johannes Thumshirn <Johannes.Thumshirn@wdc.com>
Cc:     Qu Wenruo <quwenruo.btrfs@gmx.com>,
        David Sterba <dsterba@suse.com>,
        "linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>
Subject: Re: [PATCH 01/10] btrfs: use precalculated sectorsize_bits from
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On Mon, Nov 02, 2020 at 02:31:35PM +0000, Johannes Thumshirn wrote:
> On 02/11/2020 15:20, Qu Wenruo wrote:
> > This may sounds like a nitpicking, but what about "ffs(4096) - 1"?
> > IMHO it should be a little more faster than ilog2, especially when we
> > have ensure all sector size is power of 2 already.
> 
> Looking at the actual ilog2() implementation (and considering you're 
> passing 4096, a constant) you'll end up in const_ilog() which will 
> evaluate to 9.
> 
> ffs() on the other hand on x86_64 will evaluate to a bfsl. So ffs() will
> evaluated at runtime, while ilog2() at compile time.

As the value is calculated only once for the whole filesystem lifetime,
I'm not concerned about speed but readability.