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Date: Thu, 9 Nov 2023 20:50:19 -0800
From: Eric Biggers <ebiggers@kernel.org>
To: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Hannes Reinecke <hare@suse.de>,
	Luis Chamberlain <mcgrof@kernel.org>,
	Pankaj Raghav <p.raghav@samsung.com>, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v2 7/7] buffer: Fix more functions for block size >
 PAGE_SIZE
Message-ID: <20231110045019.GB6572@sol.localdomain>
References: <20231109210608.2252323-1-willy@infradead.org>
 <20231109210608.2252323-8-willy@infradead.org>
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On Thu, Nov 09, 2023 at 09:06:08PM +0000, Matthew Wilcox (Oracle) wrote:
> Replace the shift with a divide, which is probably cheaper than first
> calculating the shift.

No.  The divs are almost certainly more expensive on most CPUs, especially when
doing two of them.  On x86_64 for example, it will be two div instructions
instead of one bsr and two shr instructions.  The two divs are much more costly.
The block size is always a power of 2; we should take advantage of that.

- Eric