From: Greg KH <gregkh@linuxfoundation.org>
To: Tao pilgrim <pilgrimtao@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>,
hch@lst.de, sth@linux.ibm.com, viro@zeniv.linux.org.uk,
clm@fb.com, jaegeuk@kernel.org, hch@infradead.org,
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dhowells@redhat.com, balbi@kernel.org, damien.lemoal@wdc.com,
bvanassche@acm.org, ming.lei@redhat.com,
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asml.silence@gmail.com, ajay.joshi@wdc.com,
linux-kernel@vger.kernel.org,
Muchun Song <songmuchun@bytedance.com>,
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gor@linux.ibm.com, borntraeger@de.ibm.com,
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Josef Bacik <josef@toxicpanda.com>,
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Subject: [Ocfs2-devel] [PATCH v2] blkdev: Replace blksize_bits() with ilog2()
Date: Mon, 1 Jun 2020 10:44:26 +0200 [thread overview]
Message-ID: <20200601084426.GB1667318@kroah.com> (raw)
In-Reply-To: <CAAWJmAZOQQQeNiTr48OSRRdO2pG+q4c=6gjT55CkWC5FN=HXmA@mail.gmail.com>
On Mon, Jun 01, 2020 at 03:22:01PM +0800, Tao pilgrim wrote:
> On Fri, May 29, 2020 at 10:13 PM Jens Axboe <axboe@kernel.dk> wrote:
> >
> > On 5/29/20 8:11 AM, Kaitao Cheng wrote:
> > > There is a function named ilog2() exist which can replace blksize.
> > > The generated code will be shorter and more efficient on some
> > > architecture, such as arm64. And ilog2() can be optimized according
> > > to different architecture.
> >
> > When you posted this last time, I said:
> >
> > "I like the simplification, but do you have any results to back up
> > that claim? Is the generated code shorter? Runs faster?"
> >
>
> Hi Jens Axboe:
>
> I did a test on ARM64.
> unsigned int ckt_blksize(int size)
> {
> return blksize_bits(size);
> }
> unsigned int ckt_ilog2(int size)
> {
> return ilog2(size);
> }
>
> When I compiled it into assembly code, I got the following result,
>
> 0000000000000088 <ckt_blksize>:
> 88: 2a0003e8 mov w8, w0
> 8c: 321d03e0 orr w0, wzr, #0x8
> 90: 11000400 add w0, w0, #0x1
> 94: 7108051f cmp w8, #0x201
> 98: 53017d08 lsr w8, w8, #1
> 9c: 54ffffa8 b.hi 90 <ckt_blksize+0x8>
> a0: d65f03c0 ret
> a4: d503201f nop
>
> 00000000000000a8 <ckt_ilog2>:
> a8: 320013e8 orr w8, wzr, #0x1f
> ac: 5ac01009 clz w9, w0
> b0: 4b090108 sub w8, w8, w9
> b4: 7100001f cmp w0, #0x0
> b8: 5a9f1100 csinv w0, w8, wzr, ne
> bc: d65f03c0 ret
>
> The generated code of ilog2 is shorter , and runs faster
But does this code path actually show up anywhere that is actually
measurable as mattering?
If so, please show that benchmark results.
thanks,
greg k-h
next prev parent reply other threads:[~2020-06-01 8:44 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-29 14:11 [Ocfs2-devel] [PATCH v2] blkdev: Replace blksize_bits() with ilog2() Kaitao Cheng
2020-05-29 14:13 ` Jens Axboe
2020-06-01 7:22 ` Tao pilgrim
2020-06-01 8:44 ` Greg KH [this message]
2020-06-02 5:51 ` Christoph Hellwig
2020-06-02 6:13 ` Greg KH
2020-05-29 14:15 ` Christoph Hellwig
2020-05-29 20:27 ` Matthew Wilcox
2020-05-29 22:27 ` Bart Van Assche
2020-05-29 22:39 ` Jens Axboe
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