Re: [PATCH v2] blkdev: Replace blksize_bits() with ilog2()

[Tao pilgrim <pilgrimtao@gmail.com>]

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


> which you handily ignored, yet sending out a new version. I'm not
> going to apply this without justification, your commit message is
> handwavy at best.
>

Do I need to write the test content into commit message?



-- 
Yours,
Kaitao Cheng