Re: [PATCH v2 1/3] scsi_cmnd: Introduce scsi_transfer_length helper

James Bottomley
2014-06-26T16:38:42Z

On June 26, 2014 11:41:48 AM EDT, "Atchley, Scott" <atchleyes@ornl.gov> wrote:
>On Jun 26, 2014, at 10:55 AM, James Bottomley
><James.Bottomley@HansenPartnership.com> wrote:
>
>> On Thu, 2014-06-26 at 16:53 +0200, Bart Van Assche wrote:
>>> On 06/11/14 11:09, Sagi Grimberg wrote:
>>>> + return xfer_len + (xfer_len >> ilog2(sector_size)) * 8;
>>>
>>> Sorry that I just noticed this now, but why is a shift-right and
>ilog2()
>>> used in the above expression instead of just dividing the transfer
>>> length by the sector size ?
>>
>> It's a performance thing. Division is really slow on most CPUs.
>> However, we know the divisor is a power of two so we re-express the
>> division as a shift, which the processor can do really fast.
>>
>> James
>
>I have done this in the past as well, but have you benchmarked it?
>Compilers typically do the right thing in this case (i.e replace
>division with shift).

The compiler can only do that for values which are reducible to constants at compile time. This is a runtime value, the compiler has no way of deducing that it will be a power of 2

James
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