On June 26, 2014 11:41:48 AM EDT, "Atchley, Scott" <atchle...@ornl.gov> wrote: >On Jun 26, 2014, at 10:55 AM, James Bottomley ><james.bottom...@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 -- Sent from my Android device with K-9 Mail. Please excuse my brevity. -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
