[pypy-dev] Integer division

[William ML Leslie]

On 1 June 2017 at 20:53, Armin Rigo <armin.rigo at gmail.com> wrote:

> Hi,
>
> On 31 May 2017 at 17:11, Tuom Larsen <tuom.larsen at gmail.com> wrote:
> >             # k = i//j # 2.12 seconds
> >             k = int(i/j) # 0.98 seconds
>
> Note first that if you don't do anything with 'k', it might be optimized
> away.
>
> I just wrote a pure C example doing the same thing, and indeed
> converting the integers to float, dividing, and then converting back
> to integer... is 2.2x times faster there too.
>
> Go figure it out.  I have no idea why the CPU behaves like that.
> Maybe Neal can provide a clue.
>

​I took a look at this earlier today.  It turns out that, on Skylake (and I
think it's similar on other recent x86_64 implementations), 80-bit floating
point division has a latency of 14 cycles, where 32 bit integer division
has a latency of 26 cycles.​  I expect this is because there are only two
hardware division units, and both are on the floating point path.

-- 
William Leslie

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