[pypy-dev] Copy of list

[Armin Rigo]

Hi Tuom,

On Tue, Sep 29, 2015 at 7:31 AM, Tuom Larsen <tuom.larsen at gmail.com> wrote:
> Please, let me rephrase my question: currently I use `[:]` because it
> is faster in CPython (0.131 usec vs 0.269 usec per loop). I almost
> don't mind changing it to `list()` because of PyPy but I was wondering
> what do PyPy developers recommend. I don't understand why is `[:]`
> twice as slow as `list()` as it seems it should do the same thing
> (create a list and copy the content).

Looking at the jit logs, it is tripped by a RPython function with a
loop in its slow-path.  Fixed in 4e688540cfe9.

There is still a bit of overhead.  For example, lst[:] is equivalent
to lst[0:9223372036854775807].  The general logic looks like this:
when doing lst[a:b], if b > len(lst) then replace b with len(lst).
This means here a check if 9223372036854775807 > len(lst)...  It is
not possible that the length of a list be that huge, but this
knowledge is not codified explicitly.

Yes, we could improve that in the future.
But this is really advanced details.  You should write 'list()' or
'[:]' as you feel more natural, or maybe as benefits the speed of
CPython if it makes an important difference there.  Using 'timeit' to
measure microbenchmarks in PyPy may or may not give a useful result.
In this case it did only after you stopped using range() and only
because we don't have more advanced optimizations that realize that
the resulting list is not needed at all.  In general, you should not
rely on it.

What you should do instead is measure how much time is spent in some
real loop of your algorithm, and compare it with variants.  (Make sure
every variant is run in its own process, otherwise the JITting of
similar pieces of code might interfere in unexpected ways.)  If you're
lucky you may be able to find a variant that is overall much faster.
If you're not, it means that what you're changing is not relevant for
performance.


A bientôt,

Armin.