[Python-Dev] The future of the wchar_t cache

[Nick Coghlan]

On Tue, 23 Oct 2018 at 00:50, Steve Dower <steve.dower at python.org> wrote:
>
> On 22Oct2018 1007, Serhiy Storchaka wrote:
> > 22.10.18 16:24, Steve Dower пише:
> >> Yes, that's true. But "should reduce ... footprint" is also an
> >> optimisation that deserves a benchmark by that standard. Also, I'm
> >> proposing keeping the 'kind' as UCS-2 when the string is created from
> >> UCS-2 data that is likely to be used as UCS-2. We would not create the
> >> UCS-1 version in this case, so it's not the same as prefilling the
> >> cache, but it would cost a bit of memory in exchange for CPU. If
> >> slicing and concatentation between matching kinds also preserved the
> >> kind, a lot of path handling code could avoid back-and-forth conversions.
> >
> > Oh, I afraid this will complicate the whole code of unicodeobject.c (and
> > several other files) a much and can introduce a lot of subtle bugs.
> >
> > For example, when you search a UCS2 string in a UCS1 string, the current
> > code returns the result fast, because a UCS1 string can't contain codes
> >  > 0xff, and a UCS2 string should contain codes > 0xff. And there are
> > many such assumptions.
>
> That doesn't change though, as we're only ever expanding the range. So
> searching a UCS2 string in a UCS2 string that doesn't contain any actual
> UCS2 characters is the only case that would be affected, and whether
> that case occurs more than the UCS2->UCS1->UCS2 conversion case is
> something we can measure (but I'd be surprised if substring searches
> occur more frequently than OS conversions).
>
> Currently, unicode_compare_eq exits early when the kinds do not match,
> and that would be a problem (but is also easily fixable). But other
> string operations already handle mismatched kinds.

If you did allow for denormalised UCS-2 strings, you'd probably want
some kind of flag on the instance to indicate that the real kind was
8-bit.

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia