Reduce temporary unicode object while adding descriptors

[methane]

BPO	29383
Nosy	@methane, @serhiy-storchaka
Files	descr-remove-getitemstring.patch descr-remove-getitemstring-2.patch dict-setitemstring.patch: removes unnecessary comment.

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/methane'
closed_at = <Date 2017-01-28.07:39:33.947>
created_at = <Date 2017-01-28.03:04:14.956>
labels = ['interpreter-core', '3.7']
title = 'Reduce temporary unicode object while adding descriptors'
updated_at = <Date 2017-01-28.07:39:33.946>
user = 'https://github.com/methane'

bugs.python.org fields:

activity = <Date 2017-01-28.07:39:33.946>
actor = 'methane'
assignee = 'methane'
closed = True
closed_date = <Date 2017-01-28.07:39:33.947>
closer = 'methane'
components = ['Interpreter Core']
creation = <Date 2017-01-28.03:04:14.956>
creator = 'methane'
dependencies = []
files = ['46441', '46443', '46444']
hgrepos = []
issue_num = 29383
keywords = ['patch']
message_count = 5.0
messages = ['286394', '286399', '286400', '286401', '286404']
nosy_count = 3.0
nosy_names = ['methane', 'python-dev', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue29383'
versions = ['Python 3.7']

[methane]

add_methods(), add_members(), and add_getset() creates PyUnicode from C string 3 times, and calls PyUnicode_InternInplace 2 times.

1. PyDict_GetItemString() at first. (PyUnicode_FromString() is called).
2. In middle, descr_new() calls PyUnicode_InternFromString().
3. PyDict_SetItemString() at last. (creates unicode and intern it).

Skipping (2) is require adding new private APIs to pass PyUnicodeObject.
But I don't think it worth enough. (I'll try it later.)
So this patch only remove last temporary unicode.
(3 PyUnicode_FromString + 2 PyUnicode_InternInplace) becomes (2 PyUnicode_FromString + 2 PyUnicode_InternInplace).

It seems ~1% startup speedup (without site).

  $ ./python -m performance.benchmarks.bm_python_startup --no-site
  default: python_startup_no_site: Median +- std dev: 12.7 ms +- 0.1 ms
  patched: python_startup_no_site: Median +- std dev: 12.6 ms +- 0.1 ms

While speedup is small, this patch removes time to think "How large this
overhead of GetItemString + SetItemString pair?" while reading code :)

Additionally, this patch removes this comment in PyDict_SetItemString:

• PyUnicode_InternInPlace(&kv); /* XXX Should we really? */

SetItemString is used to add something to namespace.
Changing this behavior affects too widely. So we should do it.

[methane]

I think I found better way.

Interned string can be get from descripter. Interning can be reduced
without adding private API.
Please don't review the first patch.

[methane]

descr-remove-getitemstring-2.patch is more compact than first patch.
(3 unicode + 2 intern) becomes (2 unicode + 1 intern).

python_startup_no_site: Median +- std dev: 12.5 ms +- 0.1 ms

[serhiy-storchaka]

descr-remove-getitemstring-2.patch LGTM.

[python-dev]

New changeset d7ec72c1620c by INADA Naoki in branch 'default':
Issue bpo-29383: reduce temporary interned unicode
https://hg.python.org/cpython/rev/d7ec72c1620c