Populate the initial per-interpreter interned_strings dict with runtime-global singleton strings

[Christopher-Chianelli]

Feature or enhancement

Re-use the runtime-global singleton strings inside the interned_strings dict to reduce duplication of
singleton strings (improving performance where such strings are used).

Pitch

bpo-46430 (#30683) caused an interesting side effect; the code
x = 'a'; x[0] is x no longer returned True. This in turn
is because there are two different cached versions of 'a':

• One that was cached when code in frozen modules was compiled
  (and is stored in the interned_dict)
• One that is stored as a runtime-global object that is used
  during function calls (and is stored in _Py_SINGLETON(strings))

However, some characters do not have this behaviour (for example,
'g', 'u', and 'z'). I suspect it because these characters are not
used in co_consts of frozen modules.

The interned_dict is per interpreter, and is initialized by
init_interned_dict(PyInterpreterState *). Currently, it is
initialized to an empty dict, which allows code in frozen modules
to use their (different and per interpreter) singleton strings
instead of the runtime-global one.

Using the synthetic test case:

def test():
 total = 0
 for ch in 'abc':
 if ch in{'a', 'c'}:
 total += 1
 return total

timeit.timeit(`test()`, globals=globals())

I get a ~5.43% improvement when the interned_strings dict reuses the runtime-global singleton strings.

Previous discussion

Linked PRs

• gh-103571: Add runtime-global strings to the initial per-interpreter interned_dict #103572

[terryjreedy]

x = 'a'; x[0] is x is True in 3.10, False in 3.11. Neither is correct as the reference declares caching-dependent behaviors like this to be implementation details.

[kumaraditya303]

It is an implementation detail, it will soon change as that dict will be made immortal and be global instead of per-interpreter so it will be automatically be resolved, closing as it is not actionable for now.