gh-127936: convert marshal module to use import/export API for ints (PEP 757)

[skirpichev]

• Issue: Remove private _PyLong_New() function #127936

Benchmark	ref-dumps	patch-dumps
dumps 1<<38	628 ns	640 ns: 1.02x slower
dumps 1<<300	635 ns	696 ns: 1.10x slower
dumps 1<<3000	2.02 us	2.28 us: 1.13x slower
Geometric mean	(ref)	1.06x slower

Benchmark hidden because not significant (1): dumps 1<<7

Benchmark	ref-loads	patch-loads
loads 1<<7	303 ns	311 ns: 1.03x slower
loads 1<<38	334 ns	388 ns: 1.16x slower
loads 1<<300	516 ns	605 ns: 1.17x slower
loads 1<<3000	2.08 us	2.56 us: 1.23x slower
Geometric mean	(ref)	1.15x slower

scripts

# bench-dumps.py

from marshal import dumps
import pyperf

values = ['1<<7', '1<<38', '1<<300', '1<<3000']
runner = pyperf.Runner()
for v in values:
    i = eval(v)
    bn = 'dumps '+v
    runner.bench_func(bn, dumps, i)

# bench-loads.py

from marshal import loads, dumps
import pyperf

values = ['1<<7', '1<<38', '1<<300', '1<<3000']
runner = pyperf.Runner()
for v in values:
    d = dumps(eval(v))
    bn = 'loads '+v
    runner.bench_func(bn, loads, d)

[skirpichev]

CC @vstinner

I suspect that major slowdown for marshal.loads() benchmarks is due to normalization&singletonization in the PyLongWriter_Finish(). In case of marshal.dumps() I suspect things were better without the value field.

[vstinner]

You need a special case for n==0, like the code that you removed, no?

[skirpichev]

I think no, it's just size=0size=1.

Edit: But 0 has one digit allocated anyway. So, all works correctly:

>>> import marshal
>>> marshal.dumps(0)
b'\xe9\x00\x00\x00\x00'
>>> marshal.loads(marshal.dumps(0))
0

[vstinner]

If n==0, 1 + (Py_ABS(n) - 1) / marshal_ratio gives size=0, no?

size=0 is annoying. _w_digitsXX() functions use size - 1 for example.

Please keep if (n == 0) return (PyObject *)_PyLong_New(0); to avoid this annoying case.

But 0 has one digit allocated anyway. So, all works correctly:

Ok, but I'm thinking about invalid/special marshal data. I would prefer to avoid a crash if possible.

[skirpichev]

if n==0, 1 + (Py_ABS(n) - 1) / marshal_ratio gives size=0, no?

No. (-1)/positive is 0 in C.

Please keep if (n == 0) return (PyObject *)_PyLong_New(0)

Then we will depend on this import.

I'm thinking about invalid/special marshal data.

An example?

[vstinner]

No. (-1)/positive is 0 in C.

Oh, I forgot that. It might help future readers to add a comment explaining that size=1 if n=0.

[vstinner]

Maybe add also an assertion: assert(size >= 1).

[vstinner]

LGTM. I'm (just) a bit unfortunate that the using PEP 757 makes the code slower.

@serhiy-storchaka @picnixz: Would you mind to review this change?