gh-158592: Skip reallocation when shrinking a small list

[corona10]

• Issue: del list[index] never reallocates the array #158592

[corona10]

cc @x42005e1f @ZeroIntensity @picnixz

Here is the data (based on unittests, bm_nbody, bm_fannkuch), why I choose this way.
Shows a similar distribution.

allocated	shrink calls	% of calls	cumulative	realloc to same capacity	bytes / real realloc
<= 4	351,958	27.8%	27.8%	351,935 (100%)	0 B
5 - 8	718,926	56.9%	84.7%	290,077 (40%)	38 B
9 - 16	158,291	12.5%	97.2%	0	45 B
17 - 32	30,160	2.4%	99.6%	0	121 B
33 - 64	4,031	0.3%	99.9%	0	258 B
65 - 128	646	0.05%	100.0%	0	404 B
> 128	501	0.04%		0	1.2 KB - 267 KB
total	1,264,513			642,012 (51%)

And here is the microbenchmark

Free-threading

main vs this PR

Benchmark	main	this PR
append+del[-1] size=0	14.8 ns	16.4 ns: 1.11x slower
append+pop size=0	20.7 ns	22.4 ns: 1.08x slower
append+del[-1] size=1	14.9 ns	16.4 ns: 1.10x slower
append+pop size=1	19.2 ns	13.7 ns: 1.40x faster
append+del[-1] size=3	15.0 ns	16.5 ns: 1.10x slower
append+pop size=3	18.3 ns	13.8 ns: 1.33x faster
append+del[-1] size=7	15.1 ns	16.3 ns: 1.08x slower
append+pop size=7	13.3 ns	13.7 ns: 1.03x slower
append+del[-1] size=15	15.0 ns	16.5 ns: 1.10x slower
append+pop size=15	13.4 ns	13.7 ns: 1.02x slower
append+del[-1] size=100	15.2 ns	16.3 ns: 1.08x slower
append+pop size=100	13.4 ns	13.7 ns: 1.02x slower
extend+del[S:] S=1	45.9 ns	40.4 ns: 1.14x faster
extend+del[S:] S=2	45.7 ns	41.0 ns: 1.11x faster
extend+del[S:] S=4	41.9 ns	43.3 ns: 1.03x slower
extend+del[S:] S=16	51.6 ns	52.3 ns: 1.01x slower
extend+del[S:] S=32	62.1 ns	63.5 ns: 1.02x slower
extend+del[S:] S=64	84.6 ns	87.5 ns: 1.03x slower
extend+del[S:] S=256	233 ns	243 ns: 1.04x slower
copy only N=32	69.3 ns	71.2 ns: 1.03x slower
del[-1] x N N=32	465 ns	573 ns: 1.23x slower
copy only N=64	90.6 ns	92.9 ns: 1.03x slower
del[-1] x N N=64	863 ns	1.04 us: 1.21x slower
copy only N=256	228 ns	232 ns: 1.02x slower
del[-1] x N N=256	3.29 us	3.71 us: 1.13x slower
del[-1] x N N=1024	12.9 us	14.2 us: 1.10x slower
copy only N=4096	6.12 us	6.23 us: 1.02x slower
del[-1] x N N=4096	70.6 us	77.6 us: 1.10x slower
copy only N=65536	108 us	111 us: 1.04x slower
del[-1] x N N=65536	1.18 ms	1.31 ms: 1.10x slower
del[0] x N N=256	12.3 us	12.9 us: 1.05x slower
del[0] x N N=4096	2.28 ms	2.31 ms: 1.02x slower
Geometric mean	(ref)	1.03x slower

Benchmark hidden because not significant (4): extend+del[S:] S=8, extend+del[S:] S=1024, extend+del[S:] S=4096, copy only N=1024

main vs #158602 alone

Benchmark	main	#158602
append+del[-1] size=0	14.8 ns	31.0 ns: 2.09x slower
append+pop size=0	20.7 ns	22.0 ns: 1.06x slower
append+del[-1] size=1	14.9 ns	23.1 ns: 1.55x slower
append+pop size=1	19.2 ns	20.5 ns: 1.07x slower
append+del[-1] size=3	15.0 ns	21.7 ns: 1.44x slower
append+pop size=3	18.3 ns	19.2 ns: 1.05x slower
append+del[-1] size=7	15.1 ns	16.1 ns: 1.07x slower
append+pop size=7	13.3 ns	13.6 ns: 1.02x slower
append+del[-1] size=15	15.0 ns	16.3 ns: 1.08x slower
append+del[-1] size=100	15.2 ns	16.1 ns: 1.06x slower
append+pop size=100	13.4 ns	13.6 ns: 1.02x slower
extend+del[S:] S=1	45.9 ns	47.4 ns: 1.03x slower
extend+del[S:] S=2	45.7 ns	46.5 ns: 1.02x slower
extend+del[S:] S=4	41.9 ns	43.0 ns: 1.03x slower
extend+del[S:] S=16	51.6 ns	52.6 ns: 1.02x slower
extend+del[S:] S=64	84.6 ns	88.7 ns: 1.05x slower
extend+del[S:] S=256	233 ns	240 ns: 1.03x slower
del[-1] x N N=32	465 ns	559 ns: 1.20x slower
del[-1] x N N=64	863 ns	1.01 us: 1.17x slower
copy only N=256	228 ns	233 ns: 1.02x slower
del[-1] x N N=256	3.29 us	3.64 us: 1.11x slower
del[-1] x N N=1024	12.9 us	14.0 us: 1.09x slower
copy only N=4096	6.12 us	6.23 us: 1.02x slower
del[-1] x N N=4096	70.6 us	75.3 us: 1.07x slower
copy only N=65536	108 us	110 us: 1.02x slower
del[-1] x N N=65536	1.18 ms	1.25 ms: 1.06x slower
del[0] x N N=256	12.3 us	12.8 us: 1.04x slower
Geometric mean	(ref)	1.08x slower

Default build

main vs this PR

Benchmark	main	this PR
append+del[-1] size=0	12.8 ns	13.9 ns: 1.08x slower
append+pop size=0	15.5 ns	16.7 ns: 1.08x slower
append+del[-1] size=1	12.7 ns	13.8 ns: 1.09x slower
append+pop size=1	12.6 ns	11.4 ns: 1.10x faster
append+del[-1] size=3	13.0 ns	13.9 ns: 1.07x slower
append+pop size=3	12.8 ns	11.4 ns: 1.13x faster
append+del[-1] size=7	13.1 ns	13.6 ns: 1.04x slower
append+pop size=7	10.7 ns	11.2 ns: 1.05x slower
append+del[-1] size=15	13.1 ns	13.5 ns: 1.03x slower
append+pop size=15	10.8 ns	11.2 ns: 1.04x slower
append+del[-1] size=100	13.1 ns	13.6 ns: 1.04x slower
append+pop size=100	10.9 ns	11.2 ns: 1.03x slower
extend+del[S:] S=1	34.3 ns	33.8 ns: 1.02x faster
extend+del[S:] S=2	35.7 ns	34.6 ns: 1.03x faster
extend+del[S:] S=16	45.5 ns	44.9 ns: 1.01x faster
extend+del[S:] S=4096	2.91 us	2.75 us: 1.06x faster
copy only N=32	62.0 ns	62.8 ns: 1.01x slower
del[-1] x N N=32	411 ns	489 ns: 1.19x slower
del[-1] x N N=64	744 ns	872 ns: 1.17x slower
copy only N=256	209 ns	206 ns: 1.01x faster
del[-1] x N N=256	2.82 us	3.14 us: 1.11x slower
del[-1] x N N=1024	10.9 us	11.7 us: 1.08x slower
del[-1] x N N=4096	57.5 us	60.1 us: 1.05x slower
copy only N=65536	68.8 us	68.3 us: 1.01x faster
del[-1] x N N=65536	983 us	1.02 ms: 1.04x slower
del[0] x N N=256	4.27 us	4.85 us: 1.14x slower
del[0] x N N=4096	616 us	627 us: 1.02x slower
Geometric mean	(ref)	1.03x slower

Benchmark hidden because not significant (9): extend+del[S:] S=4, S=8, S=32, S=64, S=256, S=1024, copy only N=64, N=1024, N=4096

main vs #158602 alone

Benchmark	main	#158602
append+del[-1] size=0	12.8 ns	64.1 ns: 5.01x slower
append+pop size=0	15.5 ns	16.0 ns: 1.03x slower
append+del[-1] size=1	12.7 ns	15.3 ns: 1.20x slower
append+del[-1] size=3	13.0 ns	15.5 ns: 1.19x slower
append+del[-1] size=7	13.1 ns	13.4 ns: 1.02x slower
append+pop size=7	10.7 ns	10.9 ns: 1.02x slower
append+del[-1] size=100	13.1 ns	13.3 ns: 1.02x slower
extend+del[S:] S=1	34.3 ns	33.4 ns: 1.03x faster
extend+del[S:] S=2	35.7 ns	34.8 ns: 1.03x faster
extend+del[S:] S=8	46.7 ns	46.1 ns: 1.01x faster
extend+del[S:] S=32	55.0 ns	53.6 ns: 1.03x faster
del[-1] x N N=32	411 ns	466 ns: 1.13x slower
del[-1] x N N=64	744 ns	853 ns: 1.15x slower
del[-1] x N N=256	2.82 us	3.18 us: 1.13x slower
del[-1] x N N=1024	10.9 us	11.5 us: 1.06x slower
del[-1] x N N=4096	57.5 us	60.4 us: 1.05x slower
del[-1] x N N=65536	983 us	1.01 ms: 1.02x slower
del[0] x N N=256	4.27 us	4.85 us: 1.13x slower
del[0] x N N=4096	616 us	627 us: 1.02x slower
Geometric mean	(ref)	1.07x slower

Benchmark hidden because not significant (17): append+pop size=1, 3, 15, 100; append+del[-1] size=15; extend+del[S:] S=4, 16, 64, 256, 1024, 4096; copy only N=32, 64, 256, 1024, 4096, 65536

Notes on the default build:

Script

import pyperf

INNER = 1000


def append_del_last(obj, inner=INNER):
    for _ in range(inner):
        obj.append(None)
        del obj[-1]


def append_pop(obj, inner=INNER):
    for _ in range(inner):
        obj.append(None)
        obj.pop()


def extend_del_slice(obj, blk, size, inner):
    for _ in range(inner):
        obj.extend(blk)
        del obj[size:]


def copy_only(blk):
    obj = blk[:]
    return obj


def del_last_repeatedly(blk, r):
    obj = blk[:]
    for _ in r:
        del obj[-1]
    return obj


def del_first_repeatedly(blk, r):
    obj = blk[:]
    for _ in r:
        del obj[0]
    return obj


def main():
    runner = pyperf.Runner()

    for size in (0, 1, 3, 7, 15, 100):
        obj = [None] * size
        runner.bench_func(f"append+del[-1] size={size}", append_del_last,
                          obj, inner_loops=INNER)
        obj = [None] * size
        runner.bench_func(f"append+pop size={size}", append_pop,
                          obj, inner_loops=INNER)

    for size in (1, 2, 4, 8, 16, 32, 64, 256, 1024, 4096):
        inner = 100 if size <= 64 else 10
        obj = [None] * size
        blk = [None] * size
        runner.bench_func(f"extend+del[S:] S={size}", extend_del_slice,
                          obj, blk, size, inner, inner_loops=inner)

    for n in (32, 64, 256, 1024, 4096, 65536):
        blk = list(range(n))
        r = range(n - 1)
        runner.bench_func(f"copy only N={n}", copy_only, blk)
        runner.bench_func(f"del[-1] x N N={n}", del_last_repeatedly, blk, r)

    for n in (256, 4096):
        blk = list(range(n))
        r = range(n - 1)
        runner.bench_func(f"del[0] x N N={n}", del_first_repeatedly, blk, r)


if __name__ == "__main__":
    main()

A nice side effect of this approach is that list.pop() benefits as well
(1.33x-1.40x faster on small lists), since it already goes through
list_resize().