Inada Naoki <songofaca...@gmail.com> added the comment:
> I do not like how much code is needed for such minor optimization.
PR 12307 is +46 lines. Benefit is about 10ns when first insertion.
$ cpython/python.opt -m perf timeit --compare-to master/python.opt
--python-names master:empty-dict2 --duplicate 100 'd={};d["a"]=1'
master: ..................... 62.6 ns +- 1.1 ns
empty-dict2: ..................... 52.5 ns +- 1.0 ns
Mean +- std dev: [master] 62.6 ns +- 1.1 ns -> [empty-dict2] 52.5 ns +- 1.0 ns:
1.19x faster (-16%)
While "1.19x faster" is significant, I agree that this optimization is "minor".
_PyDict_NewPresized() is used for important cases. So this optimization
doesn't work in most case.
But I came up with an idea. _PyDict_NewPresized(n) skips preallocation when n
fit in minsize table.
https://github.com/python/cpython/pull/12307/commits/93906f1568b08c59e0afddfc98070827c65cd0f9
With this idea, "first insertion optimization" works. Additionally, it may
help branch prediction
for `while (newsize < minsize) { newsize <<= 1 }` loop.
Total diff is still +46 lines. But it affects more cases.
$ alias compare='cpython/python.opt -m perf timeit --compare-to
master/python.opt --python-names master:empty-dict2'
$ compare -s 'def f(x, **kw): pass' --duplicate 10 -- 'f(4)' # No table
Mean +- std dev: [master] 65.4 ns +- 2.3 ns -> [empty-dict2] 64.5 ns +- 1.5 ns:
1.01x faster (-1%)
$ compare -s 'def f(x, **kw): pass' --duplicate 10 -- 'f(4, a=1)' # minsize
table is allocated
Mean +- std dev: [master] 152 ns +- 3 ns -> [empty-dict2] 144 ns +- 4 ns: 1.05x
faster (-5%)
$ compare -s 'def f(x, **kw): pass' --duplicate 10 -- 'f(4, a=1,b=2)'
Mean +- std dev: [master] 211 ns +- 3 ns -> [empty-dict2] 186 ns +- 3 ns: 1.13x
faster (-12%)
$ compare -s 'def f(x, **kw): pass' --duplicate 10 -- 'f(4, a=1,b=2,c=3)'
Mean +- std dev: [master] 248 ns +- 3 ns -> [empty-dict2] 223 ns +- 3 ns: 1.11x
faster (-10%)
$ compare -s 'def f(x, **kw): pass' --duplicate 10 -- 'f(4,
a=1,b=2,c=3,d=4,e=5)'
Mean +- std dev: [master] 327 ns +- 6 ns -> [empty-dict2] 301 ns +- 6 ns: 1.09x
faster (-8%)
$ compare -s 'def f(x, **kw): pass' --duplicate 10 -- 'f(4,
a=1,b=2,c=3,d=4,e=5,f=6)' # minsize*2 table is allocated
Mean +- std dev: [master] 431 ns +- 8 ns -> [empty-dict2] 406 ns +- 8 ns: 1.06x
faster (-6%)
Now I think PR 12307 is not so minor. Of course, same idea can be applied to
PR 12308.
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