[issue39425] list.count performance regression

[Maxwell Bernstein]

Change by Maxwell Bernstein :


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nosy: +tekknolagi
nosy_count: 5.0 -> 6.0
pull_requests: +28262
pull_request: https://github.com/python/cpython/pull/30036

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[issue39425] list.count performance regression

[STINNER Victor]

Change by STINNER Victor :


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pull_requests: +17516
pull_request: https://github.com/python/cpython/pull/18130

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[issue39425] list.count performance regression

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

> The following result is a little bit surprising:

haha, I went to exactly the same journey when reviewing this PR :)

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[issue39425] list.count performance regression

[STINNER Victor]

STINNER Victor  added the comment:

> In fact, PyObject_RichCompareBool() has a fast path if the two object 
> pointers are equal

It's documented:

https://docs.python.org/dev/c-api/object.html?highlight=pyobject_richcomparebool#c.PyObject_RichCompareBool

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[issue39425] list.count performance regression

[STINNER Victor]

STINNER Victor  added the comment:

The following result is a little bit surprising:

>>> nan=float("nan"); ([nan]*5).count(nan)
5
>>> nan == nan
False

But in fact, the optimization doesn't change the value. It was already 5 
previously.

In fact, PyObject_RichCompareBool() has a fast path if the two object pointers 
are equal:

/* Quick result when objects are the same.
   Guarantees that identity implies equality. */
if (v == w) {
if (op == Py_EQ)
return 1;
else if (op == Py_NE)
return 0;
}

In short, the optimization is good: thank you ;-)

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[issue39425] list.count performance regression

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:


New changeset 9e06d61af30bac4fcacb7973f826147ccc010392 by Pablo Galindo (Miss 
Islington (bot)) in branch '3.7':
bpo-39425: Fix list.count performance regression (GH-18119) (GH-18121)
https://github.com/python/cpython/commit/9e06d61af30bac4fcacb7973f826147ccc010392


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[issue39425] list.count performance regression

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:

Thanks for the PR, Dong-hee Na :)

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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue39425] list.count performance regression

[Pablo Galindo Salgado]

Pablo Galindo Salgado  added the comment:


New changeset fdb21609d944941f0732df72dc3d07a7a9a7efea by Pablo Galindo (Miss 
Islington (bot)) in branch '3.8':
bpo-39425: Fix list.count performance regression (GH-18119) (GH-18120)
https://github.com/python/cpython/commit/fdb21609d944941f0732df72dc3d07a7a9a7efea


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[issue39425] list.count performance regression

[miss-islington]

Change by miss-islington :


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pull_requests: +17507
pull_request: https://github.com/python/cpython/pull/18120

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[issue39425] list.count performance regression

[miss-islington]

miss-islington  added the comment:


New changeset 14d80d0b605d8b148e14458e4c1853a940071462 by Miss Islington (bot) 
(Dong-hee Na) in branch 'master':
bpo-39425: Fix list.count performance regression (GH-18119)
https://github.com/python/cpython/commit/14d80d0b605d8b148e14458e4c1853a940071462


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nosy: +miss-islington

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[issue39425] list.count performance regression

[miss-islington]

Change by miss-islington :


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pull_requests: +17508
pull_request: https://github.com/python/cpython/pull/18121

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[issue39425] list.count performance regression

[Dong-hee Na]

Change by Dong-hee Na :


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versions: +Python 3.7

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[issue39425] list.count performance regression

[Dong-hee Na]

Change by Dong-hee Na :


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nosy: +inada.naoki, pablogsal, vstinner

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[issue39425] list.count performance regression

[Dong-hee Na]

Change by Dong-hee Na :


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keywords: +patch
pull_requests: +17506
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/18119

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[issue39425] list.count performance regression

[Dong-hee Na]

New submission from Dong-hee Na :

./python.exe -m pyperf timeit -s 'a = [1]*100'  'a.count(1)'

Current Master: Mean +- std dev: 1.05 us +- 0.03 us
My patch: Mean +- std dev: 423 ns +- 11 ns

This is the side-effect of pr 17022.

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assignee: corona10
messages: 360488
nosy: corona10
priority: normal
severity: normal
status: open
title: list.count performance regression
type: performance
versions: Python 3.8, Python 3.9

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