[issue39434] Add float __floordiv__ fast path

[Dong-hee Na]

Dong-hee Na  added the comment:

@mark.dickinson

I extract the common function.
Now maintainence cost is same as AS-IS.

optimization is still work :)

AS-IS: Mean +- std dev: 360 ns +- 19 ns
TO-BE: Mean +- std dev: 185 ns +- 8 ns

what do you think?

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[issue39434] Add float __floordiv__ fast path

[Dong-hee Na]

Dong-hee Na  added the comment:

> (e.g., because someone decides to "fix" the floor float division)

Okay, what if we create a common divmod function except for creating a tuple?

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[issue39434] Add float __floordiv__ fast path

[Mark Dickinson]

Mark Dickinson  added the comment:

So the risk here is that by adding the floordiv fast path, the division code is 
duplicated, and that increases the risk of accidentally losing the invariant 
that `a // b` is interchangeable with `divmod(a, b)[0]` (e.g., because someone 
decides to "fix" the floor float division). That and the usual increased 
maintenance cost of more code.

Whether the optimization is worth the cost, I'm not sure. My gut feeling is 
not, but others may have different views.

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[issue39434] Add float __floordiv__ fast path

[Dong-hee Na]

Dong-hee Na  added the comment:

And on the other side,

>>> 3.8 // 0.0
Traceback (most recent call last):
  File "", line 1, in 
ZeroDivisionError: float divmod()

I think that people expect 
ZeroDivisionError: float floor division by zero

not the current message.
I caught this optimization issue during I investigate about the error message.

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[issue39434] Add float __floordiv__ fast path

[Dong-hee Na]

Dong-hee Na  added the comment:

> Is this worth optimizing? Floating-point floor division is a comparatively 
> rare operation.

1. I don't want to say that this should always be optimized.
2. However, this operation is a relatively primitive python operation. I think 
this optimization is easy to bring and worth it.
3. Besides, the relatively unchanged logic, so the maintenance cost is not 
expected to be large

--
nosy: +vstinner

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[issue39434] Add float __floordiv__ fast path

[Mark Dickinson]

Mark Dickinson  added the comment:

Is this worth optimising? Floating-point floor division is a comparatively rare 
operation.

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nosy: +mark.dickinson

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[issue39434] Add float __floordiv__ fast path

[Dong-hee Na]

Change by Dong-hee Na :


--
keywords: +patch
pull_requests: +17533
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/18147

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[issue39434] Add float __floordiv__ fast path

[Dong-hee Na]

Change by Dong-hee Na :


--
type:  -> performance

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[issue39434] Add float __floordiv__ fast path

[Dong-hee Na]

New submission from Dong-hee Na :

./python.exe -m pyperf timeit "a = 3.5" "b = a // 2"
AS-IS: Mean +- std dev: 377 ns +- 4 ns
my patch: Mean +- std dev: 204 ns +- 2 ns

--
assignee: corona10
messages: 360559
nosy: corona10
priority: normal
severity: normal
status: open
title: Add float __floordiv__ fast path

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