[issue46961] Caching/interning of small ints sometimes fails

[Alex Waygood]

Change by Alex Waygood :


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nosy:  -AlexWaygood

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[issue46961] Caching/interning of small ints sometimes fails

[Dennis Sweeney]

Change by Dennis Sweeney :


--
keywords: +patch
pull_requests: +29942
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/31843

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[issue46961] Caching/interning of small ints sometimes fails

[Dennis Sweeney]

Dennis Sweeney  added the comment:

I believe the issue is the usage of the x_divrem function.

x_divrem always returns fresh ints, never cached small ints. This behavior is 
relied upon in the long_true_divide function, as it mutates the returned 
quotient at the line """x->ob_digit[0] = low & ~(2U*mask-1U);""".

The other uses of x_divrem account for this when handling the *quotient*, but 
apparently missed checking for small ints in the *remainder*.

uses of x_divrem:
- long_divrem
- uses maybe_small_long on quotient
- doesn't check if remainder is small < oops
- long_rem
- throws away quotient
- doesn't check if remainder is small < oops
- long_true_divide
- modifies the quotient
- throws away remainder


Possible patches to fix it:

1) Modify long_divrem and long_rem to check for small remainders, in addition 
to the small-quotient checks they already do.
2) Modify x_divrem to check for small remainders, but still always return fresh 
quotients.
3) Modify x_divrem to return cached quotients and remainders, and modify 
long_true_divide to make a copy before mutating.

I'd lean towards #1, since that quotient check already exists.
In #2, the mismatch of checking/not checking between quotient/remainder would 
be confusing.
In #3, an extra int allocation gets added to integer true divide, which isn't 
ideal.

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[issue46961] Caching/interning of small ints sometimes fails

[Brandt Bucher]

Brandt Bucher  added the comment:

Related, except this seems to be happening in long_pow. I’ll take a look at it 
today.

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assignee:  -> brandtbucher

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[issue46961] Caching/interning of small ints sometimes fails

[Brandt Bucher]

Change by Brandt Bucher :


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nosy: +brandtbucher

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[issue46961] Caching/interning of small ints sometimes fails

[Alex Waygood]

Alex Waygood  added the comment:

I think this might be a duplicate of Issue46361?

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[issue46961] Caching/interning of small ints sometimes fails

[Steven D'Aprano]

New submission from Steven D'Aprano :

I'm reluctant to call this a bug, as small int interning/caching is an 
implementation detail and there are no hard guarantees made.

But on the other hand, it seems that the intention is that small ints such as 
0, 1 and 2 should be cached. Here are some examples where they are not. 
Intentional or a bug?

>>> x = 1
>>> y = pow(2, 31, 2**31-1)
>>> y == x
True
>>> y is x
False


>>> x = 2
>>> y = pow(2, 31, 2**31-2)
>>> y == x
True
>>> y is x
False


It also affects values which are presumably constant-folded at compile time:

>>> x = 1
>>> y = 2**31 % (2**31 - 1)
>>> z = 2**31 % (2**31 - 1)
>>> x == y == z
True
>>> x is y
False
>>> y is z
False
>>> x is z
False



But if you run the code in exec, the value is interned:

>>> code = """
... x = 1
... y = 2**31 % (2**31-1)
... """
>>> dis(code)
  2   0 LOAD_CONST   0 (1)
  2 STORE_NAME   0 (x)

  3   4 LOAD_CONST   0 (1)
  6 STORE_NAME   1 (y)
  8 LOAD_CONST   1 (None)
 10 RETURN_VALUE
>>> exec(code)
>>> x is y
True



Also affects zero:

>>> x = 0
>>> y = 2**29 % (2**29)
>>> x is y
True
>>> y = 2**30 % (2**30)
>>> x is y
False


First noted here:

https://discuss.python.org/t/cached-integer-id-on-high-calculations/14128/1


>>> sys.version
'3.10.0 (default, Oct 28 2021, 20:43:43) [GCC 8.3.1 20190223 (Red Hat 8.3.1-2)]'

--
components: Interpreter Core
messages: 414762
nosy: steven.daprano
priority: normal
severity: normal
status: open
title: Caching/interning of small ints sometimes fails
type: behavior
versions: Python 3.10

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