New submission from Xavier de Gaye <[email protected]>:
Title: testFsum failure caused by constant folding of a float expression
Description:
------------
Python (Python 3.9.0a1+ heads/master-dirty:ea9835c5d1) is built on a Linux
x86_64. This native interpreter is used to cross-compile Python (using the same
source) to Android API 24. Next the installation is done locally to DESTDIR by
running 'make install' with the env var DESTDIR set and the standard library
modules are compiled by the native interpreter during this process. The
content of DESTDIR is then copied to an arm64 android device (Huawei FWIW). The
test_math.MathTests.testFsum test fails on the android device with:
AssertionError: -4.309103330548428e+214 != -1.0
This occurs when testing '([1.7**(i+1)-1.7**i for i in range(1000)] +
[-1.7**1000], -1.0)' in test_values.
Next the test_math.py file is touched on the android device to force
recompilation of the module and testFsum becomes surprisingly successful.
Investigation:
--------------
The hexadecimal representation of 1.7**n on x86_64 and arm64 are:
* different for n in (10, 100, 1000)
* equal for n in [0, 9] or 11
on x86_64:
>>> 1.7**10
201.59939004489993
>>> (1.7**10).hex()
'0x1.9332e34080c95p+7'
on arm64:
>>> 1.7**10
201.59939004489996
>>> (1.7**10).hex()
'0x1.9332e34080c96p+7'
The output of the following foo.py module that has been run on x86_64 and arm64
are attached to this issue:
#######################
import math, dis
def test_fsum():
x = [1.7**(i+1)-1.7**i for i in range(10)] + [-1.7**10]
return x
y = test_fsum()
print(y)
print(math.fsum(y))
dis.dis(test_fsum)
#######################
The only difference between both dissasembly of test_fsum() is at bytecode 16
that loads the folded constant 1.7**10.
Conclusion:
-----------
The compilation of the expression '[1.7**(i+1)-1.7**i for i in range(1000)] +
[-1.7**1000]' on x86_64 folds '1.7**1000' to 2.8113918290273277e+230 When the
list comprehension (the first term of the expression) is executed on arm64,
then 1.7**1000 is evaluated as 2.8113918290273273e+230. On arm64 1.7**1000 -
2.8113918290273277e+230 = -4.309103330548428e+214, hence the AssertionError
above.
This is confirmed by changing testFsum to prevent constant folding by replacing
1000 in the testFsum expression with a variable whose value is 1000. In that
case the test_math module compiled on x86_64 is successful on arm64. This could
be a fix for this issue unless this fix would be hiding another problem such as
.pyc files portability across different platforms and my knowledge of IEEE 754
is too superficial to answer that point.
----------
components: Tests
files: foo.x86_64
messages: 357969
nosy: tim.peters, vstinner, xdegaye
priority: normal
severity: normal
stage: needs patch
status: open
title: testFsum failure caused by constant folding of a float expression
type: behavior
versions: Python 3.9
Added file: https://bugs.python.org/file48762/foo.x86_64
_______________________________________
Python tracker <[email protected]>
<https://bugs.python.org/issue38992>
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