[issue30104] Float rounding errors on AMD64 FreeBSD CURRENT Debug 3.x buildbot

[STINNER Victor]

New submission from STINNER Victor:

Since the build 154, many tests fail on the AMD64 FreeBSD CURRENT Debug 3.x buildbot slave because of float rounding errors. Failing tests:

* test_cmath
* test_float
* test_json
* test_marshal
* test_math
* test_statistics
* test_strtod

http://buildbot.python.org/all/builders/AMD64%20FreeBSD%20CURRENT%20Non-Debug%203.x/builds/154/steps/test/logs/stdio

Problem: none of build 154 changes are related to floats

* commit f9f87f0934ca570293ba7194bed3448a7f9bf39c
* commit 947629916a5ecb1f6f6792e9b9234e084c5bf274

It looks more like a libc change of the FreeBSD CURRENT slave.

Example of errors:

======================================================================
FAIL: test_specific_values (test.test_cmath.CMathTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/home/buildbot/python/3.x.koobs-freebsd-current.nondebug/build/Lib/test/test_cmath.py", line 420, in test_specific_values
    msg=error_message)
  File "/usr/home/buildbot/python/3.x.koobs-freebsd-current.nondebug/build/Lib/test/test_cmath.py", line 149, in rAssertAlmostEqual
    '{!r} and {!r} are not sufficiently close'.format(a, b))
AssertionError: acos0036: acos(complex(-1.0009999999999992, 0.0))
Expected: complex(3.141592653589793, -0.04471763360830684)
Received: complex(3.141592653589793, -0.04471763360829195)
Received value insufficiently close to expected value.

======================================================================
FAIL: test_floats (test.test_json.test_float.TestPyFloat)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/home/buildbot/python/3.x.koobs-freebsd-current/build/Lib/test/test_json/test_float.py", line 8, in test_floats
    self.assertEqual(float(self.dumps(num)), num)
AssertionError: 1.9275814160560202e-50 != 1.9275814160560204e-50

FAIL: test_bigcomp (test.test_strtod.StrtodTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/home/buildbot/python/3.x.koobs-freebsd-current.nondebug/build/Lib/test/test_strtod.py", line 213, in test_bigcomp
    self.check_strtod(s)
  File "/usr/home/buildbot/python/3.x.koobs-freebsd-current.nondebug/build/Lib/test/test_strtod.py", line 104, in check_strtod
    "expected {}, got {}".format(s, expected, got))
AssertionError: '0x1.8265ea9f864bcp+579' != '0x1.8265ea9f864bdp+579'
- 0x1.8265ea9f864bcp+579
?                 ^
+ 0x1.8265ea9f864bdp+579
?                 ^
 : Incorrectly rounded str->float conversion for 29865e170: expected 0x1.8265ea9f864bcp+579, got 0x1.8265ea9f864bdp+579

----------
messages: 291901
nosy: haypo, koobs
priority: normal
severity: normal
status: open
title: Float rounding errors on AMD64 FreeBSD CURRENT Debug 3.x buildbot
versions: Python 3.7

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue30104>
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