[issue31630] math.tan has poor accuracy near pi/2 on OpenBSD

[Tim Peters]

Tim Peters <tim at python.org> added the comment:

Of course the relationship is extremely delicate near pi/2.  On my Windows Python 3:

>>> import math
>>> (1.5707963267948961).hex()
'0x1.921fb54442d16p+0'
>>> math.tan(float.fromhex('0x1.921fb54442d16p+0')) # what the test expects
1978937966095219.0
>>> math.tan(float.fromhex('0x1.921fb54442d15p+0')) # input 1 ulp less
1374823386397210.2
>>> math.tan(float.fromhex('0x1.921fb54442d17p+0')) # input 1 ulp more
3530114321217157.5

Interestingly, wxMaxima on the same box reproduces the OpenBSD result:

(%i1) tan(1.5707963267948961);
(%o1) 1.978945885716843*10^15

But I don't know how Maxima (or OpenBSD) implement tan().

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue31630>
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