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bpo-44954: Fix wrong result in float.fromhex corner case (GH-27834)

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(cherry picked from commit 60b93d9e49)

Co-authored-by: Mark Dickinson <mdickinson@enthought.com>
This commit is contained in:
Miss Islington (bot) 2021-08-20 10:48:47 -07:00 • committed by GitHub
parent f0e2a46349
commit 838b0e975f
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3 changed files with 18 additions and 2 deletions
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14
Lib/test/test_float.py
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@ -1468,6 +1468,20 @@ def test_from_hex(self):
self.identical(fromHex('0X1.0000000000001fp0'), 1.0+2*EPS) self.identical(fromHex('0X1.0000000000001fp0'), 1.0+2*EPS)
self.identical(fromHex('0x1.00000000000020p0'), 1.0+2*EPS) self.identical(fromHex('0x1.00000000000020p0'), 1.0+2*EPS)
# Regression test for a corner-case bug reported in b.p.o. 44954
self.identical(fromHex('0x.8p-1074'), 0.0)
self.identical(fromHex('0x.80p-1074'), 0.0)
self.identical(fromHex('0x.81p-1074'), TINY)
self.identical(fromHex('0x8p-1078'), 0.0)
self.identical(fromHex('0x8.0p-1078'), 0.0)
self.identical(fromHex('0x8.1p-1078'), TINY)
self.identical(fromHex('0x80p-1082'), 0.0)
self.identical(fromHex('0x81p-1082'), TINY)
self.identical(fromHex('.8p-1074'), 0.0)
self.identical(fromHex('8p-1078'), 0.0)
self.identical(fromHex('-.8p-1074'), -0.0)
self.identical(fromHex('+8p-1078'), 0.0)
def test_roundtrip(self): def test_roundtrip(self):
def roundtrip(x): def roundtrip(x):
return fromHex(toHex(x)) return fromHex(toHex(x))

2
Misc/NEWS.d/next/Core and Builtins/2021-08-19-14-43-24.bpo-44954.dLn3lg.rst Normal file
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@ -0,0 +1,2 @@
Fixed a corner case bug where the result of ``float.fromhex('0x.8p-1074')``
was rounded the wrong way.

4
Objects/floatobject.c
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@ -1463,8 +1463,8 @@ float_fromhex(PyTypeObject *type, PyObject *string)
bits lsb, lsb-2, lsb-3, lsb-4, ... is 1. */ bits lsb, lsb-2, lsb-3, lsb-4, ... is 1. */
if ((digit & half_eps) != 0) { if ((digit & half_eps) != 0) {
round_up = 0; round_up = 0;
if ((digit & (3*half_eps-1)) != 0 || if ((digit & (3*half_eps-1)) != 0 || (half_eps == 8 &&
(half_eps == 8 && (HEX_DIGIT(key_digit+1) & 1) != 0)) key_digit+1 < ndigits && (HEX_DIGIT(key_digit+1) & 1) != 0))
round_up = 1; round_up = 1;
else else
for (i = key_digit-1; i >= 0; i--) for (i = key_digit-1; i >= 0; i--)