[Python-checkins] r85048 - in python/branches/py3k: Misc/NEWS Modules/mathmodule.c
mark.dickinson
python-checkins at python.org
Tue Sep 28 09:22:27 CEST 2010
Author: mark.dickinson
Date: Tue Sep 28 09:22:27 2010
New Revision: 85048
Log:
Issue #9599: Tweak loghelper algorithm to return slightly improved results for powers of 2.
Modified:
python/branches/py3k/Misc/NEWS
python/branches/py3k/Modules/mathmodule.c
Modified: python/branches/py3k/Misc/NEWS
==============================================================================
--- python/branches/py3k/Misc/NEWS (original)
+++ python/branches/py3k/Misc/NEWS Tue Sep 28 09:22:27 2010
@@ -2201,6 +2201,9 @@
Extension Modules
-----------------
+- Issue #9959: Tweak formula used for computing math.log of an integer,
+ making it marginally more accurate for exact powers of 2.
+
- Issue #9422: Fix memory leak when re-initializing a struct.Struct object.
- Issue #7900: The getgroups(2) system call on MacOSX behaves rather oddly
Modified: python/branches/py3k/Modules/mathmodule.c
==============================================================================
--- python/branches/py3k/Modules/mathmodule.c (original)
+++ python/branches/py3k/Modules/mathmodule.c Tue Sep 28 09:22:27 2010
@@ -1572,12 +1572,14 @@
"math domain error");
return NULL;
}
- /* Special case for log(1), to make sure we get an
- exact result there. */
- if (e == 1 && x == 0.5)
- return PyFloat_FromDouble(0.0);
- /* Value is ~= x * 2**e, so the log ~= log(x) + log(2) * e. */
- x = func(x) + func(2.0) * e;
+ /* Value is ~= x * 2**e, so the log ~= log(x) + log(2) * e.
+
+ It's slightly better to compute the log as log(2 * x) + log(2) * (e
+ - 1): then when 'arg' is a power of 2, 2**k say, this gives us 0.0 +
+ log(2) * k instead of log(0.5) + log(2)*(k+1), and so marginally
+ increases the chances of log(arg, 2) returning the correct result.
+ */
+ x = func(2.0 * x) + func(2.0) * (e - 1);
return PyFloat_FromDouble(x);
}
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