Issue #9599: Tweak loghelper algorithm to return slightly improved results for powers of 2.
diff --git a/Modules/mathmodule.c b/Modules/mathmodule.c
index 69e1423..3cfb5f7 100644
--- a/Modules/mathmodule.c
+++ b/Modules/mathmodule.c
@@ -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);
}