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[Bug libstdc++/48738] pow() fails to produce (some) subnormalized numbers with integer exponents
- From: "rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: Sat, 23 Apr 2011 09:21:04 +0000
- Subject: [Bug libstdc++/48738] pow() fails to produce (some) subnormalized numbers with integer exponents
- Auto-submitted: auto-generated
- References: <bug-48738-4@http.gcc.gnu.org/bugzilla/>
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48738
--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-04-23 09:20:46 UTC ---
Integer exponent powers are computed using
TYPE
NAME (TYPE x, int m)
{
unsigned int n = m < 0 ? -m : m;
TYPE y = n % 2 ? x : 1;
while (n >>= 1)
{
x = x * x;
if (n % 2)
y = y * x;
}
return m < 0 ? 1/y : y;
}
or optimal power series expansion for constant exponents starting with
GCC 4.5.
ISTR the standard allows this. You should use a double exponent to
use the real power function, pow (2., -1024.)