http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48738
--- Comment #2 from Paulo A. P. Pires <paulo1205 at yahoo dot com> 2011-04-23
13:55:48 UTC ---
(In reply to comment #1)
> 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.)
I have no objection to using such optimizations for integer exponents, and I am
aware that it . But perhaps it could be improved further, by widening the
range over which the result for an integer exponent will be consistent with the
corresponding double (or whatever) exponent.
If the code above was changed to something like what goes below, would it be
worse?
TYPE
NAME (TYPE x, int m)
{
unsigned int n;
if(m < 0){
n = -m;
x = TYPE(1) / x;
}
TYPE y = n % 2 ? x : 1;
while (n >>= 1)
{
x = x * x;
if (n % 2)
y = y * x;
}
return y;
}
