From: Charles Mills <cmills@...>
Date: 2005-05-19T00:50:14+09:00
Subject: Re: Strange behaviour of class Fixnum in exponentation


Wolfgang N�dasi-Donner wrote:
> The german Ruby forum recognized a strange behaviour of the Fixnum
class.
>
> In exponentation Fixnum is much slower than Bignum, which lets us
assume, that some kind of a problem occurs
> in the implementation. Example:
>
> >>>>> Code (Ruby) >>>>>
>
> require 'benchmark'
>
> N = 10**6
> B = 65891264863465298234965902602612457060348499377
> F = 4628
>
> Benchmark.bmbm(10) do |bm|
>     bm.report('Bignum') do N.times do
>         B ** 0.5
>     end end
>
>     bm.report('Fixnum') do N.times do
>         F ** 0.5
>     end end
> end
>
> >>>>> Result >>>>>
>                 user     system      total        real
>
> Bignum      3.856000   0.000000   3.856000 (  3.946000)
> Fixnum      7.861000   0.020000   7.881000 (  8.081000)
>
> >>>>> End of Example >>>>>
>
> Further investigations leads to the responsible C code:
>
> >>>>> Code (C) >>>>>
>
> static VALUE
> fix_pow(x, y)
>     VALUE x, y;
> {
>     if (FIXNUM_P(y)) {
>         long a, b;
>
>         b = FIX2LONG(y);
>         if (b == 0) return INT2FIX(1);
>         if (b == 1) return x;
>         a = FIX2LONG(x);
>         if (b > 0) {
>             return rb_big_pow(rb_int2big(a), y);
>         }
>         return rb_float_new(pow((double)a, (double)b));
>     }
>     return rb_num_coerce_bin(x, y);  // <-- das dauert
> }

You chould change the above code to and it should solve the speed
problem.  untested code:

static VALUE
fix_pow(x, y)
    VALUE x, y;
{
    if (FIXNUM_P(y)) {
        long a, b;

        b = FIX2LONG(y);
        if (b == 0) return INT2FIX(1);
        if (b == 1) return x;
        a = FIX2LONG(x);
        if (b > 0) {
            return rb_big_pow(rb_int2big(a), y);
        }
        return rb_float_new(pow((double)a, (double)b));
    } else if (TYPE(y) == T_FLOAT) {
        long a = FIX2LONG(x);
        return rb_float_new(pow((double)a, RFLOAT(y)->value));
    }
    return rb_num_coerce_bin(x, y);  // <-- das dauert
}

-Charlie