speed of mpq_cmp vs mpq_sub

[Marc Glisse]

On Sun, 22 May 2016, Torbjörn Granlund wrote:

> Marc Glisse <marc.glisse at inria.fr> writes:
>
>  It isn't just the case where the 2 denominators are equal, a large gcd
>  is sufficient. In the example I posted, if I multiply the denominator
>  of y by 3 and that of x by 5 (didn't check if that makes the mpq non
>  canonical, but it shouldn't really matter), I still get sub in 1.8s
>  and cmp in 25s.
>
> That seems very odd.  How can sub be so fast?

I am also surprised, but it is rather easy to reproduce. Maybe you'll see 
a bug in my test program?

-- 
Marc Glisse