[RFC] Faster implementation of Integer to string conversion.

[Simon Marlow]

Bertram Felgenhauer wrote:

> several people have noted that printing integers is slow. The attached
> patch implements a faster version of the jtos (read: positive integer
> to string) function in GHC's Num library. The algorithm is a divide and
> conquer algorithm, that converts numbers to base b by converting them
> to base b^2 first, and then splitting the digits.
> 
> Notes:
> - I've tested the speed on my computer (which has an Athlon XP processor)
>   and it seems to be of comparable speed as the original for small numbers
>   and way faster for big numbers (read: a few hundred or thousand digits).
> - The changed conversion code is a much better list producer than the
>   original one, which generates digits starting with the last.
> - There's similar code in Numeric.lhs, that deals with arbitrary bases.
>   The current patch does not deal with this. (see questions below)
> - musabi on #haskell mentioned that there's interest in replacing GMP as
>   the bignum implementation. This is independent of changing this string
>   conversion, as far as I can see.

I'm tempted to commit this patch, but I want to make sure it's not a 
performance regression for small integers.  When you say "comparable" 
performance, what does that mean exactly?  Printing small integers is by 
far the most common case.  Perhaps we should special-case the small 
integers (i.e. the S# constructor)?

Cheers,
	Simon