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Here's a version of the patch that includes optimizations to
basecase multiplication, and a streamlined x_divrem for faster division.
 With Victor's benchmark, I'm getting 43% speed increase on 64-bit
Linux/Core 2 Duo.
Note: the base patch is stable and ready for review; in contrast, the
optimizations are still in a state of flux, so the +optimizations
patch is just there as an example of what might be possible.
About using uint64_t:
the 64-bit type isn't really used very much: its main role is as the
result type of a 32-bit by 32-bit multiplication. So it might
not matter too much if it's an emulated type; what's
important is that the 32-bit by 32-bit multiply with 64-bit results
is done in a single CPU instruction. I don't know how to test for
this. Do you know of a mainstream system where this isn't true?
I'll test this tonight on 32-bit PPC and 32=bit Intel, and report back.
I don't care very much about trying to *automatically* do the right
thing for small or embedded systems: they can use the
--disable-big-digits configure option to turn 30-bit digits off.
Antoine, do you think we should be using 30-bit digits by default *only*
on 64-bit machines? I guess I could go with that, if it
can be manually overridden by the configure option.