> if MOST-POSITIVE-FIXNUM is not uniquely determined by the CPU address
> size, we need to specify more options.
> the user should be able to figure out MOST-POSITIVE-FIXNUM from this
> table for his computer.
Agreed. Now I had some time to measure clisp's speed with different
oint_data_len values. (On AMD64. "time make check-recompile".)
As you can see, fixnums larger than 32 bit _do_ cost performance, ca. 1%
But there's no measurable difference between 40 and 48 bit. So I'm setting
oint_data_len to 48 for all 64-bit CPUs.