GMP 4.3 multiplication performance

[Niels Möller]

bodrato at mail.dm.unipi.it writes:

> I do not know exactly... but mpn_submul_1 does write on memory only once,
> that's why I prefer it.

C-implementations of combination functions could use a strategy like
in the current addsub: Use a small temporary area (a suitable fraction
of the L1 cache, typically allocated on the stack), and do the
combined operation blockwise with two (for sublsh_n) function calls
per block.

What are the costs and benefits of using the C implementation of
addsub (it naturally varies with size, and the benefit should be zero
if operands fit in the L1 cache).

It would make the code so much easier to read if most or all calls to
the combination functions could be done unconditionally. 

/Niels