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There are 5 generations of 64-but s390 processors, z900, z990, z9,
z10, and z196.  The current GMP code was optimised for the two oldest,
z900 and z990.


mpn_copyi

This code makes use of a loop around MVC.  It almost surely runs very
close to optimally.  A small improvement could be done by using one
MVC for size 256 bytes, now we use two (we use an extra MVC when
copying any multiple of 256 bytes).


mpn_copyd

We have tried several feed-in variants here, branch tree, jump table
and computed goto.  The fastest (on z990) turned out to be computed
goto.

An approach not tried is EX of LMG and STMG, modifying the register set
on-the-fly.  Using that trick, we could completely avoid using
separate feed-in paths.


mpn_lshift, mpn_rshift

The current code runs at pipeline decode bandwidth on z990.


mpn_add_n, mpn_sub_n

The current code is 4-way unrolled.  It should be unrolled more, at
least 8x, in order to reach 2.5 c/l.


mpn_mul_1, mpn_addmul_1, mpn_submul_1

The current code is very naive, but due to the non-pipelined nature of
MLGR on z900 and z990, more sophisticated code would not gain much.

On z10 one would need to cluster at least 4 MLGR together, in order to
reduce stalling.

On z196, one surely want to use unrolling and pipelining, to perhaps
reach around 12 c/l.  A major issue here and on z10 is ALCGR's 3 cycle
stalling.


mpn_mul_2, mpn_addmul_2

At least for older machines (z900, z990) with very slow MLGR, we
should use Karatsuba's algorithm on 2-limb units, making mul_2 and
addmul_2 the main multiplication primitives.  The newer machines might
benefit less from this approach, perhaps in particular z10, where MLGR
clustering is more important.

With Karatsuba, one could hope for around 16 cycles per accumulated
128 cross product, on z990.