github.com/aergoio/aergo@v1.3.1/libtool/src/gmp-6.1.2/mpn/s390_64/README (about)

     1  Copyright 2011 Free Software Foundation, Inc.
     2  
     3  This file is part of the GNU MP Library.
     4  
     5  The GNU MP Library is free software; you can redistribute it and/or modify
     6  it under the terms of either:
     7  
     8    * the GNU Lesser General Public License as published by the Free
     9      Software Foundation; either version 3 of the License, or (at your
    10      option) any later version.
    11  
    12  or
    13  
    14    * the GNU General Public License as published by the Free Software
    15      Foundation; either version 2 of the License, or (at your option) any
    16      later version.
    17  
    18  or both in parallel, as here.
    19  
    20  The GNU MP Library is distributed in the hope that it will be useful, but
    21  WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
    22  or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
    23  for more details.
    24  
    25  You should have received copies of the GNU General Public License and the
    26  GNU Lesser General Public License along with the GNU MP Library.  If not,
    27  see https://www.gnu.org/licenses/.
    28  
    29  
    30  
    31  There are 5 generations of 64-but s390 processors, z900, z990, z9,
    32  z10, and z196.  The current GMP code was optimised for the two oldest,
    33  z900 and z990.
    34  
    35  
    36  mpn_copyi
    37  
    38  This code makes use of a loop around MVC.  It almost surely runs very
    39  close to optimally.  A small improvement could be done by using one
    40  MVC for size 256 bytes, now we use two (we use an extra MVC when
    41  copying any multiple of 256 bytes).
    42  
    43  
    44  mpn_copyd
    45  
    46  We have tried several feed-in variants here, branch tree, jump table
    47  and computed goto.  The fastest (on z990) turned out to be computed
    48  goto.
    49  
    50  An approach not tried is EX of LMG and STMG, modifying the register set
    51  on-the-fly.  Using that trick, we could completely avoid using
    52  separate feed-in paths.
    53  
    54  
    55  mpn_lshift, mpn_rshift
    56  
    57  The current code runs at pipeline decode bandwidth on z990.
    58  
    59  
    60  mpn_add_n, mpn_sub_n
    61  
    62  The current code is 4-way unrolled.  It should be unrolled more, at
    63  least 8x, in order to reach 2.5 c/l.
    64  
    65  
    66  mpn_mul_1, mpn_addmul_1, mpn_submul_1
    67  
    68  The current code is very naive, but due to the non-pipelined nature of
    69  MLGR on z900 and z990, more sophisticated code would not gain much.
    70  
    71  On z10 one would need to cluster at least 4 MLGR together, in order to
    72  reduce stalling.
    73  
    74  On z196, one surely want to use unrolling and pipelining, to perhaps
    75  reach around 12 c/l.  A major issue here and on z10 is ALCGR's 3 cycle
    76  stalling.
    77  
    78  
    79  mpn_mul_2, mpn_addmul_2
    80  
    81  At least for older machines (z900, z990) with very slow MLGR, we
    82  should use Karatsuba's algorithm on 2-limb units, making mul_2 and
    83  addmul_2 the main multiplication primitives.  The newer machines might
    84  benefit less from this approach, perhaps in particular z10, where MLGR
    85  clustering is more important.
    86  
    87  With Karatsuba, one could hope for around 16 cycles per accumulated
    88  128 cross product, on z990.