source/libs/gmp/gmp-src/mpn/x86/k6/divrem_1.asm

   1 dnl  AMD K6 mpn_divrem_1 -- mpn by limb division.
   2
   3 dnl  Copyright 1999-2003, 2007 Free Software Foundation, Inc.
   4
   5 dnl  This file is part of the GNU MP Library.
   6 dnl
   7 dnl  The GNU MP Library is free software; you can redistribute it and/or modify
   8 dnl  it under the terms of either:
   9 dnl
  10 dnl    * the GNU Lesser General Public License as published by the Free
  11 dnl      Software Foundation; either version 3 of the License, or (at your
  12 dnl      option) any later version.
  13 dnl
  14 dnl  or
  15 dnl
  16 dnl    * the GNU General Public License as published by the Free Software
  17 dnl      Foundation; either version 2 of the License, or (at your option) any
  18 dnl      later version.
  19 dnl
  20 dnl  or both in parallel, as here.
  21 dnl
  22 dnl  The GNU MP Library is distributed in the hope that it will be useful, but
  23 dnl  WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
  24 dnl  or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  25 dnl  for more details.
  26 dnl
  27 dnl  You should have received copies of the GNU General Public License and the
  28 dnl  GNU Lesser General Public License along with the GNU MP Library.  If not,
  29 dnl  see https://www.gnu.org/licenses/.
  30
  31 include(`../config.m4')
  32
  33
  34 C K6: 20 cycles/limb
  35
  36
  37 C mp_limb_t mpn_divrem_1 (mp_ptr dst, mp_size_t xsize,
  38 C                         mp_srcptr src, mp_size_t size, mp_limb_t divisor);
  39 C mp_limb_t mpn_divrem_1c (mp_ptr dst, mp_size_t xsize,
  40 C                          mp_srcptr src, mp_size_t size, mp_limb_t divisor,
  41 C                          mp_limb_t carry);
  42 C
  43 C The code here is basically the same as mpn/x86/divrem_1.asm, but uses loop
  44 C instead of decl+jnz, since it comes out 2 cycles/limb faster.
  45 C
  46 C A test is done to see if the high limb is less than the divisor, and if so
  47 C one less div is done.  A div is 20 cycles, so assuming high<divisor about
  48 C half the time, then this test saves half that amount.  The branch
  49 C misprediction penalty is less than that.
  50 C
  51 C Back-to-back div instructions run at 20 cycles, the same as the loop here,
  52 C so it seems there's nothing to gain by rearranging the loop.  Pairing the
  53 C mov and loop instructions was found to gain nothing.
  54 C
  55 C Enhancements:
  56 C
  57 C The low-latency K6 multiply might be thought to suit a mul-by-inverse, but
  58 C that algorithm has been found to suffer from the relatively poor carry
  59 C handling on K6 and too many auxiliary instructions.  The fractional part
  60 C however could be done at about 13 c/l, if it mattered enough.
  61
  62 defframe(PARAM_CARRY,  24)
  63 defframe(PARAM_DIVISOR,20)
  64 defframe(PARAM_SIZE,   16)
  65 defframe(PARAM_SRC,    12)
  66 defframe(PARAM_XSIZE,  8)
  67 defframe(PARAM_DST,    4)
  68
  69         TEXT
  70
  71         ALIGN(32)
  72 PROLOGUE(mpn_divrem_1c)
  73 deflit(`FRAME',0)
  74
  75         movl    PARAM_SIZE, %ecx
  76         pushl   %edi            FRAME_pushl()
  77
  78         movl    PARAM_SRC, %edi
  79         pushl   %esi            FRAME_pushl()
  80
  81         movl    PARAM_DIVISOR, %esi
  82         pushl   %ebx            FRAME_pushl()
  83
  84         movl    PARAM_DST, %ebx
  85         pushl   %ebp            FRAME_pushl()
  86
  87         movl    PARAM_XSIZE, %ebp
  88         orl     %ecx, %ecx              C size
  89
  90         movl    PARAM_CARRY, %edx
  91         jz      L(fraction)             C if size==0
  92
  93         leal    -4(%ebx,%ebp,4), %ebx   C dst one limb below integer part
  94         jmp     L(integer_top)
  95
  96 EPILOGUE()
  97
  98
  99         ALIGN(16)
 100 PROLOGUE(mpn_divrem_1)
 101 deflit(`FRAME',0)
 102
 103         movl    PARAM_SIZE, %ecx
 104         pushl   %edi            FRAME_pushl()
 105
 106         movl    PARAM_SRC, %edi
 107         pushl   %esi            FRAME_pushl()
 108
 109         movl    PARAM_DIVISOR, %esi
 110         orl     %ecx,%ecx               C size
 111
 112         jz      L(size_zero)
 113         pushl   %ebx            FRAME_pushl()
 114
 115         movl    -4(%edi,%ecx,4), %eax   C src high limb
 116         xorl    %edx, %edx
 117
 118         movl    PARAM_DST, %ebx
 119         pushl   %ebp            FRAME_pushl()
 120
 121         movl    PARAM_XSIZE, %ebp
 122         cmpl    %esi, %eax
 123
 124         leal    -4(%ebx,%ebp,4), %ebx   C dst one limb below integer part
 125         jae     L(integer_entry)
 126
 127
 128         C high<divisor, so high of dst is zero, and avoid one div
 129
 130         movl    %edx, (%ebx,%ecx,4)
 131         decl    %ecx
 132
 133         movl    %eax, %edx
 134         jz      L(fraction)
 135
 136
 137 L(integer_top):
 138         C eax   scratch (quotient)
 139         C ebx   dst+4*xsize-4
 140         C ecx   counter
 141         C edx   scratch (remainder)
 142         C esi   divisor
 143         C edi   src
 144         C ebp   xsize
 145
 146         movl    -4(%edi,%ecx,4), %eax
 147 L(integer_entry):
 148
 149         divl    %esi
 150
 151         movl    %eax, (%ebx,%ecx,4)
 152         loop    L(integer_top)
 153
 154
 155 L(fraction):
 156         orl     %ebp, %ecx
 157         jz      L(done)
 158
 159         movl    PARAM_DST, %ebx
 160
 161
 162 L(fraction_top):
 163         C eax   scratch (quotient)
 164         C ebx   dst
 165         C ecx   counter
 166         C edx   scratch (remainder)
 167         C esi   divisor
 168         C edi
 169         C ebp
 170
 171         xorl    %eax, %eax
 172
 173         divl    %esi
 174
 175         movl    %eax, -4(%ebx,%ecx,4)
 176         loop    L(fraction_top)
 177
 178
 179 L(done):
 180         popl    %ebp
 181         movl    %edx, %eax
 182         popl    %ebx
 183         popl    %esi
 184         popl    %edi
 185         ret
 186
 187
 188 L(size_zero):
 189 deflit(`FRAME',8)
 190         movl    PARAM_XSIZE, %ecx
 191         xorl    %eax, %eax
 192
 193         movl    PARAM_DST, %edi
 194
 195         cld     C better safe than sorry, see mpn/x86/README
 196
 197         rep
 198         stosl
 199
 200         popl    %esi
 201         popl    %edi
 202         ret
 203 EPILOGUE()