Annotation of OpenXM_contrib/gmp/mpn/x86/pentium/diveby3.asm, Revision 1.1
1.1 ! maekawa 1: dnl Intel P5 mpn_divexact_by3 -- mpn division by 3, expecting no remainder.
! 2: dnl
! 3: dnl P5: 15.0 cycles/limb
! 4:
! 5:
! 6: dnl Copyright (C) 2000 Free Software Foundation, Inc.
! 7: dnl
! 8: dnl This file is part of the GNU MP Library.
! 9: dnl
! 10: dnl The GNU MP Library is free software; you can redistribute it and/or
! 11: dnl modify it under the terms of the GNU Lesser General Public License as
! 12: dnl published by the Free Software Foundation; either version 2.1 of the
! 13: dnl License, or (at your option) any later version.
! 14: dnl
! 15: dnl The GNU MP Library is distributed in the hope that it will be useful,
! 16: dnl but WITHOUT ANY WARRANTY; without even the implied warranty of
! 17: dnl MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
! 18: dnl Lesser General Public License for more details.
! 19: dnl
! 20: dnl You should have received a copy of the GNU Lesser General Public
! 21: dnl License along with the GNU MP Library; see the file COPYING.LIB. If
! 22: dnl not, write to the Free Software Foundation, Inc., 59 Temple Place -
! 23: dnl Suite 330, Boston, MA 02111-1307, USA.
! 24:
! 25:
! 26: include(`../config.m4')
! 27:
! 28:
! 29: C mp_limb_t mpn_divexact_by3c (mp_ptr dst, mp_srcptr src, mp_size_t size,
! 30: C mp_limb_t carry);
! 31:
! 32: defframe(PARAM_CARRY,16)
! 33: defframe(PARAM_SIZE, 12)
! 34: defframe(PARAM_SRC, 8)
! 35: defframe(PARAM_DST, 4)
! 36:
! 37: dnl multiplicative inverse of 3, modulo 2^32
! 38: deflit(INVERSE_3, 0xAAAAAAAB)
! 39:
! 40: dnl ceil(b/3), ceil(b*2/3) and floor(b*2/3) where b=2^32
! 41: deflit(ONE_THIRD_CEIL, 0x55555556)
! 42: deflit(TWO_THIRDS_CEIL, 0xAAAAAAAB)
! 43: deflit(TWO_THIRDS_FLOOR, 0xAAAAAAAA)
! 44:
! 45: .text
! 46: ALIGN(8)
! 47:
! 48: PROLOGUE(mpn_divexact_by3c)
! 49: deflit(`FRAME',0)
! 50:
! 51: movl PARAM_SRC, %ecx
! 52: movl PARAM_SIZE, %edx
! 53:
! 54: decl %edx
! 55: jnz L(two_or_more)
! 56:
! 57: movl (%ecx), %edx
! 58: movl PARAM_CARRY, %eax C risk of cache bank clash here
! 59:
! 60: movl PARAM_DST, %ecx
! 61: subl %eax, %edx
! 62:
! 63: sbbl %eax, %eax C 0 or -1
! 64:
! 65: imull $INVERSE_3, %edx, %edx
! 66:
! 67: negl %eax C 0 or 1
! 68: cmpl $ONE_THIRD_CEIL, %edx
! 69:
! 70: sbbl $-1, %eax C +1 if edx>=ceil(b/3)
! 71: cmpl $TWO_THIRDS_CEIL, %edx
! 72:
! 73: sbbl $-1, %eax C +1 if edx>=ceil(b*2/3)
! 74: movl %edx, (%ecx)
! 75:
! 76: ret
! 77:
! 78:
! 79: L(two_or_more):
! 80: C eax
! 81: C ebx
! 82: C ecx src
! 83: C edx size-1
! 84: C esi
! 85: C edi
! 86: C ebp
! 87:
! 88: pushl %ebx FRAME_pushl()
! 89: pushl %esi FRAME_pushl()
! 90:
! 91: pushl %edi FRAME_pushl()
! 92: pushl %ebp FRAME_pushl()
! 93:
! 94: movl PARAM_DST, %edi
! 95: movl PARAM_CARRY, %esi
! 96:
! 97: movl (%ecx), %eax C src low limb
! 98: xorl %ebx, %ebx
! 99:
! 100: sub %esi, %eax
! 101: movl $TWO_THIRDS_FLOOR, %esi
! 102:
! 103: leal (%ecx,%edx,4), %ecx C &src[size-1]
! 104: leal (%edi,%edx,4), %edi C &dst[size-1]
! 105:
! 106: adcl $0, %ebx C carry, 0 or 1
! 107: negl %edx C -(size-1)
! 108:
! 109:
! 110: C The loop needs a source limb ready at the top, which leads to one limb
! 111: C handled separately at the end, and the special case above for size==1.
! 112: C There doesn't seem to be any scheduling that would keep the speed but move
! 113: C the source load and carry subtract up to the top.
! 114: C
! 115: C The destination cache line prefetching adds 1 cycle to the loop but is
! 116: C considered worthwhile. The slowdown is a factor of 1.07, but will prevent
! 117: C repeated write-throughs if the destination isn't in L1. A version using
! 118: C an outer loop to prefetch only every 8 limbs (a cache line) proved to be
! 119: C no faster, due to unavoidable branch mispreditions in the inner loop.
! 120: C
! 121: C setc is 2 cycles on P54, so an adcl is used instead. If the movl $0,%ebx
! 122: C could be avoided then the src limb fetch could pair up and save a cycle.
! 123: C This would probably mean going to a two limb loop with the carry limb
! 124: C alternately positive or negative, since an sbbl %ebx,%ebx will leave a
! 125: C value which is in the opposite sense to the preceding sbbl/adcl %ebx,%eax.
! 126: C
! 127: C A register is used for TWO_THIRDS_FLOOR because a cmp can't be done as
! 128: C "cmpl %edx, $n" with the immediate as the second operand.
! 129: C
! 130: C The "4" source displacement is in the loop rather than the setup because
! 131: C this gets L(top) aligned to 8 bytes at no cost.
! 132:
! 133: ALIGN(8)
! 134: L(top):
! 135: C eax source limb, carry subtracted
! 136: C ebx carry (0 or 1)
! 137: C ecx &src[size-1]
! 138: C edx counter, limbs, negative
! 139: C esi TWO_THIRDS_FLOOR
! 140: C edi &dst[size-1]
! 141: C ebp scratch (result limb)
! 142:
! 143: imull $INVERSE_3, %eax, %ebp
! 144:
! 145: cmpl $ONE_THIRD_CEIL, %ebp
! 146: movl (%edi,%edx,4), %eax C dst cache line prefetch
! 147:
! 148: sbbl $-1, %ebx C +1 if ebp>=ceil(b/3)
! 149: cmpl %ebp, %esi
! 150:
! 151: movl 4(%ecx,%edx,4), %eax C next src limb
! 152:
! 153: sbbl %ebx, %eax C and further -1 if ebp>=ceil(b*2/3)
! 154: movl $0, %ebx
! 155:
! 156: adcl $0, %ebx C new carry
! 157: movl %ebp, (%edi,%edx,4)
! 158:
! 159: incl %edx
! 160: jnz L(top)
! 161:
! 162:
! 163:
! 164: imull $INVERSE_3, %eax, %edx
! 165:
! 166: cmpl $ONE_THIRD_CEIL, %edx
! 167: movl %edx, (%edi)
! 168:
! 169: sbbl $-1, %ebx C +1 if edx>=ceil(b/3)
! 170: cmpl $TWO_THIRDS_CEIL, %edx
! 171:
! 172: sbbl $-1, %ebx C +1 if edx>=ceil(b*2/3)
! 173: popl %ebp
! 174:
! 175: movl %ebx, %eax
! 176: popl %edi
! 177:
! 178: popl %esi
! 179: popl %ebx
! 180:
! 181: ret
! 182:
! 183: EPILOGUE()
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