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dnl AMD K6 mpn_divrem_1 -- mpn by limb division.
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dnl Copyright 1999, 2000, 2001, 2002, 2003 Free Software Foundation, Inc.
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dnl This file is part of the GNU MP Library.
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dnl The GNU MP Library is free software; you can redistribute it and/or
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dnl modify it under the terms of the GNU Lesser General Public License as
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dnl published by the Free Software Foundation; either version 2.1 of the
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dnl License, or (at your option) any later version.
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dnl The GNU MP Library is distributed in the hope that it will be useful,
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dnl but WITHOUT ANY WARRANTY; without even the implied warranty of
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dnl MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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dnl Lesser General Public License for more details.
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dnl You should have received a copy of the GNU Lesser General Public
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dnl License along with the GNU MP Library; see the file COPYING.LIB. If
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dnl not, write to the Free Software Foundation, Inc., 51 Franklin Street,
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dnl Fifth Floor, Boston, MA 02110-1301, USA.
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include(`../config.m4')
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C mp_limb_t mpn_divrem_1 (mp_ptr dst, mp_size_t xsize,
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C mp_srcptr src, mp_size_t size, mp_limb_t divisor);
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C mp_limb_t mpn_divrem_1c (mp_ptr dst, mp_size_t xsize,
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C mp_srcptr src, mp_size_t size, mp_limb_t divisor,
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C The code here is basically the same as mpn/x86/divrem_1.asm, but uses loop
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C instead of decl+jnz, since it comes out 2 cycles/limb faster.
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C A test is done to see if the high limb is less the the divisor, and if so
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C one less div is done. A div is 20 cycles, so assuming high<divisor about
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C half the time, then this test saves half that amount. The branch
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C misprediction penalty is less than that.
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C Back-to-back div instructions run at 20 cycles, the same as the loop here,
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C so it seems there's nothing to gain by rearranging the loop. Pairing the
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C mov and loop instructions was found to gain nothing.
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C The low-latency K6 multiply might be thought to suit a mul-by-inverse, but
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C that algorithm has been found to suffer from the relatively poor carry
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C handling on K6 and too many auxiliary instructions. The fractional part
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C however could be done at about 13 c/l, if it mattered enough.
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defframe(PARAM_CARRY, 24)
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defframe(PARAM_DIVISOR,20)
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defframe(PARAM_SIZE, 16)
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defframe(PARAM_SRC, 12)
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defframe(PARAM_XSIZE, 8)
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defframe(PARAM_DST, 4)
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PROLOGUE(mpn_divrem_1c)
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pushl %edi FRAME_pushl()
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pushl %esi FRAME_pushl()
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movl PARAM_DIVISOR, %esi
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pushl %ebx FRAME_pushl()
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pushl %ebp FRAME_pushl()
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movl PARAM_XSIZE, %ebp
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movl PARAM_CARRY, %edx
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jz L(fraction) C if size==0
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leal -4(%ebx,%ebp,4), %ebx C dst one limb below integer part
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PROLOGUE(mpn_divrem_1)
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pushl %edi FRAME_pushl()
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pushl %esi FRAME_pushl()
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movl PARAM_DIVISOR, %esi
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pushl %ebx FRAME_pushl()
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movl -4(%edi,%ecx,4), %eax C src high limb
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pushl %ebp FRAME_pushl()
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movl PARAM_XSIZE, %ebp
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leal -4(%ebx,%ebp,4), %ebx C dst one limb below integer part
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C high<divisor, so high of dst is zero, and avoid one div
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movl %edx, (%ebx,%ecx,4)
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C eax scratch (quotient)
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C edx scratch (remainder)
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movl -4(%edi,%ecx,4), %eax
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movl %eax, (%ebx,%ecx,4)
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C eax scratch (quotient)
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C edx scratch (remainder)
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movl %eax, -4(%ebx,%ecx,4)
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movl PARAM_XSIZE, %ecx
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cld C better safe than sorry, see mpn/x86/README
added
removed
src/gmp/mpn/x86/k6/divrem_1.asm
