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dnl x86 mpn_divrem_1 -- mpn by limb division extending to fractional quotient.
dnl Copyright (C) 1999, 2000 Free Software Foundation, Inc.
dnl
dnl This file is part of the GNU MP Library.
dnl
dnl The GNU MP Library is free software; you can redistribute it and/or
dnl modify it under the terms of the GNU Lesser General Public License as
dnl published by the Free Software Foundation; either version 2.1 of the
dnl License, or (at your option) any later version.
dnl
dnl The GNU MP Library is distributed in the hope that it will be useful,
dnl but WITHOUT ANY WARRANTY; without even the implied warranty of
dnl MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
dnl Lesser General Public License for more details.
dnl
dnl You should have received a copy of the GNU Lesser General Public
dnl License along with the GNU MP Library; see the file COPYING.LIB. If
dnl not, write to the Free Software Foundation, Inc., 59 Temple Place -
dnl Suite 330, Boston, MA 02111-1307, USA.
dnl cycles/limb
dnl K6 20
dnl P5 44
dnl P6 39
dnl 486 approx 43 maybe
dnl
dnl
dnl The following have their own optimized divrem_1 implementations, but
dnl for reference the code here runs as follows.
dnl
dnl cycles/limb
dnl P6MMX 39
dnl K7 42
include(`../config.m4')
C mp_limb_t mpn_divrem_1 (mp_ptr dst, mp_size_t xsize,
C mp_srcptr src, mp_size_t size, mp_limb_t divisor);
C mp_limb_t mpn_divrem_1c (mp_ptr dst, mp_size_t xsize,
C mp_srcptr src, mp_size_t size, mp_limb_t divisor);
C
C Divide src,size by divisor and store the quotient in dst+xsize,size.
C Extend the division to fractional quotient limbs in dst,xsize. Return the
C remainder. Either or both xsize and size can be 0.
C
C mpn_divrem_1c takes a carry parameter which is an initial high limb,
C effectively one extra limb at the top of src,size. Must have
C carry<divisor.
C
C
C Essentially the code is the same as the division based part of
C mpn/generic/divrem_1.c, but has the following advantages.
C
C - If gcc isn't being used then divrem_1.c will get the generic C
C udiv_qrnnd() and be rather slow.
C
C - On K6, using the loop instruction is a 10% speedup, but gcc doesn't
C generate that instruction (as of gcc 2.95.2 at least).
C
C A test is done to see if the high limb is less the the divisor, and if so
C one less div is done. A div is between 20 and 40 cycles on the various
C x86s, so assuming high<divisor about half the time, then this test saves
C half that amount. The branch misprediction penalty on each chip is less
C than half a div.
C
C
C K6: Back-to-back div instructions run at 20 cycles, the same as the loop
C here, so it seems there's nothing to gain by rearranging the loop.
C Pairing the mov and loop instructions was found to gain nothing. (The
C same is true of the mpn/x86/mod_1.asm loop.)
C
C With a "decl/jnz" rather than a "loop" this code runs at 22 cycles.
C The loop_or_decljnz macro is an easy way to get a 10% speedup.
C
C The fast K6 multiply might be thought to suit a multiply-by-inverse,
C but that algorithm has been found to suffer from the releatively poor
C carry handling on K6 and too many auxiliary instructions. The
C fractional part however could be done at about 13 c/l.
C
C P5: Moving the load down to pair with the store might save 1 cycle, but
C that doesn't seem worth bothering with, since it'd be only a 2.2%
C saving.
C
C Again here the auxiliary instructions hinder a multiply-by-inverse,
C though there might be a 10-15% speedup available
defframe(PARAM_CARRY, 24)
defframe(PARAM_DIVISOR,20)
defframe(PARAM_SIZE, 16)
defframe(PARAM_SRC, 12)
defframe(PARAM_XSIZE, 8)
defframe(PARAM_DST, 4)
.text
ALIGN(16)
PROLOGUE(mpn_divrem_1c)
deflit(`FRAME',0)
movl PARAM_SIZE, %ecx
pushl %edi FRAME_pushl()
movl PARAM_SRC, %edi
pushl %esi FRAME_pushl()
movl PARAM_DIVISOR, %esi
pushl %ebx FRAME_pushl()
movl PARAM_DST, %ebx
pushl %ebp FRAME_pushl()
movl PARAM_XSIZE, %ebp
orl %ecx, %ecx
movl PARAM_CARRY, %edx
jz LF(mpn_divrem_1,fraction)
leal -4(%ebx,%ebp,4), %ebx C dst one limb below integer part
jmp LF(mpn_divrem_1,integer_top)
EPILOGUE()
PROLOGUE(mpn_divrem_1)
deflit(`FRAME',0)
movl PARAM_SIZE, %ecx
pushl %edi FRAME_pushl()
movl PARAM_SRC, %edi
pushl %esi FRAME_pushl()
movl PARAM_DIVISOR, %esi
orl %ecx,%ecx
jz L(size_zero)
pushl %ebx FRAME_pushl()
movl -4(%edi,%ecx,4), %eax C src high limb
xorl %edx, %edx
movl PARAM_DST, %ebx
pushl %ebp FRAME_pushl()
movl PARAM_XSIZE, %ebp
cmpl %esi, %eax
leal -4(%ebx,%ebp,4), %ebx C dst one limb below integer part
jae L(integer_entry)
C high<divisor, so high of dst is zero, and avoid one div
movl %edx, (%ebx,%ecx,4)
decl %ecx
movl %eax, %edx
jz L(fraction)
L(integer_top):
C eax scratch (quotient)
C ebx dst+4*xsize-4
C ecx counter
C edx scratch (remainder)
C esi divisor
C edi src
C ebp xsize
movl -4(%edi,%ecx,4), %eax
L(integer_entry):
divl %esi
movl %eax, (%ebx,%ecx,4)
loop_or_decljnz L(integer_top)
L(fraction):
orl %ebp, %ecx
jz L(done)
movl PARAM_DST, %ebx
L(fraction_top):
C eax scratch (quotient)
C ebx dst
C ecx counter
C edx scratch (remainder)
C esi divisor
C edi
C ebp
xorl %eax, %eax
divl %esi
movl %eax, -4(%ebx,%ecx,4)
loop_or_decljnz L(fraction_top)
L(done):
popl %ebp
movl %edx, %eax
popl %ebx
popl %esi
popl %edi
ret
L(size_zero):
deflit(`FRAME',8)
movl PARAM_XSIZE, %ecx
xorl %eax, %eax
movl PARAM_DST, %edi
cld C better safe than sorry, see mpn/x86/README.family
rep
stosl
popl %esi
popl %edi
ret
EPILOGUE()
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