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dnl x86 mpn_divrem_1 -- mpn by limb division extending to fractional quotient.
dnl Copyright 1999, 2000, 2001, 2002, 2003, 2007 Free Software Foundation,
dnl 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 3 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 License
dnl along with the GNU MP Library. If not, see http://www.gnu.org/licenses/.
include(`../config.m4')
C cycles/limb
C 486 approx 43 maybe
C P5 44
C P6 39
C P6MMX 39
C K6 22
C K7 42
C P4 58
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 mp_limb_t carry);
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 advantage that we get the desired divl
C instruction even when gcc is not being used (when longlong.h only has the
C rather slow generic C udiv_qrnnd().
C
C A test is done to see if the high limb is less than 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 Notes for P5:
C
C It might be thought that moving the load down to pair with the store would
C save 1 cycle, but that doesn't seem to happen in practice, and in any case
C would be a mere 2.2% saving, so it's hardly worth bothering about.
C
C A mul-by-inverse might be a possibility for P5, as done in
C mpn/x86/pentium/mod_1.asm. The number of auxiliary instructions required
C is a hinderance, but there could be a 10-15% speedup available.
C
C
C Notes for K6:
C
C K6 has its own version of this code, using loop and paying attention to
C cache line boundary crossings. The target 20 c/l can be had with the
C decl+jnz of the present code by pairing up the load and store in the
C loops. But it's considered easier not to introduce complexity just for
C that, but instead let k6 have its own code.
C
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 L(fraction)
leal -4(%ebx,%ebp,4), %ebx C dst one limb below integer part
jmp L(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)
decl %ecx
jnz 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)
decl %ecx
jnz 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
rep
stosl
popl %esi
popl %edi
ret
EPILOGUE()
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