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dnl AMD K6 mpn_divrem_1 -- mpn by limb division.
dnl Copyright 1999-2003, 2007 Free Software Foundation, Inc.
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 modify
dnl it under the terms of either:
dnl
dnl * the GNU Lesser General Public License as published by the Free
dnl Software Foundation; either version 3 of the License, or (at your
dnl option) any later version.
dnl
dnl or
dnl
dnl * the GNU General Public License as published by the Free Software
dnl Foundation; either version 2 of the License, or (at your option) any
dnl later version.
dnl
dnl or both in parallel, as here.
dnl
dnl The GNU MP Library is distributed in the hope that it will be useful, but
dnl WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
dnl or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
dnl for more details.
dnl
dnl You should have received copies of the GNU General Public License and the
dnl GNU Lesser General Public License along with the GNU MP Library. If not,
dnl see https://www.gnu.org/licenses/.
include(`../config.m4')
C K6: 20 cycles/limb
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 The code here is basically the same as mpn/x86/divrem_1.asm, but uses loop
C instead of decl+jnz, since it comes out 2 cycles/limb faster.
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 20 cycles, so assuming high<divisor about
C half the time, then this test saves half that amount. The branch
C misprediction penalty is less than that.
C
C Back-to-back div instructions run at 20 cycles, the same as the loop here,
C so it seems there's nothing to gain by rearranging the loop. Pairing the
C mov and loop instructions was found to gain nothing.
C
C Enhancements:
C
C The low-latency K6 multiply might be thought to suit a mul-by-inverse, but
C that algorithm has been found to suffer from the relatively poor carry
C handling on K6 and too many auxiliary instructions. The fractional part
C however could be done at about 13 c/l, if it mattered enough.
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(32)
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 C size
movl PARAM_CARRY, %edx
jz L(fraction) C if size==0
leal -4(%ebx,%ebp,4), %ebx C dst one limb below integer part
jmp L(integer_top)
EPILOGUE()
ALIGN(16)
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 C size
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 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 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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