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dnl Intel P6 mpn_copyd -- copy limb vector backwards.
dnl Copyright 2001, 2002 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 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 P6: 1.75 cycles/limb, or 0.75 if no overlap
C void mpn_copyd (mp_ptr dst, mp_srcptr src, mp_size_t size);
C
C An explicit loop is used because a decrementing rep movsl is a bit slow at
C 2.4 c/l. That rep movsl also has about a 40 cycle startup time, and the
C code here stands a chance of being faster if the branches predict well.
C
C The slightly strange loop form seems necessary for the claimed speed.
C Maybe load/store ordering affects it.
C
C The source and destination are checked to see if they're actually
C overlapping, since it might be possible to use an incrementing rep movsl
C at 0.75 c/l. (It doesn't suffer the bad startup time of the decrementing
C version.)
C
C Enhancements:
C
C Top speed for an all-integer copy is probably 1.0 c/l, being one load and
C one store each cycle. Unrolling the loop below would approach 1.0, but
C it'd be good to know why something like store/load/subl + store/load/jnz
C doesn't already run at 1.0 c/l. It looks like it should decode in 2
C cycles, but doesn't run that way.
defframe(PARAM_SIZE,12)
defframe(PARAM_SRC, 8)
defframe(PARAM_DST, 4)
dnl re-using parameter space
define(SAVE_ESI,`PARAM_SIZE')
define(SAVE_EDI,`PARAM_SRC')
TEXT
ALIGN(16)
PROLOGUE(mpn_copyd)
deflit(`FRAME',0)
movl PARAM_SIZE, %ecx
movl %esi, SAVE_ESI
movl PARAM_SRC, %esi
movl %edi, SAVE_EDI
movl PARAM_DST, %edi
subl $1, %ecx
jb L(zero)
movl (%esi,%ecx,4), %eax C src[size-1]
jz L(one)
movl -4(%esi,%ecx,4), %edx C src[size-2]
subl $2, %ecx
jbe L(done_loop) C 2 or 3 limbs only
C The usual overlap is
C
C high low
C +------------------+
C | dst|
C +------------------+
C +------------------+
C | src|
C +------------------+
C
C We can use an incrementing copy in the following circumstances.
C
C src+4*size<=dst, since then the regions are disjoint
C
C src==dst, clearly (though this shouldn't occur normally)
C
C src>dst, since in that case it's a requirement of the
C parameters that src>=dst+size*4, and hence the
C regions are disjoint
C
leal (%edi,%ecx,4), %edx
cmpl %edi, %esi
jae L(use_movsl) C src >= dst
cmpl %edi, %edx
movl 4(%esi,%ecx,4), %edx C src[size-2] again
jbe L(use_movsl) C src+4*size <= dst
L(top):
C eax prev high limb
C ebx
C ecx counter, size-3 down to 0 or -1, inclusive, by 2s
C edx prev low limb
C esi src
C edi dst
C ebp
movl %eax, 8(%edi,%ecx,4)
movl (%esi,%ecx,4), %eax
movl %edx, 4(%edi,%ecx,4)
movl -4(%esi,%ecx,4), %edx
subl $2, %ecx
jnbe L(top)
L(done_loop):
movl %eax, 8(%edi,%ecx,4)
movl %edx, 4(%edi,%ecx,4)
C copy low limb (needed if size was odd, but will already have been
C done in the loop if size was even)
movl (%esi), %eax
L(one):
movl %eax, (%edi)
movl SAVE_EDI, %edi
movl SAVE_ESI, %esi
ret
L(use_movsl):
C eax
C ebx
C ecx size-3
C edx
C esi src
C edi dst
C ebp
addl $3, %ecx
cld C better safe than sorry, see mpn/x86/README
rep
movsl
L(zero):
movl SAVE_ESI, %esi
movl SAVE_EDI, %edi
ret
EPILOGUE()
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