mpir/mpn/x86/pentium/dive_1.asm

dnl  Intel Pentium mpn_divexact_1 -- mpn by limb exact division.

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 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., 51 Franklin Street,
dnl  Fifth Floor, Boston, MA 02110-1301, USA.

include(`../config.m4')


C         divisor
C       odd   even
C P54:  24.5  30.5   cycles/limb
C P55:  23.0  28.0


C void mpn_divexact_1 (mp_ptr dst, mp_srcptr src, mp_size_t size,
C                      mp_limb_t divisor);
C
C Plain divl is used for small sizes, since the inverse takes a while to
C setup.  Multiplying works out faster for size>=3 when the divisor is odd,
C or size>=4 when the divisor is even.  Actually on P55 size==2 for odd or
C size==3 for even are about the same speed for both divl or mul, but the
C former is used since it will use up less code cache.
C
C The P55 speeds noted above, 23 cycles odd or 28 cycles even, are as
C expected.  On P54 in the even case the shrdl pairing nonsense (see
C mpn/x86/pentium/README) costs 1 cycle, but it's not clear why there's a
C further 1.5 slowdown for both odd and even.

defframe(PARAM_DIVISOR,16)
defframe(PARAM_SIZE,   12)
defframe(PARAM_SRC,    8)
defframe(PARAM_DST,    4)

dnl  re-use parameter space
define(VAR_INVERSE,`PARAM_DST')

	TEXT

	ALIGN(32)
PROLOGUE(mpn_divexact_1)
deflit(`FRAME',0)

	movl	PARAM_DIVISOR, %eax
	movl	PARAM_SIZE, %ecx

	pushl	%esi		FRAME_pushl()
	push	%edi		FRAME_pushl()

	movl	PARAM_SRC, %esi
	andl	$1, %eax

	movl	PARAM_DST, %edi
	addl	%ecx, %eax	C size if even, size+1 if odd

	cmpl	$4, %eax
	jae	L(mul_by_inverse)


	xorl	%edx, %edx
L(div_top):
	movl	-4(%esi,%ecx,4), %eax

	divl	PARAM_DIVISOR

	movl	%eax, -4(%edi,%ecx,4)
	decl	%ecx

	jnz	L(div_top)

	popl	%edi
	popl	%esi

	ret



L(mul_by_inverse):
	movl	PARAM_DIVISOR, %eax
	movl	$-1, %ecx

L(strip_twos):
	ASSERT(nz, `orl %eax, %eax')
	shrl	%eax
	incl	%ecx			C shift count

	jnc	L(strip_twos)

	leal	1(%eax,%eax), %edx	C d
	andl	$127, %eax		C d/2, 7 bits

	pushl	%ebx		FRAME_pushl()
	pushl	%ebp		FRAME_pushl()

ifdef(`PIC',`
	call	L(here)
L(here):
	popl	%ebp			C eip

	addl	$_GLOBAL_OFFSET_TABLE_+[.-L(here)], %ebp
	C AGI
	movl	modlimb_invert_table@GOT(%ebp), %ebp
	C AGI
	movzbl	(%eax,%ebp), %eax
',`

dnl non-PIC
	movzbl	modlimb_invert_table(%eax), %eax	C inv 8 bits
')

	movl	%eax, %ebp		C inv
	addl	%eax, %eax		C 2*inv

	imull	%ebp, %ebp		C inv*inv

	imull	%edx, %ebp		C inv*inv*d

	subl	%ebp, %eax		C inv = 2*inv - inv*inv*d
	movl	PARAM_SIZE, %ebx

	movl	%eax, %ebp
	addl	%eax, %eax		C 2*inv

	imull	%ebp, %ebp		C inv*inv

	imull	%edx, %ebp		C inv*inv*d

	subl	%ebp, %eax		C inv = 2*inv - inv*inv*d
	movl	%edx, PARAM_DIVISOR	C d without twos

	leal	(%esi,%ebx,4), %esi	C src end
	leal	(%edi,%ebx,4), %edi	C dst end

	negl	%ebx			C -size

	ASSERT(e,`	C expect d*inv == 1 mod 2^BITS_PER_MP_LIMB
	pushl	%eax	FRAME_pushl()
	imull	PARAM_DIVISOR, %eax
	cmpl	$1, %eax
	popl	%eax	FRAME_popl()')

	movl	%eax, VAR_INVERSE
	xorl	%ebp, %ebp		C initial carry bit

	movl	(%esi,%ebx,4), %eax	C src low limb
	orl	%ecx, %ecx		C shift

	movl	4(%esi,%ebx,4), %edx	C src second limb (for even)
	jz	L(odd_entry)

	shrdl(	%cl, %edx, %eax)

	incl	%ebx
	jmp	L(even_entry)


	ALIGN(8)
L(odd_top):
	C eax	scratch
	C ebx	counter, limbs, negative
	C ecx
	C edx
	C esi	src end
	C edi	dst end
	C ebp	carry bit, 0 or -1

	mull	PARAM_DIVISOR

	movl	(%esi,%ebx,4), %eax
	subl	%ebp, %edx

	subl	%edx, %eax

	sbbl	%ebp, %ebp

L(odd_entry):
	imull	VAR_INVERSE, %eax

	movl	%eax, (%edi,%ebx,4)

	incl	%ebx
	jnz	L(odd_top)


	popl	%ebp
	popl	%ebx

	popl	%edi
	popl	%esi

	ret


L(even_top):
	C eax	scratch
	C ebx	counter, limbs, negative
	C ecx	twos
	C edx
	C esi	src end
	C edi	dst end
	C ebp	carry bit, 0 or -1

	mull	PARAM_DIVISOR

	subl	%ebp, %edx		C carry bit
	movl	-4(%esi,%ebx,4), %eax	C src limb

	movl	(%esi,%ebx,4), %ebp	C and one above it

	shrdl(	%cl, %ebp, %eax)

	subl	%edx, %eax		C carry limb

	sbbl	%ebp, %ebp

L(even_entry):
	imull	VAR_INVERSE, %eax

	movl	%eax, -4(%edi,%ebx,4)
	incl	%ebx

	jnz	L(even_top)



	mull	PARAM_DIVISOR

	movl	-4(%esi), %eax		C src high limb
	subl	%ebp, %edx

	shrl	%cl, %eax

	subl	%edx, %eax		C no carry if division is exact

	imull	VAR_INVERSE, %eax

	movl	%eax, -4(%edi)		C dst high limb
	nop				C protect against cache bank clash

	popl	%ebp
	popl	%ebx

	popl	%edi
	popl	%esi

	ret

EPILOGUE()