gcc/gmp/mpn/x86_64/lshsub_n.asm

dnl  AMD64 mpn_lshsub_n.  R = 2^k(U - V).

dnl  Copyright 2006 Free Software Foundation, Inc.

dnl  This file is part of the GNU MP Library.

dnl  The GNU MP Library is free software; you can redistribute it and/or modify
dnl  it under the terms of the GNU Lesser General Public License as published
dnl  by the Free Software Foundation; either version 3 of the License, or (at
dnl  your option) any later version.

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 Lesser General Public
dnl  License for more details.

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 K8,K9:	 3.15	(mpn_sub_n + mpn_lshift costs about 4 c/l)
C K10:		 3.15	(mpn_sub_n + mpn_lshift costs about 4 c/l)
C P4:		16.5
C P6-15:	 4.35

C This was written quickly and not optimized at all, but it runs very well on
C K8.  But perhaps one could get under 3 c/l.  Ideas:
C   1) Use indexing to save the 3 LEA
C   2) Write reasonable feed-in code
C   3) Be more clever about register usage
C   4) Unroll more, handling CL negation, carry save/restore cost much now
C   5) Reschedule

C INPUT PARAMETERS
define(`rp',	`%rdi')
define(`up',	`%rsi')
define(`vp',	`%rdx')
define(`n',	`%rcx')
define(`cnt'	`%r8')

ASM_START()
	TEXT
	ALIGN(16)
PROLOGUE(mpn_lshsub_n)

	push	%r12
	push	%r13
	push	%r14
	push	%r15
	push	%rbx

	mov	n, %rax
	xor	%ebx, %ebx		C clear carry save register
	mov	%r8d, %ecx		C shift count
	xor	%r15d, %r15d		C limb carry

	mov	%eax, %r11d
	and	$3, %r11d
	je	L(4)
	sub	$1, %r11d

L(oopette):
	add	%ebx, %ebx		C restore carry flag
	mov	0(up), %r8
	lea	8(up), up
	sbb	0(vp), %r8
	mov	%r8, %r12
	sbb	%ebx, %ebx		C save carry flag
	shl	%cl, %r8
	or	%r15, %r8
	mov	%r12, %r15
	lea	8(vp), vp
	neg	%cl
	shr	%cl, %r15
	neg	%cl
	mov	%r8, 0(rp)
	lea	8(rp), rp
	sub	$1, %r11d
	jnc	L(oopette)

L(4):
	sub	$4, %rax
	jc	L(end)

	ALIGN(16)
L(oop):
	add	%ebx, %ebx		C restore carry flag

	mov	0(up), %r8
	mov	8(up), %r9
	mov	16(up), %r10
	mov	24(up), %r11

	lea	32(up), up

	sbb	0(vp), %r8
	mov	%r8, %r12
	sbb	8(vp), %r9
	mov	%r9, %r13
	sbb	16(vp), %r10
	mov	%r10, %r14
	sbb	24(vp), %r11

	sbb	%ebx, %ebx		C save carry flag

	shl	%cl, %r8
	shl	%cl, %r9
	shl	%cl, %r10
	or	%r15, %r8
	mov	%r11, %r15
	shl	%cl, %r11

	lea	32(vp), vp

	neg	%cl

	shr	%cl, %r12
	shr	%cl, %r13
	shr	%cl, %r14
	shr	%cl, %r15		C used next loop

	or	%r12, %r9
	or	%r13, %r10
	or	%r14, %r11

	neg	%cl

	mov	%r8, 0(rp)
	mov	%r9, 8(rp)
	mov	%r10, 16(rp)
	mov	%r11, 24(rp)

	lea	32(rp), rp

	sub	$4, %rax
	jnc	L(oop)
L(end):
	neg	%ebx
	shl	%cl, %rbx
	adc	%r15, %rbx
	mov	%rbx, %rax
	pop	%rbx
	pop	%r15
	pop	%r14
	pop	%r13
	pop	%r12

	ret
EPILOGUE()