;;  x86_64 mpn_sub_n -- Subtract two limb vectors of the same length > 0 and
;;  store difference in a third limb vector.
;;
;;  Copyright (C) 2006  Jason Worth Martin <jason.worth.martin@gmail.com>
;;
;;  This program is free software; you can redistribute it and/or modify
;;  it under the terms of the GNU General Public License as published by
;;  the Free Software Foundation; either version 2 of the License, or
;;  (at your option) any later version.
;;
;;  This program is distributed in the hope that it will be useful,
;;  but WITHOUT ANY WARRANTY; without even the implied warranty of
;;  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;;  GNU General Public License for more details.
;;
;;  You should have received a copy of the GNU General Public License along
;;  with this program; if not, write to the Free Software Foundation, Inc.,
;;  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
;;
;; **************************************************************************
;;
;;
;; CREDITS
;;
;; This code is based largely on Pierrick Gaudry's excellent assembly
;; support for the AMD64 architecture.  (Note that Intel64 and AMD64,
;; while using the same instruction set, have very different
;; microarchitectures.  So, this code performs very poorly on AMD64
;; machines even though it is near-optimal on Intel64.)
;;
;; Roger Golliver works for Intel and provided insightful improvements
;; particularly in using the "lea" instruction to perform additions
;; and register-to-register moves.
;;
;; Eric Bainville has a brilliant exposition of optimizing arithmetic for
;; AMD64 (http://www.bealto.it).  I adapted many of the ideas he
;; describes to Intel64.
;;
;; Agner Fog is a demigod in the x86 world.  If you are reading assembly
;; code files and you haven't heard of Agner Fog, then take a minute to
;; look over his software optimization manuals (http://www.agner.org/).
;; They are superb.
;;
;; *********************************************************************

%ifdef	__JWM_Test_Code__
%include 'yasm_mac.inc'
%else
%include '../yasm_mac.inc'
%endif

;;
;; If YASM supports lahf and sahf instructions, then we'll get rid
;; of this.
;;
%define save_CF_to_reg_a  db	0x9f
%define get_CF_from_reg_a db	0x9e
;; define(`save_CF_to_reg_a',`setc	%al')
;; define(`get_CF_from_reg_a',`bt	`$'0x0,%rax')


;;         cycles/limb
;; Hammer:     2.5 (for 1024 limbs)
;; Woodcrest:  2.6 (for 1024 limbs)	

;; INPUT PARAMETERS
;; rp	rdi
;; up	rsi
;; vp	rdx
;; n	rcx
	
	BITS	64
GLOBAL_FUNC mpn_sub_n
	push	rbp		;; Save off callee-save registers
	push	rbx
	push	r12
	push	r13
	push	r14
	push	r15

	xor	r15,r15			;; r15 will be our index, so
					;; I'll call it i here after
	save_CF_to_reg_a				;; Save CF

	mov	r9,rcx
	sub	r9,4			;; r9 = n-(i+4)

	align	16			;; aligning for loop
L_mpn_sub_n_main_loop:
	;; The goal of our main unrolled loop is to keep all the
	;; execution units as busy as possible.  Since
	;; there are three ALUs, we try to perform three
	;; adds at a time.  Of course, we will have the
	;; borrow dependency, so there is at least one
	;; clock cycle between each sbb.  However, we'll
	;; try to keep the other execution units busy
	;; with loads and stores at the same time so that
	;; our net throughput is close to one sbb per clock
	;; cycle.  Hopefully this function will have asymptotic
	;; behavior of taking 3*n clock cycles where n is the
	;; number of limbs to sub.
	;;
	;; Note that I'm using FOUR sbbs at a time, this is just
	;; because I wanted to use up all available registers since
	;; I'm hoping the out-of-order and loop-pipeline logic in
	;; the Xeon will help us out.

	;; See if we are still looping
	jle	L_mpn_sub_n_loop_done

	get_CF_from_reg_a			;; recover CF
	
	;; Load inputs into rbx and r8
	;; sub with borrow, and put result in r8
	;; then store r8 to output.
	mov	rbx,[rdx+r15*8]
	mov	r8,[rsi+r15*8]
	sbb	r8,rbx
	mov	[rdi+r15*8],r8
	
	;; Load inputs into r9 and r10
	;; sub with borrow, and put result in r10
	;; then store r10 to output.
	mov	r9,[8+rdx+r15*8]
	mov	r10,[8+rsi+r15*8]
	sbb	r10,r9
	mov	[8+rdi+r15*8],r10
	
	;; Load inputs into r11 and r12
	;; sub with borrow, and put result in r12
	;; then store r12 to output.
	mov	r11,[16+rdx+r15*8]
	mov	r12,[16+rsi+r15*8]
	sbb	r12,r11
	mov	[16+rdi+r15*8],r12

	;; Load inputs into r13 and r14
	;; sub with borrow, and put result in r14
	;; then store r14 to output.
	mov	r13,[24+rdx+r15*8]
	mov	r14,[24+rsi+r15*8]
	sbb	r14,r13
	mov	[24+rdi+r15*8],r14

	save_CF_to_reg_a			;; save CF

	mov	r10,r15
	add	r10,8
	add	r15,4		;; increment by 4.
	
	mov	r9,rcx
	sub	r9,r10		;; r9 = n-(i+4)
	jmp	L_mpn_sub_n_main_loop

L_mpn_sub_n_loop_done:
	mov	r15,rcx		;; 
	sub	r15,r9		;; r15 = n-(n-(i+4))=i+4
	sub	r15,4		;; r15 = i
	cmp	r15,rcx
L_mpn_sub_n_post_loop:
	je	L_mpn_sub_n_exit
	get_CF_from_reg_a			;; recover CF
	
	;; Load inputs into rbx and r8
	;; sub with borrow, and put result in r8
	;; then store r8 to output.
	mov	rbx,[rdx+r15*8]
	mov	r8,[rsi+r15*8]
	sbb	r8,rbx
	mov	[rdi+r15*8],r8
	save_CF_to_reg_a			;; save CF
	add	r15,1
	cmp	r15,rcx
	jmp	L_mpn_sub_n_post_loop
	
	
L_mpn_sub_n_exit:
	xor	rcx,rcx
	get_CF_from_reg_a			;; recover the CF
	mov	rax,rcx		;; Clears rax without affecting carry flag 
	adc	rax,rax		;; returns carry status.
	
	pop	r15		;; restore callee-save registers
	pop	r14
	pop	r13
	pop	r12
	pop	rbx
	pop	rbp
	ret