C arm/memxor3.asm

ifelse(<
   Copyright (C) 2013, 2015 Niels Möller

   This file is part of GNU Nettle.

   GNU Nettle is free software: you can redistribute it and/or
   modify it under the terms of either:

     * the GNU Lesser General Public License as published by the Free
       Software Foundation; either version 3 of the License, or (at your
       option) any later version.

   or

     * 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.

   or both in parallel, as here.

   GNU Nettle 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 copies of the GNU General Public License and
   the GNU Lesser General Public License along with this program.  If
   not, see http://www.gnu.org/licenses/.
>)

C Possible speedups:
C
C The ldm instruction can do load two registers per cycle,
C if the address is two-word aligned. Or three registers in two
C cycles, regardless of alignment.

C Register usage:

define(<DST>, <r0>)
define(<AP>, <r1>)
define(<BP>, <r2>)
define(<N>, <r3>)

C Temporaries r4-r7
define(<ACNT>, <r8>)
define(<ATNC>, <r10>)
define(<BCNT>, <r11>)
define(<BTNC>, <r12>)

	.syntax unified

	.file "memxor3.asm"

	.text
	.arm

	C memxor3(void *dst, const void *a, const void *b, size_t n)
	.align 2
PROLOGUE(nettle_memxor3)
	cmp	N, #0
	beq	.Lmemxor3_ret

	push	{r4,r5,r6,r7,r8,r10,r11}
	cmp	N, #7

	add	AP, N
	add	BP, N
	add	DST, N

	bcs	.Lmemxor3_large

	C Simple byte loop
.Lmemxor3_bytes:
	ldrb	r4, [AP, #-1]!
	ldrb	r5, [BP, #-1]!
	eor	r4, r5
	strb	r4, [DST, #-1]!
	subs	N, #1
	bne	.Lmemxor3_bytes

.Lmemxor3_done:
	pop	{r4,r5,r6,r7,r8,r10,r11}
.Lmemxor3_ret:
	bx	lr

.Lmemxor3_align_loop:
	ldrb	r4, [AP, #-1]!
	ldrb	r5, [BP, #-1]!
	eor	r5, r4
	strb	r5, [DST, #-1]!
	sub	N, #1

.Lmemxor3_large:
	tst	DST, #3
	bne	.Lmemxor3_align_loop

	C We have at least 4 bytes left to do here.
	sub	N, #4
	ands	ACNT, AP, #3
	lsl	ACNT, #3
	beq	.Lmemxor3_a_aligned

	ands	BCNT, BP, #3
	lsl	BCNT, #3
	bne	.Lmemxor3_uu

	C Swap
	mov	r4, AP
	mov	AP, BP
	mov	BP, r4

.Lmemxor3_au:
	C NOTE: We have the relevant shift count in ACNT, not BCNT

	C AP is aligned, BP is not
	C           v original SRC
	C +-------+------+
	C |SRC-4  |SRC   |
	C +---+---+------+
	C     |DST-4  |
	C     +-------+
	C
	C With little-endian, we need to do
	C DST[i-i] ^= (SRC[i-i] >> CNT) ^ (SRC[i] << TNC)
	rsb	ATNC, ACNT, #32
	bic	BP, #3

	ldr	r4, [BP]

	tst	N, #4
	itet	eq
	moveq	r5, r4
	subne	N, #4
	beq	.Lmemxor3_au_odd

.Lmemxor3_au_loop:
	ldr	r5, [BP, #-4]!
	ldr	r6, [AP, #-4]!
	eor	r6, r6, r4, lsl ATNC
	eor	r6, r6, r5, lsr ACNT
	str	r6, [DST, #-4]!
.Lmemxor3_au_odd:
	ldr	r4, [BP, #-4]!
	ldr	r6, [AP, #-4]!
	eor	r6, r6, r5, lsl ATNC
	eor	r6, r6, r4, lsr ACNT
	str	r6, [DST, #-4]!
	subs	N, #8
	bcs	.Lmemxor3_au_loop
	adds	N, #8
	beq	.Lmemxor3_done

	C Leftover bytes in r4, low end
	ldr	r5, [AP, #-4]
	eor	r4, r5, r4, lsl ATNC

.Lmemxor3_au_leftover:
	C Store a byte at a time
	ror	r4, #24
	strb	r4, [DST, #-1]!
	subs	N, #1
	beq	.Lmemxor3_done
	subs	ACNT, #8
	sub	AP, #1
	bne	.Lmemxor3_au_leftover
	b	.Lmemxor3_bytes

.Lmemxor3_a_aligned:
	ands	ACNT, BP, #3
	lsl	ACNT, #3
	bne	.Lmemxor3_au ;

	C a, b and dst all have the same alignment.
	subs	N, #8
	bcc	.Lmemxor3_aligned_word_end

	C This loop runs at 8 cycles per iteration. It has been
	C observed running at only 7 cycles, for this speed, the loop
	C started at offset 0x2ac in the object file.

	C FIXME: consider software pipelining, similarly to the memxor
	C loop.

.Lmemxor3_aligned_word_loop:
	ldmdb	AP!, {r4,r5,r6}
	ldmdb	BP!, {r7,r8,r10}
	subs	N, #12
	eor	r4, r7
	eor	r5, r8
	eor	r6, r10
	stmdb	DST!, {r4, r5,r6}
	bcs	.Lmemxor3_aligned_word_loop

.Lmemxor3_aligned_word_end:
	C We have 0-11 bytes left to do, and N holds number of bytes -12.
	adds	N, #4
	bcc	.Lmemxor3_aligned_lt_8
	C Do 8 bytes more, leftover is in N
	ldmdb	AP!, {r4, r5}
	ldmdb	BP!, {r6, r7}
	eor	r4, r6
	eor	r5, r7
	stmdb	DST!, {r4,r5}
	beq	.Lmemxor3_done
	b	.Lmemxor3_bytes

.Lmemxor3_aligned_lt_8:
	adds	N, #4
	bcc	.Lmemxor3_aligned_lt_4

	ldr	r4, [AP,#-4]!
	ldr	r5, [BP,#-4]!
	eor	r4, r5
	str	r4, [DST,#-4]!
	beq	.Lmemxor3_done
	b	.Lmemxor3_bytes

.Lmemxor3_aligned_lt_4:
	adds	N, #4
	beq	.Lmemxor3_done
	b	.Lmemxor3_bytes

.Lmemxor3_uu:

	cmp	ACNT, BCNT
	bic	AP, #3
	bic	BP, #3
	rsb	ATNC, ACNT, #32

	bne	.Lmemxor3_uud

	C AP and BP are unaligned in the same way

	ldr	r4, [AP]
	ldr	r6, [BP]
	eor	r4, r6

	tst	N, #4
	itet	eq
	moveq	r5, r4
	subne	N, #4
	beq	.Lmemxor3_uu_odd

.Lmemxor3_uu_loop:
	ldr	r5, [AP, #-4]!
	ldr	r6, [BP, #-4]!
	eor	r5, r6
	lsl	r4, ATNC
	eor	r4, r4, r5, lsr ACNT
	str	r4, [DST, #-4]!
.Lmemxor3_uu_odd:
	ldr	r4, [AP, #-4]!
	ldr	r6, [BP, #-4]!
	eor	r4, r6
	lsl	r5, ATNC
	eor	r5, r5, r4, lsr ACNT
	str	r5, [DST, #-4]!
	subs	N, #8
	bcs	.Lmemxor3_uu_loop
	adds	N, #8
	beq	.Lmemxor3_done

	C Leftover bytes in a4, low end
	ror	r4, ACNT
.Lmemxor3_uu_leftover:
	ror	r4, #24
	strb	r4, [DST, #-1]!
	subs	N, #1
	beq	.Lmemxor3_done
	subs	ACNT, #8
	bne	.Lmemxor3_uu_leftover
	b	.Lmemxor3_bytes

.Lmemxor3_uud:
	C Both AP and BP unaligned, and in different ways
	rsb	BTNC, BCNT, #32

	ldr	r4, [AP]
	ldr	r6, [BP]

	tst	N, #4
	ittet	eq
	moveq	r5, r4
	moveq	r7, r6
	subne	N, #4
	beq	.Lmemxor3_uud_odd

.Lmemxor3_uud_loop:
	ldr	r5, [AP, #-4]!
	ldr	r7, [BP, #-4]!
	lsl	r4, ATNC
	eor	r4, r4, r6, lsl BTNC
	eor	r4, r4, r5, lsr ACNT
	eor	r4, r4, r7, lsr BCNT
	str	r4, [DST, #-4]!
.Lmemxor3_uud_odd:
	ldr	r4, [AP, #-4]!
	ldr	r6, [BP, #-4]!
	lsl	r5, ATNC
	eor	r5, r5, r7, lsl BTNC
	eor	r5, r5, r4, lsr ACNT
	eor	r5, r5, r6, lsr BCNT
	str	r5, [DST, #-4]!
	subs	N, #8
	bcs	.Lmemxor3_uud_loop
	adds	N, #8
	beq	.Lmemxor3_done

	C FIXME: More clever left-over handling? For now, just adjust pointers.
	add	AP, AP,	ACNT, lsr #3
	add	BP, BP, BCNT, lsr #3
	b	.Lmemxor3_bytes
EPILOGUE(nettle_memxor3)