Re: [PATCH] riscv: fix memmove and optimise memcpy when misalign

[Gary Guo]

On Tue, 16 Feb 2021 22:55:51 +0000
Gary Guo <gary@xxxxxxxxxxx> wrote:

> 04091d6 introduces an assembly version of memmove but
> it does take misalignment into account (it checks if
> length is a multiple of machine word size but pointers
> need also be aligned). As a result it will generate
> misaligned load/store for the majority of cases and causes
> significant performance regression on hardware that traps
> misaligned load/store and emulate them using firmware.
> 
> The current behaviour of memcpy is that it checks if both
> src and dest pointers are co-aligned (aka congruent
> modular SZ_REG). If aligned, it will copy data word-by-word
> after first aligning pointers to word boundary. If src
> and dst are not co-aligned, however, byte-wise copy will
> be performed.
> 
> This patch fixes the memmove and optimises memcpy for
> misaligned cases. It will first align destination pointer
> to word-boundary regardless whether src and dest are
> co-aligned or not. If they indeed are, then wordwise copy
> is performed. If they are not co-aligned, then it will
> load two adjacent words from src and use shifts to assemble
> a full machine word. Some additional assembly level
> micro-optimisation is also performed to ensure more
> instructions can be compressed (e.g. prefer a0 to t6).
> 
> In my testing this speeds up memcpy 4~5x when src and dest
> are not co-aligned (which is quite common in networking),
> and speeds up memmove 1000+x by avoiding trapping to firmware.
> 
> Signed-off-by: Gary Guo <gary@xxxxxxxxxxx>
> ---
>  arch/riscv/lib/memcpy.S  | 223
> ++++++++++++++++++++++++--------------- arch/riscv/lib/memmove.S |
> 176 ++++++++++++++++++++---------- 2 files changed, 257
> insertions(+), 142 deletions(-)
> 
> diff --git a/arch/riscv/lib/memcpy.S b/arch/riscv/lib/memcpy.S
> index 51ab716253fa..00672c19ad9b 100644
> --- a/arch/riscv/lib/memcpy.S
> +++ b/arch/riscv/lib/memcpy.S
> @@ -9,100 +9,151 @@
>  /* void *memcpy(void *, const void *, size_t) */
>  ENTRY(__memcpy)
>  WEAK(memcpy)
> -	move t6, a0  /* Preserve return value */
> +	/* Save for return value */
> +	mv	t6, a0
>  
> -	/* Defer to byte-oriented copy for small sizes */
> -	sltiu a3, a2, 128
> -	bnez a3, 4f
> -	/* Use word-oriented copy only if low-order bits match */
> -	andi a3, t6, SZREG-1
> -	andi a4, a1, SZREG-1
> -	bne a3, a4, 4f
> +	/*
> +	 * Register allocation for code below:
> +	 * a0 - start of uncopied dst
> +	 * a1 - start of uncopied src
> +	 * t0 - end of uncopied dst
> +	 */
> +	add	t0, a0, a2
>  
> -	beqz a3, 2f  /* Skip if already aligned */
>  	/*
> -	 * Round to nearest double word-aligned address
> -	 * greater than or equal to start address
> +	 * Use bytewise copy if too small.
> +	 *
> +	 * This threshold must be at least 2*SZREG to ensure at
> least one
> +	 * wordwise copy is performed. It is chosen to be 16 because
> it will
> +	 * save at least 7 iterations of bytewise copy, which pays
> off the
> +	 * fixed overhead.
>  	 */
> -	andi a3, a1, ~(SZREG-1)
> -	addi a3, a3, SZREG
> -	/* Handle initial misalignment */
> -	sub a4, a3, a1
> +	li	a3, 16
> +	bltu	a2, a3, .Lbyte_copy_tail
> +
> +	/*
> +	 * Bytewise copy first to align a0 to word boundary.
> +	 */
> +	addi	a2, a0, SZREG-1
> +	andi	a2, a2, ~(SZREG-1)
> +	beq	a0, a2, 2f
>  1:
> -	lb a5, 0(a1)
> -	addi a1, a1, 1
> -	sb a5, 0(t6)
> -	addi t6, t6, 1
> -	bltu a1, a3, 1b
> -	sub a2, a2, a4  /* Update count */
> +	lb	a5, 0(a1)
> +	addi	a1, a1, 1
> +	sb	a5, 0(a0)
> +	addi	a0, a0, 1
> +	bne	a0, a2, 1b
> +2:
> +
> +	/*
> +	 * Now a0 is word-aligned. If a1 is also word aligned, we
> could perform
> +	 * aligned word-wise copy. Otherwise we need to perform
> misaligned
> +	 * word-wise copy.
> +	 */
> +	andi	a3, a1, SZREG-1
> +	bnez	a3, .Lmisaligned_word_copy
>  
> +	/* Unrolled wordwise copy */
> +	addi	t0, t0, -(16*SZREG-1)
> +	bgeu	a0, t0, 2f
> +1:
> +	REG_L	a2,        0(a1)
> +	REG_L	a3,    SZREG(a1)
> +	REG_L	a4,  2*SZREG(a1)
> +	REG_L	a5,  3*SZREG(a1)
> +	REG_L	a6,  4*SZREG(a1)
> +	REG_L	a7,  5*SZREG(a1)
> +	REG_L	t1,  6*SZREG(a1)
> +	REG_L	t2,  7*SZREG(a1)
> +	REG_L	t3,  8*SZREG(a1)
> +	REG_L	t4,  9*SZREG(a1)
> +	REG_L	t5, 10*SZREG(a1)
> +	REG_S	a2,        0(a0)
> +	REG_S	a3,    SZREG(a0)
> +	REG_S	a4,  2*SZREG(a0)
> +	REG_S	a5,  3*SZREG(a0)
> +	REG_S	a6,  4*SZREG(a0)
> +	REG_S	a7,  5*SZREG(a0)
> +	REG_S	t1,  6*SZREG(a0)
> +	REG_S	t2,  7*SZREG(a0)
> +	REG_S	t3,  8*SZREG(a0)
> +	REG_S	t4,  9*SZREG(a0)
> +	REG_S	t5, 10*SZREG(a0)
> +	REG_L	a2, 11*SZREG(a1)
> +	REG_L	a3, 12*SZREG(a1)
> +	REG_L	a4, 13*SZREG(a1)
> +	REG_L	a5, 14*SZREG(a1)
> +	REG_L	a6, 15*SZREG(a1)
> +	addi	a1, a1, 16*SZREG
> +	REG_S	a2, 11*SZREG(a0)
> +	REG_S	a3, 12*SZREG(a0)
> +	REG_S	a4, 13*SZREG(a0)
> +	REG_S	a5, 14*SZREG(a0)
> +	REG_S	a6, 15*SZREG(a0)
> +	addi	a0, a0, 16*SZREG
> +	bltu	a0, t0, 1b
>  2:
> -	andi a4, a2, ~((16*SZREG)-1)
> -	beqz a4, 4f
> -	add a3, a1, a4
> -3:
> -	REG_L a4,       0(a1)
> -	REG_L a5,   SZREG(a1)
> -	REG_L a6, 2*SZREG(a1)
> -	REG_L a7, 3*SZREG(a1)
> -	REG_L t0, 4*SZREG(a1)
> -	REG_L t1, 5*SZREG(a1)
> -	REG_L t2, 6*SZREG(a1)
> -	REG_L t3, 7*SZREG(a1)
> -	REG_L t4, 8*SZREG(a1)
> -	REG_L t5, 9*SZREG(a1)
> -	REG_S a4,       0(t6)
> -	REG_S a5,   SZREG(t6)
> -	REG_S a6, 2*SZREG(t6)
> -	REG_S a7, 3*SZREG(t6)
> -	REG_S t0, 4*SZREG(t6)
> -	REG_S t1, 5*SZREG(t6)
> -	REG_S t2, 6*SZREG(t6)
> -	REG_S t3, 7*SZREG(t6)
> -	REG_S t4, 8*SZREG(t6)
> -	REG_S t5, 9*SZREG(t6)
> -	REG_L a4, 10*SZREG(a1)
> -	REG_L a5, 11*SZREG(a1)
> -	REG_L a6, 12*SZREG(a1)
> -	REG_L a7, 13*SZREG(a1)
> -	REG_L t0, 14*SZREG(a1)
> -	REG_L t1, 15*SZREG(a1)
> -	addi a1, a1, 16*SZREG
> -	REG_S a4, 10*SZREG(t6)
> -	REG_S a5, 11*SZREG(t6)
> -	REG_S a6, 12*SZREG(t6)
> -	REG_S a7, 13*SZREG(t6)
> -	REG_S t0, 14*SZREG(t6)
> -	REG_S t1, 15*SZREG(t6)
> -	addi t6, t6, 16*SZREG
> -	bltu a1, a3, 3b
> -	andi a2, a2, (16*SZREG)-1  /* Update count */
> -
> -4:
> -	/* Handle trailing misalignment */
> -	beqz a2, 6f
> -	add a3, a1, a2
> -
> -	/* Use word-oriented copy if co-aligned to word boundary */
> -	or a5, a1, t6
> -	or a5, a5, a3
> -	andi a5, a5, 3
> -	bnez a5, 5f
> -7:
> -	lw a4, 0(a1)
> -	addi a1, a1, 4
> -	sw a4, 0(t6)
> -	addi t6, t6, 4
> -	bltu a1, a3, 7b
> +	/* Post-loop increment by 16*SZREG-1 and pre-loop decrement
> by SZREG-1 */
> +	addi	t0, t0, 15*SZREG
>  
> -	ret
> +	/* Wordwise copy */
> +	bgeu	a0, t0, 2f
> +1:
> +	REG_L	a5, 0(a1)
> +	addi	a1, a1, SZREG
> +	REG_S	a5, 0(a0)
> +	addi	a0, a0, SZREG
> +	bltu	a0, t0, 1b
> +2:
> +	addi	t0, t0, SZREG-1
>  
> -5:
> -	lb a4, 0(a1)
> -	addi a1, a1, 1
> -	sb a4, 0(t6)
> -	addi t6, t6, 1
> -	bltu a1, a3, 5b
> -6:
> +.Lbyte_copy_tail:
> +	/*
> +	 * Bytewise copy anything left.
> +	 */
> +	beq	a0, t0, 2f
> +1:
> +	lb	a5, 0(a1)
> +	addi	a1, a1, 1
> +	sb	a5, 0(a0)
> +	addi	a0, a0, 1
> +	bne	a0, t0, 1b
> +2:
> +
> +	mv	a0, t6
>  	ret
> +
> +.Lmisaligned_word_copy:
> +	/*
> +	 * Misaligned word-wise copy.
> +	 * For misaligned copy we still perform word-wise copy, but
> we need to
> +	 * use the value fetched from the previous iteration and do
> some shifts.
> +	 * This is safe because we wouldn't access more words than
> necessary.
> +	 */
> +
> +	/* Calculate shifts */
> +	slli	t3, a3, 3
> +	sub	t4, x0, t3 /* negate is okay as shift will only
> look at LSBs */ +
> +	/* Load the initial value and align a1 */
> +	andi	a1, a1, ~(SZREG-1)
> +	REG_L	a5, 0(a1)
> +
> +	addi	t0, t0, -(SZREG-1)
> +	/* At least one iteration will be executed here, no check */
> +1:
> +	srl	a4, a5, t3
> +	REG_L	a5, SZREG(a1)
> +	addi	a1, a1, SZREG
> +	sll	a2, a5, t4
> +	or	a2, a2, a4
> +	REG_S	a2, 0(a0)
> +	addi	a0, a0, SZREG
> +	bltu	a0, t0, 1b
> +
> +	/* Update pointers to correct value */
> +	addi	t0, t0, SZREG-1
> +	add	a1, a1, a3
> +
> +	j	.Lbyte_copy_tail
>  END(__memcpy)
> diff --git a/arch/riscv/lib/memmove.S b/arch/riscv/lib/memmove.S
> index 07d1d2152ba5..fbe6701dbe4a 100644
> --- a/arch/riscv/lib/memmove.S
> +++ b/arch/riscv/lib/memmove.S
> @@ -5,60 +5,124 @@
>  
>  ENTRY(__memmove)
>  WEAK(memmove)
> -        move    t0, a0
> -        move    t1, a1
> -
> -        beq     a0, a1, exit_memcpy
> -        beqz    a2, exit_memcpy
> -        srli    t2, a2, 0x2
> -
> -        slt     t3, a0, a1
> -        beqz    t3, do_reverse
> -
> -        andi    a2, a2, 0x3
> -        li      t4, 1
> -        beqz    t2, byte_copy
> -
> -word_copy:
> -        lw      t3, 0(a1)
> -        addi    t2, t2, -1
> -        addi    a1, a1, 4
> -        sw      t3, 0(a0)
> -        addi    a0, a0, 4
> -        bnez    t2, word_copy
> -        beqz    a2, exit_memcpy
> -        j       byte_copy
> -
> -do_reverse:
> -        add     a0, a0, a2
> -        add     a1, a1, a2
> -        andi    a2, a2, 0x3
> -        li      t4, -1
> -        beqz    t2, reverse_byte_copy
> -
> -reverse_word_copy:
> -        addi    a1, a1, -4
> -        addi    t2, t2, -1
> -        lw      t3, 0(a1)
> -        addi    a0, a0, -4
> -        sw      t3, 0(a0)
> -        bnez    t2, reverse_word_copy
> -        beqz    a2, exit_memcpy
> -
> -reverse_byte_copy:
> -        addi    a0, a0, -1
> -        addi    a1, a1, -1
> -
> -byte_copy:
> -        lb      t3, 0(a1)
> -        addi    a2, a2, -1
> -        sb      t3, 0(a0)
> -        add     a1, a1, t4
> -        add     a0, a0, t4
> -        bnez    a2, byte_copy
> -
> -exit_memcpy:
> -        move a0, t0
> -        move a1, t1
> -        ret
> +	/*
> +	 * Here we determine if forward copy is possible. Forward
> copy is
> +	 * preferred to backward copy as it is more cache friendly.
> +	 *
> +	 * If a0 >= a1, t0 gives their distance, if t0 >= a2 then we
> can
> +	 *   copy forward.
> +	 * If a0 < a1, we can always copy forward. This will make t0
> negative,
> +	 *   so a *unsigned* comparison will always have t0 >= a2.
> +	 *
> +	 * For forward copy we just delegate the task to memcpy.
> +	 */
> +	sub	t0, a0, a1
> +	bltu	t0, a2, 1f
> +	tail	__memcpy
> +1:
> +
> +	/*
> +	 * Register allocation for code below:
> +	 * a0 - end of uncopied dst
> +	 * a1 - end of uncopied src
> +	 * t0 - start of uncopied dst
> +	 */
> +	mv	t0, a0
> +	add	a0, a0, a2
> +	add	a1, a1, a2
> +
> +	/*
> +	 * Use bytewise copy if too small.
> +	 *
> +	 * This threshold must be at least 2*SZREG to ensure at
> least one
> +	 * wordwise copy is performed. It is chosen to be 16 because
> it will
> +	 * save at least 7 iterations of bytewise copy, which pays
> off the
> +	 * fixed overhead.
> +	 */
> +	li	a3, 16
> +	bltu	a2, a3, .Lbyte_copy_tail
> +
> +	/*
> +	 * Bytewise copy first to align t0 to word boundary.
> +	 */
> +	andi	a2, a0, ~(SZREG-1)
> +	beq	a0, a2, 2f
> +1:
> +	addi	a1, a1, -1
> +	lb	a5, 0(a1)
> +	addi	a0, a0, -1
> +	sb	a5, 0(a0)
> +	bne	a0, a2, 1b
> +2:
> +
> +	/*
> +	 * Now a0 is word-aligned. If a1 is also word aligned, we
> could perform
> +	 * aligned word-wise copy. Otherwise we need to perform
> misaligned
> +	 * word-wise copy.
> +	 */
> +	andi	a3, a1, SZREG-1
> +	bnez	a3, .Lmisaligned_word_copy
> +
> +	/* Wordwise copy */
> +	addi	t0, t0, SZREG-1
> +	bleu	a0, t0, 2f
> +1:
> +	addi	a1, a1, -SZREG
> +	REG_L	a5, 0(a1)
> +	addi	a0, a0, -SZREG
> +	REG_S	a5, 0(a0)
> +	bgtu	a0, t0, 1b
> +2:
> +	addi	t0, t0, -(SZREG-1)
> +
> +.Lbyte_copy_tail:
> +	/*
> +	 * Bytewise copy anything left.
> +	 */
> +	beq	a0, t0, 2f
> +1:
> +	addi	a1, a1, -1
> +	lb	a5, 0(a1)
> +	addi	a0, a0, -1
> +	sb	a5, 0(a0)
> +	bne	a0, t0, 1b
> +2:
> +
> +	mv	a0, t0
> +	ret
> +
> +.Lmisaligned_word_copy:
> +	/*
> +	 * Misaligned word-wise copy.
> +	 * For misaligned copy we still perform word-wise copy, but
> we need to
> +	 * use the value fetched from the previous iteration and do
> some shifts.
> +	 * This is safe because we wouldn't access more words than
> necessary.
> +	 */
> +
> +	/* Calculate shifts */
> +	slli	t3, a3, 3
> +	sub	t4, x0, t3 /* negate is okay as shift will only
> look at LSBs */ +
> +	/* Load the initial value and align a1 */
> +	andi	a1, a1, ~(SZREG-1)
> +	REG_L	a5, 0(a1)
> +
> +	addi	t0, t0, SZREG-1
> +	/* At least one iteration will be executed here, no check */
> +1:
> +	sll	a4, a5, t4
> +	addi	a1, a1, -SZREG
> +	REG_L	a5, 0(a1)
> +	srl	a2, a5, t3
> +	or	a2, a2, a4
> +	addi	a0, a0, -SZREG
> +	REG_S	a2, 0(a0)
> +	bgtu	a0, t0, 1b
> +
> +	/* Update pointers to correct value */
> +	addi	t0, t0, -(SZREG-1)
> +	add	a1, a1, a3
> +
> +	j	.Lbyte_copy_tail
> +
>  END(__memmove)

ping. It's been 3 month since submission and I really would like to see
this applied or at least have some feedbacks.

Link to the original patch in archive:
https://lore.kernel.org/linux-riscv/20210216225555.4976-1-gary@xxxxxxxxxxx/

- Gary