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

[Gary Guo]

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)
-- 
2.20.1