[PING*2][PATCH] aarch64: Improve strcmp unaligned performance

[Siddhesh Poyarekar <siddhesh@sourceware.org>]

Ping!

On Monday 04 December 2017 09:37 PM, Siddhesh Poyarekar wrote:
> Ping!
> 
> On Friday 01 December 2017 10:56 AM, Siddhesh Poyarekar wrote:
>> Replace the simple byte-wise compare in the misaligned case with a
>> dword compare with page boundary checks in place.  For simplicity I've
>> chosen a 4K page boundary so that we don't have to query the actual
>> page size on the system.
>>
>> This results in up to 3x improvement in performance in the unaligned
>> case on falkor and about 2.5x improvement on mustang as measured using
>> bench-strcmp.
>>
>> 	* sysdeps/aarch64/strcmp.S (misaligned8): Compare dword at a
>> 	time whenever possible.
>> ---
>>  sysdeps/aarch64/strcmp.S | 31 +++++++++++++++++++++++++++++--
>>  1 file changed, 29 insertions(+), 2 deletions(-)
>>
>> diff --git a/sysdeps/aarch64/strcmp.S b/sysdeps/aarch64/strcmp.S
>> index e99d662..c260e1d 100644
>> --- a/sysdeps/aarch64/strcmp.S
>> +++ b/sysdeps/aarch64/strcmp.S
>> @@ -72,6 +72,7 @@ L(start_realigned):
>>  	cbz	syndrome, L(loop_aligned)
>>  	/* End of performance-critical section  -- one 64B cache line.  */
>>  
>> +L(end):
>>  #ifndef	__AARCH64EB__
>>  	rev	syndrome, syndrome
>>  	rev	data1, data1
>> @@ -145,12 +146,38 @@ L(mutual_align):
>>  	b	L(start_realigned)
>>  
>>  L(misaligned8):
>> -	/* We can do better than this.  */
>> +	/* Align SRC1 to 8 bytes and then compare 8 bytes at a time, always
>> +	   checking to make sure that we don't access beyond page boundary in
>> +	   SRC2.  */
>> +	tst	src1, #7
>> +	b.eq	L(loop_misaligned)
>> +L(do_misaligned):
>>  	ldrb	data1w, [src1], #1
>>  	ldrb	data2w, [src2], #1
>>  	cmp	data1w, #1
>>  	ccmp	data1w, data2w, #0, cs	/* NZCV = 0b0000.  */
>> -	b.eq	L(misaligned8)
>> +	b.ne	L(done)
>> +	tst	src1, #7
>> +	b.ne	L(misaligned8)
>> +
>> +L(loop_misaligned):
>> +	/* Test if we are within the last dword of the end of a 4K page.  If
>> +	   yes then jump back to the misaligned loop to copy a byte at a time.  */
>> +	and	tmp1, src2, #0xff8
>> +	eor	tmp1, tmp1, #0xff8
>> +	cbz	tmp1, L(do_misaligned)
>> +	ldr	data1, [src1], #8
>> +	ldr	data2, [src2], #8
>> +
>> +	sub	tmp1, data1, zeroones
>> +	orr	tmp2, data1, #REP8_7f
>> +	eor	diff, data1, data2	/* Non-zero if differences found.  */
>> +	bic	has_nul, tmp1, tmp2	/* Non-zero if NUL terminator.  */
>> +	orr	syndrome, diff, has_nul
>> +	cbz	syndrome, L(loop_misaligned)
>> +	b	L(end)
>> +
>> +L(done):
>>  	sub	result, data1, data2
>>  	RET
>>  END(strcmp)
>>
>