[PING2][PATCH] aarch64: Optimized memcmp for medium to large sizes

[Siddhesh Poyarekar <siddhesh at sourceware dot org>]

Ping!

On Thursday 01 March 2018 04:48 PM, Siddhesh Poyarekar wrote:
> Ping: to summarize, these are the peak performance results for 2K+
> comparisons in the memcmp microbenchmark:
> 
> falkor: almost 2x (up to 48% time reduction)
> xgene: over 1.5x (up to 31% time reduction)
> cortex-a73: over 1.5x (up to 33% time reduction)
> cortex-a53: over 1.5x (up to 30% time reduction)
> 
> Siddhesh
> 
> On Friday 02 February 2018 10:20 AM, Siddhesh Poyarekar wrote:
>> This improved memcmp provides a fast path for compares up to 16 bytes
>> and then compares 16 bytes at a time, thus optimizing loads from both
>> sources.  The glibc memcmp microbenchmark retains performance (with an
>> error of ~1ns) for smaller compare sizes and reduces up to 31% of
>> execution time for compares up to 4K on the APM Mustang.  On Qualcomm
>> Falkor this improves to almost 48%, i.e. it is almost 2x improvement
>> for sizes of 2K and above.
>>
>> 	* sysdeps/aarch64/memcmp.S: Widen comparison to 16 bytes at a
>> 	time.
>> ---
>>  sysdeps/aarch64/memcmp.S | 66 ++++++++++++++++++++++++++++++++++++------------
>>  1 file changed, 50 insertions(+), 16 deletions(-)
>>
>> diff --git a/sysdeps/aarch64/memcmp.S b/sysdeps/aarch64/memcmp.S
>> index ecd12061b2..0804e75d99 100644
>> --- a/sysdeps/aarch64/memcmp.S
>> +++ b/sysdeps/aarch64/memcmp.S
>> @@ -34,9 +34,12 @@
>>  /* Internal variables.  */
>>  #define data1		x3
>>  #define data1w		w3
>> -#define data2		x4
>> -#define data2w		w4
>> -#define tmp1		x5
>> +#define data1h		x4
>> +#define data2		x5
>> +#define data2w		w5
>> +#define data2h		x6
>> +#define tmp1		x7
>> +#define tmp2		x8
>>  
>>  ENTRY_ALIGN (memcmp, 6)
>>  	DELOUSE (0)
>> @@ -46,39 +49,70 @@ ENTRY_ALIGN (memcmp, 6)
>>  	subs	limit, limit, 8
>>  	b.lo	L(less8)
>>  
>> -	/* Limit >= 8, so check first 8 bytes using unaligned loads.  */
>>  	ldr	data1, [src1], 8
>>  	ldr	data2, [src2], 8
>> -	and	tmp1, src1, 7
>> -	add	limit, limit, tmp1
>> +	cmp	data1, data2
>> +	b.ne	L(return)
>> +
>> +	subs	limit, limit, 8
>> +	b.gt	L(more16)
>> +
>> +	ldr	data1, [src1, limit]
>> +	ldr	data2, [src2, limit]
>> +	b	L(return)
>> +
>> +L(more16):
>> +	ldr	data1, [src1], 8
>> +	ldr	data2, [src2], 8
>>  	cmp	data1, data2
>>  	bne	L(return)
>>  
>> +	/* Jump directly to copying the last 16 bytes for 32 byte (or less)
>> +	   strings.  */
>> +	subs	limit, limit, 16
>> +	b.ls	L(last_bytes)
>> +
>> +	/* We overlap loads between 0-32 bytes at either side of SRC1 when we
>> +	   try to align, so limit it only to strings larger than 128 bytes.  */
>> +	cmp	limit, 96
>> +	b.ls	L(loop8)
>> +
>>  	/* Align src1 and adjust src2 with bytes not yet done.  */
>> +	and	tmp1, src1, 15
>> +	add	limit, limit, tmp1
>>  	sub	src1, src1, tmp1
>>  	sub	src2, src2, tmp1
>>  
>> -	subs	limit, limit, 8
>> -	b.ls	L(last_bytes)
>> -
>>  	/* Loop performing 8 bytes per iteration using aligned src1.
>>  	   Limit is pre-decremented by 8 and must be larger than zero.
>>  	   Exit if <= 8 bytes left to do or if the data is not equal.  */
>>  	.p2align 4
>>  L(loop8):
>> -	ldr	data1, [src1], 8
>> -	ldr	data2, [src2], 8
>> -	subs	limit, limit, 8
>> -	ccmp	data1, data2, 0, hi  /* NZCV = 0b0000.  */
>> +	ldp	data1, data1h, [src1], 16
>> +	ldp	data2, data2h, [src2], 16
>> +	subs	limit, limit, 16
>> +	ccmp	data1, data2, 0, hi
>> +	ccmp	data1h, data2h, 0, eq
>>  	b.eq	L(loop8)
>>  
>> +	cmp	data1, data2
>> +	bne	L(return)
>> +	mov	data1, data1h
>> +	mov	data2, data2h
>>  	cmp	data1, data2
>>  	bne	L(return)
>>  
>> -	/* Compare last 1-8 bytes using unaligned access.  */
>> +	/* Compare last 1-16 bytes using unaligned access.  */
>>  L(last_bytes):
>> -	ldr	data1, [src1, limit]
>> -	ldr	data2, [src2, limit]
>> +	add	src1, src1, limit
>> +	add	src2, src2, limit
>> +	ldp	data1, data1h, [src1]
>> +	ldp	data2, data2h, [src2]
>> +	cmp     data1, data2
>> +	bne	L(return)
>> +	mov	data1, data1h
>> +	mov	data2, data2h
>> +	cmp	data1, data2
>>  
>>  	/* Compare data bytes and set return value to 0, -1 or 1.  */
>>  L(return):
>>
>