Re: [EXTERNAL] [PATCH v7 2/4] hash: optimize compare signature for NEON

[Yoan Picchi <yoan.picchi@foss.arm.com>]

On 3/20/24 07:37, Pavan Nikhilesh Bhagavatula wrote:
>> Upon a successful comparison, NEON sets all the bits in the lane to 1
>> We can skip shifting by simply masking with specific masks.
>>
>> Signed-off-by: Yoan Picchi <yoan.picchi@arm.com>
>> Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
>> Reviewed-by: Nathan Brown <nathan.brown@arm.com>
>> ---
>>   lib/hash/arch/arm/compare_signatures.h | 24 +++++++++++-------------
>>   1 file changed, 11 insertions(+), 13 deletions(-)
>>
>> diff --git a/lib/hash/arch/arm/compare_signatures.h
>> b/lib/hash/arch/arm/compare_signatures.h
>> index 1af6ba8190..b5a457f936 100644
>> --- a/lib/hash/arch/arm/compare_signatures.h
>> +++ b/lib/hash/arch/arm/compare_signatures.h
>> @@ -30,23 +30,21 @@ compare_signatures_dense(uint16_t
>> *hitmask_buffer,
>>   	switch (sig_cmp_fn) {
>>   #if RTE_HASH_BUCKET_ENTRIES <= 8
>>   	case RTE_HASH_COMPARE_NEON: {
>> -		uint16x8_t vmat, vsig, x;
>> -		int16x8_t shift = {0, 1, 2, 3, 4, 5, 6, 7};
>> -		uint16_t low, high;
>> +		uint16x8_t vmat, hit1, hit2;
>> +		const uint16x8_t mask = {0x1, 0x2, 0x4, 0x8, 0x10, 0x20,
>> 0x40, 0x80};
>> +		const uint16x8_t vsig = vld1q_dup_u16((uint16_t const
>> *)&sig);
>>
>> -		vsig = vld1q_dup_u16((uint16_t const *)&sig);
>>   		/* Compare all signatures in the primary bucket */
>> -		vmat = vceqq_u16(vsig,
>> -			vld1q_u16((uint16_t const *)prim_bucket_sigs));
>> -		x = vshlq_u16(vandq_u16(vmat, vdupq_n_u16(0x0001)),
>> shift);
>> -		low = (uint16_t)(vaddvq_u16(x));
>> +		vmat = vceqq_u16(vsig, vld1q_u16(prim_bucket_sigs));
>> +		hit1 = vandq_u16(vmat, mask);
>> +
>>   		/* Compare all signatures in the secondary bucket */
>> -		vmat = vceqq_u16(vsig,
>> -			vld1q_u16((uint16_t const *)sec_bucket_sigs));
>> -		x = vshlq_u16(vandq_u16(vmat, vdupq_n_u16(0x0001)),
>> shift);
>> -		high = (uint16_t)(vaddvq_u16(x));
>> -		*hitmask_buffer = low | high << RTE_HASH_BUCKET_ENTRIES;
>> +		vmat = vceqq_u16(vsig, vld1q_u16(sec_bucket_sigs));
>> +		hit2 = vandq_u16(vmat, mask);
>>
>> +		hit2 = vshlq_n_u16(hit2, RTE_HASH_BUCKET_ENTRIES);
>> +		hit2 = vorrq_u16(hit1, hit2);
>> +		*hitmask_buffer = vaddvq_u16(hit2);
> 
> Since vaddv is expensive could you convert it to vshrn?
> 
> https://community.arm.com/arm-community-blogs/b/infrastructure-solutions-blog/posts/porting-x86-vector-bitmask-optimizations-to-arm-neon
> 
> https://github.com/DPDK/dpdk/blob/main/examples/l3fwd/l3fwd_neon.h#L226

Thank you for those links, it was a good read.
Unfortunatly I don't think it is a good use case here. A decent part of 
the speedup I get is by using a dense hitmask: ie every bit count with 
no padding. Using the vshrn would have 4 bits of padding, and stripping 
them would be more expensive than using a regular reduce.

> 
>>   		}
>>   		break;
>>   #endif
>> --
>> 2.25.1
>