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

[Pavan Nikhilesh Bhagavatula <pbhagavatula@marvell.com>]

> 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

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