Re: [dpdk-dev] [PATCH v2 3/4] hash: add fallback to software CRC32 implementation

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

Hi Yerden,

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Yerden Zhumabekov
> Sent: Sunday, November 16, 2014 5:59 PM
> To: dev@dpdk.org
> Subject: [dpdk-dev] [PATCH v2 3/4] hash: add fallback to software CRC32 implementation
> 
> Initially, SSE4.2 support is detected via CPUID instruction.
> 
> Added rte_hash_crc_set_alg() function to detect and set CRC32
> implementation if necessary. SSE4.2 is allowed by default. If it's
> not available, fall back to sw implementation.
> 
> Signed-off-by: Yerden Zhumabekov <e_zhumabekov@sts.kz>
> ---
>  lib/librte_hash/rte_hash_crc.h |   60 ++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 58 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/librte_hash/rte_hash_crc.h b/lib/librte_hash/rte_hash_crc.h
> index 74e2d92..178b162 100644
> --- a/lib/librte_hash/rte_hash_crc.h
> +++ b/lib/librte_hash/rte_hash_crc.h
> @@ -45,7 +45,11 @@ extern "C" {
>  #endif
> 
>  #include <stdint.h>
> +#ifdef RTE_MACHINE_CPUFLAG_SSE4_2
>  #include <nmmintrin.h>
> +#endif
> +#include <rte_cpuflags.h>
> +#include <rte_branch_prediction.h>
> 
>  /* Lookup table for software implementation of CRC32C */
>  static const uint32_t crc32c_table[256] = {
> @@ -152,8 +156,42 @@ crc32c_2words(uint64_t data, uint32_t init_val)
>  	return init_val;
>  }
> 
> +enum crc32_alg_t {
> +	CRC32_SW = 0,
> +	CRC32_SSE42,
> +	CRC32_AUTODETECT
> +};
> +
> +/* Default algorithm is left for autodetection,
> + * it is detected on first run of hash function
> + */
> +static enum crc32_alg_t crc32_alg = CRC32_AUTODETECT;
> +
> +/**
> + * Allow or disallow use of SSE4.2 instrinsics for CRC32 hash
> + * hash calculation.
> + *
> + * @param flag
> + *   unsigned integer flag
> + *   - (CRC32_SW) Don't use SSE4.2 intrinsics
> + *   - (CRC32_SSE42) Use SSE4.2 intrinsics if available, set by default
> + */
> +static inline void
> +rte_hash_crc_set_alg(enum crc32_alg_t alg)
> +{
> +	int sse42_supp = rte_cpu_get_flag_enabled(RTE_CPUFLAG_SSE4_2);
> +	enum crc32_alg_t alg_supp = sse42_supp ? CRC32_SSE42 : CRC32_SW;
> +
> +	if (alg == CRC32_SSE42)
> +		crc32_alg = alg_supp;
> +	else
> +		crc32_alg = CRC32_SW;
> +}
> +

Wonder can we define that function with __attribute__((constructor))?
Then, I suppose we can remove CRC32_AUTODETECT, and remove:
if (unlikely(crc32_alg == CRC32_AUTODETECT))
   rte_hash_crc_set_alg(CRC32_SSE42);   
from rte_hash_crc_*byte().

Konstantin

>  /**
>   * Use single crc32 instruction to perform a hash on a 4 byte value.
> + * Fall back to software crc32 implementation in case SSE4.2 is
> + * not supported
>   *
>   * @param data
>   *   Data to perform hash on.
> @@ -165,11 +203,21 @@ crc32c_2words(uint64_t data, uint32_t init_val)
>  static inline uint32_t
>  rte_hash_crc_4byte(uint32_t data, uint32_t init_val)
>  {
> -	return _mm_crc32_u32(init_val, data);
> +	if (unlikely(crc32_alg == CRC32_AUTODETECT))
> +		rte_hash_crc_set_alg(CRC32_SSE42);
> +
> +#ifdef RTE_MACHINE_CPUFLAG_SSE4_2
> +	if (likely(crc32_alg == CRC32_SSE42))
> +		return _mm_crc32_u32(init_val, data);
> +#endif
> +
> +	return crc32c_1word(data, init_val);
>  }
> 
>  /**
>   * Use single crc32 instruction to perform a hash on a 8 byte value.
> + * Fall back to software crc32 implementation in case SSE4.2 is
> + * not supported
>   *
>   * @param data
>   *   Data to perform hash on.
> @@ -181,7 +229,15 @@ rte_hash_crc_4byte(uint32_t data, uint32_t init_val)
>  static inline uint32_t
>  rte_hash_crc_8byte(uint64_t data, uint32_t init_val)
>  {
> -	return _mm_crc32_u64(init_val, data);
> +	if (unlikely(crc32_alg == CRC32_AUTODETECT))
> +		rte_hash_crc_set_alg(CRC32_SSE42);
> +
> +#ifdef RTE_MACHINE_CPUFLAG_SSE4_2
> +	if (likely(crc32_alg == CRC32_SSE42))
> +		return _mm_crc32_u64(init_val, data);
> +#endif
> +
> +	return crc32c_2words(data, init_val);
>  }
> 
>  /**
> --
> 1.7.9.5