Re: [dpdk-dev] [PATCH] hash: fix breaking strict-aliasing rules

[Bruce Richardson <bruce.richardson@intel.com>]

On Wed, Mar 18, 2015 at 10:51:12PM +0600, Yerden Zhumabekov wrote:
> Fix rte_hash_crc() function. Casting uint64_t pointer to uin32_t
> may trigger a compiler warning about breaking strict-aliasing rules.
> To avoid that, introduce a lookup table which is used to mask out
> a remainder of data.
> 
> See issue #1, http://dpdk.org/ml/archives/dev/2015-March/015174.html
> 
> Signed-off-by: Yerden Zhumabekov <e_zhumabekov@sts.kz>

Looks ok to me. Couple of minor suggestions below.

/Bruce

> ---
>  lib/librte_hash/rte_hash_crc.h |   31 +++++++++++++++----------------
>  1 file changed, 15 insertions(+), 16 deletions(-)
> 
> diff --git a/lib/librte_hash/rte_hash_crc.h b/lib/librte_hash/rte_hash_crc.h
> index 3dcd362..e81920f 100644
> --- a/lib/librte_hash/rte_hash_crc.h
> +++ b/lib/librte_hash/rte_hash_crc.h
> @@ -323,6 +323,16 @@ static const uint32_t crc32c_tables[8][256] = {{
>   0xE54C35A1, 0xAC704886, 0x7734CFEF, 0x3E08B2C8, 0xC451B7CC, 0x8D6DCAEB, 0x56294D82, 0x1F1530A5
>  }};
>  
> +static const uint64_t odd_8byte_mask[] = {

Where does the name of this variable come from, it seems unclear to me?

> +	0x00FFFFFFFFFFFFFF,
> +	0x0000FFFFFFFFFFFF,
> +	0x000000FFFFFFFFFF,
> +	0x00000000FFFFFFFF,
> +	0x0000000000FFFFFF,
> +	0x000000000000FFFF,
> +	0x00000000000000FF,
> +};
> +
>  #define CRC32_UPD(crc, n) \
>  	(crc32c_tables[(n)][(crc) & 0xFF] ^ \
>  	 crc32c_tables[(n)-1][((crc) >> 8) & 0xFF])
> @@ -535,38 +545,27 @@ static inline uint32_t
>  rte_hash_crc(const void *data, uint32_t data_len, uint32_t init_val)
>  {
>  	unsigned i;
> -	uint64_t temp = 0;
> +	uint64_t temp;

It is worth keeping variable "temp" at all, it looks to me like it could be done
away with without seriously affecting readability.

>  	const uint64_t *p64 = (const uint64_t *)data;
>  
>  	for (i = 0; i < data_len / 8; i++) {
>  		init_val = rte_hash_crc_8byte(*p64++, init_val);
>  	}
>  
> -	switch (7 - (data_len & 0x07)) {
> +	i = 7 - (data_len & 0x07);

i is not a terribly meaningful variable name, perhaps a slightly longer, more
meaningful name might improve readability.

> +	switch (i) {
>  	case 0:
> -		temp |= (uint64_t) *((const uint8_t *)p64 + 6) << 48;
> -		/* Fallthrough */
>  	case 1:
> -		temp |= (uint64_t) *((const uint8_t *)p64 + 5) << 40;
> -		/* Fallthrough */
>  	case 2:
> -		temp |= (uint64_t) *((const uint8_t *)p64 + 4) << 32;
> -		temp |= *((const uint32_t *)p64);
> +		temp = odd_8byte_mask[i] & *p64;
>  		init_val = rte_hash_crc_8byte(temp, init_val);
>  		break;
>  	case 3:
> -		init_val = rte_hash_crc_4byte(*(const uint32_t *)p64, init_val);
> -		break;
>  	case 4:
> -		temp |= *((const uint8_t *)p64 + 2) << 16;
> -		/* Fallthrough */
>  	case 5:
> -		temp |= *((const uint8_t *)p64 + 1) << 8;
> -		/* Fallthrough */
>  	case 6:
> -		temp |= *((const uint8_t *)p64);
> +		temp = odd_8byte_mask[i] & *p64;
>  		init_val = rte_hash_crc_4byte(temp, init_val);
> -		/* Fallthrough */
>  	default:
>  		break;
>  	}
> -- 
> 1.7.9.5
>