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

[Pawel Wodkowski <pawelx.wodkowski-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

On 2015-03-18 17:51, 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-8EHiFRVJVgQ@public.gmane.org>
> ---
>   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[] = {
> +	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;
>   	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);

Great idea with lookup table!
Only one question here: why keeping this magic at all now?
If you sort odd_8byte_mask opposite direction you can do something like that

	data_len &= 0x07;
	switch (data_len & 0x07) {
	case 1:
	case 2:
	case 3:
	case 4:
		temp = odd_8byte_mask[data_len] & *p64;
		init_val = rte_hash_crc_4byte(temp, init_val);
	case 5:
	case 6:
	case 7:
		temp = odd_8byte_mask[data_len] & *p64;
		init_val = rte_hash_crc_8byte(temp, init_val);
		break;
	default:
		break;
	}
Or

	data_len &= 0x07;
	if (data_len > 0) {
		temp = odd_8byte_mask[data_len] & *p64;
		if (data_len <= 4)
			init_val = rte_hash_crc_4byte(temp, init_val);
		else
			init_val = rte_hash_crc_8byte(temp, init_val);
	}
	
Is there something obvious what I am not seeing here?

Pawel

> +	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;
>   	}
>


-- 
Pawel