From: Bruce Richardson <bruce.richardson@intel.com>
To: Yerden Zhumabekov <e_zhumabekov@sts.kz>
Cc: dev@dpdk.org
Subject: Re: [dpdk-dev] [PATCH] hash: fix breaking strict-aliasing rules
Date: Thu, 19 Mar 2015 16:25:47 +0000 [thread overview]
Message-ID: <20150319162547.GA5996@bricha3-MOBL3> (raw)
In-Reply-To: <b441d020bd7830b1c66b6f4d2aa8d776a7f0bffd.1426697208.git.e_zhumabekov@sts.kz>
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
>
next prev parent reply other threads:[~2015-03-19 16:25 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-03-18 16:51 Yerden Zhumabekov
2015-03-19 16:25 ` Bruce Richardson [this message]
2015-03-19 16:31 ` Bruce Richardson
2015-03-20 3:29 ` Yerden Zhumabekov
2015-03-20 12:41 ` Pawel Wodkowski
2015-03-20 12:47 ` Pawel Wodkowski
2015-03-21 6:57 ` Жумабеков Ерден Мирзагулович
2015-03-24 13:31 ` [dpdk-dev] [PATCH v2] " Yerden Zhumabekov
2015-03-26 17:47 ` De Lara Guarch, Pablo
2015-03-27 9:26 ` Thomas Monjalon
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