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

[Yerden Zhumabekov]

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 at sts.kz>
---
 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);
+	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