[PATCH v3] hash: fix SSE comparison

[Jieqiang Wang]

__mm_cmpeq_epi16 returns 0xFFFF if the corresponding 16-bit elements are
equal. In original SSE2 implementation for function compare_signatures,
it utilizes _mm_movemask_epi8 to create mask from the MSB of each 8-bit
element, while we should only care about the MSB of lower 8-bit in each
16-bit element.
For example, if the comparison result is all equal, SSE2 path returns
0xFFFF while NEON and default scalar path return 0x5555.
Although this bug is not causing any negative effects since the caller
function solely examines the trailing zeros of each match mask, we
recommend this fix to ensure consistency with NEON and default scalar
code behaviors.

Fixes: c7d93df552c2 ("hash: use partial-key hashing")
Cc: stable at dpdk.org

v2:
1. Utilize scalar mask instead of vector mask to save extra loads (Bruce)

v3:
1. Fix coding style warnings

Signed-off-by: Feifei Wang <feifei.wang2 at arm.com>
Signed-off-by: Jieqiang Wang <jieqiang.wang at arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang at arm.com>
---
 lib/hash/rte_cuckoo_hash.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/lib/hash/rte_cuckoo_hash.c b/lib/hash/rte_cuckoo_hash.c
index d92a903bb3..19b23f2a97 100644
--- a/lib/hash/rte_cuckoo_hash.c
+++ b/lib/hash/rte_cuckoo_hash.c
@@ -1868,11 +1868,15 @@ compare_signatures(uint32_t *prim_hash_matches, uint32_t *sec_hash_matches,
 				_mm_load_si128(
 					(__m128i const *)prim_bkt->sig_current),
 				_mm_set1_epi16(sig)));
+		/* Extract the even-index bits only */
+		*prim_hash_matches &= 0x5555;
 		/* Compare all signatures in the bucket */
 		*sec_hash_matches = _mm_movemask_epi8(_mm_cmpeq_epi16(
 				_mm_load_si128(
 					(__m128i const *)sec_bkt->sig_current),
 				_mm_set1_epi16(sig)));
+		/* Extract the even-index bits only */
+		*sec_hash_matches &= 0x5555;
 		break;
 #elif defined(__ARM_NEON)
 	case RTE_HASH_COMPARE_NEON: {
-- 
2.25.1