app/testpmd: add register keyword

Currently SSE SIMD variables are declared as stack variables. Allowing
the use of keyword register for shuffle mask and address variables,
improves the mac-swap Mpps by 1 for single queue.

Test Result:
 * Platform: AMD EPYC 9554 @3.1GHz, no boost
 * Test scenarios: TEST-PMD 64B IO vs MAC-SWAP
 * NIC: broadcom P2100: loopback 2*100Gbps

<mode : Mpps Ingress: Mpps Egress>
------------------------------------------------
 - IO: 47.23 : 46.0
 - MAC-SWAP original: 45.75 : 43.8
 - MAC-SWAP register mod 45.73 : 44.83

Signed-off-by: Vipin Varghese <[email protected]>
Acked-by: Ferruh Yigit <[email protected]>

Author: vipinamd

app/test-pmd/macswap_sse.h

@@ -16,13 +16,13 @@ do_macswap(struct rte_mbuf *pkts[], uint16_t nb,
 	uint64_t ol_flags;
 	int i;
 	int r;
-	__m128i addr0, addr1, addr2, addr3;
+	register __m128i addr0, addr1, addr2, addr3;
 	/**
 	 * shuffle mask be used to shuffle the 16 bytes.
 	 * byte 0-5 wills be swapped with byte 6-11.
 	 * byte 12-15 will keep unchanged.
 	 */
-	__m128i shfl_msk = _mm_set_epi8(15, 14, 13, 12,
+	register const __m128i shfl_msk = _mm_set_epi8(15, 14, 13, 12,
 					5, 4, 3, 2,
 					1, 0, 11, 10,
 					9, 8, 7, 6);