Indicate that fallthrough is intentional in faiss (facebookresearch#2897)

Summary: Pull Request resolved: facebookresearch#2897

Reviewed By: algoriddle

Differential Revision: D46385243

fbshipit-source-id: f08b16c9db91edca53cdbf0932a990c8c1f9d0db

Author: r-barnes

faiss/utils/distances_simd.cpp

@@ -2176,10 +2176,13 @@ void pq2_8cents_table(
     switch (nout) {
         case 4:
             ip3.storeu(out + 3 * ldo);
+            [[fallthrough]];
         case 3:
             ip2.storeu(out + 2 * ldo);
+            [[fallthrough]];
         case 2:
             ip1.storeu(out + 1 * ldo);
+            [[fallthrough]];
         case 1:
             ip0.storeu(out);
     }

faiss/utils/hamming_distance/avx2-inl.h

@@ -280,21 +280,27 @@ struct HammingComputerDefault {
                     case 7:
                         accu += popcount64(a64[i] ^ b64[i]);
                         i++;
+                        [[fallthrough]];
                     case 6:
                         accu += popcount64(a64[i] ^ b64[i]);
                         i++;
+                        [[fallthrough]];
                     case 5:
                         accu += popcount64(a64[i] ^ b64[i]);
                         i++;
+                        [[fallthrough]];
                     case 4:
                         accu += popcount64(a64[i] ^ b64[i]);
                         i++;
+                        [[fallthrough]];
                     case 3:
                         accu += popcount64(a64[i] ^ b64[i]);
                         i++;
+                        [[fallthrough]];
                     case 2:
                         accu += popcount64(a64[i] ^ b64[i]);
                         i++;
+                        [[fallthrough]];
                     case 1:
                         accu += popcount64(a64[i] ^ b64[i]);
                         i++;
@@ -306,18 +312,25 @@ struct HammingComputerDefault {
             switch (remainder8) {
                 case 7:
                     accu += hamdis_tab_ham_bytes[a[6] ^ b[6]];
+                    [[fallthrough]];
                 case 6:
                     accu += hamdis_tab_ham_bytes[a[5] ^ b[5]];
+                    [[fallthrough]];
                 case 5:
                     accu += hamdis_tab_ham_bytes[a[4] ^ b[4]];
+                    [[fallthrough]];
                 case 4:
                     accu += hamdis_tab_ham_bytes[a[3] ^ b[3]];
+                    [[fallthrough]];
                 case 3:
                     accu += hamdis_tab_ham_bytes[a[2] ^ b[2]];
+                    [[fallthrough]];
                 case 2:
                     accu += hamdis_tab_ham_bytes[a[1] ^ b[1]];
+                    [[fallthrough]];
                 case 1:
                     accu += hamdis_tab_ham_bytes[a[0] ^ b[0]];
+                    [[fallthrough]];
                 default:
                     break;
             }