Add -Wimplicit-fallthrough to CFLAGS and CXXFLAGS

Use it at level 4, a bit more restrictive than the default level, and
tweak our commanding comments to FALLTHROUGH.

(However, leave zic.c alone, since it's external code; to avoid the
warnings that would appear there, change CFLAGS for that file in the
Makefile.)

Author: Julien Rouhaud <rjuju123@gmail.com>
Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/20200412081825.qyo5vwwco3fv4gdo@nol
Discussion: https://postgr.es/m/flat/E1fDenm-0000C8-IJ@gemulon.postgresql.org

3 files changed, 14 insertions, 14 deletions

diff --git a/src/backend/executor/nodeHash.c b/src/backend/executor/nodeHash.c
index 5da13ada726..4516c6346bd 100644
--- a/src/backend/executor/nodeHash.c
+++ b/src/backend/executor/nodeHash.c
@@ -256,7 +256,7 @@ MultiExecParallelHash(HashState *node)
 			 * way, wait for everyone to arrive here so we can proceed.
 			 */
 			BarrierArriveAndWait(build_barrier, WAIT_EVENT_HASH_BUILD_ALLOCATING);
-			/* Fall through. */
+			/* FALLTHROUGH */
 
 		case PHJ_BUILD_HASHING_INNER:
 
@@ -1181,13 +1181,13 @@ ExecParallelHashIncreaseNumBatches(HashJoinTable hashtable)
 				/* All other participants just flush their tuples to disk. */
 				ExecParallelHashCloseBatchAccessors(hashtable);
 			}
-			/* Fall through. */
+			/* FALLTHROUGH */
 
 		case PHJ_GROW_BATCHES_ALLOCATING:
 			/* Wait for the above to be finished. */
 			BarrierArriveAndWait(&pstate->grow_batches_barrier,
 								 WAIT_EVENT_HASH_GROW_BATCHES_ALLOCATING);
-			/* Fall through. */
+			/* FALLTHROUGH */
 
 		case PHJ_GROW_BATCHES_REPARTITIONING:
 			/* Make sure that we have the current dimensions and buckets. */
@@ -1200,7 +1200,7 @@ ExecParallelHashIncreaseNumBatches(HashJoinTable hashtable)
 			/* Wait for the above to be finished. */
 			BarrierArriveAndWait(&pstate->grow_batches_barrier,
 								 WAIT_EVENT_HASH_GROW_BATCHES_REPARTITIONING);
-			/* Fall through. */
+			/* FALLTHROUGH */
 
 		case PHJ_GROW_BATCHES_DECIDING:
 
@@ -1255,7 +1255,7 @@ ExecParallelHashIncreaseNumBatches(HashJoinTable hashtable)
 				dsa_free(hashtable->area, pstate->old_batches);
 				pstate->old_batches = InvalidDsaPointer;
 			}
-			/* Fall through. */
+			/* FALLTHROUGH */
 
 		case PHJ_GROW_BATCHES_FINISHING:
 			/* Wait for the above to complete. */
@@ -1533,13 +1533,13 @@ ExecParallelHashIncreaseNumBuckets(HashJoinTable hashtable)
 				/* Clear the flag. */
 				pstate->growth = PHJ_GROWTH_OK;
 			}
-			/* Fall through. */
+			/* FALLTHROUGH */
 
 		case PHJ_GROW_BUCKETS_ALLOCATING:
 			/* Wait for the above to complete. */
 			BarrierArriveAndWait(&pstate->grow_buckets_barrier,
 								 WAIT_EVENT_HASH_GROW_BUCKETS_ALLOCATING);
-			/* Fall through. */
+			/* FALLTHROUGH */
 
 		case PHJ_GROW_BUCKETS_REINSERTING:
 			/* Reinsert all tuples into the hash table. */
diff --git a/src/backend/executor/nodeHashjoin.c b/src/backend/executor/nodeHashjoin.c
index cc8edacdd01..6159a6957f4 100644
--- a/src/backend/executor/nodeHashjoin.c
+++ b/src/backend/executor/nodeHashjoin.c
@@ -340,7 +340,7 @@ ExecHashJoinImpl(PlanState *pstate, bool parallel)
 				else
 					node->hj_JoinState = HJ_NEED_NEW_OUTER;
 
-				/* FALL THRU */
+				/* FALLTHROUGH */
 
 			case HJ_NEED_NEW_OUTER:
 
@@ -413,7 +413,7 @@ ExecHashJoinImpl(PlanState *pstate, bool parallel)
 				/* OK, let's scan the bucket for matches */
 				node->hj_JoinState = HJ_SCAN_BUCKET;
 
-				/* FALL THRU */
+				/* FALLTHROUGH */
 
 			case HJ_SCAN_BUCKET:
 
@@ -1137,13 +1137,13 @@ ExecParallelHashJoinNewBatch(HashJoinState *hjstate)
 					if (BarrierArriveAndWait(batch_barrier,
 											 WAIT_EVENT_HASH_BATCH_ELECTING))
 						ExecParallelHashTableAlloc(hashtable, batchno);
-					/* Fall through. */
+					/* FALLTHROUGH */
 
 				case PHJ_BATCH_ALLOCATING:
 					/* Wait for allocation to complete. */
 					BarrierArriveAndWait(batch_barrier,
 										 WAIT_EVENT_HASH_BATCH_ALLOCATING);
-					/* Fall through. */
+					/* FALLTHROUGH */
 
 				case PHJ_BATCH_LOADING:
 					/* Start (or join in) loading tuples. */
@@ -1163,7 +1163,7 @@ ExecParallelHashJoinNewBatch(HashJoinState *hjstate)
 					sts_end_parallel_scan(inner_tuples);
 					BarrierArriveAndWait(batch_barrier,
 										 WAIT_EVENT_HASH_BATCH_LOADING);
-					/* Fall through. */
+					/* FALLTHROUGH */
 
 				case PHJ_BATCH_PROBING:
 
diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c
index d85cf7d93e8..371b15c14a7 100644
--- a/src/backend/executor/nodeLimit.c
+++ b/src/backend/executor/nodeLimit.c
@@ -69,7 +69,7 @@ ExecLimit(PlanState *pstate)
 			 */
 			recompute_limits(node);
 
-			/* FALL THRU */
+			/* FALLTHROUGH */
 
 		case LIMIT_RESCAN:
 
@@ -216,7 +216,7 @@ ExecLimit(PlanState *pstate)
 			}
 
 			Assert(node->lstate == LIMIT_WINDOWEND_TIES);
-			/* FALL THRU */
+			/* FALLTHROUGH */
 
 		case LIMIT_WINDOWEND_TIES:
 			if (ScanDirectionIsForward(direction))