diff --git a/configure b/configure
index 83abe872aa6..4dfe4fcfe1c 100755
--- a/configure
+++ b/configure
@@ -5552,6 +5552,97 @@ if test x"$pgac_cv_prog_CXX_cxxflags__Wmissing_format_attribute" = x"yes"; then
 fi
 
 
+
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether ${CC} supports -Wimplicit-fallthrough=4, for CFLAGS" >&5
+$as_echo_n "checking whether ${CC} supports -Wimplicit-fallthrough=4, for CFLAGS... " >&6; }
+if ${pgac_cv_prog_CC_cflags__Wimplicit_fallthrough_4+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  pgac_save_CFLAGS=$CFLAGS
+pgac_save_CC=$CC
+CC=${CC}
+CFLAGS="${CFLAGS} -Wimplicit-fallthrough=4"
+ac_save_c_werror_flag=$ac_c_werror_flag
+ac_c_werror_flag=yes
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  pgac_cv_prog_CC_cflags__Wimplicit_fallthrough_4=yes
+else
+  pgac_cv_prog_CC_cflags__Wimplicit_fallthrough_4=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+ac_c_werror_flag=$ac_save_c_werror_flag
+CFLAGS="$pgac_save_CFLAGS"
+CC="$pgac_save_CC"
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $pgac_cv_prog_CC_cflags__Wimplicit_fallthrough_4" >&5
+$as_echo "$pgac_cv_prog_CC_cflags__Wimplicit_fallthrough_4" >&6; }
+if test x"$pgac_cv_prog_CC_cflags__Wimplicit_fallthrough_4" = x"yes"; then
+  CFLAGS="${CFLAGS} -Wimplicit-fallthrough=4"
+fi
+
+
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether ${CXX} supports -Wimplicit-fallthrough=4, for CXXFLAGS" >&5
+$as_echo_n "checking whether ${CXX} supports -Wimplicit-fallthrough=4, for CXXFLAGS... " >&6; }
+if ${pgac_cv_prog_CXX_cxxflags__Wimplicit_fallthrough_4+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  pgac_save_CXXFLAGS=$CXXFLAGS
+pgac_save_CXX=$CXX
+CXX=${CXX}
+CXXFLAGS="${CXXFLAGS} -Wimplicit-fallthrough=4"
+ac_save_cxx_werror_flag=$ac_cxx_werror_flag
+ac_cxx_werror_flag=yes
+ac_ext=cpp
+ac_cpp='$CXXCPP $CPPFLAGS'
+ac_compile='$CXX -c $CXXFLAGS $CPPFLAGS conftest.$ac_ext >&5'
+ac_link='$CXX -o conftest$ac_exeext $CXXFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
+ac_compiler_gnu=$ac_cv_cxx_compiler_gnu
+
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_cxx_try_compile "$LINENO"; then :
+  pgac_cv_prog_CXX_cxxflags__Wimplicit_fallthrough_4=yes
+else
+  pgac_cv_prog_CXX_cxxflags__Wimplicit_fallthrough_4=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+ac_ext=c
+ac_cpp='$CPP $CPPFLAGS'
+ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'
+ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
+ac_compiler_gnu=$ac_cv_c_compiler_gnu
+
+ac_cxx_werror_flag=$ac_save_cxx_werror_flag
+CXXFLAGS="$pgac_save_CXXFLAGS"
+CXX="$pgac_save_CXX"
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $pgac_cv_prog_CXX_cxxflags__Wimplicit_fallthrough_4" >&5
+$as_echo "$pgac_cv_prog_CXX_cxxflags__Wimplicit_fallthrough_4" >&6; }
+if test x"$pgac_cv_prog_CXX_cxxflags__Wimplicit_fallthrough_4" = x"yes"; then
+  CXXFLAGS="${CXXFLAGS} -Wimplicit-fallthrough=4"
+fi
+
+
   # This was included in -Wall/-Wformat in older GCC versions
 
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether ${CC} supports -Wformat-security, for CFLAGS" >&5
diff --git a/configure.in b/configure.in
index ecdf1723967..94fba67261c 100644
--- a/configure.in
+++ b/configure.in
@@ -496,6 +496,8 @@ if test "$GCC" = yes -a "$ICC" = no; then
   PGAC_PROG_CXX_CFLAGS_OPT([-Wendif-labels])
   PGAC_PROG_CC_CFLAGS_OPT([-Wmissing-format-attribute])
   PGAC_PROG_CXX_CFLAGS_OPT([-Wmissing-format-attribute])
+  PGAC_PROG_CC_CFLAGS_OPT([-Wimplicit-fallthrough=4])
+  PGAC_PROG_CXX_CFLAGS_OPT([-Wimplicit-fallthrough=4])
   # This was included in -Wall/-Wformat in older GCC versions
   PGAC_PROG_CC_CFLAGS_OPT([-Wformat-security])
   PGAC_PROG_CXX_CFLAGS_OPT([-Wformat-security])
diff --git a/src/backend/access/heap/heapam_handler.c b/src/backend/access/heap/heapam_handler.c
index 56b35622f1a..f819706878f 100644
--- a/src/backend/access/heap/heapam_handler.c
+++ b/src/backend/access/heap/heapam_handler.c
@@ -814,7 +814,7 @@ heapam_relation_copy_for_cluster(Relation OldHeap, Relation NewHeap,
 				break;
 			case HEAPTUPLE_RECENTLY_DEAD:
 				*tups_recently_dead += 1;
-				/* fall through */
+				/* FALLTHROUGH */
 			case HEAPTUPLE_LIVE:
 				/* Live or recently dead, must copy it */
 				isdead = false;
diff --git a/src/backend/catalog/dependency.c b/src/backend/catalog/dependency.c
index 5565e6fc197..a6f848511fb 100644
--- a/src/backend/catalog/dependency.c
+++ b/src/backend/catalog/dependency.c
@@ -615,7 +615,7 @@ findDependentObjects(const ObjectAddress *object,
 					break;
 
 				/* Otherwise, treat this like an internal dependency */
-				/* FALL THRU */
+				/* FALLTHROUGH */
 
 			case DEPENDENCY_INTERNAL:
 
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index a518b552b3d..30142552e2e 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -12467,7 +12467,7 @@ ATExecChangeOwner(Oid relationOid, Oid newOwnerId, bool recursing, LOCKMODE lock
 		case RELKIND_TOASTVALUE:
 			if (recursing)
 				break;
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		default:
 			ereport(ERROR,
 					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
diff --git a/src/backend/commands/trigger.c b/src/backend/commands/trigger.c
index ed551ab73aa..8222cea35b8 100644
--- a/src/backend/commands/trigger.c
+++ b/src/backend/commands/trigger.c
@@ -3877,7 +3877,7 @@ AfterTriggerExecute(EState *estate,
 											 trig_tuple_slot2))
 					elog(ERROR, "failed to fetch tuple2 for AFTER trigger");
 			}
-			/* fall through */
+			/* FALLTHROUGH */
 		case AFTER_TRIGGER_FDW_REUSE:
 
 			/*
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))
diff --git a/src/backend/libpq/auth.c b/src/backend/libpq/auth.c
index 02b6c3f127c..bf9d0cca563 100644
--- a/src/backend/libpq/auth.c
+++ b/src/backend/libpq/auth.c
@@ -2123,7 +2123,7 @@ pam_passwd_conv_proc(int num_msg, const struct pam_message **msg,
 				ereport(LOG,
 						(errmsg("error from underlying PAM layer: %s",
 								msg[i]->msg)));
-				/* FALL THROUGH */
+				/* FALLTHROUGH */
 			case PAM_TEXT_INFO:
 				/* we don't bother to log TEXT_INFO messages */
 				if ((reply[i].resp = strdup("")) == NULL)
diff --git a/src/backend/optimizer/util/clauses.c b/src/backend/optimizer/util/clauses.c
index 0c6fe0115a1..55b1b788720 100644
--- a/src/backend/optimizer/util/clauses.c
+++ b/src/backend/optimizer/util/clauses.c
@@ -1582,7 +1582,7 @@ find_nonnullable_rels_walker(Node *node, bool top_level)
 				 * the intersection of the sets of nonnullable rels, just as
 				 * for OR.  Fall through to share code.
 				 */
-				/* FALL THRU */
+				/* FALLTHROUGH */
 			case OR_EXPR:
 
 				/*
@@ -1807,7 +1807,7 @@ find_nonnullable_vars_walker(Node *node, bool top_level)
 				 * the intersection of the sets of nonnullable vars, just as
 				 * for OR.  Fall through to share code.
 				 */
-				/* FALL THRU */
+				/* FALLTHROUGH */
 			case OR_EXPR:
 
 				/*
diff --git a/src/backend/parser/parse_utilcmd.c b/src/backend/parser/parse_utilcmd.c
index 75c122fe348..cbd5780a69d 100644
--- a/src/backend/parser/parse_utilcmd.c
+++ b/src/backend/parser/parse_utilcmd.c
@@ -737,7 +737,7 @@ transformColumnDefinition(CreateStmtContext *cxt, ColumnDef *column)
 							 errmsg("primary key constraints are not supported on foreign tables"),
 							 parser_errposition(cxt->pstate,
 												constraint->location)));
-				/* FALL THRU */
+				/* FALLTHROUGH */
 
 			case CONSTR_UNIQUE:
 				if (cxt->isforeign)
diff --git a/src/backend/partitioning/partprune.c b/src/backend/partitioning/partprune.c
index eac52e6ec85..473423de8d7 100644
--- a/src/backend/partitioning/partprune.c
+++ b/src/backend/partitioning/partprune.c
@@ -2571,7 +2571,7 @@ get_matching_list_bounds(PartitionPruneContext *context,
 
 		case BTGreaterEqualStrategyNumber:
 			inclusive = true;
-			/* fall through */
+			/* FALLTHROUGH */
 		case BTGreaterStrategyNumber:
 			off = partition_list_bsearch(partsupfunc,
 										 partcollation,
@@ -2606,7 +2606,7 @@ get_matching_list_bounds(PartitionPruneContext *context,
 
 		case BTLessEqualStrategyNumber:
 			inclusive = true;
-			/* fall through */
+			/* FALLTHROUGH */
 		case BTLessStrategyNumber:
 			off = partition_list_bsearch(partsupfunc,
 										 partcollation,
@@ -2853,7 +2853,7 @@ get_matching_range_bounds(PartitionPruneContext *context,
 
 		case BTGreaterEqualStrategyNumber:
 			inclusive = true;
-			/* fall through */
+			/* FALLTHROUGH */
 		case BTGreaterStrategyNumber:
 
 			/*
@@ -2934,7 +2934,7 @@ get_matching_range_bounds(PartitionPruneContext *context,
 
 		case BTLessEqualStrategyNumber:
 			inclusive = true;
-			/* fall through */
+			/* FALLTHROUGH */
 		case BTLessStrategyNumber:
 
 			/*
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index e19d5dc1a64..32f0ce936bb 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -5902,19 +5902,19 @@ bgworker_should_start_now(BgWorkerStartTime start_time)
 		case PM_RUN:
 			if (start_time == BgWorkerStart_RecoveryFinished)
 				return true;
-			/* fall through */
+			/* FALLTHROUGH */
 
 		case PM_HOT_STANDBY:
 			if (start_time == BgWorkerStart_ConsistentState)
 				return true;
-			/* fall through */
+			/* FALLTHROUGH */
 
 		case PM_RECOVERY:
 		case PM_STARTUP:
 		case PM_INIT:
 			if (start_time == BgWorkerStart_PostmasterStart)
 				return true;
-			/* fall through */
+			/* FALLTHROUGH */
 
 	}
 
diff --git a/src/backend/regex/regc_pg_locale.c b/src/backend/regex/regc_pg_locale.c
index 3cc2d4d3627..d4fe01f2d1f 100644
--- a/src/backend/regex/regc_pg_locale.c
+++ b/src/backend/regex/regc_pg_locale.c
@@ -303,7 +303,7 @@ pg_wc_isdigit(pg_wchar c)
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswdigit((wint_t) c);
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case PG_REGEX_LOCALE_1BYTE:
 			return (c <= (pg_wchar) UCHAR_MAX &&
 					isdigit((unsigned char) c));
@@ -312,7 +312,7 @@ pg_wc_isdigit(pg_wchar c)
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswdigit_l((wint_t) c, pg_regex_locale->info.lt);
 #endif
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case PG_REGEX_LOCALE_1BYTE_L:
 #ifdef HAVE_LOCALE_T
 			return (c <= (pg_wchar) UCHAR_MAX &&
@@ -339,7 +339,7 @@ pg_wc_isalpha(pg_wchar c)
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswalpha((wint_t) c);
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case PG_REGEX_LOCALE_1BYTE:
 			return (c <= (pg_wchar) UCHAR_MAX &&
 					isalpha((unsigned char) c));
@@ -348,7 +348,7 @@ pg_wc_isalpha(pg_wchar c)
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswalpha_l((wint_t) c, pg_regex_locale->info.lt);
 #endif
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case PG_REGEX_LOCALE_1BYTE_L:
 #ifdef HAVE_LOCALE_T
 			return (c <= (pg_wchar) UCHAR_MAX &&
@@ -375,7 +375,7 @@ pg_wc_isalnum(pg_wchar c)
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswalnum((wint_t) c);
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case PG_REGEX_LOCALE_1BYTE:
 			return (c <= (pg_wchar) UCHAR_MAX &&
 					isalnum((unsigned char) c));
@@ -384,7 +384,7 @@ pg_wc_isalnum(pg_wchar c)
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswalnum_l((wint_t) c, pg_regex_locale->info.lt);
 #endif
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case PG_REGEX_LOCALE_1BYTE_L:
 #ifdef HAVE_LOCALE_T
 			return (c <= (pg_wchar) UCHAR_MAX &&
@@ -411,7 +411,7 @@ pg_wc_isupper(pg_wchar c)
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswupper((wint_t) c);
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case PG_REGEX_LOCALE_1BYTE:
 			return (c <= (pg_wchar) UCHAR_MAX &&
 					isupper((unsigned char) c));
@@ -420,7 +420,7 @@ pg_wc_isupper(pg_wchar c)
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswupper_l((wint_t) c, pg_regex_locale->info.lt);
 #endif
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case PG_REGEX_LOCALE_1BYTE_L:
 #ifdef HAVE_LOCALE_T
 			return (c <= (pg_wchar) UCHAR_MAX &&
@@ -447,7 +447,7 @@ pg_wc_islower(pg_wchar c)
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswlower((wint_t) c);
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case PG_REGEX_LOCALE_1BYTE:
 			return (c <= (pg_wchar) UCHAR_MAX &&
 					islower((unsigned char) c));
@@ -456,7 +456,7 @@ pg_wc_islower(pg_wchar c)
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswlower_l((wint_t) c, pg_regex_locale->info.lt);
 #endif
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case PG_REGEX_LOCALE_1BYTE_L:
 #ifdef HAVE_LOCALE_T
 			return (c <= (pg_wchar) UCHAR_MAX &&
@@ -483,7 +483,7 @@ pg_wc_isgraph(pg_wchar c)
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswgraph((wint_t) c);
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case PG_REGEX_LOCALE_1BYTE:
 			return (c <= (pg_wchar) UCHAR_MAX &&
 					isgraph((unsigned char) c));
@@ -492,7 +492,7 @@ pg_wc_isgraph(pg_wchar c)
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswgraph_l((wint_t) c, pg_regex_locale->info.lt);
 #endif
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case PG_REGEX_LOCALE_1BYTE_L:
 #ifdef HAVE_LOCALE_T
 			return (c <= (pg_wchar) UCHAR_MAX &&
@@ -519,7 +519,7 @@ pg_wc_isprint(pg_wchar c)
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswprint((wint_t) c);
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case PG_REGEX_LOCALE_1BYTE:
 			return (c <= (pg_wchar) UCHAR_MAX &&
 					isprint((unsigned char) c));
@@ -528,7 +528,7 @@ pg_wc_isprint(pg_wchar c)
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswprint_l((wint_t) c, pg_regex_locale->info.lt);
 #endif
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case PG_REGEX_LOCALE_1BYTE_L:
 #ifdef HAVE_LOCALE_T
 			return (c <= (pg_wchar) UCHAR_MAX &&
@@ -555,7 +555,7 @@ pg_wc_ispunct(pg_wchar c)
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswpunct((wint_t) c);
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case PG_REGEX_LOCALE_1BYTE:
 			return (c <= (pg_wchar) UCHAR_MAX &&
 					ispunct((unsigned char) c));
@@ -564,7 +564,7 @@ pg_wc_ispunct(pg_wchar c)
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswpunct_l((wint_t) c, pg_regex_locale->info.lt);
 #endif
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case PG_REGEX_LOCALE_1BYTE_L:
 #ifdef HAVE_LOCALE_T
 			return (c <= (pg_wchar) UCHAR_MAX &&
@@ -591,7 +591,7 @@ pg_wc_isspace(pg_wchar c)
 		case PG_REGEX_LOCALE_WIDE:
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswspace((wint_t) c);
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case PG_REGEX_LOCALE_1BYTE:
 			return (c <= (pg_wchar) UCHAR_MAX &&
 					isspace((unsigned char) c));
@@ -600,7 +600,7 @@ pg_wc_isspace(pg_wchar c)
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return iswspace_l((wint_t) c, pg_regex_locale->info.lt);
 #endif
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case PG_REGEX_LOCALE_1BYTE_L:
 #ifdef HAVE_LOCALE_T
 			return (c <= (pg_wchar) UCHAR_MAX &&
@@ -631,7 +631,7 @@ pg_wc_toupper(pg_wchar c)
 				return pg_ascii_toupper((unsigned char) c);
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return towupper((wint_t) c);
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case PG_REGEX_LOCALE_1BYTE:
 			/* force C behavior for ASCII characters, per comments above */
 			if (c <= (pg_wchar) 127)
@@ -644,7 +644,7 @@ pg_wc_toupper(pg_wchar c)
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return towupper_l((wint_t) c, pg_regex_locale->info.lt);
 #endif
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case PG_REGEX_LOCALE_1BYTE_L:
 #ifdef HAVE_LOCALE_T
 			if (c <= (pg_wchar) UCHAR_MAX)
@@ -675,7 +675,7 @@ pg_wc_tolower(pg_wchar c)
 				return pg_ascii_tolower((unsigned char) c);
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return towlower((wint_t) c);
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case PG_REGEX_LOCALE_1BYTE:
 			/* force C behavior for ASCII characters, per comments above */
 			if (c <= (pg_wchar) 127)
@@ -688,7 +688,7 @@ pg_wc_tolower(pg_wchar c)
 			if (sizeof(wchar_t) >= 4 || c <= (pg_wchar) 0xFFFF)
 				return towlower_l((wint_t) c, pg_regex_locale->info.lt);
 #endif
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case PG_REGEX_LOCALE_1BYTE_L:
 #ifdef HAVE_LOCALE_T
 			if (c <= (pg_wchar) UCHAR_MAX)
diff --git a/src/backend/replication/logical/reorderbuffer.c b/src/backend/replication/logical/reorderbuffer.c
index 4594cf95097..02f0199ba1b 100644
--- a/src/backend/replication/logical/reorderbuffer.c
+++ b/src/backend/replication/logical/reorderbuffer.c
@@ -1589,7 +1589,7 @@ ReorderBufferCommit(ReorderBuffer *rb, TransactionId xid,
 					change = specinsert;
 					change->action = REORDER_BUFFER_CHANGE_INSERT;
 
-					/* intentionally fall through */
+					/* FALLTHROUGH */
 				case REORDER_BUFFER_CHANGE_INSERT:
 				case REORDER_BUFFER_CHANGE_UPDATE:
 				case REORDER_BUFFER_CHANGE_DELETE:
diff --git a/src/backend/replication/walreceiver.c b/src/backend/replication/walreceiver.c
index d69fb90132d..faca7327134 100644
--- a/src/backend/replication/walreceiver.c
+++ b/src/backend/replication/walreceiver.c
@@ -215,7 +215,7 @@ WalReceiverMain(void)
 		case WALRCV_STOPPING:
 			/* If we've already been requested to stop, don't start up. */
 			walrcv->walRcvState = WALRCV_STOPPED;
-			/* fall through */
+			/* FALLTHROUGH */
 
 		case WALRCV_STOPPED:
 			SpinLockRelease(&walrcv->mutex);
diff --git a/src/backend/replication/walreceiverfuncs.c b/src/backend/replication/walreceiverfuncs.c
index 4afad83539c..bf68852bf0c 100644
--- a/src/backend/replication/walreceiverfuncs.c
+++ b/src/backend/replication/walreceiverfuncs.c
@@ -183,7 +183,7 @@ ShutdownWalRcv(void)
 		case WALRCV_WAITING:
 		case WALRCV_RESTARTING:
 			walrcv->walRcvState = WALRCV_STOPPING;
-			/* fall through */
+			/* FALLTHROUGH */
 		case WALRCV_STOPPING:
 			walrcvpid = walrcv->pid;
 			break;
diff --git a/src/backend/tcop/utility.c b/src/backend/tcop/utility.c
index b1f7f6e2d01..eee544f8eb5 100644
--- a/src/backend/tcop/utility.c
+++ b/src/backend/tcop/utility.c
@@ -1883,7 +1883,7 @@ ExecDropStmt(DropStmt *stmt, bool isTopLevel)
 			if (stmt->concurrent)
 				PreventInTransactionBlock(isTopLevel,
 										  "DROP INDEX CONCURRENTLY");
-			/* fall through */
+			/* FALLTHROUGH */
 
 		case OBJECT_TABLE:
 		case OBJECT_SEQUENCE:
diff --git a/src/backend/utils/adt/formatting.c b/src/backend/utils/adt/formatting.c
index aab5802edb4..657b8f03766 100644
--- a/src/backend/utils/adt/formatting.c
+++ b/src/backend/utils/adt/formatting.c
@@ -3425,7 +3425,7 @@ DCH_from_char(FormatNode *node, const char *in, TmFromChar *out,
 			case DCH_FF5:
 			case DCH_FF6:
 				out->ff = n->key->id - DCH_FF1 + 1;
-				/* fall through */
+				/* FALLTHROUGH */
 			case DCH_US:		/* microsecond */
 				len = from_char_parse_int_len(&out->us, &s,
 											  n->key->id == DCH_US ? 6 :
diff --git a/src/backend/utils/adt/jsonb_util.c b/src/backend/utils/adt/jsonb_util.c
index 04b70c805b4..52ec80af045 100644
--- a/src/backend/utils/adt/jsonb_util.c
+++ b/src/backend/utils/adt/jsonb_util.c
@@ -637,7 +637,7 @@ pushJsonbValueScalar(JsonbParseState **pstate, JsonbIteratorToken seq,
 			break;
 		case WJB_END_OBJECT:
 			uniqueifyJsonbObject(&(*pstate)->contVal);
-			/* fall through! */
+			/* FALLTHROUGH */
 		case WJB_END_ARRAY:
 			/* Steps here common to WJB_END_OBJECT case */
 			Assert(!scalarVal);
diff --git a/src/backend/utils/adt/timestamp.c b/src/backend/utils/adt/timestamp.c
index 4caffb58040..d28d3c1628b 100644
--- a/src/backend/utils/adt/timestamp.c
+++ b/src/backend/utils/adt/timestamp.c
@@ -3866,14 +3866,14 @@ timestamp_trunc(PG_FUNCTION_ARGS)
 					tm->tm_year = ((tm->tm_year + 999) / 1000) * 1000 - 999;
 				else
 					tm->tm_year = -((999 - (tm->tm_year - 1)) / 1000) * 1000 + 1;
-				/* FALL THRU */
+				/* FALLTHROUGH */
 			case DTK_CENTURY:
 				/* see comments in timestamptz_trunc */
 				if (tm->tm_year > 0)
 					tm->tm_year = ((tm->tm_year + 99) / 100) * 100 - 99;
 				else
 					tm->tm_year = -((99 - (tm->tm_year - 1)) / 100) * 100 + 1;
-				/* FALL THRU */
+				/* FALLTHROUGH */
 			case DTK_DECADE:
 				/* see comments in timestamptz_trunc */
 				if (val != DTK_MILLENNIUM && val != DTK_CENTURY)
@@ -3883,25 +3883,25 @@ timestamp_trunc(PG_FUNCTION_ARGS)
 					else
 						tm->tm_year = -((8 - (tm->tm_year - 1)) / 10) * 10;
 				}
-				/* FALL THRU */
+				/* FALLTHROUGH */
 			case DTK_YEAR:
 				tm->tm_mon = 1;
-				/* FALL THRU */
+				/* FALLTHROUGH */
 			case DTK_QUARTER:
 				tm->tm_mon = (3 * ((tm->tm_mon - 1) / 3)) + 1;
-				/* FALL THRU */
+				/* FALLTHROUGH */
 			case DTK_MONTH:
 				tm->tm_mday = 1;
-				/* FALL THRU */
+				/* FALLTHROUGH */
 			case DTK_DAY:
 				tm->tm_hour = 0;
-				/* FALL THRU */
+				/* FALLTHROUGH */
 			case DTK_HOUR:
 				tm->tm_min = 0;
-				/* FALL THRU */
+				/* FALLTHROUGH */
 			case DTK_MINUTE:
 				tm->tm_sec = 0;
-				/* FALL THRU */
+				/* FALLTHROUGH */
 			case DTK_SECOND:
 				fsec = 0;
 				break;
@@ -4007,14 +4007,14 @@ timestamptz_trunc_internal(text *units, TimestampTz timestamp, pg_tz *tzp)
 					tm->tm_year = ((tm->tm_year + 999) / 1000) * 1000 - 999;
 				else
 					tm->tm_year = -((999 - (tm->tm_year - 1)) / 1000) * 1000 + 1;
-				/* FALL THRU */
+				/* FALLTHROUGH */
 			case DTK_CENTURY:
 				/* truncating to the century? as above: -100, 1, 101... */
 				if (tm->tm_year > 0)
 					tm->tm_year = ((tm->tm_year + 99) / 100) * 100 - 99;
 				else
 					tm->tm_year = -((99 - (tm->tm_year - 1)) / 100) * 100 + 1;
-				/* FALL THRU */
+				/* FALLTHROUGH */
 			case DTK_DECADE:
 
 				/*
@@ -4028,26 +4028,26 @@ timestamptz_trunc_internal(text *units, TimestampTz timestamp, pg_tz *tzp)
 					else
 						tm->tm_year = -((8 - (tm->tm_year - 1)) / 10) * 10;
 				}
-				/* FALL THRU */
+				/* FALLTHROUGH */
 			case DTK_YEAR:
 				tm->tm_mon = 1;
-				/* FALL THRU */
+				/* FALLTHROUGH */
 			case DTK_QUARTER:
 				tm->tm_mon = (3 * ((tm->tm_mon - 1) / 3)) + 1;
-				/* FALL THRU */
+				/* FALLTHROUGH */
 			case DTK_MONTH:
 				tm->tm_mday = 1;
-				/* FALL THRU */
+				/* FALLTHROUGH */
 			case DTK_DAY:
 				tm->tm_hour = 0;
 				redotz = true;	/* for all cases >= DAY */
-				/* FALL THRU */
+				/* FALLTHROUGH */
 			case DTK_HOUR:
 				tm->tm_min = 0;
-				/* FALL THRU */
+				/* FALLTHROUGH */
 			case DTK_MINUTE:
 				tm->tm_sec = 0;
-				/* FALL THRU */
+				/* FALLTHROUGH */
 			case DTK_SECOND:
 				fsec = 0;
 				break;
@@ -4195,33 +4195,33 @@ interval_trunc(PG_FUNCTION_ARGS)
 				case DTK_MILLENNIUM:
 					/* caution: C division may have negative remainder */
 					tm->tm_year = (tm->tm_year / 1000) * 1000;
-					/* FALL THRU */
+					/* FALLTHROUGH */
 				case DTK_CENTURY:
 					/* caution: C division may have negative remainder */
 					tm->tm_year = (tm->tm_year / 100) * 100;
-					/* FALL THRU */
+					/* FALLTHROUGH */
 				case DTK_DECADE:
 					/* caution: C division may have negative remainder */
 					tm->tm_year = (tm->tm_year / 10) * 10;
-					/* FALL THRU */
+					/* FALLTHROUGH */
 				case DTK_YEAR:
 					tm->tm_mon = 0;
-					/* FALL THRU */
+					/* FALLTHROUGH */
 				case DTK_QUARTER:
 					tm->tm_mon = 3 * (tm->tm_mon / 3);
-					/* FALL THRU */
+					/* FALLTHROUGH */
 				case DTK_MONTH:
 					tm->tm_mday = 0;
-					/* FALL THRU */
+					/* FALLTHROUGH */
 				case DTK_DAY:
 					tm->tm_hour = 0;
-					/* FALL THRU */
+					/* FALLTHROUGH */
 				case DTK_HOUR:
 					tm->tm_min = 0;
-					/* FALL THRU */
+					/* FALLTHROUGH */
 				case DTK_MINUTE:
 					tm->tm_sec = 0;
-					/* FALL THRU */
+					/* FALLTHROUGH */
 				case DTK_SECOND:
 					fsec = 0;
 					break;
diff --git a/src/backend/utils/adt/tsginidx.c b/src/backend/utils/adt/tsginidx.c
index 2d656168fca..a6d7b9805ad 100644
--- a/src/backend/utils/adt/tsginidx.c
+++ b/src/backend/utils/adt/tsginidx.c
@@ -264,7 +264,7 @@ TS_execute_ternary(GinChkVal *gcv, QueryItem *curitem, bool in_phrase)
 			/* Pass down in_phrase == true in case there's a NOT below */
 			in_phrase = true;
 
-			/* FALL THRU */
+			/* FALLTHROUGH */
 
 		case OP_AND:
 			val1 = TS_execute_ternary(gcv, curitem + curitem->qoperator.left,
diff --git a/src/backend/utils/hash/dynahash.c b/src/backend/utils/hash/dynahash.c
index 5b4b9e487f5..254c49ef9f2 100644
--- a/src/backend/utils/hash/dynahash.c
+++ b/src/backend/utils/hash/dynahash.c
@@ -1042,7 +1042,7 @@ hash_search_with_hash_value(HTAB *hashp,
 		case HASH_ENTER_NULL:
 			/* ENTER_NULL does not work with palloc-based allocator */
 			Assert(hashp->alloc != DynaHashAlloc);
-			/* FALL THRU */
+			/* FALLTHROUGH */
 
 		case HASH_ENTER:
 			/* Return existing element if found, else create one */
diff --git a/src/backend/utils/mb/mbutils.c b/src/backend/utils/mb/mbutils.c
index a8e13cacfde..4a162316057 100644
--- a/src/backend/utils/mb/mbutils.c
+++ b/src/backend/utils/mb/mbutils.c
@@ -1263,7 +1263,7 @@ pg_utf8_increment(unsigned char *charptr, int length)
 				charptr[3]++;
 				break;
 			}
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case 3:
 			a = charptr[2];
 			if (a < 0xBF)
@@ -1271,7 +1271,7 @@ pg_utf8_increment(unsigned char *charptr, int length)
 				charptr[2]++;
 				break;
 			}
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case 2:
 			a = charptr[1];
 			switch (*charptr)
@@ -1291,7 +1291,7 @@ pg_utf8_increment(unsigned char *charptr, int length)
 				charptr[1]++;
 				break;
 			}
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case 1:
 			a = *charptr;
 			if (a == 0x7F || a == 0xDF || a == 0xEF || a == 0xF4)
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 5bdc02fce2f..7bb41bbb3fd 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -8436,7 +8436,7 @@ ExecSetVariableStmt(VariableSetStmt *stmt, bool isTopLevel)
 		case VAR_SET_DEFAULT:
 			if (stmt->is_local)
 				WarnNoTransactionBlock(isTopLevel, "SET LOCAL");
-			/* fall through */
+			/* FALLTHROUGH */
 		case VAR_RESET:
 			if (strcmp(stmt->name, "transaction_isolation") == 0)
 				WarnNoTransactionBlock(isTopLevel, "RESET TRANSACTION");
diff --git a/src/common/hashfn.c b/src/common/hashfn.c
index 990f18e610b..29c846fcc31 100644
--- a/src/common/hashfn.c
+++ b/src/common/hashfn.c
@@ -178,13 +178,13 @@ hash_bytes(const unsigned char *k, int keylen)
 		{
 			case 11:
 				c += ((uint32) k[10] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 10:
 				c += ((uint32) k[9] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 9:
 				c += ((uint32) k[8] << 24);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 8:
 				/* the lowest byte of c is reserved for the length */
 				b += ka[1];
@@ -192,22 +192,22 @@ hash_bytes(const unsigned char *k, int keylen)
 				break;
 			case 7:
 				b += ((uint32) k[6] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 6:
 				b += ((uint32) k[5] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 5:
 				b += ((uint32) k[4] << 24);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 4:
 				a += ka[0];
 				break;
 			case 3:
 				a += ((uint32) k[2] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 2:
 				a += ((uint32) k[1] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 1:
 				a += ((uint32) k[0] << 24);
 				/* case 0: nothing left to add */
@@ -217,13 +217,13 @@ hash_bytes(const unsigned char *k, int keylen)
 		{
 			case 11:
 				c += ((uint32) k[10] << 24);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 10:
 				c += ((uint32) k[9] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 9:
 				c += ((uint32) k[8] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 8:
 				/* the lowest byte of c is reserved for the length */
 				b += ka[1];
@@ -231,22 +231,22 @@ hash_bytes(const unsigned char *k, int keylen)
 				break;
 			case 7:
 				b += ((uint32) k[6] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 6:
 				b += ((uint32) k[5] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 5:
 				b += k[4];
-				/* fall through */
+				/* FALLTHROUGH */
 			case 4:
 				a += ka[0];
 				break;
 			case 3:
 				a += ((uint32) k[2] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 2:
 				a += ((uint32) k[1] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 1:
 				a += k[0];
 				/* case 0: nothing left to add */
@@ -280,35 +280,35 @@ hash_bytes(const unsigned char *k, int keylen)
 		{
 			case 11:
 				c += ((uint32) k[10] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 10:
 				c += ((uint32) k[9] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 9:
 				c += ((uint32) k[8] << 24);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 8:
 				/* the lowest byte of c is reserved for the length */
 				b += k[7];
-				/* fall through */
+				/* FALLTHROUGH */
 			case 7:
 				b += ((uint32) k[6] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 6:
 				b += ((uint32) k[5] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 5:
 				b += ((uint32) k[4] << 24);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 4:
 				a += k[3];
-				/* fall through */
+				/* FALLTHROUGH */
 			case 3:
 				a += ((uint32) k[2] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 2:
 				a += ((uint32) k[1] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 1:
 				a += ((uint32) k[0] << 24);
 				/* case 0: nothing left to add */
@@ -318,35 +318,35 @@ hash_bytes(const unsigned char *k, int keylen)
 		{
 			case 11:
 				c += ((uint32) k[10] << 24);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 10:
 				c += ((uint32) k[9] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 9:
 				c += ((uint32) k[8] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 8:
 				/* the lowest byte of c is reserved for the length */
 				b += ((uint32) k[7] << 24);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 7:
 				b += ((uint32) k[6] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 6:
 				b += ((uint32) k[5] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 5:
 				b += k[4];
-				/* fall through */
+				/* FALLTHROUGH */
 			case 4:
 				a += ((uint32) k[3] << 24);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 3:
 				a += ((uint32) k[2] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 2:
 				a += ((uint32) k[1] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 1:
 				a += k[0];
 				/* case 0: nothing left to add */
@@ -417,13 +417,13 @@ hash_bytes_extended(const unsigned char *k, int keylen, uint64 seed)
 		{
 			case 11:
 				c += ((uint32) k[10] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 10:
 				c += ((uint32) k[9] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 9:
 				c += ((uint32) k[8] << 24);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 8:
 				/* the lowest byte of c is reserved for the length */
 				b += ka[1];
@@ -431,22 +431,22 @@ hash_bytes_extended(const unsigned char *k, int keylen, uint64 seed)
 				break;
 			case 7:
 				b += ((uint32) k[6] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 6:
 				b += ((uint32) k[5] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 5:
 				b += ((uint32) k[4] << 24);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 4:
 				a += ka[0];
 				break;
 			case 3:
 				a += ((uint32) k[2] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 2:
 				a += ((uint32) k[1] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 1:
 				a += ((uint32) k[0] << 24);
 				/* case 0: nothing left to add */
@@ -456,13 +456,13 @@ hash_bytes_extended(const unsigned char *k, int keylen, uint64 seed)
 		{
 			case 11:
 				c += ((uint32) k[10] << 24);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 10:
 				c += ((uint32) k[9] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 9:
 				c += ((uint32) k[8] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 8:
 				/* the lowest byte of c is reserved for the length */
 				b += ka[1];
@@ -470,22 +470,22 @@ hash_bytes_extended(const unsigned char *k, int keylen, uint64 seed)
 				break;
 			case 7:
 				b += ((uint32) k[6] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 6:
 				b += ((uint32) k[5] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 5:
 				b += k[4];
-				/* fall through */
+				/* FALLTHROUGH */
 			case 4:
 				a += ka[0];
 				break;
 			case 3:
 				a += ((uint32) k[2] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 2:
 				a += ((uint32) k[1] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 1:
 				a += k[0];
 				/* case 0: nothing left to add */
@@ -519,35 +519,35 @@ hash_bytes_extended(const unsigned char *k, int keylen, uint64 seed)
 		{
 			case 11:
 				c += ((uint32) k[10] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 10:
 				c += ((uint32) k[9] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 9:
 				c += ((uint32) k[8] << 24);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 8:
 				/* the lowest byte of c is reserved for the length */
 				b += k[7];
-				/* fall through */
+				/* FALLTHROUGH */
 			case 7:
 				b += ((uint32) k[6] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 6:
 				b += ((uint32) k[5] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 5:
 				b += ((uint32) k[4] << 24);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 4:
 				a += k[3];
-				/* fall through */
+				/* FALLTHROUGH */
 			case 3:
 				a += ((uint32) k[2] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 2:
 				a += ((uint32) k[1] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 1:
 				a += ((uint32) k[0] << 24);
 				/* case 0: nothing left to add */
@@ -557,35 +557,35 @@ hash_bytes_extended(const unsigned char *k, int keylen, uint64 seed)
 		{
 			case 11:
 				c += ((uint32) k[10] << 24);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 10:
 				c += ((uint32) k[9] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 9:
 				c += ((uint32) k[8] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 8:
 				/* the lowest byte of c is reserved for the length */
 				b += ((uint32) k[7] << 24);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 7:
 				b += ((uint32) k[6] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 6:
 				b += ((uint32) k[5] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 5:
 				b += k[4];
-				/* fall through */
+				/* FALLTHROUGH */
 			case 4:
 				a += ((uint32) k[3] << 24);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 3:
 				a += ((uint32) k[2] << 16);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 2:
 				a += ((uint32) k[1] << 8);
-				/* fall through */
+				/* FALLTHROUGH */
 			case 1:
 				a += k[0];
 				/* case 0: nothing left to add */
diff --git a/src/common/wchar.c b/src/common/wchar.c
index efaf1c155bb..352083d7bb1 100644
--- a/src/common/wchar.c
+++ b/src/common/wchar.c
@@ -1452,12 +1452,12 @@ pg_utf8_islegal(const unsigned char *source, int length)
 			a = source[3];
 			if (a < 0x80 || a > 0xBF)
 				return false;
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case 3:
 			a = source[2];
 			if (a < 0x80 || a > 0xBF)
 				return false;
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case 2:
 			a = source[1];
 			switch (*source)
@@ -1483,7 +1483,7 @@ pg_utf8_islegal(const unsigned char *source, int length)
 						return false;
 					break;
 			}
-			/* FALL THRU */
+			/* FALLTHROUGH */
 		case 1:
 			a = *source;
 			if (a >= 0x80 && a < 0xC2)
diff --git a/src/interfaces/ecpg/pgtypeslib/interval.c b/src/interfaces/ecpg/pgtypeslib/interval.c
index 48ba50bbce3..9720dfce654 100644
--- a/src/interfaces/ecpg/pgtypeslib/interval.c
+++ b/src/interfaces/ecpg/pgtypeslib/interval.c
@@ -391,7 +391,7 @@ DecodeInterval(char **field, int *ftype, int nf,	/* int range, */
 					tmask = DTK_M(TZ);
 					break;
 				}
-				/* FALL THROUGH */
+				/* FALLTHROUGH */
 
 			case DTK_DATE:
 			case DTK_NUMBER:
diff --git a/src/interfaces/libpq/fe-secure.c b/src/interfaces/libpq/fe-secure.c
index b455b45e964..062994145b6 100644
--- a/src/interfaces/libpq/fe-secure.c
+++ b/src/interfaces/libpq/fe-secure.c
@@ -375,7 +375,7 @@ retry_masked:
 				REMEMBER_EPIPE(spinfo, true);
 
 #ifdef ECONNRESET
-				/* FALL THRU */
+				/* FALLTHROUGH */
 
 			case ECONNRESET:
 #endif
diff --git a/src/pl/plpgsql/src/pl_exec.c b/src/pl/plpgsql/src/pl_exec.c
index aeb6c8fefc2..e3a588a7ef0 100644
--- a/src/pl/plpgsql/src/pl_exec.c
+++ b/src/pl/plpgsql/src/pl_exec.c
@@ -3186,7 +3186,7 @@ exec_stmt_return(PLpgSQL_execstate *estate, PLpgSQL_stmt_return *stmt)
 				/* fulfill promise if needed, then handle like regular var */
 				plpgsql_fulfill_promise(estate, (PLpgSQL_var *) retvar);
 
-				/* FALL THRU */
+				/* FALLTHROUGH */
 
 			case PLPGSQL_DTYPE_VAR:
 				{
@@ -3332,7 +3332,7 @@ exec_stmt_return_next(PLpgSQL_execstate *estate,
 				/* fulfill promise if needed, then handle like regular var */
 				plpgsql_fulfill_promise(estate, (PLpgSQL_var *) retvar);
 
-				/* FALL THRU */
+				/* FALLTHROUGH */
 
 			case PLPGSQL_DTYPE_VAR:
 				{
@@ -5423,7 +5423,7 @@ exec_eval_datum(PLpgSQL_execstate *estate,
 			/* fulfill promise if needed, then handle like regular var */
 			plpgsql_fulfill_promise(estate, (PLpgSQL_var *) datum);
 
-			/* FALL THRU */
+			/* FALLTHROUGH */
 
 		case PLPGSQL_DTYPE_VAR:
 			{
diff --git a/src/port/snprintf.c b/src/port/snprintf.c
index 83a81033a58..4deea58314c 100644
--- a/src/port/snprintf.c
+++ b/src/port/snprintf.c
@@ -464,7 +464,7 @@ nextch2:
 				/* set zero padding if no nonzero digits yet */
 				if (accum == 0 && !pointflag)
 					zpad = '0';
-				/* FALL THRU */
+				/* FALLTHROUGH */
 			case '1':
 			case '2':
 			case '3':
diff --git a/src/timezone/Makefile b/src/timezone/Makefile
index bf23ac9da97..72cc98e1dfb 100644
--- a/src/timezone/Makefile
+++ b/src/timezone/Makefile
@@ -55,6 +55,11 @@ ifeq (,$(ZIC))
 ZIC= ./zic
 endif
 
+# zic.c doesn't compile cleanly with our default
+# -Wimplicit-fallthrough=4, and we don't want to touch the file since
+# it's third-party code, so downgrade to 3 for that file.
+zic.o: CFLAGS := $(subst -Wimplicit-fallthrough=4,-Wimplicit-fallthrough=3,$(CFLAGS))
+
 zic: $(ZICOBJS) | submake-libpgport
 	$(CC) $(CFLAGS) $(ZICOBJS) $(LDFLAGS) $(LDFLAGS_EX) $(LIBS) -o $@$(X)