Add pg_memory_is_all_zeros() in memutils.h

This new function tests if a memory region starting at a given location
for a defined length is made only of zeroes.  This unifies in a single
path the all-zero checks that were happening in a couple of places of
the backend code:
- For pgstats entries of relation, checkpointer and bgwriter, where
some "all_zeroes" variables were previously used with memcpy().
- For all-zero buffer pages in PageIsVerifiedExtended().

This new function uses the same forward scan as the check for all-zero
buffer pages, applying it to the three pgstats paths mentioned above.

Author: Bertrand Drouvot
Reviewed-by: Peter Eisentraut, Heikki Linnakangas, Peter Smith
Discussion: https://postgr.es/m/ZupUDDyf1hHI4ibn@ip-10-97-1-34.eu-west-3.compute.internal

3 files changed, 8 insertions, 11 deletions

diff --git a/src/backend/utils/activity/pgstat_bgwriter.c b/src/backend/utils/activity/pgstat_bgwriter.c
index 364a7a2024a..85d53d82f26 100644
--- a/src/backend/utils/activity/pgstat_bgwriter.c
+++ b/src/backend/utils/activity/pgstat_bgwriter.c
@@ -17,6 +17,7 @@
 
 #include "postgres.h"
 
+#include "utils/memutils.h"
 #include "utils/pgstat_internal.h"
 
 
@@ -30,7 +31,6 @@ void
 pgstat_report_bgwriter(void)
 {
 	PgStatShared_BgWriter *stats_shmem = &pgStatLocal.shmem->bgwriter;
-	static const PgStat_BgWriterStats all_zeroes;
 
 	Assert(!pgStatLocal.shmem->is_shutdown);
 	pgstat_assert_is_up();
@@ -39,7 +39,8 @@ pgstat_report_bgwriter(void)
 	 * This function can be called even if nothing at all has happened. In
 	 * this case, avoid unnecessarily modifying the stats entry.
 	 */
-	if (memcmp(&PendingBgWriterStats, &all_zeroes, sizeof(all_zeroes)) == 0)
+	if (pg_memory_is_all_zeros(&PendingBgWriterStats,
+							   sizeof(struct PgStat_BgWriterStats)))
 		return;
 
 	pgstat_begin_changecount_write(&stats_shmem->changecount);
diff --git a/src/backend/utils/activity/pgstat_checkpointer.c b/src/backend/utils/activity/pgstat_checkpointer.c
index 5a3fb4a9e09..b2d8eb0d9c3 100644
--- a/src/backend/utils/activity/pgstat_checkpointer.c
+++ b/src/backend/utils/activity/pgstat_checkpointer.c
@@ -17,6 +17,7 @@
 
 #include "postgres.h"
 
+#include "utils/memutils.h"
 #include "utils/pgstat_internal.h"
 
 
@@ -29,8 +30,6 @@ PgStat_CheckpointerStats PendingCheckpointerStats = {0};
 void
 pgstat_report_checkpointer(void)
 {
-	/* We assume this initializes to zeroes */
-	static const PgStat_CheckpointerStats all_zeroes;
 	PgStatShared_Checkpointer *stats_shmem = &pgStatLocal.shmem->checkpointer;
 
 	Assert(!pgStatLocal.shmem->is_shutdown);
@@ -40,8 +39,8 @@ pgstat_report_checkpointer(void)
 	 * This function can be called even if nothing at all has happened. In
 	 * this case, avoid unnecessarily modifying the stats entry.
 	 */
-	if (memcmp(&PendingCheckpointerStats, &all_zeroes,
-			   sizeof(all_zeroes)) == 0)
+	if (pg_memory_is_all_zeros(&PendingCheckpointerStats,
+							   sizeof(struct PgStat_CheckpointerStats)))
 		return;
 
 	pgstat_begin_changecount_write(&stats_shmem->changecount);
diff --git a/src/backend/utils/activity/pgstat_relation.c b/src/backend/utils/activity/pgstat_relation.c
index 36d3adf7310..faba8b64d23 100644
--- a/src/backend/utils/activity/pgstat_relation.c
+++ b/src/backend/utils/activity/pgstat_relation.c
@@ -800,7 +800,6 @@ pgstat_twophase_postabort(TransactionId xid, uint16 info,
 bool
 pgstat_relation_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
 {
-	static const PgStat_TableCounts all_zeroes;
 	Oid			dboid;
 	PgStat_TableStatus *lstats; /* pending stats entry  */
 	PgStatShared_Relation *shtabstats;
@@ -815,11 +814,9 @@ pgstat_relation_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
 	 * Ignore entries that didn't accumulate any actual counts, such as
 	 * indexes that were opened by the planner but not used.
 	 */
-	if (memcmp(&lstats->counts, &all_zeroes,
-			   sizeof(PgStat_TableCounts)) == 0)
-	{
+	if (pg_memory_is_all_zeros(&lstats->counts,
+							   sizeof(struct PgStat_TableCounts)))
 		return true;
-	}
 
 	if (!pgstat_lock_entry(entry_ref, nowait))
 		return false;