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

src/backend/storage/page/bufpage.c

@@ -89,10 +89,8 @@ PageIsVerifiedExtended(Page page, BlockNumber blkno, int flags)
 {
 	PageHeader	p = (PageHeader) page;
 	size_t	   *pagebytes;
-	int			i;
 	bool		checksum_failure = false;
 	bool		header_sane = false;
-	bool		all_zeroes = false;
 	uint16		checksum = 0;
 
 	/*
@@ -126,18 +124,9 @@ PageIsVerifiedExtended(Page page, BlockNumber blkno, int flags)
 	}
 
 	/* Check all-zeroes case */
-	all_zeroes = true;
 	pagebytes = (size_t *) page;
-	for (i = 0; i < (BLCKSZ / sizeof(size_t)); i++)
-	{
-		if (pagebytes[i] != 0)
-		{
-			all_zeroes = false;
-			break;
-		}
-	}
 
-	if (all_zeroes)
+	if (pg_memory_is_all_zeros(pagebytes, (BLCKSZ / sizeof(size_t))))
 		return true;
 
 	/*

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);

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,
+	if (pg_memory_is_all_zeros(&PendingCheckpointerStats,
-			   sizeof(all_zeroes)) == 0)
+							   sizeof(struct PgStat_CheckpointerStats)))
 		return;
 
 	pgstat_begin_changecount_write(&stats_shmem->changecount);

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,
+	if (pg_memory_is_all_zeros(&lstats->counts,
-			   sizeof(PgStat_TableCounts)) == 0)
+							   sizeof(struct PgStat_TableCounts)))
-	{
 		return true;
-	}
 
 	if (!pgstat_lock_entry(entry_ref, nowait))
 		return false;

src/include/utils/memutils.h

@@ -189,4 +189,21 @@ extern MemoryContext BumpContextCreate(MemoryContext parent,
 #define SLAB_DEFAULT_BLOCK_SIZE		(8 * 1024)
 #define SLAB_LARGE_BLOCK_SIZE		(8 * 1024 * 1024)
 
+/*
+ * Test if a memory region starting at "ptr" and of size "len" is full of
+ * zeroes.
+ */
+static inline bool
+pg_memory_is_all_zeros(const void *ptr, size_t len)
+{
+	const char *p = (const char *) ptr;
+
+	for (size_t i = 0; i < len; i++)
+	{
+		if (p[i] != 0)
+			return false;
+	}
+	return true;
+}
+
 #endif							/* MEMUTILS_H */