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

1 files changed, 17 insertions, 0 deletions

diff --git a/src/include/utils/memutils.h b/src/include/utils/memutils.h
index cd9596ff219..3590c8bad9a 100644
--- a/src/include/utils/memutils.h
+++ b/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 */