Faster PageIsVerified() for the all zeroes case.

That's primarily useful for testing very large relations, using sparse
files.

Discussion: <20140331101001.GE13135@alap3.anarazel.de>
Reviewed-By: Peter Geoghegan

src/backend/storage/page/bufpage.c

@@ -81,7 +81,7 @@ bool
 PageIsVerified(Page page, BlockNumber blkno)
 {
 	PageHeader	p = (PageHeader) page;
-	char	   *pagebytes;
+	size_t	   *pagebytes;
 	int			i;
 	bool		checksum_failure = false;
 	bool		header_sane = false;
@@ -118,10 +118,17 @@ PageIsVerified(Page page, BlockNumber blkno)
 			return true;
 	}
 
-	/* Check all-zeroes case */
+	/*
+	 * Check all-zeroes case. Luckily BLCKSZ is guaranteed to always be a
+	 * multiple of size_t - and it's much faster to compare memory using the
+	 * native word size.
+	 */
+	StaticAssertStmt(BLCKSZ == (BLCKSZ / sizeof(size_t)) * sizeof(size_t),
+					 "BLCKSZ has to be a multiple of sizeof(size_t)");
+
 	all_zeroes = true;
-	pagebytes = (char *) page;
+	pagebytes = (size_t *) page;
-	for (i = 0; i < BLCKSZ; i++)
+	for (i = 0; i < (BLCKSZ / sizeof(size_t)); i++)
 	{
 		if (pagebytes[i] != 0)
 		{