1 files changed, 46 insertions, 9 deletions

diff --git a/src/include/storage/buf_internals.h b/src/include/storage/buf_internals.h
index 406db6be783..c3417b28ba9 100644
--- a/src/include/storage/buf_internals.h
+++ b/src/include/storage/buf_internals.h
@@ -92,29 +92,66 @@ typedef struct buftag
 {
 	Oid			spcOid;			/* tablespace oid */
 	Oid			dbOid;			/* database oid */
-	RelFileNumber relNumber;	/* relation file number */
-	ForkNumber	forkNum;		/* fork number */
+
+	/*
+	 * relForkDetails[] stores the fork number in the high 8 bits of the first
+	 * integer; the remaining 56 bits are used to store the relfilenmber.
+	 * Expanding the relfilenumber to a full 64 bits would require widening
+	 * the BufferTag, which is undesirable for performance reasons. We use
+	 * two 32-bit values here rather than a single 64-bit value to avoid
+	 * padding the struct out to a multiple of 8 bytes.
+	 */
+	uint32		relForkDetails[2];
 	BlockNumber blockNum;		/* blknum relative to begin of reln */
 } BufferTag;
 
+/* High relNumber bits in relForkDetails[0] */
+#define	BUFTAG_RELNUM_HIGH_BITS	24
+
+/* Low relNumber bits in relForkDetails[1] */
+#define	BUFTAG_RELNUM_LOW_BITS	32
+
+/* Mask to fetch high bits of relNumber from relForkDetails[0] */
+#define	BUFTAG_RELNUM_HIGH_MASK	((1U << BUFTAG_RELNUM_HIGH_BITS) - 1)
+
+/* Mask to fetch low bits of relNumber from relForkDetails[1] */
+#define	BUFTAG_RELNUM_LOW_MASK	0XFFFFFFFF
+
 static inline RelFileNumber
 BufTagGetRelNumber(const BufferTag *tag)
 {
-	return tag->relNumber;
+	uint64		relnum;
+
+	relnum = ((uint64) tag->relForkDetails[0]) & BUFTAG_RELNUM_HIGH_MASK;
+	relnum = (relnum << BUFTAG_RELNUM_LOW_BITS) | tag->relForkDetails[1];
+
+	Assert(relnum <= MAX_RELFILENUMBER);
+	return (RelFileNumber) relnum;
 }
 
 static inline ForkNumber
 BufTagGetForkNum(const BufferTag *tag)
 {
-	return tag->forkNum;
+	ForkNumber		ret;
+
+	StaticAssertStmt(MAX_FORKNUM <= INT8_MAX,
+					 "MAX_FORKNUM can't be greater than INT8_MAX");
+
+	ret = (int8) (tag->relForkDetails[0] >> BUFTAG_RELNUM_HIGH_BITS);
+	return ret;
 }
 
 static inline void
 BufTagSetRelForkDetails(BufferTag *tag, RelFileNumber relnumber,
 						ForkNumber forknum)
 {
-	tag->relNumber = relnumber;
-	tag->forkNum = forknum;
+	Assert(relnumber <= MAX_RELFILENUMBER);
+	Assert(forknum <= MAX_FORKNUM);
+
+	tag->relForkDetails[0] = (relnumber >> BUFTAG_RELNUM_LOW_BITS) &
+		BUFTAG_RELNUM_HIGH_MASK;
+	tag->relForkDetails[0] |= (forknum << BUFTAG_RELNUM_HIGH_BITS);
+	tag->relForkDetails[1] = relnumber & BUFTAG_RELNUM_LOW_MASK;
 }
 
 static inline RelFileLocator
@@ -153,9 +190,9 @@ BufferTagsEqual(const BufferTag *tag1, const BufferTag *tag2)
 {
 	return (tag1->spcOid == tag2->spcOid) &&
 		(tag1->dbOid == tag2->dbOid) &&
-		(tag1->relNumber == tag2->relNumber) &&
-		(tag1->blockNum == tag2->blockNum) &&
-		(tag1->forkNum == tag2->forkNum);
+		(tag1->relForkDetails[0] == tag2->relForkDetails[0]) &&
+		(tag1->relForkDetails[1] == tag2->relForkDetails[1]) &&
+		(tag1->blockNum == tag2->blockNum);
 }
 
 static inline bool