From f6ba238dd46416eb29ac43dadac0c69a75f106fe Mon Sep 17 00:00:00 2001
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: Tue, 28 Mar 2017 17:39:26 -0300
Subject: [PATCH] Free 3 bits of ItemPointerData.ip_posid

Since we limit block size to 32 kB, the highest offset number used in a
bufpage.h-organized page is 5461, which can be represented with 13 bits.
Therefore, the upper 3 bits in 16-bit ItemPointerData.ip_posid are
unused and this commit reserves them for other purposes.

Author: Pavan Deolasee
---
 src/include/access/htup_details.h |  2 +-
 src/include/storage/itemptr.h     | 30 +++++++++++++++++++++++++++---
 src/include/storage/off.h         | 11 ++++++++++-
 3 files changed, 38 insertions(+), 5 deletions(-)

diff --git a/src/include/access/htup_details.h b/src/include/access/htup_details.h
index 7b6285d..d3cc0ad 100644
--- a/src/include/access/htup_details.h
+++ b/src/include/access/htup_details.h
@@ -282,7 +282,7 @@ struct HeapTupleHeaderData
  * than MaxOffsetNumber, so that it can be distinguished from a valid
  * offset number in a regular item pointer.
  */
-#define SpecTokenOffsetNumber		0xfffe
+#define SpecTokenOffsetNumber		OffsetNumberPrev(OffsetNumberMask)
 
 /*
  * HeapTupleHeader accessor macros
diff --git a/src/include/storage/itemptr.h b/src/include/storage/itemptr.h
index c21d2ad..74eed4e 100644
--- a/src/include/storage/itemptr.h
+++ b/src/include/storage/itemptr.h
@@ -57,7 +57,7 @@ typedef ItemPointerData *ItemPointer;
  *		True iff the disk item pointer is not NULL.
  */
 #define ItemPointerIsValid(pointer) \
-	((bool) (PointerIsValid(pointer) && ((pointer)->ip_posid != 0)))
+	((bool) (PointerIsValid(pointer) && (((pointer)->ip_posid & OffsetNumberMask) != 0)))
 
 /*
  * ItemPointerGetBlockNumberNoCheck
@@ -84,7 +84,7 @@ typedef ItemPointerData *ItemPointer;
  */
 #define ItemPointerGetOffsetNumberNoCheck(pointer) \
 ( \
-	(pointer)->ip_posid \
+	((pointer)->ip_posid & OffsetNumberMask) \
 )
 
 /*
@@ -98,6 +98,30 @@ typedef ItemPointerData *ItemPointer;
 )
 
 /*
+ * Get the flags stored in high order bits in the OffsetNumber.
+ */
+#define ItemPointerGetFlags(pointer) \
+( \
+	((pointer)->ip_posid & ~OffsetNumberMask) >> OffsetNumberBits \
+)
+
+/*
+ * Set the flag bits. We first left-shift since flags are defined starting 0x01
+ */
+#define ItemPointerSetFlags(pointer, flags) \
+( \
+	((pointer)->ip_posid |= ((flags) << OffsetNumberBits)) \
+)
+
+/*
+ * Clear all flags.
+ */
+#define ItemPointerClearFlags(pointer) \
+( \
+	((pointer)->ip_posid &= OffsetNumberMask) \
+)
+
+/*
  * ItemPointerSet
  *		Sets a disk item pointer to the specified block and offset.
  */
@@ -105,7 +129,7 @@ typedef ItemPointerData *ItemPointer;
 ( \
 	AssertMacro(PointerIsValid(pointer)), \
 	BlockIdSet(&((pointer)->ip_blkid), blockNumber), \
-	(pointer)->ip_posid = offNum \
+	(pointer)->ip_posid = (offNum) \
 )
 
 /*
diff --git a/src/include/storage/off.h b/src/include/storage/off.h
index fe8638f..f058fe1 100644
--- a/src/include/storage/off.h
+++ b/src/include/storage/off.h
@@ -26,7 +26,16 @@ typedef uint16 OffsetNumber;
 #define InvalidOffsetNumber		((OffsetNumber) 0)
 #define FirstOffsetNumber		((OffsetNumber) 1)
 #define MaxOffsetNumber			((OffsetNumber) (BLCKSZ / sizeof(ItemIdData)))
-#define OffsetNumberMask		(0xffff)		/* valid uint16 bits */
+
+/*
+ * The biggest BLCKSZ we support is 32kB, and each ItemId takes 6 bytes.
+ * That limits the number of line pointers in a page to 32kB/6B = 5461.
+ * Therefore, 13 bits in OffsetNumber are enough to represent all valid
+ * on-disk line pointers.  Hence, we can reserve the high-order bits in
+ * OffsetNumber for other purposes.
+ */
+#define OffsetNumberBits		13
+#define OffsetNumberMask		((((uint16) 1) << OffsetNumberBits) - 1)
 
 /* ----------------
  *		support macros
-- 
2.1.4