Reduce per tuple overhead (bitmap)

This small patch makes the length of the null bitmap a multiple
of 1 byte (has been a multiple of 4 bytes previously), thus saving
MAXIMUM_ALIGNOF (4 or 8 on modern platforms) bytes per on-disk-tuple
in certain cases (depending on number of attributes and if there is
at least one null attribute).

It passes make check for 7.2 and for cvs HEAD.

Though it changes the on-disk-format, there's no need to initdb,
because data offsets are stored in t_hoff for each tuple. So this
patch can be applied to HEAD and to the 7.2 branch.

Servus
Manfred

diff -r -u ../orig/src/include/access/htup.h src/include/access/htup.h
--- ../orig/src/include/access/htup.h Mon Nov 5 18:46:31 2001
+++ src/include/access/htup.h Sun May 5 08:03:29 2002
@@ -17,7 +17,7 @@
#include "storage/bufpage.h"
#include "storage/relfilenode.h"

-#define MinHeapTupleBitmapSize 32 /* 8 * 4 */
+#define MinHeapTupleBitmapLen 1 /* in bytes, must be 2**n */

/*
* MaxHeapAttributeNumber limits the number of (user) columns in a table.
@@ -57,7 +57,7 @@

/* ^ - 31 bytes - ^ */

- bits8 t_bits[MinHeapTupleBitmapSize / 8];
+ bits8 t_bits[MinHeapTupleBitmapLen];
/* bit map of NULLs */

/* MORE DATA FOLLOWS AT END OF STRUCT */
@@ -225,8 +225,8 @@
* Computes minimum size of bitmap given number of domains.
*/
#define BITMAPLEN(NATTS) \
- ((((((int)(NATTS) - 1) >> 3) + 4 - (MinHeapTupleBitmapSize >> 3)) \
- & ~03) + (MinHeapTupleBitmapSize >> 3))
+ (((((int)(NATTS) - 1) >> 3) \
+ & ~(MinHeapTupleBitmapLen - 1)) + MinHeapTupleBitmapLen)

/*
* HeapTupleIsValid