HeapTupleHeader withoud oid

We have been discussing heap tuple header changes for a while now.
Here is my proposal for omitting the oid, when it is not needed:

First let's eliminate t_oid from HeapTupleHeaderData.

Then add the oid to the end of the structure, if and only if it is
needed.  The tricky part here is that there is a variable length field
(t_bits) and the oid has to be properly aligned.

This pseudo code snippet illustrates what I plan to do:
len = offsetof(HeapTupleHeaderData, t_bits);  /* 23 */if (hasnulls) {    len += BITMAPLEN(NumberOfAttributes);}if
(hasoid){    len += sizeof(Oid);}len = MAXALIGN(len);hoff = len;oidoff = hoff - sizeof(Oid);
 

#define HeapTupleHeaderGetOid(hth) \( *((Oid *)((char *)(hth) + (hth)->t_hoff - sizeof(Oid))) )

And this is how the structure would look like:              1         2           3    0   4     0         0  34  78
2
now  oooo<---------fix--------->.x___X___
+oid <---------fix--------->.oooox___       MAXALIGN 4
+oid <---------fix--------->.....ooooX___   MAXALIGN 8
-oid <---------fix--------->.X___

1:
now  oooo<---------fix--------->bx___X___
+oid <---------fix--------->boooox___       MAXALIGN 4
+oid <---------fix--------->b....ooooX___   MAXALIGN 8
-oid <---------fix--------->bX___

2:
now  oooo<---------fix--------->bb...X___
+oid <---------fix--------->bb...ooooX___   MAXALIGN 4 und 8
-oid <---------fix--------->bb...x___X___                                    3       4 
6:                                   2   6   0
now  oooo<---------fix--------->bbbbbb...x___X___
+oid <---------fix--------->bbbbbb...oooox___       MAXALIGN 4
+oid <---------fix--------->bbbbbb.......ooooX___   MAXALIGN 8
-oid <---------fix--------->bbbbbb...X___

where
<---------fix--------->  fixed sized part without oid, 23 bytes
oooo  oid, 4 bytes
b     one bitmap byte
.     one padding byte
x     start of data area (= hoff) with 4-byte-alignment
X     start of data area (= hoff) with 8-byte-alignment

Bytes saved on architectures with 4/8 byte alignment:                                 hoff    bytes
natts  bitmaplen  hoff72  oidoff   woo    saved      0          28/32     24    24/24    4/8
1-8    1          28/32     24    24/24    4/8
9-40   2-5        32/32     28    28/32    4/0
41-72  6-9        36/40     32    32/32    4/8

As a first step I've already posted a patch that eliminates direct
access to t_oid.  The final patch will change not much more than the
getter and setter macros.

Problems I have identified so far:

.  heap_formtuple needs a parameter bool withoid
.  Does heap_addheader *always* create a header with oid?
.  Have to check heap_xlog_xxxx routines
.  Occasionally a heap tuple header is copied by memmove.

Comments?

ServusManfred