Do I understand the storage formats?

[Angus Leeming <a.leeming@ic.ac.uk>]

This is a long and largely useless email. It's an attempt by me to understand
exactly how the different storage formats are meant to work so that I can cure
the problems with "unaligned access errors" on the Alpha. If, however, I've
got this wrong would somebody please put me straight!

Angus
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All info is stored in xtl as one long character string.

An unaligned access error occurs if "type data" spans a word boundary but does
not begin at the start of the word, for some type (char, int, double etc). On 32
bit machines a word is 4 bytes long. On 64 bit machines such as the Alpha, a
word is 8 bytes long.

Thus, storing
	char a1, a2, a3, a4, a5, a6, a7, a8, a9, a10;
is Ok, because no variable crosses a word boundary. Similarly,
	double a1, a2, a3, a4, a5, a6, a7, a8, a9, a10;
is also Ok, but attempting to store
	char a1;
	double a2, a3, a4, a5, a6, a7, a8, a9, a10;
will result in unaligned access errors for each double if no effort is made to
pad the storage (this is what happens in raw_format).

This problem can be addressed in two different ways, as instatiated in xdr.h
and giop.h

XDR_format overcomes the problem by storing each variable in a space of size
WORDLENGTH (4 bytes on an Intel, 8 bytes on an Alpha).

Since the storage of all variables begins on a word boundary, no unaligned
access errors can occur. Bingo! However, the format is clearly very inefficient
at storing chars since each char is stored with (WORDLENGTH-1) bytes of padding.

GIOP_format is much more subtle. Padding is introduced only if storage will
cross a word boundary. Thus, storing:
	char a1, a2, a3, a4, a5;
	double a6, a7, a8, a9, a10;
on an Alpha machine will require 3 bytes of padding between a5 and a6.
Thereafter, no more padding is needed. 

Clearly, this is the optimal storage format and is, therefore, the way that LyX
should go.

I think this is what the code in GIOP_format::req_align() is tyying to do,
but my brain is slowing down (long day). Am I correct?

A.