[M3devel] <*LAZYALIGN*>

[Rodney M. Bates]

Does anybody know about the status of pragma <*LAZYALIGN*>?  Is it
being used anywhere?

It is not documented in pragmas.html.  The compiler front end appears
to accept it.  (In fact, Decls.m3 contains constants that suggest
limitations on what declarations the pragma can appear on, but these
are not actually enforced.)  It liberalizes the alignment rules,
generally allowing scalars to start on any byte boundary.

Pickles have to be able to reconstruct the layout of types as the
compiler would have done it for a machine (on which a now-being-read
pickle was written) with different word size and alignment properties.
Currently, pickles are completely unaware of lazy alignment.  It
would have to be encoded in type descriptions generated by the compiler
using TipeDesc and read by pickles using RTTipe.

Most troubling to me is what looks like a linguistic Pandora's box.
The pragma can be associated with any constant, variable, or type
_declaration_ (not type definition), with the result that different
values of the same type can actually be different in their alignment
rules and thus their layout.  Similarly for different identifiers
equated to the same type.  Although the effects of this could possibly
be hidden from the programmer in purely safe code, not so with unsafe
code.  I haven't thoroughly thought this through,  but seems to me to
really fly in the face of the whole typing philosophy of the language.

For example, if pickles were to read in an object value, and there
were >1 variants of the object's type in the reading program, differing
only in the alignment rules, how would it decide which one to build?
In fact, ignoring pickles altogether and just looking at a single program,
if the object's type doesn't actually uniquely give its memory layout,
how can it be accessed correctly?

Additionally, a quick look at the compiler suggests it won't generate
correct code for whole record assignment when the LHS and RHS are the
same type but have different alignment characteristics.

The more I think about it, it seems the only workable possibilities are:

1) Require the pragma to be associated only with a type _definition_ not a
    declaration (e.g., allow RECORD <*LAZYALIGN*> ... END) and make this a
    property of the type that propagates to all names for the type and
    all variables, constants, etc.  Also, this would make this property
    a part of the type signature that pickles uses when reading, -OR-

2) Forget it altogether.

What do people think?
-- 
-------------------------------------------------------------
Rodney M. Bates, retired assistant professor
Dept. of Computer Science, Wichita State University
Wichita, KS 67260-0083
316-978-3922
rodney.bates at wichita.edu