[M3devel] <LAZYALIGN>

Sat Feb 23 19:26:55 CET 2008

kind of obnoxious of me here sorry:

I am not volunteering to do any work here. :)

I propose that declaration that interface with C be declared PACKED (if that even exists) or BITS n FOR (I know that exists; it was pointed out to me and I have been using it).
I would like to be able to say BYTES n FOR, but ok, I say BITS n * 8 FOR.

I propose there be a command line switch, or interface/module-level pragma that issues warnings or errors for any apparent need to insert any padding. 

I propose that any padding for alignment that is needed to interface with existing C code be explicitly inserted.

I propose that when you say BITS n FOR, you be able to get a warning/error if the size isn't exactly right.
I know it is already an error today if the size is too small, but I assume the compiler will grow the type to fit if it is "too big" and I'd rather not have that.
On the other hand, while I have been using BITS n FOR, I make up an arbitrary type. That is, I say:

Foo = BITS exact FOR RECORD
   blah blah 
   pad : BITS exact FOR arbitrary small type such as [0..0]
END;

While I don't want the compiler to expand Foo up to exact, I don't care about the [0..0] thing.
I guess in the interest of being less lazy and coming up with a simpler feature set, I am willing to be more precise in the padding. Or maybe a new type can be invented BITS exact for Padding. And maybe you can tack on ":= 0" to it, no matter the size. (Notice how in the socket code, the type of the padding can't be changed willy nilly because there is code to zero initialize it; seems a little bit lame to me.) 

I would probably volunteer to do some of the header repair here.

Alignment issues bother me and I think we might be in a position to solve them once and for all for Modula-3, in a reasonable simple, clear, reliable, multi-target way. 

Part of this proposal would need to distinguish at least three scenarios:
   exact layout for interfacing with existing C 
   stable layout for interface with self -- declare some new type in Modula-3 that I can pickle on multiple platforms but which Modula-3 can determine the layout of; padding can be inserted here but it must be the same across various time/space/configuration  
   don't care layout for data that will only ever be in memory and shared amongst code built by the same basic compiler with the same declarations; padding can be inserted here and vary from build to build or such 

 another case: exact layout for interfacing with existing Modula-3/pickles. 

Or maybe simply pickles need persisted metadata about the various fields, offsets, sizes?
I have never used pickles or looked at the code...

 - Jay

> Date: Sat, 23 Feb 2008 11:31:36 -0600> From: rodney.bates at wichita.edu> To: darko at darko.org> CC: m3devel at elegosoft.com> Subject: Re: [M3devel] <*LAZYALIGN*>> > I guess I misunderstood. I had the idea from the quoted text below that MAC OS X needed a mixture> of lazy and strict alignment. If every platform uses the same alignment rules for all types of all> programs on the platform, then the alignment rule property can just be made an additional field of> the RTPacking.T.> > This is much easier to do without compatibility problems, because this value is encoded in bits> in a 32-bit word in pickle headers, with unused bits that are now ignored. The alignment strategy> could just be in another bit.> > > From a post on 2-11:> ---------------------------------------------------------------------------------------------------------> That's not quite right. Certain Mac API structures need to be aligned to Motorola 68K alignment for > historical reasons. All other structures should use normal alignment to be compatible with C and > Unix interfaces. The alignment change was implemented to be supported only in packed structures as > a natural and intuitive way to "force" alignment. You could view it as a bug fix to overly > restrictive alignment rules in packed structures.> > > On 12/02/2008, at 2:46 PM, Rodney M. Bates wrote:> > > It sounds like all that Mac OS X needs is for _all_ types in an entire> > program to use the liberal packing rules. Have I understood correctly?> > I would have no grief over that.> ---------------------------------------------------------------------------------------------------------> > Darko wrote:> > The alignment rules are particular to a platform, not a type (at least > > after we get rid of the pragma), so is there no field where we can > > encode some information about the platform in the pickle header or > > something? Where is endian stored?> > > > > -- > -------------------------------------------------------------> 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
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