[M3devel] A GC/compiler/Pickle interaction bug

[Tony Hosking]

Rodney,

I've checked in fixes to RTTypeMap and clients that should restore  
functional pickling support.  For some reason my commit messages are  
awaiting moderator approval (it seems my commit id is not the same as  
my subscription to the m3commit list) before they can be sent out.   
Please make sure that unpickling now works for you.

Best regards,

Tony

On Dec 30, 2006, at 7:41 PM, Rodney M. Bates wrote:

> I tracked down a bug unpickling to the following.  When executing
> in ConvertPacking.Read, at ConvertPacking.m3:327, this code:
>
>                 WITH ref = LOOPHOLE(dest, UNTRACED REF REFANY) DO
>                   ref^ := v.readRef(nelem.refType);
>                 END;
>
> has a compiler-generated call on RTCollector.CheckStoreTraced(ref).
> CheckStoreTraced expects ref to be a normal reference to an object,
> computes the address of its header (4 bytes less), and sets the  
> dirty bit.
>
> But ConvertPacking is reading in to a field of a recently created
> object that is not first in the object.  It computes ref to point
> to the field, and CheckStoreTraced actually steps on the previous
> real field instead of the header.
>
> It looks like the actions of CheckStoreTraced are needed, but I
> don't right off hand see how to recode Convert to fix this.  It's
> use of ref can't be moved.  Presumably, there could be other unsafe
> code elsewhere using the same technique to store data.
>
> Does the compiler need a different criterion on when to generate
> the call on CheckStoreTraced?  ref is an UNTRACED REF here, but it
> still is pointing inside a traced object.   Maybe people who modify
> objects in this way need to explicitly call CheckStoreTraced?
>
> -- 
> -------------------------------------------------------------
> 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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Antony Hosking                | Associate Professor
Dept of Computer Science      | Office: +1 765 494-6001
Purdue University             | Mobile: +1 765 427-5484
250 N. University Street      | Email:  hosking at cs.purdue.edu
West Lafayette, IN 47907-2066 | http://www.cs.purdue.edu/~hosking
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