[M3devel] Using REFANYs to store integers?

[Tony Hosking]

I didn't mean to disparage your suggestion -- just pointing out some  
of the pitfalls.  Certainly, the current collectors will have  
problems if you store tagged values in REFANY fields, since it  
expects to find aligned references there.  I can see a couple of  
options that might work, some of which might use the WeakRef  
interface to control reachability.  I imagine we could come up with  
some sort of API that would support this.  It might even be an  
interesting result...

On Dec 10, 2007, at 6:33 PM, Mika Nystrom wrote:

>
> Well, come to think of it... I'm not sure I can even do my own GC.
> The idea is that the interpreter I am working on would have more
> or less unrestricted access to the M3 environment, anyhow, and that
> many of the objects on its stacks will be plain old M3 objects,
> which will be handled as opaque handles by the interpreter.  In
> other words, I'd have to have and worry about two garbage collectors
> (M3 + interpreter) if I follow your suggestion.  I was really hoping
> to avoid that.  I was also hoping to use your excellent garbage
> collector rather than having to worry about that too, in addition
> to all the other issues that come up in coding bytecode compilers,
> interpreters, with all the other runtime support etc etc.
>
> Since I clearly have to support versions of M3 that don't appreciate
> the LSB hack I am just going to have to be able to represent small
> integers as something along the lines of
>
>    TYPE SmallInteger = OBJECT v : INTEGER END
>
> anyhow.  This is not a problem, it's just going to be slow, since
> I anticipate that most of the work done by the interpreter will be
> manipulating small integers (array indices, etc.)  Tony, don't
> worry, I'm not going to give you (much of) a hard time if you come
> up with a beautiful garbage collector that's incompatible with my
> "nasty hack"!
>
> In any case, I am just thinking aloud (awrite?) and you gentlemen
> have fully satisfied my curiosity with your answers to my questions.
> Thanks!
>
>      Mika
>
> Tony Hosking writes:
>> The LSB trick is something that one could teach the current
>> implementation to handle, but I note that there is *nothing* in the
>> language definition that would mean this hack could ever be
>> portable.  Moreover, there is reason to expect that a particular GC
>> implementation might use this same trick internally, in which case
>> you have a conflict between your hack and that implementation.
>> Basically, you should not assume *any* particular representation for
>> REFANY in Modula-3.  For example, an implementation would be free to
>> use a level of indirection (ie, "handles" or an object table) to
>> implement M3 REFANY.  In sum, I think what you propose is a nasty
>> hack.  Better to implement your own GC anyway, since it is likely to
>> be much better to exploit the semantics of your interpreter/language
>> for best behavior.
>>
>> On Dec 10, 2007, at 3:21 PM, Mika Nystrom wrote:
>>
>>> Hi Rodney,
>>>
>>> You're reading my mind.  What I am thinking of doing is that I want
>>> to optimize the handling of small integers (only), exactly the way
>>> it's done in Smalltalk, that is, immutable, single-instance objects.
>>> And only the interpreter will ever dereference the objects, so it
>>> would check the LSBs---if they are set to zero, it will assume that
>>> it's a heap object, if anything else, it's a special immutable value
>>> of some sort.
>>>
>>> I'm well aware it's a very dirty trick to attempt!  But I was
>>> thinking that by doing this sort of thing I could avoid having to
>>> code my own garbage collector.  I'm not surprised that the current
>>> garbage collector has issues with it but I also cannot think of a
>>> reason why one couldn't make it ignore unaligned "pointers" unless
>>> the trick is already used for some purpose I am not aware of.
>>>
>>>     Mika
>>>
>>> "Rodney M. Bates" writes:
>>>> I presume "pointers" that are not 0 MOD 4 "refer to" some kinds
>>>> of immutable, single-instance objects and don't actually point
>>>> to heap objects in need of collection, as in Smalltalk?  I would
>>>> think it would be a simple modification of the existing M3 GC to
>>>> just make it ignore words containing misaligned pointers.
>>>>
>>>> Will only interpreted bytecode ever dereference these pointers?
>>>> Or does M3 compiler-generated code also dereference them?  Then
>>>> the compiler would also have to generate checks and do whatever
>>>> with misaligned pointers.
>>>>
>>>> Mika Nystrom wrote:
>>>>> Hello Modula-3ers...
>>>>>
>>>>> I have a question about garbage collecting, pointers, and dirty
>>>>> tricks that I'm curious if anyone (Tony??) can answer.
>>>>>
>>>>> Here it is.  I am considering writing a bytecode interpreter that
>>>>> is to run mixed into a Modula-3 environment, that is, it will be
>>>>> able to deal with bytecodes as well as Modula-3 objects.  There is
>>>>> no problem with that of course, as bytecodes are "code" and  
>>>>> Modula-3
>>>>> objects are "objects" and can live on separate stacks.  However,
>>>>> as an optimization (not an uncommon one in these types of  
>>>>> systems),
>>>>> I'd like a space and time efficient representation for "small
>>>>> integers".  A tried-and-true method (it goes back at least to the
>>>>> Smalltalk-80 runtime) is to realize that pointers (in my case
>>>>> REFANYs) always point to word-aligned addresses.  We can then use
>>>>> integers that are congruent to 1, 2, and 3 (mod 4) to represent
>>>>> other types of data.
>>>>>
>>>>> What will happen if I LOOPHOLE such integers back and forth to
>>>>> REFANY?  Will the garbage collector just ignore them?  I wrote a
>>>>> test program that does it and it doesn't crash... except when you
>>>>> hit ctrl-C, it often dies with an assertion failure in
>>>>> RTCollector.m3.. (This both with my ancient PM3 on FreeBSD and a
>>>>> relatively recent update of CM3 on PPC_DARWIN.)  If the garbage
>>>>> collector somehow disapproves of these integers, is there any
>>>>> conceivable thing that would be broken by making necessary
>>>>> adjustments
>>>>> to the garbage collector such that it would just ignore them?
>>>>> Or is there a better way of solving my problem?
>>>>>
>>>>>       Best regards,
>>>>>         Mika
>>>>>
>>>>
>>>> -- 
>>>> -------------------------------------------------------------
>>>> 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