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Re: Continuations, basic blocks, loops and register allocation

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From:
Dan Sugalski
Date:
November 16, 2004 09:30
Subject:
Re: Continuations, basic blocks, loops and register allocation
Message ID:
a06200701bdbfe8212d80@[172.24.18.155]
At 11:52 AM -0500 11/16/04, Matt Fowles wrote:
>Leo~
>
>On Tue, 16 Nov 2004 16:37:04 +0100, Leopold Toetsch <lt@toetsch.at> wrote:
>>
>>
>>  Matt Fowles <ubermatt@gmail.com> wrote:
>>  > Leo~
>>
>>  > On Tue, 16 Nov 2004 09:23:24 +0100, Leopold Toetsch <lt@toetsch.at> wrote:
>>
>>  >>     i = 0
>>  >>   lp:
>>  >>     foo()
>>  >>     inc i
>>  >>     if i < 10 goto lp
>>
>>  > There is one thing I am not sure about here.  The loop will work
>>  > correctly for each seperate invocation of the appropriate
>>  > continuation.
>>
>>  No. Imagine, foo() is not a simple function anymore. Someone e.g. Jens
>>  Rieks[1], discovers that the algoritm is simpler implemented with
>>  continuations. So inside foo() the return continuation of foo() is
>>  copyied, stored elsewhere, passed along to another function, and that
>>  one now suddenly returns via this continuation to your loop.  If this
>>  invocation of the continuation would restore registers suddenly the loop
>>  will become an infinite one, as C<i> is always restored to zero.
>>
>>  [1] Have a look at runtime/parrot/library/Streams/Sub.imc
>>
>>  leo
>>
>
>I disagree with that analysis.

You would, however, in this case be incorrect.

The loop variable must have a backing store outside of the register 
set. The contents of registers must be assumed to be unreliable when 
a continuation is continued. If we declare that they are put back 
into the state they were when the continuation was taken, which is 
reasonable though not required, the values of non-reference type 
registers (ints and floats) will be reset. The rference type 
registers (strings and PMCs) will also be reset so the pointers to 
the string/pmc structs will be what they were when the continuation 
was taken, but as they are references the referred-to thing may have 
changed and the changed value will be seen.
-- 
				Dan

--------------------------------------it's like this-------------------
Dan Sugalski                          even samurai
dan@sidhe.org                         have teddy bears and even
                                       teddy bears get drunk

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