[Patch, fortran] PR20541 - allocatable components (TR15541)

[Tobias Schlüter]

Paul Thomas <paulthomas2@wanadoo.fr> wrote on Fri, 06 Oct 2006:
>> Out of curiousity, why didn't you use alloca instead of doing all   
>> this bookkeeping?  Maybe because you would have needed bookkeeping   
>> anyway  for the case where the allocatable component would have   
>> been >  -fmax-stack-var-size?  Or is there a simpler reason?
>
> Ignorance.
>
> Having looked at the spec. for alloca, you are right, we would have had
> to branch for >  -fmax-stack-var-size; otherwise the warnings in the
> gcc manual would hold good.  I will put a TODO in the code somewhere.
>
> It would simplify the code considerably - take a look for some of the
> testcases!

For the record, I don't think a size restriction on the allocated  
components would render a patch for the TR unacceptable.  Nor do I  
think that your patch is any less acceptable because you decided to  
manually allocate and deallocate the memory.

OTOH I think it would be a good thing if you could estimate the  
probable savings in terms of code size and complexity a patch with  
alloca (or whatever one calls it when one means the portable  
equivalent) would bring.

One thing that may render this more difficult than one expects, is  
that C has no notion of contained procedures, which may interfere with  
stack-based memory allocation.  I.e. __builtin_alloca may not be well  
tested in all circumstance that are interesting to us.  Maybe a  
middle-end person can weigh in on this?

(As an aside, I wonder if we should allocate temporaries on the stack  
instead of the heap, as we do currently.  Going via a compiler builtin  
may be more transparent to the optimizers.)

- Tobi


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