[mercury-users] Structure reuse

[Nancy Mazur]

Hi Simon,

I disagree here a little bit with you :-)

>>It would be nice to be able to write code like this and have the compiler
>>determine that it can update the tree in-place, eliminating unnecessary
>>copying and memory allocation, improving locality, reducing memory usage
>>and so on. I have no idea whether that is feasible though, or whether it 
>>would require assistance from the programmer by specifying unique modes or 
>>something like that.
> 
> 
> In practice, if you want to be sure that the optimization takes place
> you'll need to specify unique modes (sorry, not working yet) rather
> than relying on automagic compiler analyses.  Relying on the compiler
> to do the work will result in difficult to track down performance bugs
> where a data structure which should be single threaded actually isn't.

Even if you let the programmer annotate unique modes by hand, I thought 
the philosophy of the Mercury compiler was always not to trust the 
programmer, and therefore an automatic verification of these annotations 
should still be done. This verification requires almost the same 
analysis as the automatic derivation.
But some cooperation human-compiler is interesting: let the compiler 
deduce automatically the reuse, let the programmer indicate some hints 
where reuse should be possible according to him, and then provide the 
adequate feedback if the two views do not match.

Nancy

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