bug#42147: 28.0.50; pure vs side-effect-free, missing optimizations?

[Mattias Engdegård]

2 juli 2020 kl. 12.59 skrev Andrea Corallo <andrea_corallo@yahoo.it>:

>> I still wonder if there is any reason to limit arithmetic constant
>> folding to the portable fixnum range. Given that we don't evaluate
>> fixnump or bignump at compile-time, what observable effects would
>> constant-folding, say, (ash 1 32) have? Advice from deeper thinkers
>> solicited!
> 
> I always thought the general idea is to respect the allocation side
> effect we have creating a bignum.  Not sure if the class of example you
> have in mind here fits this case.

Number allocation isn't a semantically visible effect and we probably don't 
want to change that. As far as I can tell, only fixnump and bignump can 
discriminate fixnums from bignums. There may be functions that only accept 
fixnums as arguments and thus fail with a different error, but I don't think we 
constant-fold any of them, and they would be easy to fix if we did.

It may be preferable to defer generation of very big numbers to run-time, to 
avoid evaluation of (ash 1 1000) at compile-time, but  such a limit should, if 
implemented, be independent of the fixnum limit (and likely higher).