[LNO, mainline] Remember rs6000 builtins decls

[Geoff Keating]

On 09/06/2004, at 5:56 AM, Dorit Naishlos wrote:

>
> I think we all agree that introducing new tree codes is better than 
> using
> builtins.
>
> The only (potential) reason for not using tree codes is for operations 
> that
> are not general, and/or which are complicated in the sense that even if
> they had a tree-code it would probably be treated by the optimizers 
> like a
> black-box which hides a certain functionality, much like a builtin
> function. At least that was my assumption... would the tree-level
> optimizers be able to handle such tree-codes (which semantics they 
> don't
> "understand") any better than builtins?
>
> I think the question here is where we draw the line between operations 
> that
> qualify to have their own tree-codes, and those that don't. You seem to
> suggest that all operations qualify to have a tree-code. We were not 
> sure
> that adding so many new tree-code would be acceptable, and therefore 
> our
> approach was:
> (1) operations that are general enough, and which semantics are simple
> enough (i.e. - the optimizers can actually do something with them) 
> should
> definitely have a tree-code.
> (2) operations which we're not sure whether they qualify to have a
> tree-code (because they are less general or represent low level 
> details or
> are relatively complicated) would be expressed using builtins, and 
> possibly
> replaced with tree-codes later on, after people agree that these 
> tree-codes
> deserve to exist. I guess we can skip directly to propose these new
> tree-codes then...

I would suggest that the criteria should be "operations which the 
optimisers need to deal with should not be a builtin".  In particular, 
anything that the vectorizer can generate should be fully represented 
as trees.

I think this is just another way of saying what you said above, which 
is that if the optimiser is going to treat it as a 'black box' then it 
might as well be a builtin function.  (Such operations are pretty 
rare---even if the exact details of the operation are complicated, 
fold() can be taught to do it.)