Re: tail call optimizations

[Geoff Keating <geoffk at cygnus dot com>]

Jeffrey A Law <law@cygnus.com> writes:

> In general, if a particular call might throw, then we can not perform
> tail call optimizations.
> 
> Why?  Because we'll have no way to know the proper return pointer since the
> callee returns directly to the parent of the caller.  And without the proper
> return pointer, how are we supposed to know where to vector the exception?

This means that you can't do tail call optimisations when the tail
call is in a region where exceptions can be caught (in that
procedure), but in that case it's not really a 'tail call'.

Is there a problem I don't see where if you have

int foo ()  { bar(); }

and bar() throws, you can't just pass the exception up to foo's
caller, just as if foo's caller had called bar directly?

> I don't know if this is related to your problem, but it is the primary
> reason why (IMHO) tail calls optimizations are not particularly useful
> for C++.

I was going to say that there would be _fewer_ tail calls in C++, but
after a little thought I decided I couldn't be sure; because
destructors may very often be tail calls.

-- 
- Geoffrey Keating <geoffk@cygnus.com>