Re: Does gcc violate the ia64 ABI?

["John David Anglin" <dave at hiauly1 dot hia dot nrc dot ca>]

> On Fri, May 16, 2003 at 05:43:18PM -0700, Richard Henderson wrote:
> > On Fri, May 16, 2003 at 03:25:08PM -0700, H. J. Lu wrote:
> > > d. At procedure return, gp must be valid (for the returning prodecure).
> > > This allows the compiler to optimize calls known to be local (i.e., the
> > > exceptions to Rule 'c').
> > 
> > I was not aware of this clause.  I'd have sworn it wasn't a part
> > of the ABI at one time...
> > 
> > Indeed, if this clause is truely correct, then virtually all of
> > the tail-call possibilities on ia64 are invalid.  Yes, statics
> > still can be done, but that's not nearly as frequent.
> 
> Gcc can skip gp save/restore across a local call. Right now, gcc
> saves and restores gp across a local call.

Your example shows a tail-call to foo.  Gp is obviously valid at the call.
There is no return in your example, so 'd' doesn't apply.

Note that skipping the gp save/restore across a local call is a "compiler"
optimization.  If you save and restore gp across local calls, then
it's likely possible to skip the save and restore across the last call in
a function if gp is not used after the last call, including the return
path to the caller.  I don't have a good feeling as to which of these
two optimizations is more beneficial.  I would judge that the latter
treatment would be in slight violation of the ia64 ABI, but the violation
would be local to a translation unit.

We currently do the latter optimization on the pa but I am wondering
if it would be better to skip the gp save/restore across local calls
instead.

Dave
-- 
J. David Anglin                                  dave.anglin@nrc-cnrc.gc.ca
National Research Council of Canada              (613) 990-0752 (FAX: 952-6602)