From: "John David Anglin" <dave@hiauly1.hia.nrc.ca>
To: gcc@gcc.gnu.org, rth@redhat.com, schwab@suse.de, hjl@lucon.org
Subject: Re: Does gcc violate the ia64 ABI?
Date: Sat, 17 May 2003 18:58:00 -0000 [thread overview]
Message-ID: <200305171858.h4HIwT9t014002@hiauly1.hia.nrc.ca> (raw)
> 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)
next reply other threads:[~2003-05-17 18:58 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-05-17 18:58 John David Anglin [this message]
2003-05-17 20:43 ` H. J. Lu
2003-05-17 23:27 ` John David Anglin
2003-05-17 23:45 ` H. J. Lu
2003-05-18 3:22 ` John David Anglin
2003-05-18 4:10 ` H. J. Lu
2003-05-18 23:00 ` John David Anglin
2003-05-19 3:08 ` Fergus Henderson
2003-05-19 6:18 ` Richard Henderson
2003-05-19 15:00 ` H. J. Lu
2003-05-19 15:06 ` Jakub Jelinek
2003-05-19 15:27 ` H. J. Lu
2003-05-19 20:43 ` Richard Henderson
-- strict thread matches above, loose matches on Subject: below --
2003-05-16 23:37 John David Anglin
2003-05-16 21:55 H. J. Lu
2003-05-16 22:10 ` Andreas Schwab
2003-05-16 22:27 ` H. J. Lu
2003-05-17 0:50 ` Richard Henderson
2003-05-17 5:52 ` H. J. Lu
2003-05-17 18:58 ` Richard Henderson
2003-05-17 22:02 ` H. J. Lu
2003-05-18 0:08 ` Richard Henderson
2003-05-18 3:10 ` H. J. Lu
2003-05-20 23:21 ` H. J. Lu
2003-05-21 3:22 ` Richard Henderson
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