From: Randolph Chung <randolph@tausq.org>
To: Daniel Jacobowitz <drow@false.org>
Cc: Jim Blandy <jimb@red-bean.com>, gdb@sources.redhat.com
Subject: Re: Argument pointers, dwarf and prologue analysis
Date: Mon, 21 Nov 2005 15:11:00 -0000 [thread overview]
Message-ID: <4381E376.5020003@tausq.org> (raw)
In-Reply-To: <20051121140809.GA18686@nevyn.them.org>
> So you're saying you have to do (A) backwards code analysis from the
> call site to compute r29, and then (B) forward code analysis from the
> function start to compute r20 relative to r29, and then (C) use dwarf
> debug information to find arguments relative to r20, and then (D) hope
> that GCC doesn't decide to use r20 for something different, since it
> doesn't seem to be a traditional "frame pointer"? There's a lot more
> prayer in that method than there really ought to be.
Yeah...., except it doesn't have to be r20, it can be any register.
> And GCC will
> probably take some severe liberties with where the set of %r29 is, so
> I'm skeptical that you can do the call site analysis.
yes, that's what i worry about too....
> How does GDB cope today? A lot of guessing and failing?
It doesn't -- basically I discovered this because on hppa64-*, if you do
a backtrace, gdb prints completely bogus arguments for anything other
than the innermost frame.
> I'd recommend ignoring the call site/ABI bits. They're too unreliable.
> The real problem is that you can't recover %r20. You're thinking in
> terms of finding the offset relative to the CFA where these things are
> stored, which is why you've got a solution that relies on analyzing the
> caller. But think about the problem in terms of the callee for a
> moment. The function doesn't care what the offset relative to the CFA
> is. It cares what the offset relative to the incoming %r29 and later
> %r20 is. And then, if it makes any calls, it had better still know
> where its own arguments went! So that should be in the debug info
> somehow.
Oh, I agree -- my first instinct is that gcc is wrong in writing the
debug info based on r20.... in fact I filed this as
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24943 before I wrote my
email to this list; but some emails I had with Dave convinced me that
the problem is not as easy as I thought it would be, so I thought I'd
bring it up here...
> We've got dwarf unwind information to play with now. Even with the new
> additions in dwarf3, it's not so good at representing what you need.
> I'd need to see a larger example to know how we could, or couldn't, do
> this.
What would you like to see?
randolph
next prev parent reply other threads:[~2005-11-21 15:11 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2005-11-21 5:50 Randolph Chung
2005-11-21 7:42 ` Jim Blandy
2005-11-21 10:32 ` Randolph Chung
2005-11-21 14:08 ` Daniel Jacobowitz
2005-11-21 15:11 ` Randolph Chung [this message]
2005-11-21 15:26 ` Daniel Jacobowitz
2005-11-22 0:07 ` Randolph Chung
2005-11-22 5:15 ` Jim Blandy
2005-11-22 6:39 ` Randolph Chung
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=4381E376.5020003@tausq.org \
--to=randolph@tausq.org \
--cc=drow@false.org \
--cc=gdb@sources.redhat.com \
--cc=jimb@red-bean.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).