Argument pointers, dwarf and prologue analysis

[Randolph Chung]

> 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