Re: Integrating DWARF2 CFA info

[Daniel Berlin <dan at www dot cgsoftware dot com>]

On Thu, 10 May 2001, Andrew Cagney wrote:

> > Well, actually, IMHO, it is.
> > The CFA info gives us the location of all saved registers for the entire
> > procedure.
> > It's always correct, even in the presence of optimization.
> > Given the pc for the frame, it can tell you exactly where all the saved
> > registers are, at that particular point in time.
> > Therefore, it is strictly better to use the CFA info, than scan the
> > instructions.
> >
> > So I see a strict hiearchy of what to use:
> > Dwarf2 CFA Info
> > <whatever else>
>
> This assumes that dwarf2 info is more correct than some other sort of info.

Also, not only is it always correct, even in the presence of optimization
(unless the compiler is broken), but accessing it and using it is
architecture independent.
All the arch dependent stuff is on the gcc side.

In other words, it removes the need for making a new gdb port have to
have functions to scan prologue and epilogue code to find register saves
and whatnot.

Also, unlike the scanning, DWARF2 CFA gives you a virtual frame for inline
functions (if you want it), so you can treat them like normal functions
calls, and the user doesn't have to care that the function was inlined.

While some other debug format may come along that can provide this
kind of info, there is absolutely no indication people are even
developing new debug formats. Most compiler vendors seem perfectly
content with DWARF2, because you can always extend it to handle some
weird new language semantics by adding new tags and attributes.
Past that, it can already (in 2.0) handle almost all optimized code
situations, and 2.1 can handle *all* of them.
What else do you need from a debug format?

And if this debug format *did* come along, that gave somehow even
better info to us (which isn't really possible, but that's besides
the point), it would still maintain a strict ordering of prefernece.

<whatever the new debug info format is>
DWARF2 CFA info
<everything else>

So I don't quite see what your objection to a stack is. There certainly is
a strict ordering, and dwarf2 info is certainly more correct than what we
do now, in all cases.  It's always better, never worse.

 --Dan