Re: Designs for better debug info in GCC

[Mark Mitchell <mark@codesourcery.com>]

Alexandre Oliva wrote:

>> Yes, please.  I would very much like to see an abstract design
>> document on what you are trying to accomplish.
> 
> Other than the ones I've already posted, here's one:
> 
> http://dwarfstd.org/Dwarf3Std.php
> 
> Seriously.  There is a standard for this stuff. 

That's the specification for the encoding format.  I agree with you that
emitting incorrect debugging information, in the sense of declaring that
the location of a variable is in one place, even though its value is not
available in that place, is bad.  In -O0 code, I consider it a serious bug.

In -O2 code, I think it's still a bug, but with our current
infrastructure, we may have little choice: we either deny all knowledge
of the variable's location, or give one that's sometimes incorrect.
Which alternative is better depends on what you're trying to do with the
information; for interactive debugging, mostly-right is probably better
than nothing, whereas for some programmatic activities, the opposite may
be true.

If your goal is to avoid the information ever being wrong -- without
worrying about whether it is complete -- there is of course a trivial
solution: do not emit the information.  That is not a serious
suggestion, but it does provide a path to a serious suggestion, which I
gave earlier: conservatively emit location information you provide based
on what you can prove at the time you generate debugging information.
For example, if the value of "x" is in a register, and you cross a call
which might clobber that register value, then emit debugging information
that says that at that point the value is unavailable.  You could
probably do this kind of thing with relatively few changes to the GCC
internal representation; you would run a pass before debug-information
generation that attempted to prove dataflow properties about variables
and told you where values could reliably be found.

Your earlier messages, however, suggest that you are trying to do
something harder: emit information that is essentially both complete (in
the sense of providing as much information as possible about the
locations and values of variables) and correct (in the sense of never
giving incorrect information).  If you want to do that, you're going to
have to answer the harder questions, like "what line number corresponds
to this address?" and "what should the debugging information say that
the value of a variable is when it has been optimized away?"

If that's still your goal, then pointing at the DWARF3 specification
doesn't help.  Diego and I are asking you to confront these fundamental
questions about what information you want to provide and what the
correctness criteria are.

-- 
Mark Mitchell
CodeSourcery
mark@codesourcery.com
(650) 331-3385 x713