Re: Designs for better debug info in GCC

[Diego Novillo <dnovillo at google dot com>]

On 11/25/07 3:43 PM, Mark Mitchell wrote:

> My suggestion (not as a GCC SC member or GCC RM, but just as a fellow
> GCC developer with an interest in improving the compiler in the same way
> that you're trying to do) is that you stop writing code and start
> writing a paper about what you're trying to do.
>
> Ignore the implementation.  Describe the problem in detail.  Narrow its
> scope if necessary.  Describe the success criteria in detail.  Ideally,
> the success criteria are mechanically checkable properties: i.e., given
> a C program as input, and optimized code + debug information as output,
> it should be possible to algorithmically prove whether the output is
> correct.

Yes, please.  I would very much like to see an abstract design document 
on what you are trying to accomplish.  I have been trying to follow this 
thread but I've gotten lost.  It's full of implementation details, 
rhetoric and high-level discussion.

I would like to see exactly what Mark is asking for.  Perhaps a 
presentation in next year's Summit?  I don't think I understand the goal 
of the project.  "Correct debugging info" means little, particularly if 
you say that it's not debuggers that you are thinking about.

It's certainly worrisome that your implementation seems to be intrusive 
to the point of brittleness.  Will every new optimization need to think 
about debug information from scratch and refrain from doing certain 
transformations?

In my simplistic view of this problem, I've always had the idea that -O0 
-g means "full debugging bliss", -O1 -g means "tolerable debugging" 
(symbols shouldn't disappear, for instance, though they do now) and -O2 
-g means "you can probably know what line+function you're executing".

But you seem to be addressing other problems.  And it even seems to me 
that you want debugging information that is capable of deconstructing 
arbitrary transformations done by the optimizers.  But I think I'm just 
lost in this thread, so a high-level design document would be perfect to 
 expose your ideas.


Diego.