[dmd-internals] Debugging with D

[Robert Clipsham]

DMD currently outputs debug info differently depending on the switch you 
pass it. Currently it works as follows:

-g Act as D, using D extensions
-gc Act as C, no D extensions

This leads to an odd situation for debugging (here I focus on 
linux/other platforms that use DWARF, as my knowledge of debugging on 
windows is rather limited). Let's assume you're using a patched version 
of gdb or a sufficiently recent version from cvs/git.

Option 1: Compile with -g.

You fire up gdb with your crashing app. Oh look, gdb fails due to the D 
extensions to DWARF which aren't supported yet (unless the hypothetical 
app doesn't use dynamic arrays at all... what sort of app are you 
writing? :D).

Option 2: Compile with -gc.

You fire up gdb with your crashing app. Gdb recognises all your files 
end with .d and overrides the language setting, and gives you demangling 
and a nice debugging environment. However C doesn't support things like 
references, so all your classes appear as MyClass* rather than MyClass, 
same goes for all your functions with ref args (See bug #4149, this 
isn't the case yet, needs a small patch which I intend to write after 
this post).

Of course neither of these options are ideal, so I'm wondering what the 
correct way to proceed is. We have several options (in no particular order):

  * Add more -g* options

Eww. Doing this we could enable acting like D without D extensions, or 
acting like C++, or any number of other methods to get around the 
problems. On the other hand, there's more options to confuse the user, 
and they'll all be wondering why they have to compile with -gturnip 
rather than -g to get working debug info.

  * Remove the D extensions

We could remove the D extensions completely and stay within the spec. 
DWARF 4 is in drafting now, we could request the few features get added 
to the standard, then add in support when it's out. GCC is outputting 
some DWARF4 already by default as of 4.5, so users aren't going to 
complain about their ancient versions of gdb or other debuggers working. 
I think this is the best option, and would volunteer to poke the 
necessary people to get what we want in there.

  * Patch GDB

This needs someone who's familiar enough with gdb to write a patch, and 
it'll add ugliness to the source code from what I've seen of gdb's 
internals... I'm not sure such a patch would get included, even if it is 
possible to write one (I've just asked some gdb dev's, I'm awaiting a 
reply).

  * Change/replace the -gc option

Like the first option, but rather than adding a new -g* option, we 
replace -gc to use a language that supports more features, and go from 
there.

What I'm looking for is a consensus of some sort of what we think is the 
best way forward for the debugging situation in D. Do you have any 
better ideas than what I've suggested, or which of them do you think is 
the best way for us to proceed?

Thanks for your input,

Robert