Re: v2 of GDB hooks for debugging GCC

[Tom Tromey <tromey at redhat dot com>]

>>>>> "David" == David Malcolm <dmalcolm@redhat.com> writes:

Tom> Naughty.

David> We chatted about this at Cauldron; I haven't yet had a chance to
David> implement the magic bullet approach we discussed there.  In the
David> meantime, is there a API I can call to determine how safe this kludge
David> is?

Not right now.  You can just call the function and catch the exception
that occurs if it can't be done.

I think you can still run into trouble sometimes.  For example if the
user puts a breakpoint in one of the functions used by the
pretty-printer, and then does "bt", hitting the breakpoint while
printing the backtrace... not sure what happens then, maybe a crash.

Tom> I think you could set up the safe-path in the gcc .gdbinit.

David> Interesting idea - but .gdbinit itself seems to get declined, so I don't
David> think this can help.

Haha, I didn't think of that :-)

David> So I implemented a similar scheme, with all the prettyprinters in a
David> top-level "gcc" holder, but doing precise string matching on the
David> "unqualified" type, like in the original patch.

David> This works as before, and presumably works with the pretty-printer
David> management facilities; running this gives sane-looking output:
[...]

Nice.

David> How would one go about toggling the enabledness of a prettyprinter?  Is
David> this something that can only be done from python?

You can use "enable pretty-printer" and "disable pretty-printer".

David> I did see references to gdb.parameter("verbose") in gdb.printing
David> - how would one go about setting this?

"set verbose on"

I think few people use this setting though; probably best to do what
you're doing now.

David> +# Convert "enum tree_code" (tree.def and tree.h) to a dict:
David> +tree_code_dict = gdb.types.make_enum_dict(gdb.lookup_type('enum tree_code'))

One other subtlety is that this doesn't interact well with all kinds of
uses of gdb.  For example if you have a running gdb, then modify enum
tree_code and rebuild, then the pretty-printers won't notice this
change.

I guess it would be nice if we had pre-built caches for this kind of
this available upstream.  But meanwhile, if you care, you can roll your
own using events to notice when to invalidate data.

David> +    def __call__(self, gdbval):
David> +        type_ = gdbval.type.unqualified()
David> +        str_type_ = str(type_)

FWIW I think for RegexpCollectionPrettyPrinter you could write a
subclass whose __call__ first dereferenced a pointer, then called
super's __call__.  But your approach is just fine.

Tom