Re: [RFC] Empty result of "info types inner": Is this a bug?

Doug Evans
2015-01-07T08:55:10Z

On Tue, Jan 6, 2015 at 1:44 AM, Doug Evans <xdje42@gmail.com> wrote:
> On Sun, Jan 4, 2015 at 7:04 PM, Joel Brobecker <brobecker@adacore.com> wrote:
>>> Technically, it's a bug that the first "info types inner" didn't
>>> print anything, but is this a bug we're willing to accept for
>>> performance reasons? I want to make the case that the answer is "No."
>>
>> My inclination as well, and you'll probably see in dwarf2read.c
>> a few added blocks of code that say "if (ada an some other langs),
>> add children into global scope. That was for things such as putting
>> a breakpoint on a nested subprogram, for instance.
>
> For completeness sake,
> another way to solve this particular bug is to traverse all the debug
> info but not expand every (matching) symtab.
> Yeah, it'll be slower, but info var/fun/types doesn't scale,
> plus regexp matching means we have to make "o::b" find "foo::bar",
> and is it better to try to make that scale, or provide the user with
> something else that does scale for the common case(s)?
> IOW, what are the common uses
> of info var/fun/types and can we require a different argument besides
> a simple regex such that the implementation can scale?
> [We'd still keep info/var/types, but the implementation, while fixing
> the bug under discussion, would not be intended to be fast for
> large programs, and nor would it result in significantly increased
> memory usage by gdb. I can imagine it just walking all the DWARF
> info but not expanding any symtabs.]

Heh.
I have a sandbox where I've been playing with ways of
lazily demangling minsyms (in the background, or just for
lookup, or whatever ... it's just a set of experiments,
and at least some of which has been looked at before).

One thing I found is that even with dwarf debug info there's
another scenario where we require minsyms.
m-static.exp has this: print 'gnu_obj_1::method()::sintvar'.
It's a static local var of method().
That doesn't work without minsyms, even with debug
info, because the variable is a static local to a function
and thus goes in the function's symtab and not
STATIC_BLOCK (as it should, though one
could I think instead put in STATIC_BLOCK, not
that I'm advocating actually doing that).

My current patch to look up a component of a symbol name
at a time doesn't handle this case, but it could, it's a
straightforward extension, and thus I can make
"print 'gnu_obj_1::method()::sintvar'" from m-static.exp
work without requiring minsyms - which I like: with debug
info we should be falling back to minsyms only when we
actually need to.

We could instead put function/method static locals in the index(/psyms),
but now we're getting into needing to do more detailed reading
of function DIEs (and thus now building psyms is getting measurably
more expensive), and I didn't plan to go down that road
(though I still plan to collect some data on it).

Thus even with a more complete index(/psyms table)
it'd be useful to have some form of my current patch
(I'll submit it sometime soon, pending what the data says).

Whether to solve "info types inner" by having it appear in
psyms/index then becomes an open question.
[The bug in this thread is a proxy for a general class of problems,
but it doesn't cover everything. E.g., one could extend "info var"
to find/print function static locals.
E.g., by traversing the debug info at the time the user invokes
info var/fun/types, maybe as an option.]