Re: [PATCH 02/11] gdb: introduce intrusive_list, make thread_info use it

[Simon Marchi <simon.marchi@polymtl.ca>]

On 2021-07-07 7:46 a.m., Pedro Alves wrote:
>> I think these are all good ideas for improvements, but I'd rather keep
>> them for later (if someone wants to implement them, I'm not sure I
>> will).  We could bike-shed for a while on how to display a thread_info,
>> what to include / what to exclude, etc.  I think that my original
>> proposal is strictly better than what we have today, in the sense that
>> today you just can't print the whole list of threads, so we don't lose
>> anything.
> 
> The discussion about the thread_info pretty printer, yes, agreed.
> 
> However, the discussion on the list printer itself, one point that we should
> settle discuss a bit more is whether it is really the right approach to make it
> show children as pointers.  
> 
> Showing pointers really looks not useful to me.  The only thing I think I get
> out of it is that there are elements in the list.
> 
> I mean, AFAICT, even the std::list printer shows objects, not pointers,
> for instance:
> 
>  (gdb) p my_list
>  $1 = std::__cxx11::list = {[0] = 1, [1] = 2, [2] = 3}
> 
> Hmm, funny, it shows the indexes, even though I don't have
> array-indexes off.  Guess it must be using "map", but I haven't checked.

In fact, it doesn't specify any display_hint.  The [%d] comes from here:

   https://gitlab.com/gnutools/gcc/-/blob/trunk/libstdc%2B%2B-v3/python/libstdcxx/v6/printers.py#L313

So it's as if the children of the list are members of a structure, with
names [0], [1], and so on.  That doesn't look totally right to me but...
I'll pretend I didn't see it.

> Imagine we were using std::list for thread_info objects.
> 
> But I don't think that that discussion should block the main change
> from going in.  I'd support moving the printer out to a separate patch,
> even.

Well, the printer mostly works, so I think it's ok to include it.  I
agree with you that printing pointers is not ideal.  For objects smaller
than thread_info, it would certainly be useful to see the objects and
not pointers.  So I will make the quick change to make the list children
be objects and pointers.  Sure, printing the thread list will be very
verbose, but the same argument I made earlier applies: it's not a
regression, since printing the whole list is simply not possible today.

Simon