Re: [PR59319] output friends in debug info

[Jason Merrill <jason@redhat.com>]

On 12/07/2017 04:04 PM, Alexandre Oliva wrote:
> On Apr  7, 2017, Alexandre Oliva <aoliva@redhat.com> wrote:
> 
>> On Mar 21, 2017, Alexandre Oliva <aoliva@redhat.com> wrote:
>>> On Jan 27, 2017, Alexandre Oliva <aoliva@redhat.com> wrote:
>>>> On Oct 19, 2016, Alexandre Oliva <aoliva@redhat.com> wrote:
>>>>> On Sep 23, 2016, Alexandre Oliva <aoliva@redhat.com> wrote:
>>>>>> On Aug 30, 2016, Alexandre Oliva <aoliva@redhat.com> wrote:
>>>>>>> Handling non-template friends is kind of easy, [...]
>>>>>> Ping?
>>>>> Ping?  (conflicts resolved, patch refreshed and retested)
>>>> Ping?  (trivial conflicts resolved)
>>> Ping?  https://gcc.gnu.org/ml/gcc-patches/2017-01/msg02112.html
>> Ping?
> Ping? (refreshed, retested)
> 
> [PR59319] output friends in debug info
> 
> Handling non-template friends is kind of easy, but it required a bit
> of infrastructure in dwarf2out to avoid (i) forcing debug info for
> unused types or functions: DW_TAG_friend DIEs are only emitted if
> their DW_AT_friend DIE is emitted, and (ii) creating DIEs for such
> types or functions just to have them discarded at the end.  To this
> end, I introduced a list (vec, actually) of types with friends,
> processed at the end of the translation unit, and a list of
> DW_TAG_friend DIEs that, when we're pruning unused types, reference
> DIEs that are still not known to be used, revisited after we finish
> deciding all other DIEs, so that we prune DIEs that would have
> referenced pruned types or functions.
> 
> Handling template friends turned out to be trickier: there's no
> representation in DWARF for templates.  I decided to give debuggers as
> much information as possible, enumerating all specializations of
> friend templates and outputting DW_TAG_friend DIEs referencing them as
> well.  I considered marking those as DW_AT_artificial, to indicate
> they're not explicitly stated in the source code, but in the end we
> decided that was not useful.  The greatest challenge was to enumerate
> all specializations of a template.  It looked trivial at first, given
> DECL_TEMPLATE_INSTANTIATIONS, but it won't list specializations of
> class-scoped functions and of nested templates.  For other templates,
> I ended up writing code to look for specializations in the hashtables
> of decl or type specializations.  That's not exactly efficient, but it
> gets the job done.

How inefficient is it, exactly?  I'm concerned about the impact on 
compile time of scanning the entire hash table for each friend 
declaration in a template instantiation.  This sounds prohibitive for 
template-heavy code that uses friends.

I wonder about changing register_specialization to fill out 
DECL_TEMPLATE_INSTANTIATIONS for more templates (even more than you 
already do).

> +  /* At DETAIL level 0, returns non-NULL if the named class TYPE has
> +     any friends, NULL otherwise.  At higher detail levels, return a
> +     tree list with the friends of the named class type.  Each
> +     TREE_VALUE contains one friend type or function decl.  For
> +     non-template friends, TREE_PURPOSE is NULL.  For template friend
> +     declarations, the returned entries depend on the DETAIL level.
> +     At level 1, and only at level 1, an entry with NULL TREE_VALUE
> +     and non-NULL TREE_PURPOSE will START the returned list to
> +     indicate the named class TYPE has at least one template friend.
> +     At level 2, each template friend will be in an entry with NULL
> +     TREE_VALUE, and with the TEMPLATE_DECL in TREE_PURPOSE.  At level
> +     3, instead of a NULL TREE_VALUE, we add one entry for each
> +     instantiation or specialization of the template that fits the
> +     template friend declaration, as long as there is at least one
> +     instantiation or specialization; if there isn't any, an entry
> +     with NULL TREE_VALUE is created.  A negative detail level will
> +     omit non-template friends from the returned list.  */

The calls I see only seem to use details 0 and 3, while I would expect 
level 3 to only be used with -g3.  What is the purpose of the negative 
level?

Jason