Re: [PATCH] Show optimized out local variables in "info locals"

Simon Marchi
2017-11-15T04:54:39Z

On 2017-11-14 05:44 PM, Yao Qi wrote:
> On Tue, Nov 14, 2017 at 5:08 PM, Simon Marchi <simon.marchi@ericsson.com> wrote:
>>
>> However, this change reveals what I think is a bug in GDB, see:
>>
>> http://lists.dwarfstd.org/pipermail/dwarf-discuss-dwarfstd.org/2017-September/004394.html
>>
>
> IMO, it is not necessary to emit DW_TAG_lexical_block in concrete instances.
> See comment #4 in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=37801
> At least, looks gcc generates unnecessary debug information, and we need
> to fix GCC somehow.
>
> Whether it is a bug in GDB or not, I don't know. The answer depends on it is
> *unnecessary* or *wrong* to have DW_TAG_lexical_block in concrete instances.

I had an email discussion with some gcc developers (Nathan Sidwell, Richard Biener,
Jason Merill) after a chat on IRC. Unfortunately, they answered privately so it's
not on dwarf-discuss. I'll try to update the thread on dwarf-discuss with their
answers tomorrow, for future reference. But the gist of it was:

Richard said:

> I think the lexical block is just the function scope itself and the inliner
> inserts this BLOCK which then corresponds to the DW_TAG_inlined_subroutine.
> I suppose we should avoid emitting that BLOCK itself as a DW_TAG_lexical_block
> but use the emitted DW_TAG_inlined_subroutine for that.
>
> Not sure if I remember the details correctly.
>
> I don't think the DWARF is invalid btw, with early LTO debug we have plenty of
> abstract origins where source and destination context don't match 1:1. We're
> just using it as a "get some more info from this DIE" link which I think is
> all that is documented as semantics (though the 'inline' term pops up too
> often there and the relation to DW_AT_specification is unclear to me though
> the latter is restricted to DW_TAG_subroutine AFAIR).

Jason said (replying to Richard):

>> I think the lexical block is just the function scope itself and the inliner
>> inserts this BLOCK which then corresponds to the DW_TAG_inlined_subroutine.
>> I suppose we should avoid emitting that BLOCK itself as a DW_TAG_lexical_block
>> but use the emitted DW_TAG_inlined_subroutine for that.
>
> Agreed. It's curious that we would generate the lexical block in the
> inlined instance and not the abstract.
>
>> I don't think the DWARF is invalid btw, with early LTO debug we have plenty of
>> abstract origins where source and destination context don't match 1:1. We're
>> just using it as a "get some more info from this DIE" link which I think is
>> all that is documented as semantics (though the 'inline' term pops up too
>> often there and the relation to DW_AT_specification is unclear to me though
>> the latter is restricted to DW_TAG_subroutine AFAIR).
>
> Also agreed, GDB ought to be able to handle this situation.
>
> So, bugs on both sides...

So even though there might be something to fix in GCC, I think we'll have to handle
the current case in GDB as well.

Simon