Debug info questy (was Re: [llvm] r256003 - [ThinLTO/LTO] Don't link in unneeded metadata)

Thu Jan 14 16:50:23 PST 2016

On Thu, Jan 14, 2016 at 4:46 PM, Adrian Prantl <aprantl at apple.com> wrote:

>
> On Jan 14, 2016, at 4:06 PM, David Blaikie <dblaikie at gmail.com> wrote:
>
>
>
> On Thu, Jan 14, 2016 at 4:00 PM, Adrian Prantl <aprantl at apple.com> wrote:
>
>>
>> On Jan 8, 2016, at 10:18 AM, David Blaikie <dblaikie at gmail.com> wrote:
>>
>>
>>
>> On Wed, Jan 6, 2016 at 10:38 AM, Adrian Prantl <aprantl at apple.com> wrote:
>>
>>>
>>> On Jan 6, 2016, at 10:27 AM, Eric Christopher <echristo at gmail.com>
>>> wrote:
>>>
>>>
>>>
>>> On Wed, Jan 6, 2016 at 10:04 AM Duncan P. N. Exon Smith <
>>> dexonsmith at apple.com> wrote:
>>>
>>>> +aprantl
>>>>
>>>> > On 2016-Jan-06, at 09:46, Teresa Johnson <tejohnson at google.com>
>>>> wrote:
>>>> >
>>>> > +dexonsmith,echristo,blaikie who are way more familiar with debug
>>>> info than I
>>>> > +pcc who also found this and contacted me off-list with a possible
>>>> solution
>>>> >
>>>> > See also https://llvm.org/bugs/show_bug.cgi?id=26037 that I updated
>>>> > with a test case and more detailed analysis.
>>>> >
>>>> > The issue is essentially that we no longer pull in a DISubprogram on
>>>> > an LTO link with this patch because the associated function was not
>>>> > linked in. However, it had been referenced by a DIImportedEntity which
>>>> > is leaving behind a DIImportedEntity with no entity:
>>>> >
>>>> > !11 = !DIImportedEntity(tag: DW_TAG_imported_declaration, scope: !8,
>>>> line: 2)
>>>> >
>>>> > which in turn causes an assert during debug generation. The 3
>>>> > solutions I mention there are:
>>>> >
>>>> > 1) Change findNeededSubprograms to look at entities and conservatively
>>>> > remove any subprograms reached from there from the UnneededSubprograms
>>>> > list
>>>> > 2) Remove any DIImportedEntity that no longer have an entity
>>>> > 3) Change the debug generation code to handle DIImportedEntity with a
>>>> > null entity
>>>> >
>>>> > pcc has a tentative solution that implements 3) above.
>>>> >
>>>> > Debug info experts - what is the right thing to do here? Is it legal
>>>> > to have a DIImportedEntity with no entity? If so, pcc's solution
>>>> > (solution 3 above) is the right one. Otherwise I should implement 1)
>>>> > or 2).
>>>>
>>>> My intuition says that (2) is the correct solution, but Adrian would
>>>> know best.
>>>>
>>>>
>>> It's a bit iffy:
>>>
>>> 3.2.3 Imported (or Renamed) Declaration Entries Some languages support
>>> the concept of importing into or making accessible in a given unit
>>> declarations made in a different module or scope. An imported declaration
>>> may sometimes be given another name. An imported declaration is represented
>>> by one or more debugging information entries with the tag
>>> DW_TAG_imported_declaration. When an overloaded entity is imported, there
>>> is one imported declaration entry for each overloading.
>>>
>>> which means that we're probably looking at 1 as we care about
>>> declarations for entities rather than defining declarations. That said,
>>> unless we change the metadata to be a declaration of a subprogram rather
>>> than a defining declaration we're going to have a bad time further down the
>>> line.
>>>
>>>
>>> Right. Dropping the imported entity because it’s not in the current
>>> translation unit would change the overload resolution in the debugger.
>>>
>>
>> I'm not sure I follow here - if the function is never emitted (which it
>> isn't in this translation unit) where would overload resolution come into
>> it?
>>
>>
>> The silly example I had in mind was something like:
>>
>> namespace N {
>>   void foo();
>> }
>>
>> void foo() {
>>   using N::foo;
>>   // stop here and evaluate "foo()"
>>
>
> Right - but in this module, there is no definition of 'foo', so there's no
> way to stop at it.
>
> In whatever other module has a definition of 'foo', we would expect the
> using declaration's debug info will be present there, no?
>
> If I'm understanding the situation correctly...
>
>
> It almost looks like we’re thinking of different situations. My
> understanding of the problem was that there is an imported entity
> referencing a subprogram that was optimized away, and the question was
> whether it was safe to remove the imported entity if the subprogram doesn’t
> exist any more. The point I was trying to make was that it is never safe to
> remove an imported entity because it changes the name lookup at the site of
> the import (::foo in my example).
>

But it's ::foo that's been optimized away, right? So in what situation are
you doing name lookup in that scope?

I think this is about the same as if the function was never called/code
generated (eg: Clang skips emitting functions (& there imported entities,
etc) if the function is not called (& it's not externally visible - so
inline, or static, etc)). Seems OK to have the same result if a function is
optimized away, or otherwise removed (in the case of ThinLTO it's not so
much that the subprogram is optimized away, but that it's not imported into
a certain module (or not needed because something else was removed, I
guess))

>
>   if (false)
>>     foo(); // optimized out
>> }
>>
>> -- adrian
>>
>> OK, /maybe/ if the subprogram had a local type that leaked out somewhere
>> (eg: through a template instantiation that lead to a global variable that
>> stayed alive despite the outer function being removed) and you were inside
>> a member function of that local type you could rely on the imported
>> declaration to provide name lookup. But in that case you'd still need to
>> preserve the subprogram for the nested type to be the child of.
>>
>> Adrian: What name resolution situation did you have in mind?
>>
>>
>> Teresa: Have you looked at/hit this issue with local types? I might be
>> able to conjure a specific example (using the aforementioned template
>> scenario) that could tickle it in that direction if that'd be helpful.
>>
>> - Dave
>>
>>
>>>
>>>
>>>
>>>> In the meantime, if (3) isn't too intrusive, it would be worth doing
>>>> temporarily to unbreak the bot...
>>>>
>>>
>>> This is absolutely ok as long as it's temporary while we figure this out.
>>>
>>>
>>> Agreed.
>>>
>>>
>>> -eric
>>>
>>>
>>>>
>>>> >
>>>> > Thanks,
>>>> > Teresa
>>>> >
>>>> >
>>>> > On Tue, Jan 5, 2016 at 4:24 PM, Ahmed Bougacha <
>>>> ahmed.bougacha at gmail.com> wrote:
>>>> >> On Fri, Dec 18, 2015 at 9:51 AM, Teresa Johnson via llvm-commits
>>>> >> <llvm-commits at lists.llvm.org> wrote:
>>>> >>> Author: tejohnson
>>>> >>> Date: Fri Dec 18 11:51:37 2015
>>>> >>> New Revision: 256003
>>>> >>>
>>>> >>> URL: http://llvm.org/viewvc/llvm-project?rev=256003&view=rev
>>>> >>> Log:
>>>> >>> [ThinLTO/LTO] Don't link in unneeded metadata
>>>> >>>
>>>> >>> Summary:
>>>> >>> Third patch split out from http://reviews.llvm.org/D14752.
>>>> >>>
>>>> >>> Only map in needed DISubroutine metadata (imported or otherwise
>>>> linked
>>>> >>> in functions and other DISubroutine referenced by inlined
>>>> instructions).
>>>> >>> This is supported for ThinLTO, LTO and llvm-link --only-needed, with
>>>> >>> associated tests for each one.
>>>> >>>
>>>> >>> Depends on D14838.
>>>> >>>
>>>> >>> Reviewers: dexonsmith, joker.eph
>>>> >>>
>>>> >>> Subscribers: davidxl, llvm-commits, joker.eph
>>>> >>>
>>>> >>> Differential Revision: http://reviews.llvm.org/D14843
>>>> >>>
>>>> >>> Added:
>>>> >>>    llvm/trunk/test/Linker/Inputs/only-needed-debug-metadata.ll
>>>> >>>    llvm/trunk/test/Linker/only-needed-debug-metadata.ll
>>>> >>> Modified:
>>>> >>>    llvm/trunk/include/llvm/Transforms/Utils/ValueMapper.h
>>>> >>>    llvm/trunk/lib/Linker/IRMover.cpp
>>>> >>>    llvm/trunk/lib/Transforms/Utils/ValueMapper.cpp
>>>> >>>    llvm/trunk/test/Linker/thinlto_funcimport_debug.ll
>>>> >>>    llvm/trunk/test/tools/gold/X86/Inputs/linkonce-weak.ll
>>>> >>>    llvm/trunk/test/tools/gold/X86/linkonce-weak.ll
>>>> >>>
>>>> >>> Modified: llvm/trunk/lib/Linker/IRMover.cpp
>>>> >>> URL:
>>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Linker/IRMover.cpp?rev=256003&r1=256002&r2=256003&view=diff
>>>> >>>
>>>> ==============================================================================
>>>> >>> --- llvm/trunk/lib/Linker/IRMover.cpp (original)
>>>> >>> +++ llvm/trunk/lib/Linker/IRMover.cpp Fri Dec 18 11:51:37 2015
>>>> >>> @@ -487,6 +495,16 @@ class IRLinker {
>>>> >>>
>>>> >>>   void linkNamedMDNodes();
>>>> >>>
>>>> >>> +  /// Populate the UnneededSubprograms set with the DISubprogram
>>>> metadata
>>>> >>> +  /// from the source module that we don't need to link into the
>>>> dest module,
>>>> >>> +  /// because the functions were not imported directly or via an
>>>> inlined body
>>>> >>> +  /// in an imported function.
>>>> >>> +  void findNeededSubprograms(ValueToValueMapTy &ValueMap);
>>>> >>
>>>> >> Hey Teresa,
>>>> >>
>>>> >> This has been causing bootstrap assertion failures, I think because a
>>>> >> DISubprogram referenced by DIImportedEntity should also be considered
>>>> >> needed.
>>>> >>
>>>> >> I filed PR26037, could you please have a look?
>>>> >>
>>>> >> Thanks!
>>>> >> -Ahmed
>>>> >
>>>> >
>>>> >
>>>> > --
>>>> > Teresa Johnson | Software Engineer | tejohnson at google.com |
>>>> 408-460-2413
>>>
>>>
>
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