Debug info questy (was Re: [llvm] r256003 - [ThinLTO/LTO] Don't link in unneeded metadata)
Duncan P. N. Exon Smith via llvm-commits
llvm-commits at lists.llvm.org
Wed Jan 6 14:20:20 PST 2016
> On 2016-Jan-06, at 14:17, Teresa Johnson via llvm-commits <llvm-commits at lists.llvm.org> wrote:
>
> On Wed, Jan 6, 2016 at 2:11 PM, Adrian Prantl <aprantl at apple.com> wrote:
>>
>>> On Jan 6, 2016, at 1:33 PM, Teresa Johnson <tejohnson at google.com> wrote:
>>>
>>> Here is the change I made which fixes the test case. I can add some
>>> comments and a test case, but let me know if this looks ok otherwise.
>>> As Eric noted, the DISubroutine for a() is marked as a definition,
>>> which it isn't actually, but this was the behavior before this patch
>>> and seems to work ok.
>>>
>>> Thanks,
>>> Teresa
>>>
>>> diff --git a/lib/Linker/IRMover.cpp b/lib/Linker/IRMover.cpp
>>> index 309690f..9900ecc 100644
>>> --- a/lib/Linker/IRMover.cpp
>>> +++ b/lib/Linker/IRMover.cpp
>>> @@ -1211,6 +1211,12 @@ void
>>> IRLinker::findNeededSubprograms(ValueToValueMapTy &ValueMap) {
>>> for (unsigned I = 0, E = CompileUnits->getNumOperands(); I != E; ++I) {
>>> auto *CU = cast<DICompileUnit>(CompileUnits->getOperand(I));
>>> assert(CU && "Expected valid compile unit");
>>> + SmallPtrSet<DISubprogram *, 8> ImportedEntitySPs;
>>> + for (auto *IE : CU->getImportedEntities()) {
>>> + assert(IE->getEntity());
>>> + if (auto *SP = dyn_cast<DISubprogram>(IE->getEntity()))
>>
>> This is silly nitpicking, but the assertion followed by a dyn_cast is redundant (cf. dyn_cast_or_null).
>
> I could use dyn_cast_or_null instead of the assert. But I wanted to
> catch a case where we somehow already ended up in the bad situation of
> an imported entity with a null entity. The dyn_cast_or_null would
> handle it silently, right?
>
I think Adrian's point is that the dyn_cast<>() already crashes on
null, so you can remove the assert() on the previous line.
> Teresa
>
>>
>> -- adrian
>>
>>> + ImportedEntitySPs.insert(SP);
>>> + }
>>> for (auto *Op : CU->getSubprograms()) {
>>> // Unless we were doing function importing and deferred metadata linking,
>>> // any needed SPs should have been mapped as they would be reached
>>> @@ -1218,7 +1224,7 @@ void
>>> IRLinker::findNeededSubprograms(ValueToValueMapTy &ValueMap) {
>>> // function bodies, or from DILocation on inlined instructions).
>>> assert(!(ValueMap.MD()[Op] && IsMetadataLinkingPostpass) &&
>>> "DISubprogram shouldn't be mapped yet");
>>> - if (!ValueMap.MD()[Op])
>>> + if (!ValueMap.MD()[Op] && !ImportedEntitySPs.count(Op))
>>> UnneededSubprograms.insert(Op);
>>> }
>>> }
>>>
>>>
>>> On Wed, Jan 6, 2016 at 10:45 AM, Teresa Johnson <tejohnson at google.com> wrote:
>>>> On Wed, Jan 6, 2016 at 10:37 AM, Teresa Johnson <tejohnson at google.com> wrote:
>>>>> Thanks Duncan and Eric.
>>>>>
>>>>> I should also be able to implement #1 very quickly and get it in today
>>>>> if we want a workaround while we figure out what is the right approach
>>>>> here. Will take a stab at that now.
>>>>
>>>> Oh, missed your point about needing to change the DISubprogram for a()
>>>> from a definition to a declaration. However, presumably that worked ok
>>>> before this patch went in, since we would have had the same issue?
>>>>
>>>>>
>>>>> (Somehow I feel the need to apologize for the overly cute subject - my
>>>>> intended "question" got turned into "questy", presumably by fat
>>>>> fingers. =( =) )
>>>>>
>>>>> Teresa
>>>>>
>>>>> On Wed, 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.
>>>>>>
>>>>>>>
>>>>>>> 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.
>>>>>>
>>>>>> -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
>>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> Teresa Johnson | Software Engineer | tejohnson at google.com | 408-460-2413
>>>>
>>>>
>>>>
>>>> --
>>>> Teresa Johnson | Software Engineer | tejohnson at google.com | 408-460-2413
>>>
>>>
>>>
>>> --
>>> Teresa Johnson | Software Engineer | tejohnson at google.com | 408-460-2413
>>
>
>
>
> --
> Teresa Johnson | Software Engineer | tejohnson at google.com | 408-460-2413
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