[llvm] r218041 - Always emit DW_AT_declaration attribute when the variable isn't a definition.

[Frédéric Riss]

> On 18 Sep 2014, at 17:48, David Blaikie <dblaikie at gmail.com> wrote:
> 
> Here's a simple example:
> 
>   namespace x {
>     int i = 3;
>   }
>   int j = x::i;
> 
> LLVM will emit a declaration for 'i' in namespace x and a definition at the CU scope referencing the declaration. With your change, we end up omitting the DW_AT_declaration on the declaration of 'i'. The debugger then thinks it's a definition without a location, so any attempt to use/print the variable produces "this value has been optimized away”.

OK, I think I see how that happened. I thought I could only add more AT_declarations with the patch, but I’m obviously removing the ones introduced for globals emitted as a pair of declaration/definition DIEs. I’ll revert half of the patch and it should fix it.

> A couple of side notes:
> 
> 1) We should ideally not use "Verify()" for any debug info functionality - it should be enough to test "isBlah" methods. Any time isBlah is true but Verify fails is a bug we should fix at some point - I think that's a valid/useful invariant, but I could be wrong.

I was just moving things around. I’ll try to see if I can find the equivalent isSomething check.

> 2) we don't actually need to separate declaration and definition in this way, just because the variable is inside a namespace - so if you happen to fix this by removing the separation of declaration from definition, that's OK too (& a minor win to reduce debug info size)

I wondered why we do this (it’s done at the exact place the patch modifies the code), but it’s too explicit to be a simple thinko. There must be some good reason for it. If you don’t know of one, I’ll try to look at this when I get some time.

Thanks for the help!
Fred

> On Thu, Sep 18, 2014 at 8:28 AM, Frédéric Riss <friss at apple.com <mailto:friss at apple.com>> wrote:
> 
>> On 18 Sep 2014, at 17:23, David Blaikie <dblaikie at gmail.com <mailto:dblaikie at gmail.com>> wrote:
>> 
>> Looks like this might've regressed some debug scenarios:
>> 
>> http://lab.llvm.org:8011/builders/clang-x86_64-ubuntu-gdb-75/builds/17377 <http://lab.llvm.org:8011/builders/clang-x86_64-ubuntu-gdb-75/builds/17377>
>> 
>> (could be r218040, but it seems like the less likely candidate of the two on the blame list)
> 
> I’ve seen it. I sent a mail to the review thread asking for some help from someone that can run the test suite.
> 
> Fred
> 
>> On Thu, Sep 18, 2014 at 2:38 AM, Frederic Riss <friss at apple.com <mailto:friss at apple.com>> wrote:
>> Author: friss
>> Date: Thu Sep 18 04:38:23 2014
>> New Revision: 218041
>> 
>> URL: http://llvm.org/viewvc/llvm-project?rev=218041&view=rev <http://llvm.org/viewvc/llvm-project?rev=218041&view=rev>
>> Log:
>> Always emit DW_AT_declaration attribute when the variable isn't a definition.
>> 
>> Summary:
>> This doesn't show up today as we don't emit decalration only variables. This
>> will be tested when the followup patches implementing import of forward
>> declared entities lands in clang.
>> 
>> Reviewers: echristo, dblaikie, aprantl
>> 
>> Subscribers: llvm-commits
>> 
>> Differential Revision: http://reviews.llvm.org/D5382 <http://reviews.llvm.org/D5382>
>> 
>> Modified:
>>     llvm/trunk/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
>> 
>> Modified: llvm/trunk/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/AsmPrinter/DwarfUnit.cpp?rev=218041&r1=218040&r2=218041&view=diff <http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/AsmPrinter/DwarfUnit.cpp?rev=218041&r1=218040&r2=218041&view=diff>
>> ==============================================================================
>> --- llvm/trunk/lib/CodeGen/AsmPrinter/DwarfUnit.cpp (original)
>> +++ llvm/trunk/lib/CodeGen/AsmPrinter/DwarfUnit.cpp Thu Sep 18 04:38:23 2014
>> @@ -1670,6 +1670,9 @@ void DwarfCompileUnit::createGlobalVaria
>>        DD->addArangeLabel(SymbolCU(this, Sym));
>>        addOpAddress(*Loc, Sym);
>>      }
>> +    // A static member's declaration is already flagged as such.
>> +    if (!SDMDecl.Verify() && !GV.isDefinition())
>> +      addFlag(*VariableDIE, dwarf::DW_AT_declaration);
>>      // Do not create specification DIE if context is either compile unit
>>      // or a subprogram.
>>      if (GVContext && GV.isDefinition() && !GVContext.isCompileUnit() &&
>> @@ -1678,9 +1681,6 @@ void DwarfCompileUnit::createGlobalVaria
>>        VariableSpecDIE = &createAndAddDIE(dwarf::DW_TAG_variable, UnitDie);
>>        addDIEEntry(*VariableSpecDIE, dwarf::DW_AT_specification, *VariableDIE);
>>        addBlock(*VariableSpecDIE, dwarf::DW_AT_location, Loc);
>> -      // A static member's declaration is already flagged as such.
>> -      if (!SDMDecl.Verify())
>> -        addFlag(*VariableDIE, dwarf::DW_AT_declaration);
>>      } else {
>>        addBlock(*VariableDIE, dwarf::DW_AT_location, Loc);
>>      }
>> 
>> 
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> 
> 

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