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

[David Blaikie]

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".

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.

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)

On Thu, Sep 18, 2014 at 8:28 AM, Frédéric Riss <friss at apple.com> wrote:

>
> On 18 Sep 2014, at 17:23, David Blaikie <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
>
> (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> 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
>> 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
>>
>> 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
>>
>> ==============================================================================
>> --- 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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>> llvm-commits at cs.uiuc.edu
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>>
>
>
>
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