[lldb-dev] SymbolFile::FindGlobalVariables

Greg Clayton via lldb-dev lldb-dev at lists.llvm.org
Fri Mar 11 17:05:30 PST 2016

> On Mar 11, 2016, at 1:02 PM, Zachary Turner <zturner at google.com> wrote:
> How large of a change do you think it would be to abstract out the location information for the variable?  As far as I can tell, our uses of this DWARFExpression on Variables are very limited:
> 1. In ValueObjectVariable::UpdateValue and ClangExpressionDeclMap::GetVariableValue, if the location is a constant value, it refers to a a host address, we just read the value out as a number.
> 2. In EntityVariable::Materialize(), we check whether it is valid.
> 3. In SymbolFileDWARF, we "evaluate" the expression.

Leave this one alone, don't abstract it since it is DWARF native.

> 4. In a few places, we check whether an input address matches the location specified.
> 5. We dump the location to stdout in a few places.
> Everything else could just as easily be private methods, because that's all that public users of DWARFExpression actually use.

Sounds like it won't be too bad.
> This seems like an easy abstraction to create.  #3 is irrelevant because that code is in SymbolFileDWARF, it could downcast from Location to DWARFLocation.  #1, 2, 4, and 5 could easily be implemented directly against a PDB.
> While I haven't tried to actually *do* either approach yet, I like the idea of creating the abstraction because it provides the native / most optimized debugging experience no matter what you're using.  For example, I can easily imagine a scenario where I have to keep the PDB open in memory to query some types of information, but I have to do a conversion of location information for Variables, and the memory usage becomes unacceptable because everything is memory twice (even though it's lazily evaluated, the memory usage would double over time).

You will abstract the location only and that is fine. For everything else we do have lldb classes that will need to be created (compile units, functions, blocks, variables). Types are done via the TypeSystem subclasses so you will need convert all types there. So feel free to abstract the DWARFExpression for variable locations only.

I have no problem with the abstraction if you think it is needed. I personally think it will be much more work, but I won't be doing it so I don't mind.


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