[lldb-dev] SymbolFile::FindGlobalVariables

Fri Mar 11 13:02:57 PST 2016

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.
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.

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).

On Fri, Mar 11, 2016 at 11:56 AM Greg Clayton <gclayton at apple.com> wrote:

> Feel free to abstract if you need to. The page you sent me to has _very_
> simple locations that would convert to DWARF expressions very easily.
> Probably less that a hundred lines of code.
>
> If you need to abstract, making a lldb_private::Location class that
> DWARFExpression would implement the needed pure virtuals. Then each things
> that contains DWARFExpression would now contain a lldb_private::LocationSP
> which would be a shared pointer to a lldb_private::Location.
> DWARFExpression has grown over the years to contain a bunch of evaluate
> variants.
>
> Just know that LLDB lazily parses things. We don't say "convert the entire
> PDB into the internal LLDB format now!". We say "get the line table for
> this one compile unit". Find the function for address "0x123000" and parse
> it. Later we will ask to get the function type and its args. Later, if we
> ever need to, we will lazily parse the blocks in the function. Nothing is
> parsed in full.
>
> Let me know what you want to do. Abstraction is great, but comes at a cost
> of breaking things when our tests don't cover everything, so that is a
> worry on my end with any large changes...
>
>
> > On Mar 11, 2016, at 11:47 AM, Zachary Turner <zturner at google.com> wrote:
> >
> > The only "spec" is the API that allows you to access the info.  There's
> no spec of the bit format.  This is probably all you are actually looking
> for though:
> >
> > The problem isn't necessarily that one is more pwoerful than the other,
> it's just that PDBs can get huge (on the order of gigabytes), and
> converting between formats is an unnecessary step that a) will be slow to
> do the conversion, b) might not map 1 to 1 between the formats, and c) it'
> already trivial (on the order of a few lines of code) to just query the PDB
> for everything you need.
> >
> > So we're talking about potentially thousands of lines of code to do
> something that would take about 10 (as well as being more efficient) with a
> proper abstraction.
> >
> > On Fri, Mar 11, 2016 at 11:43 AM Greg Clayton <gclayton at apple.com>
> wrote:
> > See my other email. You can abstract this, but it doesn't seem worth it
> unless PDB has some really powerful way to express variable locations?
> >
> > > On Mar 11, 2016, at 11:39 AM, Zachary Turner via lldb-dev <
> lldb-dev at lists.llvm.org> wrote:
> > >
> > > Can we abstract this somehow?  Converting all my debug info to DWARF
> seems like a non-starter, as it doesn't look like you can just do it
> partially, you have to go all the way (just based on glancing at the
> DWARFExpression header file)
> > >
> > > On Fri, Mar 11, 2016 at 11:38 AM Jim Ingham <jingham at apple.com> wrote:
> > > lldb uses DWARF expressions internally as a convenient language to
> represent locations of values.  We had to pick some representation, and the
> DWARF expression was powerful enough for our purposes, meant we didn't have
> to reinvent something that already existed, and had the added benefit that
> if you did your DWARF then you don't have to transcode.
> > >
> > > Jim
> > >
> > > > On Mar 11, 2016, at 11:32 AM, Zachary Turner via lldb-dev <
> lldb-dev at lists.llvm.org> wrote:
> > > >
> > > > Also why does the lldb_private::Variable() class take a
> DWARFExpression to its constructor?  Seems like this is wrong in the face
> of non-DWARF debug information.
> > > >
> > > > On Fri, Mar 11, 2016 at 11:02 AM Zachary Turner <zturner at google.com>
> wrote:
> > > > I'm trying to implement this function for PDB.  There are two
> overloads:
> > > >
> > > > uint32_t
> > > > FindGlobalVariables (const ConstString &name, const
> CompilerDeclContext *parent_decl_ctx, bool append, uint32_t max_matches,
> VariableList& variables)
> > > >
> > > > uint32_t
> > > > FindGlobalVariables(const RegularExpression& regex, bool append,
> uint32_t max_matches, VariableList& variables)
> > > >
> > > > I know how to implement the second overload, but not the first.
> What is a CompilerDeclContext?  Some comments in the DWARF implementation
> of the function seem to imply it's related to namespaces, but there's a lot
> of strange code that I don't understand.  What is the relationship between
> a namespace and a symbol file?  And why does
> `DeclContextMatchesThisSymbolFile` contain no code at all that accesses any
> property of the symbol file?  It just checks if
> decl_ctx->GetTypeSystem()->GetMinimumLanguage(nullptr) ==
> decl_ctx->GetTypeSystem(), which appears to have nothing to do with any
> symbol file.
> > > >
> > > > What user command or debugger operation results in
> FindGlobalVariables getting called with this particular overload, and how
> does it build the CompilerDeclContext?
> > > >
> > > > On another note, why is the decl context stored as void* instead of
> having an actual wrapper with an abstract interface such as
> ClangDeclContext / JavaDeclContext, etc that all inherit from
> LanguageDeclContext, and pass the LanguageDeclContext around instead of a
> void*?
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>
>
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