[lldb-dev] RFC: -flimit-debug-info + frame variable

Pavel Labath via lldb-dev lldb-dev at lists.llvm.org
Mon Jul 20 05:34:20 PDT 2020

Hello all,

With the expression evaluation part of -flimit-debug-info nearly
completed, I started looking at doing the same for the "frame variable"

I have thought this would be simpler than the expression evaluator,
since we control most of that code. However, I have hit one snag, hence
this RFC.

The problem centers around how to implement
ValueObject::GetChildMemberWithName, which is the engine of the
subobject resultion in the "frame variable" command. Currently, this
function delegates most of the work to
CompilerType::GetIndexOfChildMemberWithName, which returns a list of (!)
indexes needed to access the relevant subobject. The list aspect is
important, because the desired object can be in a base class or in a C11
anonymous struct member.

The CompilerType instance in question belongs to the type system of the
module from which we retrieved the original variable. Therein lies the
problem -- this type system does not have complete information about the
contents of the base class subobjects.

Now, my question is what to do about it. At the moment, it seems to me
that the easiest solution to this problem would be to replace
CompilerType::GetIndexOfChildMemberWithName, with two new interfaces:
- Get(IndexOf)**Direct**ChildMemberWithName -- return any direct
children with the given name
- IsTransparent -- whether to descend into the type during name lookups
(i.e., is this an anonymous struct member)

The idea is that these two functions (in conjunction with existing
methods) can provide their answers even in a -flimit-debug-info setting,
and they also provide enough information for the caller to perform the
full name lookup himself. It would first check for direct members, and
if no matches are found, (recursively) proceed to look in all the
transparent members and base classes, switching type systems if the
current one does not contain the full type definition.

The downside of that is that this would hardcode a specific, c++-based,
algorithm which may not be suitable for all languages. Swift has a
fairly simple inheritance model, so I don't think this should be a
problem there, but for example python uses a slightly different method
to resolve ambiguities. The second downside is that a faithful
implementation of the c++ model, including the virtual inheritance
dominance is going to be fairly complicated.

The first issue could be solved by moving this logic into the clang
plugin, but making it independent of any specific type system instance.
The second issue is unavoidable, except by creating a unified view of
the full type in some scratch ast context, as we do for expression

That said, it's not clear to me how faithful do we need the "frame
variable" algorithm to be. The frame variable syntax does not precisely
follow the c++ semantics anyway. And a simple "recurse into subclasses"
algorithm is going to be understandable and be "close enough" under
nearly all circumstances. Virtual inheritance is used very seldomly, and
shadowing of members defined in a base class is even rarer.

While analysing this code I've found much more serious bugs (e.g.,
accessing a transparent member fetches a random other value if the class
it is in also has base cases; fetching a transparent member in a base
class does not work at all), which seem to have existed for quite some
time without being discovered.

For that reason I am tempted to just implement a basic "recurse into
subclasses" algorithm inside ValueObject::GetChildMemberWithName, and
not bother with the virtual inheritance details, nor with being able to
customize this algorithm to the needs of a specific language.

What do you think?


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