The LLDB Debugger

# Adding Programming Language Support to LLDB

LLDB has been architected to make it straightforward to add support for a programming language. Only a small enum in core LLDB needs to be modified to make LLDB aware of a new programming language. Everything else can be supplied in derived classes that need not even be present in the core LLDB repository. This makes it convenient for developers adding language support either in branches or downstream repositories since it practically eliminates the potential for merge conflicts.

The basic steps needed are as follows:

- Add the language to the LanguageType enum
- Add a TypeSystem for the language
- Add expression evaluation support

Additionally, you may want to create a Language and LanguageRuntime plugin for your language, which enables support for advanced features like dynamic typing and data formatting.

# Add the Language to the LanguageType enum

The LanguageType enum (see lldb-enumerations.h) contains a list of every language known to LLDB. It is the one place where support for a language must live that will need to merge cleanly with core LLDB if you are developing your language support in a separate branch. When adding support for a language previously unknown to LLDB, start by adding an enumeration entry to LanguageType.

# Add a TypeSystem for the Language

Both Module and Target support the retrieval of a TypeSystem instance via GetTypeSystemForLanguage(). For Module, this method is directly on the Module instance. For Target, this is retrieved indirectly via the TypeSystemMap for the Target instance.

The TypeSystem instance returned by the Target is expected to be capable of evaluating expressions, while the TypeSystem instance returned by the Module is not. If you will support expression evaluation for your language, you could consider following one of these approaches:

- implement a single TypeSystem class that supports evaluation when given an optional Target, implementing all the expression evaluation methods on the TypeSystem in this case, OR
- create multiple TypeSystem classes, one for evaluation and one for static Module usage.

For clang and Swift, we chose to go with the latter, primarily to make it clearer that evaluation with the static Module-returned TypeSystem instances make no sense, and have them error out on those calls. But either approach is fine to pursue.

# Add Expression Evaluation Support

Expression Evaluation support is enabled by implementing the relevant methods on a TypeSystem-derived class. Search for "Expression" in the TypeSystem header to find relevant methods to implement.

# Type Completion

There are three levels of type completion, each requiring more type information:

1. Pointer size: when you have a forward decl or a reference, and that's all you need. At this stage, the pointer size is all you need.
2. Layout info: you need the size of an instance of the type, but you still don't need to know all the guts of the type.
3. Full type info. Here you need everything, because you're playing with internals of it, such as modifying a member variable.

Ensure you never complete more of a type than is needed for a given situation. This will keep your type system from doing more work than necessary.

# Creating Types

Your TypeSystem will need an approach for creating types based on a set of Modules. If your type info is going to come from DWARF info, you will want to subclass DWARFASTParser.

# Language and LanguageRuntime plugins

If you followed the steps outlined above, you already have taught LLDB a great deal about your language. And if your language's runtime model and fundamental data types don't differ much from the C model, you are pretty much done.
However, it is likely that your language offers its own data types for things like strings, arrays, ..., and probably has a notion of dynamic types, where the effective type of a variable can only be known at runtime.

These tasks are covered by two plugins:

- a LanguageRuntime plugin, which provides LLDB with a dynamic view of your language; this plugin answers questions that require a live process to acquire information (e.g. dynamic type resolution)
- a Language plugin, which provides LLDB with a static view of your language; questions that are statically knoawble and do not require a process are answered by this plugin (e.g. data formatters)