[lldb-dev] Rust language support question

Greg Clayton via lldb-dev lldb-dev at lists.llvm.org
Fri Jan 26 15:49:21 PST 2018

> On Jan 26, 2018, at 2:54 PM, Tom Tromey via lldb-dev <lldb-dev at lists.llvm.org> wrote:
> Hi.  I'm working on adding Rust language support to lldb.
> One question that's come up is the best way to handle expression
> parsing.
> On the one hand, it would be convenient to reuse an existing parser --
> the one that we discussed was the "syn" crate.  But, this is a Rust
> program.

We have great support for multiple language in LLDB. First off you will need to subclass lldb_private::TypeSystem. See lldb_private::ClangASTContext for the most complete version. Your type system allows you to use an abstract syntax tree that makes sense for your language. For clang, we actually create a clang::ASTContext using the code from the compiler and then we translate debug info from DWARF into clang types. So you would probably want to make a "lldb_private::RustASTContext". Then, if you use DWARF as your debug info, you will want to make subclass DWARFASTParser with something like DWARFASTParserRust. This is where you will translate types from DWARF back into your custom AST types. TypeSystem then becomes your main type abstraction within LLDB. There are many virtual functions in it that you must override and some that you might want to override. Types in LLDB are handed out as "lldb_private::CompilerType". This class contains a "lldb_private::TypeSystem *" plus a "void *" which can point to what ever makes sense if your lldb_private::RustASTContext class. Similarly CompilerDecl and CompilerDeclContext also have a "lldb_private::TypeSystem *" plus a "void *" so you can hand out declarations and namespaces, etc.

When it comes to expressions, we will dig up the right lldb_private::TypeSystem for a given stack frame language and then we will call lldb_private::TypeSystem::GetUserExpression(...) and your type system can evaluate your expression exactly as your language should.

> So then there's the question of how to ship it.  Directly using the syn
> crate would mean having Rust code in-tree.  Or, perhaps the Rust parts
> could be shipped as a shared library or an external executable.

You can make your lldb_private::RustASTContext have a custom setting in LLDB that specifies a path to a shared library to load. You could keep all of the functionality out of LLDB that way, just have lldb_private::RustASTContext shim over to the shared library to do the work. Or you can build it right into LLDB. It really depends on how sharable your compiler code is and if the code can be used in another program.
> Are either of these doable?  What do other language plugins do?

It is. All other languages compile in their support right now, but that is mostly due to the lack of a stable API in clang. Clang has no library interface that exports all of the details that we need, so we just build it into LLDB. Swift does the same thing: a full compiler is included in LLDB itself. Go and Java and OCaml all write their own minimal debug layer that doesn't depend on the compiler codebase at all. 

It takes some work to build your compiler so that is can share its implementation with the debugger, but the investment can pay off:
- expressions in clang can use latest language features just by updating code
- no need to maintain a separate expression parser that constantly gets out of date with the current compiler
- no need to invent a type system, just use your native AST. This also helps ensure you can recreate any types from DWARF since if some info is missing in DWARF, you won't be able to convert it back into your AST format without losing something

Swift did a slight different thing that you might want to think about: if your compiler can serialize an AST when it builds your program, you can put that serialized AST into the executable, along with debug info. When you debug, you can deserialize the AST and hand it right back to the compiler! With Swift that was great because the compiler guys would change the language and add new features. As they did this, we wouldn't always have work to do in LLDB, because they would store a blob of info in the binary, and we would hand it back to them. The debug info didn't actually have types in it, the DWARF just had mangled names for the types. We take those mangled names, and hand them to the compiler code, and it would use the mangled names to locate the type and hand it back to us. So we didn't need to know anything about the type, just hand a string to the compiler and it would hand us back a type. Swift also has generics, like a "Dictionary<String, String>" and even these types aren't each contained in the AST, but they can be re-created on the fly by the compiler code. This all was hidden behind the "there is a mangle type name, please give me a type back".

> My original plan here was to simply make the entire language support an
> external plugin.  But, from what I can tell this isn't possible -- the
> necessary DWARF-related headers aren't installed.  So maybe this could
> be changed?  This would provide us with the most flexibility I think.

I wouldn't recommend that. I would recommend making stable API to your custom shared library that lldb_private::RustASTContext knows how to load. Many of the virtual functions in TypeSystem already will show you the layers we need. The LLDB internal API is not stable and shouldn't be used or exported through to other plug-ins. It is possible, but the shear number of HUGE C++ names would make the any library export produce very large LLDB binaries. Right now we really take care to only export a sensible API. So if you need to be out of process, design an API into Rust that produces a shared library that LLDB can use and that you commit to. As you update Rust, you can then just update the shared library and then re-run LLDB. Does that sound feasible? If we need to develope an API for pluggable type systems in LLDB, maybe we can use your rust shared library as a basis for others in the future.

> A final idea is to do what I did for gdb, and simply write a new parser
> in C++.  Doable, but for me I think a last resort.

I agree that this is a last resort kind of approach. Lots of work keeping the compiler an expression parser in sync. Better to architect the compiler correctly so that the code can be re-used.

Let me know if you have an questions about anything I said above,

Greg Clayton

> thanks,
> Tom
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