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| <h1 id="lldb-internals"><a class="header" href="#lldb-internals">LLDB Internals</a></h1> |
| <p>LLDB’s debug info processing relies on a set of extensible interfaces largely defined in |
| <a href="https://github.com/llvm/llvm-project/tree/main/lldb/source/Plugins">lldb/src/Plugins</a>. These are meant to allow third-party compiler developers to add |
| language support that is loaded at run-time by LLDB, but at time of writing (Nov 2025) the public |
| API has not been settled on, so plugins exist either in LLDB itself or in standalone forks of LLDB.</p> |
| <p>Typically, language support will be written as a pipeline of these plugins: <code>*ASTParser</code> -> |
| <code>TypeSystem</code> -> <code>ExpressionParser</code>/<code>Language</code>.</p> |
| <p>Here are some existing implementations of LLDB’s plugin API:</p> |
| <ul> |
| <li><a href="https://github.com/swiftlang/llvm-project">Apple’s fork with support for Swift</a></li> |
| <li><a href="https://archive.softwareheritage.org/browse/origin/directory/?branch=refs/heads/codelldb/16.x&origin_url=https://github.com/vadimcn/llvm-project&path=lldb/source/Plugins/TypeSystem/Rust&timestamp=2023-09-11T04:55:10Z">CodeLLDB’s former fork with support for Rust</a></li> |
| <li><a href="https://github.com/Walnut356/llvm-project/tree/lldbrust/19.x">A work in progress reimplementation of Rust support</a></li> |
| <li><a href="https://github.com/tromey/lldb/tree/a0fc10ce0dacb3038b7302fff9f6cb8cb34b37c6/source/Plugins/ExpressionParser/Rust">A Rust expression parser plugin</a>. |
| This was written before the <code>TypeSystem</code> API was created. Due to the freeform nature of expression parsing, the |
| underlyng lexing, parsing, function calling, etc. should still offer valuable insights.</li> |
| </ul> |
| <h2 id="rust-support-and-typesystemclang"><a class="header" href="#rust-support-and-typesystemclang">Rust Support and TypeSystemClang</a></h2> |
| <p>As mentioned in the debug info overview, LLDB has partial Rust support. To further clarify, Rust |
| uses the plugin-pipeline that was built for C/C++ (though it contains some helpers for Rust enum |
| types), which relies directly on the <code>clang</code> compiler’s representation of types. This imposes heavy |
| restrictions on how much we can change when LLDB’s output doesn’t match what we want. Some |
| workarounds can help, but at the end of the day Rust’s needs are secondary compared to making sure |
| C and C++ compilation and debugging work correctly.</p> |
| <p>LLDB is receptive to adding a <code>TypeSystemRust</code>, but it is a massive undertaking. This section serves |
| to not only document how we currently interact with <a href="https://github.com/llvm/llvm-project/tree/main/lldb/source/Plugins/TypeSystem/Clang"><code>TypeSystemClang</code></a>, but also as light |
| guidance on implementing a <code>TypeSystemRust</code> in the future.</p> |
| <p>It is worth noting that a <code>TypeSystem</code> directly interacting with the target language’s compiler is |
| the intention, but it is not a requirement. One can create all the necessary supporting types within |
| their plugin implementation.</p> |
| <blockquote> |
| <p>Note: LLDB’s documentation, including comments in the source code, is pretty sparse. Trying to |
| understand how language support works by reading <code>TypeSystemClang</code>’s implementation is somewhat |
| difficult due to the added requirement of understanding the <code>clang</code> compiler’s internals. It is |
| recommended to look at the 2 <code>TypeSystemRust</code> implementations listed above, as they are written |
| “from scratch” without leveraging a compiler’s type representation. They are relatively close to |
| the minimum necessary to implement language support.</p> |
| </blockquote> |
| <h2 id="dwarf-vs-pdb"><a class="header" href="#dwarf-vs-pdb">DWARF vs PDB</a></h2> |
| <p>LLDB is unique in being able to handle both DWARF and PDB debug information. This does come with |
| some added complexity. To complicate matters further, PDB support is split between <code>dia</code>, which |
| relies on the <code>msdia140.dll</code> library distributed with Visual Studio, and <code>native</code>, which is written |
| from scratch using publicly available information about the PDB format.</p> |
| <blockquote> |
| <p>Note: <code>dia</code> was the default up to LLDB version 21. <code>native</code> is the new default as of |
| LLDB 22’s release. There are plans to deprecate and completely remove the <code>dia</code>-based plugins. As |
| such, only <code>native</code> parsing will be discussed below. For progress, please see |
| <a href="https://discourse.llvm.org/t/rfc-removing-the-dia-pdb-plugin-from-lldb/87827">this discourse thread</a> and the relevant <a href="https://github.com/llvm/llvm-project/issues/114906">tracking issue</a>.</p> |
| <p><code>native</code> can be toggled via the <code>plugin.symbol-file.pdb.reader</code> setting added in LLDB 22 or using |
| the environment variable <code>LLDB_USE_NATIVE_PDB_READER=0/1</code></p> |
| </blockquote> |
| <h2 id="debug-node-parsing"><a class="header" href="#debug-node-parsing">Debug Node Parsing</a></h2> |
| <p>The first step is to process the raw debug nodes into something usable. This primarily occurs in |
| the <a href="https://github.com/llvm/llvm-project/tree/main/lldb/source/Plugins/SymbolFile/DWARF"><code>DWARFASTParser</code></a> and <a href="https://github.com/llvm/llvm-project/tree/main/lldb/source/Plugins/SymbolFile/NativePDB"><code>PdbAstBuilder</code></a> classes. These classes are fed a |
| deserialized form of the debug info generated from <a href="https://github.com/llvm/llvm-project/blob/main/lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.h"><code>SymbolFileDWARF</code></a> and |
| <a href="https://github.com/llvm/llvm-project/blob/main/lldb/source/Plugins/SymbolFile/NativePDB/SymbolFileNativePDB.h"><code>SymbolFileNativePDB</code></a> respectively. The <code>SymbolFile</code> implementers make almost no |
| transformations to the underlying debug info before passing it to the parsers. For both PDB and |
| DWARF, the debug info is read using LLVM’s debug info handlers.</p> |
| <p>The parsers translate the nodes into more convenient formats for LLDB’s purposes. For <code>clang</code>, these |
| formats are <code>clang::QualType</code>, <code>clang::Decl</code>, and <code>clang::DeclContext</code>, which are the types <code>clang</code> |
| uses internally when compiling C and C++. Again, using the compiler’s representation of types is not a |
| requirement, but the plugin system was built with it as a possibility.</p> |
| <blockquote> |
| <p>Note: The above types will be referred to language-agnostically as <code>LangType</code>, <code>Decl</code>, and |
| <code>DeclContext</code> when the specific implementation details of <code>TypeSystemClang</code> are not relevant.</p> |
| </blockquote> |
| <p><code>LangType</code> represents a type. This includes information such as the name of the type, the size and |
| alignment, its classification (e.g. struct, primitive, pointer), its qualifiers (e.g. |
| <code>const</code>, <code>volatile</code>), template arguments, function argument and return types, etc. <a href="https://github.com/Walnut356/llvm-project/blob/13bcfd502452606d69faeea76aec3a06db554af9/lldb/source/Plugins/TypeSystem/Rust/TypeSystemRust.h#L618">Here</a> |
| is an example of what a <code>RustType</code> might look like.</p> |
| <p><code>Decl</code> represents any kind of declaration. It could be a type, a variable, a static field of a |
| struct, the value that a static or const is initialized with, etc.</p> |
| <p><code>DeclContext</code> more or less represents a scope. <code>DeclContext</code>s typically contain <code>Decl</code>s and other |
| <code>DeclContexts</code>, though the relationship isn’t that straight forward. For example, a function can be |
| both a <code>Decl</code> (because function signatures are types), <strong>and</strong> a <code>DeclContext</code> (because functions |
| contain variable declarations, nested functions declarations, etc.).</p> |
| <p>The translation process can be quite verbose, but is usually straightforward. Much of the work here |
| is dependant on the exact information needed to fill out <code>LangType</code>, <code>Decl</code>, and <code>DeclContext</code>.</p> |
| <p>Once a node is translated, a pointer to it is type-erased (<code>void*</code>) and wrapped in <code>CompilerType</code>, |
| <code>CompilerDecl</code>, or <code>CompilerDeclContext</code>. These wrappers associate the them with the <code>TypeSystem</code> |
| that owns them. Methods on these objects delegates to the <code>TypeSystem</code>, which casts the <code>void*</code> back |
| to the appropriate <code>LangType*</code>/<code>Decl*</code>/<code>DeclContext*</code> and operates on the internals. In Rust terms, |
| the relationship looks something like this:</p> |
| <pre><code class="language-Rust">struct CompilerType { |
| inner_type: *mut c_void, |
| type_system: Arc<dyn TypeSystem>, |
| } |
| |
| impl CompilerType { |
| pub fn get_byte_size(&self) -> usize { |
| self.type_system.get_byte_size(self.lang_type) |
| } |
| |
| } |
| |
| ... |
| |
| impl TypeSystem for TypeSystemLang { |
| pub fn get_byte_size(lang_type: *mut c_void) -> usize { |
| let lang_type = lang_type as *mut LangType; |
| |
| // Operate on the internals of the LangType to |
| // determine its size |
| ... |
| } |
| } |
| </code></pre> |
| <h2 id="type-systems"><a class="header" href="#type-systems">Type Systems</a></h2> |
| <p>The <a href="https://github.com/llvm/llvm-project/blob/main/lldb/include/lldb/Symbol/TypeSystem.h#L69"><code>TypeSystem</code> interface</a> has 3 major purposes:</p> |
| <ol> |
| <li>Act as the “sole authority” of a language’s types. This allows the type system to be added to |
| LLDB’s “pool” of type systems. When an executable is loaded, the target language is determined, and |
| the pool is queried to find a <code>TypeSystem</code> that claims it can handle the language. One can also use |
| the <code>TypeSystem</code> to retrieve the backing <code>SymbolFile</code>, search for types, and synthesize basic types |
| that might not exist in the debug info (e.g. primitives, arrays-of-<code>T</code>, pointers-to-<code>T</code>).</li> |
| <li>Manage the lifetimes of the <code>LangType</code>, <code>Decl</code>, and <code>DeclContext</code> objects</li> |
| <li>Customize the “defaults” of how those types appear and how they can be interacted with.</li> |
| </ol> |
| <p>The first two functions are pretty straightforward so we will focus on the third.</p> |
| <p>Many of the functions in the <code>TypeSystem</code> interface will look familiar if you have worked with the |
| visualizer scripts. These functions underpin <code>SBType</code> the <code>SBValue</code> functions with matching names. |
| For example, <code>TypeSystem::GetFormat</code> returns the default format for the type if no custom formatter |
| has been applied to it.</p> |
| <p>Of particular note are <code>GetIndexOfChildWithName</code> and <code>GetNumChildren</code>. The <code>TypeSystem</code> versions of |
| these functions operate on a <em>type</em>, not a value like the <code>SBValue</code> versions. The values returned |
| from the <code>TypeSystem</code> functions dictate what parts of the struct can be interacted with <em>at all</em> by |
| the rest of LLDB. If a field is ommitted, that field effectively no longer exists to LLDB.</p> |
| <p>Additionally, since they do not work with objects, there is no underlying memory to inspect or |
| interpret. Essentially, this means these functions do not have the same purpose as their equivalent |
| <code>SyntheticProvider</code> functions. There is no way to determine how many elements a <code>Vec</code> has or what |
| address those elements live at. It is also not possible to determine the value of the discriminant |
| of a sum-type.</p> |
| <p>Ideally, the <code>TypeSystem</code> should expose types as they appear in the debug info with as few |
| alterations as possible. LLDB’s synthetics and frontend can handle making the type pretty. If some |
| piece of information is useless, the Rust compiler should be altered to not output that debug info |
| in the first place.</p> |
| <h2 id="expression-parsing"><a class="header" href="#expression-parsing">Expression Parsing</a></h2> |
| <p>The <code>TypeSystem</code> is typically written to have a counterpart that can handle expression parsing. It |
| requires implementing a few extra functions in the <code>TypeSystem</code> interface. The bulk of the |
| expression parsing code should live in <a href="https://github.com/llvm/llvm-project/tree/main/lldb/source/Plugins/ExpressionParser">lldb/source/Plugins/ExpressionParser</a>.</p> |
| <p>There isn’t too much of note about the parser. It requires implementing a simple interpreter that |
| can handle (possibly simplified) Rust syntax. They operate on <code>lldb::ValueObject</code>s, which are the |
| objects that underpin <code>SBValue</code>.</p> |
| <h2 id="language"><a class="header" href="#language">Language</a></h2> |
| <p>The <a href="https://github.com/llvm/llvm-project/tree/main/lldb/source/Plugins/Language"><code>Language</code> plugins</a> are the C++ equivalent to the Python visualizer scripts. They |
| operate on <code>SBValue</code> objects for the same purpose: creating synthetic children and pretty-printing. |
| The <a href="https://github.com/llvm/llvm-project/tree/main/lldb/source/Plugins/Language/CPlusPlus">CPlusPlusLanguage’s implementations</a> for the LibCxx types are great resources to |
| learn how visualizers should be written.</p> |
| <p>These plugins can access LLDB’s private internals (including the underlying <code>TypeSystem</code>), so |
| synthetic/summary providers written as a <code>Language</code> plugin can provide higher quality output than |
| their python equivalent.</p> |
| <p>While debug node parsing, type systems, and expression parsing are all closely tied to eachother, |
| the <code>Language</code> plugin is encapsulated more and thus can be written “standalone” for any language |
| that an existing type system supports. Due to the lower barrier of entry, a <code>RustLanguage</code> plugin |
| may be a good stepping stone towards full language support in LLDB.</p> |
| <h2 id="visualizers"><a class="header" href="#visualizers">Visualizers</a></h2> |
| <p>WIP</p> |
| |
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