# MLIR

Multi-Level IR Compiler Framework

# MLIR : Language Server Protocol

This document describes the tools and utilities related to supporting LSP IDE language extensions for various MLIR-related languages. An LSP language extension is generally comprised of two components; a language client and a language server. A language client is a piece of code that interacts with the IDE that you are using, such as VSCode. A language server acts as the backend for queries that the client may want to perform, such as “Find Definition”, “Find References”, etc.

## MLIR LSP Language Server : mlir-lsp-server¶

MLIR provides an implementation of an LSP language server for .mlir text files in the form of the mlir-lsp-server tool. This tool interacts with the MLIR C++ API to support rich language queries, such as “Find Definition”.

### Supporting custom dialects and passes ¶

mlir-lsp-server, like many other MLIR based tools, relies on having the appropriate dialects registered to be able to parse in the custom assembly formats used in the textual .mlir files. The mlir-lsp-server found within the main MLIR repository provides support for all of the upstream MLIR dialects and passes. Downstream and out-of-tree users will need to provide a custom mlir-lsp-server executable that registers the entities that they are interested in. The implementation of mlir-lsp-server is provided as a library, making it easy for downstream users to register their dialect/passes and simply call into the main implementation. A simple example is shown below:

#include "mlir/Tools/mlir-lsp-server/MlirLspServerMain.h"

int main(int argc, char **argv) {
mlir::DialectRegistry registry;
registerMyDialects(registry);
registerMyPasses();
return mlir::failed(mlir::MlirLspServerMain(argc, argv, registry));
}


See the Editor Plugins section below for details on how to setup support in a few known LSP clients, such as vscode.

### Features ¶

This section details a few of the features that the MLIR language server provides. The screenshots are shown in VSCode, but the exact feature set available will depend on your editor client.

#### Diagnostics ¶

The language server actively runs verification on the IR as you type, showing any generated diagnostics in-place.

##### Automatically insert expected- diagnostic checks ¶

MLIR provides infrastructure for checking expected diagnostics, which is heavily utilized when defining IR parsing and verification. The language server provides code actions for automatically inserting the checks for diagnostics it knows about.

#### Code completion ¶

The language server provides suggestions as you type, offering completions for dialect constructs (such as attributes, operations, and types), block names, SSA value names, keywords, and more.

#### Cross-references ¶

Cross references allow for navigating the use/def chains of SSA values (i.e. operation results and block arguments), Symbols, and Blocks.

##### Find definition ¶

Jump to the definition of the IR entity under the cursor. A few examples are shown below:

• SSA Values

• Symbol References

The definition of an operation will also take into account the source location attached, allowing for navigating into the source file that generated the operation.

##### Find references ¶

Show all references of the IR entity under the cursor.

#### Hover ¶

Hover over an IR entity to see more information about it. The exact information displayed is dependent on the type of IR entity under the cursor. For example, hovering over an Operation may show its generic format.

The language server will also inform the editor about the structure of symbol tables within the IR. This allows for jumping directly to the definition of a symbol, such as a func.func, within the file.

#### Bytecode Editing and Inspection ¶

The language server provides support for interacting with MLIR bytecode files, enabling IDEs to transparently view and edit bytecode files in the same way as textual .mlir files.

## PDLL LSP Language Server : mlir-pdll-lsp-server¶

MLIR provides an implementation of an LSP language server for .pdll text files in the form of the mlir-pdll-lsp-server tool. This tool interacts with the PDLL C++ API to support rich language queries, such as code completion and “Find Definition”.

### Compilation Database ¶

Similarly to clangd, and language servers for various other programming languages, the PDLL language server relies on a compilation database to provide build-system information for .pdll files. This information includes, for example, the include directories available for that file. This database allows for the server to interact with .pdll files using the same configuration as when building.

#### Format ¶

A PDLL compilation database is a YAML file, conventionally named pdll_compile_commands.yml, that contains a set of FileInfo documents providing information for individiual .pdll files.

Example:

--- !FileInfo:
filepath: "/home/user/llvm/mlir/lib/Dialect/Arith/IR/ArithCanonicalization.pdll"
includes: "/home/user/llvm/mlir/lib/Dialect/Arith/IR;/home/user/llvm/mlir/include"

• filepath: - Absolute file path of the file.
• includes: - Semi-colon delimited list of absolute include directories.

#### Build System Integration ¶

Per convention, PDLL compilation databases should be named pdll_compile_commands.yml and placed at the top of the build directory. When using CMake and mlir_pdll, a compilation database is generally automatically built and placed in the appropriate location.

### Features ¶

This section details a few of the features that the PDLL language server provides. The screenshots are shown in VSCode, but the exact feature set available will depend on your editor client.

#### Diagnostics ¶

The language server actively runs verification as you type, showing any generated diagnostics in-place.

#### Code completion and signature help ¶

The language server provides suggestions as you type based on what constraints, rewrites, dialects, operations, etc are available in this context. The server also provides information about the structure of constraint and rewrite calls, operations, and more as you fill them in.

#### Cross-references ¶

Cross references allow for navigating the code base.

##### Find definition ¶

Jump to the definition of a symbol under the cursor:

If ODS information is available, we can also jump to the definition of operation names and more:

##### Find references ¶

Show all references of the symbol under the cursor.

#### Hover ¶

Hover over a symbol to see more information about it, such as its type, documentation, and more.

If ODS information is available, we can also show information directly from the operation definitions:

The language server will also inform the editor about the structure of symbols within the IR.

#### View intermediate output ¶

The language server provides support for introspecting various intermediate stages of compilation, such as the AST, the .mlir containing the generated PDL, and the generated C++ glue. This is a custom LSP extension, and is not necessarily provided by all IDE clients.

#### Inlay hints ¶

The language server provides additional information inline with the source code. Editors usually render this using read-only virtual text snippets interspersed with code. Hints may be shown for:

• types of local variables
• names of operand and result groups
• constraint and rewrite arguments

## TableGen LSP Language Server : tblgen-lsp-server¶

MLIR provides an implementation of an LSP language server for .td text files in the form of the tblgen-lsp-server tool. This tool interacts with the TableGen C++ API to support rich language queries, such as “Find Definition”.

### Compilation Database ¶

Similarly to clangd, and language servers for various other programming languages, the TableGen language server relies on a compilation database to provide build-system information for .td files. This information includes, for example, the include directories available for that file. This database allows for the server to interact with .td files using the same configuration as when building.

#### Format ¶

A TableGen compilation database is a YAML file, conventionally named tablegen_compile_commands.yml, that contains a set of FileInfo documents providing information for individiual .td files.

Example:

--- !FileInfo:
filepath: "/home/user/llvm/mlir/lib/Dialect/Arith/IR/ArithCanonicalization.td"
includes: "/home/user/llvm/mlir/lib/Dialect/Arith/IR;/home/user/llvm/mlir/include"

• filepath: - Absolute file path of the file.
• includes: - Semi-colon delimited list of absolute include directories.

#### Build System Integration ¶

Per convention, TableGen compilation databases should be named tablegen_compile_commands.yml and placed at the top of the build directory. When using CMake and mlir_tablegen, a compilation database is generally automatically built and placed in the appropriate location.

### Features ¶

This section details a few of the features that the TableGen language server provides. The screenshots are shown in VSCode, but the exact feature set available will depend on your editor client.

#### Diagnostics ¶

The language server actively runs verification as you type, showing any generated diagnostics in-place.

#### Cross-references ¶

Cross references allow for navigating the code base.

##### Find definition ¶

Jump to the definition of a symbol under the cursor:

##### Find references ¶

Show all references of the symbol under the cursor.

#### Hover ¶

Hover over a symbol to see more information about it, such as its type, documentation, and more.

Hovering over an overridden field will also show you information such as documentation from the base value:

## Language Server Design ¶

The design of the various language servers provided by MLIR are effectively the same, and are largely comprised of three different components:

• Communication and Transport (via JSON-RPC)
• Language Server Protocol
• Language-Specific Server

### Communication and Transport ¶

The language server, such as mlir-lsp-server, communicates with the language client via JSON-RPC over stdin/stdout. In the code, this is the JSONTransport class. This class knows nothing about the Language Server Protocol, it only knows that JSON-RPC messages are coming in and JSON-RPC messages are going out. The handling of incoming and outgoing LSP messages is left to the MessageHandler class. This class routes incoming messages to handlers in the Language Server Protocol layer for interpretation, and packages outgoing messages for transport. This class also has limited knowledge of the LSP, and only has information about the three main classes of messages: notifications, calls, and replies.

### Language Server Protocol ¶

LSPServer handles the interpretation of the finer LSP details. This class registers handlers for LSP messages and then forwards to the Language-Specific Server for processing. The intent of this component is to hold all of the necessary glue when communicating from the LSP world to the language-specific world (e.g. MLIR, PDLL, etc.). In most cases, the LSP message handlers simply forward directly to the Language-Specific Server. In some cases, however, the impedance mismatch between the two requires more complicated glue code.

### Language-Specific Server ¶

The language specific server, such as MLIRServer or PDLLServer, provides the internal implementation of all of LSP queries for a specific language. These are the classes that directly interacts with the C++ API for the language, including parsing text files, interpreting definition/reference information, etc.

## Editor Plugins ¶

LSP Language plugins are available for many popular editors, and in principle the language servers provided by MLIR should work with any of them, though feature sets and interfaces may vary. Below are a set of plugins that are known to work:

### Visual Studio Code ¶

Provides language IDE features for MLIR related languages: MLIR, PDLL, and TableGen

#### .mlir - MLIR textual assembly format: ¶

The MLIR extension adds language support for the MLIR textual assembly format:

##### Features ¶
• Syntax highlighting for .mlir files and mlir markdown blocks
• go-to-definition and cross references
• Detailed information when hovering over IR entities
• Outline and navigation of symbols and symbol tables
• Code completion
• Live parser and verifier diagnostics
##### Setup ¶
###### mlir-lsp-server¶

The various .mlir language features require the mlir-lsp-server language server. If mlir-lsp-server is not found within your workspace path, you must specify the path of the server via the mlir.server_path setting. The path of the server may be absolute or relative within your workspace.

#### .pdll - MLIR PDLL pattern files: ¶

The MLIR extension adds language support for the PDLL pattern language.

##### Features ¶
• Syntax highlighting for .pdll files and pdll markdown blocks
• go-to-definition and cross references
• Types and documentation on hover
• Code completion and signature help
• View intermediate AST, MLIR, or C++ output
##### Setup ¶
###### mlir-pdll-lsp-server¶

The various .pdll language features require the mlir-pdll-lsp-server language server. If mlir-pdll-lsp-server is not found within your workspace path, you must specify the path of the server via the mlir.pdll_server_path setting. The path of the server may be absolute or relative within your workspace.

###### Project setup ¶

To properly understand and interact with .pdll files, the language server must understand how the project is built (compile flags). pdll_compile_commands.yml files related to your project should be provided to ensure files are properly processed. These files can usually be generated by the build system, and the server will attempt to find them within your build/ directory. If not available in or a unique location, additional pdll_compile_commands.yml files may be specified via the mlir.pdll_compilation_databases setting. The paths of these databases may be absolute or relative within your workspace.

#### .td - TableGen files: ¶

The MLIR extension adds language support for the TableGen language.

##### Features ¶
• Syntax highlighting for .td files and tablegen markdown blocks
• go-to-definition and cross references
• Types and documentation on hover
##### Setup ¶
###### tblgen-lsp-server¶

The various .td language features require the tblgen-lsp-server language server. If tblgen-lsp-server is not found within your workspace path, you must specify the path of the server via the mlir.tablegen_server_path setting. The path of the server may be absolute or relative within your workspace.

###### Project setup ¶

To properly understand and interact with .td files, the language server must understand how the project is built (compile flags). tablegen_compile_commands.yml files related to your project should be provided to ensure files are properly processed. These files can usually be generated by the build system, and the server will attempt to find them within your build/ directory. If not available in or a unique location, additional tablegen_compile_commands.yml files may be specified via the mlir.tablegen_compilation_databases setting. The paths of these databases may be absolute or relative within your workspace.

#### Contributing ¶

This extension is actively developed within the LLVM monorepo, at mlir/utils/vscode. As such, contributions should follow the normal LLVM guidelines, with code reviews sent to phabricator.

When developing or deploying this extension within the LLVM monorepo, a few extra setup steps are required:

• Copy mlir/utils/textmate/mlir.json to the extension directory and rename to grammar.json.
• Copy llvm/utils/textmate/tablegen.json to the extension directory and rename to tablegen-grammar.json.
• Copy https://mlir.llvm.org//LogoAssets/logo/PNG/full_color/mlir-identity-03.png to the extension directory and rename to icon.png.

Please follow the existing code style when contributing to the extension, we recommend to run npm run format before sending a patch.