A VS Code extension for running LLVM lit RUN commands interactively — on a single MLIR section, a text selection, or a Python lit test — with the output displayed in a side panel with full LSP support.

Features

Run a single section — Place your cursor inside a // ----- delimited section in an MLIR file and press Ctrl+Shift+M. The extension extracts that section, runs the file's RUN command on it (stripping -split-input-file and | FileCheck), and shows the output alongside.
Run a selection — Select any MLIR text and run the command on just that selection.
Run Python lit tests — Works on .py files with # RUN: directives too.
LSP on output — Output is written to a .mlir file, so your MLIR LSP server (e.g. mlir-lsp-server) attaches automatically. You get syntax highlighting, go-to-definition, hover info, and diagnostics on the transformed output.
Right-click menu — "Lit: Run on selection/section" appears in the editor context menu for .mlir and .py files.
Automatic substitution resolution — %PYTHON is resolved from VS Code's selected Python interpreter. Other lit substitutions are resolved dynamically at activation time.

How It Works

MLIR Section Extraction

MLIR lit tests use // ----- separators with the -split-input-file flag to define independent test cases in a single file:

// RUN: my-opt -some-pass -split-input-file %s | FileCheck %s

func.func @test_a() { ... }

// -----

func.func @test_b() { ... }

When you press Ctrl+Shift+M with your cursor in @test_b, the extension:

Extracts only the @test_b section
Writes it to a temp file
Runs my-opt -some-pass <temp-file> (no -split-input-file, no FileCheck)
Shows the output in a side-by-side .mlir tab named filename → test_b

Substitution Resolution (Hybrid Approach)

Lit tests use %-substitutions like %s, %t, %PYTHON, and project-specific variables. The extension resolves these through three layers:

Variable	Source
`%s`	Replaced with the input file path
`%t`	Replaced with a temp output path
`%PYTHON`	VS Code Python extension's selected interpreter
Other `%`-vars	Lit echo trick: creates a temp `.mlir` in `build/test/` with `// RUN: echo "MARKER:%var"`, runs `lit -a` on it, parses the output
Any variable	User override via `litRunner.substitutions` setting (highest priority)

Output as Real Files

Output is written to /tmp/lit-runner-output/<name>.mlir:

LSP support — MLIR LSP servers only attach to file: scheme documents. Virtual documents (lit-output: scheme) get syntax highlighting but no go-to-def, hover, or diagnostics.
Re-running — Same section → same filename → file gets overwritten and reverted in the editor automatically.

Settings

Setting	Type	Default	Description
`litRunner.substitutions`	`object`	`{}`	Extra `%`-substitution overrides as key-value pairs

Example in .vscode/settings.json:

{
  "litRunner.substitutions": {
    "%MY_CUSTOM_TOOL": "/opt/tools/bin/my-tool",
    "%PYTHON": "/usr/bin/python3.12"
  }
}

Keybinding

Default: Ctrl+Shift+M (Cmd+Shift+M on Mac)

To customize, add to keybindings.json:

{
  "key": "ctrl+alt+r",
  "command": "lit-runner.run",
  "when": "editorTextFocus"
}

Commands

Command	Title	Description
`lit-runner.run`	Lit: Run on selection/section	Run the RUN command on the current section or selection
`lit-runner.refreshSubstitutions`	Lit: Refresh substitutions	Re-resolve all lit substitutions (useful after rebuilding)

Requirements

VS Code ≥ 1.80
lit on PATH (for the echo trick; falls back to static defaults if unavailable)
The tool referenced in your RUN line (e.g. mlir-opt) must be on PATH or referenced by absolute path
(Optional) MLIR extension for LSP on output files
(Optional) Python extension for %PYTHON resolution

License

MIT

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