[Mlir-commits] [mlir] Add a tutorial on mlir-opt (PR #96105)

[Jeremy Kun]

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+# Using `mlir-opt`
+
+`mlir-opt` is a command-line entry point for running passes and lowerings on MLIR code.
+This tutorial will explain how to use `mlir-opt`, show some examples of its usage,
+and mention some useful tips for working with it.
+
+Prerequisites:
+
+- [Building MLIR from source](/getting_started/)
+- [MLIR Language Reference](/docs/LangRef/)
+
+[TOC]
+
+## `mlir-opt` basics
+
+The `mlir-opt` tool loads a textual IR or bytecode into an in-memory structure,
+and optionally executes a sequence of passes
+before serializing back the IR (textual form by default).
+It is intended as a testing and debugging utility.
+
+After building the MLIR project,
+the `mlir-opt` binary (located in `build/bin`)
+is the entry point for running passes and lowerings,
+as well as emitting debug and diagnostic data.
+
+Running `mlir-opt` with no flags will consume textual or bytecode IR
+from standard in, parse and run verifiers on it,
+and write the textual format back to standard out.
+This is a good way to test if an input MLIR is well-formed.
+
+`mlir-opt --help` shows a complete list of flags
+(there are nearly 1000).
+Each pass has its own flag,
+though it is recommended to use `--pass-pipeline`
+to run passes rather than bare flags.
+
+## Running a pass
+
+Next we run [`convert-to-llvm`](/docs/Passes/#-convert-to-llvm),
+which converts all supported dialects to the `llvm` dialect,
+on the following IR:
+
+```mlir
+// mlir/test/Examples/mlir-opt/ctlz.mlir
+func.func @main(%arg0: i32) -> i32 {
+  %0 = math.ctlz %arg0 : i32
+  func.return %0 : i32
+}
+```
+
+After building MLIR, and from the `llvm-project` base directory, run
+
+```bash
+build/bin/mlir-opt --pass-pipeline="builtin.module(convert-math-to-llvm)" mlir/test/Examples/mlir-opt/ctlz.mlir
+```
+
+which produces
+
+```mlir
+module {
----------------
j2kun wrote:

Added explicit modules

https://github.com/llvm/llvm-project/pull/96105