[mlir][ods] Add documentation on how to use sharded op definitions (NFC) (#89664)

This adds explanations and instructions on how to set up a dialect for
sharded op definitions to the MLIR documentation.

Author: Jeff Niu

mlir/docs/DefiningDialects/Operations.md

@@ -1114,6 +1114,100 @@ void process(AddOp op, ArrayRef<Value> newOperands) {
 }
 ```
 
+#### Sharded Operation Definitions
+
+Large dialects with many operations may struggle with C++ compile time of
+generated op definitions, due to large compilation units. `mlir-tblgen`
+provides the ability to shard op definitions by splitting them up evenly
+by passing `-op-shard-count` to `-gen-op-defs` and `-gen-op-decls`. The tool
+will generate a single include file for the definitions broken up by
+`GET_OP_DEFS_${N}` where `${N}` is the shard number. A shard can be compiled in
+a single compilation unit by adding a file like this to your dialect library:
+
+```c++
+#include "mlir/IR/Operation.h"
+// Add any other required includes.
+
+// Utilities shared by generated op definitions: custom directive parsers,
+// printers, etc.
+#include "OpUtils.h"
+
+#define GET_OP_DEFS_0
+#include "MyDialectOps.cpp.inc"
+```
+
+Note: this requires restructing shared utility functions within the dialect
+library so they can be shared by multiple compilation units. I.e. instead of
+defining `static` methods in the same source file, you should declare them in a
+shared header and define them in their own source file.
+
+The op registration hooks are also sharded, because the template instantiation
+can take a very long time to compile. Operations should be registered in your
+dialect like:
+
+```c++
+void MyDialect::initialize() {
+  registerMyDialectOperations(this);
+}
+```
+
+CMake and Bazel functions are included to make sharding dialects easier.
+Assuming you have organized your operation utility functions into their own
+header, define a file that looks like the one above, but without the `#define`:
+
+```c++
+// MyDialectOps.cpp
+#include "mlir/IR/Operation.h"
+
+#include "OpUtils.h"
+
+#include "MyDialectOps.cpp.inc"
+```
+
+In CMake, remove the manual `mlir_tablegen` invocations and replace them with:
+
+```cmake
+set(LLVM_TARGET_DEFINITIONS MyDialectOps.td)
+add_sharded_ops(MyDialectOps 8) # shard the op definitions by 8
+
+add_mlir_library(MyDialect
+  MyDialect.cpp
+  MyDialectOpDefs.cpp
+  ${SHARDED_SRCS}
+
+  DEPENDS
+  MLIRTestOpsShardGen
+)
+```
+
+This will automatically duplicate the `MyDialectOps.cpp` source file and add the
+`#define` up the number of shards indicated.
+
+It is recommended that any out-of-line op member functions (like verifiers) be
+defined in a separate source file. In this example, it is called
+`MyDialectOpDefs.cpp`.
+
+In Bazel, remove the `-gen-op-defs` and `-gen-op-decls` invocations, and add
+
+```bazel
+gentbl_sharded_ops(
+    name = "MyDialectOpSrcs",
+    hdr_out = "MyDialectOps.h.inc",
+    shard_count = 8,
+    sharder = "//mlir:mlir-src-sharder",
+    src_file = "MyDialectOps.cpp",
+    src_out = "MyDialectOps.cpp.inc",
+    tblgen = "//mlir:mlir-tblgen",
+    td_file = "MyDialectOps.td",
+    deps = [":MyDialectOpsTdFiles"],
+)
+
+cc_library(
+    name = "MyDialect",
+    srcs = glob(["MyDialect/*.cpp"]) + [":MyDialectOpSrcs"]
+)
+```
+
 ## Constraints
 
 Constraint is a core concept in table-driven operation definition: operation