[mlir] Overhaul C/Python registration APIs to properly scope registration/loading activities.

Since the very first commits, the Python and C MLIR APIs have had mis-placed registration/load functionality for dialects, extensions, etc. This was done pragmatically in order to get bootstrapped and then just grew in. Downstreams largely bypass and do their own thing by providing various APIs to register things they need. Meanwhile, the C++ APIs have stabilized around this and it would make sense to follow suit.

The thing we have observed in canonical usage by downstreams is that each downstream tends to have native entry points that configure its installation to its preferences with one-stop APIs. This patch leans in to this approach with `RegisterEverything.h` and `mlir._mlir_libs._mlirRegisterEverything` being the one-stop entry points for the "upstream packages". The `_mlir_libs.__init__.py` now allows customization of the environment and Context by adding "initialization modules" to the `_mlir_libs` package. If present, `_mlirRegisterEverything` is treated as such a module. Others can be added by downstreams by adding a `_site_initialize_{i}.py` module, where '{i}' is a number starting with zero. The number will be incremented and corresponding module loaded until one is not found. Initialization modules can:

* Perform load time customization to the global environment (i.e. registering passes, hooks, etc).
* Define a `register_dialects(registry: DialectRegistry)` function that can extend the `DialectRegistry` that will be used to bootstrap the `Context`.
* Define a `context_init_hook(context: Context)` function that will be added to a list of callbacks which will be invoked after dialect registration during `Context` initialization.

Note that the `MLIRPythonExtension.RegisterEverything` is not included by default when building a downstream (its corresponding behavior was prior). For downstreams which need the default MLIR initialization to take place, they must add this back in to their Python CMake build just like they add their own components (i.e. to `add_mlir_python_common_capi_library` and `add_mlir_python_modules`). It is perfectly valid to not do this, in which case, only the things explicitly depended on and initialized by downstreams will be built/packaged. If the downstream has not been set up for this, it is recommended to simply add this back for the time being and pay the build time/package size cost.

CMake changes:
* `MLIRCAPIRegistration` -> `MLIRCAPIRegisterEverything` (renamed to signify what it does and force an evaluation: a number of places were incidentally linking this very expensive target)
* `MLIRPythonSoure.Passes` removed (without replacement: just drop)
* `MLIRPythonExtension.AllPassesRegistration` removed (without replacement: just drop)
* `MLIRPythonExtension.Conversions` removed (without replacement: just drop)
* `MLIRPythonExtension.Transforms` removed (without replacement: just drop)

Header changes:
* `mlir-c/Registration.h` is deleted. Dialect registration functionality is now in `IR.h`. Registration of upstream features are in `mlir-c/RegisterEverything.h`. When updating MLIR and a couple of downstreams, I found that proper usage was commingled so required making a choice vs just blind S&R.

Python APIs removed:
  * mlir.transforms and mlir.conversions (previously only had an __init__.py which indirectly triggered `mlirRegisterTransformsPasses()` and `mlirRegisterConversionPasses()` respectively). Downstream impact: Remove these imports if present (they now happen as part of default initialization).
  * mlir._mlir_libs._all_passes_registration, mlir._mlir_libs._mlirTransforms, mlir._mlir_libs._mlirConversions. Downstream impact: None expected (these were internally used).

C-APIs changed:
  * mlirRegisterAllDialects(MlirContext) now takes an MlirDialectRegistry instead. It also used to trigger loading of all dialects, which was already marked with a TODO to remove -- it no longer does, and for direct use, dialects must be explicitly loaded. Downstream impact: Direct C-API users must ensure that needed dialects are loaded or call `mlirContextLoadAllAvailableDialects(MlirContext)` to emulate the prior behavior. Also see the `ir.c` test case (e.g. `  mlirContextGetOrLoadDialect(ctx, mlirStringRefCreateFromCString("func"));`).
  * mlirDialectHandle* APIs were moved from Registration.h (which now is restricted to just global/upstream registration) to IR.h, arguably where it should have been. Downstream impact: include correct header (likely already doing so).

C-APIs added:
  * mlirContextLoadAllAvailableDialects(MlirContext): Corresponds to C++ API with the same purpose.

Python APIs added:
  * mlir.ir.DialectRegistry: Mapping for an MlirDialectRegistry.
  * mlir.ir.Context.append_dialect_registry(MlirDialectRegistry)
  * mlir.ir.Context.load_all_available_dialects()
  * mlir._mlir_libs._mlirAllRegistration: New native extension that exposes a `register_dialects(MlirDialectRegistry)` entry point and performs all upstream pass/conversion/transforms registration on init. In this first step, we eagerly load this as part of the __init__.py and use it to monkey patch the Context to emulate prior behavior.
  * Type caster and capsule support for MlirDialectRegistry

This should make it possible to build downstream Python dialects that only depend on a subset of MLIR. See: #56037

Here is an example PR, minimally adapting IREE to these changes: https://github.com/iree-org/iree/pull/9638/files In this situation, IREE is opting to not link everything, since it is already configuring the Context to its liking. For projects that would just like to not think about it and pull in everything, add `MLIRPythonExtension.RegisterEverything` to the list of Python sources getting built, and the old behavior will continue.

Reviewed By: mehdi_amini, ftynse

Differential Revision: https://reviews.llvm.org/D128593

Author: stellaraccident

mlir/examples/standalone/include/Standalone-c/Dialects.h

@@ -9,7 +9,7 @@
 #ifndef STANDALONE_C_DIALECTS_H
 #define STANDALONE_C_DIALECTS_H
 
-#include "mlir-c/Registration.h"
+#include "mlir-c/IR.h"
 
 #ifdef __cplusplus
 extern "C" {

mlir/examples/standalone/python/CMakeLists.txt

@@ -40,6 +40,9 @@ add_mlir_python_common_capi_library(StandalonePythonCAPI
   RELATIVE_INSTALL_ROOT "../../../.."
   DECLARED_SOURCES
     StandalonePythonSources
+    # TODO: Remove this in favor of showing fine grained registration once
+    # available.
+    MLIRPythonExtension.RegisterEverything
     MLIRPythonSources.Core
 )
 
@@ -52,6 +55,9 @@ add_mlir_python_modules(StandalonePythonModules
   INSTALL_PREFIX "python_packages/standalone/mlir_standalone"
   DECLARED_SOURCES
     StandalonePythonSources
+    # TODO: Remove this in favor of showing fine grained registration once
+    # available.
+    MLIRPythonExtension.RegisterEverything
     MLIRPythonSources
   COMMON_CAPI_LINK_LIBS
     StandalonePythonCAPI

mlir/examples/standalone/test/CAPI/CMakeLists.txt

@@ -6,7 +6,9 @@ add_mlir_aggregate(StandaloneCAPITestLib
   SHARED
   EMBED_LIBS
   MLIRCAPIIR
-  MLIRCAPIRegistration
+  # TODO: Remove this in favor of showing fine grained dialect registration
+  # (once available).
+  MLIRCAPIRegisterEverything
   StandaloneCAPI
 )
 

mlir/examples/standalone/test/CAPI/standalone-capi-test.c

@@ -11,14 +11,22 @@
 
 #include <stdio.h>
 
-#include "mlir-c/IR.h"
 #include "Standalone-c/Dialects.h"
+#include "mlir-c/IR.h"
+#include "mlir-c/RegisterEverything.h"
+
+static void registerAllUpstreamDialects(MlirContext ctx) {
+  MlirDialectRegistry registry = mlirDialectRegistryCreate();
+  mlirRegisterAllDialects(registry);
+  mlirContextAppendDialectRegistry(ctx, registry);
+  mlirDialectRegistryDestroy(registry);
+}
 
 int main(int argc, char **argv) {
   MlirContext ctx = mlirContextCreate();
   // TODO: Create the dialect handles for the builtin dialects and avoid this.
   // This adds dozens of MB of binary size over just the standalone dialect.
-  mlirRegisterAllDialects(ctx);
+  registerAllUpstreamDialects(ctx);
   mlirDialectHandleRegisterDialect(mlirGetDialectHandle__standalone__(), ctx);
 
   MlirModule module = mlirModuleCreateParse(

mlir/include/mlir-c/Bindings/Python/Interop.h

@@ -64,6 +64,8 @@
   MAKE_MLIR_PYTHON_QUALNAME("ir.Attribute._CAPIPtr")
 #define MLIR_PYTHON_CAPSULE_CONTEXT                                            \
   MAKE_MLIR_PYTHON_QUALNAME("ir.Context._CAPIPtr")
+#define MLIR_PYTHON_CAPSULE_DIALECT_REGISTRY                                   \
+  MAKE_MLIR_PYTHON_QUALNAME("ir.DialectRegistry._CAPIPtr")
 #define MLIR_PYTHON_CAPSULE_EXECUTION_ENGINE                                   \
   MAKE_MLIR_PYTHON_QUALNAME("execution_engine.ExecutionEngine._CAPIPtr")
 #define MLIR_PYTHON_CAPSULE_INTEGER_SET                                        \
@@ -172,6 +174,28 @@ static inline MlirContext mlirPythonCapsuleToContext(PyObject *capsule) {
   return context;
 }
 
+/** Creates a capsule object encapsulating the raw C-API MlirDialectRegistry.
+ * The returned capsule does not extend or affect ownership of any Python
+ * objects that reference the context in any way.
+ */
+static inline PyObject *
+mlirPythonDialectRegistryToCapsule(MlirDialectRegistry registry) {
+  return PyCapsule_New(registry.ptr, MLIR_PYTHON_CAPSULE_DIALECT_REGISTRY,
+                       NULL);
+}
+
+/** Extracts an MlirDialectRegistry from a capsule as produced from
+ * mlirPythonDialectRegistryToCapsule. If the capsule is not of the right type,
+ * then a null context is returned (as checked via mlirContextIsNull). In such a
+ * case, the Python APIs will have already set an error. */
+static inline MlirDialectRegistry
+mlirPythonCapsuleToDialectRegistry(PyObject *capsule) {
+  void *ptr =
+      PyCapsule_GetPointer(capsule, MLIR_PYTHON_CAPSULE_DIALECT_REGISTRY);
+  MlirDialectRegistry registry = {ptr};
+  return registry;
+}
+
 /** Creates a capsule object encapsulating the raw C-API MlirLocation.
  * The returned capsule does not extend or affect ownership of any Python
  * objects that reference the location in any way. */

mlir/include/mlir-c/Dialect/Async.h

@@ -10,7 +10,7 @@
 #ifndef MLIR_C_DIALECT_ASYNC_H
 #define MLIR_C_DIALECT_ASYNC_H
 
-#include "mlir-c/Registration.h"
+#include "mlir-c/IR.h"
 #include "mlir-c/Support.h"
 
 #ifdef __cplusplus

mlir/include/mlir-c/Dialect/ControlFlow.h

@@ -10,7 +10,7 @@
 #ifndef MLIR_C_DIALECT_CONTROLFLOW_H
 #define MLIR_C_DIALECT_CONTROLFLOW_H
 
-#include "mlir-c/Registration.h"
+#include "mlir-c/IR.h"
 
 #ifdef __cplusplus
 extern "C" {

mlir/include/mlir-c/Dialect/Func.h

@@ -18,7 +18,7 @@
 #ifndef MLIR_C_DIALECT_FUNC_H
 #define MLIR_C_DIALECT_FUNC_H
 
-#include "mlir-c/Registration.h"
+#include "mlir-c/IR.h"
 
 #ifdef __cplusplus
 extern "C" {

mlir/include/mlir-c/Dialect/GPU.h

@@ -10,7 +10,7 @@
 #ifndef MLIR_C_DIALECT_GPU_H
 #define MLIR_C_DIALECT_GPU_H
 
-#include "mlir-c/Registration.h"
+#include "mlir-c/IR.h"
 #include "mlir-c/Support.h"
 
 #ifdef __cplusplus

mlir/include/mlir-c/Dialect/LLVM.h

@@ -11,7 +11,6 @@
 #define MLIR_C_DIALECT_LLVM_H
 
 #include "mlir-c/IR.h"
-#include "mlir-c/Registration.h"
 
 #ifdef __cplusplus
 extern "C" {