Add a basic C API for the MLIR PassManager as well as a basic TableGen backend for creating passes

[mehdi_amini]

Summary

This is exposing the basic functionalities (create, nest, addPass, run) of
the PassManager through the C API in the new header: include/mlir-c/Pass.h.

In order to exercise it in the unit-test, a basic TableGen backend is
also provided to generate a simple C wrapper around the pass
constructor. It is used to expose the libTransforms passes to the C API.

[stellaraccident]

Naming/style/doc nits.

I am interested to see where you take the pass constructor generation/registry/etc but don't need to know more for this patch.

mlir/include/mlir-c/Pass.h

55

Here and below: s/pass_manager/passManager/

63

Possibly add a comment to the end: To create a second-level nested pass manager, use ...other function....

64

I had to read this several times and refer to the C++ documentation to grok. I think that I would have understood it better if the name of this function read like a factory/constructor. e.g. mlirPassManagerNestNew or mlirPassManagerCreateNested

65

s/operation/operationName/ ?

75

Ditto on naming style with the top-level nest. Also, I think this makes more sense directly under mlirPassManagerNest.

76

s/operation/operationName/ ?

mlir/include/mlir-c/Transforms.h

19

I believe that this is the first time that a public mlir-c header file includes something from the mlir tree. That is going to be unfortunate when it comes to packaging and installation (i.e. only wanting to expose C headers).

I'm fine to wait and see what you do here if you are just staging things to get started. Possibly drop a TODO.

mlir/lib/CAPI/IR/Pass.cpp

28

s/pass_manager/passManager/

[mehdi_amini]

mlir/include/mlir-c/Pass.h

64

Isn't "Create" conveying a notion of ownership? The returned pass manager here is attached to the provided one.

mlir/include/mlir-c/Transforms.h

19

This is purely a CMake question at the moment: our TableGen plumbing does not allow to emit the output somewhere else than where the .td is located.

[mehdi_amini]

Address comments

[mehdi_amini]

Clang-format

[ftynse]

LGTM provided that we discuss how to generalize the tablegen infra and file organization wrt individual libraries.

mlir/include/mlir-c/Transforms.h

19

We will likely need to restructure Tablegen infra a bit. I was also pondering the question whether we want to have Tablegen backends for bindings to live under tools/mlir-tblgen or under bindings/.

mlir/include/mlir/Transforms/CAPI.h

9–14 ↗

(On Diff #302505)

Do we actually need this file? There is only one user currently, and this user is a test, so I wouldn't mind if it included .inc directly. We don't have a CAPI.h under IR

mlir/lib/Transforms/CAPIPasses.cpp

1 ↗

(On Diff #302505)

So far, we tried not to add C API related files to the "main" libraries. The risk is to have C-related functionality creep into C++ and make it hard to separate the two. I suppose this is due to Tablegen setup not being able to emit files elsewhere, so I am happy to discuss the future layering.

mlir/test/CAPI/pass.c

51

Nit: EXIT_FAILURE ?

86

Nit: camelBack

102

Nit: 80 cols

mlir/tools/mlir-tblgen/PassCAPIGen.cpp

41

Ultra-nit: add a trailing dot to the comment.

[mehdi_amini]

Address Alex's comments

[mehdi_amini]

Fix clang-tidy header guards

[mehdi_amini]

mlir/include/mlir/Transforms/CAPI.h

9–14 ↗

(On Diff #302505)

Yes this file is bogus, deleting

mlir/lib/Transforms/CAPIPasses.cpp

1 ↗

(On Diff #302505)

Moved to lib/CAPI now

[mehdi_amini]

mlir/include/mlir-c/Pass.h

64

Ping on this naming question?
@ftynse an opinion?

mlir/include/mlir-c/Transforms.h

19

I am not sure, do you want to address this now?

[ftynse]

mlir/include/mlir-c/Pass.h

64

Indeed, Create indicates ownership transfer to the caller. We use Get when there is no ownership transfer, e.g. for types and attributes that may be allocated under the hood. Not sure if is the best choice here.

mlir/include/mlir-c/Transforms.h

19

Nah, just mentioning that it is something we need to think about eventually.

[mehdi_amini]

mlir/include/mlir-c/Pass.h

64

What about mlirPassManagerGetNested() or mlirPassManagerGetNestedUnder() ?

[mehdi_amini]

Rename mlirOpPassManagerGetNestedUnder

[stellaraccident]

mlir/include/mlir-c/Pass.h

64

I like mlirPassManagerGetNested(). "Get" as the verb makes more sense to me vs "nest".

[awarzynski]

Hi @mehdi_amini , thank you for working on this!

Sadly This has caused our Flang buildbots to start failing: http://lab.llvm.org:8011/#/builders/33/builds/478

I'm guessing that you haven't tested with BUILD_SHARED_LIBS? The fix seemed trivial and took the liberty of pushing a fix: https://github.com/llvm/llvm-project/commit/d007bbd986d9ab004c34efaca27aff4a7633b9e9.

I hope that people don't mind. It's not clear to me _why_ static builds are fine (even without the fix). Perhaps I missed something obvious?

Also, if you do mind such ad-hoc fixes, that's a very valuable feedback for me :)

[mehdi_amini]

In D90667#2373257, @awarzynski wrote:

Sadly This has caused our Flang buildbots to start failing: http://lab.llvm.org:8011/#/builders/33/builds/478

I'm guessing that you haven't tested with BUILD_SHARED_LIBS?

Indeed!

The fix seemed trivial and took the liberty of pushing a fix: https://github.com/llvm/llvm-project/commit/d007bbd986d9ab004c34efaca27aff4a7633b9e9.

Thanks! In the future, if your bot is broken and the fix isn't trivial, feel free to push a revert for my patch. Getting the bot back to green is the priority of course.

I hope that people don't mind. It's not clear to me _why_ static builds are fine (even without the fix). Perhaps I missed something obvious?

The reasons why this is working in static library is that linking is delayed to the final binary: as long as the missing dependency is also present in the final binary link it'll be fine.
With the shared library build, for each library we link it in isolation and resolve the dependencies at this point.

[awarzynski]

In D90667#2374428, @mehdi_amini wrote:

The reasons why this is working in static library is that linking is delayed to the final binary: as long as the missing dependency is also present in the final binary link it'll be fine.
With the shared library build, for each library we link it in isolation and resolve the dependencies at this point.

This makes a lot of sense, thanks for explaining!

mlir/include/mlir-c/Pass.h

+/*===-- mlir-c/Pass.h - C API to Pass Management ------------------*- C -*-===*\
+|*                                                                            *|
+|* Part of the LLVM Project, under the Apache License v2.0 with LLVM          *|
+|* Exceptions.                                                                *|
+|* See https://llvm.org/LICENSE.txt for license information.                  *|
+|* SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception                    *|
+|*                                                                            *|
+|*===----------------------------------------------------------------------===*|
+|*                                                                            *|
+|* This header declares the C interface to MLIR pass manager.                 *|
+|*                                                                            *|
+\*===----------------------------------------------------------------------===*/
+
+#ifndef MLIR_C_PASS_H
+#define MLIR_C_PASS_H
+
+#include "mlir-c/IR.h"
+#include "mlir-c/Support.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/*============================================================================*/
+/** Opaque type declarations.
+ *
+ * Types are exposed to C bindings as structs containing opaque pointers. They
+ * are not supposed to be inspected from C. This allows the underlying
+ * representation to change without affecting the API users. The use of structs
+ * instead of typedefs enables some type safety as structs are not implicitly
+ * convertible to each other.
+ *
+ * Instances of these types may or may not own the underlying object. The
+ * ownership semantics is defined by how an instance of the type was obtained.
+ */
+/*============================================================================*/
+
+#define DEFINE_C_API_STRUCT(name, storage)                                     \
+  struct name {                                                                \
+    storage *ptr;                                                              \
+  };                                                                           \
+  typedef struct name name
+
+DEFINE_C_API_STRUCT(MlirPass, void);
+DEFINE_C_API_STRUCT(MlirPassManager, void);
+DEFINE_C_API_STRUCT(MlirOpPassManager, void);
+
+#undef DEFINE_C_API_STRUCT
+
+/** Create a new top-level PassManager. */
+MlirPassManager mlirPassManagerCreate(MlirContext ctx);
+
+/** Destroy the provided PassManager. */
+void mlirPassManagerDestroy(MlirPassManager passManager);
+
+/** Run the provided `passManager` on the given `module`. */
+MlirLogicalResult mlirPassManagerRun(MlirPassManager passManager,
+                                     MlirModule module);
+
+/** Nest an OpPassManager under the top-level PassManager, the nested
+ * passmanager will only run on operations matching the provided name.
+ * The returned OpPassManager will be destroyed when the parent is destroyed.
+ * To further nest more OpPassManager under the newly returned one, see
+ * `mlirOpPassManagerNest` below. */
+MlirOpPassManager mlirPassManagerGetNestedUnder(MlirPassManager passManager,
+                                                MlirStringRef operationName);
+
+/** Nest an OpPassManager under the provided OpPassManager, the nested
+ * passmanager will only run on operations matching the provided name.
+ * The returned OpPassManager will be destroyed when the parent is destroyed. */
+MlirOpPassManager mlirOpPassManagerGetNestedUnder(MlirOpPassManager passManager,
+                                                  MlirStringRef operationName);
+
+/** Add a pass and transfer ownership to the provided top-level mlirPassManager.
+ * If the pass is not a generic operation pass or a ModulePass, a new
+ * OpPassManager is implicitly nested under the provided PassManager. */
+void mlirPassManagerAddOwnedPass(MlirPassManager passManager, MlirPass pass);
+
+/** Add a pass and transfer ownership to the provided mlirOpPassManager. If the
+ * pass is not a generic operation pass or matching the type of the provided
+ * PassManager, a new OpPassManager is implicitly nested under the provided
+ * PassManager. */
+void mlirOpPassManagerAddOwnedPass(MlirOpPassManager passManager,
+                                   MlirPass pass);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif // MLIR_C_PASS_H

mlir/include/mlir-c/Transforms.h

+/*===-- mlir-c/Transforms.h - Helpers for C API to Core MLIR ------*- C -*-===*\
+|*                                                                            *|
+|* Part of the LLVM Project, under the Apache License v2.0 with LLVM          *|
+|* Exceptions.                                                                *|
+|* See https://llvm.org/LICENSE.txt for license information.                  *|
+|* SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception                    *|
+|*                                                                            *|
+|*===----------------------------------------------------------------------===*|
+|*                                                                            *|
+|* This header declares the registration and creation method for              *|
+|* transformation passes.                                                     *|
+|*                                                                            *|
+\*===----------------------------------------------------------------------===*/
+
+#ifndef MLIR_C_TRANSFORMS_H
+#define MLIR_C_TRANSFORMS_H
+
+#include "mlir/Transforms/Transforms.capi.h.inc"
+
+#endif // MLIR_C_TRANSFORMS_H

mlir/include/mlir/CAPI/IR.h

 //===- IR.h - C API Utils for Core MLIR classes -----------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This file contains declarations of implementation details of the C API for
 // core MLIR classes. This file should not be included from C++ code other than
 // C API implementation nor from C code.
 //
 //===----------------------------------------------------------------------===//
 
-#ifndef MLIR_INCLUDE_MLIR_CAPI_IR_H
-#define MLIR_INCLUDE_MLIR_CAPI_IR_H
+#ifndef MLIR_CAPI_IR_H
+#define MLIR_CAPI_IR_H
 
 #include "mlir/CAPI/Wrap.h"
 #include "mlir/IR/Identifier.h"
 #include "mlir/IR/MLIRContext.h"
 #include "mlir/IR/Module.h"
 #include "mlir/IR/Operation.h"
 
 DEFINE_C_API_PTR_METHODS(MlirContext, mlir::MLIRContext)
 DEFINE_C_API_PTR_METHODS(MlirDialect, mlir::Dialect)
 DEFINE_C_API_PTR_METHODS(MlirOperation, mlir::Operation)
 DEFINE_C_API_PTR_METHODS(MlirBlock, mlir::Block)
 DEFINE_C_API_PTR_METHODS(MlirOpPrintingFlags, mlir::OpPrintingFlags)
 DEFINE_C_API_PTR_METHODS(MlirRegion, mlir::Region)
 
 DEFINE_C_API_METHODS(MlirAttribute, mlir::Attribute)
 DEFINE_C_API_METHODS(MlirIdentifier, mlir::Identifier)
 DEFINE_C_API_METHODS(MlirLocation, mlir::Location)
 DEFINE_C_API_METHODS(MlirModule, mlir::ModuleOp)
 DEFINE_C_API_METHODS(MlirType, mlir::Type)
 DEFINE_C_API_METHODS(MlirValue, mlir::Value)
 
-#endif // MLIR_INCLUDE_MLIR_CAPI_IR_H
+#endif // MLIR_CAPI_IR_H

mlir/include/mlir/CAPI/Pass.h

+//===- IR.h - C API Utils for Core MLIR classes -----------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file contains declarations of implementation details of the C API for
+// core MLIR classes. This file should not be included from C++ code other than
+// C API implementation nor from C code.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef MLIR_CAPI_PASS_H
+#define MLIR_CAPI_PASS_H
+
+#include "mlir-c/Pass.h"
+
+#include "mlir/CAPI/Wrap.h"
+#include "mlir/Pass/Pass.h"
+#include "mlir/Pass/PassManager.h"
+
+DEFINE_C_API_PTR_METHODS(MlirPass, mlir::Pass)
+DEFINE_C_API_PTR_METHODS(MlirPassManager, mlir::PassManager)
+DEFINE_C_API_PTR_METHODS(MlirOpPassManager, mlir::OpPassManager)
+
+#endif // MLIR_CAPI_PASS_H

mlir/include/mlir/Transforms/CMakeLists.txt

 
 set(LLVM_TARGET_DEFINITIONS Passes.td)
 mlir_tablegen(Passes.h.inc -gen-pass-decls -name Transforms)
+mlir_tablegen(Transforms.capi.h.inc -gen-pass-capi-header --prefix Transforms)
+mlir_tablegen(Transforms.capi.cpp.inc -gen-pass-capi-impl --prefix Transforms)
 add_public_tablegen_target(MLIRTransformsPassIncGen)
 
 add_mlir_doc(Passes -gen-pass-doc GeneralPasses ./)

mlir/lib/CAPI/CMakeLists.txt

 add_subdirectory(IR)
 add_subdirectory(Registration)
 add_subdirectory(Standard)
+add_subdirectory(Transforms)

mlir/lib/CAPI/IR/CMakeLists.txt

 # Main API.
 add_mlir_library(MLIRCAPIIR
   AffineExpr.cpp
   AffineMap.cpp
   Diagnostics.cpp
   IR.cpp
+  Pass.cpp
   StandardAttributes.cpp
   StandardTypes.cpp
   Support.cpp
 
   ADDITIONAL_HEADER_DIRS
   ${MLIR_MAIN_INCLUDE_DIR}/mlir-c
 
   LINK_LIBS PUBLIC
   MLIRIR
   MLIRParser
   MLIRSupport
   )

mlir/lib/CAPI/IR/Pass.cpp

+//===- Pass.cpp - C Interface for General Pass Management APIs ------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "mlir-c/Pass.h"
+
+#include "mlir/CAPI/IR.h"
+#include "mlir/CAPI/Pass.h"
+#include "mlir/CAPI/Support.h"
+#include "mlir/CAPI/Utils.h"
+#include "mlir/Pass/PassManager.h"
+
+using namespace mlir;
+
+/* ========================================================================== */
+/* PassManager/OpPassManager APIs. */
+/* ========================================================================== */
+
+MlirPassManager mlirPassManagerCreate(MlirContext ctx) {
+  return wrap(new PassManager(unwrap(ctx)));
+}
+
+void mlirPassManagerDestroy(MlirPassManager passManager) {
+  delete unwrap(passManager);
+}
+
+MlirLogicalResult mlirPassManagerRun(MlirPassManager passManager,
+                                     MlirModule module) {
+  return wrap(unwrap(passManager)->run(unwrap(module)));
+}
+
+MlirOpPassManager mlirPassManagerGetNestedUnder(MlirPassManager passManager,
+                                                MlirStringRef operationName) {
+  return wrap(&unwrap(passManager)->nest(unwrap(operationName)));
+}
+
+MlirOpPassManager mlirOpPassManagerGetNestedUnder(MlirOpPassManager passManager,
+                                                  MlirStringRef operationName) {
+  return wrap(&unwrap(passManager)->nest(unwrap(operationName)));
+}
+
+void mlirPassManagerAddOwnedPass(MlirPassManager passManager, MlirPass pass) {
+  unwrap(passManager)->addPass(std::unique_ptr<Pass>(unwrap(pass)));
+}
+
+void mlirOpPassManagerAddOwnedPass(MlirOpPassManager passManager,
+                                   MlirPass pass) {
+  unwrap(passManager)->addPass(std::unique_ptr<Pass>(unwrap(pass)));
+}

mlir/lib/CAPI/Transforms/CMakeLists.txt

+add_mlir_library(MLIRCAPITransforms
+
+  Passes.cpp
+
+  ADDITIONAL_HEADER_DIRS
+  ${MLIR_MAIN_INCLUDE_DIR}/mlir-c
+
+  LINK_LIBS PUBLIC
+  MLIRTransforms
+  )

mlir/lib/CAPI/Transforms/Passes.cpp

+//===- CAPIPAsses.cpp - C API for Tranformations Passes -------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "mlir/Transforms/Passes.h"
+#include "mlir/CAPI/Pass.h"
+#include "mlir/Pass/Pass.h"
+
+using namespace mlir;
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include "mlir/Transforms/Transforms.capi.cpp.inc"
+
+#ifdef __cplusplus
+}
+#endif

mlir/test/CAPI/CMakeLists.txt

+set(LLVM_OPTIONAL_SOURCES
+  ir.c
+  pass.c
+)
+
 set(LLVM_LINK_COMPONENTS
   Core
   Support
   )
 
 add_llvm_executable(mlir-capi-ir-test
   ir.c
   )
 llvm_update_compile_flags(mlir-capi-ir-test)
 
 get_property(dialect_libs GLOBAL PROPERTY MLIR_DIALECT_LIBS)
 target_link_libraries(mlir-capi-ir-test
   PRIVATE
   MLIRCAPIIR
   MLIRCAPIRegistration
   MLIRCAPIStandard
   ${dialect_libs})
+
+
+add_llvm_executable(mlir-capi-pass-test
+  pass.c
+  )
+llvm_update_compile_flags(mlir-capi-pass-test)
+
+get_property(dialect_libs GLOBAL PROPERTY MLIR_DIALECT_LIBS)
+target_link_libraries(mlir-capi-pass-test
+  PRIVATE
+  MLIRCAPIIR
+  MLIRCAPIRegistration
+  MLIRCAPIStandard
+  MLIRCAPITransforms
+  ${dialect_libs})

mlir/test/CAPI/pass.c

+/*===- pass.c - Simple test of C APIs -------------------------------------===*\
+|*                                                                            *|
+|* Part of the LLVM Project, under the Apache License v2.0 with LLVM          *|
+|* Exceptions.                                                                *|
+|* See https://llvm.org/LICENSE.txt for license information.                  *|
+|* SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception                    *|
+|*                                                                            *|
+\*===----------------------------------------------------------------------===*/
+
+/* RUN: mlir-capi-pass-test 2>&1 | FileCheck %s
+ */
+
+#include "mlir-c/Pass.h"
+#include "mlir-c/IR.h"
+#include "mlir-c/Registration.h"
+#include "mlir-c/Transforms.h"
+
+#include <assert.h>
+#include <math.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+void testRunPassOnModule() {
+  MlirContext ctx = mlirContextCreate();
+  mlirRegisterAllDialects(ctx);
+
+  MlirModule module =
+      mlirModuleCreateParse(ctx,
+                            // clang-format off
+"func @foo(%arg0 : i32) -> i32 {                                            \n"
+"  %res = addi %arg0, %arg0 : i32                                           \n"
+"  return %res : i32                                                        \n"
+"}");
+  // clang-format on
+  if (mlirModuleIsNull(module))
+    exit(EXIT_FAILURE);
+
+  // Run the print-op-stats pass on the top-level module:
+  // CHECK-LABEL: Operations encountered:
+  // CHECK: func              , 1
+  // CHECK: module_terminator , 1
+  // CHECK: std.addi          , 1
+  // CHECK: std.return        , 1
+  {
+    MlirPassManager pm = mlirPassManagerCreate(ctx);
+    MlirPass printOpStatPass = mlirCreateTransformsPrintOpStats();
+    mlirPassManagerAddOwnedPass(pm, printOpStatPass);
+    MlirLogicalResult success = mlirPassManagerRun(pm, module);
+    if (mlirLogicalResultIsFailure(success))
+      exit(EXIT_FAILURE);
+    mlirPassManagerDestroy(pm);
+  }
+  mlirModuleDestroy(module);
+  mlirContextDestroy(ctx);
+}
+
+void testRunPassOnNestedModule() {
+  MlirContext ctx = mlirContextCreate();
+  mlirRegisterAllDialects(ctx);
+
+  MlirModule module =
+      mlirModuleCreateParse(ctx,
+                            // clang-format off
+"func @foo(%arg0 : i32) -> i32 {                                            \n"
+"  %res = addi %arg0, %arg0 : i32                                           \n"
+"  return %res : i32                                                        \n"
+"}                                                                          \n"
+"module {                                                                   \n"
+"  func @bar(%arg0 : f32) -> f32 {                                          \n"
+"    %res = addf %arg0, %arg0 : f32                                         \n"
+"    return %res : f32                                                      \n"
+"  }                                                                        \n"
+"}");
+  // clang-format on
+  if (mlirModuleIsNull(module))
+    exit(1);
+
+  // Run the print-op-stats pass on functions under the top-level module:
+  // CHECK-LABEL: Operations encountered:
+  // CHECK-NOT: module_terminator
+  // CHECK: func              , 1
+  // CHECK: std.addi          , 1
+  // CHECK: std.return        , 1
+  {
+    MlirPassManager pm = mlirPassManagerCreate(ctx);
+    MlirOpPassManager nestedFuncPm = mlirPassManagerGetNestedUnder(
+        pm, mlirStringRefCreateFromCString("func"));
+    MlirPass printOpStatPass = mlirCreateTransformsPrintOpStats();
+    mlirOpPassManagerAddOwnedPass(nestedFuncPm, printOpStatPass);
+    MlirLogicalResult success = mlirPassManagerRun(pm, module);
+    if (mlirLogicalResultIsFailure(success))
+      exit(2);
+    mlirPassManagerDestroy(pm);
+  }
+  // Run the print-op-stats pass on functions under the nested module:
+  // CHECK-LABEL: Operations encountered:
+  // CHECK-NOT: module_terminator
+  // CHECK: func              , 1
+  // CHECK: std.addf          , 1
+  // CHECK: std.return        , 1
+  {
+    MlirPassManager pm = mlirPassManagerCreate(ctx);
+    MlirOpPassManager nestedModulePm = mlirPassManagerGetNestedUnder(
+        pm, mlirStringRefCreateFromCString("module"));
+    MlirOpPassManager nestedFuncPm = mlirOpPassManagerGetNestedUnder(
+        nestedModulePm, mlirStringRefCreateFromCString("func"));
+    MlirPass printOpStatPass = mlirCreateTransformsPrintOpStats();
+    mlirOpPassManagerAddOwnedPass(nestedFuncPm, printOpStatPass);
+    MlirLogicalResult success = mlirPassManagerRun(pm, module);
+    if (mlirLogicalResultIsFailure(success))
+      exit(2);
+    mlirPassManagerDestroy(pm);
+  }
+
+  mlirModuleDestroy(module);
+  mlirContextDestroy(ctx);
+}
+
+int main() {
+  testRunPassOnModule();
+  testRunPassOnNestedModule();
+  return 0;
+}

mlir/test/CMakeLists.txt

configure_lit_site_cfg(
   MAIN_CONFIG
   ${CMAKE_CURRENT_SOURCE_DIR}/Unit/lit.cfg.py
   )
 
 set(MLIR_TEST_DEPENDS
   FileCheck count not
   MLIRUnitTests
   mlir-capi-ir-test
+  mlir-capi-pass-test
   mlir-cpu-runner
   mlir-edsc-builder-api-test
   mlir-linalg-ods-gen
   mlir-opt
   mlir-reduce
   mlir-sdbm-api-test
   mlir-tblgen
   mlir-translate

mlir/test/lit.cfg.py

 llvm_config.with_environment('PATH', config.llvm_tools_dir, append_path=True)
 
 tool_dirs = [config.mlir_tools_dir, config.llvm_tools_dir]
 tools = [
     'mlir-opt',
     'mlir-tblgen',
     'mlir-translate',
     'mlir-capi-ir-test',
+    'mlir-capi-pass-test',
     'mlir-edsc-builder-api-test',
 ]
 
 # The following tools are optional
 tools.extend([
     ToolSubst('%PYTHON', config.python_executable, unresolved='ignore'),
     ToolSubst('toy-ch1', unresolved='ignore'),
     ToolSubst('toy-ch2', unresolved='ignore'),

mlir/tools/mlir-tblgen/CMakeLists.txt

add_tablegen(mlir-tblgen MLIR
   LLVMIRConversionGen.cpp
   LLVMIRIntrinsicGen.cpp
   mlir-tblgen.cpp
   OpDefinitionsGen.cpp
   OpDocGen.cpp
   OpFormatGen.cpp
   OpInterfacesGen.cpp
   OpenMPCommonGen.cpp
-  PassGen.cpp
+  PassCAPIGen.cpp
   PassDocGen.cpp
+  PassGen.cpp
   RewriterGen.cpp
   SPIRVUtilsGen.cpp
   StructsGen.cpp
   TypeDefGen.cpp
   )
 
 set_target_properties(mlir-tblgen PROPERTIES FOLDER "Tablegenning")
 target_link_libraries(mlir-tblgen
   PRIVATE
   MLIRSupportIndentedOstream
   MLIRTableGen)
 
 mlir_check_all_link_libraries(mlir-tblgen)

mlir/tools/mlir-tblgen/PassCAPIGen.cpp

+//===- Pass.cpp - MLIR pass registration generator ------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// PassCAPIGen uses the description of passes to generate C API for the passes.
+//
+//===----------------------------------------------------------------------===//
+
+#include "mlir/TableGen/GenInfo.h"
+#include "mlir/TableGen/Pass.h"
+#include "llvm/ADT/StringExtras.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/FormatVariadic.h"
+#include "llvm/TableGen/Error.h"
+#include "llvm/TableGen/Record.h"
+
+using namespace mlir;
+using namespace mlir::tblgen;
+
+static llvm::cl::OptionCategory
+    passGenCat("Options for -gen-pass-capi-header and -gen-pass-capi-impl");
+static llvm::cl::opt<std::string>
+    groupName("prefix",
+              llvm::cl::desc("The prefix to use for this group of passes. The "
+                             "form will be mlirCreate<prefix><passname>, the "
+                             "prefix can avoid conflicts across libraries."),
+              llvm::cl::cat(passGenCat));
+
+const char *const passDecl = R"(
+/* Create {0} Pass. */
+MlirPass mlirCreate{0}{1}();
+
+)";
+
+const char *const fileHeader = R"(
+/* Autogenerated by mlir-tblgen; don't manually edit. */
+
+#include "mlir-c/Pass.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+)";
+
+const char *const fileFooter = R"(
+
+#ifdef __cplusplus
+}
+#endif
+)";
+
+/// Emit TODO
+static bool emitCAPIHeader(const llvm::RecordKeeper &records, raw_ostream &os) {
+  os << fileHeader;
+  for (const auto *def : records.getAllDerivedDefinitions("PassBase")) {
+    Pass pass(def);
+    StringRef defName = pass.getDef()->getName();
+    os << llvm::formatv(passDecl, groupName, defName);
+  }
+  os << fileFooter;
+  return false;
+}
+
+const char *const passCreateDef = R"(
+MlirPass mlirCreate{0}{1}() {
+  return wrap({2}.release());
+}
+
+)";
+
+static bool emitCAPIImpl(const llvm::RecordKeeper &records, raw_ostream &os) {
+  os << "/* Autogenerated by mlir-tblgen; don't manually edit. */";
+  for (const auto *def : records.getAllDerivedDefinitions("PassBase")) {
+    Pass pass(def);
+    StringRef defName = pass.getDef()->getName();
+    os << llvm::formatv(passCreateDef, groupName, defName,
+                        pass.getConstructor());
+  }
+  return false;
+}
+
+static mlir::GenRegistration genCAPIHeader("gen-pass-capi-header",
+                                           "Generate pass C API header",
+                                           &emitCAPIHeader);
+
+static mlir::GenRegistration genCAPIImpl("gen-pass-capi-impl",
+                                         "Generate pass C API implementation",
+                                         &emitCAPIImpl);