[mlir] Add Cpp emitter

This upstreams the Cpp emitter, initially presented with [1], from [2]
to MLIR core. Together with the previously upstreamed EmitC dialect [3],
the target allows to translate MLIR to C/C++.

[1] https://reviews.llvm.org/D76571
[2] https://github.com/iml130/mlir-emitc
[3] https://reviews.llvm.org/D103969

Co-authored-by: Jacques Pienaar <jpienaar@google.com>
Co-authored-by: Simon Camphausen <simon.camphausen@iml.fraunhofer.de>
Co-authored-by: Oliver Scherf <oliver.scherf@iml.fraunhofer.de>

Reviewed By: jpienaar

Differential Revision: https://reviews.llvm.org/D104632

Author: marbre

mlir/docs/Dialects/emitc.md

@@ -0,0 +1,34 @@
+The EmitC dialect allows to convert operations from other MLIR dialects to
+EmitC ops. Those can be translated to C/C++ via the Cpp emitter.
+
+The following convention is followed:
+
+* If template arguments are passed to an `emitc.call` operation,
+  C++ is generated.
+* If tensors are used, C++ is generated.
+* If multiple return values are used within in a functions or an
+  `emitc.call` operation, C++11 is required.
+* If floating-point type template arguments are passed to an `emitc.call`
+  operation, C++20 is required.
+* Else the generated code is compatible with C99.
+
+These restrictions are neither inherent to the EmitC dialect itself nor to the
+Cpp emitter and therefore need to be considered while implementing conversions.
+
+After the conversion, C/C++ code can be emitted with `mlir-translate`. The tool
+support translating MLIR to C/C++ by passing `-mlir-to-cpp`.
+Furthermore, code with variables declared at top can be generated by passing 
+`-mlir-to-cpp-with-variable-declarations-at-top`.
+
+Besides operations part of the EmitC dialect, the Cpp targets supports
+translating the following operations:
+
+* 'std' Dialect
+  * `std.br`
+  * `std.call`
+  * `std.cond_br`
+  * `std.constant`
+  * `std.return`
+* 'scf' Dialect
+  * `scf.for`
+  * `scf.yield`

mlir/include/mlir/Dialect/EmitC/IR/EmitCBase.td

@@ -22,6 +22,12 @@ include "mlir/IR/OpBase.td"
 def EmitC_Dialect : Dialect {
   let name = "emitc";
   let cppNamespace = "::mlir::emitc";
+
+  let summary = "Dialect to generate C/C++ from MLIR.";
+  let description = [{
+    [include "Dialects/emitc.md"]
+  }];
+
   let hasConstantMaterializer = 1;
 }
 

mlir/include/mlir/InitAllTranslations.h

@@ -18,6 +18,7 @@ namespace mlir {
 
 void registerFromLLVMIRTranslation();
 void registerFromSPIRVTranslation();
+void registerToCppTranslation();
 void registerToLLVMIRTranslation();
 void registerToSPIRVTranslation();
 
@@ -28,6 +29,7 @@ inline void registerAllTranslations() {
   static bool initOnce = []() {
     registerFromLLVMIRTranslation();
     registerFromSPIRVTranslation();
+    registerToCppTranslation();
     registerToLLVMIRTranslation();
     registerToSPIRVTranslation();
     return true;

mlir/include/mlir/Target/Cpp/CppEmitter.h

@@ -0,0 +1,34 @@
+//===- CppEmitter.h - Helpers to create C++ emitter -------------*- 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 defines helpers to emit C++ code using the EmitC dialect.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef MLIR_TARGET_CPP_CPPEMITTER_H
+#define MLIR_TARGET_CPP_CPPEMITTER_H
+
+#include "mlir/IR/BuiltinTypes.h"
+#include "mlir/IR/Value.h"
+#include "llvm/ADT/ScopedHashTable.h"
+#include "llvm/Support/raw_ostream.h"
+#include <stack>
+
+namespace mlir {
+namespace emitc {
+
+/// Translates the given operation to C++ code. The operation or operations in
+/// the region of 'op' need almost all be in EmitC dialect. The parameter
+/// 'declareVariablesAtTop' enforces that all variables for op results and block
+/// arguments are declared at the beginning of the function.
+LogicalResult translateToCpp(Operation *op, raw_ostream &os,
+                             bool declareVariablesAtTop = false);
+} // namespace emitc
+} // namespace mlir
+
+#endif // MLIR_TARGET_CPP_CPPEMITTER_H

mlir/lib/Target/CMakeLists.txt

@@ -1,2 +1,3 @@
+add_subdirectory(Cpp)
 add_subdirectory(SPIRV)
 add_subdirectory(LLVMIR)

mlir/lib/Target/Cpp/CMakeLists.txt

@@ -0,0 +1,14 @@
+add_mlir_translation_library(MLIRTargetCpp
+  TranslateRegistration.cpp
+  TranslateToCpp.cpp
+
+  ADDITIONAL_HEADER_DIRS
+  ${EMITC_MAIN_INCLUDE_DIR}/emitc/Target/Cpp
+
+  LINK_LIBS PUBLIC
+  MLIREmitC
+  MLIRIR
+  MLIRSCF
+  MLIRStandard
+  MLIRSupport
+  )

mlir/lib/Target/Cpp/TranslateRegistration.cpp

@@ -0,0 +1,48 @@
+//===- TranslateRegistration.cpp - Register translation -------------------===//
+//
+// 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/Dialect/EmitC/IR/EmitC.h"
+#include "mlir/Dialect/SCF/SCF.h"
+#include "mlir/Dialect/StandardOps/IR/Ops.h"
+#include "mlir/IR/BuiltinOps.h"
+#include "mlir/IR/Dialect.h"
+#include "mlir/Target/Cpp/CppEmitter.h"
+#include "mlir/Translation.h"
+#include "llvm/Support/CommandLine.h"
+
+using namespace mlir;
+
+namespace mlir {
+
+//===----------------------------------------------------------------------===//
+// Cpp registration
+//===----------------------------------------------------------------------===//
+
+void registerToCppTranslation() {
+  static llvm::cl::opt<bool> declareVariablesAtTop(
+      "declare-variables-at-top",
+      llvm::cl::desc("Declare variables at top when emitting C/C++"),
+      llvm::cl::init(false));
+
+  TranslateFromMLIRRegistration reg(
+      "mlir-to-cpp",
+      [](ModuleOp module, raw_ostream &output) {
+        return emitc::translateToCpp(
+            module, output,
+            /*declareVariablesAtTop=*/declareVariablesAtTop);
+      },
+      [](DialectRegistry &registry) {
+        // clang-format off
+        registry.insert<emitc::EmitCDialect,
+                        StandardOpsDialect,
+                        scf::SCFDialect>();
+        // clang-format on
+      });
+}
+
+} // namespace mlir