[MLIR, OpenMP] Translation of OpenMP barrier construct to LLVM IR

Summary:
This patch adds support for translation of the OpenMP barrier construct to LLVM
IR. The OpenMP IRBuilder is used for this translation. In this patch the code
for translation is added to the existing LLVM dialect translation to LLVM IR.

The patch includes code changes and a testcase.

Reviewers: jdoerfert, nicolasvasilache, ftynse, rriddle, mehdi_amini

Reviewed By: ftynse, rriddle, mehdi_amini

Differential Revision: https://reviews.llvm.org/D72962

mlir/include/mlir/Target/LLVMIR/ModuleTranslation.h

@@ -19,6 +19,7 @@
 #include "mlir/IR/Module.h"
 #include "mlir/IR/Value.h"
 
+#include "llvm/Frontend/OpenMP/OMPIRBuilder.h"
 #include "llvm/IR/BasicBlock.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/IRBuilder.h"
@@ -103,6 +104,11 @@ private:
   /// A converter for translating debug information.
   std::unique_ptr<detail::DebugTranslation> debugTranslation;
 
+  /// Builder for LLVM IR generation of OpenMP constructs.
+  std::unique_ptr<llvm::OpenMPIRBuilder> ompBuilder;
+  /// Precomputed pointer to OpenMP dialect.
+  const Dialect *ompDialect;
+
   /// Mappings between llvm.mlir.global definitions and corresponding globals.
   DenseMap<Operation *, llvm::GlobalValue *> globalsMapping;
 

mlir/lib/Dialect/LLVMIR/CMakeLists.txt

@@ -4,8 +4,8 @@ add_mlir_dialect_library(MLIRLLVMIR
   ADDITIONAL_HEADER_DIRS
   ${MLIR_MAIN_INCLUDE_DIR}/mlir/Dialect/LLVMIR
   )
-add_dependencies(MLIRLLVMIR MLIRLLVMOpsIncGen MLIRLLVMConversionsIncGen LLVMAsmParser LLVMCore LLVMSupport)
-target_link_libraries(MLIRLLVMIR LLVMAsmParser LLVMCore LLVMSupport MLIRIR)
+add_dependencies(MLIRLLVMIR MLIRLLVMOpsIncGen MLIRLLVMConversionsIncGen MLIROpenMP LLVMFrontendOpenMP LLVMAsmParser LLVMCore LLVMSupport)
+target_link_libraries(MLIRLLVMIR LLVMAsmParser LLVMCore LLVMSupport LLVMFrontendOpenMP MLIROpenMP MLIRIR)
 
 add_mlir_dialect_library(MLIRNVVMIR
   IR/NVVMDialect.cpp

mlir/lib/Target/LLVMIR/ModuleTranslation.cpp

@@ -15,12 +15,14 @@
 
 #include "DebugTranslation.h"
 #include "mlir/Dialect/LLVMIR/LLVMDialect.h"
+#include "mlir/Dialect/OpenMP/OpenMPDialect.h"
 #include "mlir/IR/Attributes.h"
 #include "mlir/IR/Module.h"
 #include "mlir/IR/StandardTypes.h"
 #include "mlir/Support/LLVM.h"
 
 #include "llvm/ADT/SetVector.h"
+#include "llvm/Frontend/OpenMP/OMPIRBuilder.h"
 #include "llvm/IR/BasicBlock.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DerivedTypes.h"
@@ -271,7 +273,9 @@ ModuleTranslation::ModuleTranslation(Operation *module,
                                      std::unique_ptr<llvm::Module> llvmModule)
     : mlirModule(module), llvmModule(std::move(llvmModule)),
       debugTranslation(
-          std::make_unique<DebugTranslation>(module, *this->llvmModule)) {
+          std::make_unique<DebugTranslation>(module, *this->llvmModule)),
+      ompDialect(
+          module->getContext()->getRegisteredDialect<omp::OpenMPDialect>()) {
   assert(satisfiesLLVMModule(mlirModule) &&
          "mlirModule should honor LLVM's module semantics.");
 }
@@ -374,6 +378,21 @@ LogicalResult ModuleTranslation::convertOperation(Operation &opInst,
     return success();
   }
 
+  if (opInst.getDialect() == ompDialect) {
+    if (!ompBuilder) {
+      ompBuilder =
+          std::move(std::make_unique<llvm::OpenMPIRBuilder>(*llvmModule));
+      ompBuilder->initialize();
+    }
+
+    if (isa<omp::BarrierOp>(opInst)) {
+      ompBuilder->CreateBarrier(builder.saveIP(), llvm::omp::OMPD_barrier);
+      return success();
+    }
+    return opInst.emitError("unsupported OpenMP operation: ")
+           << opInst.getName();
+  }
+
   return opInst.emitError("unsupported or non-LLVM operation: ")
          << opInst.getName();
 }

mlir/test/Target/openmp-llvm.mlir

@@ -0,0 +1,10 @@
+// RUN: mlir-translate -mlir-to-llvmir %s | FileCheck %s
+
+// CHECK-LABEL: define void @empty()
+// CHECK: [[OMP_THREAD:%.*]] = call i32 @__kmpc_global_thread_num(%struct.ident_t* @{{[0-9]+}})
+// CHECK-NEXT:  call void @__kmpc_barrier(%struct.ident_t* @{{[0-9]+}}, i32 [[OMP_THREAD]])
+// CHECK-NEXT:    ret void
+llvm.func @empty() {
+  omp.barrier
+  llvm.return
+}