Remove dependency from LLVM Dialect on the OpenMP dialect

The OmpDialect is in practice optional during translation to LLVM IR: the code is tolerant
to have a "nullptr" when not present / needed.

The dependency still exists on the export to LLVMIR.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D88351

Author: joker-eph

mlir/include/mlir/Dialect/LLVMIR/LLVMOpBase.td

@@ -25,10 +25,6 @@ def LLVM_Dialect : Dialect {
   let name = "llvm";
   let cppNamespace = "::mlir::LLVM";
 
-  /// FIXME: at the moment this is a dependency of the translation to LLVM IR,
-  /// not really one of this dialect per-se.
-  let dependentDialects = ["omp::OpenMPDialect"];
-
   let hasRegionArgAttrVerify = 1;
   let hasOperationAttrVerify = 1;
   let extraClassDeclaration = [{

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

@@ -123,7 +123,9 @@ class ModuleTranslation {
 
   /// Builder for LLVM IR generation of OpenMP constructs.
   std::unique_ptr<llvm::OpenMPIRBuilder> ompBuilder;
-  /// Precomputed pointer to OpenMP dialect.
+  /// Precomputed pointer to OpenMP dialect. Note this can be nullptr if the
+  /// OpenMP dialect hasn't been loaded (it is always loaded if there are OpenMP
+  /// operations in the module though).
   const Dialect *ompDialect;
 
   /// Mappings between llvm.mlir.global definitions and corresponding globals.

mlir/lib/Dialect/LLVMIR/CMakeLists.txt

@@ -18,12 +18,10 @@ add_mlir_dialect_library(MLIRLLVMIR
   BitReader
   BitWriter
   Core
-  FrontendOpenMP
 
   LINK_LIBS PUBLIC
   MLIRCallInterfaces
   MLIRControlFlowInterfaces
-  MLIROpenMP
   MLIRIR
   MLIRSideEffectInterfaces
   MLIRSupport

mlir/lib/Target/CMakeLists.txt

@@ -51,6 +51,7 @@ add_mlir_translation_library(MLIRTargetLLVMIR
   IRReader
 
   LINK_LIBS PUBLIC
+  MLIROpenMP
   MLIRTargetLLVMIRModuleTranslation
   )
 

mlir/lib/Target/LLVMIR/ConvertToLLVMIR.cpp

@@ -41,6 +41,8 @@ void registerToLLVMIRTranslation() {
         llvmModule->print(output, nullptr);
         return success();
       },
-      [](DialectRegistry &registry) { registry.insert<LLVM::LLVMDialect>(); });
+      [](DialectRegistry &registry) {
+        registry.insert<LLVM::LLVMDialect, omp::OpenMPDialect>();
+      });
 }
 } // namespace mlir

mlir/lib/Target/LLVMIR/ModuleTranslation.cpp

@@ -302,7 +302,7 @@ ModuleTranslation::ModuleTranslation(Operation *module,
     : mlirModule(module), llvmModule(std::move(llvmModule)),
       debugTranslation(
           std::make_unique<DebugTranslation>(module, *this->llvmModule)),
-      ompDialect(module->getContext()->getOrLoadDialect<omp::OpenMPDialect>()),
+      ompDialect(module->getContext()->getLoadedDialect("omp")),
       typeTranslator(this->llvmModule->getContext()) {
   assert(satisfiesLLVMModule(mlirModule) &&
          "mlirModule should honor LLVM's module semantics.");
@@ -639,9 +639,8 @@ LogicalResult ModuleTranslation::convertOperation(Operation &opInst,
     return success();
   }
 
-  if (opInst.getDialect() == ompDialect) {
+  if (ompDialect && opInst.getDialect() == ompDialect)
     return convertOmpOperation(opInst, builder);
-  }
 
   return opInst.emitError("unsupported or non-LLVM operation: ")
          << opInst.getName();