[flang] Lower omp.workshare to other omp constructs (llvm#101446)

Add a new pass that lowers an `omp.workshare` with its binding `omp.workshare.loop_wrapper` loop nests into other OpenMP constructs that can be lowered to LLVM.

More specifically, in order to preserve the sequential execution semantics of the code contained, it wraps portions that needs to be executed on a single thread in `omp.single` blocks, converts code that must be parallelized into `omp.wsloop` nests and inserts the appropriate synchronization.

Author: ivanradanov

flang/include/flang/Optimizer/OpenMP/Passes.h

@@ -25,6 +25,11 @@ namespace flangomp {
 #define GEN_PASS_REGISTRATION
 #include "flang/Optimizer/OpenMP/Passes.h.inc"
 
+/// Impelements the logic specified in the 2.8.3  workshare Construct section of
+/// the OpenMP standard which specifies what statements or constructs shall be
+/// divided into units of work.
+bool shouldUseWorkshareLowering(mlir::Operation *op);
+
 } // namespace flangomp
 
 #endif // FORTRAN_OPTIMIZER_OPENMP_PASSES_H

flang/include/flang/Optimizer/OpenMP/Passes.td

@@ -50,4 +50,9 @@ def FunctionFilteringPass : Pass<"omp-function-filtering"> {
   ];
 }
 
+// Needs to be scheduled on Module as we create functions in it
+def LowerWorkshare : Pass<"lower-workshare", "::mlir::ModuleOp"> {
+  let summary = "Lower workshare construct";
+}
+
 #endif //FORTRAN_OPTIMIZER_OPENMP_PASSES

flang/include/flang/Optimizer/Passes/Pipelines.h

@@ -123,7 +123,8 @@ void createDefaultFIROptimizerPassPipeline(mlir::PassManager &pm,
 /// \param optLevel - optimization level used for creating FIR optimization
 ///   passes pipeline
 void createHLFIRToFIRPassPipeline(
-    mlir::PassManager &pm, llvm::OptimizationLevel optLevel = defaultOptLevel);
+    mlir::PassManager &pm, bool enableOpenMP,
+    llvm::OptimizationLevel optLevel = defaultOptLevel);
 
 /// Create a pass pipeline for handling certain OpenMP transformations needed
 /// prior to FIR lowering.

flang/include/flang/Tools/CrossToolHelpers.h

@@ -123,6 +123,7 @@ struct MLIRToLLVMPassPipelineConfig : public FlangEPCallBacks {
       false; ///< Set no-signed-zeros-fp-math attribute for functions.
   bool UnsafeFPMath = false; ///< Set unsafe-fp-math attribute for functions.
   bool NSWOnLoopVarInc = false; ///< Add nsw flag to loop variable increments.
+  bool EnableOpenMP = false; ///< Enable OpenMP lowering.
 };
 
 struct OffloadModuleOpts {

flang/lib/Frontend/FrontendActions.cpp

@@ -715,7 +715,11 @@ void CodeGenAction::lowerHLFIRToFIR() {
   pm.enableVerifier(/*verifyPasses=*/true);
 
   // Create the pass pipeline
-  fir::createHLFIRToFIRPassPipeline(pm, level);
+  fir::createHLFIRToFIRPassPipeline(
+      pm,
+      ci.getInvocation().getFrontendOpts().features.IsEnabled(
+          Fortran::common::LanguageFeature::OpenMP),
+      level);
   (void)mlir::applyPassManagerCLOptions(pm);
 
   if (!mlir::succeeded(pm.run(*mlirModule))) {
@@ -828,6 +832,10 @@ void CodeGenAction::generateLLVMIR() {
     config.VScaleMax = vsr->second;
   }
 
+  if (ci.getInvocation().getFrontendOpts().features.IsEnabled(
+          Fortran::common::LanguageFeature::OpenMP))
+    config.EnableOpenMP = true;
+
   if (ci.getInvocation().getLoweringOpts().getNSWOnLoopVarInc())
     config.NSWOnLoopVarInc = true;
 

flang/lib/Optimizer/OpenMP/CMakeLists.txt

@@ -5,6 +5,7 @@ add_flang_library(FlangOpenMPTransforms
   MapsForPrivatizedSymbols.cpp
   MapInfoFinalization.cpp
   MarkDeclareTarget.cpp
+  LowerWorkshare.cpp
 
   DEPENDS
   FIRDialect