[flang][OpenMP] Restore reduction processor behavior broken by #145837 (#150178)

Fixes #149089 and #149700.

Before #145837, when processing a reduction symbol not yet supported by
OpenMP lowering, the reduction processor would simply skip filling in
the reduction symbols and variables. With #145837, this behvaior was
slightly changed because the reduction symbols are populated before
invoking the reduction processor (this is more convenient to shared the
code with `do concurrent`).

This PR restores the previous behavior.

flang/include/flang/Lower/Support/ReductionProcessor.h

@@ -124,7 +124,7 @@ public:
   /// Creates a reduction declaration and associates it with an OpenMP block
   /// directive.
   template <typename OpType, typename RedOperatorListTy>
-  static void processReductionArguments(
+  static bool processReductionArguments(
       mlir::Location currentLocation, lower::AbstractConverter &converter,
       const RedOperatorListTy &redOperatorList,
       llvm::SmallVectorImpl<mlir::Value> &reductionVars,

flang/lib/Lower/Bridge.cpp

@@ -2125,9 +2125,10 @@ private:
 
     llvm::SmallVector<mlir::Value> reduceVars;
     Fortran::lower::omp::ReductionProcessor rp;
-    rp.processReductionArguments<fir::DeclareReductionOp>(
+    bool result = rp.processReductionArguments<fir::DeclareReductionOp>(
         toLocation(), *this, info.reduceOperatorList, reduceVars,
         reduceVarByRef, reductionDeclSymbols, info.reduceSymList);
+    assert(result && "Failed to process `do concurrent` reductions");
 
     doConcurrentLoopOp.getReduceVarsMutable().assign(reduceVars);
     doConcurrentLoopOp.setReduceSymsAttr(

flang/lib/Lower/OpenMP/ClauseProcessor.cpp

@@ -1116,11 +1116,12 @@ bool ClauseProcessor::processInReduction(
         collectReductionSyms(clause, inReductionSyms);
 
         ReductionProcessor rp;
-        rp.processReductionArguments<mlir::omp::DeclareReductionOp>(
+        if (!rp.processReductionArguments<mlir::omp::DeclareReductionOp>(
-            currentLocation, converter,
+                currentLocation, converter,
-            std::get<typename omp::clause::ReductionOperatorList>(clause.t),
+                std::get<typename omp::clause::ReductionOperatorList>(clause.t),
-            inReductionVars, inReduceVarByRef, inReductionDeclSymbols,
+                inReductionVars, inReduceVarByRef, inReductionDeclSymbols,
-            inReductionSyms);
+                inReductionSyms))
+          inReductionSyms.clear();
 
         // Copy local lists into the output.
         llvm::copy(inReductionVars, std::back_inserter(result.inReductionVars));
@@ -1461,10 +1462,12 @@ bool ClauseProcessor::processReduction(
         }
 
         ReductionProcessor rp;
-        rp.processReductionArguments<mlir::omp::DeclareReductionOp>(
+        if (!rp.processReductionArguments<mlir::omp::DeclareReductionOp>(
-            currentLocation, converter,
+                currentLocation, converter,
-            std::get<typename omp::clause::ReductionOperatorList>(clause.t),
+                std::get<typename omp::clause::ReductionOperatorList>(clause.t),
-            reductionVars, reduceVarByRef, reductionDeclSymbols, reductionSyms);
+                reductionVars, reduceVarByRef, reductionDeclSymbols,
+                reductionSyms))
+          reductionSyms.clear();
         // Copy local lists into the output.
         llvm::copy(reductionVars, std::back_inserter(result.reductionVars));
         llvm::copy(reduceVarByRef, std::back_inserter(result.reductionByref));
@@ -1486,11 +1489,12 @@ bool ClauseProcessor::processTaskReduction(
         collectReductionSyms(clause, taskReductionSyms);
 
         ReductionProcessor rp;
-        rp.processReductionArguments<mlir::omp::DeclareReductionOp>(
+        if (!rp.processReductionArguments<mlir::omp::DeclareReductionOp>(
-            currentLocation, converter,
+                currentLocation, converter,
-            std::get<typename omp::clause::ReductionOperatorList>(clause.t),
+                std::get<typename omp::clause::ReductionOperatorList>(clause.t),
-            taskReductionVars, taskReduceVarByRef, taskReductionDeclSymbols,
+                taskReductionVars, taskReduceVarByRef, taskReductionDeclSymbols,
-            taskReductionSyms);
+                taskReductionSyms))
+          taskReductionSyms.clear();
         // Copy local lists into the output.
         llvm::copy(taskReductionVars,
                    std::back_inserter(result.taskReductionVars));

flang/lib/Lower/Support/ReductionProcessor.cpp

@@ -39,7 +39,7 @@ namespace lower {
 namespace omp {
 
 // explicit template declarations
-template void ReductionProcessor::processReductionArguments<
+template bool ReductionProcessor::processReductionArguments<
     mlir::omp::DeclareReductionOp, omp::clause::ReductionOperatorList>(
     mlir::Location currentLocation, lower::AbstractConverter &converter,
     const omp::clause::ReductionOperatorList &redOperatorList,
@@ -48,7 +48,7 @@ template void ReductionProcessor::processReductionArguments<
     llvm::SmallVectorImpl<mlir::Attribute> &reductionDeclSymbols,
     const llvm::SmallVectorImpl<const semantics::Symbol *> &reductionSymbols);
 
-template void ReductionProcessor::processReductionArguments<
+template bool ReductionProcessor::processReductionArguments<
     fir::DeclareReductionOp, llvm::SmallVector<fir::ReduceOperationEnum>>(
     mlir::Location currentLocation, lower::AbstractConverter &converter,
     const llvm::SmallVector<fir::ReduceOperationEnum> &redOperatorList,
@@ -607,7 +607,7 @@ static bool doReductionByRef(mlir::Value reductionVar) {
 }
 
 template <typename OpType, typename RedOperatorListTy>
-void ReductionProcessor::processReductionArguments(
+bool ReductionProcessor::processReductionArguments(
     mlir::Location currentLocation, lower::AbstractConverter &converter,
     const RedOperatorListTy &redOperatorList,
     llvm::SmallVectorImpl<mlir::Value> &reductionVars,
@@ -627,10 +627,10 @@ void ReductionProcessor::processReductionArguments(
               std::get_if<omp::clause::ProcedureDesignator>(&redOperator.u)) {
         if (!ReductionProcessor::supportedIntrinsicProcReduction(
                 *reductionIntrinsic)) {
-          return;
+          return false;
         }
       } else {
-        return;
+        return false;
       }
     }
   }
@@ -765,6 +765,8 @@ void ReductionProcessor::processReductionArguments(
 
   if (isDoConcurrent)
     builder.restoreInsertionPoint(dcIP);
+
+  return true;
 }
 
 const semantics::SourceName

flang/test/Lower/OpenMP/wsloop-reduction-non-intrinsic.f90

@@ -0,0 +1,25 @@
+! Tests reduction processor behavior when a reduction symbol is not supported.
+
+! RUN: %flang_fc1 -emit-hlfir -fopenmp -o - %s 2>&1 | FileCheck %s
+
+subroutine foo
+  implicit none
+  integer :: k, i
+
+  interface max
+    function max(m1,m2)
+      integer :: m1, m2
+    end function
+  end interface
+
+  !$omp do reduction (max: k)
+  do i=1,10
+  end do
+  !$omp end do
+end
+
+! Verify that unsupported reduction is ignored.
+! CHECK: omp.wsloop 
+! CHECK-SAME: private(@{{[^[:space:]]+}} %{{[^[:space:]]+}}
+! CHECK-SAME:         -> %{{[^[:space:]]+}} : !{{[^[:space:]]+}}) {
+! CHECK: }