[flang][OpenMP] Diagnose invalid reduction modifiers

flang/lib/Semantics/check-omp-structure.cpp

@@ -2309,6 +2309,7 @@ void OmpStructureChecker::Enter(const parser::OmpClause::Reduction &x) {
   if (CheckReductionOperators(x)) {
     CheckReductionTypeList(x);
   }
+  CheckReductionModifier(x);
 }
 
 bool OmpStructureChecker::CheckReductionOperators(
@@ -2393,6 +2394,64 @@ void OmpStructureChecker::CheckReductionTypeList(
   }
 }
 
+void OmpStructureChecker::CheckReductionModifier(
+    const parser::OmpClause::Reduction &x) {
+  using ReductionModifier = parser::OmpReductionClause::ReductionModifier;
+  const auto &maybeModifier{std::get<std::optional<ReductionModifier>>(x.v.t)};
+  if (!maybeModifier || *maybeModifier == ReductionModifier::Default) {
+    // No modifier, or the default one is always ok.
+    return;
+  }
+  ReductionModifier modifier{*maybeModifier};
+  const DirectiveContext &dirCtx{GetContext()};
+  if (dirCtx.directive == llvm::omp::Directive::OMPD_loop) {
+    // [5.2:257:33-34]
+    // If a reduction-modifier is specified in a reduction clause that
+    // appears on the directive, then the reduction modifier must be
+    // default.
+    context_.Say(GetContext().clauseSource,
+        "REDUCTION modifier on LOOP directive must be DEFAULT"_err_en_US);
+  }
+  if (modifier == ReductionModifier::Task) {
+    // "Task" is only allowed on worksharing or "parallel" directive.
+    static llvm::omp::Directive worksharing[]{
+        llvm::omp::Directive::OMPD_do, llvm::omp::Directive::OMPD_scope,
+        llvm::omp::Directive::OMPD_sections,
+        // There are more worksharing directives, but they do not apply:
+        // "for" is C++ only,
+        // "single" and "workshare" don't allow reduction clause,
+        // "loop" has different restrictions (checked above).
+    };
+    if (dirCtx.directive != llvm::omp::Directive::OMPD_parallel &&
+        !llvm::is_contained(worksharing, dirCtx.directive)) {
+      context_.Say(GetContext().clauseSource,
+          "Modifier 'TASK' on REDUCTION clause is only allowed with "
+          "PARALLEL or worksharing directive"_err_en_US);
+    }
+  } else if (modifier == ReductionModifier::Inscan) {
+    // "Inscan" is only allowed on worksharing-loop, worksharing-loop simd,
+    // or "simd" directive.
+    // The worksharing-loop directives are OMPD_do and OMPD_for. Only the
+    // former is allowed in Fortran.
+    switch (dirCtx.directive) {
+    case llvm::omp::Directive::OMPD_do: // worksharing-loop
+    case llvm::omp::Directive::OMPD_do_simd: // worksharing-loop simd
+    case llvm::omp::Directive::OMPD_simd: // "simd"
+      break;
+    default:
+      context_.Say(GetContext().clauseSource,
+          "Modifier 'INSCAN' on REDUCTION clause is only allowed with "
+          "worksharing-loop, worksharing-loop simd, "
+          "or SIMD directive"_err_en_US);
+    }
+  } else {
+    // Catch-all for potential future modifiers to make sure that this
+    // function is up-to-date.
+    context_.Say(GetContext().clauseSource,
+        "Unexpected modifier on REDUCTION clause"_err_en_US);
+  }
+}
+
 void OmpStructureChecker::CheckIntentInPointerAndDefinable(
     const parser::OmpObjectList &objectList, const llvm::omp::Clause clause) {
   for (const auto &ompObject : objectList.v) {

flang/lib/Semantics/check-omp-structure.h

@@ -205,6 +205,7 @@ class OmpStructureChecker
   bool CheckIntrinsicOperator(
       const parser::DefinedOperator::IntrinsicOperator &);
   void CheckReductionTypeList(const parser::OmpClause::Reduction &);
+  void CheckReductionModifier(const parser::OmpClause::Reduction &);
   void CheckMasterNesting(const parser::OpenMPBlockConstruct &x);
   void ChecksOnOrderedAsBlock();
   void CheckBarrierNesting(const parser::OpenMPSimpleStandaloneConstruct &x);

flang/test/Lower/OpenMP/invalid-reduction-modifier.f90

@@ -1,6 +1,4 @@
-!Remove the --crash below once we can diagnose the issue more gracefully.
-!REQUIRES: asserts
-!RUN: not --crash %flang_fc1 -fopenmp -emit-hlfir -o - %s
+!RUN: not %flang_fc1 -fopenmp -emit-hlfir -o - %s
 
 ! Check that we reject the "task" reduction modifier on the "simd" directive.
 

flang/test/Semantics/OpenMP/reduction-modifiers.f90

@@ -0,0 +1,89 @@
+! RUN: %python %S/../test_errors.py %s %flang_fc1 -fopenmp -fopenmp-version=52
+
+subroutine mod_task1(x)
+  integer, intent(inout) :: x
+
+  !Correct: "parallel" directive.
+  !$omp parallel reduction(task, +:x)
+  do i = 1, 100
+    x = foo(i)
+  enddo
+  !$omp end parallel
+end
+
+subroutine mod_task2(x)
+  integer, intent(inout) :: x
+
+  !Correct: worksharing directive.
+  !$omp sections reduction(task, +:x)
+  do i = 1, 100
+    x = foo(i)
+  enddo
+  !$omp end sections
+end
+
+subroutine mod_task3(x)
+  integer, intent(inout) :: x
+
+  !ERROR: Modifier 'TASK' on REDUCTION clause is only allowed with PARALLEL or worksharing directive
+  !$omp simd reduction(task, +:x)
+  do i = 1, 100
+    x = foo(i)
+  enddo
+  !$omp end simd
+end
+
+subroutine mod_inscan1(x)
+  integer, intent(inout) :: x
+
+  !Correct: worksharing-loop directive
+  !$omp do reduction(inscan, +:x)
+  do i = 1, 100
+    x = foo(i)
+  enddo
+  !$omp end do
+end
+
+subroutine mod_inscan2(x)
+  integer, intent(inout) :: x
+
+  !Correct: worksharing-loop simd directive
+  !$omp do simd reduction(inscan, +:x)
+  do i = 1, 100
+    x = foo(i)
+  enddo
+  !$omp end do simd
+end
+
+subroutine mod_inscan3(x)
+  integer, intent(inout) :: x
+
+  !Correct: "simd" directive
+  !$omp simd reduction(inscan, +:x)
+  do i = 1, 100
+    x = foo(i)
+  enddo
+  !$omp end simd
+end
+
+subroutine mod_inscan4(x)
+  integer, intent(inout) :: x
+
+  !ERROR: Modifier 'INSCAN' on REDUCTION clause is only allowed with worksharing-loop, worksharing-loop simd, or SIMD directive
+  !$omp parallel reduction(inscan, +:x)
+  do i = 1, 100
+    x = foo(i)
+  enddo
+  !$omp end parallel
+end
+
+subroutine mod_inscan5(x)
+  integer, intent(inout) :: x
+
+  !ERROR: Modifier 'INSCAN' on REDUCTION clause is only allowed with worksharing-loop, worksharing-loop simd, or SIMD directive
+  !$omp sections reduction(inscan, +:x)
+  do i = 1, 100
+    x = foo(i)
+  enddo
+  !$omp end sections
+end