[flang][OpenMP] Improve handling of REQUIRES ATOMIC_DEFAULT_MEM_ORDER

According to OpenMP 5.0 rules, the ACQ_REL ordering coming from a
REQUIRES directive may need to be replaced with ACQUIRE or RELEASE
depending on the directive in the ATOMIC construct. This was not
done, leading to an incorrect "memory-order" clause appearing in the
generated HLFIR.

This may need to be relaxed a bit to fully comply with later spec
versions, that will be done in a future PR.

Author: kparzysz

flang/lib/Semantics/rewrite-directives.cpp

@@ -112,9 +112,22 @@ bool OmpRewriteMutator::Pre(parser::OpenMPAtomicConstruct &x) {
 
   // Add a memory order clause to the atomic directive.
   atomicDirectiveDefaultOrderFound_ = true;
+  llvm::omp::Clause kind{x.GetKind()};
   switch (*defaultMemOrder) {
   case common::OmpMemoryOrderType::Acq_Rel:
-    clauseList->v.emplace_back(parser::OmpClause{parser::OmpClause::AcqRel{}});
+    // FIXME: Implement 5.0 rules, pending clarification on later spec
+    // versions.
+    // [5.0:62:22-26]
+    if (kind == llvm::omp::Clause::OMPC_read) {
+      clauseList->v.emplace_back(
+          parser::OmpClause{parser::OmpClause::Acquire{}});
+    } else if (kind == llvm::omp::Clause::OMPC_update && x.IsCapture()) {
+      clauseList->v.emplace_back(
+          parser::OmpClause{parser::OmpClause::AcqRel{}});
+    } else {
+      clauseList->v.emplace_back(
+          parser::OmpClause{parser::OmpClause::Release{}});
+    }
     break;
   case common::OmpMemoryOrderType::Relaxed:
     clauseList->v.emplace_back(parser::OmpClause{parser::OmpClause::Relaxed{}});

flang/test/Lower/OpenMP/requires-atomic-default-mem-order.f90

@@ -0,0 +1,22 @@
+!RUN: %flang_fc1 -emit-hlfir %openmp_flags -fopenmp-version=50 %s -o - | FileCheck %s
+
+module m
+!$omp requires atomic_default_mem_order(acq_rel)
+
+contains
+
+!CHECK: %[[V:[0-9]+]]:2 = hlfir.declare {{.*}} {uniq_name = "_QMmFf00Ev"}
+!CHECK: %[[X:[0-9]+]]:2 = hlfir.declare {{.*}} {uniq_name = "_QMmFf00Ex"}
+!CHECK: omp.atomic.read %[[V]]#0 = %[[X]]#0 memory_order(acquire)
+!CHECK: omp.atomic.write %[[X]]#0 = %{{[0-9]+}} memory_order(release)
+
+subroutine f00(x, v)
+  integer :: x, v
+  !$omp atomic read
+    v = x
+
+  !$omp atomic write
+    x = v
+end
+
+end module

flang/test/Semantics/OpenMP/requires-atomic02.f90

@@ -12,7 +12,7 @@ program requires
 
   ! CHECK-LABEL: OpenMPAtomicConstruct
   ! CHECK: OmpClause -> Read
-  ! CHECK: OmpClause -> AcqRel
+  ! CHECK: OmpClause -> Acquire
   !$omp atomic read
   i = j
 
@@ -36,7 +36,7 @@ program requires
 
   ! CHECK-LABEL: OpenMPAtomicConstruct
   ! CHECK: OmpClause -> Write
-  ! CHECK: OmpClause -> AcqRel
+  ! CHECK: OmpClause -> Release
   !$omp atomic write
   i = j
 
@@ -60,7 +60,7 @@ program requires
 
   ! CHECK-LABEL: OpenMPAtomicConstruct
   ! CHECK: OmpClause -> Update
-  ! CHECK: OmpClause -> AcqRel
+  ! CHECK: OmpClause -> Release
   !$omp atomic update
   i = i + j
 
@@ -79,7 +79,7 @@ program requires
   i = i + j
 
   ! CHECK-LABEL: OpenMPAtomicConstruct
-  ! CHECK: OmpClause -> AcqRel
+  ! CHECK: OmpClause -> Release
   !$omp atomic
   i = i + j