[flang-commits] [flang] [flang][OpenMP] Detect conflicting data-sharing clauses for common blocks (PR #213913)

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https://github.com/ejose02 updated https://github.com/llvm/llvm-project/pull/213913

>From be1cae524cccc96c75d65d528daddcfba15acf4b Mon Sep 17 00:00:00 2001
From: ejose <ejose at amd.com>
Date: Tue, 4 Aug 2026 10:13:24 +0000
Subject: [PATCH] [flang][OpenMP] Detect conflicting data-sharing clauses for
 common blocks

OpenMP treats a named common block in a data-sharing clause as equivalent to listing every explicit member. Extend CheckMultipleAppearances to detect conflicts between a common block and its members, and register all explicit members when a common block is listed. Added a regression test.
---
 flang/lib/Semantics/resolve-directives.cpp    | 41 +++++++++++++++++--
 .../OpenMP/common-block-data-sharing.f90      | 11 +++++
 2 files changed, 48 insertions(+), 4 deletions(-)
 create mode 100644 flang/test/Semantics/OpenMP/common-block-data-sharing.f90

diff --git a/flang/lib/Semantics/resolve-directives.cpp b/flang/lib/Semantics/resolve-directives.cpp
index 15bb84d7e486f..6ce165b0fa4c6 100644
--- a/flang/lib/Semantics/resolve-directives.cpp
+++ b/flang/lib/Semantics/resolve-directives.cpp
@@ -3194,7 +3194,7 @@ void OmpAttributeVisitor::ResolveOmpCommonBlock(
   Symbol *originalCB{ResolveOmpCommonBlockName(&cbName)};
   if (auto *symbol{cbResolved ? name.symbol : originalCB}) {
     if (!dataCopyingAttributeFlags.test(ompFlag)) {
-      CheckMultipleAppearances(name, *symbol, Symbol::Flag::OmpCommonBlock);
+      CheckMultipleAppearances(name, *symbol, ompFlag);
     }
     // 2.15.3 When a named common block appears in a list, it has the
     // same meaning as if every explicit member of the common block
@@ -3311,14 +3311,47 @@ void OmpAttributeVisitor::CheckMultipleAppearances(
       target = &details->symbol();
     }
   }
-  if (HasDataSharingAttributeObject(target->GetUltimate()) &&
-      !WithMultipleAppearancesOmpException(symbol, ompFlag)) {
+  const Symbol &ultimate = target->GetUltimate();
+  auto hasConflict{[&](const Symbol &sym) {
+    if (!HasDataSharingAttributeObject(sym)) {
+      return false;
+    }
+    if (WithMultipleAppearancesOmpException(sym, ompFlag)) {
+      return false;
+    }
+    if (auto prevFlag{GetContext().FindSymbolWithDSA(sym)}) {
+      return !((ompFlag == Symbol::Flag::OmpFirstPrivate &&
+                     *prevFlag == Symbol::Flag::OmpLastPrivate) ||
+          (ompFlag == Symbol::Flag::OmpLastPrivate &&
+              *prevFlag == Symbol::Flag::OmpFirstPrivate));
+    }
+    return true;
+  }};
+  bool conflicts = hasConflict(ultimate);
+  if (!conflicts) {
+    if (const Symbol *commonBlock = FindCommonBlockContaining(ultimate)) {
+      conflicts = hasConflict(*commonBlock);
+    } else if (const auto *details = ultimate.detailsIf<CommonBlockDetails>()) {
+      for (const auto &object : details->objects()) {
+        if (hasConflict(object->GetUltimate())) {
+          conflicts = true;
+          break;
+        }
+      }
+    }
+  }
+  if (conflicts) {
     context_.Say(name.source,
         "'%s' appears in more than one data-sharing clause "
         "on the same OpenMP directive"_err_en_US,
         name.ToString());
   } else {
-    AddDataSharingAttributeObject(target->GetUltimate());
+    AddDataSharingAttributeObject(ultimate);
+    if (const auto *details{ultimate.detailsIf<CommonBlockDetails>()}) {
+      for (const auto &object : details->objects()) {
+        AddDataSharingAttributeObject(object->GetUltimate());
+      }
+    }
   }
 }
 
diff --git a/flang/test/Semantics/OpenMP/common-block-data-sharing.f90 b/flang/test/Semantics/OpenMP/common-block-data-sharing.f90
new file mode 100644
index 0000000000000..f0ddf8e048441
--- /dev/null
+++ b/flang/test/Semantics/OpenMP/common-block-data-sharing.f90
@@ -0,0 +1,11 @@
+! RUN: %python %S/../test_errors.py %s %flang -fopenmp
+
+! a common block in a data-sharing clause is equivalent to
+! listing every explicit member of the common block.
+
+subroutine common_block_dsa()
+  common /c/ x, y
+  !ERROR: 'x' appears in more than one data-sharing clause on the same OpenMP directive
+  !$omp parallel private(/c/) shared(x)
+  !$omp end parallel
+end subroutine