[flang][cuda] Add restriction on implicit data transfer

flang/include/flang/Evaluate/tools.h

@@ -1226,18 +1226,24 @@ bool CheckForCoindexedObject(parser::ContextualMessages &,
     const std::optional<ActualArgument> &, const std::string &procName,
     const std::string &argName);
 
-/// Check if any of the symbols part of the expression has a cuda data
-/// attribute.
-inline bool HasCUDAAttrs(const Expr<SomeType> &expr) {
+// Get the number of distinct symbols with CUDA attribute in the expression.
+template <typename A> inline int GetNbOfCUDASymbols(const A &expr) {
+  semantics::UnorderedSymbolSet symbols;
   for (const Symbol &sym : CollectSymbols(expr)) {
     if (const auto *details =
             sym.GetUltimate().detailsIf<semantics::ObjectEntityDetails>()) {
       if (details->cudaDataAttr()) {
-        return true;
+        symbols.insert(sym);
       }
     }
   }
-  return false;
+  return symbols.size();
+}
+
+// Check if any of the symbols part of the expression has a CUDA data
+// attribute.
+template <typename A> inline bool HasCUDAAttrs(const A &expr) {
+  return GetNbOfCUDASymbols(expr) > 0;
 }
 
 } // namespace Fortran::evaluate

flang/lib/Semantics/check-cuda.cpp

@@ -9,12 +9,14 @@
 #include "check-cuda.h"
 #include "flang/Common/template.h"
 #include "flang/Evaluate/fold.h"
+#include "flang/Evaluate/tools.h"
 #include "flang/Evaluate/traverse.h"
 #include "flang/Parser/parse-tree-visitor.h"
 #include "flang/Parser/parse-tree.h"
 #include "flang/Parser/tools.h"
 #include "flang/Semantics/expression.h"
 #include "flang/Semantics/symbol.h"
+#include "flang/Semantics/tools.h"
 
 // Once labeled DO constructs have been canonicalized and their parse subtrees
 // transformed into parser::DoConstructs, scan the parser::Blocks of the program
@@ -413,4 +415,18 @@ void CUDAChecker::Enter(const parser::CUFKernelDoConstruct &x) {
   }
 }
 
+void CUDAChecker::Enter(const parser::AssignmentStmt &x) {
+  const evaluate::Assignment *assign{semantics::GetAssignment(x)};
+  int nbLhs{evaluate::GetNbOfCUDASymbols(assign->lhs)};
+  int nbRhs{evaluate::GetNbOfCUDASymbols(assign->rhs)};
+  auto lhsLoc{std::get<parser::Variable>(x.t).GetSource()};
+
+  // device to host transfer with more than one device object on the rhs is not
+  // legal.
+  if (nbLhs == 0 && nbRhs > 1) {
+    context_.Say(lhsLoc,
+        "More than one reference to a CUDA object on the right hand side of the assigment"_err_en_US);
+  }
+}
+
 } // namespace Fortran::semantics

flang/lib/Semantics/check-cuda.h

@@ -17,6 +17,7 @@ struct Program;
 class Messages;
 struct Name;
 class CharBlock;
+struct AssignmentStmt;
 struct ExecutionPartConstruct;
 struct ExecutableConstruct;
 struct ActionStmt;
@@ -38,6 +39,7 @@ class CUDAChecker : public virtual BaseChecker {
   void Enter(const parser::FunctionSubprogram &);
   void Enter(const parser::SeparateModuleSubprogram &);
   void Enter(const parser::CUFKernelDoConstruct &);
+  void Enter(const parser::AssignmentStmt &);
 
 private:
   SemanticsContext &context_;

flang/test/Semantics/cuf11.cuf

@@ -0,0 +1,12 @@
+! RUN: %python %S/test_errors.py %s %flang_fc1
+
+subroutine sub1()
+  real, device :: adev(10), bdev(10)
+  real :: ahost(10)
+
+!ERROR: More than one reference to a CUDA object on the right hand side of the assigment
+  ahost = adev + bdev
+
+  ahost = adev + adev
+
+end subroutine