Added atomic operations.

src/TNL/Algorithms/AtomicOperations.h

+/***************************************************************************
+                          AtomicOperations.h  -  description
+                             -------------------
+    begin                : Feb 26, 2020
+    copyright            : (C) 2020 by Tomas Oberhuber et al.
+    email                : tomas.oberhuber@fjfi.cvut.cz
+ ***************************************************************************/
+
+/* See Copyright Notice in tnl/Copyright */
+
+// Implemented by: Tomas Oberhuber, Jakub Klinkovsky
+
+#pragma once
+
+#include <TNL/Devices/Sequential.h>
+#include <TNL/Devices/Host.h>
+#include <TNL/Devices/Cuda.h>
+
+namespace TNL {
+namespace Algorithms {
+
+template< typename Device >
+struct AtomicOperations{};
+
+template<>
+struct AtomicOperations< Devices::Host >
+{
+   template< typename Value >
+   static void add( Value& v, const Value& a )
+   {
+#pragma omp atomic update
+      v += a;
+   }
+};
+
+template<>
+struct AtomicOperations< Devices::Cuda >
+{
+   template< typename Value >
+   __cuda_callable__
+   static void add( Value& v, const Value& a )
+   {
+#ifdef HAVE_CUDA
+#if __CUDA_ARCH__ < 600
+      if( std::is_same< Value, double >::value )
+      {
+         unsigned long long int* v_as_ull = ( unsigned long long int* ) &v;
+         unsigned long long int old = *v_as_ull, assumed;
+
+         do
+         {
+            assumed = old;
+            old = atomicCAS( v_as_ull,
+                             assumed,
+                             __double_as_longlong( s + __longlong_as_double( assumed ) ) ) ;
+
+         // Note: uses integer comparison to avoid hang in case of NaN (since NaN != NaN)
+         }
+         while( assumed != old );
+         return;
+      }
+#endif
+      atomicAdd( &v, a );
+#endif
+   }
+
+};
+
+} //namespace Algorithms
+} //namespace TNL
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