Make ReduceData::value safer

Although ReduceData::value is meant to be called only once, the user might
call it multiple times. This will result in the wrong result for sum
reduction because of how the value function is implemented. In this PR, we
fix this defect by allowing the function be called multiple times.

Src/Base/AMReX_Reduce.H

@@ -666,25 +666,29 @@ public:
     template <typename D>
     typename D::Type value (D & reduce_data)
     {
+        auto hp = reduce_data.hostPtr();
+
+        if (m_result_is_ready) {
+            return *hp;
+        }
+
         using ReduceTuple = typename D::Type;
         auto const& stream = Gpu::gpuStream();
-        auto hp = reduce_data.hostPtr();
         auto dp = reduce_data.devicePtr();
         auto const& nblocks = reduce_data.nBlocks();
 #if defined(AMREX_USE_SYCL)
         if (reduce_data.maxStreamIndex() == 0 && nblocks[0] <= 4096) {
             const int N = nblocks[0];
             if (N == 0) {
                 Reduce::detail::for_each_init<0, ReduceTuple, Ps...>(*hp);
-                return *hp;
             } else {
                 Gpu::PinnedVector<ReduceTuple> tmp(N);
                 Gpu::dtoh_memcpy_async(tmp.data(), dp, sizeof(ReduceTuple)*N);
                 Gpu::streamSynchronize();
                 for (int i = 1; i < N; ++i) {
                     Reduce::detail::for_each_local<0, ReduceTuple, Ps...>(tmp[0], tmp[i]);
                 }
-                return tmp[0];
+                *hp = tmp[0];
             }
         } else
 #endif
@@ -738,9 +742,14 @@ public:
             });
 #endif
             Gpu::streamSynchronize();
-            return *hp;
         }
+
+        m_result_is_ready = true;
+        return *hp;
     }
+
+private:
+    bool m_result_is_ready = false;
 };
 
 namespace Reduce {
@@ -1135,15 +1144,20 @@ public:
     template <typename D>
     typename D::Type value (D & reduce_data)
     {
-        using ReduceTuple = typename D::Type;
         auto& rrv = reduce_data.reference();
-        if (rrv.size() > 1) {
-            for (int i = 1, N = rrv.size(); i < N; ++i) {
-                Reduce::detail::for_each_local<0, ReduceTuple, Ps...>(rrv[0], rrv[i]);
+        if (! m_result_is_ready) {
+            using ReduceTuple = typename D::Type;
+            if (rrv.size() > 1) {
+                for (int i = 1, N = rrv.size(); i < N; ++i) {
+                    Reduce::detail::for_each_local<0, ReduceTuple, Ps...>(rrv[0], rrv[i]);
+                }
             }
+            m_result_is_ready = true;
         }
         return rrv[0];
     }
+
+    bool m_result_is_ready = false;
 };
 
 namespace Reduce {