Use double buffer for block_scan (#2436)

src/targets/gpu/device/include/migraphx/gpu/device/scan.hpp

@@ -43,24 +43,32 @@ template <index_int N,
 __device__ void block_scan(index idx, Op op, T init, ForStride fs, Input input, Output output)
 {
     using type = decltype(input(deduce_for_stride(fs)));
-    MIGRAPHX_DEVICE_SHARED type buffer[N];
+    MIGRAPHX_DEVICE_SHARED type buffer[2][N];
     type x = init;
     fs([&](auto i) {
+        index_int iout = 0;
+        index_int iin  = 1;
         if(idx.local == 0)
-            buffer[idx.local] = op(input(i), x);
+            buffer[iout][idx.local] = op(input(i), x);
         else
-            buffer[idx.local] = input(i);
+            buffer[iout][idx.local] = input(i);
         __syncthreads();
         for(index_int s = 1; s < idx.nlocal(); s *= 2)
         {
-            if(idx.local + s < idx.nlocal())
+            iout = 1 - iout;
+            iin  = 1 - iin;
+            if(idx.local >= s)
             {
-                buffer[idx.local + s] = op(buffer[idx.local], buffer[idx.local + s]);
+                buffer[iout][idx.local] = op(buffer[iin][idx.local], buffer[iin][idx.local - s]);
+            }
+            else
+            {
+                buffer[iout][idx.local] = buffer[iin][idx.local];
             }
             __syncthreads();
         }
-        x = buffer[idx.nlocal() - 1];
-        output(i, buffer[idx.local]);
+        x = buffer[iout][idx.nlocal() - 1];
+        output(i, buffer[iout][idx.local]);
     });
 }