Make implicit copy of TrackingRecHitsSoACollection<TrackerTraits> to host asynchronous

Author: makortel

DataFormats/TrackingRecHitSoA/interface/alpaka/TrackingRecHitsSoACollection.h

@@ -51,17 +51,22 @@ namespace cms::alpakatools {
       assert(deviceData.nHits() == hostData.nHits());
       assert(deviceData.offsetBPIX2() == hostData.offsetBPIX2());
 #endif
-      // Update the contents address of the phiBinner histo container after the copy from device happened
-      alpaka::wait(queue);
+      return hostData;
+    }
+
+    // Update the contents address of the phiBinner histo container after the copy from device happened
+    static void postCopy(TrackingRecHitHost<TrackerTraits>& hostData) {
+      // Don't bother if zero hits
+      if (hostData.view().metadata().size() == 0) {
+        return;
+      }
       typename TrackingRecHitSoA<TrackerTraits>::PhiBinnerView pbv;
       pbv.assoc = &(hostData.view().phiBinner());
       pbv.offSize = -1;
       pbv.offStorage = nullptr;
       pbv.contentSize = hostData.nHits();
       pbv.contentStorage = hostData.view().phiBinnerStorage();
       hostData.view().phiBinner().initStorage(pbv);
-
-      return hostData;
     }
   };
 }  // namespace cms::alpakatools

DataFormats/TrackingRecHitSoA/test/alpaka/Hits_test.cc

@@ -51,12 +51,16 @@ int main() {
       // requires c++23 to make cms::alpakatools::CopyToHost compile using if constexpr
       // see https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2593r0.html
       TrackingRecHitHost<pixelTopology::Phase1> const& host_collection = tkhit;
+      // wait for the kernel to complete
+      alpaka::wait(queue);
 #else
-      TrackingRecHitHost<pixelTopology::Phase1> host_collection =
-          cms::alpakatools::CopyToHost<TrackingRecHitDevice<pixelTopology::Phase1, Device> >::copyAsync(queue, tkhit);
-#endif
-      // wait for the kernel and the potential copy to complete
+      using CopyT = cms::alpakatools::CopyToHost<TrackingRecHitDevice<pixelTopology::Phase1, Device> >;
+      TrackingRecHitHost<pixelTopology::Phase1> host_collection = CopyT::copyAsync(queue, tkhit);
+      // wait for the kernel and the copy to complete
       alpaka::wait(queue);
+      CopyT::postCopy(host_collection);
+#endif
+
       assert(tkhit.nHits() == nHits);
       assert(tkhit.offsetBPIX2() == 22);  // set in the kernel
       assert(tkhit.nHits() == host_collection.nHits());