diff a/src/hotspot/share/gc/shenandoah/shenandoahArguments.cpp b/src/hotspot/share/gc/shenandoah/shenandoahArguments.cpp
--- a/src/hotspot/share/gc/shenandoah/shenandoahArguments.cpp
+++ b/src/hotspot/share/gc/shenandoah/shenandoahArguments.cpp
@@ -206,10 +206,19 @@
               "<= ShenandoahAllocRateSampleWindow (%u)",
         ShenandoahMomentaryAllocRateSampleWindow, ShenandoahRecentAllocRateSampleWindow,
         ShenandoahAllocRateSampleWindow));
   }
 
+  if (Arguments::is_valhalla_enabled()) {
+    // Flat atomic payloads may contain embedded oops. Current Valhalla code does not handle
+    // it well, missing the GC barriers. As the temporary kludge, disable compressed oops:
+    // this would make flat atomic payloads contain at most one oop, which would be treated
+    // as the single oop field.
+    log_warning(gc)("Shenandoah disables compressed oops to avoid breaking with Valhalla");
+    FLAG_SET_ERGO(UseCompressedOops, false);
+  }
+
   FullGCForwarding::initialize_flags(MaxHeapSize);
 }
 
 size_t ShenandoahArguments::conservative_max_heap_alignment() {
   static_assert(is_power_of_2(ShenandoahHeapRegion::MAX_REGION_SIZE), "Max region size must be a power of 2.");