src/hotspot/share/gc/shenandoah/shenandoahGenerationalHeap.cpp

@@ -240,11 +240,18 @@
  oop ShenandoahGenerationalHeap::try_evacuate_object(oop p, Thread* thread, ShenandoahHeapRegion* from_region,
                                          ShenandoahAffiliation target_gen) {
    bool alloc_from_lab = true;
    bool has_plab = false;
    HeapWord* copy = nullptr;
-   size_t size = ShenandoahForwarding::size(p);
+ 
+   markWord mark = p->mark();
+   if (ShenandoahForwarding::is_forwarded(mark)) {
+     return ShenandoahForwarding::get_forwardee(p);
+   }
+   size_t old_size = ShenandoahForwarding::size(p);
+   size_t size = p->copy_size(old_size, mark);
+ 
    bool is_promotion = (target_gen == OLD_GENERATION) && from_region->is_young();
  
  #ifdef ASSERT
    if (ShenandoahOOMDuringEvacALot &&
        (os::random() & 1) == 0) { // Simulate OOM every ~2nd slow-path call

@@ -336,13 +343,23 @@
    if (ShenandoahEvacTracking) {
      evac_tracker()->begin_evacuation(thread, size * HeapWordSize, from_region->affiliation(), target_gen);
    }
  
    // Copy the object:
-   Copy::aligned_disjoint_words(cast_from_oop<HeapWord*>(p), copy, size);
+   Copy::aligned_disjoint_words(cast_from_oop<HeapWord*>(p), copy, old_size);
    oop copy_val = cast_to_oop(copy);
  
+   // Initialize the identity hash on the copy before installing the forwarding
+   // pointer, using the mark word we captured earlier. We must do this before
+   // the CAS so that the copy is fully initialized when it becomes visible to
+   // other threads. Using the captured mark (rather than re-reading the copy's
+   // mark) avoids races with other threads that may have evacuated p and
+   // installed a forwarding pointer in the meantime.
+   if (UseCompactObjectHeaders && mark.is_hashed_not_expanded()) {
+     copy_val->set_mark(copy_val->initialize_hash_if_necessary(p, mark.klass(), mark));
+   }
+ 
    // Update the age of the evacuated object
    if (target_gen == YOUNG_GENERATION && is_aging_cycle()) {
      ShenandoahHeap::increase_object_age(copy_val, from_region->age() + 1);
    }