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

@@ -3126,11 +3126,23 @@
  }
  
  
  HeapWord* ShenandoahFreeSet::allocate(ShenandoahAllocRequest& req, bool& in_new_region) {
    shenandoah_assert_heaplocked();
-   if (ShenandoahHeapRegion::requires_humongous(req.size())) {
+   // A freshly allocated mutator object is not yet hashed and may grow by one word
+   // when an identity hash-code is injected during a future GC copy. Classify it by
+   // its potential expanded size so an object that could outgrow a region is placed
+   // as humongous from the start, rather than allocated as a regular object and then
+   // needing to become humongous after it grows. Every other request carries an
+   // already-final size:
+   //  - a GC copy (_alloc_shared_gc*) uses an already-expanded copy_size(),
+   //  - a CDS allocation (_alloc_cds) requests a raw archive range; its objects are
+   //    written at their final, already hash-expanded size (copy_size_cds) and are
+   //    not grown at load time,
+   //  - a LAB buffer (TLAB/GCLAB/PLAB, bounded by max_tlab_size_words()) never grows.
+   const bool may_expand_for_hash = (req.type() == ShenandoahAllocRequest::_alloc_shared);
+   if (ShenandoahHeapRegion::requires_humongous(req.size(), may_expand_for_hash)) {
      switch (req.type()) {
        case ShenandoahAllocRequest::_alloc_shared:
        case ShenandoahAllocRequest::_alloc_shared_gc:
          in_new_region = true;
          return allocate_contiguous(req, /* is_humongous = */ true);