diff --git a/runtime/gc/space/space.h b/runtime/gc/space/space.h
index 0a87a16..dcf5357 100644
--- a/runtime/gc/space/space.h
+++ b/runtime/gc/space/space.h
@@ -203,9 +203,17 @@
   // Allocate num_bytes without allowing growth. If the allocation
   // succeeds, the output parameter bytes_allocated will be set to the
   // actually allocated bytes which is >= num_bytes.
+  // Alloc can be called from multiple threads at the same time and must be thread-safe.
   virtual mirror::Object* Alloc(Thread* self, size_t num_bytes, size_t* bytes_allocated,
                                 size_t* usable_size) = 0;
 
+  // Thread-unsafe allocation for when mutators are suspended, used by the semispace collector.
+  virtual mirror::Object* AllocThreadUnsafe(Thread* self, size_t num_bytes, size_t* bytes_allocated,
+                                            size_t* usable_size)
+      EXCLUSIVE_LOCKS_REQUIRED(Locks::mutator_lock_) {
+    return Alloc(self, num_bytes, bytes_allocated, usable_size);
+  }
+
   // Return the storage space required by obj.
   virtual size_t AllocationSize(mirror::Object* obj, size_t* usable_size) = 0;