While working on JDK-8238247, I realized that CodeHeap::blob_count() miscalculates the blob count. If you look at the usages of CodeHeap::_blob_count field, it is only incremented, never decremented. Which is almost fine, but it leads to blob_count miscalculation: when block is freed and put on free list and re-acquired from the free list, it is counted as new blob all over again.

So, either way should be done:
 a) Decrement _blob_count on addition to the free list, which would make CodeHeap::blob_count() report the number of alive blobs;
 b) Stop incrementing blob_count on removal from the free list, which would make CodeHeap::blob_count() report the number of alive+free blobs.

Sample patch for option (a):

diff -r e4e6c1846d31 src/hotspot/share/memory/heap.cpp
--- a/src/hotspot/share/memory/heap.cpp Fri Jan 31 19:00:14 2020 +0100
+++ b/src/hotspot/share/memory/heap.cpp Fri Jan 31 19:00:17 2020 +0100
@@ -609,10 +609,13 @@
 void CodeHeap::add_to_freelist(HeapBlock* a) {
   FreeBlock* b = (FreeBlock*)a;
   size_t  bseg = segment_for(b);
   _freelist_length++;
 
+  _blob_count--;
+  assert(_blob_count >= 0, "sanity");
+
   assert(b != _freelist, "cannot be removed twice");
 
   // Mark as free and update free space count
   _freelist_segments += b->length();
   b->set_free();