NFC: Extract MetaCommon from ChunkRecord

src/mem/corealloc.h

@@ -322,7 +322,8 @@ namespace snmalloc
       auto start_of_slab = pointer_align_down<void>(
         p, snmalloc::sizeclass_to_slab_size(sizeclass));
       // TODO Add bounds correctly here
-      chunk_record->chunk = capptr::Chunk<void>(start_of_slab.unsafe_ptr());
+      chunk_record->meta_common.chunk =
+        capptr::Chunk<void>(start_of_slab.unsafe_ptr());
 
 #ifdef SNMALLOC_TRACING
       std::cout << "Slab " << start_of_slab.unsafe_ptr()

src/mem/localalloc.h

@@ -541,7 +541,8 @@ namespace snmalloc
          * a CapPtr<void, Chunk> to this region internally even while it's
          * allocated.
          */
-        slab_record->chunk = capptr::Chunk<void>(p_tame.unsafe_ptr());
+        slab_record->meta_common.chunk =
+          capptr::Chunk<void>(p_tame.unsafe_ptr());
         check_init(
           [](
             CoreAlloc* core_alloc,

src/mem/metaslab.h

@@ -9,6 +9,11 @@
 
 namespace snmalloc
 {
+  struct MetaCommon
+  {
+    capptr::Chunk<void> chunk;
+  };
+
   // The Metaslab represent the status of a single slab.
   // This can be either a short or a standard slab.
   class alignas(CACHELINE_SIZE) Metaslab

src/mem/slaballocator.h

@@ -17,9 +17,13 @@ namespace snmalloc
    */
   struct ChunkRecord
   {
+    MetaCommon meta_common;
     std::atomic<ChunkRecord*> next;
-    capptr::Chunk<void> chunk;
   };
+  static_assert(std::is_standard_layout_v<ChunkRecord>);
+  static_assert(
+    offsetof(ChunkRecord, meta_common) == 0,
+    "ChunkRecord and Metaslab must share a common prefix");
 
   /**
    * How many slab sizes that can be provided.
@@ -97,7 +101,7 @@ namespace snmalloc
 
       if (chunk_record != nullptr)
       {
-        auto slab = chunk_record->chunk;
+        auto slab = chunk_record->meta_common.chunk;
         state.memory_in_stacks -= slab_size;
         auto meta = reinterpret_cast<Metaslab*>(chunk_record);
 #ifdef SNMALLOC_TRACING
@@ -138,8 +142,8 @@ namespace snmalloc
     {
       auto& state = SharedStateHandle::get_slab_allocator_state(&local_state);
 #ifdef SNMALLOC_TRACING
-      std::cout << "Return slab:" << p->chunk.unsafe_ptr() << " slab_sizeclass "
-                << slab_sizeclass << " size "
+      std::cout << "Return slab:" << p->meta_common.chunk.unsafe_ptr()
+                << " slab_sizeclass " << slab_sizeclass << " size "
                 << slab_sizeclass_to_size(slab_sizeclass)
                 << " memory in stacks " << state.memory_in_stacks << std::endl;
 #endif