Rework free list so that 0 is the placeholder.

This is needed because in some configurations the constructor for the
global placeholder is not called before the first allocation (i.e. when
other globals call the allocator in their constructor) and so we ended
up following a null pointer.

src/mem/alloc.h

@@ -1072,7 +1072,7 @@ namespace snmalloc
       assert(sizeclass < NUM_SMALL_CLASSES);
       auto& fl = small_fast_free_lists[sizeclass];
       auto head = fl.value;
-      if (likely((reinterpret_cast<size_t>(head) & 1) == 0))
+      if (likely(head != nullptr))
       {
         void* p = head;
         // Read the next slot from the memory that's about to be allocated.

src/mem/metaslab.h

@@ -146,10 +146,10 @@ namespace snmalloc
     {
 #ifndef NDEBUG
       size_t length = 0;
-      void* curr = pointer_offset(slab, head);
-      void* curr_slow = pointer_offset(slab, head);
+      void* curr = (head == 1) ? nullptr : pointer_offset(slab, head);
+      void* curr_slow = (head == 1) ? nullptr : pointer_offset(slab, head);
       bool both = false;
-      while ((reinterpret_cast<size_t>(curr) & 1) == 0)
+      while (curr != nullptr)
       {
         curr = follow_next(curr);
         if (both)
@@ -200,8 +200,8 @@ namespace snmalloc
       UNUSED(length);
 
       // Walk bump-free-list-segment accounting for unused space
-      void* curr = pointer_offset(slab, head);
-      while ((address_cast(curr) & 1) == 0)
+      void* curr = (head == 1) ? nullptr : pointer_offset(slab, head);
+      while (curr != nullptr)
       {
         // Check we are looking at a correctly aligned block
         void* start = curr;

src/mem/slab.h

@@ -7,7 +7,7 @@ namespace snmalloc
   struct FreeListHead
   {
     // Use a value with bottom bit set for empty list.
-    void* value = pointer_offset<void*>(nullptr, 1);
+    void* value = nullptr;
   };
 
   class Slab
@@ -98,7 +98,8 @@ namespace snmalloc
             }
             else
             {
-              Metaslab::store_next(curr, pointer_offset(this, bumpptr));
+              Metaslab::store_next(
+                curr, (bumpptr == 1) ? nullptr : pointer_offset(this, bumpptr));
             }
             curr = pointer_offset(this, bumpptr);
             bumpptr = newbumpptr;
@@ -106,7 +107,7 @@ namespace snmalloc
           }
 
           assert(curr != nullptr);
-          Metaslab::store_next(curr, pointer_offset<void*>(nullptr, 1));
+          Metaslab::store_next(curr, nullptr);
         }
       }
 
@@ -177,7 +178,8 @@ namespace snmalloc
       assert(meta.valid_head(is_short()));
 
       // Set the next pointer to the previous head.
-      Metaslab::store_next(p, pointer_offset(this, head));
+      Metaslab::store_next(
+        p, (head == 1) ? nullptr : pointer_offset(this, head));
       meta.debug_slab_invariant(is_short(), this);
       return true;
     }