Used start of a slab for link.

difference.md

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+# Difference from published paper
+
+This document outlines the changes that have diverged from the published paper
+on `snmalloc`.
+
+  1.  Link no longer terminates the bump-free list.  The paper describes a 
+      complex invariant for how the final element of the bump-free list can 
+      also be the link node.
+
+      We now have a much simpler invariant.  The link is either 1, signifying 
+      the block is completely full.  Or not 1, signifying it has at least one
+      free element at the offset contained in link, and that contains the DLL
+      node for this sizeclass.
+
+      The bump-free list contains additional free elements, and the remaining
+      bump allocated space.
+
+      The value 1, is never a valid bump allocation value, as we initially
+      allocate the first entry as the link, so we can use 1 as the no more bump
+      space value.