Changes the representation in Metaslab Used

Used is now set to 1, when the slab is full.  This means that the test
for
 used - 1 == 0
can be used to detect leaving full, and entering empty, reducing fast
path deallocation branchs by 1.

src/mem/metaslab.h

@@ -61,14 +61,15 @@ namespace snmalloc
       used++;
     }
 
-    void sub_use()
+    /**
+     * Removes a use, if the slab is either
+     *  - empty after removing the use, or
+     *  - was full before the substraction
+     * this returns true, otherwise returns false.
+     **/
+    bool sub_use()
     {
-      used--;
-    }
-
-    void set_unused()
-    {
-      used = 0;
+      return (--used) == 0;
     }
 
     bool is_unused()
@@ -78,7 +79,9 @@ namespace snmalloc
 
     bool is_full()
     {
-      return link == 1;
+      auto result = link == 1;
+      assert(!result || head == 1);
+      return result;
     }
 
     void set_full()
@@ -86,6 +89,9 @@ namespace snmalloc
       assert(head == 1);
       assert(link != 1);
       link = 1;
+      // Set used to 1, so that "sub_use" will return true after calling
+      // set_full
+      used = 1;
     }
 
     SlabLink* get_link(Slab* slab)
@@ -175,23 +181,21 @@ namespace snmalloc
     void debug_slab_invariant(bool is_short, Slab* slab)
     {
 #if !defined(NDEBUG) && !defined(SNMALLOC_CHEAP_CHECKS)
-      size_t size = sizeclass_to_size(sizeclass);
-      size_t offset = get_initial_offset(sizeclass, is_short);
-
-      if (is_unused())
-        return;
-
-      size_t accounted_for = used * size + offset;
-
       if (is_full())
       {
-        // All the blocks must be used.
-        assert(SLAB_SIZE == accounted_for);
         // There is no free list to validate
         // 'link' value is not important if full.
         return;
       }
 
+      if (is_unused())
+        return;
+
+      size_t size = sizeclass_to_size(sizeclass);
+      size_t offset = get_initial_offset(sizeclass, is_short);
+      size_t accounted_for = used * size + offset;
+
+
       // Block is not full
       assert(SLAB_SIZE > accounted_for);
 

src/mem/sizeclass.h

@@ -8,6 +8,7 @@ namespace snmalloc
   // We use size_t as it generates better code.
   using sizeclass_t = size_t;
   //  using sizeclass_t = uint8_t;
+  using sizeclass_compress_t = uint8_t;
 
   constexpr static uint16_t get_initial_offset(sizeclass_t sc, bool is_short);
   constexpr static size_t sizeclass_to_size(sizeclass_t sizeclass);

src/mem/slab.h

@@ -163,14 +163,9 @@ namespace snmalloc
         error("Detected potential double free.");
 #endif
       meta.debug_slab_invariant(is_short(), this);
-      meta.sub_use();
 
-      bool was_full = meta.is_full();
-      if (unlikely(was_full))
-        return false;
-
-      bool is_unused = meta.is_unused();
-      if (unlikely(is_unused))
+      
+      if (unlikely(meta.sub_use()))
         return false;
 
       // Update the head and the next pointer in the free list.
@@ -198,13 +193,10 @@ namespace snmalloc
     {
       Metaslab& meta = super->get_meta(this);
 
-      bool was_full = meta.is_full();
-      bool is_unused = meta.is_unused();
-
-      if (was_full)
+      if (meta.is_full())
       {
         // We are not on the sizeclass list.
-        if (is_unused)
+        if (meta.allocated == 1)
         {
           // Dealloc on the superslab.
           if (is_short())
@@ -217,6 +209,7 @@ namespace snmalloc
         assert(meta.head == 1);
         //        assert(meta.fully_allocated(is_short()));
         meta.link = index;
+        meta.used = meta.allocated - 1;
 
         // Push on the list of slabs for this sizeclass.
         sl->insert_back(meta.get_link(this));
@@ -224,18 +217,13 @@ namespace snmalloc
         return Superslab::NoSlabReturn;
       }
 
-      if (is_unused)
-      {
-        // Remove from the sizeclass list and dealloc on the superslab.
-        sl->remove(meta.get_link(this));
+      // Remove from the sizeclass list and dealloc on the superslab.
+      sl->remove(meta.get_link(this));
 
-        if (is_short())
-          return super->dealloc_short_slab(memory_provider);
+      if (is_short())
+        return super->dealloc_short_slab(memory_provider);
 
-        return super->dealloc_slab(this, memory_provider);
-      }
-
-      abort();
+      return super->dealloc_slab(this, memory_provider);
     }
 
     bool is_short()