Major refactor of snmalloc (#343)

# Pagemap
 
The Pagemap now stores all the meta-data for the object allocation. The meta-data in the pagemap is effectively a triple of the sizeclass, the remote allocator, and a pointer to a 64 byte block of meta-data for this chunk of memory. By storing the pointer to a block, it allows the pagemap to handle multiple slab sizes without branching on the fast path. There is one entry in the pagemap per 16KiB of address space, but by using the same entry in the pagemap for 4 adjacent entries, then we can treat a 64KiB range can be treated as a single slab of allocations.

This change also means there is almost no capability amplification required by the implementation on CHERI for finding meta-data. The only amplification is required, when we change the way a chunk is used to a size of object allocation.


# Backend

There is a second major aspect of the refactor that there is now a narrow API that abstracts the Pagemap, PAL and address space management. This should better enable the compartmentalisation and makes it easier to produce alternative backends for various research directions. This is a template parameter that can be used to specialised by the front-end in different ways.

# Thread local state

The thread local state has been refactored into two components, one (called 'localalloc') that is stored directly in the TLS and is constant initialised, and one that is allocated in the address space (called 'coreallloc') which is lazily created and pooled.

# Difference

This removes Superslabs/Medium slabs as there meta-data is now part of the pagemap.

src/mem/allocstats.h

@@ -1,13 +1,12 @@
 #pragma once
 
 #include "../ds/bits.h"
-#include "../mem/sizeclass.h"
+#include "../mem/sizeclasstable.h"
 
 #include <cstdint>
 
 #ifdef USE_SNMALLOC_STATS
 #  include "../ds/csv.h"
-#  include "sizeclass.h"
 
 #  include <cstring>
 #  include <iostream>
@@ -18,11 +17,13 @@ namespace snmalloc
   template<size_t N, size_t LARGE_N>
   struct AllocStats
   {
+    constexpr AllocStats() = default;
+
     struct CurrentMaxPair
     {
-      size_t current = 0;
-      size_t max = 0;
-      size_t used = 0;
+      size_t current{0};
+      size_t max{0};
+      size_t used{0};
 
       void inc()
       {
@@ -34,7 +35,9 @@ namespace snmalloc
 
       void dec()
       {
-        SNMALLOC_ASSERT(current > 0);
+        // Split stats means this is not true.
+        // TODO reestablish checks, when we sanitise the stats.
+        //        SNMALLOC_ASSERT(current > 0);
         current--;
       }
 
@@ -64,11 +67,13 @@ namespace snmalloc
 
     struct Stats
     {
+      constexpr Stats() = default;
+
       CurrentMaxPair count;
       CurrentMaxPair slab_count;
-      uint64_t time = Aal::tick();
-      uint64_t ticks = 0;
-      double online_average = 0;
+      uint64_t time{0};
+      uint64_t ticks{0};
+      double online_average{0};
 
       bool is_empty()
       {
@@ -413,5 +418,13 @@ namespace snmalloc
           << csv.endl;
     }
 #endif
+
+    void start()
+    {
+#ifdef USE_SNMALLOC_STATS
+      for (size_t i = 0; i < N; i++)
+        sizeclass[i].time = Aal::tick();
+#endif
+    }
   };
 } // namespace snmalloc