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/ds/aba.h

@@ -13,7 +13,7 @@
  */
 namespace snmalloc
 {
-#ifndef NDEBUG
+#if !defined(NDEBUG) && !defined(SNMALLOC_DISABLE_ABA_VERIFY)
   // LL/SC typically can only perform one operation at a time
   // check this on other platforms using a thread_local.
   inline thread_local bool operation_in_flight = false;
@@ -24,24 +24,20 @@ namespace snmalloc
   // fall back to locked implementation.
 #if defined(PLATFORM_IS_X86) && \
   !(defined(GCC_NOT_CLANG) && defined(OPEN_ENCLAVE))
-  template<
-    typename T,
-    Construction c = RequiresInit,
-    template<typename> typename Ptr = Pointer,
-    template<typename> typename AtomicPtr = AtomicPointer>
+  template<typename T, Construction c = RequiresInit>
   class ABA
   {
   public:
     struct alignas(2 * sizeof(std::size_t)) Linked
     {
-      Ptr<T> ptr;
-      uintptr_t aba;
+      T* ptr{nullptr};
+      uintptr_t aba{0};
     };
 
     struct Independent
     {
-      AtomicPtr<T> ptr;
-      std::atomic<uintptr_t> aba;
+      std::atomic<T*> ptr{nullptr};
+      std::atomic<uintptr_t> aba{0};
     };
 
     static_assert(
@@ -59,13 +55,9 @@ namespace snmalloc
     };
 
   public:
-    ABA()
-    {
-      if constexpr (c == RequiresInit)
-        init(nullptr);
-    }
+    constexpr ABA() : independent() {}
 
-    void init(Ptr<T> x)
+    void init(T* x)
     {
       independent.ptr.store(x, std::memory_order_relaxed);
       independent.aba.store(0, std::memory_order_relaxed);
@@ -75,7 +67,7 @@ namespace snmalloc
 
     Cmp read()
     {
-#  ifndef NDEBUG
+#  if !defined(NDEBUG) && !defined(SNMALLOC_DISABLE_ABA_VERIFY)
       if (operation_in_flight)
         error("Only one inflight ABA operation at a time is allowed.");
       operation_in_flight = true;
@@ -97,12 +89,12 @@ namespace snmalloc
        */
       Cmp(Linked old, ABA* parent) : old(old), parent(parent) {}
 
-      Ptr<T> ptr()
+      T* ptr()
       {
         return old.ptr;
       }
 
-      bool store_conditional(Ptr<T> value)
+      bool store_conditional(T* value)
       {
 #  if defined(_MSC_VER) && defined(SNMALLOC_VA_BITS_64)
         auto result = _InterlockedCompareExchange128(
@@ -127,7 +119,7 @@ namespace snmalloc
 
       ~Cmp()
       {
-#  ifndef NDEBUG
+#  if !defined(NDEBUG) && !defined(SNMALLOC_DISABLE_ABA_VERIFY)
         operation_in_flight = false;
 #  endif
       }
@@ -137,7 +129,7 @@ namespace snmalloc
     };
 
     // This method is used in Verona
-    Ptr<T> peek()
+    T* peek()
     {
       return independent.ptr.load(std::memory_order_relaxed);
     }
@@ -146,19 +138,15 @@ namespace snmalloc
   /**
    * Naive implementation of ABA protection using a spin lock.
    */
-  template<
-    typename T,
-    Construction c = RequiresInit,
-    template<typename> typename Ptr = Pointer,
-    template<typename> typename AtomicPtr = AtomicPointer>
+  template<typename T, Construction c = RequiresInit>
   class ABA
   {
-    AtomicPtr<T> ptr = nullptr;
+    std::atomic<T*> ptr = nullptr;
     std::atomic_flag lock = ATOMIC_FLAG_INIT;
 
   public:
     // This method is used in Verona
-    void init(Ptr<T> x)
+    void init(T* x)
     {
       ptr.store(x, std::memory_order_relaxed);
     }
@@ -170,7 +158,7 @@ namespace snmalloc
       while (lock.test_and_set(std::memory_order_acquire))
         Aal::pause();
 
-#  ifndef NDEBUG
+#  if !defined(NDEBUG) && !defined(SNMALLOC_DISABLE_ABA_VERIFY)
       if (operation_in_flight)
         error("Only one inflight ABA operation at a time is allowed.");
       operation_in_flight = true;
@@ -184,12 +172,12 @@ namespace snmalloc
       ABA* parent;
 
     public:
-      Ptr<T> ptr()
+      T* ptr()
       {
         return parent->ptr;
       }
 
-      bool store_conditional(Ptr<T> t)
+      bool store_conditional(T* t)
       {
         parent->ptr = t;
         return true;
@@ -198,14 +186,14 @@ namespace snmalloc
       ~Cmp()
       {
         parent->lock.clear(std::memory_order_release);
-#  ifndef NDEBUG
+#  if !defined(NDEBUG) && !defined(SNMALLOC_DISABLE_ABA_VERIFY)
         operation_in_flight = false;
 #  endif
       }
     };
 
     // This method is used in Verona
-    Ptr<T> peek()
+    T* peek()
     {
       return ptr.load(std::memory_order_relaxed);
     }