Initial commit of snmalloc

History squashed from internal development.

Internal history has commit hash:
  e27a0e485c44a5003a802de2661ce3b21e120316

src/mem/remoteallocator.h

@@ -0,0 +1,64 @@
+#pragma once
+
+#include "../ds/mpscq.h"
+#include "../mem/allocconfig.h"
+
+#include <atomic>
+
+namespace snmalloc
+{
+  struct Remote
+  {
+    static const uint64_t SIZECLASS_SHIFT = 56;
+    static const uint64_t SIZECLASS_MASK = 0xffULL << SIZECLASS_SHIFT;
+    static const uint64_t TARGET_MASK = ~SIZECLASS_MASK;
+
+    static_assert(SIZECLASS_MASK == 0xff00'0000'0000'0000ULL);
+
+    using alloc_id_t = size_t;
+    union
+    {
+      std::atomic<Remote*> next;
+      Remote* non_atomic_next;
+    };
+
+    uint64_t value;
+
+    void set_target_id(alloc_id_t id)
+    {
+      assert(id == (id & TARGET_MASK));
+      value = (id & TARGET_MASK) | (value & SIZECLASS_MASK);
+    }
+
+    void set_sizeclass(uint8_t sizeclass)
+    {
+      value = (value & TARGET_MASK) |
+        ((static_cast<uint64_t>(sizeclass) << SIZECLASS_SHIFT) &
+         SIZECLASS_MASK);
+    }
+
+    alloc_id_t target_id()
+    {
+      return value & TARGET_MASK;
+    }
+
+    uint8_t sizeclass()
+    {
+      return (value & SIZECLASS_MASK) >> SIZECLASS_SHIFT;
+    }
+  };
+
+  struct RemoteAllocator
+  {
+    using alloc_id_t = Remote::alloc_id_t;
+    // Store the message queue on a separate cacheline. It is mutable data that
+    // is read by other threads.
+    alignas(CACHELINE_SIZE) MPSCQ<Remote> message_queue;
+
+    alloc_id_t id()
+    {
+      return static_cast<alloc_id_t>(
+        reinterpret_cast<uintptr_t>(&message_queue));
+    }
+  };
+}