Initial commit of snmalloc

History squashed from internal development.

Internal history has commit hash:
  e27a0e485c44a5003a802de2661ce3b21e120316

src/test/perf/external_pointer/externalpointer.cc

@@ -0,0 +1,78 @@
+#include <snmalloc.h>
+#include <test/measuretime.h>
+#include <test/xoroshiro.h>
+#include <unordered_set>
+
+using namespace snmalloc;
+
+constexpr size_t count_log = 20;
+constexpr size_t count = 1 << count_log;
+// Pre allocate all the objects
+size_t* objects[count];
+
+NOINLINE void setup(xoroshiro::p128r64& r, Alloc* alloc)
+{
+  for (size_t i = 0; i < count; i++)
+  {
+    size_t rand = (size_t)r.next();
+    size_t offset = bits::clz(rand);
+    if (offset > 30)
+      offset = 30;
+    size_t size = (rand & 15) << offset;
+    if (size < 16)
+      size = 16;
+    // store object
+    objects[i] = (size_t*)alloc->alloc(size);
+    // Store allocators size for this object
+    *objects[i] = Alloc::alloc_size(objects[i]);
+  }
+}
+
+NOINLINE void teardown(Alloc* alloc)
+{
+  // Deallocate everything
+  for (size_t i = 0; i < count; i++)
+  {
+    alloc->dealloc(objects[i]);
+  }
+
+  current_alloc_pool()->debug_check_empty();
+}
+
+void test_external_pointer(xoroshiro::p128r64& r)
+{
+  auto* alloc = ThreadAlloc::get();
+
+  setup(r, alloc);
+
+  DO_TIME("External pointer queries ", {
+    for (size_t i = 0; i < 10000000; i++)
+    {
+      size_t rand = (size_t)r.next();
+      size_t oid = rand & (((size_t)1 << count_log) - 1);
+      size_t* external_ptr = objects[oid];
+      size_t size = *external_ptr;
+      size_t offset = (size >> 4) * (rand & 15);
+      size_t interior_ptr = ((size_t)external_ptr) + offset;
+      void* calced_external = Alloc::external_pointer((void*)interior_ptr);
+      if (calced_external != external_ptr)
+        abort();
+    }
+  });
+
+  teardown(alloc);
+}
+
+int main(int, char**)
+{
+  xoroshiro::p128r64 r;
+#if NDEBUG
+  size_t nn = 30;
+#else
+  size_t nn = 3;
+#endif
+
+  for (size_t n = 0; n < nn; n++)
+    test_external_pointer(r);
+  return 0;
+}