Natural alignment for USE_MALLOC (#248)

* Add concept of natural alignment to tests.

snmalloc naturally aligns blocks very heavily, so that
the largest power-of-two in the rounded size is the alignment.
This checks that in the test, and provides a method for
finding the natural alignment of a block.

* Improve USE_MALLOC to provide alignment

snmalloc provides a lot of alginment guarantees. This ensures that when
we pass through to the system allocator we still get those alignment
guarantees.

The commit also fixes the tests to work with USE_MALLOC, and builds a
set of unit tests for ctest to check behaviour.

src/override/malloc.cc

@@ -24,6 +24,18 @@ using namespace snmalloc;
 
 extern "C"
 {
+  void SNMALLOC_NAME_MANGLE(check_start)(void* ptr)
+  {
+#if !defined(NDEBUG) && !defined(SNMALLOC_PASS_THROUGH)
+    if (Alloc::external_pointer<Start>(ptr) != ptr)
+    {
+      error("Using pointer that is not to the start of an allocation");
+    }
+#else
+    UNUSED(ptr);
+#endif
+  }
+
   SNMALLOC_EXPORT void* SNMALLOC_NAME_MANGLE(__malloc_end_pointer)(void* ptr)
   {
     return Alloc::external_pointer<OnePastEnd>(ptr);
@@ -36,6 +48,7 @@ extern "C"
 
   SNMALLOC_EXPORT void SNMALLOC_NAME_MANGLE(free)(void* ptr)
   {
+    SNMALLOC_NAME_MANGLE(check_start)(ptr);
     ThreadAlloc::get_noncachable()->dealloc(ptr);
   }
 
@@ -79,24 +92,26 @@ extern "C"
       SNMALLOC_NAME_MANGLE(free)(ptr);
       return nullptr;
     }
-#ifndef NDEBUG
-    // This check is redundant, because the check in memcpy will fail if this
-    // is skipped, but it's useful for debugging.
-    if (Alloc::external_pointer<Start>(ptr) != ptr)
-    {
-      error(
-        "Calling realloc on pointer that is not to the start of an allocation");
-    }
-#endif
+
+    SNMALLOC_NAME_MANGLE(check_start)(ptr);
+
     size_t sz = Alloc::alloc_size(ptr);
     // Keep the current allocation if the given size is in the same sizeclass.
     if (sz == round_size(size))
+    {
+#ifdef SNMALLOC_PASS_THROUGH
+      // snmallocs alignment guarantees can be broken by realloc in pass-through
+      // this is not exercised, by existing clients, but is tested.
+      if (pointer_align_up(ptr, natural_alignment(size)) == ptr)
+        return ptr;
+#else
       return ptr;
-
+#endif
+    }
     void* p = SNMALLOC_NAME_MANGLE(malloc)(size);
     if (p != nullptr)
     {
-      SNMALLOC_ASSERT(p == Alloc::external_pointer<Start>(p));
+      SNMALLOC_NAME_MANGLE(check_start)(p);
       sz = bits::min(size, sz);
       memcpy(p, ptr, sz);
       SNMALLOC_NAME_MANGLE(free)(ptr);