src: fix pointer alignment

The NgLibMemoryManager::ReallocImpl method prefixes the allocated
memory with its size, and returns a pointer to the region after it.
This pointer can however no longer be suitably aligned.

On Arm 32bits this resulted in unaligned accesses, since the NEON
vst1.64 instruction was used with a not properly aligned addresses.
A reproducer is available at [1].

Correct this by allocating the maximum of the the size of the size_t
and the max alignment.

[1] victronenergy/venus#1559.

Author: jhofstee

src/node_mem-inl.h

@@ -8,6 +8,8 @@
 
 namespace node {
 namespace mem {
+  static constexpr size_t kReserveSizeAndAlign = std::max(sizeof(size_t),
+      alignof(max_align_t));
 
 template <typename Class, typename AllocatorStruct>
 AllocatorStruct NgLibMemoryManager<Class, AllocatorStruct>::MakeAllocator() {
@@ -30,19 +32,19 @@ void* NgLibMemoryManager<Class, T>::ReallocImpl(void* ptr,
   char* original_ptr = nullptr;
 
   // We prepend each allocated buffer with a size_t containing the full
-  // size of the allocation.
-  if (size > 0) size += sizeof(size_t);
+  // size of the allocation, while keeping the returned pointer aligned.
+  if (size > 0) size += kReserveSizeAndAlign;
 
   if (ptr != nullptr) {
     // We are free()ing or re-allocating.
-    original_ptr = static_cast<char*>(ptr) - sizeof(size_t);
+    original_ptr = static_cast<char*>(ptr) - kReserveSizeAndAlign;
     previous_size = *reinterpret_cast<size_t*>(original_ptr);
     // This means we called StopTracking() on this pointer before.
     if (previous_size == 0) {
       // Fall back to the standard Realloc() function.
       char* ret = UncheckedRealloc(original_ptr, size);
       if (ret != nullptr)
-        ret += sizeof(size_t);
+        ret += kReserveSizeAndAlign;
       return ret;
     }
   }
@@ -62,7 +64,7 @@ void* NgLibMemoryManager<Class, T>::ReallocImpl(void* ptr,
     manager->env()->external_memory_accounter()->Update(
         manager->env()->isolate(), new_size);
     *reinterpret_cast<size_t*>(mem) = size;
-    mem += sizeof(size_t);
+    mem += kReserveSizeAndAlign;
   } else if (size == 0) {
     manager->DecreaseAllocatedSize(previous_size);
     manager->env()->external_memory_accounter()->Decrease(
@@ -96,7 +98,7 @@ void* NgLibMemoryManager<Class, T>::CallocImpl(size_t nmemb,
 template <typename Class, typename T>
 void NgLibMemoryManager<Class, T>::StopTrackingMemory(void* ptr) {
   size_t* original_ptr = reinterpret_cast<size_t*>(
-      static_cast<char*>(ptr) - sizeof(size_t));
+      static_cast<char*>(ptr) - kReserveSizeAndAlign);
   Class* manager = static_cast<Class*>(this);
   manager->DecreaseAllocatedSize(*original_ptr);
   manager->env()->external_memory_accounter()->Decrease(