We do not need both malloc and realloc.

ptr = malloc(size)

is equivalent to

ptr = realloc(NULL, size)

Yet, when I look at the code (haven't actually try it yet) I see some differences that suggests that we would not be able to do it right now. For example, our os:malloc() has this:

  if (DumpSharedSpaces) {
    // Need to deterministically fill all the alignment gaps in C++ structures.
    ::memset(inner_ptr, 0, size);
  } else {
    DEBUG_ONLY(::memset(inner_ptr, uninitBlockPad, size);)
  }

and os:realloc() only has this (but only if NMT is enabled):

#ifdef ASSERT
    size_t old_size = free_info.size;
    if (old_size < size) {
      // We also zap the newly extended region.
      ::memset((char*)new_inner_ptr + old_size, uninitBlockPad, size - old_size);
    }
#endif

Investigate and remove os:malloc() if we can, so we only need to support one codepath.