[MLton] x86_64 port status

[Matthew Fluet]

> > I'm hoping that they are benign.  What's going on is that we are allocating 
> > space for a C struct in ML as a Word8.word array or a Word8.word vector. 
> > These are passed through the FFI as pointers:
> >
> >  typedef unsigned char* Pointer;
> >  #define Array(t) Pointer
> >  #define Ref(t) Pointer
> >  #define Vector(t) const Pointer
> 
> May break aliasing rules. Use unions. This also ensures
> correct alignment.

I was under the impression that 'unsigned char*' implies that it may alias 
with anything, so that this specifically doesn't break aliasing rules.

Union with what?  The point is that the callers of these functions are not 
C/gcc; they are the MLton codegen (which, admittedly, might be C/gcc, but 
equally well might not).  We can't guarantee anything more than what the 
codegen's guarantee: namely, that the start of ML objects will be 4-byte 
aligned.  (It is a compile time option to ensure 8-byte alignment, but we 
need to be conservative at compile-compile time.)