Re: snapshot of Lua 4.1 (work)

[David Jones <djones@...>]

In message <001d01c0d9cc$24d21f40$8402a8c0@johndell>, "John Belmonte" writes:
> Roberto wrote:
> > >     union L_Umaxalign { double d; void *s; long l; };
> > >
> > > should probably be:
> > >
> > >     union L_Umaxalign { lua_Number d; void *s; long l; };
> >
> > This double is to ensure proper alignment for userdata and strings
> > *outside* Lua. (Notice that strings will always be aligned, too.)
>  [deletia]
> Anyway, I think that union is not generally sufficient.  On my machine the
> largest data type requiring alignment is 16 bytes (a vector).  (By no
> coincidence this is also the alignment of the stack and heap.)  Maybe you
> could make it a build option, something like:
> 
>     #ifdef LUA_ALIGNMENT
>         union L_Umaxalign { char[LUA_ALIGNMENT]; };
>     #else
>         union L_Umaxalign { double d; void *s; long l; };
>     #endif

No, because a char[n] array for some n won't allow you to guarantee any
particular alignment (though it may be true for your architecture that a
char[16] is always 16 byte aligned).  If the user wants to override the
alignment union then they need to specify a type:

#ifdef LUSER_ALIGNMENT_T
  union L_Umaxalign { LUSER_ALIGNMENT_T luser_dummy_member; };
#else
  ...

(Thank goodness for typedef).

Cheers,
 drj