Comment # 6 on bug 1042012 from Richard Biener

So as I thought GCC is taking advantage of

 #ifdef _MSC_VER
 __declspec(align(16))
 #endif
 struct cdata {
     const struct ctype type
 #ifdef __GNUC__
       __attribute__ ((aligned(16)))
 #endif
       ;
 };

when loading from cd->type but

    cd = (struct cdata*) lua_touserdata(L, idx);

lua_touserdata doesn't honor that alignment.  You can probably see that
at the point of the crash %eax is not aligned to 16 bytes.