https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114171
--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Iain Buclaw from comment #4)
> Removed druntime dependency.
> ---
> import gcc.builtins;
> struct Token {
> string label;
> }
> struct BreakStatement {
> ulong pad;
> Token label;
> }
>
> pragma(inline, false)
> auto newclass()
> {
> void *p = __builtin_malloc(BreakStatement.sizeof);
> __builtin_memset(p, 0, BreakStatement.sizeof);
> return cast(BreakStatement*) p;
> }
>
> int main ()
> {
> auto bn = newclass();
> return bn.label is Token.init;
> }
> ---
>
>
>
> Roughly the equivalent C++
> ---
> struct Token {
> struct {
> __SIZE_TYPE__ length;
> const char *ptr;
> } label;
> };
> struct BreakStatement {
> __UINT64_TYPE__ pad;
> Token label;
> };
>
> __attribute__((noinline))
> BreakStatement *newclass()
> {
> void *p = __builtin_malloc(sizeof(BreakStatement));
> __builtin_memset(p, 0, sizeof(BreakStatement));
> return (BreakStatement*) p;
> }
>
> int main ()
> {
> auto bn = newclass();
> auto init = Token();
> return *(__uint128_t*)&bn->label == *(__uint128_t*)&init;
> }
> ---
Unless gdc somehow guarantees bn->label and init are 128bit aligned
then "casting" this way is broken. You can of course use
build_aligned_type to build a properly (mis-)aligned type to use
to dereference to.
