Re: Passing empty "tag" structs

[Jonathan Wakely <jwakely dot gcc at gmail dot com>]

On Fri, 7 Sep 2018 at 08:06, David Brown wrote:
>
> In C++ programming, it is sometimes helpful to have empty structs acting
> as tags.  An example is "struct nothrow_t {}".
>
> When parameters of these types - such as "nothrow", are passed to
> functions the compiler passes them as a value 0.  Since the type cannot
> hold any kind of value, surely it could be passed without any value at
> all being placed in the corresponding parameter register or stack slot?
> Or do the rules of C++ require a value here?

The ABI dictates it, and in GCC 8 it was fixed, see
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60336

> Going one step further, if a function takes empty tag parameters such as:
>
>         foo(tag1_t, tag2_t, int x);
>
> the first two parameters take up valuable register (or stack) slots for
> no useful information.  Could this function be mangled to be, in effect:
>
>         foo__tag1_t__tag2_t(int x);

No, the ABI also dictates how to mangle functions.