Re: [Tinycc-devel] [PATCH] stdatomic: ld/st/xchg/cmpxchg on simple types

[Michael Matz]

Hello,

On Fri, 19 Mar 2021, Dmitry Selyutin wrote:

> Some complex types can still be small and simple enough to fit into
> register. Other compilers allow some operations on these types, and it
> seems to be quite a reasonable choice. From now on, we should be able
> to compile the following artificial example:
>
>    struct combo {
>        uint16_t lo;
>        uint16_t hi;
>    };
>
>    struct combo load(const _Atomic(struct combo) *atom)
>    {
>        return atomic_load(atom);
>    }

Is the above atomic_load supposed to be similar to the GCC __atomic_load 
intrinsic?  If so, then it doesn't return a value.  It rather takes a 
pointer to the return value, hence structs aren't passed around, pointers 
are, and as such don't need special calling convention massaging.  I.e. 
it's declared similar to this:

void __atomic_load (size_t size, void *mem, void *return, int model);

Even when you encode the size directly and hence have a return type it's 
uintN, and hence not convertible to a struct type:

  struct combo c;
  c = __atomic_load_4(atom, __ATOMIC_SEQ_CST);

doesn't compile (and it shouldn't).

>    void store(_Atomic(struct combo) *atom, struct combo value)
>    {
>        atomic_store(atom, value);
>    }

Same for atomic_store and the rest: it doesn't take 'value' by value but 
by reference.

So, what exactly are you trying to implement?  Please make a full 
compilable (with GCC) example.



Ciao,
Michael.