On Wed, 24 May 2023 at 09:41, Xi Ruoyao <xry111@xry111.site> wrote:

> Wang Lei raised some concerns about Itanium C++ ABI, so let's ask a C++
> expert here...
>
> Jonathan: AFAIK the standard and the Itanium ABI treats an empty class
> as size 1


Only as a complete object, not as a subobject.


> in order to guarantee unique address, so for the following:
>
> class Empty {};
> class Test { Empty empty; double a, b; };
>

There is no need to have a unique address here, so Test::empty and Test::a
have the same address. It's a potentially-overlapping subobject.

For the Itanium ABI, sizeof(Test) == 2 * sizeof(double).



> When we pass "Test" via registers, we may only allocate the registers
> for Test::a and Test::b, and complete ignore Test::empty because there
> is no addresses of registers.  Is this correct or not?
>

I think that's a decision for the loongarch psABI. In principle, there's no
reason a register has to be used to pass Test::empty, since you can't read
from it or write to it.



>
> On Wed, 2023-05-24 at 14:45 +0800, Xi Ruoyao via Gcc-patches wrote:
> > On Wed, 2023-05-24 at 14:04 +0800, Lulu Cheng wrote:
> > > An empty struct type that is not non-trivial for the purposes of
> > > calls
> > > will be treated as though it were the following C type:
> > >
> > > struct {
> > >   char c;
> > > };
> > >
> > > Before this patch was added, a structure parameter containing an
> > > empty structure and
> > > less than three floating-point members was passed through one or two
> > > floating-point
> > > registers, while nested empty structures are ignored. Which did not
> > > conform to the
> > > calling convention.
> >
> > No, it's a deliberate decision I've made in
> > https://gcc.gnu.org/r12-8294.  And we already agreed "the ABI needs to
> > be updated" when we applied r12-8294, but I've never improved my
> > English
> > skill to revise the ABI myself :(.
> >
> > We are also using the same "de-facto" ABI throwing away the empty
> > struct
> > for Clang++ (https://reviews.llvm.org/D132285).  So we should update
> > the
> > spec here, instead of changing every implementation.
> >
> > The C++ standard treats the empty struct as size 1 for ensuring the
> > semantics of pointer comparison operations.  When we pass it through
> > the
> > registers, there is no need to really consider the empty field because
> > there is no pointers to registers.
> >
>
>