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 in order to guarantee unique address, so for the following:
class Empty {};
class Test { Empty empty; double a, b; };
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?
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.
>
--
Xi Ruoyao <[email protected]>
School of Aerospace Science and Technology, Xidian University
