On Wed, 2023-05-24 at 18:07 +0800, Lulu Cheng wrote:
>
> 在 2023/5/24 下午5:25, Xi Ruoyao 写道:
> > On Wed, 2023-05-24 at 16:47 +0800, Lulu Cheng wrote:
> > > 在 2023/5/24 下午2:45, Xi Ruoyao 写道:
> > > > 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.
> > > >
> > > I think that the rules for passing parameters to empty structures or
> > > nested empty structures should be unified,
> > There is no need to unify them because "passing a struct" is already
> > different from "passing its members one by one". Say:
> >
> > int f1(int a, int b);
> >
> > and
> >
> > int f2(struct {int a, b;} ab);
> >
> > "a" and "b" are already passed differently.
> I mean I think that empty structs in st1 and st2 should be treated the
> same way in the way of passing parameters.
> >
> > > but the current implementation in gcc is as follows(in C++):
> > >
> > > Compare the two structures,the current implementation is as follows:
> > >
> > > struct st1
> > > {
> > > struct empty {} e1;
> > > long a;
> > > long b;
> > > };
> > >
> > > passed by reference.
> > >
> > >
> > > struct st2
> > > {
> > > struct empty {} e1;
> > > double f0;
> > > double f1;
> > > };
> > >
> > > passed through two floating-point registers.
> > Well this is nasty, but it is the same behavior as RISC-V:
> > https://godbolt.org/z/fEexq148r
> >
> > I deliberately made our logic similar to RISC-V in r12-8294 because
> > "there seems no reason to do it differently". Maybe I was wrong and we
> > should have ignored st1::e1 as well (but IIRC we were running out of
> > time for GCC 12 release so we didn't have time to consider this :( ).
> >
> > But now it's better to "keep the current behavior as-is" because:
> >
> > 1. The current behavior of GCC and Clang already matches and the
> > behavior is kept since the day one GCC and Clang supports LoongArch. So
> > there is currently no ABI incompatibility in practice, but changing the
> > behavior will introduce an ABI incompatibility.
>
> The parameter passing rules for a single empty structure are different
> in GCC and Clang.
>
> eg:
>
> void test (struct empty, int a);
>
> In GCC, the empty structure is passed through $a0, and the variable a is
> passed through $a1,
>
> but Clang passes a through $a0, and the empty structure is ignored.
>
> > 2. Changing the behavior will make the compiler more complex, and
> > slower.
> > 3. Changing the behavior will need a -Wpsabi warning according to the
> > GCC policy, leading to more boring code (and more slow-down) in the
> > compiler.
>
> I really understand and thank you for your concerns, we have also
> considered the issue of compatibility.
>
> Before the modification, we made an assessment. The colleagues of the
> operating system
>
> built a total of 3,300 linux basic packages, and only one package was
> affected by this modification.
>
> This is why GCC fixes this as a bug without adding -Wpsabi.
If you are really determined to do this, then OK. I'm in a very bad
mood and I don't want to spend my mental strength on debating (esp. on a
corner case unlikely to affect "real" code) anymore.
But remember to add a entry in GCC 14 changes.html, and test this thing:
struct Empty {};
struct Something : Empty
{
double a, b;
};
If we are not careful enough we may introduce a ABI mismatch between -
std=c++14 and -std=c++17 here. See https://gcc.gnu.org/PR94383.
--
Xi Ruoyao <xry...@xry111.site>
School of Aerospace Science and Technology, Xidian University
