Re: [PATCH] LoongArch: Fix the problem of structure parameter passing in C++. This structure has empty structure members and less than three floating point members.

[Lulu Cheng <chenglulu@loongson.cn>]

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,

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.

Judging from the size of the structure, the size of st2 is already 
larger than 2xGRLEN, should be passed by reference.