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 2023/5/25 上午10:52, WANG Xuerui 写道:
>
> On 2023/5/25 10:46, Lulu Cheng wrote:
>>
>> 在 2023/5/25 上午4:15, Jason Merrill 写道:
>>> On Wed, May 24, 2023 at 5:00 AM Jonathan Wakely via Gcc-patches 
>>> <gcc-patches@gcc.gnu.org <mailto:gcc-patches@gcc.gnu.org>> wrote:
>>>
>>>     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.
>>>
>>>
>>> Also as a data member 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).
>>>
>>>
>>> That would be true if Test::empty were marked [[no_unique_address]], 
>>> but without that attribute, sizeof(Test) is actually 3 * 
>>> 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.
>>>
>>>
>>> Agreed.  The Itanium C++ ABI has nothing to say about how registers 
>>> are allocated for parameter passing; this is a matter for the psABI.
>>>
>>> And there is no need for a psABI to allocate a register for 
>>> Test::empty because it contains no data.
>>>
>>> In the x86_64 psABI, Test above is passed in memory because of its 
>>> size ("the size of the aggregate exceeds two eightbytes...").  But
>>>
>>> struct Test2 { Empty empty; double a; };
>>>
>>> is passed in a single floating-point register; the Test2::empty 
>>> subobject is not passed anywhere, because its eightbyte is 
>>> classified as NO_CLASS, because there is no actual data there.
>>
>>
>>
>>>
>>> I know nothing about the LoongArch psABI, but going out of your way 
>>> to assign a register to an empty class seems like a mistake.
>>
>> MIPS64 and ARM64 also allocate parameter registers for empty structs. 
>> https://godbolt.org/z/jT4cY3T5o
>>
>> Our original intention is not to pass this empty structure member, 
>> but to make the following two structures treat empty structure members
>>
>> in the same way in the process of passing parameters.
>>
>> struct st1
>> {
>>      struct empty {} e1;
>>      long a;
>>      long b;
>> };
>>
>> struct st2
>> {
>>      struct empty {} e1;
>>      double f0;
>>      double f1;
>> };
>
> Then shouldn't we try to avoid the extra register in all cases, 
> instead of wasting one regardless? ;-)

https://godbolt.org/z/eK5T3Erbs

Compared with the situation of x86-64, if it is necessary not to pass 
empty structure members, it is difficult to achieve uniform processing.