xry111 added a comment.
In D151298#4367458 <https://reviews.llvm.org/D151298#4367458>, @wangleiat wrote:
>> I think the paragraph means:
>>
>> class Empty {};
>> int test(Empty empty, int a);
>>
>> Then we should put `a` into `a1`, not `a0`. And we are indeed doing so.
>
> yes. with this patch, `a` will be passed with `a1` register.
>
>> I mean now GCC and Clang have the same behavior, so it's easier to just
>> document the behavior in our psABI doc instead of making both Clang and GCC
>> rigidly following the interpretation of psABI (which is currently unclear
>> about zero-sized fields now) anyway.
>>
>> And the current behavior of GCC and Clang treating a class containing two
>> floating-point members and some empty fields is same as RISC-V, so it's
>> highly unlikely we are violating the C++ standard or IA64 C++ ABI (or the
>> entire RISC-V ecosystem would be violating them). The only issue is our
>> psABI is unclear about empty fields, and the easiest way to solve the issue
>> is revising the psABI (maybe, just "borrowing" some paragraphs from RISC-V
>> psABI if there is no copyright issue).
>
> If we want to ignore empty structures, it is not enough to only modify psABI,
> because the current implementations of gcc and clang do not ignore empty
> structures in all cases. For example:
>
> struct { struct{}; int i; }; // in this case, the empty struct is not
> ignored.
> struct { struct{}; float f; }; // ignored.
This is the same behavior as RISC-V. While attempting to pass a struct through
FPRs, the empty field is ignored. But if passing through FPR does not work and
it's passed through GPRs, the empty fields are not ignored:
https://godbolt.org/z/T1PKoxbYM
> Whether to ignore empty structures or not can affect the testing of gdb.
> @seehearfeel knows more details.
I guess we should be able to fix it for GDB because they must have fixed it for
RISC-V already.
Repository:
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