r350920 - [Sema] Make canPassInRegisters return true if the CXXRecordDecl passed

[John McCall via cfe-commits]

On 16 Jan 2019, at 20:03, Richard Smith wrote:

> On Wed, 16 Jan 2019 at 16:20, John McCall via cfe-commits <
> cfe-commits at lists.llvm.org> wrote:
>
>> On 16 Jan 2019, at 18:32, Richard Smith wrote:
>>
>>> On Wed, 16 Jan 2019 at 09:10, John McCall via cfe-commits <
>>> cfe-commits at lists.llvm.org> wrote:
>>>
>>>> On 16 Jan 2019, at 9:13, Aaron Ballman wrote:
>>>>
>>>>> On Wed, Jan 16, 2019 at 1:57 AM Akira Hatanaka <ahatanaka at apple.com>
>>>>> wrote:
>>>>>>
>>>>>> Yes, the behavior of the compiler doesn’t match what’s explained
>>>>>> in the documentation anymore.
>>>>>>
>>>>>> Please take a look at the attached patch, which updates the
>>>>>> documentation.
>>>>>
>>>>> Patch mostly LGTM, but I did have one wording suggestion.
>>>>>
>>>>>> diff --git a/include/clang/Basic/AttrDocs.td
>>>>>> b/include/clang/Basic/AttrDocs.td
>>>>>> index 5773a92c9c..ca3cfcf9b2 100644
>>>>>> --- a/include/clang/Basic/AttrDocs.td
>>>>>> +++ b/include/clang/Basic/AttrDocs.td
>>>>>> @@ -2478,15 +2478,20 @@ def TrivialABIDocs : Documentation {
>>>>>>    let Category = DocCatVariable;
>>>>>>    let Content = [{
>>>>>>  The ``trivial_abi`` attribute can be applied to a C++ class, struct,
>>>>>> or union.
>>>>>> -It instructs the compiler to pass and return the type using the C
>>>>>> ABI for the
>>>>>> +``trivial_abi`` has the following effects:
>>>>>> +
>>>>>> +- It instructs the compiler to pass and return the type using the C
>>>>>> ABI for the
>>>>>>  underlying type when the type would otherwise be considered
>>>>>> non-trivial for the
>>>>>>  purpose of calls.
>>>>>> -A class annotated with `trivial_abi` can have non-trivial
>>>>>> destructors or copy/move constructors without automatically becoming
>>>>>> non-trivial for the purposes of calls. For example:
>>>>>> +- It makes the destructor and copy and move constructors of the
>>>>>> class trivial
>>>>>> +that would otherwise be considered non-trivial under the C++ ABI
>>>>>> rules.
>>>>>
>>>>> How about: It makes the destructor, copy constructors, and move
>>>>> constructors of the class trivial even if they would otherwise be
>>>>> non-trivial under the C++ ABI rules.
>>>>
>>>> Let's not say that it makes them trivial, because it doesn't.  It causes
>>>> their immediate non-triviality to be ignored for the purposes of
>>>> deciding
>>>> whether the type can be passed in registers.
>>>>
>>>
>>> Given the attribute now forces the type to be passed in registers, I
>> think
>>> it'd be more to the point to say that it makes the triviality of those
>>> special members be ignored when deciding whether to pass a type with a
>>> subobject of this type in registers.
>>
>> Wait, it forces it to be passed in registers?  I thought the design
>> was that it didn't override the non-trivial-abi-ness of subobjects;
>> see all the discussion of trivially_relocatable.
>>
>
> The attribute is ill-formed if applied to a class that has a subobject that
> can't be passed in registers (or that has a vptr). And then as a weird
> special case we don't instantiate the attribute when instantiating a class
> if it would be ill-formed (well, we instantiate it and then remove it
> again, but the effect is the same).
>
> The commit at the start of this email chain switches us from the "just
> override the trivialness for the purposes of the ABI" model to /also/
> forcing the type to be passed in registers (the type would otherwise not be
> passed in registers in some corner cases, such as if all its copy/move
> special members are deleted).

I see.  Alright, I accept your wording, then.

John.