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

[Richard Smith via cfe-commits]

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).
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