trivial_abi

[Richard Smith via cfe-commits]

On 2 January 2018 at 15:33, John McCall via cfe-commits <
cfe-commits at lists.llvm.org> wrote:

> Hey, Richard et al.  Akira and I were talking about the right ABI rule for
> deciding can-pass-in-registers-ness for structs in the presence of
> trivial_abi, and I think I like Akira's approach but wanted to get your
> input.
>
> The current definition in Itanium is:
>
>   *non-trivial for the purposes of calls*
>
> A type is considered non-trivial for the purposes of calls if:
>
>    - it has a non-trivial copy constructor, move constructor, or
>    destructor, or
>
> I'm assuming we're implicitly excluding deleted functions here. (I'd
prefer to make that explicit; this has been the source of a number of ABI
mismatches.)

>
>    - all of its copy and move constructors are deleted.
>
>
> I'd suggest modifying this to:
>
> A type is considered non-trivial for the purposes of calls if:
> - if has a copy constructor, move constructor, or destructor which is
> non-trivial for the purposes of calls, or
> - all of its copy and move constructors are deleted and it does not have
> the trivial_abi attribute.
>
> A copy/move constructor is considered trivial for the purposes of calls if:
> - it is user-provided and
> - the class has the trivial_abi attribute and
> - a defaulted definition of the constructor would be trivial for the
> purposes of calls; or
>

We'd need to say what happens if the function in question cannot validly be
defaulted for any of the reasons in [dcl.fct.def.default]. Do we try to
infer whether it's a copy or move constructor, and use the rules for a
defaulted copy or move constructor? Or do we just say that's never trivial
for the purposes of calls? Or something else? Eg:

struct [[clang::trivial_abi]] A {
  A(A && = make());
};

Here, A::A(A&&) cannot validly be defaulted. Is A trivial for the purpose
of calls? Likewise:

struct [[clang::trivial_abi]] B {
  B(...);
};
struct C {
  volatile B b;
};

Here, C's copy constructor calls B::B(...). Is C trivial for the purpose of
calls? (OK, Clang crashes on that example today. But still...)

I'd be uncomfortable making the rules in [dcl.fct.def.default] part of the
ABI; they seem to be changing relatively frequently. Perhaps we could say
"if the function is a copy constructor ([class.copy.ctor]/1), then consider
what an implicitly-declared defaulted copy constructor would do; if it's a
move constructor ([class.copy.ctor]/2), then consider what an
implicitly-declared defaulted move constructor would do; otherwise, it's
not trivial for the purpose of calls". That'd mean A is trivial for the
purpose of calls and C is not, which I think is probably the right answer.

- it is not user-provided and
> - the class has no virtual functions and no virtual base classes, and
> - the constructor used to copy/move each direct base class subobject is
> trivial for the purposes of calls, and
> - for each non-static data member that is of class type (or array
> thereof), the constructor selected to copy/move that member is trivial for
> the purposes of calls.
>
> A destructor is considered trivial for the purposes of calls if:
> - it is not user-provided or the class has the trivial_abi attribute, and
> - the destructor is not virtual, and
> - all of the direct base classes of its class have destructors that are
> trivial for the purposes of calls, and
> - for all of the non-static data members of its class that are of class
> type (or array thereof), each such class is trivial for the purposes of
> calls.
>
> These definitions are intended to follow [class.copy.ctor]p11 and
> [class.dtor]p6 except for the special rules applicable to trivial_abi
> classes.
>

If I could rephrase: a *tor is considered trivial for for the purposes of
calls if it is either defaulted or the class has the trivial_abi attribute,
and the defaulted definition would satisfy the language rule for being
trivial but with the word "trivial" replaced by "trivial for the purposes
of calls". So only effect of the trivial_abi attribute is to "undo" the
non-triviality implied by a user-provided *tor when computing triviality
for the purpose of calls.

I think that's a reasonable rule, if we have a satisfactory notion of
"defaulted definition".

I'm not sure about the "defaulted definition" rule for copy/move
> constructors in trivial_abi classes.  The intent is to allow class
> temploids with trivial_abi that are instantiated to contain non-trivial
> classes to just silently become non-trivial.  I was thinking at first that
> it would be nice to have a general rule that trivial_abi classes only
> contain trivial_abi subobjects, but unfortunately that's not consistent
> with the standard triviality rule in some silly corner cases: a
> trivially-copyable class can have a non-trivially-copyable subobject if it
> happens to copy that subobject with a trivial copy constructor.  I couldn't
> think of a better way of capturing this than the "defaulted definition"
> rule.  I considered using the actual initializers used by the constructor,
> but that would introduce a lot of new complexity: suddenly we'd be asking
> about triviality for an arbitrary constructor, and copy/move elision make
> the question somewhat ambiguous anyway.
>

Per the above examples, I don't think you can escape asking about
triviality for an arbitrary constructor if you take this path.

Another option, similar to your general rule, would be to say that a type
is considered trivial for the purpose of calls if either: (1) it is trivial
for the purpose of calls under the current Itanium ABI rule, or (2) it has
the trivial_abi attribute and all members and base classes have types that
are trivial for the purpose of calls. That would sidestep the "defaulted
definition" complexity entirely, and while it differs from the way that the
language computes triviality normally, it doesn't seem fundamentally
unreasonable: when we're thinking about triviality for the purpose of
calls, there's notionally a call to the trivial copy/move ctor being
elided, not a call to an arbitrary ctor selected by overload resolution,
and we'd just be pushing that effect from the class itself to its
subobjects with this attribute.


> I'm also not sure about the right rules about virtual methods.  Should we
> allow polymorphic classes to be made trivial by application of the
> attribute?
>

I think that it probably doesn't make much sense to pass dynamic classes
indirectly unless we can avoid passing the vptr; otherwise I'd expect we'd
use too many registers for it to be worthwhile. Perhaps as a compromise, we
could make the attribute ill-formed if used on a class definition that
introduces any virtual bases or explicitly declares any member functions as
'virtual'. That gives us the room to make this decision later if we find we
want to.
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