On Wed, Jan 3, 2018 at 12:24 PM, Akira Hatanaka <ahatan...@apple.com> wrote:
> On Jan 2, 2018, at 9:42 AM, David Blaikie via cfe-commits < > cfe-commits@lists.llvm.org> wrote: > > > > On Tue, Dec 19, 2017 at 9:43 PM Akira Hatanaka <ahata...@gmail.com> wrote: > >> On Tue, Dec 12, 2017 at 12:12 PM, John McCall <rjmcc...@gmail.com> wrote: >> >>> On Tue, Dec 12, 2017 at 1:45 PM, David Blaikie <dblai...@gmail.com> w >>> rote: >>> >>>> On Mon, Dec 11, 2017 at 5:38 PM John McCall <rjmcc...@gmail.com> wrote: >>>> >>>>> On Mon, Dec 11, 2017 at 6:19 PM, David Blaikie <dblai...@gmail.com> w >>>>> rote: >>>>> >>>>>> On Mon, Dec 11, 2017 at 3:16 PM John McCall via Phabricator < >>>>>> revi...@reviews.llvm.org> wrote: >>>>>> >>>>>>> rjmccall added a comment. >>>>>>> >>>>>>> In https://reviews.llvm.org/D41039#951648, @ahatanak wrote: >>>>>>> >>>>>>> > I had a discussion with Duncan today and he pointed out that >>>>>>> perhaps we shouldn't allow users to annotate a struct with >>>>>>> "trivial_abi" if >>>>>>> one of its subobjects is non-trivial and is not annotated with >>>>>>> "trivial_abi" since that gives users too much power. >>>>>>> > >>>>>>> > Should we error out or drop "trivial_abi" from struct Outer when >>>>>>> the following code is compiled? >>>>>>> > >>>>>>> > struct Inner1 { >>>>>>> > ~Inner1(); // non-trivial >>>>>>> > int x; >>>>>>> > }; >>>>>>> > >>>>>>> > struct __attribute__((trivial_abi)) Outer { >>>>>>> > ~Outer(); >>>>>>> > Inner1 x; >>>>>>> > }; >>>>>>> > >>>>>>> > >>>>>>> > The current patch doesn't error out or drop the attribute, but the >>>>>>> patch would probably be much simpler if we didn't allow it. >>>>>>> >>>>>>> >>>>>>> I think it makes sense to emit an error if there is provably a >>>>>>> non-trivial-ABI component. However, for class temploids I think that >>>>>>> diagnostic should only fire on the definition, not on instantiations; >>>>>>> for >>>>>>> example: >>>>>>> >>>>>>> template <class T> struct __attribute__((trivial_abi)) holder { >>>>>>> T value; >>>>>>> ~holder() {} >>>>>>> }; >>>>>>> holder<std::string> hs; // this instantiation should be legal >>>>>>> despite the fact that holder<std::string> cannot be trivial-ABI. >>>>>>> >>>>>>> But we should still be able to emit the diagnostic in template >>>>>>> definitions, e.g.: >>>>>>> >>>>>>> template <class T> struct __attribute__((trivial_abi)) >>>>>>> named_holder { >>>>>>> std::string name; // there are no instantiations of this >>>>>>> template that could ever be trivial-ABI >>>>>>> T value; >>>>>>> ~named_holder() {} >>>>>>> }; >>>>>>> >>>>>>> The wording should be something akin to the standard template rule >>>>>>> that a template is ill-formed if it has no valid instantiations, no >>>>>>> diagnostic required. >>>>>>> >>>>>>> I would definitely like to open the conversation about the name of >>>>>>> the attribute. I don't think we've used "abi" in an existing attribute >>>>>>> name; usually it's more descriptive. And "trivial" is a weighty word in >>>>>>> the standard. I'm not sure I have a great counter-proposal off the top >>>>>>> of >>>>>>> my head, though. >>>>>>> >>>>>> >>>>>> Agreed on both counts (would love a better name, don't have any >>>>>> stand-out candidates off the top of my head). >>>>>> >>>>>> I feel like a more descriptive term about the property of the object >>>>>> would make me happier - something like "address_independent_identity" >>>>>> (s/identity/value/?) though, yeah, that's not spectacular by any stretch. >>>>>> >>>>> >>>>> Incidentally, your comments are not showing up on Phabricator for some >>>>> reason. >>>>> >>>> >>>> Yeah, Phab doesn't do a great job looking on the mailing list for >>>> interesting replies - I forget, maybe it only catches bottom post or top >>>> post but not infix replies or something... >>>> >>> >>> Well, fortunately the mailing list is also archived. :) >>> >>> >>>> The term "trivially movable" suggests itself, with two caveats: >>>>> - What we're talking about is trivial *destructive* movability, >>>>> i.e. that the combination of moving the value to a new object and not >>>>> destroying the old object can be done trivially, which is not quite the >>>>> same as trivial movability in the normal C++ sense, which I guess could be >>>>> a property that someone theoretically might care about (if the type is >>>>> trivially destructed, but it isn't copyable for semantic reasons?). >>>>> - Trivial destructive movability is a really common property, and >>>>> it's something that a compiler would really like to optimize based on even >>>>> in cases where trivial_abi can't be adopted for binary-compatibility >>>>> reasons. Stealing the term for the stronger property that the type is >>>>> trivially destructively movable *and its ABI should reflect that in a >>>>> specific way* would be unfortunate. >>>>> >>>> >>>> Fair point. >>>> >>>> >>>>> "trivially_movable" is a long attribute anyway, and >>>>> "trivially_destructively_movable" is even worse. >>>>> >>>>> Maybe that second point is telling us that this isn't purely >>>>> descriptive — it's inherently talking about the ABI, not just the >>>>> semantics >>>>> of the type. I might be talking myself into accepting trivial_abi if we >>>>> don't end up with a better suggestion. >>>>> >>>> >>>> *nod* I think if you want to slice this that way (ensuring there's >>>> space to support describing a similar property for use only in >>>> non-ABI-breaking contexts as well) it seems like ABI is the term to use >>>> somewhere in the name. >>>> >>> >>> Yeah. We just had a little internal conversation about it, and the idea >>> of "address_invariant_abi" came up, but I'm not sure it has enough to >>> recommend it over "trivial_abi" to justify the longer name. >>> >>> >>>> Random thing that occurred to me: is it actually reasonable to enforce >>>>> trivial_abi correctness in a non-template context? Templates aren't the >>>>> only case where a user could reasonably want to add trivial_abi and just >>>>> have it be suppressed if it's wrong. Imagine if some stdlib made >>>>> std::string trivial_abi; someone might reasonably want to make my >>>>> named_holder example above trivial_abi as well, with the expectation that >>>>> it would only have an effect on targets where std::string was trivial_abi. >>>>> At the very least, I'm concerned that we might be opening ourselves up to >>>>> a >>>>> need to add supporting features, like a way to be conditionally >>>>> trivial_abi >>>>> based on context. >>>>> >>>> >>>> Fair point, much like the quirky but useful behavior of "= default". >>>> Good point about non-dependent contexts still being relevant to this >>>> subjective behavior... >>>> >>>> I was already leaning towards this being a warning rather than an error >>>> - this situation leans me moreso that way & possibly suppressing the >>>> warning when the types are split between system and non-system headers (if >>>> the attribute's on a type in a non-system header, but the type that's >>>> blocking it from being active is in a system header, don't warn). >>>> >>> >>> That's an interesting idea. >>> >>> >> OK, so if a trivial_abi class has a component that is not trivial, we >> drop the trivial_abi from the containing class. Also, we may decide not to >> error out or warn if the attribute is on a template or the non-trivial >> class comes from a system header. >> >> What would the rules be for propagating the trivial_abi property >> outwards? I tried to extend the existing standard rules for determining >> whether a special function is trivial to trivial_abi or whether a type >> should be forced to be passed indirectly in my first patch, but it looks >> like that approach is too complicated. >> > > What makes it too complicated? That would seem unfortunate to diverge > here, I would think.. > > > > Based on my understanding of the discussion yesterday, I think we’ve > decided not to diverge from the standard: > > http://lists.llvm.org/pipermail/cfe-commits/Week-of- > Mon-20180101/thread.html#214043 > > My patch keeps track of the triviality of special functions using two > bitfields in CXXRecordDecl (HasTrivialSpecialMembers and > HasTrivialSpecialMembersForCall) and two flags in FunctionDecl (IsTrivial > and IsTrivialForCall) because it needs to distinguish between triviality > according to the current standard’s definition and triviality for the > purpose of calls. Those flags have to be updated in various places, which I > thought might be too complicated. > Well, we're not changing the definition of triviality, and the intent is that we're not changing the definition of triviality-for-calls (as standardized by Itanium) in the absence of the trivial_abi attribute. Under the definition we developed in that thread, I think your new tracking bits are unavoidable. John. > > I'm thinking about using the following rules instead. What do you think? >> >> trivial_abi=true for a class if it meets the following conditions: >> >> 1. it has attribute "trivial_abi" or >> 2. if it doesn't have attribute "trivial_abi", it must meet all of the >> following conditions: >> 2-1. it doesn't have any virtual functions or virtual bases and >> 2-2. it doesn't have any members that would cause the type to be >> non-trivial (e.g., __weak objc pointers) and >> 2-3. it doesn't have any user-provided copy constructors, move >> constructors, or destructors (explicitly defaulted methods are OK) and >> 2-4. trivial_abi=true for all of its base classes and members >> >> These rules apply only when the class or the type of one of its members >> or base classes has attribute "trivial_abi". The existing rules in the >> standard will be used to determine whether clang should force a type to be >> passed indirectly if none of the classes in the class hierarchy have >> attribute "trivial_abi". >> >> >>> John. >>> >> >> _______________________________________________ > cfe-commits mailing list > cfe-commits@lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits > > > -- I suppose you'd like my real thoughts as well.
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