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> wrote: > >> 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... > > 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. > 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). > > https://reviews.llvm.org/D41039 >
_______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
