On Fri, Jul 17, 2015 at 10:25 AM, Bob Wilson <bob.wil...@apple.com> wrote:
> > On Jul 17, 2015, at 10:17 AM, Reid Kleckner <r...@google.com> wrote: > > On Fri, Jul 17, 2015 at 4:45 AM, Aaron Ballman <aa...@aaronballman.com> > wrote: > >> On Thu, Jul 16, 2015 at 8:42 PM, Bob Wilson <bob.wil...@apple.com> wrote: >> > Clang used to silently ignore __declspec(novtable) for all platforms, >> but it is now implemented for Windows. However, we don’t check if the >> target is Windows. This does not work when using the Itanium ABI, where the >> class layout for complex class hierarchies is stored in the vtable. Leaving >> the vtable uninitialized on non-Windows platforms does not work in that >> case. It might be possible to honor the novtable attribute in some simple >> cases and either report an error or ignore it in more complex situations, >> but it’s not clear if that would be worthwhile. There is also value in >> having a simple and predictable behavior, so I am proposed the attached >> patch which simply ignores novtable on non-Windows platforms. >> > > I think it might actually be worth making it work. I have vague > recollections of Chromium developers wondering how to do the equivalent > size saving optimization on non-Windows targets. We'd have to pin down what > makes a "complex" class hierarchy. I'm assuming the fix would be to emit > the vptr store if the class has virtual bases. > > >> MSVC supports an Itanium build target. What does __declspec(novtable) >> do there with the complex class layouts? >> >> I don't have Visual Studio installed with support for Itanium, >> otherwise I would test this myself. >> > > I think Bob is talking about the Itanium C++ ABI, which I don't think MSVC > ever implemented. If they did, I wouldn't be surprised if they simply > ignored this declspec. > > > Yes — I meant the Itanium C++ ABI. > > If someone else wants to sign up to implement this properly, I have no > objections. In the meantime, my patch would fix the miscompiles that result > from the current behavior. I’d still like to go ahead with it. > My only qualm with the patch is that it wouldn't engage for MingW targets. It LGTM but the predicate needs tweaking to focus on the MSVC compatible targets.. > > _______________________________________________ > cfe-commits mailing list > cfe-commits@cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits > >
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