On Fri, Jul 17, 2015 at 10:38 AM, Bob Wilson <bob.wil...@apple.com> wrote:
> > On Jul 17, 2015, at 10:34 AM, David Majnemer <david.majne...@gmail.com> > wrote: > > > > 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.. > > > That makes sense. The “TargetWindows” predicate is also used for the > dllexport and dllimport declspecs. Would it make sense to treat those in > the same way? It has been a while since I looked at MinGW. > MingW needs dllimport and dllexports so its OK for them to be using the predicate.
_______________________________________________ cfe-commits mailing list cfe-commits@cs.uiuc.edu http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
