On Fri, Jul 17, 2015 at 4:10 PM, Bob Wilson <bob.wil...@apple.com> wrote:
> > On Jul 17, 2015, at 11:46 AM, David Majnemer <david.majne...@gmail.com> > wrote: > > > > On Fri, Jul 17, 2015 at 11:17 AM, Bob Wilson <bob.wil...@apple.com> wrote: > >> >> On Jul 17, 2015, at 10:40 AM, David Majnemer <david.majne...@gmail.com> >> wrote: >> >> >> >> 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. >> >> >> I looked into changing this, but there’s nothing to be done. The >> canonical representation of MinGW in the target triples is *-*-windows-gnu, >> and the predicate is checking the OS component. >> > > Right, we need a new predicate keyed off > of llvm::Triple::isKnownWindowsMSVCEnvironment. > > > You want novtable to be ignored for MinGW? (Sorry — I’ve used MinGW in the > past, but I don’t know what C++ ABI it uses.) > MinGW uses the itanium C++ ABI, as does the windows-itanium target. > _______________________________________________ > cfe-commits mailing list > cfe-commits@cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits > > -- Saleem Abdulrasool compnerd (at) compnerd (dot) org
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