> 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
> <mailto:bob.wil...@apple.com>> wrote:
>
>> On Jul 17, 2015, at 10:34 AM, David Majnemer <david.majne...@gmail.com
>> <mailto:david.majne...@gmail.com>> wrote:
>>
>>
>>
>> On Fri, Jul 17, 2015 at 10:25 AM, Bob Wilson <bob.wil...@apple.com
>> <mailto:bob.wil...@apple.com>> wrote:
>>
>>> On Jul 17, 2015, at 10:17 AM, Reid Kleckner <r...@google.com
>>> <mailto:r...@google.com>> wrote:
>>>
>>> On Fri, Jul 17, 2015 at 4:45 AM, Aaron Ballman <aa...@aaronballman.com
>>> <mailto:aa...@aaronballman.com>> wrote:
>>> On Thu, Jul 16, 2015 at 8:42 PM, Bob Wilson <bob.wil...@apple.com
>>> <mailto: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.
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