[PATCH] ignore __declspec(novtable) on non-Windows platforms

David Majnemer david.majnemer at gmail.com
Fri Jul 17 11:46:05 PDT 2015


On Fri, Jul 17, 2015 at 11:17 AM, Bob Wilson <bob.wilson at apple.com> wrote:

>
> On Jul 17, 2015, at 10:40 AM, David Majnemer <david.majnemer at gmail.com>
> wrote:
>
>
>
> On Fri, Jul 17, 2015 at 10:38 AM, Bob Wilson <bob.wilson at apple.com> wrote:
>
>>
>> On Jul 17, 2015, at 10:34 AM, David Majnemer <david.majnemer at gmail.com>
>> wrote:
>>
>>
>>
>> On Fri, Jul 17, 2015 at 10:25 AM, Bob Wilson <bob.wilson at apple.com>
>> wrote:
>>
>>>
>>> On Jul 17, 2015, at 10:17 AM, Reid Kleckner <rnk at google.com> wrote:
>>>
>>> On Fri, Jul 17, 2015 at 4:45 AM, Aaron Ballman <aaron at aaronballman.com>
>>> wrote:
>>>
>>>> On Thu, Jul 16, 2015 at 8:42 PM, Bob Wilson <bob.wilson at 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.
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