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

[Richard Smith]

It seems to me that we should be basing the check on the TargetCXXABI
rather than whether the target is Windows.

On Fri, Jul 17, 2015 at 2:19 PM, Saleem Abdulrasool <compnerd at compnerd.org>
wrote:

> On Fri, Jul 17, 2015 at 4:10 PM, Bob Wilson <bob.wilson at apple.com> wrote:
>
>>
>> On Jul 17, 2015, at 11:46 AM, David Majnemer <david.majnemer at gmail.com>
>> wrote:
>>
>>
>>
>> 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.
>>
>>
>> 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.
>
>
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>
>
> --
> Saleem Abdulrasool
> compnerd (at) compnerd (dot) org
>
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