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

[Richard Smith]

On Fri, Jul 17, 2015 at 3:43 PM, David Majnemer <david.majnemer at gmail.com>
wrote:

>
>
> On Fri, Jul 17, 2015 at 3:13 PM, Richard Smith <richard at metafoo.co.uk>
> wrote:
>
>> It seems to me that we should be basing the check on the TargetCXXABI
>> rather than whether the target is Windows.
>>
>
> That's why I suggested to use llvm::Triple::isKnownWindowsMSVCEnvironment,
> it's what we use to set the CXX ABI:
> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Basic/TargetInfo.cpp?revision=242198&view=markup#l87
>

That's only the default; targets are permitted to override it, and many of
them do so. ItaniumWindowsARMleTargetInfo sets a non-MS C++ ABI, for
instance.

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
>>>
>>> _______________________________________________
>>> cfe-commits mailing list
>>> cfe-commits at cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
>>>
>>>
>>
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