[PATCH] D155809: [NFC] [Clang] Fix strict weak ordering in ItaniumVTableBuilder

Tue Aug 1 08:21:32 PDT 2023

aaron.ballman added a comment.

In D155809#4527890 <https://reviews.llvm.org/D155809#4527890>, @danlark wrote:

> In D155809#4527847 <https://reviews.llvm.org/D155809#4527847>, @aaron.ballman wrote:
>
>> In D155809#4527199 <https://reviews.llvm.org/D155809#4527199>, @danlark wrote:
>>
>>> In D155809#4521494 <https://reviews.llvm.org/D155809#4521494>, @rsmith wrote:
>>>
>>>> This looks correct to me, but it's still a little subtle. Perhaps it'd be clearer to map the method to an integer (0 for copy assignment, 1 for move assignment, 2 for destructor, 3 for equality comparison), and then order them by that integer? That'd be more obviously a strict weak order.
>>>
>>>
>>>
>>> In D155809#4520765 <https://reviews.llvm.org/D155809#4520765>, @shafik wrote:
>>>
>>>> I am not sure about this change but I think we at least need a test and this does not seem non-functional if it prevents a crash.
>>>
>>> This is NFC as it only prevents further assert to fire when stable_sort compares the element with itself
>>
>> Richard's suggestion makes sense to me as a clarifying change to the code. I also agree with Shafik -- if this prevents an assertion from firing in practice, then it's a functional change that should come with tests. Or are you saying the assertion isn't happening in practice and this is a defensive change?
>
> The assertion happens in debug libcxx mode after https://reviews.llvm.org/D150264.  This is a defensive change, in practice, 2 same functions cannot happen in this comparator, this is only for preventing assertions on line 1568 <https://github.com/llvm/llvm-project/blob/2773098ee3187d5f9daca8938d57657dd89dd36f/clang/lib/AST/VTableBuilder.cpp#L1569>

I apologize, but I'm still confused. If this assertion triggers in practice in debug modes with libc++, we should be able to make a stand-alone reproducer that we test as part of these changes within Clang.

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https://reviews.llvm.org/D155809