[PATCH] D158145: [clang] Update NumFunctionDeclBits for FunctionDeclBitfields

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman accepted this revision.
aaron.ballman added a comment.
This revision is now accepted and ready to land.

In D158145#4594569 <https://reviews.llvm.org/D158145#4594569>, @cor3ntin wrote:

> In D158145#4594549 <https://reviews.llvm.org/D158145#4594549>, @danix800 wrote:
>
>> I also investigated whether we could count those bits at compile time and statically assert on them,
>> because a small typo or missed update could spend us a lot of time to dig for the cause.
>>
>> My first step is trying to count number of bits for a single bitfield, this is promising based on this <https://gist.github.com/RMDarth/6357ddd6e09b4117efe84fc3347a732a> but
>> with a restriction, it only works on `struct` (default public fields), not `class` (default to private fields).
>>
>> If we can implement this `bitsizeof` then we could have:
>>
>>   enum { NumFunctionDeclBits = offsetof(FunctionDeclBitfields, SClass)
>>                              + offsetof(FunctionDeclBitfields, IsInline)
>>                              + ... };
>>
>> This can automatically update total number of bits if any of the existing one is updated.
>>
>> The second step is trying to enumerate all bit fields at compile time so that we can totally fix this kind
>> of issue, but it seems not possible.
>>
>> Any suggestions or advices? Is it even worth it to do it like this?
>
> It would be great to be able to stattic_assert these sort of bugs. I think @aaron did some research in this area, it might be worth asking him how far he got!

I was never able to find a conforming way to do it without implementing something like a "bitwidth of" operator, which is something I was planning to propose to WG14 anyway for use with `_BitInt`. I've not implemented the extension yet though, and even if I did, it wouldn't help us with non-Clang host compilers, so I eventually gave up trying to automatically calculate the values.

Changes LGTM!


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https://reviews.llvm.org/D158145