RFR: 8274400: HotSpot Style Guide should permit use of alignof [v3]

[Kim Barrett]

On Tue, 17 Jan 2023 09:05:00 GMT, Xin Liu <xliu at openjdk.org> wrote:

> > > What is the use-case for permitting use of `alignof`? Usually we don't add such permits without some use-case in mind.
> > > [...]
> > > I'm not opposed to permitting the use of `alignof`, but wondering why we might need it.
> > 
> > 
> > Responding to myself, I've found a usage category where `alignof` would be appropriate. We have various places where we are aligning a pointer to the size of some type, where it would be more appropriate to align to the alignment of that type. See, for example, `Message::calc_size()` (logging/logAsyncWriter.hpp), which uses `sizeof(void*)` where it seems like it should be using `alignof(Message)`.
> 
> Alignment of 'sizeof(void*)' here is a performance hint. We would like to see the address of next Message is aligned of pointer size. It eases the load instruction of 'Message::_output'. It's more effective to load from the aligned address.
> 
> ```
>     static constexpr size_t calc_size(size_t message_len) {
>       return align_up(sizeof(Message) + message_len + 1, sizeof(void*));
>     }
> ```
> 
> In this context, alignof(Message) is correct but a little bit wasteful.

How is it wasteful?  I think alignof(Message) is exactly what should be used, since the calculation is used
to determine the positioning of the start of a Message object in memory.  Given the current definition of
Message the two are probably the same, but that's kind of accidental.

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PR: https://git.openjdk.org/jdk/pull/11761