RFR: 8314488: Compiling the JDK with C++17 [v3]

[Stefan Karlsson]

On Mon, 25 Aug 2025 09:42:45 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> Please review this change to use C++17 for building C++ parts of the JDK. In
>> particular this affects HotSpot. This change also includes an update to the
>> HotSpot Style Guide regarding C++17 features and their use in HotSpot code.
>> 
>> Testing: mach5 tier1-8
>> 
>> This change includes a modification of the Style Guide. Rough consensus among
>> the HotSpot Group members is required to make such a change. Only Group
>> members should vote for approval (via the github PR), though reasoned
>> objections or comments from anyone will be considered. A decision on this
>> proposal will not be made before Friday 5-September-2025 at 12h00 UTC.
>> 
>> Since we're piggybacking on github PRs here, please use the PR review process
>> to approve (click on Review Changes > Approve), rather than sending a "vote:
>> yes" email reply that would be normal for a CFV.
>
> Kim Barrett has updated the pull request incrementally with one additional commit since the last revision:
> 
>   dholmes tweaks

Thanks for taking on this task! I read through it and added a few comments, but none of them are crucial to get addressed. I haven't followed and checked the links.

doc/hotspot-style.md line 1481:

> 1479: ## Excluded Features
> 1480: 
> 1481: ### Structured Bindings

FWIW, there was a recent discussions about memory APIs in `os::` that returned both a size and an error code. I'm a little bit intrigued to see if Structured Bindings would have been able to give us slightly more cleaner call-site code. I'll still my curiosity after the support for C++17 is in.

doc/hotspot-style.md line 1509:

> 1507: with files, and already has adequate mechanisms for its needs. Rewriting in
> 1508: terms of this new library would not be a good use of resources. Having a mix
> 1509: of the existing usage and uses of this new library would just be confusing.

I'm not sure the style guide should be opinionated about we choose to use our resources. Could this be slightly tweaked to not say that?

doc/hotspot-style.md line 1567:

> 1565: in HotSpot code because of the
> 1566: [no implicit boolean](#avoid-implicit-conversions-to-bool)
> 1567: guideline.)

> (Note that conversion to `bool` isn't needed ...

Pre-existing: This section sounds weird to me. It more or less says "it isn't needed because we forbid it". I think people can still feel the need to use it. Maybe this could be changed to say something like:

> (Note that conversion to `bool` isn't permitted
in HotSpot code because of the
[no implicit boolean](#avoid-implicit-conversions-to-bool)
guideline.)

or something like that. (This doesn't have to be fixed in this PR if you disagree)

-------------

Marked as reviewed by stefank (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/26884#pullrequestreview-3150965151
PR Review Comment: https://git.openjdk.org/jdk/pull/26884#discussion_r2297826334
PR Review Comment: https://git.openjdk.org/jdk/pull/26884#discussion_r2297814769
PR Review Comment: https://git.openjdk.org/jdk/pull/26884#discussion_r2297837433