RFR: 8314488: Compiling the JDK with C++17

[Kim Barrett]

On Thu, 26 Jun 2025 02:01:55 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> I'm hijacking the PR mechanism as a way to discuss new C++17 features that can
> be more easily structured and captured than bare email. Once discussion
> settles down I'll turn the results into HotSpot Style Guide changes. I don't
> intend to integrate any version of this document to the OpenJDK repository.

doc/cpp17-features.md line 128:

> 126: [n4147](http://wg21.link/n4147)
> 127: [n4424](http://wg21.link/n4424)
> 128: [p0386r2](http://wg21.link/p0386r2)

After looking at inline variables more carefully as part of writing it up for
the style guide, I'm inclined to move inline variables to the undecided or
forbidden category. They seem to be a solution to a problem I think we don't
really have.

Basically, they let you provide the full definition for a non-local variable
in a header file, rather than declaring it in the header and defining it in
an associated .cpp file. The motivating use-case is a component or library
that is header only, to simplify client use by avoiding build issues. But
that's just not a thing we care about for HotSpot code.

So what WE get from an inline variable is having a complete declaration and
definition in one place, rather than separating the declaration from the
definition (so probably saving a little bit of code), at the cost of putting
the initialization expression in the header (which might require pulling other
stuff into the header), and maybe making the compiler work a little harder.

The benefit of that seems minimal at best.

The one part of it that seems slightly useful is that a constexpr variable is
implicitly inline, so doesn't need a definition without an initializer in a
.cpp file if the variable is ODR-used. Indeed, that's now treated as a
duplicate defintion and deprecated usage. But we haven't had any warnings, so
guessing we don't actually have any of those. We can take advantage of this
for future usage.

Any other opinions?

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PR Review Comment: https://git.openjdk.org/jdk/pull/25992#discussion_r2281251684