Not-yet-discussed in HotSpot style guide: C++ Lambdas

[Kim Barrett]

> On Aug 14, 2021, at 5:28 AM, Andrew Haley <aph-open at littlepinkcloud.com> wrote:
> 
> https://github.com/openjdk/jdk/blob/master/doc/hotspot-style.md says:
> 
> "Undecided Features
> 
> This list is incomplete; it serves to explicitly call out some
> features that have not yet been discussed.  ...  Lambdas"
> 
> I can't see any reason to forbid Lambdas from a performance point of
> view, and they can add to readability and maintainability in C++ for
> the same reasons that they do in Java.
> 
> I have a job I'm working on (in the AArch64 back end) where Lambdas
> would be very useful.  How about I give it a try, and I'll produce the
> patch for review, as an initial proposal? Then anyone can put in their
> 2c worth.

Funny you should mention lambdas.  There has been a fair amount of
discussion around them internally here at Oracle.  And there are some
folks trying them out in not-yet-ready-for-production work.  I’ve drafted
a style-guide modification reflecting that discussion, along with a couple
examples, and am planning to open a PR in the next few days.

You can see the current work in progress here:

https://github.com/kimbarrett/openjdk-jdk/tree/permit_lambda

Note that I'm planning to rebase and do some commit refactoring / reordering
before opening the PR.

There is one downside that has recently been noted.  Stack frame names
in stack traces where lambdas are involved are pretty indecipherable.