RFR: 8314644: Change "Rvalue references and move semantics" into an accepted feature

[daniel.daugherty at oracle.com]

A couple of questions:

- Are these C++ features supported in all of the compilers that we 
use/support
   in OpenJDK? I mean the Tier1 platforms that Oracle supports in our CI 
and the
   other platforms that OpenJDK supports in the GHA setups.
- How do we gauge the quality of support for these C++ features in the 
set of
   compilers that we use/support in OpenJDK?
- Is it possible, feasible and desirable to create standalone tests for 
these
   C++ features so that we can use the new tests to evaluate compiler 
upgrades?

Dan

On 8/22/23 8:23 AM, Johan Sjölen wrote:
> Hi,
>
> I'd like to propose that rvalue references and move semantics are now considered permitted in the style guide. This change would allow for move constructors to be written. This enables more performant code, if the move ctr is less expensive than the copy ctr, but also more correct code. For the latter part, look at "8314571: GrowableArray should move its old data and not copy it". Here we can avoid using copy assignment, instead using move constructors, which more accurately reflects what is happening: The old elements are in fact moved, and not copied.
>
> Two useful std functions will become available to us with this change:
>
> 1. `std::move`, for explicitly moving a value. This is a slightly more powerful `static_cast<T&&>(T)`, in that it also handles `T&` corectly.
> 2. `std::forward`, which simplifies the usage of perfect forwarding. Perfect forwarding is a technique where in copying is minimized. To quote Scott Meyers ( https://cppandbeyond.com/2011/04/25/session-announcement-adventures-in-perfect-forwarding/ ):
>
>> Perfecting forwarding is an important C++0x technique built atop rvalue references.  It allows move semantics to be automatically applied, even when the source and the destination of a move are separated by intervening function calls.  Common examples include constructors and setter functions that forward arguments they receive to the data members of the class they are initializing or setting, as well as standard library functions like make_shared, which “perfect-forwards” its arguments to the class constructor of whatever object the to-be-created shared_ptr is to point to.
> Looking forward to your feedback, thank you.
> Johan
>
> -------------
>
> Commit messages:
>   - Also update html
>   - Move over Rvalue references and move semantics to permitted features
>
> Changes: https://git.openjdk.org/jdk/pull/15386/files
>   Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15386&range=00
>    Issue: https://bugs.openjdk.org/browse/JDK-8314644
>    Stats: 8 lines in 2 files changed: 5 ins; 3 del; 0 mod
>    Patch: https://git.openjdk.org/jdk/pull/15386.diff
>    Fetch: git fetch https://git.openjdk.org/jdk.git pull/15386/head:pull/15386
>
> PR: https://git.openjdk.org/jdk/pull/15386