move | powerful programming language that runs on any ES3 | Interpreter library

std::vector and other containers (except std::array) are specified to have a copy constructor. This is not specified to be conditional on whether or not the element type is copyable. Only instantiation of the copy constructor's definition is forbidden if the element type is not copyable.

As a result std::is_copy_constructible_v on the container will always be true. There is no way to test whether an instantiation of a definition would be well-formed with a type trait.

It would be possible to specify that the copy constructor is not declared or excluded from overload resolution if the element type is not copyable. However, that would come with a trade-off which is explained in detail in this blog post: https://quuxplusone.github.io/blog/2020/02/05/vector-is-copyable-except-when-its-not/.

In short, if we want to be able to use the container with an incomplete type, e.g. recursively like

This is not possible in C++11 without overloading sum for & and && qualifiers. (In which case you can determine the value category from the qualifier of the particular overload.)

*this is, just like the result of any indirection, a lvalue, and is also what an implicit member function call is called on.

This will be fixed in C++23 via introduction of an explicit object parameter for which usual forwarding can be applied: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p0847r7.html

It is correct that the constructor template is generally a better match for the constructor call with argument of type DerivedFoo& or Foo& than the copy constructors are, since it doesn't require a const conversion.

However, [over.match.funcs.general]/8 essentially (almost) says, in more general wording, that an inherited constructor that would have the form of a move or copy constructor is excluded from overload resolution, even if it is instantiated from a constructor template. Therefore the template constructor will not be considered.

Therefore the implicit copy constructor of DerivedFoo will be chosen by overload resolution for

It's a global outage in JCenter. You can monitor status at https://status.gradle.com. It replaces the bintray status page which seems is now fully sunset and returns a 502 error.

UPDATE Jan 13, 06:35 UTC

JCenter is now back online, and systems are fully operational.

UPDATE Jan 20

Gradle Plugin resolution outage postmortem

https://blog.gradle.org/plugins-jcenter

Following this incident, the Gradle Plugin Portal now uses a JCenter mirror hosted by Gradle instead of JCenter directly. This should shield users from short JCenter outages for libraries that have been cached by the mirror. We saw another short outage of JCenter over the weekend and this did not appear to impact Gradle Plugin Portal users.

It looks like GCC's naïveté about store-forwarding stalls is hurting its auto-vectorization strategy here. See also Store forwarding by example for some practical benchmarks on Intel with hardware performance counters, and What are the costs of failed store-to-load forwarding on x86? Also Agner Fog's x86 optimization guides.

(gcc -O3 enables -ftree-vectorize and a few other options not included by -O2, e.g. if-conversion to branchless cmov, which is another way -O3 can hurt with data patterns GCC didn't expect. By comparison, Clang enables auto-vectorization even at -O2, although some of its optimizations are still only on at -O3.)

It's doing 64-bit loads (and branching to store or not) on pairs of ints. This means, if we swapped the last iteration, this load comes half from that store, half from fresh memory, so we get a store-forwarding stall after every swap. But bubble sort often has long chains of swapping every iteration as an element bubbles far, so this is really bad.

(Bubble sort is bad in general, especially if implemented naively without keeping the previous iteration's second element around in a register. It can be interesting to analyze the asm details of exactly why it sucks, so it is fair enough for wanting to try.)

Anyway, this is pretty clearly an anti-optimization you should report on GCC Bugzilla with the "missed-optimization" keyword. Scalar loads are cheap, and store-forwarding stalls are costly. (Can modern x86 implementations store-forward from more than one prior store? no, nor can microarchitectures other than in-order Atom efficiently load when it partially overlaps with one previous store, and partially from data that has to come from the L1d cache.)

Even better would be to keep buf[x+1] in a register and use it as buf[x] in the next iteration, avoiding a store and load. (Like good hand-written asm bubble sort examples, a few of which exist on Stack Overflow.)

If it wasn't for the store-forwarding stalls (which AFAIK GCC doesn't know about in its cost model), this strategy might be about break-even. SSE 4.1 for a branchless pmind / pmaxd comparator might be interesting, but that would mean always storing and the C source doesn't do that.

If this strategy of double-width load had any merit, it would be better implemented with pure integer on a 64-bit machine like x86-64, where you can operate on just the low 32 bits with garbage (or valuable data) in the upper half. E.g.,

Unfortunately (?), the easiest way to know for sure what those categories map to is to look at OpenJDK source code. The NMT tag you are looking for is mtServiceability. This would show that "serviceability" are basically diagnostic interfaces in JDK/JVM: JVMTI, heap dumps, etc.

But the same kind of thing is clear from observing that stack trace sample you are showing mentions ThreadStackTrace::dump_stack_at_safepoint -- that is something that dumps the thread information, for example for jstack, heap dump, etc. If you have a suspicion for the memory leak in that code, you might try to build a MCVE demonstrating it, and submitting the bug against OpenJDK, or showing it to a fellow OpenJDK developer. You probably know better what your application is doing to cause thread dumps, focus there.

That being said, I don't see any obvious memory leaks in StackFrameInfo, neither can I reproduce any leak with stress tests, so maybe what you are seeing is "just" thread dumping over the larger and larger thread stacks. Or you capture it when thread dump is happening. Or... It is hard to say without the MCVE.

Update: After playing with MCVE, I realized that it reproduces with 17.0.1, but not with either mainline development JDK, or JDK 18 EA, or JDK 17.0.2 EA. I tested with 17.0.2 EA before, so was not seeing it, dang. Bisection between 17.0.1 and 17.0.2 EA shows it was fixed with JDK-8273902 backport. 17.0.2 releases this week, so the bug should disappear after you upgrade.

The first one should probably be:

I think this is the open CWG issue 2077.

Basically,

The value category of init + *first doesn't matter.

init in init + *first is a lvalue.

So if init + *first calls an operator+ overload taking the parameter by-value, it will cause a copy construction of that parameter

But the value of init is not required anymore after init + *first, so it makes sense to move it into the parameter instead.

Similarly a operator+ overload taking its first argument by rvalue-reference might be used to allow modification of the argument by the operation.

This is what std::move achieves here.

The standard specifies this behavior since C++20.