[libcxx-commits] [libcxx] [libc++] Optimize __hash_table copy constructors and assignment (PR #151951)

[Nico Weber via libcxx-commits]

nico wrote:

This one also changes behavior, right? I think before this change, the copy of an unordered_foo had the same iteration order as the original unordered_foo, and after this change it's no longer true.

(This is fine by the standard, but still nice to mention in release notes.)

For example, this program used to print

```
url_1
url_2
```

but after this change it prints

```
url_2
url_1
```:

```
% cat repro.cc 
#include <unordered_map>
#include <string>
#include <vector>

struct Context {
  explicit Context(std::string session_id) : session_id(session_id) {}
  Context(const Context&) = default;

  std::string session_id;
  std::unordered_map<std::string, std::vector<std::string>> url_passages_map;
};

void f(Context c) {
  for (auto const& u : c.url_passages_map)
    printf("%s\n", u.first.c_str());
}

int main() {
  Context context("1");
  context.url_passages_map.insert({"url_1", {"passage_11", "passage_12"}});
  context.url_passages_map.insert({"url_2", {"passage_21", "passage_22"}});
  f(context); // Copies.
}
```

https://github.com/llvm/llvm-project/pull/151951