[libcxx-commits] [libcxx] [libc++] Optimize __tree::find and __tree::__erase_unique (PR #152370)

[Nico Weber via libcxx-commits]

nico wrote:

We have this function that tries to find all bookmark nodes with a given URL:

```
std::vector<BookmarkNode*> BookmarkModel::GetNodesByURL(GURL url) {
  std::vector<BookmarkNode*> nodes;
  auto i = nodes_ordered_by_url_set_.find<GURL>(url);
  while (i != nodes_ordered_by_url_set_.end() && (*i)->url() == url) {
    nodes.push_back(*i);
    ++i;
  }
  return nodes;
}
```

`nodes_ordered_by_url_set_` looks like so:

```
  // Used to order BookmarkNodes by URL as well as lookups using GURL.
  class NodeUrlComparator {
   public:
    // Required by std::set to support GURL-based lookups.
    using is_transparent = void;

    bool operator()(const BookmarkNode* n1, const BookmarkNode* n2) const {
      return n1->url() < n2->url();
    }
    bool operator()(const BookmarkNode* n1, const GURL& url2) const {
      return n1->url() < url2;
    }
    bool operator()(const GURL& url1, const BookmarkNode* n2) const {
      return url1 < n2->url();
    }
  };

  using NodesOrderedByUrlSet = std::multiset<BookmarkNode*, NodeUrlComparator>;
  NodesOrderedByUrlSet nodes_ordered_by_url_set_;
```

The whole thing as a standalone repro (the .sh is meant to run on a mac; needs minor tweaks to run on linux):
[repro.zip](https://github.com/user-attachments/files/21971130/repro.zip) This includes a single standalone .cc file that prints `count: 2` without this commit but `count: 1` with it. It also includes a .sh script to compile libc++ and link the included .cc file against the just-built libc++. You probably have a local script for the latter part that you like better :)

>From what I understand, our code is incorrect, and we should use e.g. `equal_range()` instead. I'm only mentioning this here in case other folks run into it.

(But, see question in the comment above please :))


https://github.com/llvm/llvm-project/pull/152370