[all-commits] [llvm/llvm-project] 4fc5b6: [libc++] Optimize {std, ranges}::for_each for itera...

[Nikolas Klauser via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 4fc5b6d8c4d8a27e8b6c32a112d865de4935d401
      https://github.com/llvm/llvm-project/commit/4fc5b6d8c4d8a27e8b6c32a112d865de4935d401
  Author: Nikolas Klauser <nikolasklauser at berlin.de>
  Date:   2025-12-12 (Fri, 12 Dec 2025)

  Changed paths:
    M libcxx/docs/ReleaseNotes/22.rst
    M libcxx/include/__algorithm/for_each.h
    M libcxx/include/__algorithm/ranges_for_each.h
    M libcxx/include/__algorithm/specialized_algorithms.h
    M libcxx/include/__tree
    M libcxx/include/map
    M libcxx/include/set
    M libcxx/test/benchmarks/algorithms/nonmodifying/for_each.bench.cpp
    A libcxx/test/std/algorithms/alg.nonmodifying/alg.foreach/for_each.associative.pass.cpp
    M libcxx/test/std/algorithms/alg.nonmodifying/alg.foreach/for_each.pass.cpp
    A libcxx/test/std/algorithms/alg.nonmodifying/alg.foreach/ranges.for_each.associative.pass copy.cpp

  Log Message:
  -----------
  [libc++] Optimize {std,ranges}::for_each for iterating over __trees (#164405)

This patch optimizes how `for_each` iterates over trees by using
recursion and storing pointers to the next nodes on the stack. This
avoids pointer chasing through the `__parent_` pointer, reducing cache
misses. It also makes use of the compiler being able tail-call optimize
the recursive function, removing back-tracking the iterators have to do.

```
Benchmark                                                      old             new    Difference    % Difference
--------------------------------------------------  --------------  --------------  ------------  --------------
rng::for_each(map<int>)/32                                   35.19           26.67         -8.52         -24.21%
rng::for_each(map<int>)/50                                   64.13           40.68        -23.45         -36.57%
rng::for_each(map<int>)/8                                     5.06            6.49          1.43          28.21%
rng::for_each(map<int>)/8192                              22893.89         9266.68     -13627.21         -59.52%
rng::for_each(map<int>::iterator)/32                         35.51           26.88         -8.63         -24.31%
rng::for_each(map<int>::iterator)/50                         64.39           41.24        -23.15         -35.95%
rng::for_each(map<int>::iterator)/8                           5.12            5.93          0.81          15.80%
rng::for_each(map<int>::iterator)/8192                    21283.14         9736.83     -11546.31         -54.25%
rng::for_each(multimap<int>)/32                              35.22           26.61         -8.61         -24.45%
rng::for_each(multimap<int>)/50                              64.10           40.07        -24.03         -37.49%
rng::for_each(multimap<int>)/8                                5.08            6.69          1.61          31.70%
rng::for_each(multimap<int>)/8192                         23130.44         9026.16     -14104.28         -60.98%
rng::for_each(multimap<int>::iterator)/32                    35.40           25.08        -10.32         -29.15%
rng::for_each(multimap<int>::iterator)/50                    64.19           38.15        -26.04         -40.56%
rng::for_each(multimap<int>::iterator)/8                      5.04            5.25          0.22           4.31%
rng::for_each(multimap<int>::iterator)/8192               22875.97         9392.08     -13483.89         -58.94%
rng::for_each(multiset<int>)/32                              35.82           27.11         -8.72         -24.33%
rng::for_each(multiset<int>)/50                              62.92           41.59        -21.32         -33.89%
rng::for_each(multiset<int>)/8                                4.79            6.79          2.00          41.70%
rng::for_each(multiset<int>)/8192                         22642.68         9280.95     -13361.73         -59.01%
rng::for_each(multiset<int>::iterator)/32                    35.76           26.71         -9.04         -25.28%
rng::for_each(multiset<int>::iterator)/50                    63.44           39.00        -24.44         -38.53%
rng::for_each(multiset<int>::iterator)/8                      4.90            5.21          0.30           6.18%
rng::for_each(multiset<int>::iterator)/8192               19930.45         9867.60     -10062.85         -50.49%
rng::for_each(set<int>)/32                                   35.90           27.30         -8.60         -23.96%
rng::for_each(set<int>)/50                                   63.15           40.75        -22.40         -35.47%
rng::for_each(set<int>)/8                                     4.77            6.83          2.06          43.23%
rng::for_each(set<int>)/8192                              20262.77         9381.57     -10881.20         -53.70%
rng::for_each(set<int>::iterator)/32                         36.02           26.42         -9.60         -26.64%
rng::for_each(set<int>::iterator)/50                         63.29           37.97        -25.32         -40.01%
rng::for_each(set<int>::iterator)/8                           4.72            5.22          0.50          10.50%
rng::for_each(set<int>::iterator)/8192                    20041.91         9831.91     -10210.00         -50.94%
```



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