[libcxx-commits] [libcxx] [libc++] Tiny optimizations for is_permutation (PR #129565)

[Imad Aldij via libcxx-commits]

imdj wrote:

So, I compared few of the benchmarks results in pairs and ranked the top 10 with largest relative changes. The results overall match your findings @ldionne :

```
------------------------------------------------------------------------------------------------------------------------
Benchmark                                                         | Bad CPU (ns) | Good CPU (ns) | Abs Diff   | Change (%)     
------------------------------------------------------------------------------------------------------------------------
rng::is_permutation(vector<int>) (4leg) (shuffled)/1024             711634.00      698098.00      13536.00     1.90           
rng::is_permutation(vector<int>) (4leg, pred) (shuffled)/8          120.00         122.00         2.00         1.67           
std::is_permutation(list<int>) (3leg, pred) (common prefix)/8192    41729.00       41151.00       578.00       1.39           
rng::is_permutation(list<int>) (4leg, pred) (common prefix)/8192    41816.00       41271.00       545.00       1.30           
std::is_permutation(list<int>) (4leg) (common prefix)/8192          41871.00       41359.00       512.00       1.22           
rng::is_permutation(list<int>) (4leg) (common prefix)/8192          41802.00       41295.00       507.00       1.21           
std::is_permutation(list<int>) (4leg, pred) (common prefix)/8192    41976.00       41490.00       486.00       1.16           
std::is_permutation(list<int>) (4leg, pred) (common prefix)/1024    3841.00        3875.00        34.00        0.89           
std::is_permutation(vector<int>) (4leg, pred) (shuffled)/8          120.00         121.00         1.00         0.83           
std::is_permutation(vector<int>) (3leg) (shuffled)/8                98.80          98.10          0.70         0.71
```

https://github.com/llvm/llvm-project/pull/129565