[libcxx-commits] [libcxx] [libc++][ranges] optimize the performance of `ranges::starts_with` (PR #84570)

[Louis Dionne via libcxx-commits]

ldionne wrote:

When benchmarking locally, I get the following results for `std::vector`:

```
Comparing build/default/libcxx/test/benchmarks/algorithms/nonmodifying/Output/starts_with.bench.cpp.dir/benchmark-result.json to build/candidate/libcxx/test/benchmarks/algorithms/nonmodifying/Output/starts_with.bench.cpp.dir/benchmark-result.json
Benchmark                                               Time             CPU      Time Old      Time New       CPU Old       CPU New
------------------------------------------------------------------------------------------------------------------------------------
rng::starts_with(vector<int>)/8                      -0.0314         -0.0314             2             2             2             2
rng::starts_with(vector<int>)/1000                   +0.1724         +0.1719            99           116            99           115
rng::starts_with(vector<int>)/1024                   +0.1791         +0.1791           101           119           101           119
rng::starts_with(vector<int>)/8192                   +0.1894         +0.1893           821           976           821           976
rng::starts_with(vector<int>)/1048576                +0.0984         +0.0984        113321        124466        113308        124457
OVERALL_GEOMEAN                                      +5.1211         +5.1148             0             0             0             0
```

Using `ranges::equal` seems to be consistently worse than using `ranges::mismatch`.

https://github.com/llvm/llvm-project/pull/84570