[perf] path::parent_path() is slow · Issue #88 · gulrak/filesystem · GitHub
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[perf] path::parent_path() is slow #88

Description

@tstack

Describe the bug
The path::parent_path() method is unreasonably slow compared to something like POSIX's dirname(3).

To Reproduce
Using this file:

#include <libgen.h>
#include <chrono>
#include "ghc/filesystem.hpp"

int main(int argc, char *argv[])
{
  {
    std::string path_str = "/a/really/long/path/to/test/performance/of/parent_path";

    {
      ghc::filesystem::path path(path_str);
      auto start = std::chrono::system_clock::now();

      for (int lpc = 0; lpc < 1000000; lpc++) {
        auto par = path.parent_path();
      }
      auto stop = std::chrono::system_clock::now();

      printf("ghc duration: %d\n", (int32_t) (stop - start).count());
    }
    {
      auto start = std::chrono::system_clock::now();

      for (int lpc = 0; lpc < 1000000; lpc++) {
        auto par = (char *) malloc(path_str.length() + 1);

        strcpy(par, path_str.c_str());
        dirname(par);
        free(par);
      }
      auto stop = std::chrono::system_clock::now();

      printf("dirname(3) duration: %d\n", (int32_t) (stop - start).count());
    }
  }
}

Compiled with:

$ g++ --std=c++14 -O3 path_perf.cc

On my 2013 iMac with a 3.5Ghz Core i7, I get the following results:

ghc duration: 2337478
dirname(3) duration: 94162

Expected behavior
The performance of parent_path() should be a lot better.

Additional context
I'm using this library in https://github.com/tstack/lnav and parent_path() turned out to be a major source of slowness when a lot of files were open.

(This library is great, thanks for making it!)

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