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Comment by anonymoushn | Hacker News Reader

For a solution that contains this stuff

  const u8 *file_lo;
  file_lo = (const u8*)mmap(0,250000000ull,PROT_READ,MAP_PRIVATE|MAP_POPULATE,0,0);
  const u8 *file_hi = file_lo + 250000000ull;
  u64 count = 0;
  while (file_lo < file_hi) {
    if (*file_lo == 127) {
        ++count;
    }
    file_lo++;
  }

I got a bit under 54ms. The solution in the article runs in a bit under 16ms.

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Nice. Did some quick tests with your code on site, got score of ~34000 - best solution is around 14700, so this one is only 2.3 times slower.

Used clang with -Ofast -march=native -static. Funnily, gcc gets only 54000 with the same options, 1.6 times slower.

Wow, changing `count` type from uint64_t to uint32_t or int radically changes results - now gcc gets 26500 and clang gets 25000, that's just 1.7 times slower than current best solution.

So you can get 25k with following code, clang -Ofast -std=c++17 -march=native -static

    #include <iostream>
    #include <cstdint>
    #include <sys/mman.h>
    #include <unistd.h>

    int main() {
      auto file_lo = (const uint8_t*)mmap(0,250000000ull,PROT_READ,MAP_PRIVATE|MAP_POPULATE,STDIN_FILENO,0);
      int count = 0;
      for (uint32_t i = 0; i < 250000000; ++i) {
        if (file_lo[i] == 127) {
            ++count;
        }
      }
      std::cout << count << std::endl;
      _exit(0);
      return 0;
    }

Neat! I'll add the best solution without explicit SIMD/asm in this thread to the post after I wake up, it's a great datapoint.

Bit rusty here but what if you replaced

    if (*file_lo == 127) {
        ++count;

with

    count += (*file_lo == 127);

That might save you the occasional branch mis-prediction, and might possibly open up some hardware-level loop optimisations. Any difference?

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