float roundtrip conversion test

gistfile1.txt

#include <atomic>
#include <vector>
#include <thread>
#include <string>
#include <cstdio>
#include <charconv>

bool test(float f) {
#if 0
  float f2 = std::stof(std::to_string(f));
  return (f == f2);
#else
  char buffer[40];
  float f2;
  auto res = std::to_chars(buffer, buffer+sizeof(buffer), f);
  auto res2 = std::from_chars(buffer, res.ptr, f2);
  return f == f2;
#endif
}

int main() {
  std::atomic<size_t> failcount;
  std::vector<std::thread> threads;
  threads.reserve(std::thread::hardware_concurrency());
  for (size_t n = 0; n < threads.capacity(); n++) {
    threads.emplace_back([&,id = n]{
      size_t fails = 0;
      for (uint32_t top = 0x0; top != 0xFF000000; top += 0x800000) {
        for (uint32_t i = (id << 18); i < ((id+1) << 18); i++) {
          uint32_t val = i + top;
          float f;
          memcpy(&f, &val, 4);
          if (not test(f)) fails++;
        }
      }
      failcount += fails;
      printf("Thread %zu failed %zu\n", id, fails);
    });
  }
  size_t fails2 = 0;
  for (uint32_t val : { 0x7F800000U, 0x7F800001U, 0xFF800000U, 0xFF800001U }) {
    float f;
    memcpy(&f, &val, 4);
    if (not test(f)) fails2++;
  }
  printf("nan/inf %zu failed\n", fails2);
  failcount += fails2;
  for (auto& t : threads) t.join();
  printf("%zu fail\n", failcount.load());
}