round.c

#include <stdint.h>
#include <fenv.h>

float hs_word2float32_old(uint64_t x) {
  return (float)x;
}

float hs_word2float32_new(uint64_t x) {
  int r = fegetround();
  fesetround(FE_TOWARDZERO);
  volatile float y = (float)x;
  fesetround(r);
  return y;
}

timeit.cpp

#include <iostream>
#include <cstdint>
#include <chrono>
#include <functional>

float hs_word2float32_old(uint64_t x);
float hs_word2float32_new(uint64_t x);

using namespace std::chrono;

template<typename clock = high_resolution_clock>
duration<double> time_it(std::function<void()> f)
{
  typename clock::time_point start = clock::now();
  for (int i=0; i<100000000; i++) {
    f();
  }
  typename clock::time_point end = clock::now();
  return end - start;
}

int main() {
  auto t_old = time_it([](){ hs_word2float32_old(42); });
  auto t_new = time_it([](){ hs_word2float32_new(42); });
  std::cout << t_old.count() << std::endl;
  std::cout << t_new.count() << std::endl;
}