Why C++ optimizer has problems with these temporary variables, C++ (gcc)

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#include <algorithm>
#include <chrono>
#include <random>
#include <iomanip>
#include <iostream>
#include <vector>

using namespace std;
using namespace std::chrono;

vector<int> v(1'000'000);

int f0()
{
  int n = 0;

for (int i = 1; i < v.size()-1; ++i) {
    int a = v[i-1];
    int b = v[i];
    int c = v[i+1];

if (a < b  &&  b < c)
      ++n;
  }

return n;
}

int f1()
{
  int n = 0;

for (int i = 1; i < v.size()-1; ++i) 
    if (v[i-1] < v[i]  &&  v[i] < v[i+1])
      ++n;

return n;
}

int main()
{
  auto benchmark = [](int (*f)()) {
    const int N = 100;

volatile long long result = 0;
    vector<long long>  timings(N);

for (int i = 0; i < N; ++i) {
      auto t0 = high_resolution_clock::now(); 
      result += f();
      auto t1 = high_resolution_clock::now();

timings[i] = duration_cast<nanoseconds>(t1-t0).count();
    }
    
    sort(timings.begin(), timings.end());
    cout << fixed << setprecision(6) << timings.front()/1'000'000.0 << "ms min\n";
    cout << timings[timings.size()/2]/1'000'000.0 << "ms median\n" << "Result: " << result/N << "\n\n";
  };

mt19937                    generator   (31415);   // deterministic seed
  uniform_int_distribution<> distribution(0, 1023);

for (auto& e: v) 
    e = distribution(generator);

benchmark(f0);
  benchmark(f1);

cout << "\ndone\n";

return 0;
}

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Compilation time: 0.92 sec, absolute running time: 0.94 sec, cpu time: 0.83 sec, memory peak: 5 Mb, absolute service time: 1,86 sec