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         Boot ID: 7f7f08a039ed40d2b473aed355f1e17b
Operating System: Ubuntu 22.04.2 LTS
          Kernel: Linux 5.19.0-45-generic
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 Hardware Vendor: HONOR
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 10:28:24 up 24 min,  1 user,  load average: 1.70, 1.21, 0.79

perftune/sieve.c

@@ -10,11 +10,15 @@ static int  primes[N];
 
 int *sieve(int n) {
   assert(n + 1 < N);
-  for (int i = 0; i <= n; i++)
+  is_prime[1] = false; is_prime[2] = true; is_prime[3] = true; //ini
+  for (int i = 5; i <= n; i+=2)
     is_prime[i] = true;
 
-  for (int i = 2; i <= n; i++) {
-    if(is_prime[i]) continue;
+  for(int j = 4; j <= n; j += 2)
+    is_prime[j] = false;
+
+  for (int i = 3; i <= n; i+=2) {
+    if(!is_prime[i]) continue;  //use prime only
     for (int j = i + i + i; j <= n; j += (i+i)) {
       is_prime[j] = false;
     }