Make primes.c a bazillion times faster.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@4810

diff --git a/auxprogs/primes.c b/auxprogs/primes.c
index 2ad817227f52980d9f6439f6b7ba0bbb4c5c308d..bf2fb82d9b1497bbf53cc7f88c7de5d086f2512c 100644 (file)
--- a/auxprogs/primes.c
+++ b/auxprogs/primes.c
@@ -1,10 +1,12 @@
 
 #include <stdio.h>
+#include <math.h>
 
 int isprime ( int n )
 {
    int m;
-   for (m = 2; m < n; m++)
+   int sqrt_n = sqrt(n);
+   for (m = 2; m <= sqrt_n+1; m++)  // +1 in case of obscure rounding error
       if ((n % m) == 0) return 0;
    return 1;
 }