String comparision performance in Java and C - string

Question

I need to measure performance of a function that compares two strings. My task is to write it in Java and C and compare execution time. For testing purposes I generated a txt file with 100000 random strings varying from 100 to 200 characters each. Using them I invoke comparision function 20'000'000 times. In Java it takes ~500ms while in C the execution time is 0ms (Im doing exactly the same tests on exaclty the same data in both languages). Even if I increase it to 20'000'000'000 calls in C, it still measures 0ms duration. How is it possible? Am I missing something important?
implementaton in Java
public class StringComparer {
public static boolean compareStrings(String string1, String string2) {
if(string1.length() != string2.length()) {
return false;
}
for (int i = 0; i < string1.length(); i++) {
if(string1.charAt(i) != string2.charAt(i)) {
return false;
}
}
return true;
}
}
implementation in C
bool string_compare(char* s1, char* s2)
{
int i = 0;
while (s1[i] != NULL && s1[i] == s2[i])
i++;
return s1[i] == s2[i];
}
This is the code I use to test performance in C
#include <stdio.h>
#include <stdbool.h>
#include <stdlib.h>
#include <windows.h>
#define NUMBER_OF_WORDS 100000
#define MAX_WORD_LENGTH 200
long long milliseconds_now() {
static LARGE_INTEGER s_frequency;
BOOL s_use_qpc = QueryPerformanceFrequency(&s_frequency);
if (s_use_qpc) {
LARGE_INTEGER now;
QueryPerformanceCounter(&now);
return (1000LL * now.QuadPart) / s_frequency.QuadPart;
}
else {
return GetTickCount();
}
}
int main()
{
char* fileName = "tests.txt";
FILE *file = fopen(fileName, "r");
char* words[NUMBER_OF_WORDS];
long long i, j;
for (i = 0; i < NUMBER_OF_WORDS; i++) {
words[i] = (char*)malloc((MAX_WORD_LENGTH + 1) * sizeof(char));
fgets(words[i], MAX_WORD_LENGTH + 1, file);
}
long long repeats = 10000000000 / NUMBER_OF_WORDS;
long long start = milliseconds_now();
for (i = 0; i < repeats; i++)
{
for (j = 0; j < NUMBER_OF_WORDS - 1; j++)
{
;
}
}
long long loopDuration = milliseconds_now() - start;
start = milliseconds_now();
for (i = 0; i < repeats; i++)
{
for (j = 0; j < NUMBER_OF_WORDS - 1; j++)
{
string_compare(words[j], words[j + 1]); //compare different strings
string_compare(words[j], words[j]); //compare the same strings
}
}
long long customFunctionDuration = milliseconds_now() - start;
printf("Loop duration: %lld\n", loopDuration);
printf("Custom function duration: %lld - %lld = %lld ms", customFunctionDuration, loopDuration, customFunctionDuration - loopDuration);
return 0;
}

Answer 1

Your code's observable behavior is precisely the same as code that does nothing at all. You need to make the result of the string comparisons part of your program's observable behavior so they can't be optimized to nothing. Try tallying the number of times strings match and the number of times the strings don't match and outputting that number.