How Can I Precisely Measure Code Execution Time in C ?

Measuring Execution Time with Precision in C

Accurate execution time measurement is essential in optimizing code performance. In C , the clock() function provides a way to calculate elapsed time. However, it can return unreliable results with short code snippets.

Limitations of clock()

For small inputs or single-line statements, clock() often reports execution times of "0 seconds." This discrepancy arises because the clock() function has a granularity of milliseconds or nanoseconds, depending on the system. For execution durations shorter than this granularity, clock() rounds down to zero.

Solution: Using GetTimeMs64()

To overcome this limitation, consider using the utility function GetTimeMs64(). This function accurately measures the time elapsed since the Unix epoch, in milliseconds, and works on both Windows and Linux platforms.

The GetTimeMs64() function utilizes the system clock on Windows using GetSystemTimeAsFileTime(). On Linux, it employs gettimeofday() to retrieve the current time. It converts the obtained values to milliseconds and subtracts the offset from the Unix epoch to align with the Unix time standard.

Implementation

To use GetTimeMs64(), simply include the following code in your program:

#include <Windows.h>
// or
#include <sys/time.h>
#include <ctime>

uint64 GetTimeMs64()
{
    // Code from provided solution goes here
}

To calculate the execution time of a code snippet:

1. Call GetTimeMs64() before executing the code.
2. Execute the code.
3. Call GetTimeMs64() again after executing the code.
4. Subtract the first time from the second time to obtain the elapsed time in milliseconds.

Example Usage:

uint64 startTime = GetTimeMs64();
// Your code here
uint64 endTime = GetTimeMs64();
cout << "Execution time: " << (endTime - startTime) / 1000.0 << " seconds" << endl;

This method provides accurate execution time measurements even for small code snippets or short statements. It allows developers to optimize their code effectively and precisely gauge performance improvements.

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