Benchmarks of different ways to calculate the end parameters of a while loop.

while_loop_comparisons.c

#include <stdio.h>
#include <conio.h>

#ifdef WIN32

#include <windows.h>
double get_time()
{
	LARGE_INTEGER t, f;
	QueryPerformanceCounter(&t);
	QueryPerformanceFrequency(&f);
	return (double)t.QuadPart / (double)f.QuadPart;
}
#else
#include <sys/time.h>
#include <sys/resource.h>

double get_time()
{
	struct timeval t;
	struct timezone tzp;
	gettimeofday(&t, &tzp);
	return t.tv_sec + t.tv_usec*1e-6;
}
#endif

#define LIMIT 100000000
#define TEST_COUNT 50

struct test_result
{
	double start;
	double end;
} typedef tr;

tr test1a()
{
	tr result;
	volatile int i;
	volatile int limit;

	i = LIMIT;

	result.start = get_time();
	while (i--)
	{
	}
	result.end = get_time();

	return result;
}

tr test1b()
{
	tr result;
	int i;
	int limit;

	i = LIMIT;

	result.start = get_time();
	while (i--)
	{
	}
	result.end = get_time();

	return result;
}

tr test2a()
{
	tr result;
	volatile int i;
	volatile int limit;

	i = LIMIT;

	result.start = get_time();
	while (i-- > 0)
	{
	}
	result.end = get_time();

	return result;
}

tr test2b()
{
	tr result;
	int i;
	int limit;

	i = LIMIT;

	result.start = get_time();
	while (i-- > 0)
	{
	}
	result.end = get_time();

	return result;
}

tr test3a()
{
	tr result;
	volatile int i;
	volatile int limit;

	limit = LIMIT;
	i = 0;

	result.start = get_time();
	do
	{
	} while (++i < limit);
	result.end = get_time();

	return result;
}

tr test3b()
{
	tr result;
	int i;
	int limit;

	limit = LIMIT;
	i = 0;

	result.start = get_time();
	do
	{
	} while (++i < limit);
	result.end = get_time();

	return result;
}

void print_test_result(tr result)
{
	printf("Start: %f\nEnd: %-7f\nTotal: %f\n", result.start, result.end, result.end - result.start);
}

void print_test_result_avg(char* test_name, tr* result)
{
	size_t count = sizeof(*result) / sizeof(result) - 1;
	double avg = 0.0;
	int i = count;

	while (i--)
	{
		//print_test_result(result[i]);
		avg += result[i].end - result[i].start;
	}

	avg /= count;

	printf("\r%-29s average: %f\n", test_name, avg);
}

void print_progress(char* test_name)
{
	static int curr = 0;
	char* prog_chars = "|/-\\";
	curr = (curr + 1) % 4;
	char c = prog_chars[curr];

	printf("\r%s ... %c ", test_name, c);
}

void run_tests()
{
	tr result[TEST_COUNT];
	int i;
	char* test_name;
	// #region while(i)
	i = TEST_COUNT;
	test_name = "while(i--) (volatile)";
	while (i--)
	{
		print_progress(test_name);
		result[i] = test1a();
	}
	print_test_result_avg(test_name, result);

	i = TEST_COUNT;
	test_name = "while(i--) (non-volatile)";
	while (i--)
	{
		print_progress(test_name);
		result[i] = test1a();
	}
	print_test_result_avg(test_name, result);
	// #endregion

	// #region while(i>0)
	i = TEST_COUNT;
	test_name = "while(i>0) (volatile)";
	while (i--)
	{
		print_progress(test_name);
		result[i] = test2a();
	}
	print_test_result_avg(test_name, result);

	i = TEST_COUNT;
	test_name = "while(i>0) (non-volatile)";
	while (i--)
	{
		print_progress(test_name);
		result[i] = test2a();
	}
	print_test_result_avg(test_name, result);
	// #endregion

	// #region while(i<limit)
	i = TEST_COUNT;
	test_name = "while(i<limit) (volatile)";
	while (i--)
	{
		print_progress(test_name);
		result[i] = test3a();
	}
	print_test_result_avg(test_name, result);

	i = TEST_COUNT;
	test_name = "while(i<limit) (non-volatile)";
	while (i--)
	{
		print_progress(test_name);
		result[i] = test3a();
	}
	print_test_result_avg(test_name, result);
	// #endregion

	printf("\nDone.\n");
}

int main()
{
	do
	{
		run_tests();

		printf("Press 'r' to run tests again.\n");
	} while (getch() == 'r');

	return 0;
}