allzeros.c

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#include <stdbool.h>
#include <stddef.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <time.h>

#define LOOPS 10000000
#define BLCKSZ 48

typedef union AlignedBlock
{
	char		data[BLCKSZ];
	double		force_align_d;
	int64_t		force_align_i64;
} AlignedBlock;

static inline bool
isallzeros(const void *ptr, size_t len)
{
	const char *p = (const char *) ptr;
	const char *end = &p[len];
	const char *aligned_end = (const char *) ((uintptr_t) end & (~(sizeof(size_t) - 1)));

	while (((uintptr_t) p & (sizeof(size_t) - 1)) != 0)
	{
		if (p == end)
			return true;
		if (*p++ != 0)
			return false;
	}
	for (; p < aligned_end - (sizeof(size_t) * 7); p += sizeof(size_t) * 8)
	{
		if ((((size_t *) p)[0] != 0) |
			(((size_t *) p)[1] != 0) |
			(((size_t *) p)[2] != 0) |
			(((size_t *) p)[3] != 0) |
			(((size_t *) p)[4] != 0) |
			(((size_t *) p)[5] != 0) |
			(((size_t *) p)[6] != 0) |
			(((size_t *) p)[7] != 0))
			return false;
	}
	while (p < end)
	{
		if (*p++ != 0)
			return false;
	}
	return true;
}

static inline bool
isallzeros_v2(const void *ptr, size_t len)
{
	const char *p = (const char *) ptr;
	const char *end = &p[len];
	const char *aligned_end = (const char *) ((uintptr_t) end & (~(sizeof(size_t) - 1)));

	while (((uintptr_t) p & (sizeof(size_t) - 1)) != 0)
	{
		if (p == end)
			return true;
		if (*p++ != 0)
			return false;
	}
	for (; p < aligned_end - (sizeof(size_t) * 7); p += sizeof(size_t) * 8)
	{
		if (((size_t *) p)[0] != 0 |
			((size_t *) p)[1] != 0 |
			((size_t *) p)[2] != 0 |
			((size_t *) p)[3] != 0 |
			((size_t *) p)[4] != 0 |
			((size_t *) p)[5] != 0 |
			((size_t *) p)[6] != 0 |
			((size_t *) p)[7] != 0)
			return false;
	}
	for (; p < aligned_end; p += sizeof(size_t))
	{
		if (*(size_t *)p != 0)
			return false;
	}
	while (p < end)
	{
		if (*p++ != 0)
			return false;
	}
	return true;
}

#define NANOSEC_PER_SEC 1000000000

// Returns difference in nanoseconds
int64_t
get_clock_diff(struct timespec *t1, struct timespec *t2)
{
	int64_t nanosec = (t1->tv_sec - t2->tv_sec) * NANOSEC_PER_SEC;
	nanosec += (t1->tv_nsec - t2->tv_nsec);

	return nanosec;
}

int main()
{
	AlignedBlock *pagebytes;
	volatile bool result;
	struct timespec start,end;
	int64_t char_time, size_t_time;

	pagebytes = (AlignedBlock *) malloc(sizeof(AlignedBlock));
	memset(pagebytes, 0, sizeof(AlignedBlock));

	clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start);

	for (int i = 0; i < LOOPS; i++)
	{
		result = isallzeros(pagebytes, sizeof(AlignedBlock));
	}

	clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &end);
	char_time = get_clock_diff(&end, &start);
	printf("allzeros: done in %ld nanoseconds\n", char_time);

	clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start);

	for (int i = 0; i < LOOPS; i++)
	{
		result = isallzeros_v2(pagebytes, sizeof(AlignedBlock));
	}

	clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &end);
	size_t_time = get_clock_diff(&end, &start);
	printf("allzeros_v2: done in %ld nanoseconds (%g times faster)\n", size_t_time, (double) char_time / size_t_time);

	return 0;
}