Integer logs

log.c

#include <stdio.h>
#include <stdint.h>
#include <string.h>

#define PRIME ((1ul << 31) - 1)
#define GEN ((1ul << 11) - 1)

unsigned
fastlog(uint64_t x)
{
	float xf = x;
	uint32_t xf_bin;
	memcpy(&xf_bin, &xf, sizeof(xf_bin));
	return ((xf_bin >> 23) & 0xFF) - 127;
}

unsigned
cpulog(uint64_t x)
{
	return 63 - __builtin_clzll(x);
}

uint32_t
cycle(uint32_t prev)
{
	uint64_t x = prev;
	x *= (uint64_t) GEN;
	return x % PRIME;
}

uint32_t
myrandom(uint32_t *state)
{
	uint32_t n = (*state = cycle(*state));
	uint32_t s = (*state = cycle(*state));
	return n >> (s % 31);
}

void
analyse(unsigned (*logfunc)(uint64_t))
{
	uint32_t state = GEN;
	int logctr[31] = {0};
	long long i;

	for (i = 0; i < (1ull << 22); ++i) {
		logctr[(*logfunc)(myrandom(&state))]++;
	}
	for (i = 0; i < 31; ++i) {
		printf("%lld:\t%d\n", i, logctr[i]);
	}
}

int
main(void)
{
	analyse(&cpulog);
	analyse(&fastlog);
	return 0;
}