[Kiki]

hehehe ... mo nimbrung ... bikin fungsi logaritma pake SSE, masalahnya :

1. nggak gitu akurat, maklum cuma evaluasi maclaurin series cuma 4 suku saja
2. Only work for x where 0 < x < 2 (open interval)
3. No guarantee it is faster

ini kodenya :

#include <cstdio>
#include <cmath>
#include <xmmintrin.h>



float maclaurin(float val){
	__declspec(align(16) ) float tmp[4] = { 1.0F,1.0F,1.0F,0.0F};
	__declspec(align(16) ) float multiplier[4] = { 1.0F,-0.5F,0.33333333F,-0.25F};
	tmp[3] = val;
	__m128 vec1 = _mm_load_ps(tmp);
	__m128 vec2 = _mm_shuffle_ps( vec1,vec1,0xF4);
	__m128 vec3 = _mm_shuffle_ps( vec1,vec1,0xFC);
	__m128 vec4 = _mm_shuffle_ps( vec1,vec1,0xFF);
	__m128 acc  = _mm_mul_ps(vec1, vec2);
	acc  = _mm_mul_ps(acc, vec3);
	vec1 = _mm_load_ps(multiplier);
	acc  = _mm_mul_ps(acc, vec4);
	acc  = _mm_mul_ps(acc,vec1);
	vec1 = _mm_shuffle_ps(acc,acc, 0xE4);
	vec2 = _mm_shuffle_ps(acc,acc, 0x39);
	vec3 = _mm_shuffle_ps(acc,acc, 0x4E);
	vec4 = _mm_shuffle_ps(acc,acc, 0x93);
	acc  = _mm_add_ps(vec1,vec2);
	acc  = _mm_add_ps( acc, vec3);
	acc  = _mm_add_ps(acc, vec4);
	float res;
	_mm_store_ss(&res,acc);
	return res;
}

float my_log(float x){
	return maclaurin(x - 1);
}

int main(){
	float reference = logf(1.3F);
	float target    = my_log(1.3F);
	printf(" reference = %g , target = %g\n", reference, target);
}

di komputer gw, outputnya :

reference = 0.262364 , target = 0.261975

not bad ...