AVX Intrinsics use

[Valerio_L_]

Hello,

I'm evaluating my CPU board performances with a simple program that execute a specific number of single precision multiplication, with and without the AVX intrinsic functions.

But the results shows that with AVX the performances do not increase. Hereafter my code, compiled with the following command:

icpc -g -O2 -ipp=common -axCORE-AVX2 -xCORE-AVX2 -mtune=broadwell -MMD -MP -MF"mainParal_intel.d" -MT"mainParal_intel.d" -c -o "mainParal_intel.o" "../mainParal_intel.cpp"

#define NUM_LOOP (int)34608000

int main(void)
{
	struct timespec t1, t2;
	float timeApp;
	unsigned long int ii;

	__m256 vec1,vec2,vec3,vec4;

	pSrc  = ippsMalloc_32f(NUM_LOOP);
	pMolt = ippsMalloc_32f(NUM_LOOP);
	pOut  = ippsMalloc_32f(NUM_LOOP);

	for (ii=0;ii<NUM_LOOP;ii++)
	{
		pSrc[ii] = rand_float();

	}

	for (ii=0;ii<NUM_LOOP;ii++)
	{
		pMolt[ii] = rand_float();
	}

	clock_gettime(CLOCK_REALTIME, &t1);

	for (int j=0;j<NUM_LOOP;j+=8)
	{
		vec1=_mm256_load_ps(pSrc+j);
		vec2=_mm256_load_ps(pMolt+j);
		vec3=_mm256_mul_ps(vec1,vec2);
		_mm256_store_ps(pOut+j,vec3);
	}

	clock_gettime(CLOCK_REALTIME, &t2);

	timeApp  = (float)(t2.tv_sec - t1.tv_sec)*1000.0f + (float)(t2.tv_nsec - t1.tv_nsec)/1000000.0f;//tempo in ms
	printf("Execution time with intrinsics   : %f ms\n",timeApp);

	clock_gettime(CLOCK_REALTIME, &t1);

	for (int j=0;j<NUM_LOOP;j++)
	{
		pOut = pSrc*pMolt;
	}

	clock_gettime(CLOCK_REALTIME, &t2);

	timeApp  = (float)(t2.tv_sec - t1.tv_sec)*1000.0f + (float)(t2.tv_nsec - t1.tv_nsec)/1000000.0f;//tempo in ms
	printf("Execution time without intrinsics: %f ms\n", timeApp);

	return 0;
}

The output is:

Execution time with intrinsics   : 34.625843 ms
Execution time without intrinsics: 29.946192 ms

What wrong with what I do??

Thanks

Valerio.

[TimP]

One possibility is less effective unrolling with intrinsic. You could check with unroll0 or by writing out intrinsic with unroll.

[jimdempseyatthecove]

In addition to, or in lieu of, unrolling with intrinsic, by placing

__m256 vec1,vec2,vec3,vec4;
 

inside the scope of the for loop you will permit the compiler to treat those vectors as temporary. What this will do is (potentially) permit the compiler optimization to unroll the loop without intrinsic (or #pragma), as the values of these variables need not be maintained outside the scope of the loop. As written, the values of these variables following the loop must hold the values of the last iteration of the loop. It doesn't hurt to aid the compiler in producing efficient code.

Jim Dempsey