I am attempting to use SIMD instructions to speed up a dot product calculation in my C code. However, the run times of my functions are approximately equal. It'd be great if someone could explain why and how to speed up the calculation.
Specifically, I'm attempting to calculate the dot product of two arrays with about 10,000 elements in them. My regular C function is as follows:
float my_dotProd( float const * const x, float const * const y, size_t const N ){
// N is the number of elements in the arrays
size_t i;
float out=0;
for( i=0; i < N; ++i ){
out += x[i] * y[i];
}
return out;
}
My function where I use AVX SIMD commands is as follows:
void my_malloc( size_t nBytes, void ** ptrPtr ){
int boundary = 32;
posix_memalign( ptrPtr, boundary, nBytes );
}
float cimpl_sum_m128( __m128 x ){
float out;
__m128 sum = x;
sum = _mm_hadd_ps( sum, sum );
sum = _mm_hadd_ps( sum, sum );
out = _mm_cvtss_f32( sum );
return out;
}
float my_sum_m256( __m256 x ){
float out1, out2;
__m128 hi = _mm256_extractf128_ps(x, 1);
__m128 lo = _mm256_extractf128_ps(x, 0);
out1 = cimpl_sum_m128( hi );
out2 = cimpl_sum_m128( lo );
return out1 + out2;
}
float my_dotProd( float const * const x, float const * const y, size_t const N ){
// N is the number of elements in the arrays
size_t i=0;
float out=0;
float *tmp;
__m256 summed, *l, *r;
if( N > 7 ){
my_malloc( sizeof(float) * 8, (void**) &tmp );
summed = _mm256_set1_ps(0.0f);
l = (__m256*) x;
r = (__m256*) y;
for( i=0; i < N-7; i+=8, ++l, ++r ){
summed = _mm256_add_ps( summed, _mm256_mul_ps( *l, *r ) );
}
_mm256_store_ps( tmp, summed );
out += my_sum_m256( summed );
free( tmp );
}
for( ; i < N; ++i ){
out += x[i] * y[i];
}
return out;
}
My test routine is:
int test_dotProd(){
float *x, *y;
size_t i, N;
float answer, result;
float err;
N = 100000; // Fails
my_malloc( sizeof(float) * N, (void**) &x );
my_malloc( sizeof(float) * N, (void**) &y );
answer = 0;
for( i=0; i<N; ++i ){
x[i]=i; y[i]=i;
answer += (float)i * (float)i;
}
result = my_dotProd( x, y, N );
err = fabs( result - answer ) / answer;
free( x );
free( y );
return err < 5e-7;
}
And I'm using clock to measure the runtime like this:
timeStart = clock();
testStatus = test_dotProd();
timeTaken = (int)( clock() - timeStart );
I realize that the my_sum_m256 operation could be made more efficient, but I think that should be a small effect on the runtime. I would have guessed that the SIMD code would have been about eight times as fast. Any thoughts?
Thank you all for your help :)
mallocfor tmp. That will be a lot slower than just using stack. – Sami Kuhmonen