I'm trying to enhance the performance of my code by using the 256bit vector (Intel intrinsics - AVX).
I have an I7 Gen.4 (Haswell architecture) processor supporting SSE1 to SSE4.2 and AVX/AVX2 Extensions.
This is the code snippet that I'm trying to enhance:
/* code snipet */
kfac1 = kfac + factor; /* 7 cycles for 7 additions */
kfac2 = kfac1 + factor;
kfac3 = kfac2 + factor;
kfac4 = kfac3 + factor;
kfac5 = kfac4 + factor;
kfac6 = kfac5 + factor;
kfac7 = kfac6 + factor;
k1fac1 = k1fac + factor1; /* 7 cycles for 7 additions */
k1fac2 = k1fac1 + factor1;
k1fac3 = k1fac2 + factor1;
k1fac4 = k1fac3 + factor1;
k1fac5 = k1fac4 + factor1;
k1fac6 = k1fac5 + factor1;
k1fac7 = k1fac6 + factor1;
k2fac1 = k2fac + factor2; /* 7 cycles for 7 additions */
k2fac2 = k2fac1 + factor2;
k2fac3 = k2fac2 + factor2;
k2fac4 = k2fac3 + factor2;
k2fac5 = k2fac4 + factor2;
k2fac6 = k2fac5 + factor2;
k2fac7 = k2fac6 + factor2;
/* code snipet */
From the Intel Manuals, I found this.
an integer addition ADD takes 1 cycle (latency).
a vector of 8 integers (32 bit) takes 1 cycle also.
So I've tried ton make it this way:
fac = _mm256_set1_epi32 (factor )
fac1 = _mm256_set1_epi32 (factor1)
fac2 = _mm256_set1_epi32 (factor2)
v1 = _mm256_set_epi32 (0,kfac6,kfac5,kfac4,kfac3,kfac2,kfac1,kfac)
v2 = _mm256_set_epi32 (0,k1fac6,k1fac5,k1fac4,k1fac3,k1fac2,k1fac1,k1fac)
v3 = _mm256_set_epi32 (0,k2fac6,k2fac5,k2fac4,k2fac3,k2fac2,k2fac1,k2fac)
res1 = _mm256_add_epi32 (v1,fac) ////////////////////
res2 = _mm256_add_epi32 (v2,fa1) // just 3 cycles //
res3 = _mm256_add_epi32 (v3,fa2) ////////////////////
But the problem is that these factors are going to be used as tables indexes ( table[kfac] ... ). So i have to extract the factor as seperate integers again. I wonder if there is any possible way to do it??