I have been investigating the use of the new gather instructions of the AVX2 instruction set. Specifically, I decided to benchmark a simple problem, where one floating point array is permuted and added to another. In c, this can be implemented as
void vectortest(double * a,double * b,unsigned int * ind,unsigned int N)
{
int i;
for(i=0;i<N;++i)
{
a[i]+=b[ind[i]];
}
}
I compile this function with g++ -O3 -march=native. Now, I implement this in assembly in three ways. For simplicity I assume that the length of the arrays N is divisible by four. The simple, non-vectorized implementation:
align 4
global vectortest_asm
vectortest_asm:
;; double * a = rdi
;; double * b = rsi
;; unsigned int * ind = rdx
;; unsigned int N = rcx
push rax
xor rax,rax
loop: sub rcx, 1
mov eax, [rdx+rcx*4] ;eax = ind[rcx]
vmovq xmm0, [rdi+rcx*8] ;xmm0 = a[rcx]
vaddsd xmm0, [rsi+rax*8] ;xmm1 += b[rax] ( and b[rax] = b[eax] = b[ind[rcx]])
vmovq [rdi+rcx*8], xmm0
cmp rcx, 0
jne loop
pop rax
ret
The loop vectorised without the gather instruction:
loop: sub rcx, 4
mov eax,[rdx+rcx*4] ;first load the values from array b to xmm1-xmm4
vmovq xmm1,[rsi+rax*8]
mov eax,[rdx+rcx*4+4]
vmovq xmm2,[rsi+rax*8]
mov eax,[rdx+rcx*4+8]
vmovq xmm3,[rsi+rax*8]
mov eax,[rdx+rcx*4+12]
vmovq xmm4,[rsi+rax*8]
vmovlhps xmm1,xmm2 ;now collect them all to ymm1
vmovlhps xmm3,xmm4
vinsertf128 ymm1,ymm1,xmm3,1
vaddpd ymm1, ymm1, [rdi+rcx*8]
vmovupd [rdi+rcx*8], ymm1
cmp rcx, 0
jne loop
And finally, an implementation using vgatherdpd:
loop: sub rcx, 4
vmovdqu xmm2,[rdx+4*rcx] ;load the offsets from array ind to xmm2
vpcmpeqw ymm3,ymm3 ;set ymm3 to all ones, since it acts as the mask in vgatherdpd
vgatherdpd ymm1,[rsi+8*xmm2],ymm3 ;now gather the elements from array b to ymm1
vaddpd ymm1, ymm1, [rdi+rcx*8]
vmovupd [rdi+rcx*8], ymm1
cmp rcx, 0
jne loop
I benchmark these functions on a machine with a Haswell cpu (Xeon E3-1245 v3). Some typical results are (times in seconds):
Array length 100, function called 100000000 times.
Gcc version: 6.67439
Nonvectorized assembly implementation: 6.64713
Vectorized without gather: 4.88616
Vectorized with gather: 9.32949
Array length 1000, function called 10000000 times.
Gcc version: 5.48479
Nonvectorized assembly implementation: 5.56681
Vectorized without gather: 4.70103
Vectorized with gather: 8.94149
Array length 10000, function called 1000000 times.
Gcc version: 7.35433
Nonvectorized assembly implementation: 7.66528
Vectorized without gather: 7.92428
Vectorized with gather: 8.873
The gcc and the nonvectorized assembly version are very close to each other. (I also checked the assembly output of gcc, which is quite similar to my hand coded version.) The vectorization gives some benefit for small arrays, but is slower for large arrays. The big surprise (to me at least) is that the version using vgatherpdp is so slow. So, my question is, why? Am I doing something stupid here? Can someone provide an example where the gathering instruction would actually give a performance benefit over just doing multiple loading operations? If not, what is the point of actually having such an instruction?
The test code, complete with a makefile for g++ and nasm, is available at https://github.com/vanhala/vectortest.git in case somebody wants to try this out.
