On Sunday, 18 August 2013 at 05:26:00 UTC, Manu wrote:
movups is not good. It'll be a lot faster (and portable) if you
use movaps.
Process looks something like:
* do the first few from a[0] until a's alignment interval as
scalar
* load the left of b's aligned pair
* loop for each aligned vector in a
- load a[n..n+4] aligned
- load the right of b's pair
- combine left~right and shift left to match elements
against a
- left = right
* perform stragglers as scalar
Your benchmark is probably misleading too, because I suspect
you are
passing directly alloc-ed arrays into the function (which are
16 byte
aligned).
movups will be significantly slower if the pointers supplied
are not 16
byte aligned.
Also, results vary significantly between chip manufacturers and
revisions.
I have tried to write fast implementation with aligned loads:
1. I have now idea how to shift (rotate) 32-bytes avx vector
without XOP instruction set (XOP available only for AMD).
2. I have tried to use one vmovaps and [one vmovups]/[two
vinsertf128] with 16-bytes aligned arrays (previously iterates
with a). It works slower then two vmovups (because loop tricks).
Now I have 300 lines of slow dotProduct code =)
4. Condition for small arrays works good.
I think it is better to use:
1. vmovups if it is available with condition for small arrays
2. version like from phobos if vmovups is not avalible
3. special version for small static size arrays
I think version for static size arrays can be easily done for
phobos, processors can unroll such code. And dot product
optimized for complex numbers can be done too.
Best regards
Ilya
