I've written vectorized versions of some functions that are currently the bottleneck of an algorithm, using Eigen's facilities to do so.
I've also checked that AVX is enabled by making sure that EIGEN_VECTORIZE_AVX is defined after including Eigen.
However, it seems that my function never gets called with Packet8f (AVX), if the data size is not a multiple of 8. Instead, it gets called with Packet4f (SSE).
Here is a small repro: https://gist.github.com/bitonic/e89561cb21837b4dee8b5f49e1303919 . Here I define an operation using Packet4f and Packet8f, and then count how many times each gets called with an array of size 8 and 9. When the array is of size 8, the Packet8f version gets called once, as expected. When it's of size 9, the Packet4f version gets called twice instead, plus a single call to the non-vectorized version. I've tested this code on Eigen's current master 1d0c45122a5c4c5c1c4309f904120e551bacad02.
I've dug a bit and I believe that packet selection is happening here: https://gitlab.com/libeigen/eigen/blob/1d0c45122a5c4c5c1c4309f904120e551bacad02/Eigen/src/Core/util/XprHelper.h#L197 .
If I understand correctly, if the size of the data is not dynamic and not a multiple of 8 (that's the value of unpacket_traits<Packet8f>::size), the half-packet will be selected, which matches what the reproduction above shows.
If my understanding is correct, why is that the case? Shouldn't the full packet be selected, with the remaining elements working with the non-vectorized operation?
Could it be that that condition is wrong, and should be a >= comparison instead, e.g. something like
template<int Size, typename PacketType,
bool Stop = Size==Dynamic || Size >= unpacket_traits<PacketType>::size || is_same<PacketType,typename unpacket_traits<PacketType>::half>::value>
struct find_best_packet_helper;
instead of
template<int Size, typename PacketType,
bool Stop = Size==Dynamic || (Size%unpacket_traits<PacketType>::size)==0 || is_same<PacketType,typename unpacket_traits<PacketType>::half>::value>
struct find_best_packet_helper;
I've verified that with the fix above the problem goes away.
However I might be misunderstanding what is going on here, since I'm not very well versed in Eigen internals.
I have confirmed that this is due to how Eigen selects the packet type, see https://gitlab.com/libeigen/eigen/merge_requests/46 for a fix.