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5ca10480b0
See <https://stackoverflow.com/questions/59709148/ensuring-that-eigen-uses-avx-vectorization-for-a-certain-operation> for an explanation of the problem this solves. In short, for some reason, before this commit the half-packet is selected when the array / matrix size is not a multiple of `unpacket_traits<PacketType>::size`, where `PacketType` starts out being the full Packet. For example, for some data of 100 `float`s, `Packet4f` will be selected rather than `Packet8f`, because 100 is not a multiple of 8, the size of `Packet8f`. This commit switches to selecting the half-packet if the size is less than the packet size, which seems to make more sense. As I stated in the SO post I'm not sure that I'm understanding the issue correctly, but this fix resolves the issue in my program. Moreover, `make check` passes, with the exception of line 614 and 616 in `test/packetmath.cpp`, which however also fail on master on my machine: CHECK_CWISE1_IF(PacketTraits::HasBessel, numext::bessel_i0, internal::pbessel_i0); ... CHECK_CWISE1_IF(PacketTraits::HasBessel, numext::bessel_i1, internal::pbessel_i1);