- the first prefetch is actually harmful on Haswell with FMA,
but it is the most beneficial on ARM.
- the second prefetch... I was very stupid and multiplied by sizeof(scalar)
and offset of a scalar* pointer. The old offset was 64; pk = 8, so 64=pk*8.
So this effectively restores the older offset. Actually, there were
two prefetches here, one with offset 48 and one with offset 64. I could not
confirm any benefit from this strange 48 offset on either the haswell or
my ARM device.
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