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The patch works by altering the gebp lhs packing routines to also
consider ½ and ¼ packet lenght rows when packing, besides the original
whole package and row-by-row attempts. Finally, gebp itself will try
to fit a fraction of a packet at a time if:
i) ½ and/or ¼ packets are available for the current context (e.g. AVX2
and SSE-sized SIMD register for x86)
ii) The matrix's height is favorable to it (it may be it's too small
in that dimension to take full advantage of the current/maximum
packet width or it may be the case that last rows may take
advantage of smaller packets before gebp goes row-by-row)
This helps mitigate huge slowdowns one had on AVX512 builds when
compared to AVX2 ones, for some dimensions. Gains top at an extra 1x
in throughput. This patch is a complement to changeset 4ad359237aeb519dbd4b55eba43057b37988838c
.
Since packing is changed, Eigen users which would go for very
low-level API usage, like TensorFlow, will have to be adapted to work
fine with the changes.