* First version of AVX2 Flash attention
I simply took the Zen4 implementation and converted
platform specific stuff to methods of a struct providing
data loading/storing, conversions, multiply, add, etc.
Most likely not optimal as the Zen4 strategy has been
designed based on having 32 512-bit registers, so basically
we can have 4X more data stored in vector registers compared
to AVX2 with 16 x 256-bit.
It still gives a small speedup (~4% at 2048 tokens) for Gemma-2b.
* Fix Zenn4 parts broken via the AVX2 change
* Try smaller q_step - no improvement
* Fix ARM_NEON
I had forgotten to guard the AVX2/Zen4 implementation against __aarch64__
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Co-authored-by: Iwan Kawrakow <iwan.kawrakow@gmail.com>
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