The P/Q left side optimization in the delta syndrome simply involves
repeatedly multiplying a value by polynomial 'x' in GF(2^8). Given
that 'x * x * x * x' equals 'x^4' even in the polynomial world, we
can accelerate this substantially by performing up to 4 such operations
at once, using the NEON instructions for polynomial multiplication.
Results on a Cortex-A57 running in 64-bit mode:
Before:
-------
raid6: neonx1 xor() 1680 MB/s
raid6: neonx2 xor() 2286 MB/s
raid6: neonx4 xor() 3162 MB/s
raid6: neonx8 xor() 3389 MB/s
After:
------
raid6: neonx1 xor() 2281 MB/s
raid6: neonx2 xor() 3362 MB/s
raid6: neonx4 xor() 3787 MB/s
raid6: neonx8 xor() 4239 MB/s
While we're at it, simplify MASK() by using a signed shift rather than
a vector compare involving a temp register.
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
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