kernel implementation quirks between a0, a1, a3

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It's all about optimizations. Whenever possible we try to avoid swaps in the kernel, especially in fast hash kernels. For pure kernels, we need to stick to the original idea of having an openssl-like interface, otherwise it would not make sense to have it in the first place. Therefore the OPTS_TYPE_PT_GENERATE_BE gets removed automatically in case you're not using -O manually, that's why you need to have dedicated code for it.