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Faster frame decompression #487
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It was absolutely terrible and its results were more misleading than helpful.
This commits improves how we do compression in several ways:
* in decompression, use pointers directly, without indices. This makes
the codegen slightly better by using less registers
* in decompression, remove all panics, and return errors instead
* in decompression, properly verify every unsafe condition to prevent
any undefined bevavior with malformed bitpacks. Also mark the
decompression functions as `unsafe`, and document them
* avx2 compression and avx512 decompression. Note these unfortunately
do not greatly affect overall speed, because it is dominated mostly
by lz4.
* change the way we select the correct function to run at runtime. We
are now using function pointers, instead of if statements. Because
now we have 3 implementations for each function, using if statements
would create a cascade of ifs that could potentially be pretty slow,
specially for those using the fallback implementations.
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