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optimizations.md

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Optimizations

This document tracks which optimizations have been done after the initial implementation passed corpus tests and a good amount of fuzzing.

Introducing more unsafe code:

These optimizations introduced more unsafe code. These should yield significant improvements, or else they are not really worth it.

Optimizing bitreader with byteorder which uses ptr::copy_nonoverlapping

  • Reverse bitreader_reversed::get_bits was identified by linux perf tool using about 36% of the whole time

  • Benchmark: decode enwik9

  • Before: about 14.7 seconds

  • After: about 12.2 seconds with about 25% of the time used for get_bits()

Optimizing decodebuffer::repeat with ptr::copy_nonoverlapping

  • decodebuffer::repeate was identified by linux perf tool using about 28% of the whole time

  • Benchmark: decode enwik9

  • Before: about 9.9 seconds

  • After: about 9.4 seconds

Use custom ringbuffer in the decodebuffer

The decode buffer must be able to do two things efficiently

  • Collect bytes from the front
  • Copy bytes from the contents to the end

The stdlibs VecDequeu and Vec can each do one but not the other efficiently. So a custom implementation of a ringbuffer was written.

Introducing NO additional unsafe code

These are just nice to have

Even better bitreaders

Studying this material lead to a big improvement in bitreader speed