Stagger Stepping in Negative Levels #2921
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This replicates the behavior of @terrelln's `ZSTD_fast` implementation. That is, it always looks at adjacent pairs of positions, and only applies the acceleration every other position. This produces a more fine-grained acceleration.
This avoids an additional addition, at the cost of an additional variable.
Old Benchmark`silesia.tar` on GCC-10:
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The position updates are rewritten from `ip[N] = ip[N-1] + step` to be `ip[N] = ip[N-2] + step`. This lets us only deal with the asymmetric spacing of gaps at setup and then we only have to keep a single `step` variable. This seems to work quite well on GCC and Clang!
Old BenchmarkThe new version shows the following performance:gcc-10:
clang-13:
The templating does at ~10KB to the library size. |
I couldn't find a good way to spread `ip0` and `ip1` apart when we accelerate due to incompressible inputs. (The methods I tried slowed things down quite a bit.) Since we aren't splaying ip0 and ip1 apart (which would be like `0_1_2_3_`, as opposed to the `01__23__` we were actually doing), it's a big ambitious to increment `step` by 2. Instead, let's increment it by 1, which has the benefit sliiightly improving compression. Speed remains pretty much unchanged.
Cyan4973
approved these changes
Dec 14, 2021
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#2749 returned the negative levels to pre-#1562 behavior. I.e., skipping
stepSizepositions every iteration. This PR emulates #1562-like stepping by applying thestepSizeskip every other position.This PR addresses #2827.
Benchmarks
silesia.tarongcc-10:silesia.taronclang-13:Status
I believe this PR is ready for merge.