ZSTD_fast_noDict: Avoid Safety Check When Writing ip1 into Table
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This commit avoids checking whether a hashtable write is safe in two of the three match-found paths in `ZSTD_compressBlock_fast_noDict_generic`. This pro- duces a ~0.5% speed-up in compression. A comment in the code describes why we can skip this check in the other two paths (the repcode check and the first match check in the unrolled loop). A downside is that in the new position where we make this check, we have not yet computed `mLength`. We therefore have to avoid writing *possibly* dangerous positions, rather than the old check which only avoids writing *actually* dangerous positions. This leads to a miniscule loss in ratio (remember that this scenario can only been triggered in very negative levels or under incomp- ressibility acceleration).
felixhandte
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ip1 into Table
Cyan4973
reviewed
May 10, 2022
Refactoring this way avoids the bad write in the case that `step > 4`, and is a bit more straightforward. It also seems to perform better!
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Cyan4973
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Cool optimization @felixhandte ! |
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This commit avoids checking whether a hashtable write is safe in two of the
three match-found paths in
ZSTD_compressBlock_fast_noDict_generic. This pro-duces a ~1% speed-up in compression.
This is a branch I've wanted to kill for a long time. It's very rarely useful.
Recent discussion with @embg brought this topic up again, and I finally had an
idea for how to do it (mostly).
A comment in the code describes why we can skip this check in the other two
paths (the repcode check and the first match check in the unrolled loop).
A downside is that in the new position where we make this check, we have not
yet computed
mLength. We therefore have to avoid writing possibly dangerouspositions, rather than the old check which only avoids writing actually
dangerous positions. This leads to a miniscule loss in ratio (remember that
this scenario can only been triggered in very negative levels or under incomp-
ressibility acceleration).