Fix an off-by-one error in the compressor that emits corrupt blocks if:
* Zstd is compiled in 32-bit mode
* The windowLog == 25 exactly
* An offset of 2^25-3, 2^25-2, 2^25-1, or 2^25 is emitted
* The bitstream had 7 bits leftover before writing the offset
This bug has been present since before v1.0, but wasn't able to easily
be triggered, since until somewhat recently zstd wasn't able to find
matches that were within 128KB of the window size.
Add a test case, and fix 2 bugs in `ZSTD_compressSequences()`:
* The `ZSTD_isRLE()` check was incorrect. It wouldn't produce
corruption, but it could waste CPU and not emit RLE even if the block
was RLE
* One windowSize was `1 << windowLog`, not `1u << windowLog`
Thanks to @tansy for finding the issue, and giving us a reproducer!
Fixes Issue #3350.
Use a switch statement to select the search function instead of an
indirect function call. This results in a sizable performance win.
This PR is a modification of the approach taken in PR #2828.
When I measured performance for that commit, it was neutral.
However, I now see a performance regression on gcc, but still
neutral on clang. I'm measuring on the same platform, but with
newer compilers. The new approach beats both the current dev
branch and the baseline before PR #2828 was merged.
This PR is necessary for Issue #3275, to update zstd in the kernel.
Without this PR there is a large regression in greedy - btlazy2
compression speed. With this PR it is about neutral.
gcc version: 12.2.0
clang version: 14.0.6
dataset: silesia.tar
| Compiler | Level | Dev Speed (MB/s) | PR Speed (MB/s) | Delta |
|----------|-------|------------------|-----------------|--------|
| gcc | 5 | 102.6 | 113.7 | +10.8% |
| gcc | 7 | 66.6 | 74.8 | +12.3% |
| gcc | 9 | 51.5 | 58.9 | +14.3% |
| gcc | 13 | 14.3 | 14.3 | +0.0% |
| clang | 5 | 108.1 | 114.8 | +6.2% |
| clang | 7 | 68.5 | 72.3 | +5.5% |
| clang | 9 | 53.2 | 56.2 | +5.6% |
| clang | 13 | 14.3 | 14.7 | +2.8% |
The binary size stays just about the same for clang and gcc, measured
using the `size` command:
| Compiler | Branch | Text | Data | BSS | Total |
|----------|--------|---------|------|-----|---------|
| gcc | dev | 1127950 | 3312 | 280 | 1131542 |
| gcc | PR | 1123422 | 2512 | 280 | 1126214 |
| clang | dev | 1046254 | 3256 | 216 | 1049726 |
| clang | PR | 1048198 | 2296 | 216 | 1050710 |
* Intial commit to address 3090. Added support to decompress empty block
* Update zstd_decompress_block.c
Addressed review comments for the case of 'set_basic'
* Update lib/decompress/zstd_decompress_block.c
Co-authored-by: Nick Terrell <nickrterrell@gmail.com>
* Update lib/decompress/zstd_decompress_block.c
Co-authored-by: Nick Terrell <nickrterrell@gmail.com>
Co-authored-by: Nick Terrell <nickrterrell@gmail.com>
* first attempt at fast DMS short cache
* significant wins for some scenarios
* fix all clang regressions
* nits
* fix 1.5% gcc11 regression on hot 110Kdict scenario
* fix CI
* nit
* Add tags to doublefast hash table
* use tags in doublefast DMS
* Fix CI
* Clean up some hardcoded logic / constants
* Switch forCCtx to an enum
* nit
* add short cache to ip+1 long search
* Move tag size into hashLog
* Minor nits
* Truncate dictionaries greater than 16MB in short cache mode
* Helper function for tag comparison
* Cap short cache hashLog at 24 to prevent overflow
* size_t dictTagsMatch -> int dictTagsMatch
* nit
* Clean up and comment dictionary truncation
* Move ZSTD_tableFillPurpose_e next to ZSTD_dictTableLoadMethod_e
* Comment and expand helper functions
* Asserts and documentation
* nit
This assert slows the loop down by 10x. We can get similar
coverage by asserting at the beginning & end of the loop.
We need this fix because Debian compiles zstd with asserts
enabled. Separately, we should ask them why, and if they would
consider disabling asserts in their builds. Since we don't
optimize for assert enabled builds.
Fixes Issue #3150.
We found that movemask is not used properly or consumes too much CPU.
This effort helps to optimize the movemask emulation on ARM.
For level 8-9 we saw 3-5% improvements. For level 10 we say 1.5%
improvement.
The key idea is not to use pure movemasks but to have groups of bits.
For rowEntries == 16, 32 we are going to have groups of size 4 and 2
respectively. It means that each bit will be duplicated within the group
Then we do AND to have only one bit set in the group so that iteration
with lowering bit `a &= (a - 1)` works as well.
Also, aarch64 does not have rotate instructions for 16 bit, only for 32
and 64, that's why we see more improvements for level 8-9.
vshrn_n_u16 instruction is used to achieve that: vshrn_n_u16 shifts by
4 every u16 and narrows to 8 lower bits. See the picture below. It's
also used in
[Folly](c570259008/folly/container/detail/F14Table.h (L446)).
It also uses 2 cycles according to Neoverse-N{1,2} guidelines.
64 bit movemask is already well optimized. We have ongoing experiments
but were not able to validate other implementations work reliably faster.
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).
