clang only claims compatibility with gcc 4.2.
Consequently, recent patch which reserved DYNAMIC_BMI2 for gcc >= 4.8
also disabled it for clang.
fix : __clang__ is now enough to enable DYNAMIC_BMI2
(associated with other existing conditions : x64/x64, !bmi2)
which was not done properly by gcc 4.8
resulting in major performance difference.
ex :
zstd -b1 silesia.tar
before : dec 680 MB/s
after : dec 710 MB/s (without bmi2)
after : dec 770 MB/s (with DYNAMIC_BMI2)
Update code documentation, and properly names a few "magic constants".
Also, HUF_compress_internal() gets a cleaner way
to determine size of tables inside workspace.
* `ZSTD_ldm_generateSequences()` generates the LDM sequences and
stores them in a table. It should work with any chunk size, but
is currently only called one block at a time.
* `ZSTD_ldm_blockCompress()` emits the pre-defined sequences, and
instead of encoding the literals directly, it passes them to a
secondary block compressor. The code to handle chunk sizes greater
than the block size is currently commented out, since it is unused.
The next PR will uncomment exercise this code.
* During optimal parsing, ensure LDM `minMatchLength` is at least
`targetLength`. Also don't emit repcode matches in the LDM block
compressor. Enabling the LDM with the optimal parser now actually improves
the compression ratio.
* The compression ratio is very similar to before. It is very slightly
different, because the repcode handling is slightly different. If I remove
immediate repcode checking in both branches the compressed size is exactly
the same.
* The speed looks to be the same or better than before.
Up Next (in a separate PR)
--------------------------
Allow sequence generation to happen prior to compression, and produce more
than a block worth of sequences. Expose some API for zstdmt to consume.
This will test out some currently untested code in
`ZSTD_ldm_blockCompress()`.
This makes it easier to edit for maintenance and evolutions
(I plan to experiment modifications in huffman decompression functions).
The methology followed seems broadly applicable to other BMI2 modules.
Performance was tracked rigorously at each step,
there is no noticeable loss (nor win) of performance compared to `#include` version.
Note however that 4X decoder variants tend to be extremely sensitive to code alignment.
This source code resulted in pretty good performance for gcc 7.2 and 7.3,
but future changes (even in other parts of the code) might trigger the issue again.
