When I did FAST_DEC_LOOP performance test, I found the
offset check is much more than v1.8.3
You will see the condition check difference via lzbench with dickens test case.
v1.8.3 34959
v.1.9.x 1055885
After investigate the code, we could see the difference.
v.1.8.3 SKIP the condition check if
if condition is true in:
https://github.com/lz4/lz4/blob/v1.8.3/lib/lz4.c#L1463
AND below condition is true
https://github.com/lz4/lz4/blob/v1.8.3/lib/lz4.c#L1478\
The offset check should be invoked.
v1.9.3
The offset check code will be invoked in every loop which lead to downgrade.
So the fix would be move this check to specific condition
to avoid useless condition check.
After this change, the call number is same as v1.8.3
and deprecate LZ4_decompress_fast(),
with deprecation warnings enabled by default.
Note that, as a consequence of the fix,
LZ4_decompress_fast is now __slower__ than LZ4_decompress_safe().
That's because, since it doesn't know the input buffer size,
it must progress more cautiously into the input buffer
to ensure to out-of-bound read.
between lz4.c and lz4hc.c .
was left in a strange state after the "amalgamation" patch.
Now only 3 directives remain,
same name across both implementations,
single definition place.
Might allow some light simplification due to reduced nb of states possible.
when one block was not compressible,
it would tag the context as `dirty`,
resulting in compression automatically bailing out of all future blocks,
making the rest of the frame uncompressible.
since Visual 2017,
worries about potential overflow, which are actually impossible.
Replaced (c * a) by (c ? a : 0).
Will likely replaced a * by a cmov.
Probably harmless for performance.
make it possible to generate LZ4-compressed block
with a controlled maximum offset (necessarily <= 65535).
This could be useful for compatibility with decoders
using a very limited memory budget (<64 KB).
Answer #154
it fails on x86 32-bit mode :
Visual reports an alignment of 8-bytes (even with alignof())
but actually only align LZ4_stream_t on 4 bytes.
The alignment check then fails, resulting in missed initialization.
- promoted LZ4_resetStream_fast() to stable
- moved LZ4_resetStream() into deprecate, but without triggering a compiler warning
- update all sources to no longer rely on LZ4_resetStream()
note : LZ4_initStream() proposal is slightly different :
it's able to initialize any buffer, provided that it's large enough.
To this end, it accepts a void*, and returns an LZ4_stream_t*.
For small offsets of size 1, 2, 4 and 8, we can set a single uint64_t,
and then use it to do a memset() variation. In particular, this makes
the somewhat-common RLE (offset 1) about 2-4x faster than the previous
implementation - we avoid not only the load blocked by store, but also
avoid the loads entirely.
Generally we want our wildcopy loops to look like the
memcpy loops from our libc, but without the final byte copy checks.
We can unroll a bit to make long copies even faster.
The only catch is that this affects the value of FASTLOOP_SAFE_DISTANCE.
We've already checked that we are more than FASTLOOP_SAFE_DISTANCE
away from the end, so this branch can never be true, we will have
already jumped to the second decode loop.
Use LZ4_wildCopy16 for variable-length literals. For literal counts that
fit in the flag byte, copy directly. We can also omit oend checks for
roughly the same reason as the previous shortcut: We check once that both
match length and literal length fit in FASTLOOP_SAFE_DISTANCE, including
wildcopy distance.
Add an LZ4_wildCopy16, that will wildcopy, potentially smashing up
to 16 bytes, and use it for match copy. On x64, this avoids many
blocked loads due to store forwarding, similar to issue #411.
Copy the main loop, and change checks such that op is always less
than oend-SAFE_DISTANCE. Currently these are added for the literal
copy length check, and for the match copy length check.
Otherwise the first loop is exactly the same as the second. Follow on
diffs will optimize the first copy loop based on this new requirement.
I also tried instead making a separate inlineable function for the copy
loop (similar to existing partialDecode flags, etc), but I think the
changes might be significant enough to warrent doubling the code, instead
pulling out common functionality to separate functions.
This is the basic transformation that will allow several following optimisations.
Dictionaries don't need to be > 4 bytes, they need to be >= 4 bytes. This test
was overly conservative.
Also removes the test in `LZ4_attach_dictionary()`.
Fixes a mismatch in behavior between loading into the context (via
`LZ4_loadDict()`) a very small (<= 4 bytes) non-contiguous dictionary, versus
attaching it with `LZ4_attach_dictionary()`.
Before this patch, this divergence could be reproduced by running
```
make -C tests fuzzer MOREFLAGS="-m32"
tests/fuzzer -v -s1239 -t3146
```
Making sure these two paths behave exactly identically is an easy way to test
the correctness of the attach path, so it's desirable that this remain an
unpolluted, high signal test.
so "funny" thing with cppcheck
is that no 2 versions give the same list of warnings.
On Mac, I'm using v1.81, which had all warnings fixed.
On Travis CI, it's v1.61, and it complains about a dozen more/different things.
On Linux, it's v1.72, and it finds a completely different list of a half dozen warnings.
Some of these seems to be bugs/limitations in cppcheck itself.
The TravisCI version v1.61 seems unable to understand %zu correctly, and seems to assume it means %u.
