which remained undetected so far,
as it requires a fairly large number of conditions to be triggered,
starting with enabling Block checksum, which is disabled by default,
and which usage is known to be extremely rare.
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.
