`CFLAGS=-O0 make`
will now use `-O0` instead of enforcing `-O3`
which used to be the behavior before introduction of `libzstd.mk`.
This should result in faster tests,
since a few tests depend on this capability for faster roundtrips.
since this is effectively what is stored in this field (== matchLength - MINMATCH).
This makes it clearer what needs to be done when reading from / writing to this field.
the variable has only very limited usage,
being only used once at the beginning of the block for prefetching only,
hence the error had no impact on compression ratio.
This saves some 1.7Kb in rodata section (x86_64, zstd tool),
while assembler code stays the same except
the type of a few load/extend instructions.
Should not have negative performance implications.
mostly for maintenance convenience.
Performance wise, there is very little change,
slightly faster for slog 3 & 4,
neutral or very slightly negative for slot 5 & 6.
I couldn't find a good way to spread `ip0` and `ip1` apart when we accelerate
due to incompressible inputs. (The methods I tried slowed things down quite a
bit.)
Since we aren't splaying ip0 and ip1 apart (which would be like `0_1_2_3_`, as
opposed to the `01__23__` we were actually doing), it's a big ambitious to
increment `step` by 2. Instead, let's increment it by 1, which has the benefit
sliiightly improving compression. Speed remains pretty much unchanged.
The position updates are rewritten from `ip[N] = ip[N-1] + step` to be
`ip[N] = ip[N-2] + step`. This lets us only deal with the asymmetric spacing
of gaps at setup and then we only have to keep a single `step` variable.
This seems to work quite well on GCC and Clang!
This replicates the behavior of @terrelln's `ZSTD_fast` implementation. That
is, it always looks at adjacent pairs of positions, and only applies the
acceleration every other position. This produces a more fine-grained
acceleration.
Yann Collet