it's speed optimization as the inner code also checks that previous hash value
is not same as new hash value. Essentially those two checks together makes the
compression a little more efficient as it can remember matches further apart.
As far as I remember from my tests, the secondary path was triggered only twice
in very long uncompressed file, but the gain in compression rate was still noticeable.
The advantage of this over hidden is for example that the compiler can
safely assume that pointers to functions declared internal can never be
passed externally. This allows the compiler to consider optimizations
otherwise impossible.
- select the CMAKE_BUILD_TYPE "Release" by default if none has been set,
to ensure maximum generic optimisation possible on the host platform
- add WITH_NATIVE_INSTRUCTIONS to build with -march=native or its equivalent
option with other compilers (when we identify those alternatives)
- NATIVEFLAG (-march=native) will be used instead of -msseN/-mpclmul when
defined/requested
TODO: discuss whether -msseN/-mpclmul should be used only for the files that
need them instead of globally, while NATIVEFLAG can (is supposed to) be used
globally.
The checks currently assume that instructions that build also execute.
This is not necessarily true: building with -msse4 on an AMD CPU (a C60)
that only has SSE4a leads to a crash in deflateInit2 when the compiler
apparently uses an unsupported instruction to set
s->hash_bits = memLevel + 7;
By storing whether or not a block has been opened (or terminated), the
static trees used for the block and the end block markers can be emitted
appropriately.
When using deflate_quick() in a streaming fashion and the output buffer
runs out of space while the input buffer still has data, deflate_quick()
would emit partial symbols. Force the deflate_quick() loop to terminate
for a flush before any further processing is done, returning to the main
deflate() routine to do its thing.
This permits deflateParams to change the strategy and level right
after deflateInit, without having to wait until a header has been
written. The parameters can be changed immediately up until the
first deflate call that consumes any input data.
This avoids unnecessary filling of bytes in the sliding window
buffer when switching from level zero to a non-zero level. This
also provides a consistent indication of deflate having taken
input for a later commit ...
