GCC docs promise that it works and a few other compilers do
too. Clang/LLVM is documented source code only but unsurprisingly
it behaves the same as others on x86-64 at least. But the
certainly-portable way is good enough here so use that.
The x32 port has a x86-64 ABI in term of all registers but uses only
32bit pointer like x86-32. The assembly optimisation fails to compile on
x32. Given the state of x32 I suggest to exclude it from the
optimisation rather than trying to fix it.
Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
It's used only for basic bittrees and fixed-size reverse bittree
because those showed a clear benefit on x86-64 with GCC and Clang.
The other methods were more mixed and thus are commented out but
they should be tested on other archs.
Now extra buffer space is reserved so that repeating bytes for
any single match will never need to copy from two places (both
the beginning and the end of the buffer). This simplifies
dict_repeat() and helps a little with speed.
This seems to reduce .lzma decompression time about 2 %, so
with .xz and CRC it could be slightly less. The small things
add up still.
It's not completely obvious if this is better in the decoder.
It should be good if compiler can avoid creating a branch
(like using CMOV on x86).
This also makes lzma_encoder.c use the new macros.
The new decoder resumes the first decoder loop in the Resumable mode.
Then, the code executes in Non-resumable mode until it detects that it
cannot guarantee to have enough input/output to decode another symbol.
The Resumable mode is how the decoder has always worked. Before decoding
every input bit, it checks if there is enough space and will save its
location to be resumed later. When the decoder has more input/output,
it jumps back to the correct sequence in the Resumable mode code.
When the input/output buffers are large, the Resumable mode is much
slower than the Non-resumable because it has more branches and is harder
for the compiler to optimize since it is in a large switch block.
Early benchmarking shows significant time improvement (8-10% on gcc and
clang x86) by using the Non-resumable code as much as possible.
The new "safe" range decoder mode is the same as old range decoder, but
now the default behavior of the range decoder will not check if there is
enough input or output to complete the operation. When the buffers are
close to fully consumed, the "safe" operations must be used instead. This
will improve speed because it will reduce the number of branches needed
for most of the range decoder operations.
The footer template from Doxygen has the closing </body> </html>
as Doxygen doesn't add them otherwise.
target="_blank" was omitted as it's not useful here but
it can be slightly annoying as one cannot just go back
in the browser history.
Since the footer links to the license file in the same
directory and not to CC website, the rel attributes
can be omitted.
