Optimized assembler lzma decompress

[Adam Tuja]

I stumped upon optimized assembler decompression routine that can be compiled 
under linux with nasm or gas. In 18.03 version of sdk lzma's author introduced 
x86_64 assembler version of LzmaDec_DecodeReal_3() function that makes 
decompression around 30% faster in comparison to C version. This function had 
been adapted to nasm by Peter Hyman in his `lrzip-next' project[1]. There is 
also gas version made by Conor McCarthy in his `fxz'[2] and `fast-lzma2'[3]. 
Original, masm, version can be compiled with asmc[4] and is used in new p7zip 
stage[5].

So I thought if this optimization can improve lzma decompression in those 
project it can do it as well in lzip. I have tested it (with fxz to be more 
precise) and it really is much faster (about 30%-32%). So I propose to take a 
look into it as it is bit of attention.


[1] 
https://github.com/pete4abw/lrzip-next/raw/master/src/lzma/ASM/x86/LzmaDecOpt.asm
[2] 
https://github.com/conor42/fxz/raw/master/src/liblzma/lzma/lzma_dec_x86_64.S; 
(I think this one is the most promissing as gcc/gas candidate)
[3] https://github.com/conor42/fast-lzma2/raw/master/lzma_dec_x86_64.S, 
https://github.com/conor42/fast-lzma2/raw/master/lzma_dec_x86_64.asm
[4] https://github.com/nidud/asmc
[5] https://github.com/jinfeihan57/p7zip/raw/7zip_21.02/Asm/x86/LzmaDecOpt.asm