Description: The LZMA2 (.xz) compression format supports normalization filters for some data types, which improve compression efficiency. One such filter is the x86 branch-call-jump filter, which converts relative jump addresses to absolute addresses (both i386 and x86_64). This operation is designed to be always reversible, so there's no risk of compressing non-executable data with this flag.

Since most Arch packages contian executables, this translates into improved compression ratios. In the worst case -- no executable code -- it can increase the file size slightly, but that increase is usually very small.

All that's needed to enable this is to call xz with two extra arguments: --x86 --lzma2 (in this order)

I did a small test to compare the compression ratios and speed. Eclipse here is a worst-case package that contains no x86 code, which increased 0.01% in size. chromium decreased by 4.7%, firefox 2.9%, libreoffice-common by 6.3% and libreoffice-writer by 12.9% (!)

-rw-r--r-- 1 marti marti 25742360 Feb 17 14:11 chromium-17.0.963.56-1-x86_64.pkg.tar.xz
-rw-r--r-- 1 marti marti 24520560 Feb 17 14:11 chromium-17.0.963.56-1-x86_64.pkg.tar.xz.x86
-rw-r--r-- 1 marti marti 177386192 Feb 17 15:16 eclipse-3.7.1-2-x86_64.pkg.tar.xz
-rw-r--r-- 1 marti marti 177402980 Feb 17 15:16 eclipse-3.7.1-2-x86_64.pkg.tar.xz.x86
-rw-r--r-- 1 marti marti 13895816 Feb 17 14:11 firefox-10.0.1-1-x86_64.pkg.tar.xz
-rw-r--r-- 1 marti marti 13498660 Feb 17 14:11 firefox-10.0.1-1-x86_64.pkg.tar.xz.x86
-rw-r--r-- 1 marti marti 63470324 Feb 17 14:14 libreoffice-common-3.5.0-1-x86_64.pkg.tar.xz
-rw-r--r-- 1 marti marti 59469704 Feb 17 14:14 libreoffice-common-3.5.0-1-x86_64.pkg.tar.xz.x86
-rw-r--r-- 1 marti marti 1832272 Feb 17 14:14 libreoffice-writer-3.5.0-1-x86_64.pkg.tar.xz
-rw-r--r-- 1 marti marti 1596652 Feb 17 14:14 libreoffice-writer-3.5.0-1-x86_64.pkg.tar.xz.x86

The timings show a similar pattern. Packages with x86 code now compress faster, though the difference is less significant. In the worst case, Eclipse compresses 1.3% slower.

time xz --keep --x86 --lzma2 -S .xz.x86 chromium-17.0.963.56-1-x86_64.pkg.tar
52,03s user 0,20s system 99% cpu 52,301 total
time xz --keep --lzma2 -S .xz chromium-17.0.963.56-1-x86_64.pkg.tar
53,04s user 0,16s system 99% cpu 53,231 total
time xz --keep --x86 --lzma2 -S .xz.x86 firefox-10.0.1-1-x86_64.pkg.tar
20,39s user 0,12s system 99% cpu 20,603 total
time xz --keep --lzma2 -S .xz firefox-10.0.1-1-x86_64.pkg.tar
20,54s user 0,06s system 99% cpu 20,646 total
time xz --keep --x86 --lzma2 -S .xz.x86 libreoffice-common-3.5.0-1-x86_64.pkg.tar
113,03s user 0,30s system 99% cpu 1:53,75 total
time xz --keep --lzma2 -S .xz libreoffice-common-3.5.0-1-x86_64.pkg.tar
115,90s user 0,33s system 99% cpu 1:57,02 total
time xz --keep --x86 --lzma2 -S .xz.x86 libreoffice-writer-3.5.0-1-x86_64.pkg.tar
3,43s user 0,02s system 99% cpu 3,478 total
time xz --keep --lzma2 -S .xz libreoffice-writer-3.5.0-1-x86_64.pkg.tar
3,45s user 0,03s system 99% cpu 3,483 total
time xz --keep --x86 --lzma2 -S .xz.x86 eclipse-3.7.1-2-x86_64.pkg.tar
78,05s user 0,72s system 99% cpu 1:19,35 total
time xz --keep --lzma2 -S .xz eclipse-3.7.1-2-x86_64.pkg.tar
77,04s user 0,50s system 98% cpu 1:18,56 total