diff options
| -rw-r--r-- | src/xz/xz.1 | 66 |
1 files changed, 29 insertions, 37 deletions
diff --git a/src/xz/xz.1 b/src/xz/xz.1 index 5e11a332..62bab507 100644 --- a/src/xz/xz.1 +++ b/src/xz/xz.1 @@ -1674,14 +1674,16 @@ and \fB\-\-x86\fR[\fB=\fIoptions\fR] .PD 0 .TP -\fB\-\-powerpc\fR[\fB=\fIoptions\fR] -.TP -\fB\-\-ia64\fR[\fB=\fIoptions\fR] -.TP \fB\-\-arm\fR[\fB=\fIoptions\fR] .TP \fB\-\-armthumb\fR[\fB=\fIoptions\fR] .TP +\fB\-\-arm64\fR[\fB=\fIoptions\fR] +.TP +\fB\-\-powerpc\fR[\fB=\fIoptions\fR] +.TP +\fB\-\-ia64\fR[\fB=\fIoptions\fR] +.TP \fB\-\-sparc\fR[\fB=\fIoptions\fR] .PD Add a branch/call/jump (BCJ) filter to the filter chain. @@ -1690,7 +1692,7 @@ in the filter chain. .IP "" A BCJ filter converts relative addresses in the machine code to their absolute counterparts. -This doesn't change the size of the data, +This doesn't change the size of the data but it increases redundancy, which can help LZMA2 to produce 0\(en15\ % smaller .B .xz @@ -1699,21 +1701,8 @@ The BCJ filters are always reversible, so using a BCJ filter for wrong type of data doesn't cause any data loss, although it may make the compression ratio slightly worse. -.IP "" -It is fine to apply a BCJ filter on a whole executable; -there's no need to apply it only on the executable section. -Applying a BCJ filter on an archive that contains both executable -and non-executable files may or may not give good results, -so it generally isn't good to blindly apply a BCJ filter when -compressing binary packages for distribution. -.IP "" -These BCJ filters are very fast and -use insignificant amount of memory. -If a BCJ filter improves compression ratio of a file, -it can improve decompression speed at the same time. -This is because, on the same hardware, -the decompression speed of LZMA2 is roughly -a fixed number of bytes of compressed data per second. +The BCJ filters are very fast and +use an insignificant amount of memory. .IP "" These BCJ filters have known problems related to the compression ratio: @@ -1722,24 +1711,24 @@ the compression ratio: Some types of files containing executable code (for example, object files, static libraries, and Linux kernel modules) have the addresses in the instructions filled with filler values. -These BCJ filters will still do the address conversion, +These BCJ filters (except ARM64) will still do the address conversion, which will make the compression worse with these files. +The ARM64 filter doesn't have this problem. .IP \(bu 3 -Applying a BCJ filter on an archive containing multiple similar -executables can make the compression ratio worse than not using -a BCJ filter. -This is because the BCJ filter doesn't detect the boundaries -of the executable files, and doesn't reset -the address conversion counter for each executable. +If a BCJ filter is applied on an archive, +it is possible that it makes the compression ratio +worse than not using a BCJ filter. +For example, if there are similar or even identical executables +then filtering will likely make the files less similar +and thus compression is worse. +The contents of non-executable files in the same archive can matter too. +In practice one has to try with and without a BCJ filter to see +which is better in each situation. .RE .IP "" -Both of the above problems will be fixed -in the future in a new filter. -The old BCJ filters will still be useful in embedded systems, -because the decoder of the new filter will be bigger -and use more memory. -.IP "" Different instruction sets have different alignment: +the executable file must be aligned to a multiple of +this value in the input data to make the filter work. .RS .RS .PP @@ -1749,11 +1738,12 @@ l n l l n l. Filter;Alignment;Notes x86;1;32-bit or 64-bit x86 +ARM;4; +ARM-Thumb;2; +ARM64;4;4096-byte alignment is best PowerPC;4;Big endian only -ARM;4;Little endian only -ARM-Thumb;2;Little endian only -IA-64;16;Big or little endian -SPARC;4;Big or little endian +IA-64;16;Itanium +SPARC;4; .TE .RE .RE @@ -1764,6 +1754,8 @@ the LZMA2 options are set to match the alignment of the selected BCJ filter. For example, with the IA-64 filter, it's good to set .B pb=4 +or even +.B pb=4,lp=4,lc=0 with LZMA2 (2^4=16). The x86 filter is an exception; it's usually good to stick to LZMA2's default |