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