[tor-bugs] #15710 [Tor Browser]: Consider enabling the BCJ filter when packaging bundles.

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#15710: Consider enabling the BCJ filter when packaging bundles.
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 Reporter:  yawning                  |          Owner:  tbb-team
     Type:  enhancement              |         Status:  new
 Priority:  minor                    |      Milestone:
Component:  Tor Browser              |        Version:
 Keywords:  xz, bundle size, gitian  |  Actual Points:
Parent ID:                           |         Points:
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 xz (and xz embedded) supports branch/call/jump filters that is supposed to
 reduce compressed executable size.  The documentation hints that the
 compression ratio can end up being worse in certain situations, but as far
 as I can tell, it helps.

 Numbers (tested with `tor-browser-linux64-4.5a5_en-US.tar.xz`):

  * Current settings: 43667412 (1.0)
  * `xz -9`: 43463244 (0.99)
  * `xz --x86 --lzma2`: 42621816 (0.976)
  * `xz --x86 --lzma2=preset=9`: 42402308 (0.971)

 The "extreme" setting doesn't help the bundle compression enough to be
 worth it, and it would increase the already long build time even further.

 The improvement isn't absolutely massive, but ~1 MiB is ~1 MiB.  Since we
 only currently build x86(_64) bundles, adding `export XZ_OPT="--x86
 --lzma2"` or similar to the top of `dtar.sh` should be sufficient, adjust
 presets to taste.

 (There is also an Little Endian ARM BCJ filter, along with filters for
 architectures that we don't care about.)

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/15710>
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