[tor-bugs] #26475 [Applications/Tor Browser]: ESR60-based Tor Browser bundles are not built reproducibly with Stylo enabled using rustc > 1.25.0

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#26475: ESR60-based Tor Browser bundles are not built reproducibly with Stylo
enabled using rustc > 1.25.0
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 Reporter:  gk                                   |          Owner:  tbb-
                                                 |  team
     Type:  defect                               |         Status:  new
 Priority:  Immediate                            |      Milestone:
Component:  Applications/Tor Browser             |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  tbb-rbm, TorBrowserTeam201810,       |  Actual Points:
  GeorgKoppen201810                              |
Parent ID:                                       |         Points:
 Reviewer:                                       |        Sponsor:
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Comment (by alexcrichton):

 Oh sorry about this, that's a good discovery! Those settings definitely
 engage a lot of LLVM infrastructure that's not otherwise engaged, which
 could help explain why something nondeterministic is coming out the other
 end.

 The settings in `bootstrap` are pretty confusing, but what's happening
 here is either rustc is compiled with 16 codegen units (each crate turns
 into 16 object files) which are then optimized as a set with ThinLTO or
 each compiler crate is compiled into one object file and no ThinLTO is
 used.

 To clarify is the compiler's own binary nondeterministic when the
 compiler's crates are built with 16 CGUs + ThinLTO? Or is the compiler's
 binary deterministic but its output deterministic?

 In terms of impact of these settings:

 * 1 CGU builds are basically equivalent to 16 CGUs + ThinLTO. The 1 CGU
 build is slower to compile the compiler (less opportunities for
 parallelism), but is likely 1%-ish faster than the 16 CGUs + ThinLTO.
 * The 16 CGUs + ThinLTO is the default setting (as you've found)
 * If 16 CGUs are used without ThinLTO, the resulting compiler is probably
 horrendously slow (lots of missed inlining opportunities).

 Or put another way, if you disable ThinLTO you'll want to be sure to also
 compile with one codegen unit, which should happen in rustbuild via the
 above block.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/26475#comment:45>
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