[tor-bugs] #32053 [Applications/Tor Browser]: Tor Browser bundles based on Firefox 68 ESR are not reproducible (LLVM optimization issue)

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#32053: Tor Browser bundles based on Firefox 68 ESR are not reproducible (LLVM
optimization issue)
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 Reporter:  gk                                   |          Owner:  gk
     Type:  defect                               |         Status:
                                                 |  assigned
 Priority:  Immediate                            |      Milestone:
Component:  Applications/Tor Browser             |        Version:
 Severity:  Critical                             |     Resolution:
 Keywords:  TorBrowserTeam201911, tbb-9.0-must,  |  Actual Points:
  tbb-9.0-issues, tbb-regression, tbb-9.0.1-can  |
Parent ID:                                       |         Points:
 Reviewer:                                       |        Sponsor:
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Comment (by gk):

 Replying to [comment:47 gk]:
 > Replying to [comment:43 gk]:
 > > Okay, some status update. Bisecting gets hard. I am down to building
 stylo with Rust 1.30.0 and am at LLVM's
 b1546da0e8849d58fcdcf17fa1f2fab0cdae70a4 and have not reached the bottom
 yet.
 >
 > Building with Rust 1.29.2 I am down to LLVM's
 8c59921ca3284ced1c358c4c86ec2c830db0bd70 and still have not reached the
 bottom...
 >
 > > While exploring ways to move further here I'll investigate whether we
 can just move the `jump-threading` optimization out of `-O1`, say, into
 `-O3` so the we are actually do not use that one.
 >
 > I tried that, surprisingly enough that does not fix the issue. I
 inspected the log of the optimization passes that got run and no jump-
 threading showed up (so, that part worked). However, I still got non-
 reproducible outcomes. I gonna ask on the LLVM bug whether that's expected
 and if not whether that could give us some lead where to look closer to
 understand the bug better.

 So, it turns out the issue Alex was hitting and reporting to LLVM is a new
 determinism problem. I verified that the commit he used + the patch
 applied gives deterministic output, which is promising as we can
 theoretically bisect our way to the fix for the bug *we* are hitting.
 However, Eli over in the LLVM bug said there were some determinism related
 fixes and it might be hard to figure out which is affecting us. :(

 So, I am back with bisecting the optimization options with the patch from
 Eli applied to rule out that the jump-threading issue is affecting us.
 That takes considerable time on my machines, though, alas. :(

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/32053#comment:48>
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