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This patch contains two backports for reproducibility issues found and fixed in LLVM's optimization code. The patch from bug 42574 might not be necessary in our context as I only witnessed problems when compiling Stylo with an older Rust (1.32.0) and then omptimizing with LLVM 8. However, it's hard to exclude any mismatching builds we encountered so far from being affected by that issue, so I think it is safer to backport that patch as well. Bug 43909 is an issue we and Mozilla definitely hit and very likely responsible for at least the vast majority of different builds we got over the course of the last weeks and months. Many thanks to Alex Crichton for the invaluable help in tracking this issue down.
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