Commits on May 12, 2018

1. Set PrepareForThinLTO flag when using ThinLTO …

   The LLVM PassManager has a PrepareForThinLTO flag, which is intended
   when compilation occurs in conjunction with linking by ThinLTO. The
   flag has two effects:
   
    * The NameAnonGlobal pass is run after all other passes, which
      ensures that all globals have a name.
    * In optimized builds, a number of late passes (mainly related to
      vectorization and unrolling) are disabled, on the rationale that
      these a) will increase codesize of the intermediate artifacts
      and b) will be run by ThinLTO again anyway.
   
   This patch enables the use of PrepareForThinLTO if Thin or ThinLocal
   linking is used.
   
   The background for this change is the CI failure in rust-lang#49479, which
   we assume to be caused by the NameAnonGlobal pass not being run.
   As this changes which passes LLVM runs, this might have performance
   (or other) impact, so we want to land this separately.

   

   nikic committed May 12, 2018
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