Commits on Sep 30, 2017

1. rustc: Enable LTO and multiple codegen units …

   This commit is a refactoring of the LTO backend in Rust to support compilations
   with multiple codegen units. The immediate result of this PR is to remove the
   artificial error emitted by rustc about `-C lto -C codegen-units-8`, but longer
   term this is intended to lay the groundwork for LTO with incremental compilation
   and ultimately be the underpinning of ThinLTO support.
   
   The problem here that needed solving is that when rustc is producing multiple
   codegen units in one compilation LTO needs to merge them all together.
   Previously only upstream dependencies were merged and it was inherently relied
   on that there was only one local codegen unit. Supporting this involved
   refactoring the optimization backend architecture for rustc, namely splitting
   the `optimize_and_codegen` function into `optimize` and `codegen`. After an LLVM
   module has been optimized it may be blocked and queued up for LTO, and only
   after LTO are modules code generated.
   
   Non-LTO compilations should look the same as they do today backend-wise, we'll
   spin up a thread for each codegen unit and optimize/codegen in that thread. LTO
   compilations will, however, send the LLVM module back to the coordinator thread
   once optimizations have finished. When all LLVM modules have finished optimizing
   the coordinator will invoke the LTO backend, producing a further list of LLVM
   modules. Currently this is always a list of one LLVM module. The coordinator
   then spawns further work to run LTO and code generation passes over each module.
   
   In the course of this refactoring a number of other pieces were refactored:
   
   * Management of the bytecode encoding in rlibs was centralized into one module
     instead of being scattered across LTO and linking.
   * Some internal refactorings on the link stage of the compiler was done to work
     directly from `CompiledModule` structures instead of lists of paths.
   * The trans time-graph output was tweaked a little to include a name on each
     bar and inflate the size of the bars a little

   

   alexcrichton committed Sep 30, 2017
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