Commits on Sep 26, 2017

1. rustc: Default 32 codegen units at O0 …

   This commit changes the default of rustc to use 32 codegen units when compiling
   in debug mode, typically an opt-level=0 compilation. Since their inception
   codegen units have matured quite a bit, gaining features such as:
   
   * Parallel translation and codegen enabling codegen units to get worked on even
     more quickly.
   * Deterministic and reliable partitioning through the same infrastructure as
     incremental compilation.
   * Global rate limiting through the `jobserver` crate to avoid overloading the
     system.
   
   The largest benefit of codegen units has forever been faster compilation through
   parallel processing of modules on the LLVM side of things, using all the cores
   available on build machines that typically have many available. Some downsides
   have been fixed through the features above, but the major downside remaining is
   that using codegen units reduces opportunities for inlining and optimization.
   This, however, doesn't matter much during debug builds!
   
   In this commit the default number of codegen units for debug builds has been
   raised from 1 to 32. This should enable most `cargo build` compiles that are
   bottlenecked on translation and/or code generation to immediately see speedups
   through parallelization on available cores.
   
   Work is being done to *always* enable multiple codegen units (and therefore
   parallel codegen) but it requires rust-lang#44841 at least to be landed and stabilized,
   but stay tuned if you're interested in that aspect!

   

   alexcrichton committed Sep 26, 2017
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