Commits on Jul 10, 2015

1. trans: Use LLVM's writeArchive to modify archives …

   We have previously always relied upon an external tool, `ar`, to modify archives
   that the compiler produces (staticlibs, rlibs, etc). This approach, however, has
   a number of downsides:
   
   * Spawning a process is relatively expensive for small compilations
   * Encoding arguments across process boundaries often incurs unnecessary overhead
     or lossiness. For example `ar` has a tough time dealing with files that have
     the same name in archives, and the compiler copies many files around to ensure
     they can be passed to `ar` in a reasonable fashion.
   * Most `ar` programs found do **not** have the ability to target arbitrary
     platforms, so this is an extra tool which needs to be found/specified when
     cross compiling.
   
   The LLVM project has had a tool called `llvm-ar` for quite some time now, but it
   wasn't available in the standard LLVM libraries (it was just a standalone
   program). Recently, however, in LLVM 3.7, this functionality has been moved to a
   library and is now accessible by consumers of LLVM via the `writeArchive`
   function.
   
   This commit migrates our archive bindings to no longer invoke `ar` by default
   but instead make a library call to LLVM to do various operations. This solves
   all of the downsides listed above:
   
   * Archive management is now much faster, for example creating a "hello world"
     staticlib is now 6x faster (50ms => 8ms). Linking dynamic libraries also
     recently started requiring modification of rlibs, and linking a hello world
     dynamic library is now 2x faster.
   * The compiler is now one step closer to "hassle free" cross compilation because
     no external tool is needed for managing archives, LLVM does the right thing!
   
   This commit does not remove support for calling a system `ar` utility currently.
   We will continue to maintain compatibility with LLVM 3.5 and 3.6 looking forward
   (so the system LLVM can be used wherever possible), and in these cases we must
   shell out to a system utility. All nightly builds of Rust, however, will stop
   needing a system `ar`.

   

   alexcrichton committed Jul 10, 2015
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