Commits on Feb 12, 2018

1. rustc: Persist LLVM's Linker in Fat LTO …

   This commit updates our Fat LTO logic to tweak our custom wrapper around LLVM's
   "link modules" functionality. Previously whenever the
   `LLVMRustLinkInExternalBitcode` function was called it would call LLVM's
   `Linker::linkModules` wrapper. Internally this would crate an instance of a
   `Linker` which internally creates an instance of an `IRMover`. Unfortunately for
   us the creation of `IRMover` is somewhat O(n) with the input module. This means
   that every time we linked a module it was O(n) with respect to the entire module
   we had built up!
   
   Now the modules we build up during LTO are quite large, so this quickly started
   creating an O(n^2) problem for us! Discovered in rust-lang#48025 it turns out this has
   always been a problem and we just haven't noticed it. It became particularly
   worse recently though due to most libraries having 16x more object files than
   they previously did (1 -> 16).
   
   This commit fixes this performance issue by preserving the `Linker` instance
   across all links into the main LLVM module. This means we only create one
   `IRMover` and allows LTO to progress much speedier.
   
   From the `cargo-cache` project in rust-lang#48025 a **full build** locally when from
   5m15s to 2m24s. Looking at the timing logs each object file was linked in in
   single-digit millisecond rather than hundreds, clearly being a nice improvement!
   
   Closes rust-lang#48025

   

   alexcrichton committed Feb 12, 2018
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