Commits on Sep 15, 2017

1. rustc: Preallocate when building the dep graph …

   This commit alters the `query` function in the dep graph module to preallocate
   memory using `with_capacity` instead of relying on automatic growth. Discovered
   in rust-lang#44576 it was found that for the syntex_syntax clean incremental benchmark
   the peak memory usage was found when the dep graph was being saved, particularly
   the `DepGraphQuery` data structure itself. PRs like rust-lang#44142 which add more
   queries end up just making this much larger!
   
   I didn't see an immediately obvious way to reduce the size of the
   `DepGraphQuery` object, but it turns out that `with_capacity` helps quite a bit!
   Locally 831 MB was used [before] this commit, and 770 MB is in use at the peak
   of the compiler [after] this commit. That's a nice 7.5% improvement! This won't
   quite make up for the losses in rust-lang#44142 but I figured it's a good start.
   
   [before]: https://gist.github.com/alexcrichton/2d2b9c7a65503761925c5a0bcfeb0d1e
   [before]: https://gist.github.com/alexcrichton/6da51f2a6184bfb81694cc44f06deb5b

   

   alexcrichton committed Sep 15, 2017
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