Stress test for nested module hierarchies

I'm optimizing a case that is well approximated by
multiple layers of modules where every module in a layer
imports all the modules in the layer below.

It turns out I regressed performance on such cases in 7fea7121.
I'm adding a test case to track improvements and prevent
future regressions.

Test Plan: ./validate

Reviewers: simonmar, austin, bgamari

Reviewed By: simonmar

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3575

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/deSugar/DsUsage.hs b/compiler/deSugar/DsUsage.hs
index 25d8254f55..8158a8e122 100644
--- a/compiler/deSugar/DsUsage.hs
+++ b/compiler/deSugar/DsUsage.hs
@@ -16,7 +16,7 @@ import Module
 import Outputable
 import Util
 import UniqSet
-import UniqDFM
+import UniqFM
 import Fingerprint
 import Maybes
 
@@ -37,7 +37,7 @@ mkDependencies
  = do
       -- Template Haskell used?
       th_used <- readIORef th_var
-      let dep_mods = eltsUDFM (delFromUDFM (imp_dep_mods imports)
+      let dep_mods = modDepsElts (delFromUFM (imp_dep_mods imports)
                                            (moduleName mod))
                 -- M.hi-boot can be in the imp_dep_mods, but we must remove
                 -- it before recording the modules on which this one depends!