Let the simplifier know that seq# forces

Add a special case in `simplAlt` to record that the result of
`seq#` is in WHNF.

Reviewers: simonmar, bgamari, simonpj

Reviewed By: simonpj

Subscribers: simonpj, rwbarton, thomie, carter

GHC Trac Issues: #15226

Differential Revision: https://phabricator.haskell.org/D4796

2 files changed, 40 insertions, 0 deletions

diff --git a/testsuite/tests/perf/should_run/T15226.hs b/testsuite/tests/perf/should_run/T15226.hs
new file mode 100644
index 0000000000..4c09114b89
--- /dev/null
+++ b/testsuite/tests/perf/should_run/T15226.hs
@@ -0,0 +1,30 @@
+-- T15226
+import Control.Exception (evaluate)
+
+-- Just in case Prelude.repeat changes for some reason.
+import Prelude hiding (repeat)
+
+-- We want to be sure that the compiler *doesn't* know that
+-- all the elements of the list are in WHNF, because if it
+-- does, PrelRules may erase the seq#'s altogether.
+repeat :: a -> [a]
+repeat a = res
+  where res = a : res
+{-# NOINLINE repeat #-}  -- Belt *and* suspenders
+
+silly :: [Int] -> IO ()
+silly = foldr go (pure ())
+  where
+    go x r = do
+      x' <- evaluate x
+      evaluate (x' + 3)  -- GHC should know that x' has been evaluated,
+                         -- so this calculation will be erased entirely.
+                         -- Otherwise, we'll create a thunk to pass to
+                         -- evaluate.
+      r
+
+main :: IO ()
+-- 10,000,000 repetitions take only a twentieth of a second,
+-- but allocations go up dramatically if the result is not
+-- known evaluated.
+main = silly $ take 10000000 $ repeat 1
diff --git a/testsuite/tests/perf/should_run/all.T b/testsuite/tests/perf/should_run/all.T
index 9fd997f633..b248dd56f7 100644
--- a/testsuite/tests/perf/should_run/all.T
+++ b/testsuite/tests/perf/should_run/all.T
@@ -574,3 +574,13 @@ test('T14936',
                        (wordsize(64), 51792, 5) ])],
      compile_and_run,
      ['-O2'])
+
+test('T15226',
+    [stats_num_field('bytes allocated',
+                    [ (wordsize(64), 41040, 5) ]),
+                    # 2018-06-06   41040  Let the simplifier know the result
+                    #                     of seq# is in WHNF
+                    # initial  400041040
+     only_ways(['normal'])],
+    compile_and_run,
+    ['-O'])