StgCse: Do not re-use trivial case scrutinees

as they might be marked as one-shot, and suddenly we’d evaluate them
multiple times. This came up in #13536 (test cases included).

The solution was layed out by SPJ in ticket:13536#comment:12.

Differential Revision: https://phabricator.haskell.org/D3437

4 files changed, 20 insertions, 0 deletions

diff --git a/testsuite/tests/simplStg/should_run/T13536.hs b/testsuite/tests/simplStg/should_run/T13536.hs
new file mode 100644
index 0000000000..cf70f46163
--- /dev/null
+++ b/testsuite/tests/simplStg/should_run/T13536.hs
@@ -0,0 +1,17 @@
+import Debug.Trace
+
+newtype Id a = Id a
+
+
+unId True _ = Nothing -- make lazy
+unId False (Just (Id x)) = (Just x)
+unId False Nothing = Nothing
+{-# NOINLINE unId #-}
+
+val n = trace "evalued once, as it should" (Just (Id n))
+{-# NOINLINE val #-}
+
+foo b n = unId b (val n)
+{-# NOINLINE foo #-}
+
+main = print (foo False 1)
diff --git a/testsuite/tests/simplStg/should_run/T13536.stderr b/testsuite/tests/simplStg/should_run/T13536.stderr
new file mode 100644
index 0000000000..638b7f82c1
--- /dev/null
+++ b/testsuite/tests/simplStg/should_run/T13536.stderr
@@ -0,0 +1 @@
+evalued once, as it should
diff --git a/testsuite/tests/simplStg/should_run/T13536.stdout b/testsuite/tests/simplStg/should_run/T13536.stdout
new file mode 100644
index 0000000000..f8e0357378
--- /dev/null
+++ b/testsuite/tests/simplStg/should_run/T13536.stdout
@@ -0,0 +1 @@
+Just 1
diff --git a/testsuite/tests/simplStg/should_run/all.T b/testsuite/tests/simplStg/should_run/all.T
index 3d4f4a3763..b24da84ef2 100644
--- a/testsuite/tests/simplStg/should_run/all.T
+++ b/testsuite/tests/simplStg/should_run/all.T
@@ -10,3 +10,4 @@ def f( name, opts ):
 setTestOpts(f)
 
 test('T9291', normal, compile_and_run, [''])
+test('T13536', normal, compile_and_run, [''])