Zap usage info in CSE (Trac #10218)

Trac #10218 reports a subtle bug that turned out to be:

- CSE invalidated the usage information computed
  by earlier demand analysis, by increasing sharing

- that made a single-entry thunk into a multi-entry thunk

- and with -feager-blackholing, that led to <<loop>>

The patch fixes it by making the CSE pass zap usage information for
let-bound identifiers.   It can be restored by -flate-dmd-anal.

(But making -flate-dmd-anal the default needs some careful work;
see Trac #7782.)

3 files changed, 20 insertions, 0 deletions

diff --git a/testsuite/tests/stranal/should_run/T10218.hs b/testsuite/tests/stranal/should_run/T10218.hs
new file mode 100644
index 0000000000..572c6fdebf
--- /dev/null
+++ b/testsuite/tests/stranal/should_run/T10218.hs
@@ -0,0 +1,18 @@
+{-# OPTIONS_GHC -feager-blackholing #-}
+
+module Main where
+
+{-# NOINLINE foo #-}
+foo :: Bool -> Int -> Int -> Int
+foo True  _ x = 1
+foo False _ x = x+1
+
+{-# NOINLINE bar #-}
+bar :: Int -> (Int,Int)
+bar x = let y1 = x * 2
+            y2 = x * 2
+        in (foo False y1 y2,foo False y2 y1)
+
+main = print (fst p + snd p)
+  where
+    p = bar 3
diff --git a/testsuite/tests/stranal/should_run/T10218.stdout b/testsuite/tests/stranal/should_run/T10218.stdout
new file mode 100644
index 0000000000..8351c19397
--- /dev/null
+++ b/testsuite/tests/stranal/should_run/T10218.stdout
@@ -0,0 +1 @@
+14
diff --git a/testsuite/tests/stranal/should_run/all.T b/testsuite/tests/stranal/should_run/all.T
index 7f64f85a11..8a82ce86a5 100644
--- a/testsuite/tests/stranal/should_run/all.T
+++ b/testsuite/tests/stranal/should_run/all.T
@@ -9,3 +9,4 @@ test('T2756b', normal, compile_and_run, [''])
 test('T7649', normal, compile_and_run, [''])
 test('T9254', normal, compile_and_run, [''])
 test('T10148', normal, compile_and_run, [''])
+test('T10218', normal, compile_and_run, [''])