Preserve specialisations despite CSE

Trac #15445 showed that, as a result of CSE, a function with an
automatically generated specialisation RULE could be inlined
before the RULE had a chance to fire.

This patch attaches a NOINLINE[2] activation to the Id, during
CSE, to stop this happening.

See Note [Delay inlining after CSE]

---- Historical note ---

This patch is simpler and more direct than an earlier
version:

  commit 2110738b280543698407924a16ac92b6d804dc36
  Author: Simon Peyton Jones <simonpj@microsoft.com>
  Date:   Mon Jul 30 13:43:56 2018 +0100

  Don't inline functions with RULES too early

We had to revert this patch because it made GHC itself slower.

Why? It delayed inlining of /all/ functions with RULES, and that was
very bad in TcFlatten.flatten_ty_con_app

* It delayed inlining of liftM
* That delayed the unravelling of the recursion in some dictionary
  bindings.
* That delayed some eta expansion, leaving
     flatten_ty_con_app = \x y. let <stuff> in \z. blah
* That allowed the float-out pass to put sguff between
  the \y and \z.
* And that permanently stopped eta expasion of the function,
  even once <stuff> was simplified.

-- End of historical note ---

6 files changed, 37 insertions, 4 deletions

diff --git a/testsuite/tests/numeric/should_compile/T14465.stdout b/testsuite/tests/numeric/should_compile/T14465.stdout
index 32cf35639c..e6dfa5fc6c 100644
--- a/testsuite/tests/numeric/should_compile/T14465.stdout
+++ b/testsuite/tests/numeric/should_compile/T14465.stdout
@@ -93,7 +93,7 @@ plusOne :: Natural -> Natural
 plusOne = \ (n :: Natural) -> plusNatural n M.minusOne1
 
 -- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
-one :: Natural
+one [InlPrag=NOUSERINLINE[2]] :: Natural
 [GblId,
  Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True,
          WorkFree=True, Expandable=True,
diff --git a/testsuite/tests/numeric/should_compile/T7116.stdout b/testsuite/tests/numeric/should_compile/T7116.stdout
index 681d171350..fc0159a1ec 100644
--- a/testsuite/tests/numeric/should_compile/T7116.stdout
+++ b/testsuite/tests/numeric/should_compile/T7116.stdout
@@ -65,7 +65,7 @@ dr
       case x of { GHC.Types.D# x1 -> GHC.Types.D# (GHC.Prim.+## x1 x1) }
 
 -- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
-dl :: Double -> Double
+dl [InlPrag=NOUSERINLINE[2]] :: Double -> Double
 [GblId,
  Arity=1,
  Caf=NoCafRefs,
@@ -97,7 +97,7 @@ fr
       }
 
 -- RHS size: {terms: 1, types: 0, coercions: 0, joins: 0/0}
-fl :: Float -> Float
+fl [InlPrag=NOUSERINLINE[2]] :: Float -> Float
 [GblId,
  Arity=1,
  Caf=NoCafRefs,
diff --git a/testsuite/tests/simplCore/should_compile/T15445.hs b/testsuite/tests/simplCore/should_compile/T15445.hs
new file mode 100644
index 0000000000..36bf61dbbb
--- /dev/null
+++ b/testsuite/tests/simplCore/should_compile/T15445.hs
@@ -0,0 +1,8 @@
+module T15445 where
+
+import T15445a
+
+
+foo :: IO ()
+foo = do { print (plusTwoRec [1..10 :: Int])
+         ; print (plusTwoRec' [1..20 :: Int]) }
diff --git a/testsuite/tests/simplCore/should_compile/T15445.stderr b/testsuite/tests/simplCore/should_compile/T15445.stderr
new file mode 100644
index 0000000000..d5deac5a59
--- /dev/null
+++ b/testsuite/tests/simplCore/should_compile/T15445.stderr
@@ -0,0 +1,13 @@
+Rule fired: Class op + (BUILTIN)
+Rule fired: Class op fromInteger (BUILTIN)
+Rule fired: integerToInt (BUILTIN)
+Rule fired: SPEC plusTwoRec (T15445a)
+Rule fired: SPEC $fShow[] (GHC.Show)
+Rule fired: Class op >> (BUILTIN)
+Rule fired: Class op show (BUILTIN)
+Rule fired: SPEC plusTwoRec (T15445a)
+Rule fired: Class op enumFromTo (BUILTIN)
+Rule fired: Class op show (BUILTIN)
+Rule fired: Class op enumFromTo (BUILTIN)
+Rule fired: eftIntList (GHC.Enum)
+Rule fired: eftIntList (GHC.Enum)
diff --git a/testsuite/tests/simplCore/should_compile/T15445a.hs b/testsuite/tests/simplCore/should_compile/T15445a.hs
new file mode 100644
index 0000000000..02e5baceb5
--- /dev/null
+++ b/testsuite/tests/simplCore/should_compile/T15445a.hs
@@ -0,0 +1,10 @@
+module T15445a where
+
+{-# SPECIALIZE plusTwoRec :: [Int] -> [Int] #-}
+plusTwoRec :: Num a => [a] -> [a]
+plusTwoRec [] = []
+plusTwoRec (x:xs) = x+2:plusTwoRec xs
+
+plusTwoRec' :: Num a => [a] -> [a]
+plusTwoRec' [] = []
+plusTwoRec' (x:xs) = x+2:plusTwoRec' xs
diff --git a/testsuite/tests/simplCore/should_compile/all.T b/testsuite/tests/simplCore/should_compile/all.T
index 188f6432fa..1284b7c3d4 100644
--- a/testsuite/tests/simplCore/should_compile/all.T
+++ b/testsuite/tests/simplCore/should_compile/all.T
@@ -318,6 +318,8 @@ test('T15005', normal, compile, ['-O'])
 # we omit profiling because it affects the optimiser and makes the test fail
 test('T15056', [extra_files(['T15056a.hs']), omit_ways(['profasm'])], multimod_compile, ['T15056', '-O -v0 -ddump-rule-firings'])
 test('T15186', normal, multimod_compile, ['T15186', '-v0'])
-test('T15453', normal, compile, ['-O1'])
 test('T15517', normal, compile, ['-O0'])
 test('T15517a', normal, compile, ['-O0'])
+test('T15453', normal, compile, ['-dcore-lint -O1'])
+test('T15445', normal, multimod_compile, ['T15445', '-v0 -O -ddump-rule-firings'])
+