Fix over-eager inlining in SimpleOptwip/T19347

In GHC.Core.SimpleOpt, I found that its inlining could duplicate
an arbitary redex inside a lambda!  Consider (\xyz. x+y).  The
occurrence-analysis treats the lamdda as a group, and says that
both x and y occur once, even though the occur under the lambda-z.
See Note [Occurrence analysis for lambda binders] in OccurAnal.

When the lambda is under-applied in a call, the Simplifier is
careful to zap the occ-info on x,y, because they appear under the \z.
(See the call to zapLamBndrs in simplExprF1.)  But SimpleOpt
missed this test, resulting in #19347.

So this patch
* commons up the binder-zapping in GHC.Core.Utils.zapLamBndrs.
* Calls this new function from GHC.Core.Opt.Simplify
* Adds a call to zapLamBndrs to GHC.Core.SimpleOpt.simple_app

This change makes test T12990 regress somewhat, but it was always
very delicate, so I'm going to put up with that.

In this voyage I also discovered a small, rather unrelated infelicity
in the Simplifier:

* In GHC.Core.Opt.Simplify.simplNonRecX we should apply isStrictId
  to the OutId not the InId. See Note [Dark corner with levity polymorphism]

It may never "bite", because SimpleOpt should have inlined all
the levity-polymorphic compulsory inlnings already, but somehow
it bit me at one point and it's generally a more solid thing
to do.

Fixing the main bug increases runtime allocation in test
perf/should_run/T12990, for (acceptable) reasons explained in a
comement on

Metric Increase:
    T12990

1 files changed, 29 insertions, 4 deletions

diff --git a/compiler/GHC/Core/Utils.hs b/compiler/GHC/Core/Utils.hs
index a67955ad2b..fcffaa553a 100644
--- a/compiler/GHC/Core/Utils.hs
+++ b/compiler/GHC/Core/Utils.hs
@@ -40,8 +40,8 @@ module GHC.Core.Utils (
         cheapEqExpr, cheapEqExpr', eqExpr,
         diffExpr, diffBinds,
 
-        -- * Eta reduction
-        tryEtaReduce,
+        -- * Lambdas and eta reduction
+        tryEtaReduce, zapLamBndrs,
 
         -- * Manipulating data constructors and types
         exprToType, exprToCoercion_maybe,
@@ -99,7 +99,7 @@ import GHC.Utils.Panic
 import GHC.Data.FastString
 import GHC.Data.Maybe
 import GHC.Data.List.SetOps( minusList )
-import GHC.Types.Basic     ( Arity )
+import GHC.Types.Basic     ( Arity, FullArgCount )
 import GHC.Utils.Misc
 import GHC.Data.Pair
 import Data.ByteString     ( ByteString )
@@ -2521,9 +2521,34 @@ to the rule that
 we can eta-reduce    \x. f x  ===>  f
 
 This turned up in #7542.
+-}
 
+{- *********************************************************************
+*                                                                      *
+                  Zapping lambda binders
+*                                                                      *
+********************************************************************* -}
 
-************************************************************************
+zapLamBndrs :: FullArgCount -> [Var] -> [Var]
+-- If (\xyz. t) appears under-applied to only two arguments,
+-- we must zap the occ-info on x,y, because they appear under the \x
+-- See Note [Occurrence analysis for lambda binders] in GHc.Core.Opt.OccurAnal
+--
+-- NB: both `arg_count` and `bndrs` include both type and value args/bndrs
+zapLamBndrs arg_count bndrs
+  | no_need_to_zap = bndrs
+  | otherwise      = zap_em arg_count bndrs
+  where
+    no_need_to_zap = all isOneShotBndr (drop arg_count bndrs)
+
+    zap_em :: FullArgCount -> [Var] -> [Var]
+    zap_em 0 bs = bs
+    zap_em _ [] = []
+    zap_em n (b:bs) | isTyVar b = b              : zap_em (n-1) bs
+                    | otherwise = zapLamIdInfo b : zap_em (n-1) bs
+
+
+{- *********************************************************************
 *                                                                      *
 \subsection{Determining non-updatable right-hand-sides}
 *                                                                      *