compiler/GHC/Core/Opt/Arity.hs

@@ -231,7 +231,7 @@ typeOneShot ty
 -- See Note [The state-transformer hack] in "GHC.Core.Opt.Arity"
 idStateHackOneShotInfo :: Id -> OneShotInfo
 idStateHackOneShotInfo id
-  = case idOneShotInfo id of
+  = case idOneShotInfo id of  -- Test idOneShotInfo first because it is fast
        OneShotLam                                  -> OneShotLam
        NoOneShotInfo | isStateHackType (idType id) -> OneShotLam
                      | otherwise                   -> NoOneShotInfo

compiler/GHC/Core/Opt/Simplify/Utils.hs

@@ -1423,8 +1423,14 @@ preInlineUnconditionally env top_lvl bndr rhs rhs_env
     one_occ OneOcc{ occ_n_br = 1, occ_in_lam = in_lam, occ_int_cxt = int_cxt }
        | is_value_lam rhs, IsInteresting <- int_cxt
        = True
-       | NotInsideLam <- in_lam    -- Once things are flattened to top level, don't
-       , not (isTopLevel top_lvl)  -- re-inline them.  See Note [Floating to the top]
+       | NotInsideLam <- in_lam
+       , not (isTopLevel top_lvl) || not (exprIsExpandable rhs)
+         -- Inline used-once things; except expandable things at top level
+         -- These may arise from user code e.g.
+         --     x = [1,2,3]
+         --     y = length x    -- Want this to fuse
+         -- Real world example: the [1..] in 'preparse' in nofibl/real/cacheprof
+         -- See Note [Floating to the top]
        = True
        | otherwise
        = False
@@ -1436,7 +1442,6 @@ preInlineUnconditionally env top_lvl bndr rhs rhs_env
 
 {-
     one_occ OneOcc{ occ_n_br   = 1
-
                   , occ_in_lam = NotInsideLam }   = isNotTopLevel top_lvl || early_phase
     one_occ OneOcc{ occ_n_br   = 1
                   , occ_in_lam = IsInsideLam

testsuite/tests/perf/compiler/LargeRecord.hs

@@ -60,7 +60,6 @@ type BigFieldList =
   '[ "f1" := Int
    , "f2" := Int
    , "f3" := Int
-{-
    , "f4" := Int
    , "f5" := Int
    , "f6" := Int
@@ -73,7 +72,6 @@ type BigFieldList =
    , "f13" := Int
    , "f14" := Int
    , "f15" := Int
--}
    ]
 
 bigRec :: Record BigFieldList
@@ -81,7 +79,6 @@ bigRec =
     #f1   := 1
   & #f2   := 2
   & #f3   := 3
-{-
   & #f4   := 4
   & #f5   := 5
   & #f6   := 6
@@ -94,7 +91,6 @@ bigRec =
   & #f13  := 13
   & #f14  := 14
   & #f15  := 15
--}
   & rnil
 
 main :: IO ()