Drop dead core that was kept alive by RULES in CorePrep (#4962)

diff --git a/compiler/basicTypes/IdInfo.lhs b/compiler/basicTypes/IdInfo.lhs
index 1c01ba4..ec1f122 100644 (file)
--- a/compiler/basicTypes/IdInfo.lhs
+++ b/compiler/basicTypes/IdInfo.lhs
@@ -46,6 +46,7 @@ module IdInfo (
        
        -- ** The SpecInfo type
        SpecInfo(..),
+       emptySpecInfo,
        isEmptySpecInfo, specInfoFreeVars,
        specInfoRules, seqSpecInfo, setSpecInfoHead,
         specInfo, setSpecInfo,

diff --git a/compiler/coreSyn/CorePrep.lhs b/compiler/coreSyn/CorePrep.lhs
index 540fa2d..42379b4 100644 (file)
--- a/compiler/coreSyn/CorePrep.lhs
+++ b/compiler/coreSyn/CorePrep.lhs
@@ -18,6 +18,7 @@ import CoreFVs
 import CoreMonad       ( endPass, CoreToDo(..) )
 import CoreSyn
 import CoreSubst
+import OccurAnal        ( occurAnalyseExpr )
 import Type
 import Coercion
 import TyCon
@@ -248,6 +249,61 @@ always fully applied, and the bindings are just there to support
 partial applications. But it's easier to let them through.
 
 
+Note [Dead code in CorePrep]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Imagine that we got an input program like this:
+
+  f :: Show b => Int -> (Int, b -> Maybe Int -> Int)
+  f x = (g True (Just x) + g () (Just x), g)
+    where
+      g :: Show a => a -> Maybe Int -> Int
+      g _ Nothing = x
+      g y (Just z) = if z > 100 then g y (Just (z + length (show y))) else g y unknown
+
+After specialisation and SpecConstr, we would get something like this:
+
+  f :: Show b => Int -> (Int, b -> Maybe Int -> Int)
+  f x = (g$Bool_True_Just x + g$Unit_Unit_Just x, g)
+    where
+      {-# RULES g $dBool = g$Bool 
+                g $dUnit = g$Unit #-}
+      g = ...
+      {-# RULES forall x. g$Bool True (Just x) = g$Bool_True_Just x #-}
+      g$Bool = ...
+      {-# RULES forall x. g$Unit () (Just x) = g$Unit_Unit_Just x #-}
+      g$Unit = ...
+      g$Bool_True_Just = ...
+      g$Unit_Unit_Just = ...
+
+Note that the g$Bool and g$Unit functions are actually dead code: they are only kept
+alive by the occurrence analyser because they are referred to by the rules of g,
+which is being kept alive by the fact that it is used (unspecialised) in the returned pair.
+
+However, at the CorePrep stage there is no way that the rules for g will ever fire,
+and it really seems like a shame to produce an output program that goes to the trouble
+of allocating a closure for the unreachable g$Bool and g$Unit functions.
+
+The way we fix this is to:
+ * In cloneBndr, drop all unfoldings/rules
+ * In deFloatTop, run the occurrence analyser on each top-level RHS to drop
+   the dead local bindings
+
+The reason we don't just OccAnal the whole output of CorePrep is that the tidier
+ensures that all top-level binders are GlobalIds, so they don't show up in the free
+variables any longer. So if you run the occurrence analyser on the output of CoreTidy
+(or later) you e.g. turn this program:
+
+  Rec {
+  f = ... f ...
+  }
+
+Into this one:
+
+  f = ... f ...
+
+(Since f is not considered to be free in its own RHS.)
+
+
 %************************************************************************
 %*                                                                     *
                The main code
@@ -901,8 +957,12 @@ deFloatTop :: Floats -> [CoreBind]
 deFloatTop (Floats _ floats)
   = foldrOL get [] floats
   where
-    get (FloatLet b) bs = b:bs
+    get (FloatLet b) bs = occurAnalyseRHSs b : bs
     get b            _  = pprPanic "corePrepPgm" (ppr b)
+    
+    -- See Note [Dead code in CorePrep]
+    occurAnalyseRHSs (NonRec x e) = NonRec x (occurAnalyseExpr e)
+    occurAnalyseRHSs (Rec xes)    = Rec [(x, occurAnalyseExpr e) | (x, e) <- xes]
 
 -------------------------------------------
 canFloatFromNoCaf ::  Floats -> CpeRhs -> Maybe (Floats, CpeRhs)
@@ -1012,7 +1072,13 @@ cloneBndr  :: CorePrepEnv -> Var -> UniqSM (CorePrepEnv, Var)
 cloneBndr env bndr
   | isLocalId bndr
   = do bndr' <- setVarUnique bndr <$> getUniqueM
-       return (extendCorePrepEnv env bndr bndr', bndr')
+       
+       -- We are going to OccAnal soon, so drop (now-useless) rules/unfoldings
+       -- so that we can drop more stuff as dead code.
+       -- See also Note [Dead code in CorePrep]
+       let bndr'' = bndr' `setIdUnfolding` noUnfolding
+                          `setIdSpecialisation` emptySpecInfo
+       return (extendCorePrepEnv env bndr bndr'', bndr'')
 
   | otherwise  -- Top level things, which we don't want
                -- to clone, have become GlobalIds by now