X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2FcoreSyn%2FCoreSubst.lhs;h=321ea8fc3603a37936a9a8ea9db6983815cc956a;hb=8100cd4395e46ae747be4298c181a4730d6206bc;hp=ac5617658b8918b449a3c1dd7c6b7a64778ea4eb;hpb=2a3c28723075eb36e3faa2bfb6c46d934e05f208;p=ghc-hetmet.git

diff --git a/compiler/coreSyn/CoreSubst.lhs b/compiler/coreSyn/CoreSubst.lhs
index ac56176..321ea8f 100644
--- a/compiler/coreSyn/CoreSubst.lhs
+++ b/compiler/coreSyn/CoreSubst.lhs
@@ -1,7 +1,9 @@
 %
+% (c) The University of Glasgow 2006
 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
 %
-\section[CoreUtils]{Utility functions on @Core@ syntax}
+
+Utility functions on @Core@ syntax
 
 \begin{code}
 module CoreSubst (
@@ -24,29 +26,23 @@ module CoreSubst (
 
 #include "HsVersions.h"
 
-import CoreSyn		( Expr(..), Bind(..), Note(..), CoreExpr, CoreBind,
-			  CoreRule(..), hasUnfolding, noUnfolding
-			)
-import CoreFVs		( exprFreeVars )
-import CoreUtils	( exprIsTrivial )
+import CoreSyn
+import CoreFVs
+import CoreUtils
 
-import qualified Type	( substTy, substTyVarBndr )
-import Type		( Type, tyVarsOfType, TvSubstEnv, TvSubst(..), mkTyVarTy )
+import qualified Type
+import Type     ( Type, TvSubst(..), TvSubstEnv )
 import VarSet
 import VarEnv
-import Var		( setVarUnique, isId )
-import Id		( idType, idInfo, setIdType, maybeModifyIdInfo, isLocalId )
-import IdInfo		( IdInfo, SpecInfo(..), specInfo, setSpecInfo, isEmptySpecInfo,
-			  unfoldingInfo, setUnfoldingInfo, seqSpecInfo,
-			  WorkerInfo(..), workerExists, workerInfo, setWorkerInfo
-			)
-import Unique		( Unique )
-import UniqSupply	( UniqSupply, uniqFromSupply, uniqsFromSupply )
-import Var		( Var, Id, TyVar, isTyVar )
-import Maybes		( orElse, isNothing )
+import Id
+import Var      ( Var, TyVar, setVarUnique )
+import IdInfo
+import Unique
+import UniqSupply
+import Maybes
 import Outputable
 import PprCore		()		-- Instances
-import Util		( mapAccumL )
+import Util
 import FastTypes
 \end{code}
 
@@ -75,8 +71,51 @@ data Subst
 	--	- make it empty because all the free vars of the subst are fresh,
 	--		and hence can't possibly clash.a
 	--
-	-- INVARIANT 2: The substitution is apply-once; see notes with
+	-- INVARIANT 2: The substitution is apply-once; see Note [Apply once] with
 	--		Types.TvSubstEnv
+	--
+	-- INVARIANT 3: See Note [Extending the Subst]
+
+{-
+Note [Extending the Subst]
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+For a core Subst, which binds Ids as well, we make a different choice for Ids
+than we do for TyVars.  
+
+For TyVars, see Note [Extending the TvSubst] with Type.TvSubstEnv
+
+For Ids, we have a different invariant
+	The IdSubstEnv is extended *only* when the Unique on an Id changes
+	Otherwise, we just extend the InScopeSet
+
+In consequence:
+
+* In substIdBndr, we extend the IdSubstEnv only when the unique changes
+
+* If the TvSubstEnv and IdSubstEnv are both empty, substExpr does nothing
+  (Note that the above rule for substIdBndr maintains this property.  If
+   the incoming envts are both empty, then substituting the type and
+   IdInfo can't change anything.)
+
+* In lookupIdSubst, we *must* look up the Id in the in-scope set, because
+  it may contain non-trivial changes.  Example:
+	(/\a. \x:a. ...x...) Int
+  We extend the TvSubstEnv with [a |-> Int]; but x's unique does not change
+  so we only extend the in-scope set.  Then we must look up in the in-scope
+  set when we find the occurrence of x.
+
+Why do we make a different choice for the IdSubstEnv than the TvSubstEnv?
+
+* For Ids, we change the IdInfo all the time (e.g. deleting the
+  unfolding), and adding it back later, so using the TyVar convention
+  would entail extending the substitution almost all the time
+
+* The simplifier wants to look up in the in-scope set anyway, in case it 
+  can see a better unfolding from an enclosing case expression
+
+* For TyVars, only coercion variables can possibly change, and they are 
+  easy to spot
+-}
 
 type IdSubstEnv = IdEnv CoreExpr
 
@@ -124,31 +163,14 @@ extendTvSubstList (Subst in_scope ids tvs) prs = Subst in_scope ids (extendVarEn
 lookupIdSubst :: Subst -> Id -> CoreExpr
 lookupIdSubst (Subst in_scope ids tvs) v 
   | not (isLocalId v) = Var v
-  | otherwise	      = case lookupVarEnv ids v of
-			  Just e  -> e
-			  Nothing -> Var v 
-
-{-	We used to have to look up in the in-scope set, 
-	because GADTs were implicit in the intermediate language
-	But with FC, the type of an Id does not change in its scope
-	The worst that can happen if we don't look up in the in-scope set
-	is that we don't propagate IdInfo as vigorously as we might.
-	But that'll happen (when it's useful) in SimplEnv.substId
-
-	If you put this back in, you should worry about the
-		Just e -> e
-	case above too!
-
-    case lookupInScope in_scope v of {
-	-- Watch out!  Must get the Id from the in-scope set,
-	-- because its type there may differ
-	Just v  -> Var v ;
-	Nothing -> WARN( True, ptext SLIT("CoreSubst.lookupIdSubst") <+> ppr v ) 
-		   Var v
--}
+  | Just e  <- lookupVarEnv ids       v = e
+  | Just v' <- lookupInScope in_scope v = Var v'
+	-- Vital! See Note [Extending the Subst]
+  | otherwise = WARN( True, ptext SLIT("CoreSubst.lookupIdSubst") <+> ppr v ) 
+		Var v
 
 lookupTvSubst :: Subst -> TyVar -> Type
-lookupTvSubst (Subst _ ids tvs) v = lookupVarEnv tvs v `orElse` mkTyVarTy v
+lookupTvSubst (Subst _ ids tvs) v = lookupVarEnv tvs v `orElse` Type.mkTyVarTy v
 
 ------------------------------
 isInScope :: Var -> Subst -> Bool
@@ -279,7 +301,8 @@ substIdBndr rec_subst subst@(Subst in_scope env tvs) old_id
 	| otherwise	 = setIdType id1 (substTy subst old_ty)
 
     old_ty = idType old_id
-    no_type_change = isEmptyVarEnv tvs || isEmptyVarSet (tyVarsOfType old_ty)
+    no_type_change = isEmptyVarEnv tvs || 
+                     isEmptyVarSet (Type.tyVarsOfType old_ty)
 
 	-- new_id has the right IdInfo
 	-- The lazy-set is because we're in a loop here, with 
@@ -292,7 +315,9 @@ substIdBndr rec_subst subst@(Subst in_scope env tvs) old_id
     new_env | no_change = delVarEnv env old_id
 	    | otherwise = extendVarEnv env old_id (Var new_id)
 
-    no_change = False -- id1 == old_id && isNothing mb_new_info && no_type_change
+    no_change = id1 == old_id
+	-- See Note [Extending the Subst]
+	-- *not* necessary to check mb_new_info and no_type_change
 \end{code}
 
 Now a variant that unconditionally allocates a new unique.
@@ -361,7 +386,7 @@ substTy (Subst in_scope id_env tv_env) ty
 \begin{code}
 substIdType :: Subst -> Id -> Id
 substIdType subst@(Subst in_scope id_env tv_env) id
-  | isEmptyVarEnv tv_env || isEmptyVarSet (tyVarsOfType old_ty) = id
+  | isEmptyVarEnv tv_env || isEmptyVarSet (Type.tyVarsOfType old_ty) = id
   | otherwise	= setIdType id (substTy subst old_ty)
 		-- The tyVarsOfType is cheaper than it looks
 		-- because we cache the free tyvars of the type
@@ -425,5 +450,5 @@ substVarSet subst fvs
   where
     subst_fv subst fv 
 	| isId fv   = exprFreeVars (lookupIdSubst subst fv)
-	| otherwise = tyVarsOfType (lookupTvSubst subst fv)
+	| otherwise = Type.tyVarsOfType (lookupTvSubst subst fv)
 \end{code}