Two improvements to optCoercion

[ghc-hetmet.git] / compiler / coreSyn / CoreUtils.lhs

diff --git a/compiler/coreSyn/CoreUtils.lhs b/compiler/coreSyn/CoreUtils.lhs
index 1590978..ed8d6f2 100644 (file)
--- a/compiler/coreSyn/CoreUtils.lhs
+++ b/compiler/coreSyn/CoreUtils.lhs
@@ -36,7 +36,7 @@ module CoreUtils (
        hashExpr,
 
        -- * Equality
-       cheapEqExpr, 
+       cheapEqExpr, eqExpr,
 
        -- * Manipulating data constructors and types
        applyTypeToArgs, applyTypeToArg,
@@ -655,6 +655,11 @@ exprOkForSpeculation (Var v)     = isUnLiftedType (idType v)
                                 && not (isTickBoxOp v)
 exprOkForSpeculation (Note _ e)  = exprOkForSpeculation e
 exprOkForSpeculation (Cast e _)  = exprOkForSpeculation e
+
+exprOkForSpeculation (Case e _ _ alts) 
+  =  exprOkForSpeculation e  -- Note [exprOkForSpeculation: case expressions]
+  && all (\(_,_,rhs) -> exprOkForSpeculation rhs) alts
+
 exprOkForSpeculation other_expr
   = case collectArgs other_expr of
        (Var f, args) -> spec_ok (idDetails f) args
@@ -699,6 +704,36 @@ isDivOp DoubleDivOp      = True
 isDivOp _                = False
 \end{code}
 
+Note [exprOkForSpeculation: case expressions]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
+
+It's always sound for exprOkForSpeculation to return False, and we
+don't want it to take too long, so it bales out on complicated-looking
+terms.  Notably lets, which can be stacked very deeply; and in any 
+case the argument of exprOkForSpeculation is usually in a strict context,
+so any lets will have been floated away.
+
+However, we keep going on case-expressions.  An example like this one
+showed up in DPH code:
+    foo :: Int -> Int
+    foo 0 = 0
+    foo n = (if n < 5 then 1 else 2) `seq` foo (n-1)
+
+If exprOkForSpeculation doesn't look through case expressions, you get this:
+    T.$wfoo =
+      \ (ww :: GHC.Prim.Int#) ->
+        case ww of ds {
+          __DEFAULT -> case (case <# ds 5 of _ {
+                          GHC.Bool.False -> lvl1; 
+                          GHC.Bool.True -> lvl})
+                       of _ { __DEFAULT ->
+                       T.$wfoo (GHC.Prim.-# ds_XkE 1) };
+          0 -> 0
+        }
+
+The inner case is redundant, and should be nuked.
+
+
 %************************************************************************
 %*                                                                     *
              exprIsHNF, exprIsConLike
@@ -913,7 +948,9 @@ cheapEqExpr (Cast e1 t1) (Cast e2 t2)
   = e1 `cheapEqExpr` e2 && t1 `coreEqCoercion` t2
 
 cheapEqExpr _ _ = False
+\end{code}
 
+\begin{code}
 exprIsBig :: Expr b -> Bool
 -- ^ Returns @True@ of expressions that are too big to be compared by 'cheapEqExpr'
 exprIsBig (Lit _)      = False
@@ -925,7 +962,53 @@ exprIsBig (Cast e _)   = exprIsBig e       -- Hopefully coercions are not too big!
 exprIsBig _            = True
 \end{code}
 
+\begin{code}
+eqExpr :: InScopeSet -> CoreExpr -> CoreExpr -> Bool
+-- Compares for equality, modulo alpha
+eqExpr in_scope e1 e2
+  = go (mkRnEnv2 in_scope) e1 e2
+  where
+    go _   (Lit lit1)  (Lit lit2)   = lit1 == lit2
+    go env (Type t1)   (Type t2)    = coreEqType2 env t1 t2
+    go env (Var v1)    (Var v2)     = rnOccL env v1 == rnOccR env v2
+    go env (Cast e1 t1) (Cast e2 t2) = go env e1 e2 && coreEqCoercion2 env t1 t2
+    go env (App f1 a1)  (App f2 a2)  = go env f1 f2 && go env a1 a2
+
+    go env (Lam b1 e1)  (Lam b2 e2)  
+      =  coreEqType2 env (varType b1) (varType b2)     -- Will return False for Id/TyVar combination
+      && go (rnBndr2 env b1 b2) e1 e2
+
+    go env (Case e1 b1 _ a1) (Case e2 b2 _ a2)
+      =  go env e1 e2
+      && coreEqType2 env (idType b1) (idType b2)
+      && all2 (go_alt (rnBndr2 env b1 b2)) a1 a2
+   
+    go env (Let (NonRec b1 r1) e1) (Let (NonRec b2 r2) e2)
+      =  go env r1 r2  -- No need to check binder types, since RHSs match
+      && go (rnBndr2 env b1 b2) e1 e2  
+
+    go env (Let (Rec p1) e1) (Let (Rec p2) e2)
+      | equalLength p1 p2
+      =  all2 (go env') rs1 rs2 && go env' e1 e2
+      where
+         (bs1,rs1) = unzip p1
+         (bs2,rs2) = unzip p2
+         env' = rnBndrs2 env bs1 bs2
+
+    go env (Note n1 e1) (Note n2 e2) = go_note n1 n2 && go env e1 e2
+
+    go _ _ _ = False
 
+    -----------
+    go_alt env (c1, bs1, e1) (c2, bs2, e2)
+      = c1 == c2 && go (rnBndrs2 env bs1 bs2) e1 e2
+
+    -----------
+    go_note (SCC cc1)     (SCC cc2)     = cc1==cc2
+    go_note (CoreNote s1) (CoreNote s2) = s1==s2
+    go_note _ _ = False
+\end{code}
+    
 
 %************************************************************************
 %*                                                                     *