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Teach SpecConstr about Cast

This patch teaches SpecConstr about casts; see Note [SpecConstr for casts]
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commit 0da2eb6b04a57947fd2b32da9a648ad5eeeb6596 1 parent b041525
simonpj@microsoft.com authored
Showing with 46 additions and 10 deletions.
  1. +46 −10 compiler/specialise/SpecConstr.lhs
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56 compiler/specialise/SpecConstr.lhs
@@ -19,6 +19,7 @@ import PprCore ( pprRules )
import WwLib ( mkWorkerArgs )
import DataCon ( dataConRepArity, dataConUnivTyVars )
import Type ( Type, tyConAppArgs )
+import Coercion ( coercionKind )
import Rules ( matchN )
import Id ( Id, idName, idType, isDataConWorkId_maybe,
mkUserLocal, mkSysLocal, idUnfolding, isLocalId )
@@ -298,6 +299,24 @@ may avoid allocating it altogether. Just like for constructors.
Looks cool, but probably rare...but it might be easy to implement.
+
+Note [SpecConstr for casts]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider
+ data family T a :: *
+ data instance T Int = T Int
+
+ foo n = ...
+ where
+ go (T 0) = 0
+ go (T n) = go (T (n-1))
+
+The recursive call ends up looking like
+ go (T (I# ...) `cast` g)
+So we want to spot the construtor application inside the cast.
+That's why we have the Cast case in argToPat
+
+
-----------------------------------------------------
Stuff not yet handled
-----------------------------------------------------
@@ -466,14 +485,19 @@ extendCaseBndrs env case_bndr scrut con alt_bndrs
[(b,how_bound) | b <- case_bndr:alt_bndrs] }
-- Record RecArg for the components iff the scrutinee is RecArg
+ -- I think the only reason for this is to keep the usage envt small
+ -- so is it worth it at all?
-- [This comment looks plain wrong to me, so I'm ignoring it
-- "Also forget if the scrutinee is a RecArg, because we're
-- now in the branch of a case, and we don't want to
-- record a non-scrutinee use of v if we have
-- case v of { (a,b) -> ...(f v)... }" ]
- how_bound = case scrut of
- Var v -> lookupVarEnv cur_scope v `orElse` Other
- other -> Other
+ how_bound = get_how scrut
+ where
+ get_how (Var v) = lookupVarEnv cur_scope v `orElse` Other
+ get_how (Cast e _) = get_how e
+ get_how (Note _ e) = get_how e
+ get_how other = Other
extend_data_con data_con =
extendCons env1 scrut case_bndr (CV con vanilla_args)
@@ -547,9 +571,10 @@ data ArgOcc = NoOcc -- Doesn't occur at all; or a type argument
{- Note [ScrutOcc]
-An occurrence of ScrutOcc indicates that the thing is *only* taken apart or applied.
+An occurrence of ScrutOcc indicates that the thing, or a `cast` version of the thing,
+is *only* taken apart or applied.
- Functions, litersl: ScrutOcc emptyUFM
+ Functions, literal: ScrutOcc emptyUFM
Data constructors: ScrutOcc subs,
where (subs :: UniqFM [ArgOcc]) gives usage of the *pattern-bound* components,
@@ -563,7 +588,7 @@ A pattern binds b, x::a, y::b, z::b->a, but not 'a'!
-}
instance Outputable ArgOcc where
- ppr (ScrutOcc xs) = ptext SLIT("scrut-occ") <> parens (ppr xs)
+ ppr (ScrutOcc xs) = ptext SLIT("scrut-occ") <> ppr xs
ppr UnkOcc = ptext SLIT("unk-occ")
ppr BothOcc = ptext SLIT("both-occ")
ppr NoOcc = ptext SLIT("no-occ")
@@ -663,9 +688,12 @@ scExpr env e@(App _ _)
----------------------
scScrut :: ScEnv -> CoreExpr -> ArgOcc -> UniqSM (ScUsage, CoreExpr)
-- Used for the scrutinee of a case,
--- or the function of an application
-scScrut env e@(Var v) occ = returnUs (varUsage env v occ, e)
-scScrut env e occ = scExpr env e
+-- or the function of an application.
+-- Remember to look through casts
+scScrut env e@(Var v) occ = returnUs (varUsage env v occ, e)
+scScrut env (Cast e co) occ = do { (usg, e') <- scScrut env e occ
+ ; returnUs (usg, Cast e' co) }
+scScrut env e occ = scExpr env e
----------------------
@@ -726,7 +754,8 @@ specialise :: ScEnv
specialise env fn bndrs body body_usg
= do { let (_, bndr_occs) = lookupOccs body_usg bndrs
- ; mb_calls <- mapM (callToPats (scope env) bndr_occs)
+ ; mb_calls <- -- pprTrace "specialise" (ppr fn <+> ppr bndrs <+> ppr bndr_occs) $
+ mapM (callToPats (scope env) bndr_occs)
(lookupVarEnv (calls body_usg) fn `orElse` [])
; let good_calls :: [([Var], [CoreArg])]
@@ -882,6 +911,13 @@ argToPat in_scope con_env (Let _ arg) arg_occ
-- Here we can specialise for f (\y -> ...)
-- because the rule-matcher will look through the let.
+argToPat in_scope con_env (Cast arg co) arg_occ
+ = do { (interesting, arg') <- argToPat in_scope con_env arg arg_occ
+ ; if interesting then
+ return (interesting, Cast arg' co)
+ else
+ wildCardPat (snd (coercionKind co)) }
+
argToPat in_scope con_env arg arg_occ
| is_value_lam arg
= return (True, arg)
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