path: root/ghc/compiler/specialise/Rules.lhs

%
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
\section[CoreRules]{Transformation rules}

\begin{code}
module Rules (
	RuleBase, emptyRuleBase, 
	extendRuleBase, extendRuleBaseList, addRuleBaseFVs, 
	ruleBaseIds, ruleBaseFVs,
	pprRuleBase,

        lookupRule, addRule, addIdSpecialisations
    ) where

#include "HsVersions.h"

import CoreSyn		-- All of it
import OccurAnal	( occurAnalyseRule )
import CoreFVs		( exprFreeVars, ruleRhsFreeVars, ruleLhsFreeIds )
import CoreUnfold	( isCheapUnfolding, unfoldingTemplate )
import CoreUtils	( eqExpr )
import PprCore		( pprCoreRule )
import Subst		( Subst, InScopeSet, mkInScopeSet, lookupSubst, extendSubst,
			  substEnv, setSubstEnv, emptySubst, isInScope,
			  bindSubstList, unBindSubstList, substInScope, uniqAway
			)
import Id		( Id, idUnfolding, idSpecialisation, setIdSpecialisation ) 
import Var		( isId )
import VarSet
import VarEnv
import Type		( mkTyVarTy )
import qualified Unify	( match )

import Outputable
import Maybe		( isJust, isNothing, fromMaybe )
import Util		( sortLt )
\end{code}


%************************************************************************
%*									*
\subsection[specialisation-IdInfo]{Specialisation info about an @Id@}
%*									*
%************************************************************************

A @CoreRule@ holds details of one rule for an @Id@, which
includes its specialisations.

For example, if a rule for @f@ contains the mapping:
\begin{verbatim}
	forall a b d. [Type (List a), Type b, Var d]  ===>  f' a b
\end{verbatim}
then when we find an application of f to matching types, we simply replace
it by the matching RHS:
\begin{verbatim}
	f (List Int) Bool dict ===>  f' Int Bool
\end{verbatim}
All the stuff about how many dictionaries to discard, and what types
to apply the specialised function to, are handled by the fact that the
Rule contains a template for the result of the specialisation.

There is one more exciting case, which is dealt with in exactly the same
way.  If the specialised value is unboxed then it is lifted at its
definition site and unlifted at its uses.  For example:

	pi :: forall a. Num a => a

might have a specialisation

	[Int#] ===>  (case pi' of Lift pi# -> pi#)

where pi' :: Lift Int# is the specialised version of pi.


%************************************************************************
%*									*
\subsection{Matching}
%*									*
%************************************************************************

\begin{code}
matchRules :: InScopeSet -> [CoreRule] -> [CoreExpr] -> Maybe (RuleName, CoreExpr)
-- See comments on matchRule
matchRules in_scope [] args = Nothing
matchRules in_scope (rule:rules) args
  = case matchRule in_scope rule args of
	Just result -> Just result
	Nothing	    -> matchRules in_scope rules args


matchRule :: InScopeSet -> CoreRule -> [CoreExpr] -> Maybe (RuleName, CoreExpr)

-- If (matchRule rule args) returns Just (name,rhs)
-- then (f args) matches the rule, and the corresponding
-- rewritten RHS is rhs
--
-- The bndrs and rhs is occurrence-analysed
--
-- 	Example
--
-- The rule
--	forall f g x. map f (map g x) ==> map (f . g) x
-- is stored
--	CoreRule "map/map" 
--		 [f,g,x]		-- tpl_vars
--		 [f,map g x]		-- tpl_args
--		 map (f.g) x)		-- rhs
--	  
-- Then the call: matchRule the_rule [e1,map e2 e3]
--	  = Just ("map/map", (\f,g,x -> rhs) e1 e2 e3)
--
-- Any 'surplus' arguments in the input are simply put on the end
-- of the output.
--
-- ASSUMPTION (A):
--	A1. No top-level variable is bound in the target
--	A2. No template variable  is bound in the target
--	A3. No lambda bound template variable is free in any subexpression of the target
--
-- To see why A1 is necessary, consider matching
--	\x->f 	   against    \f->f
-- When we meet the lambdas we substitute [f/x] in the template (a no-op),
-- and then erroneously succeed in matching f against f.
--
-- To see why A2 is needed consider matching 
--	forall a. \b->b	   against   \a->3
-- When we meet the lambdas we substitute [a/b] in the template, and then
-- erroneously succeed in matching what looks like the template variable 'a' against 3.
--
-- A3 is needed to validate the rule that says
--	(\x->E) matches F
-- if
--	(\x->E)	matches (\x->F x)


matchRule in_scope rule@(BuiltinRule match_fn) args = match_fn args

matchRule in_scope rule@(Rule rn tpl_vars tpl_args rhs) args
  = go tpl_args args emptySubst
	-- We used to use the in_scope set, but I don't think that's necessary
	-- After all, the result is going to be simplified again with that in_scope set
 where
   tpl_var_set = mkVarSet tpl_vars

   -----------------------
	-- Do the business
   go (tpl_arg:tpl_args) (arg:args) subst = match tpl_arg arg tpl_var_set (go tpl_args args) subst

	-- Two easy ways to terminate
   go [] []	    subst = Just (rn, app_match subst (mkLams tpl_vars rhs) tpl_vars)
   go [] args	    subst = Just (rn, app_match subst (mkLams tpl_vars rhs) tpl_vars `mkApps` args)

	-- One tiresome way to terminate: check for excess unmatched
	-- template arguments
   go tpl_args []   subst = Nothing	-- Failure


   -----------------------
   app_match subst fn vs = foldl go fn vs
	where	
	  senv    = substEnv subst
	  go fn v = case lookupSubstEnv senv v of
			Just (DoneEx ex)  -> fn `App` ex 
			Just (DoneTy ty)  -> fn `App` Type ty
			-- Substitution should bind them all!


   -----------------------
{-	The code below tries to match even if there are more 
	template args than real args.

	I now think this is probably a bad idea.
	Should the template (map f xs) match (map g)?  I think not.
	For a start, in general eta expansion wastes work.
	SLPJ July 99

      = case eta_complete tpl_args (mkVarSet leftovers) of
	    Just leftovers' -> Just (rn, mkLams done (mkLams leftovers' rhs), 
				     mk_result_args subst done)
	    Nothing	    -> Nothing	-- Failure
      where
	(done, leftovers) = partition (\v -> isJust (lookupSubstEnv subst_env v))
				      (map zapOccInfo tpl_vars)
		-- Zap the occ info 
	subst_env = substEnv subst
   						
   -----------------------
   eta_complete [] vars = ASSERT( isEmptyVarSet vars )
			  Just []
   eta_complete (Type ty:tpl_args) vars
	= case getTyVar_maybe ty of
		Just tv |  tv `elemVarSet` vars
			-> case eta_complete tpl_args (vars `delVarSet` tv) of
				Just vars' -> Just (tv:vars')
				Nothing    -> Nothing
		other   -> Nothing

   eta_complete (Var v:tpl_args) vars
	| v `elemVarSet` vars
	= case eta_complete tpl_args (vars `delVarSet` v) of
		Just vars' -> Just (v:vars')
		Nothing    -> Nothing

   eta_complete other vars = Nothing


zapOccInfo bndr | isTyVar bndr = bndr
		| otherwise    = zapLamIdInfo bndr
-}
\end{code}

\begin{code}
type Matcher result =  VarSet			-- Template variables
	     	    -> (Subst -> Maybe result)	-- Continuation if success
		    -> Subst  -> Maybe result	-- Substitution so far -> result
-- The *SubstEnv* in these Substs apply to the TEMPLATE only 

-- The *InScopeSet* in these Substs gives variables bound so far in the
--	target term.  So when matching forall a. (\x. a x) against (\y. y y)
--	while processing the body of the lambdas, the in-scope set will be {y}.
--	That lets us do the occurs-check when matching 'a' against 'y'

match :: CoreExpr		-- Template
      -> CoreExpr		-- Target
      -> Matcher result

match_fail = Nothing

match (Var v1) e2 tpl_vars kont subst
  = case lookupSubst subst v1 of
	Nothing	| v1 `elemVarSet` tpl_vars  	-- v1 is a template variable
		-> if (any (`isInScope` subst) (varSetElems (exprFreeVars e2))) then
			 match_fail		-- Occurs check failure
						-- e.g. match forall a. (\x-> a x) against (\y. y y)
		   else
			 kont (extendSubst subst v1 (DoneEx e2))


		| eqExpr (Var v1) e2		 -> kont subst
			-- v1 is not a template variable, so it must be a global constant

	Just (DoneEx e2')  | eqExpr e2'       e2 -> kont subst

	other -> match_fail

match (Lit lit1) (Lit lit2) tpl_vars kont subst
  | lit1 == lit2
  = kont subst

match (App f1 a1) (App f2 a2) tpl_vars kont subst
  = match f1 f2 tpl_vars (match a1 a2 tpl_vars kont) subst

match (Lam x1 e1) (Lam x2 e2) tpl_vars kont subst
  = bind [x1] [x2] (match e1 e2) tpl_vars kont subst

-- This rule does eta expansion
--		(\x.M)  ~  N 	iff	M  ~  N x
-- See assumption A3
match (Lam x1 e1) e2 tpl_vars kont subst
  = bind [x1] [x1] (match e1 (App e2 (mkVarArg x1))) tpl_vars kont subst

-- Eta expansion the other way
--	M  ~  (\y.N)	iff   \y.M y  ~  \y.N
--			iff   M	y     ~  N
-- Remembering that by (A), y can't be free in M, we get this
match e1 (Lam x2 e2) tpl_vars kont subst
  = bind [new_id] [x2] (match (App e1 (mkVarArg new_id)) e2) tpl_vars kont subst
  where
    new_id = uniqAway (substInScope subst) x2
	-- This uniqAway is actually needed.  Here's the example:
	--  rule:	foldr (mapFB (:) f) [] = mapList
	--  target:	foldr (\x. mapFB k f x) []
	--	      where
	--		k = \x. mapFB ... x
	-- The first \x is ok, but when we inline k, hoping it might
	-- match (:) we find a second \x.

match (Case e1 x1 alts1) (Case e2 x2 alts2) tpl_vars kont subst
  = match e1 e2 tpl_vars case_kont subst
  where
    case_kont subst = bind [x1] [x2] (match_alts alts1 (sortLt lt_alt alts2))
				     tpl_vars kont subst

match (Type ty1) (Type ty2) tpl_vars kont subst
  = match_ty ty1 ty2 tpl_vars kont subst

match (Note (Coerce to1 from1) e1) (Note (Coerce to2 from2) e2)
      tpl_vars kont subst
  = (match_ty to1   to2   tpl_vars $
     match_ty from1 from2 tpl_vars $
     match e1 e2 tpl_vars kont) subst


{-	I don't buy this let-rule any more
	The let rule fails on matching
		forall f,x,xs. f (x:xs)
	against
		f (let y = e in (y:[]))
	because we just get x->y, which is bogus.

-- This is an interesting rule: we simply ignore lets in the 
-- term being matched against!  The unfolding inside it is (by assumption)
-- already inside any occurrences of the bound variables, so we'll expand
-- them when we encounter them.  Meanwhile, we can't get false matches because
-- (also by assumption) the term being matched has no shadowing.
match e1 (Let bind e2) tpl_vars kont subst
  = match e1 e2 tpl_vars kont subst
-}

-- Here is another important rule: if the term being matched is a
-- variable, we expand it so long as its unfolding is a WHNF
-- (Its occurrence information is not necessarily up to date,
--  so we don't use it.)
match e1 (Var v2) tpl_vars kont subst
  | isCheapUnfolding unfolding
  = match e1 (unfoldingTemplate unfolding) tpl_vars kont subst
  where
    unfolding = idUnfolding v2


-- We can't cope with lets in the template

match e1 e2 tpl_vars kont subst = match_fail


------------------------------------------
match_alts [] [] tpl_vars kont subst
  = kont subst
match_alts ((c1,vs1,r1):alts1) ((c2,vs2,r2):alts2) tpl_vars kont subst
  | c1 == c2
  = bind vs1 vs2 (match r1 r2) tpl_vars
		 (match_alts alts1 alts2 tpl_vars kont)
		 subst
match_alts alts1 alts2 tpl_vars kont subst = match_fail

lt_alt (con1, _, _) (con2, _, _) = con1 < con2

----------------------------------------
bind :: [CoreBndr]	-- Template binders
     -> [CoreBndr]	-- Target binders
     -> Matcher result
     -> Matcher result
-- This makes uses of assumption (A) above.  For example,
-- this would fail:
--	Template: (\x.y)	(y is free)
--	Target  : (\y.y)	(y is bound)
-- We rename x to y in the template... but then erroneously
-- match y against y.  But this can't happen because of (A)
bind vs1 vs2 matcher tpl_vars kont subst
  = WARN( not (all not_in_subst vs1), bug_msg )
    matcher tpl_vars kont' subst'
  where
    kont' subst'' = kont (unBindSubstList subst'' vs1 vs2)
    subst'        = bindSubstList subst vs1 vs2

	-- The unBindSubst relies on no shadowing in the template
    not_in_subst v = isNothing (lookupSubst subst v)
    bug_msg = sep [ppr vs1, ppr vs2]

----------------------------------------
match_ty ty1 ty2 tpl_vars kont subst
  = case Unify.match False {- for now: KSW 2000-10 -} ty1 ty2 tpl_vars Just (substEnv subst) of
	Nothing    -> match_fail
	Just senv' -> kont (setSubstEnv subst senv') 

----------------------------------------
matches [] [] tpl_vars kont subst 
  = kont subst
matches (e:es) (e':es') tpl_vars kont subst
  = match e e' tpl_vars (matches es es' tpl_vars kont) subst
matches es es' tpl_vars kont subst 
  = match_fail

----------------------------------------
mkVarArg :: CoreBndr -> CoreArg
mkVarArg v | isId v    = Var v
	   | otherwise = Type (mkTyVarTy v)
\end{code}

%************************************************************************
%*									*
\subsection{Adding a new rule}
%*									*
%************************************************************************

\begin{code}
addRule :: CoreRules -> Id -> CoreRule -> CoreRules

-- Insert the new rule just before a rule that is *less specific*
-- than the new one; or at the end if there isn't such a one.
-- In this way we make sure that when looking up, the first match
-- is the most specific.
--
-- We make no check for rules that unify without one dominating
-- the other.   Arguably this would be a bug.

addRule (Rules rules rhs_fvs) id rule@(BuiltinRule _)
  = Rules (rule:rules) rhs_fvs
	-- Put it at the start for lack of anything better

addRule (Rules rules rhs_fvs) id rule
  = Rules (insertRule rules new_rule) (rhs_fvs `unionVarSet` new_rhs_fvs)
  where
    new_rule    = occurAnalyseRule rule
    new_rhs_fvs = ruleRhsFreeVars new_rule `delVarSet` id
	-- Hack alert!
	-- Don't include the Id in its own rhs free-var set.
	-- Otherwise the occurrence analyser makes bindings recursive
	-- that shoudn't be.  E.g.
	--	RULE:  f (f x y) z  ==>  f x (f y z)

insertRule rules new_rule@(Rule _ tpl_vars tpl_args _)
  = go rules
  where
    tpl_var_set = mkInScopeSet (mkVarSet tpl_vars)
	-- Actually we should probably include the free vars of tpl_args,
	-- but I can't be bothered

    go []					= [new_rule]
    go (rule:rules) | new_is_more_specific rule = (new_rule:rule:rules)
		    | otherwise		        = rule : go rules

    new_is_more_specific rule = isJust (matchRule tpl_var_set rule tpl_args)

addIdSpecialisations :: Id -> [CoreRule] -> Id
addIdSpecialisations id rules
  = setIdSpecialisation id new_specs
  where
    new_specs = foldr add (idSpecialisation id) rules
    add rule rules = addRule rules id rule
\end{code}


%************************************************************************
%*									*
\subsection{Preparing the rule base
%*									*
%************************************************************************

\begin{code}
lookupRule :: InScopeSet -> Id -> [CoreExpr] -> Maybe (RuleName, CoreExpr)
lookupRule in_scope fn args
  = case idSpecialisation fn of
	Rules rules _ -> matchRules in_scope rules args
\end{code}


%************************************************************************
%*									*
\subsection{Getting the rules ready}
%*									*
%************************************************************************

\begin{code}
data RuleBase = RuleBase
		    IdSet	-- Ids with their rules in their specialisations
				-- Held as a set, so that it can simply be the initial
				-- in-scope set in the simplifier

		    IdSet	-- Ids (whether local or imported) mentioned on 
				-- LHS of some rule; these should be black listed

	-- This representation is a bit cute, and I wonder if we should
	-- change it to use (IdEnv CoreRule) which seems a bit more natural

ruleBaseIds (RuleBase ids _) = ids
ruleBaseFVs (RuleBase _ fvs) = fvs

emptyRuleBase = RuleBase emptyVarSet emptyVarSet

addRuleBaseFVs :: RuleBase -> IdSet -> RuleBase
addRuleBaseFVs (RuleBase rules fvs) extra_fvs 
  = RuleBase rules (fvs `unionVarSet` extra_fvs)

extendRuleBaseList :: RuleBase -> [(Id,CoreRule)] -> RuleBase
extendRuleBaseList rule_base new_guys
  = foldl extendRuleBase rule_base new_guys

extendRuleBase :: RuleBase -> (Id,CoreRule) -> RuleBase
extendRuleBase (RuleBase rule_ids rule_fvs) (id, rule)
  = RuleBase (extendVarSet rule_ids new_id)
	     (rule_fvs `unionVarSet` extendVarSet lhs_fvs id)
  where
    new_id = setIdSpecialisation id (addRule old_rules id rule)

    old_rules = idSpecialisation (fromMaybe id (lookupVarSet rule_ids id))
	-- Get the old rules from rule_ids if the Id is already there, but
	-- if not, use the Id from the incoming rule.  If may be a PrimOpId,
	-- in which case it may have rules in its belly already.  Seems
	-- dreadfully hackoid.

    lhs_fvs = ruleLhsFreeIds rule
	-- Finds *all* the free Ids of the LHS, not just
	-- locally defined ones!!

pprRuleBase :: RuleBase -> SDoc
pprRuleBase (RuleBase rules _) = vcat [ pprCoreRule (ppr id) rs
				      | id <- varSetElems rules,
					rs <- rulesRules $ idSpecialisation id ]
\end{code}