/ghc-7.0.4/compiler/simplCore/CSE.lhs

%
% (c) The AQUA Project, Glasgow University, 1993-1998
%
\section{Common subexpression}

\begin{code}
module CSE (
	cseProgram
    ) where

#include "HsVersions.h"

import Id		( Id, idType, idInlineActivation, zapIdOccInfo )
import CoreUtils	( hashExpr, eqExpr, exprIsBig, mkAltExpr, exprIsCheap )
import DataCon		( isUnboxedTupleCon )
import Type		( tyConAppArgs )
import CoreSyn
import VarEnv	
import Outputable
import StaticFlags	( opt_PprStyle_Debug )
import BasicTypes	( isAlwaysActive )
import Util		( lengthExceeds )
import UniqFM
import FastString

import Data.List
\end{code}


			Simple common sub-expression
			~~~~~~~~~~~~~~~~~~~~~~~~~~~~
When we see
	x1 = C a b
	x2 = C x1 b
we build up a reverse mapping:   C a b  -> x1
				 C x1 b -> x2
and apply that to the rest of the program.

When we then see
	y1 = C a b
	y2 = C y1 b
we replace the C a b with x1.  But then we *dont* want to
add   x1 -> y1  to the mapping.  Rather, we want the reverse, y1 -> x1
so that a subsequent binding
	y2 = C y1 b
will get transformed to C x1 b, and then to x2.  

So we carry an extra var->var substitution which we apply *before* looking up in the
reverse mapping.


Note [Shadowing]
~~~~~~~~~~~~~~~~
We have to be careful about shadowing.
For example, consider
	f = \x -> let y = x+x in
	              h = \x -> x+x
	          in ...

Here we must *not* do CSE on the inner x+x!  The simplifier used to guarantee no
shadowing, but it doesn't any more (it proved too hard), so we clone as we go.
We can simply add clones to the substitution already described.

However, we do NOT clone type variables.  It's just too hard, because then we need
to run the substitution over types and IdInfo.  No no no.  Instead, we just throw

(In fact, I think the simplifier does guarantee no-shadowing for type variables.)


Note [Case binders 1]
~~~~~~~~~~~~~~~~~~~~~~
Consider

	f = \x -> case x of wild { 
			(a:as) -> case a of wild1 {
				    (p,q) -> ...(wild1:as)...

Here, (wild1:as) is morally the same as (a:as) and hence equal to wild.
But that's not quite obvious.  In general we want to keep it as (wild1:as),
but for CSE purpose that's a bad idea.

So we add the binding (wild1 -> a) to the extra var->var mapping.
Notice this is exactly backwards to what the simplifier does, which is
to try to replaces uses of 'a' with uses of 'wild1'

Note [Case binders 2]
~~~~~~~~~~~~~~~~~~~~~~
Consider
	case (h x) of y -> ...(h x)...

We'd like to replace (h x) in the alternative, by y.  But because of
the preceding [Note: case binders 1], we only want to add the mapping
	scrutinee -> case binder
to the reverse CSE mapping if the scrutinee is a non-trivial expression.
(If the scrutinee is a simple variable we want to add the mapping
	case binder -> scrutinee 
to the substitution

Note [Unboxed tuple case binders]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consider
	case f x of t { (# a,b #) -> 
	case ... of
	  True -> f x
	  False -> 0 }

We must not replace (f x) by t, because t is an unboxed-tuple binder.
Instead, we shoudl replace (f x) by (# a,b #).  That is, the "reverse mapping" is
	f x --> (# a,b #)
That is why the CSEMap has pairs of expressions.

Note [CSE for INLINE and NOINLINE]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
We are careful to do no CSE inside functions that the user has marked as
INLINE or NOINLINE.  In terms of Core, that means 

	a) we do not do CSE inside an InlineRule

	b) we do not do CSE on the RHS of a binding b=e
	   unless b's InlinePragma is AlwaysActive

Here's why (examples from Roman Leshchinskiy).  Consider

	yes :: Int
	{-# NOINLINE yes #-}
	yes = undefined

	no :: Int
	{-# NOINLINE no #-}
	no = undefined

	foo :: Int -> Int -> Int
	{-# NOINLINE foo #-}
	foo m n = n

	{-# RULES "foo/no" foo no = id #-}

	bar :: Int -> Int
	bar = foo yes

We do not expect the rule to fire.  But if we do CSE, then we get
yes=no, and the rule does fire.  Worse, whether we get yes=no or
no=yes depends on the order of the definitions.

In general, CSE should probably never touch things with INLINE pragmas
as this could lead to surprising results.  Consider

	{-# INLINE foo #-}
	foo = <rhs>

	{-# NOINLINE bar #-}
	bar = <rhs>	-- Same rhs as foo

If CSE produces
	foo = bar
then foo will never be inlined (when it should be); but if it produces
	bar = foo
bar will be inlined (when it should not be). Even if we remove INLINE foo,
we'd still like foo to be inlined if rhs is small. This won't happen
with foo = bar.

Not CSE-ing inside INLINE also solves an annoying bug in CSE. Consider
a worker/wrapper, in which the worker has turned into a single variable:
	$wf = h
	f = \x -> ...$wf...
Now CSE may transform to
	f = \x -> ...h...
But the WorkerInfo for f still says $wf, which is now dead!  This won't
happen now that we don't look inside INLINEs (which wrappers are).


%************************************************************************
%*									*
\section{Common subexpression}
%*									*
%************************************************************************

\begin{code}
cseProgram :: [CoreBind] -> [CoreBind]
cseProgram binds = cseBinds emptyCSEnv binds

cseBinds :: CSEnv -> [CoreBind] -> [CoreBind]
cseBinds _   []     = []
cseBinds env (b:bs) = (b':bs')
		    where
		      (env1, b') = cseBind  env  b
		      bs'        = cseBinds env1 bs

cseBind :: CSEnv -> CoreBind -> (CSEnv, CoreBind)
cseBind env (NonRec b e) = let (env', (b',e')) = do_one env (b, e)
			   in (env', NonRec b' e')
cseBind env (Rec pairs)  = let (env', pairs') = mapAccumL do_one env pairs
			   in (env', Rec pairs')
			 

do_one :: CSEnv -> (Id, CoreExpr) -> (CSEnv, (Id, CoreExpr))
do_one env (id, rhs) 
  = case lookupCSEnv env rhs' of
	Just (Var other_id) -> (extendSubst env' id other_id, 	  (id', Var other_id))
	Just other_expr     -> (env', 			      	  (id', other_expr))
	Nothing             -> (addCSEnvItem env' rhs' (Var id'), (id', rhs'))
  where
    (env', id') = addBinder env id
    rhs' | isAlwaysActive (idInlineActivation id) = cseExpr env' rhs
	 | otherwise			          = rhs
		-- See Note [CSE for INLINE and NOINLINE]

tryForCSE :: CSEnv -> CoreExpr -> CoreExpr
tryForCSE _   (Type t) = Type t
tryForCSE env expr     = case lookupCSEnv env expr' of
			    Just smaller_expr -> smaller_expr
 			    Nothing  	      -> expr'
		       where
	 	         expr' = cseExpr env expr

cseExpr :: CSEnv -> CoreExpr -> CoreExpr
cseExpr _   (Type t)               = Type t
cseExpr _   (Lit lit)              = Lit lit
cseExpr env (Var v)		   = Var (lookupSubst env v)
cseExpr env (App f a)        	   = App (cseExpr env f) (tryForCSE env a)
cseExpr env (Note n e)       	   = Note n (cseExpr env e)
cseExpr env (Cast e co)            = Cast (cseExpr env e) co
cseExpr env (Lam b e)	     	   = let (env', b') = addBinder env b
				     in Lam b' (cseExpr env' e)
cseExpr env (Let bind e)    	   = let (env', bind') = cseBind env bind
				     in Let bind' (cseExpr env' e)
cseExpr env (Case scrut bndr ty alts) = Case scrut' bndr'' ty (cseAlts env' scrut' bndr bndr'' alts)
				   where
				     scrut' = tryForCSE env scrut
				     (env', bndr') = addBinder env bndr
				     bndr'' = zapIdOccInfo bndr'
					-- The swizzling from Note [Case binders 2] may
					-- cause a dead case binder to be alive, so we
					-- play safe here and bring them all to life

cseAlts :: CSEnv -> CoreExpr -> CoreBndr -> CoreBndr -> [CoreAlt] -> [CoreAlt]

cseAlts env scrut' bndr _bndr' [(DataAlt con, args, rhs)]
  | isUnboxedTupleCon con
	-- Unboxed tuples are special because the case binder isn't
	-- a real value.  See Note [Unboxed tuple case binders]
  = [(DataAlt con, args'', tryForCSE new_env rhs)]
  where
    (env', args') = addBinders env args
    args'' = map zapIdOccInfo args'	-- They should all be ids
	-- Same motivation for zapping as [Case binders 2] only this time
	-- it's Note [Unboxed tuple case binders]
    new_env | exprIsCheap scrut' = env'
	    | otherwise 	 = extendCSEnv env' scrut' tup_value
    tup_value = mkAltExpr (DataAlt con) args'' (tyConAppArgs (idType bndr))

cseAlts env scrut' bndr bndr' alts
  = map cse_alt alts
  where
    (con_target, alt_env)
	= case scrut' of
		Var v' -> (v',     extendSubst env bndr v')	-- See Note [Case binders 1]
								-- map: bndr -> v'

		_      ->  (bndr', extendCSEnv env scrut' (Var  bndr'))	-- See Note [Case binders 2]
									-- map: scrut' -> bndr'

    arg_tys = tyConAppArgs (idType bndr)

    cse_alt (DataAlt con, args, rhs)
	| not (null args)
		-- Don't try CSE if there are no args; it just increases the number
		-- of live vars.  E.g.
		--	case x of { True -> ....True.... }
		-- Don't replace True by x!  
		-- Hence the 'null args', which also deal with literals and DEFAULT
	= (DataAlt con, args', tryForCSE new_env rhs)
	where
	  (env', args') = addBinders alt_env args
	  new_env       = extendCSEnv env' (mkAltExpr (DataAlt con) args' arg_tys)
					   (Var con_target)

    cse_alt (con, args, rhs)
	= (con, args', tryForCSE env' rhs)
	where
	  (env', args') = addBinders alt_env args
\end{code}


%************************************************************************
%*									*
\section{The CSE envt}
%*									*
%************************************************************************

\begin{code}
data CSEnv = CS CSEMap InScopeSet (IdEnv Id)
			-- Simple substitution

type CSEMap = UniqFM [(CoreExpr, CoreExpr)]	-- This is the reverse mapping
	-- It maps the hash-code of an expression e to list of (e,e') pairs
	-- This means that it's good to replace e by e'
	-- INVARIANT: The expr in the range has already been CSE'd

emptyCSEnv :: CSEnv
emptyCSEnv = CS emptyUFM emptyInScopeSet emptyVarEnv

lookupCSEnv :: CSEnv -> CoreExpr -> Maybe CoreExpr
lookupCSEnv (CS cs in_scope _) expr
  = case lookupUFM cs (hashExpr expr) of
	Nothing -> Nothing
	Just pairs -> lookup_list pairs
  where
  -- In this lookup we use full expression equality
  -- Reason: when expressions differ we generally find out quickly
  --         but I found that cheapEqExpr was saying (\x.x) /= (\y.y),
  -- 	     and this kind of thing happened in real programs
    lookup_list :: [(CoreExpr,CoreExpr)] -> Maybe CoreExpr
    lookup_list []                                   = Nothing
    lookup_list ((e,e'):es) | eqExpr in_scope e expr = Just e'
        		    | otherwise	             = lookup_list es

addCSEnvItem :: CSEnv -> CoreExpr -> CoreExpr -> CSEnv
addCSEnvItem env expr expr' | exprIsBig expr = env
			    | otherwise      = extendCSEnv env expr expr'
   -- We don't try to CSE big expressions, because they are expensive to compare
   -- (and are unlikely to be the same anyway)

extendCSEnv :: CSEnv -> CoreExpr -> CoreExpr -> CSEnv
extendCSEnv (CS cs in_scope sub) expr expr'
  = CS (addToUFM_C combine cs hash [(expr, expr')]) in_scope sub
  where
    hash = hashExpr expr
    combine old new 
	= WARN( result `lengthExceeds` 4, short_msg $$ nest 2 long_msg ) result
	where
	  result = new ++ old
	  short_msg = ptext (sLit "extendCSEnv: long list, length") <+> int (length result)
	  long_msg | opt_PprStyle_Debug = (text "hash code" <+> text (show hash)) $$ ppr result 
		   | otherwise	        = empty

lookupSubst :: CSEnv -> Id -> Id
lookupSubst (CS _ _ sub) x = case lookupVarEnv sub x of
			       Just y  -> y
			       Nothing -> x

extendSubst :: CSEnv -> Id  -> Id -> CSEnv
extendSubst (CS cs in_scope sub) x y = CS cs in_scope (extendVarEnv sub x y)

addBinder :: CSEnv -> Id -> (CSEnv, Id)
addBinder (CS cs in_scope sub) v
  | not (v `elemInScopeSet` in_scope) = (CS cs (extendInScopeSet in_scope v)  sub,		       v)
  | isId v			      = (CS cs (extendInScopeSet in_scope v') (extendVarEnv sub v v'), v')
  | otherwise			      = WARN( True, ppr v )
				        (CS emptyUFM in_scope		      sub,		       v)
	-- This last case is the unusual situation where we have shadowing of
	-- a type variable; we have to discard the CSE mapping
	-- See Note [Shadowing]
  where
    v' = uniqAway in_scope v

addBinders :: CSEnv -> [Id] -> (CSEnv, [Id])
addBinders env vs = mapAccumL addBinder env vs
\end{code}