2 % (c) The AQUA Project, Glasgow University, 1993-1998
4 \section{Common subexpression}
11 #include "HsVersions.h"
13 import CmdLineOpts ( DynFlag(..), DynFlags )
14 import Id ( Id, idType, idWorkerInfo )
15 import IdInfo ( workerExists )
16 import CoreUtils ( hashExpr, cheapEqExpr, exprIsBig, mkAltExpr, exprIsCheap )
17 import DataCon ( isUnboxedTupleCon )
18 import Type ( tyConAppArgs )
21 import CoreLint ( showPass, endPass )
23 import Util ( mapAccumL, lengthExceeds )
28 Simple common sub-expression
29 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
33 we build up a reverse mapping: C a b -> x1
35 and apply that to the rest of the program.
40 we replace the C a b with x1. But then we *dont* want to
41 add x1 -> y1 to the mapping. Rather, we want the reverse, y1 -> x1
42 so that a subsequent binding
44 will get transformed to C x1 b, and then to x2.
46 So we carry an extra var->var substitution which we apply *before* looking up in the
52 We have to be careful about shadowing.
54 f = \x -> let y = x+x in
58 Here we must *not* do CSE on the inner x+x! The simplifier used to guarantee no
59 shadowing, but it doesn't any more (it proved too hard), so we clone as we go.
60 We can simply add clones to the substitution already described.
62 However, we do NOT clone type variables. It's just too hard, because then we need
63 to run the substitution over types and IdInfo. No no no. Instead, we just throw
65 (In fact, I think the simplifier does guarantee no-shadowing for type variables.)
68 [Note: case binders 1]
69 ~~~~~~~~~~~~~~~~~~~~~~
72 f = \x -> case x of wild {
73 (a:as) -> case a of wild1 {
74 (p,q) -> ...(wild1:as)...
76 Here, (wild1:as) is morally the same as (a:as) and hence equal to wild.
77 But that's not quite obvious. In general we want to keep it as (wild1:as),
78 but for CSE purpose that's a bad idea.
80 So we add the binding (wild1 -> a) to the extra var->var mapping.
81 Notice this is exactly backwards to what the simplifier does, which is
82 to try to replaces uses of a with uses of wild1
84 [Note: case binders 2]
85 ~~~~~~~~~~~~~~~~~~~~~~
87 case (h x) of y -> ...(h x)...
89 We'd like to replace (h x) in the alternative, by y. But because of
90 the preceding [Note: case binders 1], we only want to add the mapping
91 scrutinee -> case binder
92 to the reverse CSE mapping if the scrutinee is a non-trivial expression.
93 (If the scrutinee is a simple variable we want to add the mapping
94 case binder -> scrutinee
97 [Note: unboxed tuple case binders]
98 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
100 case f x of t { (# a,b #) ->
105 We must not replace (f x) by t, because t is an unboxed-tuple binder.
106 Instead, we shoudl replace (f x) by (# a,b #). That is, the "reverse mapping" is
108 That is why the CSEMap has pairs of expressions.
111 %************************************************************************
113 \section{Common subexpression}
115 %************************************************************************
118 cseProgram :: DynFlags -> [CoreBind] -> IO [CoreBind]
120 cseProgram dflags binds
122 showPass dflags "Common sub-expression";
123 let { binds' = cseBinds emptyCSEnv binds };
124 endPass dflags "Common sub-expression" Opt_D_dump_cse binds'
127 cseBinds :: CSEnv -> [CoreBind] -> [CoreBind]
129 cseBinds env (b:bs) = (b':bs')
131 (env1, b') = cseBind env b
132 bs' = cseBinds env1 bs
134 cseBind :: CSEnv -> CoreBind -> (CSEnv, CoreBind)
135 cseBind env (NonRec b e) = let (env', (b',e')) = do_one env (b, e)
136 in (env', NonRec b' e')
137 cseBind env (Rec pairs) = let (env', pairs') = mapAccumL do_one env pairs
138 in (env', Rec pairs')
142 = case lookupCSEnv env rhs' of
143 Just (Var other_id) -> (extendSubst env' id other_id, (id', Var other_id))
144 Just other_expr -> (env', (id', other_expr))
145 Nothing -> (addCSEnvItem env' rhs' (Var id'), (id', rhs'))
147 (env', id') = addBinder env id
148 rhs' | not (workerExists (idWorkerInfo id)) = cseExpr env' rhs
150 -- Hack alert: don't do CSE on wrapper RHSs.
151 -- Otherwise we find:
153 -- f = \x -> ...$wf...
156 -- But the WorkerInfo for f still says $wf, which is now dead!
160 tryForCSE :: CSEnv -> CoreExpr -> CoreExpr
161 tryForCSE env (Type t) = Type t
162 tryForCSE env expr = case lookupCSEnv env expr' of
163 Just smaller_expr -> smaller_expr
166 expr' = cseExpr env expr
168 cseExpr :: CSEnv -> CoreExpr -> CoreExpr
169 cseExpr env (Type t) = Type t
170 cseExpr env (Lit lit) = Lit lit
171 cseExpr env (Var v) = Var (lookupSubst env v)
172 cseExpr env (App f a) = App (cseExpr env f) (tryForCSE env a)
173 cseExpr env (Note n e) = Note n (cseExpr env e)
174 cseExpr env (Lam b e) = let (env', b') = addBinder env b
175 in Lam b' (cseExpr env' e)
176 cseExpr env (Let bind e) = let (env', bind') = cseBind env bind
177 in Let bind' (cseExpr env' e)
178 cseExpr env (Case scrut bndr ty alts) = Case scrut' bndr' ty (cseAlts env' scrut' bndr bndr' alts)
180 scrut' = tryForCSE env scrut
181 (env', bndr') = addBinder env bndr
184 cseAlts env scrut' bndr bndr' [(DataAlt con, args, rhs)]
185 | isUnboxedTupleCon con
186 -- Unboxed tuples are special because the case binder isn't
187 -- a real values. See [Note: unboxed tuple case binders]
188 = [(DataAlt con, args', tryForCSE new_env rhs)]
190 (env', args') = addBinders env args
191 new_env | exprIsCheap scrut' = env'
192 | otherwise = extendCSEnv env' scrut' tup_value
193 tup_value = mkAltExpr (DataAlt con) args' (tyConAppArgs (idType bndr))
195 cseAlts env scrut' bndr bndr' alts
198 (con_target, alt_env)
200 Var v' -> (v', extendSubst env bndr v') -- See [Note: case binder 1]
203 other -> (bndr', extendCSEnv env scrut' (Var bndr')) -- See [Note: case binder 2]
204 -- map: scrut' -> bndr'
206 arg_tys = tyConAppArgs (idType bndr)
208 cse_alt (DataAlt con, args, rhs)
210 -- Don't try CSE if there are no args; it just increases the number
211 -- of live vars. E.g.
212 -- case x of { True -> ....True.... }
213 -- Don't replace True by x!
214 -- Hence the 'null args', which also deal with literals and DEFAULT
215 = (DataAlt con, args', tryForCSE new_env rhs)
217 (env', args') = addBinders alt_env args
218 new_env = extendCSEnv env' (mkAltExpr (DataAlt con) args' arg_tys)
221 cse_alt (con, args, rhs)
222 = (con, args', tryForCSE env' rhs)
224 (env', args') = addBinders alt_env args
228 %************************************************************************
230 \section{The CSE envt}
232 %************************************************************************
235 data CSEnv = CS CSEMap InScopeSet (IdEnv Id)
236 -- Simple substitution
238 type CSEMap = UniqFM [(CoreExpr, CoreExpr)] -- This is the reverse mapping
239 -- It maps the hash-code of an expression e to list of (e,e') pairs
240 -- This means that it's good to replace e by e'
241 -- INVARIANT: The expr in the range has already been CSE'd
243 emptyCSEnv = CS emptyUFM emptyInScopeSet emptyVarEnv
245 lookupCSEnv :: CSEnv -> CoreExpr -> Maybe CoreExpr
246 lookupCSEnv (CS cs _ _) expr
247 = case lookupUFM cs (hashExpr expr) of
249 Just pairs -> lookup_list pairs expr
251 lookup_list :: [(CoreExpr,CoreExpr)] -> CoreExpr -> Maybe CoreExpr
252 lookup_list [] expr = Nothing
253 lookup_list ((e,e'):es) expr | cheapEqExpr e expr = Just e'
254 | otherwise = lookup_list es expr
256 addCSEnvItem env expr expr' | exprIsBig expr = env
257 | otherwise = extendCSEnv env expr expr'
258 -- We don't try to CSE big expressions, because they are expensive to compare
259 -- (and are unlikely to be the same anyway)
261 extendCSEnv (CS cs in_scope sub) expr expr'
262 = CS (addToUFM_C combine cs hash [(expr, expr')]) in_scope sub
265 combine old new = WARN( result `lengthExceeds` 4, text "extendCSEnv: long list:" <+> ppr result )
270 lookupSubst (CS _ _ sub) x = case lookupVarEnv sub x of
274 extendSubst (CS cs in_scope sub) x y = CS cs in_scope (extendVarEnv sub x y)
276 addBinder :: CSEnv -> Id -> (CSEnv, Id)
277 addBinder env@(CS cs in_scope sub) v
278 | not (v `elemInScopeSet` in_scope) = (CS cs (extendInScopeSet in_scope v) sub, v)
279 | isId v = (CS cs (extendInScopeSet in_scope v') (extendVarEnv sub v v'), v')
280 | not (isId v) = WARN( True, ppr v )
281 (CS emptyUFM in_scope sub, v)
282 -- This last case is the unusual situation where we have shadowing of
283 -- a type variable; we have to discard the CSE mapping
284 -- See "IMPORTANT NOTE" at the top
286 v' = uniqAway in_scope v
288 addBinders :: CSEnv -> [Id] -> (CSEnv, [Id])
289 addBinders env vs = mapAccumL addBinder env vs