CSE.hs 14.7 KB
Newer Older
Austin Seipp's avatar
Austin Seipp committed
1 2 3
{-
(c) The AQUA Project, Glasgow University, 1993-1998

4
\section{Common subexpression}
Austin Seipp's avatar
Austin Seipp committed
5
-}
6

7 8
{-# LANGUAGE CPP #-}

9
module CSE (cseProgram) where
10 11 12

#include "HsVersions.h"

13
import CoreSubst
14
import Var              ( Var )
15 16
import Id               ( Id, idType, idUnfolding, idInlineActivation
                        , zapIdOccInfo, zapIdUsageInfo )
17
import CoreUtils        ( mkAltExpr
18 19
                        , exprIsTrivial, exprOkForSpeculation
                        , stripTicksE, stripTicksT, mkTicks )
20
import Type             ( tyConAppArgs, isUnliftedType )
21 22
import CoreSyn
import Outputable
23
import BasicTypes       ( isAlwaysActive )
24
import TrieMap
25 26

import Data.List
27

Austin Seipp's avatar
Austin Seipp committed
28
{-
29 30
                        Simple common sub-expression
                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
31
When we see
32 33
        x1 = C a b
        x2 = C x1 b
34
we build up a reverse mapping:   C a b  -> x1
35
                                 C x1 b -> x2
36 37 38
and apply that to the rest of the program.

When we then see
39 40
        y1 = C a b
        y2 = C y1 b
41
we replace the C a b with x1.  But then we *dont* want to
42
add   x1 -> y1  to the mapping.  Rather, we want the reverse, y1 -> x1
43
so that a subsequent binding
44 45
        y2 = C y1 b
will get transformed to C x1 b, and then to x2.
46

47
So we carry an extra var->var substitution which we apply *before* looking up in the
48 49 50
reverse mapping.


simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
51 52
Note [Shadowing]
~~~~~~~~~~~~~~~~
53
We have to be careful about shadowing.
54
For example, consider
55 56 57
        f = \x -> let y = x+x in
                      h = \x -> x+x
                  in ...
58

59 60 61 62
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.

63

64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89
Note [CSE for bindings]
~~~~~~~~~~~~~~~~~~~~~~~
Let-bindings have two cases, implemnted by cseRhs.

* Trivial RHS:
     let x = y in ...(h x)....

  Here we want to extend the /substitution/ with x -> y, so that the
  (h x) in the body might CSE with an enclosing (let v = h y in ...).
  NB: the substitution maps InIds, so we extend the substitution with
      a biding for the original InId 'x'

  How can we have a trivial RHS? Doens't the simplifier inline them?

    - First, the original RHS might have been (g z) which has CSE'd
      with an enclosing (let y = g z in ...).  This is super-important.
      See Trac #5996:
         x1 = C a b
         x2 = C x1 b
         y1 = C a b
         y2 = C y1 b
      Here we CSE y1's rhs to 'x1', and then we must add (y1->x1) to
      the substitution so that we can CSE the binding for y2.

    - Second, we use cseRHS for case expression scrutinees too;
      see Note [CSE for case expressions]
90

91 92
* Non-trivial RHS
     let x = h y in ...(h y)...
93

94 95
  Here we want to extend the /reverse mapping (cs_map)/ so that
  we CSE the (h y) call to x.
96

97 98 99 100 101 102
Notice that
  - the trivial-RHS situation extends the substitution (cs_subst)
  - the non-trivial-RHS situation extends the reverse mapping (cs_map)

Note [CSE for case expressions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
103
Consider
104 105 106 107 108 109 110 111 112 113 114 115
  case scrut_expr of x { ...alts... }
This is very like a strict let-binding
  let !x = scrut_expr in ...
So we use (cseRhs x scrut_expr) to process scrut_expr and x, and as a
result all the stuff under Note [CSE for bindings] applies directly.

For example:

* Trivial scrutinee
     f = \x -> case x of wild {
                 (a:as) -> case a of wild1 {
                             (p,q) -> ...(wild1:as)...
116

117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134
  Here, (wild1:as) is morally the same as (a:as) and hence equal to
  wild. But that's not quite obvious.  In the rest of the compiler we
  want to keep it as (wild1:as), but for CSE purpose that's a bad
  idea.

  By using cseRhs we add the binding (wild1 -> a) to the substitution,
  which does exactly the right thing.

  (Notice this is exactly backwards to what the simplifier does, which
  is to try to replaces uses of 'a' with uses of 'wild1'.)

  This is the main reason that cseRHs is called with a trivial rhs.

* Non-trivial scrutinee
     case (f x) of y { pat -> ...let y = f x in ... }

  By using cseRhs we'll add (f x :-> y) to the cs_map, and
  thereby CSE the inner (f x) to y.
135

136 137
Note [CSE for INLINE and NOINLINE]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
138 139 140
There are some subtle interactions of CSE with functions that the user
has marked as INLINE or NOINLINE. (Examples from Roman Leshchinskiy.)
Consider
141

142
        yes :: Int  {-# NOINLINE yes #-}
143
        yes = undefined
144

145
        no :: Int   {-# NOINLINE no #-}
146
        no = undefined
147

148
        foo :: Int -> Int -> Int  {-# NOINLINE foo #-}
149
        foo m n = n
150

151
        {-# RULES "foo/no" foo no = id #-}
152

153 154
        bar :: Int -> Int
        bar = foo yes
155

156
We do not expect the rule to fire.  But if we do CSE, then we risk
Gabor Greif's avatar
Gabor Greif committed
157
getting yes=no, and the rule does fire.  Actually, it won't because
158
NOINLINE means that 'yes' will never be inlined, not even if we have
Gabor Greif's avatar
Gabor Greif committed
159
yes=no.  So that's fine (now; perhaps in the olden days, yes=no would
Simon Peyton Jones's avatar
Simon Peyton Jones committed
160
have substituted even if 'yes' was NOINLINE).
161

162
But we do need to take care.  Consider
163

164 165
        {-# NOINLINE bar #-}
        bar = <rhs>     -- Same rhs as foo
166

167 168
        foo = <rhs>

169
If CSE produces
170
        foo = bar
171 172 173 174 175 176 177 178 179 180 181 182 183 184 185
then foo will never be inlined to <rhs> (when it should be, if <rhs>
is small).  The conclusion here is this:

   We should not add
       <rhs> :-> bar
  to the CSEnv if 'bar' has any constraints on when it can inline;
  that is, if its 'activation' not always active.  Otherwise we
  might replace <rhs> by 'bar', and then later be unable to see that it
  really was <rhs>.

Note that we do not (currently) do CSE on the unfolding stored inside
an Id, even if is a 'stable' unfolding.  That means that when an
unfolding happens, it is always faithful to what the stable unfolding
originally was.

186 187
Note [CSE for stable unfoldings]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
188
Consider
189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226
   {-# Unf = Stable (\pq. build blah) #-}
   foo = x

Here 'foo' has a stable unfolding, but its (optimised) RHS is trivial.
(Turns out that this actually happens for the enumFromTo method of
the Integer instance of Enum in GHC.Enum.)  Then we obviously do NOT
want to extend the substitution with (foo->x)!   See similar
SimplUtils Note [Stable unfoldings and postInlineUnconditionally].

Nor do we want to change the reverse mapping. Suppose we have

   {-# Unf = Stable (\pq. build blah) #-}
   foo = <expr>
   bar = <expr>

There could conceivably be merit in rewriting the RHS of bar:
   bar = foo
but now bar's inlining behaviour will change, and importing
modules might see that.  So it seems dodgy and we don't do it.

Note [Corner case for case expressions]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Consdider
   case x |> co of (y::Array# Int) { ... }

Is it ok to extend the substutition with (y -> x |> co)?
Because y is of unlifted type, this is only OK if (x |> co) is
ok-for-speculation, else we'll destroy the let/app invariant.
But surely it is ok-for-speculation, becasue it's a trivial
expression, and x's type is also unlifted, presumably.

Well, maybe not if you are using unsafe casts.  I actually found
a case where we had
   (x :: HValue) |> (UnsafeCo :: HValue ~ Array# Int)
This is a vanishingly strange corner case, but we still have
to check.

We do the check in cseRhs, but it can't fire when cseRhs is called
Gabor Greif's avatar
Gabor Greif committed
227
from a let-binding, because they are always ok-for-speculation.  Never
228 229
mind!

230

Austin Seipp's avatar
Austin Seipp committed
231 232
************************************************************************
*                                                                      *
233
\section{Common subexpression}
Austin Seipp's avatar
Austin Seipp committed
234 235 236
*                                                                      *
************************************************************************
-}
237

238
cseProgram :: CoreProgram -> CoreProgram
239
cseProgram binds = snd (mapAccumL cseBind emptyCSEnv binds)
240 241

cseBind :: CSEnv -> CoreBind -> (CSEnv, CoreBind)
242
cseBind env (NonRec b e)
243
  = (env2, NonRec b2 e2)
244
  where
245 246 247
    e1               = tryForCSE env e
    (env1, b1)       = addBinder env b
    (env2, (b2, e2)) = addBinding env1 b b1 e1
248 249

cseBind env (Rec pairs)
250
  = (env2, Rec pairs')
251
  where
252 253 254 255 256 257 258 259 260 261 262 263 264 265
    (bndrs, rhss)  = unzip pairs
    (env1, bndrs1) = addRecBinders env bndrs
    rhss1          = map (tryForCSE env1) rhss
                     -- Process rhss in extended env1
    (env2, pairs') = mapAccumL cse_rhs env1 (zip3 bndrs bndrs1 rhss1)
    cse_rhs env (b, b1, e1) = addBinding env b b1 e1

addBinding :: CSEnv                      -- Includes InId->OutId cloning
           -> InId
           -> OutId -> OutExpr           -- Processed binding
           -> (CSEnv, (OutId, OutExpr))  -- Final env and binding
-- Extend the CSE env with a mapping [rhs -> out-id]
-- unless we can instead just substitute [in-id -> rhs]
addBinding env in_id out_id rhs'
266 267 268
  | no_cse      = (env,                              (out_id, rhs'))
  | ok_to_subst = (extendCSSubst env in_id rhs',     (out_id, rhs'))
  | otherwise   = (extendCSEnv env rhs' id_expr', (zapped_id, rhs'))
269
  where
270 271 272
    id_expr'  = varToCoreExpr out_id
    zapped_id = zapIdUsageInfo out_id
       -- Putting the Id into the cs_map makes it possible that
273
       -- it'll become shared more than it is now, which would
274 275
       -- invalidate (the usage part of) its demand info.
       --    This caused Trac #100218.
276 277 278 279 280 281
       -- Easiest thing is to zap the usage info; subsequently
       -- performing late demand-analysis will restore it.  Don't zap
       -- the strictness info; it's not necessary to do so, and losing
       -- it is bad for performance if you don't do late demand
       -- analysis

282 283 284 285
    no_cse = not (isAlwaysActive (idInlineActivation out_id))
             -- See Note [CSE for INLINE and NOINLINE]
          || isStableUnfolding (idUnfolding out_id)
             -- See Note [CSE for stable unfoldings]
Peter Wortmann's avatar
Peter Wortmann committed
286

287 288 289 290 291
    -- See Note [CSE for bindings]
    ok_to_subst = exprIsTrivial rhs'
               && (not (isUnliftedType (idType out_id))
                   || exprOkForSpeculation rhs')
               -- See Note [Corner case for case expressions]
292

293
tryForCSE :: CSEnv -> InExpr -> OutExpr
294
tryForCSE env expr
295 296 297 298 299 300 301
  | exprIsTrivial expr'              = expr'       -- No point
  | Just e <- lookupCSEnv env expr'' = mkTicks ticks e
  | otherwise                        = expr'
    -- The varToCoreExpr is needed if we have
    --   case e of xco { ...case e of yco { ... } ... }
    -- Then CSE will substitute yco -> xco;
    -- but these are /coercion/ variables
302
  where
303
    expr'  = cseExpr env expr
304
    expr'' = stripTicksE tickishFloatable expr'
305 306 307 308 309
    ticks  = stripTicksT tickishFloatable expr'
    -- We don't want to lose the source notes when a common sub
    -- expression gets eliminated. Hence we push all (!) of them on
    -- top of the replaced sub-expression. This is probably not too
    -- useful in practice, but upholds our semantics.
310

311
cseExpr :: CSEnv -> InExpr -> OutExpr
312 313
cseExpr env (Type t)               = Type (substTy (csEnvSubst env) t)
cseExpr env (Coercion c)           = Coercion (substCo (csEnvSubst env) c)
twanvl's avatar
twanvl committed
314
cseExpr _   (Lit lit)              = Lit lit
315 316
cseExpr env (Var v)                = lookupSubst env v
cseExpr env (App f a)              = App (cseExpr env f) (tryForCSE env a)
317
cseExpr env (Tick t e)             = Tick t (cseExpr env e)
318
cseExpr env (Cast e co)            = Cast (cseExpr env e) (substCo (csEnvSubst env) co)
319 320 321 322
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)
323 324 325 326
cseExpr env (Case e bndr ty alts)  = cseCase env e bndr ty alts

cseCase :: CSEnv -> InExpr -> InId -> InType -> [InAlt] -> OutExpr
cseCase env scrut bndr ty alts
327
  = Case scrut2 bndr3 ty (map cse_alt alts)
328
  where
329 330
    scrut1 = tryForCSE env scrut

331 332 333 334 335
    bndr1 = zapIdOccInfo bndr
      -- Zapping the OccInfo is needed because the extendCSEnv
      -- in cse_alt may mean that a dead case binder
      -- becomes alive, and Lint rejects that
    (env1, bndr2)              = addBinder env bndr1
336 337
    (alt_env, (bndr3, scrut2)) = addBinding env1 bndr bndr2 scrut1
         -- addBinding: see Note [CSE for case expressions]
338

339 340
    con_target :: OutExpr
    con_target = lookupSubst alt_env bndr
341

342 343
    arg_tys :: [OutType]
    arg_tys = tyConAppArgs (idType bndr3)
344

345
    cse_alt (DataAlt con, args, rhs)
346 347 348 349 350 351 352 353 354
        | 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
355 356
          new_env       = extendCSEnv env' con_expr con_target
          con_expr      = mkAltExpr (DataAlt con) args' arg_tys
357 358

    cse_alt (con, args, rhs)
359 360 361
        = (con, args', tryForCSE env' rhs)
        where
          (env', args') = addBinders alt_env args
362

Austin Seipp's avatar
Austin Seipp committed
363 364 365
{-
************************************************************************
*                                                                      *
366
\section{The CSE envt}
Austin Seipp's avatar
Austin Seipp committed
367 368 369
*                                                                      *
************************************************************************
-}
370

371 372 373 374
data CSEnv
  = CS { cs_subst :: Subst  -- Maps InBndrs to OutExprs
            -- The substitution variables to
            -- /trivial/ OutExprs, not arbitrary expressions
375

376 377 378 379
       , cs_map   :: CoreMap OutExpr   -- The reverse mapping
            -- Maps a OutExpr to a /trivial/ OutExpr
            -- The key of cs_map is stripped of all Ticks
       }
380 381 382 383

emptyCSEnv :: CSEnv
emptyCSEnv = CS { cs_map = emptyCoreMap, cs_subst = emptySubst }

384
lookupCSEnv :: CSEnv -> OutExpr -> Maybe OutExpr
385
lookupCSEnv (CS { cs_map = csmap }) expr
386 387 388 389 390 391 392
  = lookupCoreMap csmap expr

extendCSEnv :: CSEnv -> OutExpr -> OutExpr -> CSEnv
extendCSEnv cse expr triv_expr
  = cse { cs_map = extendCoreMap (cs_map cse) sexpr triv_expr }
  where
    sexpr = stripTicksE tickishFloatable expr
393 394 395 396

csEnvSubst :: CSEnv -> Subst
csEnvSubst = cs_subst

397
lookupSubst :: CSEnv -> Id -> OutExpr
398
lookupSubst (CS { cs_subst = sub}) x = lookupIdSubst (text "CSE.lookupSubst") sub x
399

400 401
extendCSSubst :: CSEnv -> Id  -> CoreExpr -> CSEnv
extendCSSubst cse x rhs = cse { cs_subst = extendSubst (cs_subst cse) x rhs }
402 403

addBinder :: CSEnv -> Var -> (CSEnv, Var)
404
addBinder cse v = (cse { cs_subst = sub' }, v')
405 406
                where
                  (sub', v') = substBndr (cs_subst cse) v
407 408

addBinders :: CSEnv -> [Var] -> (CSEnv, [Var])
409
addBinders cse vs = (cse { cs_subst = sub' }, vs')
410 411
                where
                  (sub', vs') = substBndrs (cs_subst cse) vs
412

413
addRecBinders :: CSEnv -> [Id] -> (CSEnv, [Id])
414
addRecBinders cse vs = (cse { cs_subst = sub' }, vs')
415 416
                where
                  (sub', vs') = substRecBndrs (cs_subst cse) vs