Simplify.lhs 70.8 KB
Newer Older
1
%
2
% (c) The AQUA Project, Glasgow University, 1993-1998
3 4 5 6
%
\section[Simplify]{The main module of the simplifier}

\begin{code}
7
{-# OPTIONS -w #-}
8 9 10
-- The above warning supression flag is a temporary kludge.
-- While working on this module you are encouraged to remove it and fix
-- any warnings in the module. See
Ian Lynagh's avatar
Ian Lynagh committed
11
--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
12 13
-- for details

14
module Simplify ( simplTopBinds, simplExpr ) where
15

16
#include "HsVersions.h"
17

simonpj@microsoft.com's avatar
Wibble  
simonpj@microsoft.com committed
18
import DynFlags
19
import SimplMonad
20
import Type hiding	( substTy, extendTvSubst )
21
import SimplEnv	
22 23
import SimplUtils
import Id
24
import Var
25 26
import IdInfo
import Coercion
27
import FamInstEnv	( topNormaliseType )
simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
28
import DataCon		( dataConRepStrictness, dataConUnivTyVars )
29
import CoreSyn
30
import NewDemand	( isStrictDmd )
31
import PprCore		( pprParendExpr, pprCoreExpr )
32
import CoreUnfold	( mkUnfolding, callSiteInline )
33
import CoreUtils
34
import Rules		( lookupRule )
35
import BasicTypes	( isMarkedStrict )
36
import CostCentre	( currentCCS )
37
import TysPrim		( realWorldStatePrimTy )
38
import PrelInfo		( realWorldPrimId )
39
import BasicTypes	( TopLevelFlag(..), isTopLevel, 
40
			  RecFlag(..), isNonRuleLoopBreaker )
41
import Maybes		( orElse )
42
import Data.List	( mapAccumL )
43
import Outputable
44
import Util
45 46 47
\end{code}


48 49
The guts of the simplifier is in this module, but the driver loop for
the simplifier is in SimplCore.lhs.
50 51


52 53 54 55 56 57 58 59
-----------------------------------------
	*** IMPORTANT NOTE ***
-----------------------------------------
The simplifier used to guarantee that the output had no shadowing, but
it does not do so any more.   (Actually, it never did!)  The reason is
documented with simplifyArgs.


60 61 62 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 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129
-----------------------------------------
	*** IMPORTANT NOTE ***
-----------------------------------------
Many parts of the simplifier return a bunch of "floats" as well as an
expression. This is wrapped as a datatype SimplUtils.FloatsWith.

All "floats" are let-binds, not case-binds, but some non-rec lets may
be unlifted (with RHS ok-for-speculation).



-----------------------------------------
	ORGANISATION OF FUNCTIONS
-----------------------------------------
simplTopBinds
  - simplify all top-level binders
  - for NonRec, call simplRecOrTopPair
  - for Rec,    call simplRecBind

	
	------------------------------
simplExpr (applied lambda)	==> simplNonRecBind
simplExpr (Let (NonRec ...) ..) ==> simplNonRecBind
simplExpr (Let (Rec ...)    ..) ==> simplify binders; simplRecBind

	------------------------------
simplRecBind	[binders already simplfied]
  - use simplRecOrTopPair on each pair in turn

simplRecOrTopPair [binder already simplified]
  Used for: recursive bindings (top level and nested)
	    top-level non-recursive bindings
  Returns: 
  - check for PreInlineUnconditionally
  - simplLazyBind

simplNonRecBind
  Used for: non-top-level non-recursive bindings
	    beta reductions (which amount to the same thing)
  Because it can deal with strict arts, it takes a 
	"thing-inside" and returns an expression

  - check for PreInlineUnconditionally
  - simplify binder, including its IdInfo
  - if strict binding
	simplStrictArg
	mkAtomicArgs
	completeNonRecX
    else
	simplLazyBind
	addFloats

simplNonRecX:	[given a *simplified* RHS, but an *unsimplified* binder]
  Used for: binding case-binder and constr args in a known-constructor case
  - check for PreInLineUnconditionally
  - simplify binder
  - completeNonRecX
 
	------------------------------
simplLazyBind:	[binder already simplified, RHS not]
  Used for: recursive bindings (top level and nested)
	    top-level non-recursive bindings
	    non-top-level, but *lazy* non-recursive bindings
	[must not be strict or unboxed]
  Returns floats + an augmented environment, not an expression
  - substituteIdInfo and add result to in-scope 
	[so that rules are available in rec rhs]
  - simplify rhs
  - mkAtomicArgs
  - float if exposes constructor or PAP
130
  - completeBind
131 132 133 134 135 136


completeNonRecX:	[binder and rhs both simplified]
  - if the the thing needs case binding (unlifted and not ok-for-spec)
	build a Case
   else
137
	completeBind
138 139
	addFloats

140
completeBind: 	[given a simplified RHS]
141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198
	[used for both rec and non-rec bindings, top level and not]
  - try PostInlineUnconditionally
  - add unfolding [this is the only place we add an unfolding]
  - add arity



Right hand sides and arguments
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In many ways we want to treat 
	(a) the right hand side of a let(rec), and 
	(b) a function argument
in the same way.  But not always!  In particular, we would
like to leave these arguments exactly as they are, so they
will match a RULE more easily.
	
	f (g x, h x)	
	g (+ x)

It's harder to make the rule match if we ANF-ise the constructor,
or eta-expand the PAP:

	f (let { a = g x; b = h x } in (a,b))
	g (\y. + x y)

On the other hand if we see the let-defns

	p = (g x, h x)
	q = + x

then we *do* want to ANF-ise and eta-expand, so that p and q
can be safely inlined.   

Even floating lets out is a bit dubious.  For let RHS's we float lets
out if that exposes a value, so that the value can be inlined more vigorously.
For example

	r = let x = e in (x,x)

Here, if we float the let out we'll expose a nice constructor. We did experiments
that showed this to be a generally good thing.  But it was a bad thing to float
lets out unconditionally, because that meant they got allocated more often.

For function arguments, there's less reason to expose a constructor (it won't
get inlined).  Just possibly it might make a rule match, but I'm pretty skeptical.
So for the moment we don't float lets out of function arguments either.


Eta expansion
~~~~~~~~~~~~~~
For eta expansion, we want to catch things like

	case e of (a,b) -> \x -> case a of (p,q) -> \y -> r

If the \x was on the RHS of a let, we'd eta expand to bring the two
lambdas together.  And in general that's a good thing to do.  Perhaps
we should eta expand wherever we find a (value) lambda?  Then the eta
expansion at a let RHS can concentrate solely on the PAP case.
199 200


201 202 203 204 205 206 207
%************************************************************************
%*									*
\subsection{Bindings}
%*									*
%************************************************************************

\begin{code}
208
simplTopBinds :: SimplEnv -> [InBind] -> SimplM [OutBind]
209

210
simplTopBinds env binds
211 212 213 214 215 216
  = do	{ 	-- Put all the top-level binders into scope at the start
		-- so that if a transformation rule has unexpectedly brought
		-- anything into scope, then we don't get a complaint about that.
		-- It's rather as if the top-level binders were imported.
	; env <- simplRecBndrs env (bindersOfBinds binds)
	; dflags <- getDOptsSmpl
217 218
	; let dump_flag = dopt Opt_D_dump_inlinings dflags || 
			  dopt Opt_D_dump_rule_firings dflags
219 220 221
	; env' <- simpl_binds dump_flag env binds
	; freeTick SimplifierDone
	; return (getFloats env') }
222
  where
223 224
	-- We need to track the zapped top-level binders, because
	-- they should have their fragile IdInfo zapped (notably occurrence info)
225
	-- That's why we run down binds and bndrs' simultaneously.
226 227 228
	--
	-- The dump-flag emits a trace for each top-level binding, which
	-- helps to locate the tracing for inlining and rule firing
229 230 231 232 233
    simpl_binds :: Bool -> SimplEnv -> [InBind] -> SimplM SimplEnv
    simpl_binds dump env []	      = return env
    simpl_binds dump env (bind:binds) = do { env' <- trace dump bind $
						     simpl_bind env bind
					   ; simpl_binds dump env' binds }
234

235 236
    trace True  bind = pprTrace "SimplBind" (ppr (bindersOf bind))
    trace False bind = \x -> x
simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
237

238 239 240 241
    simpl_bind env (Rec pairs)  = simplRecBind      env  TopLevel pairs
    simpl_bind env (NonRec b r) = simplRecOrTopPair env' TopLevel b b' r
	where
	  (env', b') = addLetIdInfo env b (lookupRecBndr env b)
242 243 244 245 246 247 248 249 250 251 252 253 254 255
\end{code}


%************************************************************************
%*									*
\subsection{Lazy bindings}
%*									*
%************************************************************************

simplRecBind is used for
	* recursive bindings only

\begin{code}
simplRecBind :: SimplEnv -> TopLevelFlag
256 257 258
	     -> [(InId, InExpr)]
	     -> SimplM SimplEnv
simplRecBind env top_lvl pairs
259 260
  = do	{ let (env_with_info, triples) = mapAccumL add_info env pairs
	; env' <- go (zapFloats env_with_info) triples
261 262 263
	; return (env `addRecFloats` env') }
	-- addFloats adds the floats from env', 
	-- *and* updates env with the in-scope set from env'
264
  where
265 266 267 268 269 270
    add_info :: SimplEnv -> (InBndr,InExpr) -> (SimplEnv, (InBndr, OutBndr, InExpr))
	-- Substitute in IdInfo, agument envt
    add_info env (bndr, rhs) = (env, (bndr, bndr', rhs))
	where
	  (env', bndr') = addLetIdInfo env bndr (lookupRecBndr env bndr)

271
    go env [] = return env
272
	
273 274
    go env ((old_bndr, new_bndr, rhs) : pairs)
	= do { env <- simplRecOrTopPair env top_lvl old_bndr new_bndr rhs
275
	     ; go env pairs }
276 277
\end{code}

278
simplOrTopPair is used for
279 280 281 282 283 284 285 286
	* recursive bindings (whether top level or not)
	* top-level non-recursive bindings

It assumes the binder has already been simplified, but not its IdInfo.

\begin{code}
simplRecOrTopPair :: SimplEnv
	     	  -> TopLevelFlag
287
	     	  -> InId -> OutBndr -> InExpr	-- Binder and rhs
288
	     	  -> SimplM SimplEnv	-- Returns an env that includes the binding
289

290 291 292 293
simplRecOrTopPair env top_lvl old_bndr new_bndr rhs
  | preInlineUnconditionally env top_lvl old_bndr rhs  	-- Check for unconditional inline
  = do	{ tick (PreInlineUnconditionally old_bndr)
	; return (extendIdSubst env old_bndr (mkContEx env rhs)) }
294 295

  | otherwise
296
  = simplLazyBind env top_lvl Recursive old_bndr new_bndr rhs env 
297 298 299 300 301
	-- May not actually be recursive, but it doesn't matter
\end{code}


simplLazyBind is used for
302 303 304
  * [simplRecOrTopPair] recursive bindings (whether top level or not)
  * [simplRecOrTopPair] top-level non-recursive bindings
  * [simplNonRecE]	non-top-level *lazy* non-recursive bindings
305 306 307

Nota bene:
    1. It assumes that the binder is *already* simplified, 
308
       and is in scope, and its IdInfo too, except unfolding
309 310 311 312 313 314 315 316 317 318

    2. It assumes that the binder type is lifted.

    3. It does not check for pre-inline-unconditionallly;
       that should have been done already.

\begin{code}
simplLazyBind :: SimplEnv
	      -> TopLevelFlag -> RecFlag
	      -> InId -> OutId		-- Binder, both pre-and post simpl
319
					-- The OutId has IdInfo, except arity, unfolding
320
	      -> InExpr -> SimplEnv 	-- The RHS and its environment
321
	      -> SimplM SimplEnv
322

323
simplLazyBind env top_lvl is_rec bndr bndr1 rhs rhs_se
324 325 326 327 328
  = do	{ let	rhs_env     = rhs_se `setInScope` env
		(tvs, body) = collectTyBinders rhs
	; (body_env, tvs') <- simplBinders rhs_env tvs
		-- See Note [Floating and type abstraction]
		-- in SimplUtils
329

330
  	-- Simplify the RHS; note the mkRhsStop, which tells 
331
	-- the simplifier that this is the RHS of a let.
332 333 334
	; let rhs_cont = mkRhsStop (applyTys (idType bndr1) (mkTyVarTys tvs'))
	; (body_env1, body1) <- simplExprF body_env body rhs_cont

335
	-- ANF-ise a constructor or PAP rhs
336
	; (body_env2, body2) <- prepareRhs body_env1 body1
337

338 339 340 341 342
	; (env', rhs')
	    <-	if not (doFloatFromRhs top_lvl is_rec False body2 body_env2)
		then				-- No floating, just wrap up!
		     do	{ rhs' <- mkLam tvs' (wrapFloats body_env2 body2)
			; return (env, rhs') }
343

344 345 346 347 348 349
		else if null tvs then 		-- Simple floating
		     do	{ tick LetFloatFromLet
			; return (addFloats env body_env2, body2) }

		else  				-- Do type-abstraction first
		     do	{ tick LetFloatFromLet
350
			; (poly_binds, body3) <- abstractFloats tvs' body_env2 body2
351 352 353 354 355
			; rhs' <- mkLam tvs' body3
			; return (extendFloats env poly_binds, rhs') }

	; completeBind env' top_lvl bndr bndr1 rhs' }
\end{code}
356 357 358 359 360 361 362 363 364 365 366 367 368

A specialised variant of simplNonRec used when the RHS is already simplified, 
notably in knownCon.  It uses case-binding where necessary.

\begin{code}
simplNonRecX :: SimplEnv
	     -> InId 		-- Old binder
	     -> OutExpr		-- Simplified RHS
	     -> SimplM SimplEnv

simplNonRecX env bndr new_rhs
  = do	{ (env, bndr') <- simplBinder env bndr
	; completeNonRecX env NotTopLevel NonRecursive
simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
369
			  (isStrictId bndr) bndr bndr' new_rhs }
370 371 372 373 374 375 376 377 378 379

completeNonRecX :: SimplEnv
		-> TopLevelFlag -> RecFlag -> Bool
	        -> InId 		-- Old binder
		-> OutId		-- New binder
	     	-> OutExpr		-- Simplified RHS
	     	-> SimplM SimplEnv

completeNonRecX env top_lvl is_rec is_strict old_bndr new_bndr new_rhs
  = do 	{ (env1, rhs1) <- prepareRhs (zapFloats env) new_rhs
380 381 382 383 384
	; (env2, rhs2) <- 
		if doFloatFromRhs top_lvl is_rec is_strict rhs1 env1
		then do	{ tick LetFloatFromLet
			; return (addFloats env env1, rhs1) }	-- Add the floats to the main env
		else return (env, wrapFloats env1 rhs1)		-- Wrap the floats around the RHS
385 386 387 388 389 390 391 392 393 394 395
	; completeBind env2 NotTopLevel old_bndr new_bndr rhs2 }
\end{code}

{- No, no, no!  Do not try preInlineUnconditionally in completeNonRecX
   Doing so risks exponential behaviour, because new_rhs has been simplified once already
   In the cases described by the folowing commment, postInlineUnconditionally will 
   catch many of the relevant cases.
  	-- This happens; for example, the case_bndr during case of
	-- known constructor:  case (a,b) of x { (p,q) -> ... }
	-- Here x isn't mentioned in the RHS, so we don't want to
	-- create the (dead) let-binding  let x = (a,b) in ...
396
	--
397 398 399
	-- Similarly, single occurrences can be inlined vigourously
	-- e.g.  case (f x, g y) of (a,b) -> ....
	-- If a,b occur once we can avoid constructing the let binding for them.
400

401 402 403 404 405 406 407
   Furthermore in the case-binding case preInlineUnconditionally risks extra thunks
	-- Consider 	case I# (quotInt# x y) of 
	--		  I# v -> let w = J# v in ...
	-- If we gaily inline (quotInt# x y) for v, we end up building an
	-- extra thunk:
	--		  let w = J# (quotInt# x y) in ...
	-- because quotInt# can fail.
408

409 410 411 412
  | preInlineUnconditionally env NotTopLevel bndr new_rhs
  = thing_inside (extendIdSubst env bndr (DoneEx new_rhs))
-}

413
----------------------------------
414 415 416 417 418 419 420 421 422
prepareRhs takes a putative RHS, checks whether it's a PAP or
constructor application and, if so, converts it to ANF, so that the 
resulting thing can be inlined more easily.  Thus
	x = (f a, g b)
becomes
	t1 = f a
	t2 = g b
	x = (t1,t2)

423 424 425 426 427 428
We also want to deal well cases like this
	v = (f e1 `cast` co) e2
Here we want to make e1,e2 trivial and get
	x1 = e1; x2 = e2; v = (f x1 `cast` co) v2
That's what the 'go' loop in prepareRhs does

429 430 431
\begin{code}
prepareRhs :: SimplEnv -> OutExpr -> SimplM (SimplEnv, OutExpr)
-- Adds new floats to the env iff that allows us to return a good RHS
simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
432
prepareRhs env (Cast rhs co)	-- Note [Float coercions]
433 434
  | (ty1, ty2) <- coercionKind co	-- Do *not* do this if rhs has an unlifted type
  , not (isUnLiftedType ty1)		-- see Note [Float coercions (unlifted)]
435 436 437 438
  = do	{ (env', rhs') <- makeTrivial env rhs
	; return (env', Cast rhs' co) }

prepareRhs env rhs
439 440
  = do	{ (is_val, env', rhs') <- go 0 env rhs 
	; return (env', rhs') }
441
  where
442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461
    go n_val_args env (Cast rhs co)
	= do { (is_val, env', rhs') <- go n_val_args env rhs
	     ; return (is_val, env', Cast rhs' co) }
    go n_val_args env (App fun (Type ty))
	= do { (is_val, env', rhs') <- go n_val_args env fun
	     ; return (is_val, env', App rhs' (Type ty)) }
    go n_val_args env (App fun arg)
	= do { (is_val, env', fun') <- go (n_val_args+1) env fun
	     ; case is_val of
		True -> do { (env'', arg') <- makeTrivial env' arg
			   ; return (True, env'', App fun' arg') }
		False -> return (False, env, App fun arg) }
    go n_val_args env (Var fun)
	= return (is_val, env, Var fun)
	where
	  is_val = n_val_args > 0	-- There is at least one arg
					-- ...and the fun a constructor or PAP
		 && (isDataConWorkId fun || n_val_args < idArity fun)
    go n_val_args env other
	= return (False, env, other)
462 463
\end{code}

464

465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486
Note [Float coercions]
~~~~~~~~~~~~~~~~~~~~~~
When we find the binding
	x = e `cast` co
we'd like to transform it to
	x' = e
	x = x `cast` co		-- A trivial binding
There's a chance that e will be a constructor application or function, or something
like that, so moving the coerion to the usage site may well cancel the coersions
and lead to further optimisation.  Example:

     data family T a :: *
     data instance T Int = T Int

     foo :: Int -> Int -> Int
     foo m n = ...
        where
          x = T m
          go 0 = 0
          go n = case x of { T m -> go (n-m) }
		-- This case should optimise

487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502
Note [Float coercions (unlifted)]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
BUT don't do [Float coercions] if 'e' has an unlifted type. 
This *can* happen:

     foo :: Int = (error (# Int,Int #) "urk") 
		  `cast` CoUnsafe (# Int,Int #) Int

If do the makeTrivial thing to the error call, we'll get
    foo = case error (# Int,Int #) "urk" of v -> v `cast` ...
But 'v' isn't in scope!  

These strange casts can happen as a result of case-of-case
	bar = case (case x of { T -> (# 2,3 #); F -> error "urk" }) of
		(# p,q #) -> p+q

503 504 505 506 507 508 509 510 511 512 513 514

\begin{code}
makeTrivial :: SimplEnv -> OutExpr -> SimplM (SimplEnv, OutExpr)
-- Binds the expression to a variable, if it's not trivial, returning the variable
makeTrivial env expr
  | exprIsTrivial expr
  = return (env, expr)
  | otherwise		-- See Note [Take care] below
  = do 	{ var <- newId FSLIT("a") (exprType expr)
	; env <- completeNonRecX env NotTopLevel NonRecursive 
				 False var var expr
	; return (env, substExpr env (Var var)) }
515
\end{code}
516 517


518 519 520 521 522 523
%************************************************************************
%*									*
\subsection{Completing a lazy binding}
%*									*
%************************************************************************

524 525 526 527 528
completeBind
  * deals only with Ids, not TyVars
  * takes an already-simplified binder and RHS
  * is used for both recursive and non-recursive bindings
  * is used for both top-level and non-top-level bindings
529 530 531 532 533 534 535 536

It does the following:
  - tries discarding a dead binding
  - tries PostInlineUnconditionally
  - add unfolding [this is the only place we add an unfolding]
  - add arity

It does *not* attempt to do let-to-case.  Why?  Because it is used for
537 538
  - top-level bindings (when let-to-case is impossible) 
  - many situations where the "rhs" is known to be a WHNF
539 540
		(so let-to-case is inappropriate).

541 542
Nor does it do the atomic-argument thing

543
\begin{code}
544 545 546 547 548 549 550 551 552 553
completeBind :: SimplEnv
	     -> TopLevelFlag		-- Flag stuck into unfolding
	     -> InId 			-- Old binder
	     -> OutId -> OutExpr	-- New binder and RHS
	     -> SimplM SimplEnv
-- completeBind may choose to do its work 
--	* by extending the substitution (e.g. let x = y in ...)
--	* or by adding to the floats in the envt

completeBind env top_lvl old_bndr new_bndr new_rhs
554
  | postInlineUnconditionally env top_lvl new_bndr occ_info new_rhs unfolding
555 556 557 558 559 560
		-- Inline and discard the binding
  = do	{ tick (PostInlineUnconditionally old_bndr)
	; -- pprTrace "postInlineUnconditionally" (ppr old_bndr <+> ppr new_bndr <+> ppr new_rhs) $
	  return (extendIdSubst env old_bndr (DoneEx new_rhs)) }
	-- Use the substitution to make quite, quite sure that the
	-- substitution will happen, since we are going to discard the binding
561 562 563

  |  otherwise
  = let
564
	-- 	Arity info
565 566
  	new_bndr_info = idInfo new_bndr `setArityInfo` exprArity new_rhs

567
	-- 	Unfolding info
568 569 570 571 572
	-- Add the unfolding *only* for non-loop-breakers
	-- Making loop breakers not have an unfolding at all 
	-- means that we can avoid tests in exprIsConApp, for example.
	-- This is important: if exprIsConApp says 'yes' for a recursive
	-- thing, then we can get into an infinite loop
573 574

	-- 	Demand info
575 576 577 578 579 580 581 582 583 584
	-- If the unfolding is a value, the demand info may
	-- go pear-shaped, so we nuke it.  Example:
	--	let x = (a,b) in
	--	case x of (p,q) -> h p q x
	-- Here x is certainly demanded. But after we've nuked
	-- the case, we'll get just
	--	let x = (a,b) in h a b x
	-- and now x is not demanded (I'm assuming h is lazy)
	-- This really happens.  Similarly
	--	let f = \x -> e in ...f..f...
585
	-- After inlining f at some of its call sites the original binding may
586 587 588 589 590 591 592 593
	-- (for example) be no longer strictly demanded.
	-- The solution here is a bit ad hoc...
 	info_w_unf = new_bndr_info `setUnfoldingInfo` unfolding
        final_info | loop_breaker		= new_bndr_info
		   | isEvaldUnfolding unfolding = zapDemandInfo info_w_unf `orElse` info_w_unf
		   | otherwise			= info_w_unf

	final_id = new_bndr `setIdInfo` final_info
594 595 596 597
    in
		-- These seqs forces the Id, and hence its IdInfo,
		-- and hence any inner substitutions
    final_id					`seq`
598
    -- pprTrace "Binding" (ppr final_id <+> ppr unfolding) $
599
    return (addNonRec env final_id new_rhs)
600
  where 
601
    unfolding    = mkUnfolding (isTopLevel top_lvl) new_rhs
602
    loop_breaker = isNonRuleLoopBreaker occ_info
603 604
    old_info     = idInfo old_bndr
    occ_info     = occInfo old_info
SamB's avatar
SamB committed
605
\end{code}
606 607 608



609 610 611 612 613 614
%************************************************************************
%*									*
\subsection[Simplify-simplExpr]{The main function: simplExpr}
%*									*
%************************************************************************

615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652
The reason for this OutExprStuff stuff is that we want to float *after*
simplifying a RHS, not before.  If we do so naively we get quadratic
behaviour as things float out.

To see why it's important to do it after, consider this (real) example:

	let t = f x
	in fst t
==>
	let t = let a = e1
		    b = e2
		in (a,b)
	in fst t
==>
	let a = e1
	    b = e2
	    t = (a,b)
	in
	a	-- Can't inline a this round, cos it appears twice
==>
	e1

Each of the ==> steps is a round of simplification.  We'd save a
whole round if we float first.  This can cascade.  Consider

	let f = g d
	in \x -> ...f...
==>
	let f = let d1 = ..d.. in \y -> e
	in \x -> ...f...
==>
	let d1 = ..d..
	in \x -> ...(\y ->e)...

Only in this second round can the \y be applied, and it 
might do the same again.


653
\begin{code}
654
simplExpr :: SimplEnv -> CoreExpr -> SimplM CoreExpr
655
simplExpr env expr = simplExprC env expr (mkBoringStop expr_ty')
656
		   where
657
		     expr_ty' = substTy env (exprType expr)
658
	-- The type in the Stop continuation, expr_ty', is usually not used
659
	-- It's only needed when discarding continuations after finding
660 661
	-- a function that returns bottom.
	-- Hence the lazy substitution
662

663

664 665 666
simplExprC :: SimplEnv -> CoreExpr -> SimplCont -> SimplM CoreExpr
	-- Simplify an expression, given a continuation
simplExprC env expr cont 
667 668 669 670 671 672 673 674 675 676 677
  = -- pprTrace "simplExprC" (ppr expr $$ ppr cont {- $$ ppr (seIdSubst env) -} $$ ppr (seFloats env) ) $
    do	{ (env', expr') <- simplExprF (zapFloats env) expr cont
	; -- pprTrace "simplExprC ret" (ppr expr $$ ppr expr') $
	  -- pprTrace "simplExprC ret3" (ppr (seInScope env')) $
	  -- pprTrace "simplExprC ret4" (ppr (seFloats env')) $
          return (wrapFloats env' expr') }

--------------------------------------------------
simplExprF :: SimplEnv -> InExpr -> SimplCont
	   -> SimplM (SimplEnv, OutExpr)

678 679 680
simplExprF env e cont 
  = -- pprTrace "simplExprF" (ppr e $$ ppr cont $$ ppr (seTvSubst env) $$ ppr (seIdSubst env) {- $$ ppr (seFloats env) -} ) $
    simplExprF' env e cont
681
    				     
682
simplExprF' env (Var v)	       cont = simplVar env v cont
683 684 685 686
simplExprF' env (Lit lit)      cont = rebuild env (Lit lit) cont
simplExprF' env (Note n expr)  cont = simplNote env n expr cont
simplExprF' env (Cast body co) cont = simplCast env body co cont
simplExprF' env (App fun arg)  cont = simplExprF env fun $
687
				      ApplyTo NoDup arg env cont
688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705

simplExprF' env expr@(Lam _ _) cont 
  = simplLam env (map zap bndrs) body cont
	-- The main issue here is under-saturated lambdas
	--   (\x1. \x2. e) arg1
	-- Here x1 might have "occurs-once" occ-info, because occ-info
	-- is computed assuming that a group of lambdas is applied
	-- all at once.  If there are too few args, we must zap the 
	-- occ-info.
  where
    n_args   = countArgs cont
    n_params = length bndrs
    (bndrs, body) = collectBinders expr
    zap | n_args >= n_params = \b -> b	
	| otherwise	     = \b -> if isTyVar b then b
				     else zapLamIdInfo b
	-- NB: we count all the args incl type args
	-- so we must count all the binders (incl type lambdas)
706

707
simplExprF' env (Type ty) cont
708
  = ASSERT( contIsRhsOrArg cont )
709 710
    do	{ ty' <- simplType env ty
	; rebuild env (Type ty') cont }
711

712
simplExprF' env (Case scrut bndr case_ty alts) cont
713 714 715
  | not (switchIsOn (getSwitchChecker env) NoCaseOfCase)
  = 	-- Simplify the scrutinee with a Select continuation
    simplExprF env scrut (Select NoDup bndr alts env cont)
716

717 718
  | otherwise
  = 	-- If case-of-case is off, simply simplify the case expression
719
	-- in a vanilla Stop context, and rebuild the result around it
720 721
    do	{ case_expr' <- simplExprC env scrut case_cont
	; rebuild env case_expr' cont }
722
  where
723
    case_cont = Select NoDup bndr alts env (mkBoringStop case_ty')
724
    case_ty'  = substTy env case_ty	-- c.f. defn of simplExpr
725

726 727 728 729
simplExprF' env (Let (Rec pairs) body) cont
  = do	{ env <- simplRecBndrs env (map fst pairs)
		-- NB: bndrs' don't have unfoldings or rules
		-- We add them as we go down
730

731 732
	; env <- simplRecBind env NotTopLevel pairs
	; simplExprF env body cont }
733

734 735
simplExprF' env (Let (NonRec bndr rhs) body) cont
  = simplNonRecE env bndr (rhs, env) ([], body) cont
736 737

---------------------------------
738 739 740
simplType :: SimplEnv -> InType -> SimplM OutType
	-- Kept monadic just so we can do the seqType
simplType env ty
741 742
  = -- pprTrace "simplType" (ppr ty $$ ppr (seTvSubst env)) $
    seqType new_ty   `seq`   returnSmpl new_ty
743
  where
744
    new_ty = substTy env ty
745 746 747
\end{code}


748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771
%************************************************************************
%*									*
\subsection{The main rebuilder}
%*									*
%************************************************************************

\begin{code}
rebuild :: SimplEnv -> OutExpr -> SimplCont -> SimplM (SimplEnv, OutExpr)
-- At this point the substitution in the SimplEnv should be irrelevant
-- only the in-scope set and floats should matter
rebuild env expr cont
  = -- pprTrace "rebuild" (ppr expr $$ ppr cont $$ ppr (seFloats env)) $
    case cont of
      Stop {}		      	   -> return (env, expr)
      CoerceIt co cont	      	   -> rebuild env (mkCoerce co expr) cont
      Select _ bndr alts se cont   -> rebuildCase (se `setFloats` env) expr bndr alts cont
      StrictArg fun ty info cont   -> rebuildCall env (fun `App` expr) (funResultTy ty) info cont
      StrictBind b bs body se cont -> do { env' <- simplNonRecX (se `setFloats` env) b expr
					 ; simplLam env' bs body cont }
      ApplyTo _ arg se cont	   -> do { arg' <- simplExpr (se `setInScope` env) arg
				         ; rebuild env (App expr arg') cont }
\end{code}


772 773 774 775 776 777 778
%************************************************************************
%*									*
\subsection{Lambdas}
%*									*
%************************************************************************

\begin{code}
779 780
simplCast :: SimplEnv -> InExpr -> Coercion -> SimplCont
	  -> SimplM (SimplEnv, OutExpr)
781
simplCast env body co cont
782 783 784
  = do	{ co' <- simplType env co
	; simplExprF env body (addCoerce co' cont) }
  where
785 786
       addCoerce co cont = add_coerce co (coercionKind co) cont

simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
787 788 789
       add_coerce co (s1, k1) cont 	-- co :: ty~ty
         | s1 `coreEqType` k1 = cont	-- is a no-op

790 791
       add_coerce co1 (s1, k2) (CoerceIt co2 cont)
         | (l1, t1) <- coercionKind co2
792 793 794 795 796 797 798 799 800 801 802 803
                -- 	coerce T1 S1 (coerce S1 K1 e)
		-- ==>
		--	e, 			if T1=K1
		--	coerce T1 K1 e,		otherwise
		--
		-- For example, in the initial form of a worker
		-- we may find 	(coerce T (coerce S (\x.e))) y
		-- and we'd like it to simplify to e[y/x] in one round 
		-- of simplification
         , s1 `coreEqType` t1  = cont		 -- The coerces cancel out  
         | otherwise           = CoerceIt (mkTransCoercion co1 co2) cont
    
804 805 806 807 808 809 810 811 812 813 814
       add_coerce co (s1s2, t1t2) (ApplyTo dup (Type arg_ty) arg_se cont)
		-- (f `cast` g) ty  --->   (f ty) `cast` (g @ ty)
		-- This implements the PushT rule from the paper
	 | Just (tyvar,_) <- splitForAllTy_maybe s1s2
	 , not (isCoVar tyvar)
	 = ApplyTo dup (Type ty') (zapSubstEnv env) (addCoerce (mkInstCoercion co ty') cont)
	 where
	   ty' = substTy arg_se arg_ty

	-- ToDo: the PushC rule is not implemented at all

815
       add_coerce co (s1s2, t1t2) (ApplyTo dup arg arg_se cont)
816
         | not (isTypeArg arg)  -- This implements the Push rule from the paper
817
         , isFunTy s1s2	  -- t1t2 must be a function type, becuase it's applied
818 819 820 821 822 823 824 825 826 827 828 829
                -- co : s1s2 :=: t1t2
		--	(coerce (T1->T2) (S1->S2) F) E
		-- ===> 
		--	coerce T2 S2 (F (coerce S1 T1 E))
		--
		-- t1t2 must be a function type, T1->T2, because it's applied
		-- to something but s1s2 might conceivably not be
		--
		-- When we build the ApplyTo we can't mix the out-types
		-- with the InExpr in the argument, so we simply substitute
		-- to make it all consistent.  It's a bit messy.
		-- But it isn't a common case.
830 831
		--
		-- Example of use: Trac #995
832
         = ApplyTo dup new_arg (zapSubstEnv env) (addCoerce co2 cont)
833 834 835 836 837
         where
           -- we split coercion t1->t2 :=: s1->s2 into t1 :=: s1 and 
           -- t2 :=: s2 with left and right on the curried form: 
           --    (->) t1 t2 :=: (->) s1 s2
           [co1, co2] = decomposeCo 2 co
838
           new_arg    = mkCoerce (mkSymCoercion co1) arg'
839 840
	   arg'       = substExpr arg_se arg

841
       add_coerce co _ cont = CoerceIt co cont
842 843
\end{code}

844

845 846 847 848 849
%************************************************************************
%*									*
\subsection{Lambdas}
%*									*
%************************************************************************
850 851

\begin{code}
852 853 854 855
simplLam :: SimplEnv -> [InId] -> InExpr -> SimplCont
	 -> SimplM (SimplEnv, OutExpr)

simplLam env [] body cont = simplExprF env body cont
856 857

      	-- Type-beta reduction
858 859 860 861 862
simplLam env (bndr:bndrs) body (ApplyTo _ (Type ty_arg) arg_se cont)
  = ASSERT( isTyVar bndr )
    do	{ tick (BetaReduction bndr)
	; ty_arg' <- simplType (arg_se `setInScope` env) ty_arg
	; simplLam (extendTvSubst env bndr ty_arg') bndrs body cont }
863 864

	-- Ordinary beta reduction
865 866 867
simplLam env (bndr:bndrs) body (ApplyTo _ arg arg_se cont)
  = do	{ tick (BetaReduction bndr)	
	; simplNonRecE env bndr (arg, arg_se) (bndrs, body) cont }
868

869
	-- Not enough args, so there are real lambdas left to put in the result
870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901
simplLam env bndrs body cont
  = do	{ (env, bndrs') <- simplLamBndrs env bndrs
	; body' <- simplExpr env body
	; new_lam <- mkLam bndrs' body'
	; rebuild env new_lam cont }

------------------
simplNonRecE :: SimplEnv 
	     -> InId 			-- The binder
	     -> (InExpr, SimplEnv)	-- Rhs of binding (or arg of lambda)
	     -> ([InId], InExpr)	-- Body of the let/lambda
					--	\xs.e
	     -> SimplCont
	     -> SimplM (SimplEnv, OutExpr)

-- simplNonRecE is used for
--  * non-top-level non-recursive lets in expressions
--  * beta reduction
--
-- It deals with strict bindings, via the StrictBind continuation,
-- which may abort the whole process
--
-- The "body" of the binding comes as a pair of ([InId],InExpr)
-- representing a lambda; so we recurse back to simplLam
-- Why?  Because of the binder-occ-info-zapping done before 
-- 	 the call to simplLam in simplExprF (Lam ...)

simplNonRecE env bndr (rhs, rhs_se) (bndrs, body) cont
  | preInlineUnconditionally env NotTopLevel bndr rhs
  = do	{ tick (PreInlineUnconditionally bndr)
	; simplLam (extendIdSubst env bndr (mkContEx rhs_se rhs)) bndrs body cont }

simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
902
  | isStrictId bndr
903 904 905 906
  = do	{ simplExprF (rhs_se `setFloats` env) rhs 
		     (StrictBind bndr bndrs body env cont) }

  | otherwise
907 908 909 910
  = do	{ (env1, bndr1) <- simplNonRecBndr env bndr
	; let (env2, bndr2) = addLetIdInfo env1 bndr bndr1
	; env3 <- simplLazyBind env2 NotTopLevel NonRecursive bndr bndr2 rhs rhs_se
	; simplLam env3 bndrs body cont }
911 912
\end{code}

913

914 915 916 917 918 919
%************************************************************************
%*									*
\subsection{Notes}
%*									*
%************************************************************************

sof's avatar
sof committed
920
\begin{code}
921 922
-- Hack alert: we only distinguish subsumed cost centre stacks for the 
-- purposes of inlining.  All other CCCSs are mapped to currentCCS.
923
simplNote env (SCC cc) e cont
924 925
  = do 	{ e' <- simplExpr (setEnclosingCC env currentCCS) e
	; rebuild env (mkSCC cc e') cont }
926 927 928

-- See notes with SimplMonad.inlineMode
simplNote env InlineMe e cont
929
  | Just (inside, outside) <- splitInlineCont cont  -- Boring boring continuation; see notes above
930
  = do	{ 			-- Don't inline inside an INLINE expression
931 932
	  e' <- simplExprC (setMode inlineMode env) e inside
	; rebuild env (mkInlineMe e') outside }
933 934 935 936

  | otherwise  	-- Dissolve the InlineMe note if there's
		-- an interesting context of any kind to combine with
		-- (even a type application -- anything except Stop)
937
  = simplExprF env e cont
938 939

simplNote env (CoreNote s) e cont
940 941
  = simplExpr env e    `thenSmpl` \ e' ->
    rebuild env (Note (CoreNote s) e') cont
942 943 944
\end{code}


945 946
%************************************************************************
%*									*
947
\subsection{Dealing with calls}
948 949
%*									*
%************************************************************************
950

951
\begin{code}
952
simplVar env var cont
953 954 955
  = case substId env var of
	DoneEx e	 -> simplExprF (zapSubstEnv env) e cont
	ContEx tvs ids e -> simplExprF (setSubstEnv env tvs ids) e cont
956
	DoneId var1      -> completeCall (zapSubstEnv env) var1 cont
957
		-- Note [zapSubstEnv]
958 959 960 961 962 963 964 965
		-- The template is already simplified, so don't re-substitute.
		-- This is VITAL.  Consider
		--	let x = e in
		--	let y = \z -> ...x... in
		--	\ x -> ...y...
		-- We'll clone the inner \x, adding x->x' in the id_subst
		-- Then when we inline y, we must *not* replace x by x' in
		-- the inlined copy!!
966

967
---------------------------------------------------------
968
--	Dealing with a call site
969

970
completeCall env var cont
971 972 973 974 975 976 977 978 979 980 981 982
  = do	{ dflags <- getDOptsSmpl
	; let	(args,call_cont) = contArgs cont
		-- The args are OutExprs, obtained by *lazily* substituting
		-- in the args found in cont.  These args are only examined
		-- to limited depth (unless a rule fires).  But we must do
		-- the substitution; rule matching on un-simplified args would
		-- be bogus

	------------- First try rules ----------------
	-- Do this before trying inlining.  Some functions have 
	-- rules *and* are strict; in this case, we don't want to 
	-- inline the wrapper of the non-specialised thing; better
983
	-- to call the specialised thing instead.
984
	--
985 986 987
	-- We used to use the black-listing mechanism to ensure that inlining of 
	-- the wrapper didn't occur for things that have specialisations till a 
	-- later phase, so but now we just try RULES first
988
	--
989 990
	-- Note [Rules for recursive functions]
	-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
991 992 993 994 995 996 997 998 999 1000 1001
	-- You might think that we shouldn't apply rules for a loop breaker: 
	-- doing so might give rise to an infinite loop, because a RULE is
	-- rather like an extra equation for the function:
	--	RULE:		f (g x) y = x+y
	--	Eqn:		f a     y = a-y
	--
	-- But it's too drastic to disable rules for loop breakers.  
	-- Even the foldr/build rule would be disabled, because foldr 
	-- is recursive, and hence a loop breaker:
	--	foldr k z (build g) = g k z
	-- So it's up to the programmer: rules can cause divergence
1002
	; rules <- getRules
1003
	; let	in_scope   = getInScope env
1004
		maybe_rule = case activeRule dflags env of
1005 1006 1007 1008 1009 1010
				Nothing     -> Nothing	-- No rules apply
				Just act_fn -> lookupRule act_fn in_scope 
							  rules var args 
	; case maybe_rule of {
	    Just (rule, rule_rhs) -> 
		tick (RuleFired (ru_name rule))			`thenSmpl_`
1011
		(if dopt Opt_D_dump_rule_firings dflags then
1012
		   pprTrace "Rule fired" (vcat [
1013
			text "Rule:" <+> ftext (ru_name rule),
1014
			text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
1015 1016
			text "After: " <+> pprCoreExpr rule_rhs,
			text "Cont:  " <+> ppr call_cont])
1017 1018
		 else
			id)		$
1019 1020
		simplExprF env rule_rhs (dropArgs (ruleArity rule) cont)
		-- The ruleArity says how many args the rule consumed
1021
	
1022 1023 1024 1025 1026 1027 1028 1029 1030 1031
	  ; Nothing -> do	-- No rules

	------------- Next try inlining ----------------
	{ let	arg_infos = [interestingArg arg | arg <- args, isValArg arg]
		n_val_args = length arg_infos
	      	interesting_cont = interestingCallContext (notNull args)
						  	  (notNull arg_infos)
						  	  call_cont
	 	active_inline = activeInline env var
		maybe_inline  = callSiteInline dflags active_inline
1032
				       var arg_infos interesting_cont
1033 1034 1035 1036
	; case maybe_inline of {
	    Just unfolding  	-- There is an inlining!
	      ->  do { tick (UnfoldingDone var)
		     ; (if dopt Opt_D_dump_inlinings dflags then
1037
			   pprTrace ("Inlining done" ++ showSDoc (ppr var)) (vcat [
1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052
				text "Before:" <+> ppr var <+> sep (map pprParendExpr args),
				text "Inlined fn: " <+> nest 2 (ppr unfolding),
				text "Cont:  " <+> ppr call_cont])
			 else
				id)
		       simplExprF env unfolding cont }

	    ; Nothing -> 		-- No inlining!

	------------- No inlining! ----------------
	-- Next, look for rules or specialisations that match
	--
	rebuildCall env (Var var) (idType var) 
		    (mkArgInfo var n_val_args call_cont) cont
    }}}}
1053

1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069
rebuildCall :: SimplEnv
	    -> OutExpr -> OutType	-- Function and its type
	    -> (Bool, [Bool])		-- See SimplUtils.mkArgInfo
	    -> SimplCont
	    -> SimplM (SimplEnv, OutExpr)
rebuildCall env fun fun_ty (has_rules, []) cont
  -- When we run out of strictness args, it means
  -- that the call is definitely bottom; see SimplUtils.mkArgInfo
  -- Then we want to discard the entire strict continuation.  E.g.
  --	* case (error "hello") of { ... }
  --	* (error "Hello") arg
  --	* f (error "Hello") where f is strict
  --	etc
  -- Then, especially in the first of these cases, we'd like to discard
  -- the continuation, leaving just the bottoming expression.  But the
  -- type might not be right, so we may have to add a coerce.
1070
  | not (contIsTrivial cont)	 -- Only do this if there is a non-trivial
1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096
  = return (env, mk_coerce fun)  -- contination to discard, else we do it
  where				 -- again and again!
    cont_ty = contResultType cont
    co      = mkUnsafeCoercion fun_ty cont_ty
    mk_coerce expr | cont_ty `coreEqType` fun_ty = fun
		   | otherwise = mkCoerce co fun

rebuildCall env fun fun_ty info (ApplyTo _ (Type arg_ty) se cont)
  = do	{ ty' <- simplType (se `setInScope` env) arg_ty
	; rebuildCall env (fun `App` Type ty') (applyTy fun_ty ty') info cont }

rebuildCall env fun fun_ty (has_rules, str:strs) (ApplyTo _ arg arg_se cont)
  | str || isStrictType arg_ty		-- Strict argument
  = -- pprTrace "Strict Arg" (ppr arg $$ ppr (seIdSubst env) $$ ppr (seInScope env)) $
    simplExprF (arg_se `setFloats` env) arg
	       (StrictArg fun fun_ty (has_rules, strs) cont)
		-- Note [Shadowing]

  | otherwise				-- Lazy argument
	-- DO NOT float anything outside, hence simplExprC
	-- There is no benefit (unlike in a let-binding), and we'd
	-- have to be very careful about bogus strictness through 
	-- floating a demanded let.
  = do	{ arg' <- simplExprC (arg_se `setInScope` env) arg
			     (mkLazyArgStop arg_ty has_rules)
	; rebuildCall env (fun `App` arg') res_ty (has_rules, strs) cont }
1097
  where
1098
    (arg_ty, res_ty) = splitFunTy fun_ty
1099

1100 1101
rebuildCall env fun fun_ty info cont
  = rebuild env fun cont
1102
\end{code}
1103

1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126
Note [Shadowing]
~~~~~~~~~~~~~~~~
This part of the simplifier may break the no-shadowing invariant
Consider
	f (...(\a -> e)...) (case y of (a,b) -> e')
where f is strict in its second arg
If we simplify the innermost one first we get (...(\a -> e)...)
Simplifying the second arg makes us float the case out, so we end up with
	case y of (a,b) -> f (...(\a -> e)...) e'
So the output does not have the no-shadowing invariant.  However, there is
no danger of getting name-capture, because when the first arg was simplified
we used an in-scope set that at least mentioned all the variables free in its
static environment, and that is enough.

We can't just do innermost first, or we'd end up with a dual problem:
	case x of (a,b) -> f e (...(\a -> e')...)

I spent hours trying to recover the no-shadowing invariant, but I just could
not think of an elegant way to do it.  The simplifier is already knee-deep in
continuations.  We have to keep the right in-scope set around; AND we have
to get the effect that finding (error "foo") in a strict arg position will
discard the entire application and replace it with (error "foo").  Getting
all this at once is TOO HARD!
1127

1128 1129
%************************************************************************
%*									*
1130
		Rebuilding a cse expression
1131 1132
%*									*
%************************************************************************
1133

1134 1135 1136
Blob of helper functions for the "case-of-something-else" situation.

\begin{code}
1137
---------------------------------------------------------
1138
-- 	Eliminate the case if possible
1139

1140 1141 1142
rebuildCase :: SimplEnv
	    -> OutExpr		-- Scrutinee
	    -> InId		-- Case binder
1143
	    -> [InAlt]		-- Alternatives (inceasing order)
1144
	    -> SimplCont
1145
	    -> SimplM (SimplEnv, OutExpr)
1146

1147 1148 1149 1150
--------------------------------------------------
--	1. Eliminate the case if there's a known constructor
--------------------------------------------------

1151 1152 1153 1154
rebuildCase env scrut case_bndr alts cont
  | Just (con,args) <- exprIsConApp_maybe scrut	
	-- Works when the scrutinee is a variable with a known unfolding
	-- as well as when it's an explicit constructor application
1155
  = knownCon env scrut (DataAlt con) args case_bndr alts cont
1156

1157 1158
  | Lit lit <- scrut	-- No need for same treatment as constructors
			-- because literals are inlined more vigorously
1159
  = knownCon env scrut (LitAlt lit) [] case_bndr alts cont
1160

1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199 1200

--------------------------------------------------
--	2. Eliminate the case if scrutinee is evaluated
--------------------------------------------------

rebuildCase env scrut case_bndr [(con,bndrs,rhs)] cont
  -- See if we can get rid of the case altogether
  -- See the extensive notes on case-elimination above
  -- mkCase made sure that if all the alternatives are equal, 
  -- then there is now only one (DEFAULT) rhs
 | all isDeadBinder bndrs	-- bndrs are [InId]

	-- Check that the scrutinee can be let-bound instead of case-bound
 , exprOkForSpeculation scrut
		-- OK not to evaluate it
		-- This includes things like (==# a# b#)::Bool
		-- so that we simplify 
		-- 	case ==# a# b# of { True -> x; False -> x }
		-- to just
		--	x
		-- This particular example shows up in default methods for
		-- comparision operations (e.g. in (>=) for Int.Int32)
	|| exprIsHNF scrut			-- It's already evaluated
	|| var_demanded_later scrut		-- It'll be demanded later

--      || not opt_SimplPedanticBottoms)	-- Or we don't care!
--	We used to allow improving termination by discarding cases, unless -fpedantic-bottoms was on,
-- 	but that breaks badly for the dataToTag# primop, which relies on a case to evaluate
-- 	its argument:  case x of { y -> dataToTag# y }
--	Here we must *not* discard the case, because dataToTag# just fetches the tag from
--	the info pointer.  So we'll be pedantic all the time, and see if that gives any
-- 	other problems
--	Also we don't want to discard 'seq's
  = do	{ tick (CaseElim case_bndr)
	; env <- simplNonRecX env case_bndr scrut
	; simplExprF env rhs cont }
  where
	-- The case binder is going to be evaluated later, 
	-- and the scrutinee is a simple variable
    var_demanded_later (Var v) = isStrictDmd (idNewDemandInfo case_bndr)
1201 1202 1203
    		       	         && not (isTickBoxOp v)	
				    -- ugly hack; covering this case is what 
				    -- exprOkForSpeculation was intended for.
1204 1205 1206 1207 1208 1209 1210 1211
    var_demanded_later other   = False


--------------------------------------------------
--	3. Catch-all case
--------------------------------------------------

rebuildCase env scrut case_bndr alts cont
1212 1213 1214
  = do	{ 	-- Prepare the continuation;
		-- The new subst_env is in place
	  (env, dup_cont, nodup_cont) <- prepareCaseCont env alts cont
1215

1216
	-- Simplify the alternatives
1217
	; (scrut', case_bndr', alts') <- simplAlts env scrut case_bndr alts dup_cont
1218
	; let res_ty' = contResultType dup_cont
1219
	; case_expr <- mkCase scrut' case_bndr' res_ty' alts'
sof's avatar
sof committed
1220

1221 1222
	-- Notice that rebuildDone returns the in-scope set from env, not alt_env
	-- The case binder *not* scope over the whole returned case-expression
1223
	; rebuild env case_expr nodup_cont }
1224
\end{code}
1225

1226 1227 1228 1229 1230
simplCaseBinder checks whether the scrutinee is a variable, v.  If so,
try to eliminate uses of v in the RHSs in favour of case_bndr; that
way, there's a chance that v will now only be used once, and hence
inlined.

simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
1231 1232
Note [no-case-of-case]
~~~~~~~~~~~~~~~~~~~~~~
1233 1234 1235
There is a time we *don't* want to do that, namely when
-fno-case-of-case is on.  This happens in the first simplifier pass,
and enhances full laziness.  Here's the bad case:
1236 1237 1238 1239 1240
	f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
If we eliminate the inner case, we trap it inside the I# v -> arm,
which might prevent some full laziness happening.  I've seen this
in action in spectral/cichelli/Prog.hs:
	 [(m,n) | m <- [1..max], n <- [1..max]]
1241 1242
Hence the check for NoCaseOfCase.

simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
1243 1244 1245 1246
Note [Suppressing the case binder-swap]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
There is another situation when it might make sense to suppress the
case-expression binde-swap. If we have
1247 1248 1249 1250 1251 1252 1253 1254 1255

    case x of w1 { DEFAULT -> case x of w2 { A -> e1; B -> e2 }
	           ...other cases .... }

We'll perform the binder-swap for the outer case, giving

    case x of w1 { DEFAULT -> case w1 of w2 { A -> e1; B -> e2 } 
	           ...other cases .... }

1256 1257 1258 1259
But there is no point in doing it for the inner case, because w1 can't
be inlined anyway.  Furthermore, doing the case-swapping involves
zapping w2's occurrence info (see paragraphs that follow), and that
forces us to bind w2 when doing case merging.  So we get
1260 1261 1262 1263 1264 1265 1266 1267 1268 1269 1270 1271

    case x of w1 { A -> let w2 = w1 in e1
		   B -> let w2 = w1 in e2
	           ...other cases .... }

This is plain silly in the common case where w2 is dead.

Even so, I can't see a good way to implement this idea.  I tried
not doing the binder-swap if the scrutinee was already evaluated
but that failed big-time:

	data T = MkT !Int
1272

1273 1274 1275
	case v of w  { MkT x ->
	case x of x1 { I# y1 ->
	case x of x2 { I# y2 -> ...
1276

1277 1278 1279 1280 1281
Notice that because MkT is strict, x is marked "evaluated".  But to
eliminate the last case, we must either make sure that x (as well as
x1) has unfolding MkT y1.  THe straightforward thing to do is to do
the binder-swap.  So this whole note is a no-op.

simonpj@microsoft.com's avatar
simonpj@microsoft.com committed
1282 1283
Note [zapOccInfo]
~~~~~~~~~~~~~~~~~
1284 1285 1286 1287
If we replace the scrutinee, v, by tbe case binder, then we have to nuke
any occurrence info (eg IAmDead) in the case binder, because the
case-binder now effectively occurs whenever v does.  AND we have to do
the same for the pattern-bound variables!  Example:
1288

1289
	(case x of { (a,b) -> a }) (case x of { (p,q) -> q })
1290

1291 1292
Here, b and p are dead.  But when we move the argment inside the first
case RHS, and eliminate the second case, we get
1293