Commit ee643698 authored by Simon Peyton Jones's avatar Simon Peyton Jones

Refactor filterAlts into two parts

This splits filterAlts into two:
 - filterAlts
 - refineDefaultAlt

No change in functionality
parent 0899911c
This diff is collapsed.
......@@ -62,10 +62,8 @@ import MonadUtils
import Outputable
import FastString
import Pair
import ListSetOps ( minusList )
import Control.Monad ( when )
import Data.List ( partition )
......@@ -1669,107 +1667,27 @@ of the inner case y, which give us nowhere to go!
prepareAlts :: OutExpr -> OutId -> [InAlt] -> SimplM ([AltCon], [InAlt])
-- The returned alternatives can be empty, none are possible
prepareAlts scrut case_bndr' alts
| Just (tc, tys) <- splitTyConApp_maybe (varType case_bndr')
-- Case binder is needed just for its type. Note that as an
-- OutId, it has maximum information; this is important.
-- Test simpl013 is an example
= do { us <- getUniquesM
; let (imposs_deflt_cons', refined_deflt, alts')
= filterAlts us (varType case_bndr') imposs_cons alts
(combining_done, imposs_deflt_cons'', alts'')
= combineIdenticalAlts imposs_deflt_cons' alts'
; when refined_deflt $ tick (FillInCaseDefault case_bndr')
; when combining_done $ tick (AltMerge case_bndr')
; return (imposs_deflt_cons'', alts'') }
; let (idcs1, alts1) = filterAlts tc tys imposs_cons alts
(yes2, alts2) = refineDefaultAlt us tc tys idcs1 alts1
(yes3, idcs3, alts3) = combineIdenticalAlts idcs1 alts2
-- "idcs" stands for "impossible default data constructors"
-- i.e. the constructors that can't match the default case
; when yes2 $ tick (FillInCaseDefault case_bndr')
; when yes3 $ tick (AltMerge case_bndr')
; return (idcs3, alts3) }
| otherwise -- Not a data type, so nothing interesting happens
= return ([], alts)
imposs_cons = case scrut of
Var v -> otherCons (idUnfolding v)
_ -> []
{- Note [Combine identical alternatives]
If several alternatives are identical, merge them into a single
DEFAULT alternative. I've occasionally seen this making a big
case e of =====> case e of
C _ -> f x D v -> ....v....
D v -> ....v.... DEFAULT -> f x
DEFAULT -> f x
The point is that we merge common RHSs, at least for the DEFAULT case.
[One could do something more elaborate but I've never seen it needed.]
To avoid an expensive test, we just merge branches equal to the *first*
alternative; this picks up the common cases
a) all branches equal
b) some branches equal to the DEFAULT (which occurs first)
The case where Combine Identical Alternatives transformation showed up
was like this (base/Foreign/C/Err/Error.hs):
x | p `is` 1 -> e1
| p `is` 2 -> e2
where @is@ was something like
p `is` n = p /= (-1) && p == n
This gave rise to a horrible sequence of cases
case p of
(-1) -> $j p
1 -> e1
DEFAULT -> $j p
and similarly in cascade for all the join points!
NB: it's important that all this is done in [InAlt], *before* we work
on the alternatives themselves, because Simpify.simplAlt may zap the
occurrence info on the binders in the alternatives, which in turn
defeats combineIdenticalAlts (see Trac #7360).
Note [Care with impossible-constructors when combining alternatives]
Suppose we have (Trac #10538)
data T = A | B | C
... case x::T of
A -> e2
B -> e1
When calling combineIdentialAlts, we'll have computed that the "impossible
constructors" for the DEFAULT alt is {A,B}, since if x is A or B we'll
take the other alternatives. But suppose we combine B into the DEFAULT,
to get
... case x::T of
A -> e2
Then we must be careful to trim the impossible constructors to just {A},
else we risk compiling 'e1' wrong!
combineIdenticalAlts :: [AltCon] -> [InAlt] -> (Bool, [AltCon], [InAlt])
-- See Note [Combine identical alternatives]
-- See Note [Care with impossible-constructors when combining alternatives]
-- True <=> we did some combining, result is a single DEFAULT alternative
combineIdenticalAlts imposs_cons ((_con1,bndrs1,rhs1) : con_alts)
| all isDeadBinder bndrs1 -- Remember the default
, not (null eliminated_alts) -- alternative comes first
= (True, imposs_cons', deflt_alt : filtered_alts)
(eliminated_alts, filtered_alts) = partition identical_to_alt1 con_alts
deflt_alt = (DEFAULT, [], mkTicks (concat tickss) rhs1)
imposs_cons' = imposs_cons `minusList` map fstOf3 eliminated_alts
cheapEqTicked e1 e2 = cheapEqExpr' tickishFloatable e1 e2
identical_to_alt1 (_con,bndrs,rhs)
= all isDeadBinder bndrs && rhs `cheapEqTicked` rhs1
tickss = map (stripTicksT tickishFloatable . thirdOf3) eliminated_alts
combineIdenticalAlts imposs_cons alts
= (False, imposs_cons, alts)
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